摘要:

Characterisation and Correction of Instrumental Bias in Inductively Coupled Plasma Quadrupole Mass Spectrometry for Accurate Measurement of Lead Isotope Ratios IAN S. BEGLEY† AND BARRY L. SHARP Department of Chemistry, L oughborough University, L oughborough, L eicestershire, L E11 3TU, UK The accuracy and external precision of isotope ratio was found to impose limitations upon the accuracy and precision of measured isotope ratios. Mass dependent effects measurements can be limited by any of a number of instrumental bias factors which include mass bias, pulse pile- are well known in quadrupole based ICP-MS, the most common of these being mass bias.5–7 In this study, those forms up, background effects and mass scale shift.Measured isotope ratios are generally biased from their true value by the of instrumental instability and bias which limit the accuracy and external precision of isotope ratio measurements made by interaction of several of these instrumental bias factors.Attainment of accurate isotope ratio measurement requires quadrupole based ICP-MS are characterised. Methods for minimising the influences of the various instrumental bias careful consideration of all possible causes of instrumental bias and adoption of methods for their elimination or correction. factors have been applied to Pb isotope ratio measurement to yield high accuracy and precision. The various forms of instrumental bias observed in measurement of the Pb isotope ratios for NIST SRM 981 Natural Lead ( Isotopic) were minimised to reveal EXPERIMENTAL 204Pb5206Pb=0.0592±0.0002, 207Pb5206Pb=0.9148±0.0007, Instrumentation 208Pb5206Pb=2.1709±0.0015 (n=5) compared with certified values of 0.059042±0.000037, 0.91464±0.00033 and The ICP used employed a 27 MHz crystal-controlled supply 2.1681±0.0008, respectively.The thallium mass bias (Plasma-Therm, Kresson, NJ, USA, Model 2500F) with an concentration procedure was employed in obtaining this data. automatic impedance-matching network.The ICP was oper- All uncertainties given as 2s. ated at 1300W and the plasma, auxiliary and nebuliser gas flow rates were 12.0, 0.0 and 0.75 l min-1, respectively. A glass Keywords: Inductively coupled plasma mass spectrometry; concentric nebuliser (J.E. Meinhard, Santa Cruz, CA, USA, isotope ratio measurement; mass scale shift; mass bias; lead Model TR-30-CZ), peristaltic pump (Gilson, Villiers Le Bel, France, Model Minipuls 2) and water-cooled single pass spray The use of quadrupole mass analysers in ICP-MS fundamen- chamber, of the Surrey design,8 were used for sample introduc- tally limits the accuracy and precision to which isotope ratio tion.The plasma source was centred about the sampling orifice, measurements may be made. Quadrupole mass analysers are which was located approximately 12 mm from the load coil. specifically designed for, and hence perform optimally in, the The quadrupole mass analyser was a VG Micromass scanning of relatively large mass ranges.As such they are (Altrincham, Cheshire, UK) Model 12-12S, fitted within the inherently less stable than magnetic sector mass analysers and vacuum system of the ICP-MS instrument described by Date less well suited to highly precise and accurate isotope ratio and Hutchinson.9 The efficiency of ion transmission through measurement. Quadrupole mass analysers are none the less this vacuum system is lower than that given by commercially capable of providing sufficient accuracy to be useful in many manufactured ICP-MS instruments.The ICP-MS instrument isotope ratio studies. The precision of isotope ratio measure- used had a sensitivity of 2×106 counts s-1 per mg l-1. The ments made by quadrupole ICP-MS has at best approached original cones and lens stack were replaced with those of 0.1% RSD,1 and then, only for isotopes of similar abundance the type utilised in VG PlasmaQuad II instruments (VG (e.g. 107Ag5109Ag). For isotope ratios involving an isotope of Elemental, Winsford, Cheshire, UK). The detector was a low abundance, such as 204Pb, a precision of 0.2 to 1.0% RSD continuous dynode channel electron multiplier (Galileo, is typical.2 The accuracy and precision of 207Pb5206Pb ratio Sturbridge, MA, USA, Model 4870 V), positioned at right measurements for NIST SRM 981 Natural Lead (Isotopic), angles to the instrument axis. The mass resolution was set at made by ICP-MS, have generally been 0.2 to 0.3%.Accuracies 0.9 u, defined as the peak width at 10% of the peak height. for isotope ratio measurement of 0.25% are attainable for isotope ratios in the range 0.2 to 5,3 although 0.5% is more Reagents typical. Stock solutions were 10000 mg ml-1 Specpure ICP solutions In a previous study, we investigated methods for the effective (Johnson Matthey, Royston, Herts, UK). High purity water use of data acquisition parameters to gain enhancement of the used for dilution of stock solutions was obtained by passing precision of isotope ratio measurements made by ICP-MS.4 A demineralised water through a laboratory-reagent grade water measurement precision of 0.05% RSD was realised, using system (Liquipure, Bicester, Oxfordshire, UK), operated at optimised data acquisition parameters, for the 107Ag5109Ag 18MV.All working solutions were prepared so as to contain isotope ratio. However, even under optimised conditions, 1% (v/v) nitric acid of ultrapure reagent grade.instability of the mass spectrometer or its associated ion optics Reference material NIST SRM 981 was obtained from Promochem (Welwyn Garden City, Herts, UK). Working † Present address: Department of Chemistry, Scottish Crop Research Institute, Invergowrie, Dundee, DD4 6RA, UK. solutions of NIST SRM 981 were prepared with the aid of Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (395–402) 395polypropylene beakers and volumetric flasks (Analytical pump noise.‘Detector’ refers to internal noise (Johnson and shot noise) arising within the measurement circuits used in Supplies, Little Easton, Derbyshire, UK). All solutions were stored in polypropylene bottles (Analytical Supplies). detection and counting of ions. Designated as ‘Pick-Up’ are external and interference noise components, such as those associated with the ac line frequency and its harmonics. RESULTS AND DISCUSSION Methods for the identification and subsequent reduction of these noise types have been described previously.4 Sources of Instrumental Noise and Offset The impacts of ‘Background’, ‘Mass Bias’, ‘Pulse Pile-Up’ T ransfer function and ‘Mass Scale Shift’ combine to give an offset, which is equivalent to the bias observed in the measured ratio, prior to The measurement of an isotope ratio is depicted as a hypotheticorrection.Each of these offsets has a number of contributing cal transfer function in Fig. 1. The true ratio is envisaged as factors, for instance, ‘Spectral Interference’ and ‘Analyte being altered by both noise and offset (each being the overall Contamination’ are given as inputs to ‘Background’. That is sum of a number of contributing factors) to produce the to say, the background is determined by the sum of analyte measured ratio. contamination and spectral interference from molecular species The four sources of noise, given in Fig. 1, are all likely to and ions other than those of the analyte element.To help contribute to the imprecision of isotope ratio measurement. simplify the hypothetical transfer function, in this context, ‘Poisson’ refers to the uncertainty associated with the rate of analyte contamination relates to sample-to-sample memory arrival of ions at the detector, often referred to as the counting effects and/or contamination occurring during sample prep- statistic. ‘Ion Source’ relates to the various forms of internal aration, evident in the procedural blank, which if present may noise arising during sample introduction and plasma processes, cause the measured isotope signature to differ from that of the including audible-frequency noise associated with gas dynamic phenomena occurring in the plasma discharge and peristaltic sample material.A number of correction methods for minimis- Fig. 1 Hypothetical transfer function for isotope ratio measurement by inductively coupled plasma quadrupole mass spectrometry. 396 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12ation of offset are indicated in Fig. 1. Correction of mass bias, pulse pile-up and mass scale shift can be made post-acquisition, if data acquisition is undertaken by an appropriate method. Noise Instabilities associated with the ICP ion source may arise from either a change in energy transfer from the plasma to the sample, or variation in the efficiency of the nebulisation and transportation of sample.10 As these should influence both isotopes coherently, the direct influence of instability arising from the ICP ion source upon the isotope ratio, should be rendered insignificant by use of appropriate data acquisition parameters.4 Mermet and Ivaldi11 have shown that ratioing of simultaneously measured emission line intensities, obtained by ICP-AES, is effective in cancellation of flicker noise in cases for which a correlation coefficient of >0.95 exists between the line intensities. The data acquisition parameters previously found to be optimum for measurement of a single isotope ratio (dwell time=10.24 ms; points per peak=3; settle time=2 ms; number of sweeps=1600)4 were used for determination of the 207Pb5206Pb isotope ratio over a period of 4 h.By plotting the Fig. 2 Influence of isotope mass upon observed mass shift for cobalt, indium, cerium, lead and uranium, each at a concentration of signal intensity data obtained for the 206Pb and 207Pb isotope 50 ng ml-1.intensities against one another, a linear correlation coefficient of 0.9995 was obtained. This high degree of correlation indicates that noise arising from instability of the nebuliser and/or the ICP may be cancelled when measuring the 207Pb5206Pb isotope ratio. The contribution made by ion source to noise, as depicted in Fig. 1, can hence be assumed to be negligible for isotope ratio measurements made using appropriate data acquisition parameters.Mass scale shift The stability of the mass scale calibration (the relationship between measured mass, represented by a 32 bit channel address, and actual mass) is generally limited by the constancy of the dc and rf potentials applied to the quadrupole rod pairs.12 A change in the order of 0.05% in the amplitude of the rf potential, for a m/z of 100, would cause a mass shift of approximately 0.05 u. The daily shift in mass scale calibration for quadrupole mass analysers employed in ICP-MS is typically±0.10 u, however, ambient temperature change could easily cause additional shift.13 As the resolution of the quadrupole mass analyser is inversely proportional to the ratio of the Fig. 3 Mass spectrum for Pb obtained in the scanning mode across 512 data channels. Spectrum has been smoothed using Fourier spectral dc to rf potential, any shift in mass scale calibration caused by analysis with Blackman windowing to remove random noise.change in one, but not both, of the applied potentials would also cause the resolution to change. A multi-element solution comprising Be, Mg, Co, In, Ce, Pb Fourier smoothed mass spectrum for Pb shown in Fig. 3 was utilised to simulate the effect of mass shift. The 206Pb and and U, each at a concentration of 50 ng ml-1, was used to examine the mass scale calibration, at regular intervals over a 208Pb isotope peaks were removed from the mass spectrum and cross correlated to identify the central channel addresses period of 5.5 h.Mass scans were undertaken across the mass range 4.7 to 240.3 u, utilising 4096 data acquisition channels, for which the degree of overlap between isotope peaks was maximum. Both isotopes were integrated across a width of to give a step size of approximately 0.058 u. Such a low mass resolution was insufficient to allow direct monitoring of mass 0.4 u about these central channel addresses to obtain the value of the 208Pb5206Pb isotope ratio in the absence of mass shift.scale shift. Regression analysis was performed upon each mass scan to provide a linear relationship between channel number By moving the 0.4 u wide integration windows across both isotope peaks in equal increments of 0.0134 u [1 digital-to- and actual mass. The linear equations produced for each mass scan allowed calculation of the observed mass for each isotope analogue converter (DAC) step], the apex of the peaks moved away from the centre of the integration window and drift in and, hence, the mass shift relative to the initial mass scan.Fig. 2 shows the mass shift observed for the 59Co, 115In, 140Ce, mass location was simulated. From the data presented in Fig. 4, it may be seen that a mass shift of 0.1 u caused the 208Pb and 238U isotopes over the period of the experiment. It may be seen that the magnitude of the mass shift increased observed isotope ratio to change by approximately 0.5%.Movement of the integration window to higher masses with isotope mass. To a first approximation, doubling of the isotope mass caused the severity of the mass shift to increase caused a rise in the observed isotope ratio while the reverse was true for movement to lower masses (Fig. 4). Thus, it may two fold, as may be seen by comparison of the 59Co and 140Ce or the 115In and 238U isotopes in Fig. 2. be concluded that the observed change in the 206Pb5208Pb isotope ratio is a consequence of a quantisation error in To assess the influence of mass scale shift upon isotope ratio measurement, while retaining a constant mass resolution, the selection of mass by the multi-channel analyser.In this instance, Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 397Fig. 5 Relative inaccuracy in isotope ratio A5B arising from incorrect Fig. 4 Influence of mass shift upon the 206Pb and 208Pb isotope selection of the dead time used in dead time correction.A dead time intensities and the 208Pb5206Pb isotope ratio obtained by simulation of 20 ns has been applied whereas a 25 ns dead time provides accurate of mass shift based upon movement of the integration windows across correction. the isotope peaks. Background the quantisation error results in the nominal centre of the Initially, it is necessary to establish whether background makes 206Pb isotope peak lying slightly to the left (lower mass) of the a significant contribution to the offset of the measured isotope centre of the initial integration window, relative to that of the ratio from its true value.This may be done by scanning the 208Pb isotope peak. mass region of interest for a procedural blank. In the absence of background, be it from spectral interference, analyte contamination or a combination of both, the isotope ratio(s) Dead time should be normally distributed about unity. If the isotope At count rates of above approximately 0.4 MHz, continuous ratios for the procedural blank deviate significantly from unity, dynode electron multipliers, such as the Galileo 4870V used then background can be assumed to have a measurable impact herein, count fewer events than actually occur.The interval upon the offset (high background count rates, for example during which the electron multiplier is ‘hung-up’ is termed the across a number of masses, yielding a ratio of unity, would dead time. In addition, the counting logic circuitry also exhibits still affect the measured ratio, but this situation is normally a finite response time, sometimes referred to as sag,14 which indicative of instrumental problems).From the values of the may result from pulse broadening in the amplifier, the limited isotope ratios obtained, it should be possible to identify rise and fall times of the discriminator, or the maximum clock whether the background arises from analyte contamination rate of the counter.Most pulse counting systems show dead and/or is due to spectral interference. times of 10 to 30 ns. If the value(s) of the isotope ratio(s) of interest for the If the count rate is <0.4 MHz then the probability Pn of n procedural blank are similar to those expected for the analyte, ion current pulses occurring during the dead time D is governed then analyte contamination is liable to add to the offset. It by Poisson statistics. If the count has a Poisson distribution, may be possible to take action to identify and reduce analyte the measured count rate N may be corrected for dead time D contamination arising from sample preparation or within the utilising the equation: sample introduction system.Otherwise, blank subtraction of all subsequent data may be necessary, however, this may result N¾= N (1-ND) (1) in deterioration of the precision of isotope ratios. If the value(s) of the isotope ratio(s) of interest for the procedural blank are indicative of isobaric interference, which cannot be removed where N¾ is the estimated count rate.by pre-treatment of samples, it may be necessary to apply It is necessary to precisely determine the dead time D to background correction. Generally, an additional isotope which minimise the dependence of isotope ratios upon analyte conprovides a measure of the level of isobaric interference on the centration and isotopic abundance. In Fig. 5, the dead time isotope(s) of interest will be included in the data acquisition has been under-estimated by 5 ns, a dead time correction of procedure.Arithmetic calculation can then be used to deter- 20 ns has been applied, whereas a 25 ns dead time provides mine the proportion of the total ion count arising from the accurate correction. It may be seen, from Fig. 5, that isotope isobaric interference. ratios can become biased by several parts per thousand as a consequence of inaccurate selection of the dead time utilised in dead time correction.Since the gradients of the lines shown Mass bias in Fig. 5 increase as the ratio of A to B increases, it may be assumed that the further the measured isotope ratio lies from Mass bias derives from the differential transmission of ions of different mass from the point at which they enter the sampling unity the more susceptible the ratio is to inaccurate correction for dead time. Hence, accurate measurement of the 204Pb5206Pb device until they are finally detected by the electron multiplier.The majority of mass bias effects occur in the interface region isotope ratio in natural abundance materials is more difficult to achieve than is accurate 207Pb5206Pb isotope ratio measure- of the ICP-MS, but some additional bias may occur in the quadrupole mass filter, unlike in sector mass analysers where ment, by pulse counting. 398 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12transmission in the magnetic and electrostatic sectors is largely likely to show similar mass bias effects to those for Pb. Variation in the 194Pt5196Pt and 194Pt5195Pt isotope ratios free of bias.Mass bias may be measured and hence corrected by use of an external standard of known isotopic composition observed during 80 acquisitions, each of a duration of 2 min, is shown in Fig. 6. The change in the 194Pt5196Pt isotope ratio or a fixed, constant ratio within the sample. External mass bias correction involving use of an external standard has been is approximately double that observed in the 194Pt5195Pt isotope ratio, over the same period.If a linear relationship widely used in ICP-MS. Estimation of mass bias by use of an external standard is advantageous as: (i) mass bias can be were to exist between mass bias and mass shift the variation in the 194Pt5196Pt isotope ratio would be anticipated as being measured at the correct masses (position upon the mass response curve is equivalent for standard and sample) and twice that observed in the 194Pt5195Pt isotope ratio.The best fitting linear relationship for the data given in Fig. 6 has a (ii) mass bias can be measured at the relative isotope abundances to be found in samples (pulse pile-up effects are gradient of 1.82 (r2=0.67). The similarity between the predicted (2) and observed (1.82) gradients suggests that linear correction generally equivalent for standard and sample, although this does not apply for isotope dilution).for mass bias is at least as valid as either of the other methods for correction of mass bias. Internal mass bias correction using Measurement of mass bias and analysis of samples are however displaced in time and are therefore susceptible to eqn. 2 resulted in improvement in accuracy, but reduction in the precision of the Pt isotope ratios as a consequence of shifts variation in the magnitude of the mass bias. Regular monitoring and updating of the mass bias correction is necessary to arising from sources other than mass bias.prevent long term instability. Additionally, differences between the mass response curves for the standard and sample arising Lead Isotope Ratio Measurement from matrix effects are not accounted for by external mass bias correction. Noise reduction Internal mass bias correction is beneficial in that it: (i) pro- The data acquisition parameters selected for high accuracy Pb vides near continual monitoring of change in mass bias and isotope ratio measurement are summarised in Table 1.Data (ii) can correct for non-spectral interferences caused by acquisition was undertaken in the peak jumping mode because matrix effects. a greater proportion of the available acquisition time could be Internal mass bias correction is however based upon mass and isotope abundance which differ from those of the isotope(s) of interest. Additionally, internal mass bias correction generally requires the element of interest to have three or more isotopes and for at least two of these isotopes to have not undergone any form of isotopic fractionation in nature.There are three algorithms which may be applied in correction of mass bias: these are based upon linear, power law and exponential relationships between mass bias and mass difference. 16 If mass bias is assumed to be a linear function of mass difference, to the precision of the data, mass bias correction may be performed using eqn. 2: (A/B)corr=(A/B)obs(1+an) (2) where (A/B)corr is the mass bias corrected ratio of isotopes A and B, (A/B)obs is the observed ratio of isotopes A and B, a is the bias per unit mass and n is the mass difference between isotopes A and B (u). If mass bias is, however, a power law function of mass difference, eqn. 3 may be utilised to correct for mass bias: (A/B)corr=(A/B)obs(1+a)n (3) Finally, should an exponential relationship exist between mass bias and mass difference, mass bias correction may be performed using eqn. 4: Fig. 6 Relationship between the 194Pt5196Pt and 194Pt5195Pt isotope ratios measured over 80 consecutive, 2 min integrations. The line (A/B)corr=(A/B)obs expan (4) representing the best fitting linear relationship has a gradient of 1.82. Taylor et al.16 found the power law and exponential functions to be more successful in correction of mass bias than the linear Table 1 Data acquisition parameters selected for use in Pb isotope function for U isotope ratio measurement, by multiple ratio measurement based upon noise spectral analysis4 collector ICP-MS.Acquisition mode peak jumping Although all three algorithms may yield equivalent mass Quadrupole rest mass 200 u bias factors under normal circumstances it is feasible that Quadrupole settle time 2 ms inaccuracy may arise from use of an inappropriate algorithm Detector dead time 32.5 ns in estimation of mass bias by internal mass bias correction.As Discriminator level 40 internal mass bias correction is most useful in instances in which the magnitude of the mass bias varies with time, it was Number of isotopes 4 Dwell time per channel 10.24 ms proposed that the appropriateness of an algorithm could be Points per peak 3 determined by monitoring variation in the mass bias of two Acquisition time 120 s isotope ratios over time. Replicate measurements 10 Platinum was selected to identify whether mass bias was a Abundance sensitivity* <5 ppm linear function of mass as it supplied two isotope ratios which are fixed and constant in nature and are close to unity.* Overspill for isotopes differing by 1 u using three points per peak positioned at m-0.05, m and m+0.05 u. Platinum is also only 10 u removed from Pb, and as such, is Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 399spent in accumulation of counts on the isotopes of interest, were somewhat higher, averaging 0.12% RSD for 205Tl5203Tl and 0.13% RSD for 207Pb5206Pb (n=5).allowing the theoretical counting statistic to be minimised. A dwell time of 10.24 ms was favoured, for all isotopes, independent of their isotopic abundance, to give a high frequency cut- Mass calibration off of 49 Hz.4 The narrow mass range being studied allowed Lying at the upper end of the mass range, the Pb isotopes are use of a 2 ms settle time, compared to the more commonly susceptible to mass scale shift resulting from instability of the used 10 ms.quadrupole mass analyser (see Fig. 2). As previously men- Three acquisition points per peak were utilised because it tioned, use of three data acquisition points per isotope allowed provided a crude method of identifying mass shift, and hence mass scale shift to be monitored. In instances where mass shift suspect isotopic data. Additionally, three points per peak was observed, re-calibration of the mass scale was undertaken allowed the maximum accumulated count to approximate prior to analysis of the sample which followed.The isotopes 48×106 ion counts, thrice that attainable using a single point utilised in mass calibration were 203Tl, 205Tl, 206Pb, 207Pb, per peak, since the multi-channel analyser limits the maximum and 208Pb. count per channel to 16×106 ion counts. Use of three points The nature of mass scale shift is such that the larger the per peak was also beneficial to the attainment of a sizeable difference in mass of the measured isotopes, the greater the count for 204Pb, which with a natural abundance of 1.4%, influence upon the isotope ratio, as was shown in Fig. 2. For is a primary restriction to the theoretical counting statistic. this reason, the lightest and heaviest isotopes being measured To permit internal mass bias correction, the 203Tl and 205Tl were ratioed to give a monitor ratio for correction of mass isotopes were included in the data acquisition procedure.The scale shift. Any integration which gave a monitor ratio out by number of isotopes to be measured thus increases from four 2s of the mean value (n=10), for integrations within the to six. Seven if the nature of the sample material had necessi- determination were rejected as outliers. This test was under- tated correction for isobaric interference from 204Hg on the taken iteratively, never more than twice, until all remaining minor Pb isotope. Utilising three acquisition points per peak monitor ratios occurred within 2s.Similar procedures for data and a dwell time of 10.24 ms, sequential measurement of six reduction are routinely used in isotope ratio measurements isotopes proved to be ineffective in minimisation of instrumen- made by sector based mass analysers. Statistical outliers were tal noise. With an elapse time of 195 ms, the ratioing frequency found in the monitor ratio (e.g. 203Tl5207Pb), but no outliers of isotopes is inadequate for reduction of low frequency noise, were observed in either the Tl or Pb isotope ratios. as the band-pass extends to frequencies as low as 2.6 Hz.4 The external precision of 207Pb5206Pb isotope ratio measure- To attain a measurement precision approaching counting ment before and after removal of outliers, based upon use of statistic levels for Tl mass bias corrected Pb isotope ratios, it 203Tl5207Pb as a monitor ratio, for five samples of 1000 ng ml-1 was found necessary to measure the Pb isotope ratios on an of NIST SRM 981 are compared in Table 2.Removal of individual basis. Hence, both Tl isotopes and two of the four statistical outliers resulted in improvement in measurement Pb isotopes were measured within any given data acquisition precision, while the mean measured value of the 207Pb5206Pb procedure. For both the 205Tl5203Tl and Pb isotope ratios, isotope ratio remained unchanged, to within the precision of noise reduction extended to a frequency of 5.2 Hz.4 the data.Similar reduction in the external precision of the Between the lower and upper cut-off frequencies given by 204Pb5206Pb and 208Pb5206Pb isotope ratios was obtained by the ratioing of isotopes and the dwell time, respectively, noise removal of statistical outliers based upon use of 203Tl5206Pb reduction occurs as the result of the summation of sweeps.4 and 203Tl5208Pb as monitor ratios, respectively. While isotope peak profiles are added coherently in the multichannel analyser upon summing of sweeps, noise components Dead time correction are reduced by smoothing.All noise frequencies other than those associated with the sweep rate and its harmonics are The instrumental dead time for Pb isotope ratio measurement attenuated by the summation of sweeps. A total of 916 sweeps was determined by calculation of the observed mass bias for were acquired and summed in the multi-channel analyser the 205Tl5203Tl and 208Pb5207Pb isotope ratios.These isotope during the acquisition time. As a result, the width of the ratios were chosen as they have similar values, 205Tl5203Tl= bandpass at the reciprocal of the sweep time (7.64 Hz for a 2.3871 and 208Pb5207Pb=2.3704, for NIST SRM 981. The data sweep time of 0.131 ms) and associated harmonics (15.28 Hz, acquisition parameters, with the exception of the dead time, 22.92 Hz, etc.) was just 0.0072 Hz.4 given in Table 1 were utilised. The observed mass bias factors The precision of isotope ratio measurements is influenced for the 205Tl5203Tl and 208Pb5207Pb isotope ratios were found by noise and instabilities originating in both the ICP and the to converge at a dead time of approximately 32.5 ns.quadrupole mass filter. Noise originating in the ICP is coherent in all isotopes of a given element, therefore, these coherent Table 2 Improvement in external precision given by removal of noise sources can be removed from measured isotope ratios statistical outliers for the 207Pb5206Pb isotope ratio, based upon use by use of suitable data acquisition parameters.If the quantity of 203Tl5207Pb as a monitor ratio, for a solution containing of noise originating in the quadrupole mass filter were negli- 1000 ng ml-1 of Tl and NIST SRM 981 gible, the precision of isotope ratio measurements would approximate the theoretical counting statistic. Noise originat- 207Pb5206Pb ing in the quadrupole mass filter can be minimised by use of Sample Uncorrected Mass scale shift corrected a fixed m/z. Using the data acquisition parameters given in Table 1, the measurement precision for the Tl and Pb isotope 1 0.9155 0.9153 (8)* 2 0.9155 0.9149 (8) ratios, obtained for a fixed mass position upon the apex of the 3 0.9141 0.9143 (9) 206Pb isotope peak (count rate 0.5 MHz), by switching the 4 0.9146 0.9146 (10) quadrupole mass analyser into remote preventing it from 5 0.9141 0.9148 (9) ‘jumping’, averaged 0.08% RSD for 205Tl5203Tl and 0.06% Mean 0.9147 0.9148 RSD for 207Pb5206Pb (n=3).These compared favourably with Standard deviation 0.0007 0.0004 the theoretical counting statistic, which for the first determination had a value of 0.05% RSD. The RSDs of 10 replicate * Number of integrations remaining following removal of statistical outliers. integrations forming a determination, for normal operation 400 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Mass bias correction Interference effects Lead suffers from only one isobaric interference, 204Pb being The weaknesses of the thallium correction method are: the acquisition time necessary to measure the Tl isotopes, which overlapped by 204Hg.Working solutions of 1000 ng ml-1 of NIST SRM 981 were found to contain Hg in insufficient could otherwise be spent accumulating counts on the Pb isotopes; and the reduction in precision of the Pb isotope quantities to necessitate measurement of 202Hg (29.8%) for correction of 204Hg (6.8%) on 204Pb.The 201Hg5202Hg isotope ratios which can result from correction. It is necessary to be mindful of the need to precisely correct for dead time and ratio has a fixed value of 0.571, which is sufficiently removed from unity to be indicative of Hg contamination. The ion assure that noise is minimised in both Tl and Pb isotopes if the thallium correction method is to be used effectively. It is counts observed on masses 201 and 202 u were found to be comparable at 60 counts s-1 for a solution containing worthy of note that Walder and co-workers19,20 have successfully used the thallium correction method with simultaneous 1000 ng ml-1 of NIST SRM 981, and therefore no correction for the effect of the less abundant 204Hg isotope was considered measurement of the isotopes using Faraday detectors to reproducibly measure 207Pb5206Pb in NIST SRM 981 to within to be necessary.To measure the level of Pb contamination present in the Tl 0.001%.20 Mass bias corrections were performed using the thallium stock solution, the 206Pb and 207Pb isotopes and their ratio were determined for solutions containing 500, 750 and correction method, validated by Ketterer et al.,17 by spiking each of the Pb samples with Tl to a concentration of 1000 ng ml-1 of Tl.The ion count on mass 206 u varied from 59 counts s-1 for 500 ng ml-1 of Tl to 63 counts s-1 for 500 ng ml-1. The 205Tl5203Tl isotope ratio has a fixed and known value of 2.3871.Measured mass bias factors for both 1000 ng ml-1 of Tl. The 207Pb5206Pb isotope ratio had a value of 0.9 at all three Tl concentrations, which is suggestive of the 205Tl5203Tl isotope ratio and the 207Pb5206Pb isotope ratio for 1000 ng ml-1 of NIST SRM 981, having a certified value a small quantity of Pb contamination being present in the solutions. As the level of Pb contamination associated with of 0.91464, may be seen from Table 3 to be of the order of 0.73% u-1.the Tl content of solutions was considered to be below that which would allow precise correction, no corrective action was The Tl correction method assumes mass bias to be a power law function of mass.2,17–20 Since a linear relationship was taken. A solution containing 1% (v/v) nitric acid in high purity water was used as the procedural blank and measured using observed between mass bias and mass for Pt isotope ratio measurement (Fig. 6), the use of both linear and power law the same data acquisition parameters as the samples. Background correction was performed by subtraction of the algorithms in mass bias correction was investigated. Table 4 shows that the values of the 207Pb5206Pb isotope ratio obtained ion counts obtained for the procedural blank from those for subsequent samples. following Tl mass bias correction using either algorithm are equivalent, to within the precision of the data. The power law algorithm (eqn. 4) was used in all subsequent mass bias Accuracy and precision corrections. Isotopic data obtained for five consecutive determinations of a solution containing 1000 ng ml-1 of NIST SRM 981, is given in Table 5. Samples were analysed for 20 min, the analysis Table 3 Measured mass bias factors for the 205Tl5203Tl and comprising ten measurements each of 2 min duration. Each of 207Pb5206Pb isotope ratios, for solution containing 1000 ng ml-1 of Tl the Pb isotope ratios was measured individually using the and NIST SRM 981 optimum data acquisition parameters as detailed in Table 1.The measured isotope ratios for NIST SRM 981 are shown to Bias factor (% u-1) be in good agreement with the certified ratios, following mass Sample 205Tl5203Tl 207Pb5206Pb bias, mass scale shift and dead time corrections. The measurement precision (% RSD) for the Pb isotope ratios, given in 1 0.66 0.75 Table 5, is that obtained for 1s of the mean of the five replicate 2 0.69 0.78 3 0.74 0.69 determinations.The counting statistic, however, represents the 4 0.74 0.74 uncertainty associated with measurement for a single inte- 5 0.77 0.70 gration. An external measurement precision of 0.038% RSD Mean 0.72 0.73 was obtained for the 207Pb5206Pb isotope ratio following RSD* (%) 0.087 0.038 correction of offsets, compared with 0.12% RSD for the uncorrected data. * Calculated for 1 standard deviation. Table 5 Accuracy and precision of Pb isotope ratio measurement for Table 4 Comparison of linear and power law algorithms for correc- NIST SRM 981 tion of mass bias observed in the 207Pb5206Pb isotope ratio by thallium mass bias correction Sample 204Pb5206Pb 207Pb5206Pb 208Pb5206Pb 1 0.0593 0.9153 2.1706 207Pb5206Pb 2 0.0591 0.9150 2.1715 3 0.0592 0.9143 2.1715 Mass bias algorithm 4 0.0593 0.9146 2.1698 5 0.0593 0.9148 2.1712 Sample Uncorrected Linear Power law Mean 0.0592 0.9148 2.1709 1 0.9215 0.9150 0.9155 RSD* (%) 0.12 0.038 0.033 2 0.9218 0.9149 0.9155 Counting statistic (%) 0.15 0.048 0.041 3 0.9209 0.9136 0.9141 4 0.9214 0.9140 0.9146 Certified value 0.059042 0.91464 2.1681 5 0.9211 0.9135 0.9141 2s 0.000037 0.00033 0.0008 Mean 0.9213 0.9142 0.9147 * Calculated for 1 standard deviation for isotope ratios measured Standard deviation 0.0004 0.0007 0.0007 to five decimal places. Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 401CONCLUSIONS ing this work and the Engineering and Physical Sciences Research Council for provision of a studentship to I.S.B.Application of the methodology outlined for high accuracy isotope ratio measurement to the determination of the 207Pb5206Pb isotope ratio for five samples of NIST SRM 981 revealed 0.9148±0.0004 (1s), compared with a certified value REFERENCES of 0.91464. The bias from the certified value is 0.013%, well 1 Ince, A. T., Williams, J. G., and Gray, A. L., J. Anal. At. Spectrom., within 1s (RSD=0.043%). To the best of our knowledge, the 1993, 8, 899.accuracy and precision of the Pb isotope ratios, given in 2 Ketterer, M. E., J. Anal. At. Spectrom., 1992, 7, 1125. Table 5, are equivalent to or better than those published 3 Fassett, J. D., and Paulsen, P. J., Anal. Chem., 1989, 61, 643A. elsewhere for measurement by quadrupole based ICP-MS. 4 Begley, I. S., and Sharp, B. L., J. Anal. At. Spectrom., 1994, 9, 171. Measurements made using multiple collector, magnetic sector 5 Gillson, G. R., Douglas, D.J., Fulford, J. E., Halligan, K. W., and Tanner, S. D., Anal. Chem., 1988, 60, 1472. based ICP-MS instruments are of a superior accuracy and 6 Vaughan, M. A., and Horlick, G., Spectrochim. Acta, Part B, precision.19,20 Utilising a standard nebuliser and spray chamber 1990, 45, 1301. configuration, the multiple-collector ICP-MS instrument used 7 Ross, B. S., and Hieftje, G. M., Spectrochim. Acta, Part B, 1991, by Walder et al.19 provided a precision of 0.011% RSD and 46, 1263.bias of -0.058% for 207Pb5206Pb. These are not far removed 8 Williams, J. G., in Handbook of Inductively Coupled Plasma Mass from the accuracy and precision given herein. In this study, Spectrometry, ed. Jarvis, K. E., Gray, A. L., and Houk, R. S., Blackie and Son Ltd., Glasgow and London, 1992. each sample analysis used approximately 15 mg of Pb (uptake 9 Date, A. R., and Hutchison, D., J. Anal. At. Spectrom., 1987, 2, 269. of 0.75 ml min-1 of 1000 ng ml-1 Pb for 20 min), compared 10 Carre�, M., Poussel, E., and Mermet, J.-M., J. Anal. At. Spectrom., with 200 ng of Pb for multiple-collector ICP-MS.19 The rela- 1992, 7, 791. tively large amount of Pb required reflects the low count 11 Mermet, J.-M., and Ivaldi, J. C., J. Anal. At. Spectrom., 1993, 8, 795. sensitivity of our ICP-MS instrument, at 2×106 counts s-1 12 Bley, W. G., Vacuum, 1988, 38, 103. per mg l-1 being 10–20 fold below that of modern commer- 13 Batey, J., VG Elemental, Winsford, Cheshire, UK, personal communication. cial systems. 14 Russ, G. P., and Bazan, J. M., Spectrochim. Acta, Part B, 1987, The bias, expressed as a percentage difference from the 42, 49. certified value, for the 208Pb5206Pb and 204Pb5206Pb isotope 15 Vincent, C. H., Random Pulse T rains, T heir Measurement and ratios, given in Table 5, was 0.13 and 0.30% (n=5), respectively. Statistical Properties, Peter Peregrinus, London, 1973. Certified values for NIST SRM 981 are 2.1681 for 208Pb5206Pb 16 Taylor, P. D. P., De Bie`vre, P., Walder, A. J., and Entwistle, A., and 0.059042 for 204Pb5206Pb. As is the case for these measured J. Anal. At. Spectrom., 1995, 10, 395. 17 Ketterer, M. E., Peters, M. J., and Tisdale, P. J., J. Anal. At. Pb isotope ratios, accuracy generally decreases as the true Specom., 1991, 6, 439. value of the isotope ratio deviates from unity and the mass 18 Longerich, H. P., Fryer, B. J., and Strong, D. F., Spectrochim. separation between isotopes increases despite best attempts to Acta, Part B, 1987, 42, 39. remove bias. This study demonstrates the high precision and 19 Walder, A. J., Platzner, I., and Freedman, P. A., J. Anal. At. accuracy which can be achieved for analysis of standard Spectrom., 1993, 8, 19. solutions by careful planning of experiments. Use of the given 20 Walder, A. J., Koller, D., Reed, N. M., Hutton, R. C., and Freedman, P. A., J. Anal. At. Spectrom., 1993, 8, 1037. methodology is likely to provide the best possible isotope data attainable by ICP-MS for any given sample type. Paper 6/05078F Received July 22, 1996 The authors would like to thank the British Geological Survey Accepted November 11, 1996 for loan of the ICP-MS instrument, VG Elemental for support- 402 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a605078f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Multi-element Speciation of Tea Infusion Using Cation-exchange Separation and Size-exclusion Chromatography in Combination with Inductively Coupled Plasma Mass Spectrometry KNUT E. ØDEGA° RD AND WALTER LUND* Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315 Oslo, Norway The speciation of metals in tea infusion was studied using studied Al in tea infusions using a kinetic ion-exchange procedure. Owen et al.13 determined Al species in tea infusions ICP-MS. Twenty-four elements were studied in tea leaves and tea infusion, and the extraction efficiency of the infusion and simulated gastro-intestinal digests using size-exclusion chromatography (SEC) in combination with ICP-MS.Powell process was calculated. Cation-exchange separation based on solid-phase extraction cartridges was used for studying the et al.14 studied the gastro-intestinal availability of Al from tea, using ultrafiltration as their analytical tool. The results of these charge of the dissolved species.Size-exclusion chromatography was used to obtain information about the molecular size of the studies indicate that Al in tea infusion is fully or partly bound to organics, probably of the polyphenolic thearubigen type.15 metal–organic complexes. The organic material eluted from the column was detected by UV spectrometry at 227 nm. The In this work, 24 elements in tea infusion were studied using two procedures: SEC at a pH close to that of a tea infusion results indicate that Mg, Mn and Rb are present in tea infusion as cations, which are probably not associated with was used in combination with ICP-MS, to obtain information about the molecular size of the metal–organic complexes, and organic material.Also, Ca, Fe, Co, Ni, Cu, Zn, Sr and Ba appear to be present mainly in cationic form, but there is in a cation-exchange separation was performed to study the charge of the species. Cation exchange has been widely used addition a certain non-cationic fraction.For Fe, Ni, Cu and Zn, the non-cationic species may be metal–organic complexes for the speciation of Al in natural waters.16 in the size range 4000–6000 Da, whereas Sr, Ba and Pb are associated with even larger molecules. Al is the metal with the EXPERIMENTAL most marked non-cationic behaviour in tea infusion; it is Instrumentation associated with molecules in two size ranges, viz., 4000–6000 and 6500–8500 Da, respectively. The ICP-MS system was a Perkin-Elmer SCIEX (Norwalk, CT, USA) Elan 5000 with an IBM PS/2 77 486DX2 computer Keywords: Inductively coupled plasma mass spectrometry; size- and Elan 5000 software (Xenix platform), and a Perkin-Elmer exclusion chromatography; cation-exchange separation; multi- AS90 autosampler.The data processing was carried out on an element speciation ; tea infusion IBM PS/2 486 with Origin 3.5/Windows 3.1, and a Macintosh Performa450 computer with ClarisWorks 2.1Hv3 and JMP 2.0.Tea is one of the world’s most popular beverages.1,2 The The chromatographic system consisted of the Shimadzu properties of tea arise from a combination of a large number (Tokyo, Japan) units LC-6A pump, SPD-6AV UV–visible of constituents. Tea contains 4–9% of inorganic matter; about spectrophotometric detector and C-R3A integrator, and a one third of the solids are extracted during the brewing Rheodyne (Cotati, CA, USA) 7125 injector with a loop volume process.1 The concentration of metal ions in a cup of tea of 210 ml (calibrated). The size-exclusion column was a depends on the amount present in the original leaves as well Pharmacia (Uppsala, Sweden) Superdex 75 HR 10/30 as the solubility of the elements and the time allowed for the (300×10 mm id), with a range of 1000–75000 Da.infusion process. The major elements in a tea infusion are Mg, The equipment used for cation exchange consisted of a Al, K, Ca and Mn.3–6 The relatively high concentration of Al Rheodyne 5020 injector, with two loops (Teflon) in series and has been the subject of some concern;7 for many people, tea is total volume 1.040 ml (calibrated), a Gilson (Villiers-le-Bel, the major source of Al in the diet.The effects of alimentary France) Minipuls-3 peristaltic pump with 0.76 mm id (black) intake of Al on humans have been debated;8,9 little Al is pump tubing (Elkay) and an Alltech (Deerfield, IL, USA) absorbed across the gastro-intestinal tract and the renal Maxi-Clean IC-Na (0.5 ml bed volume) strong cation-exchange excretion of Al is fairly effective;10 however, toxic effects can cartridge.occur for patients with chronic renal failure.11 The syringe used for filling the injection loops of the size- For studies of the bioavailability of metal ions, information exclusion and cation-exchange systems was always equipped about the chemical species present is required. Speciation with a 0.45 mm/13 mm cellulose acetate syringe filter.Dry tea analysis is seldom an easy task, because the separation steps was decomposed in Parr (Moline, IL, USA) Model 4782 required may disturb the species originally present in the bombs (23 ml volume), which were heated in a Husqvarna sample. This is particularly true for coordination complexes (Sarpsborg, Norway) 800 W household microwave oven. where metals are bound to large molecules of natural origin, such as in tea. However, provided that the operational aspect Reagents of speciation analysis is kept in mind, it should be possible to obtain relevant information about such species.One of the All reagents were of analytical-reagent grade, and distilled, de-ionized water was used throughout. Single-element 1 g l-1 aims of this work was to investigate this possibility. A few speciation studies have been carried out for tea stock standard solutions of Mg, Al, Mn, Fe, Cr and Sn from Spectrascan (Teknolab, Dro�bak, Norway) were used, and in infusions, all regarding the speciation of Al.French et al.12 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (403–408) 403addition the multi-element standard solution No. VI from The tea infusion was filtered through a 0.45 mm syringe filter when the 1.04 ml sample loop was filled; the sample was then Merck (Darmstadt, Germany).The concentrations in the multielement solution were 10 mg l-1 (23 elements), 100 mg l-1 (6 injected into the aqueous carrier stream.A 6.0 ml volume of the eluate was collected and diluted to 10 ml. The eluate from elements) and 1 g l-1 (Ca). Working standard solutions with lower concentrations were prepared fresh daily by dilution. the first injection was discarded. Experiments showed that 6.0 ml was a sufficient volume for collecting all the metal that For calibration of the size-exclusion column, the Pharmacia low molecular weight calibration kit was used; the kit contained was not retained by the column.It was found that the cartridge could be used for at least five injections without exceeding its blue dextran 2000 (2000 000), bovine serum albumin (67000), ovalbumin (43000), chymotrypsinogen A (25000) and ribo- exchange capacity. The exchange capacity was studied by repeated injections of a standard solution containing 12 mg l-1 nuclease A (13700). In addition, aprotinin from bovine lung (6500), vitamin B12 (cyanocobalamin) (1355) and sodium azide Mg, 4 mg l-1 Al, 4 mg l-1 Mn, 160 mg l-1 Zn and 40 mg l-1 Ba (the concentrations are similar to those found in tea (all from Fluka, Buchs, Switzerland) were used.The tea samples were Lipton (London, UK) Yellow Label infusion). More than 99.95% of each metal was retained even for the fifth injection of the standard solution. tea bags (2 g tea per bag), which were purchased in a general store. All results presented refer to the same batch of 50 tea bags. Size-exclusion chromatography Procedures A 0.1 mol l-1 ammonium acetate buffer at a pH of 5.5, which is close to the pH of a tea infusion, was used as the mobile Cleaning phase.The flow rate of the mobile phase was 1.0 ml min-1; All equipment was carried through a thorough cleaning pro- the eluent was de-gassed for 10 min with helium. Before the cedure, starting with 2–5% m/v nitric acid and ending with analysis, ammonium acetate buffer was addedo the tea repeated rinsing with distilled, de-ionized water.Special clean- infusion (1 part of 1.0 mol l-1 buffer to 9 parts of infusion), to ing procedures were requiredfor the cation-exchange cartridges match the sample matrix exactly with that of the mobile phase; and the size-exclusion column, as described in the appro- in this way a negative dip in the UV chromatogram around priate sections. the total permeation volume was avoided. The tea infusion was filtered through a 0.45 mm syringe filter when the 210 ml sample loop was filled; the sample was then injected into the Decomposition of tea leaves mobile phase.After 15 min, 210 ml of 1.0 mol l-1 sodium hydroxide were A 0.05 g amount of tea leaves was weighed in each PTFE injected twice, with a 30 s interval, to clean the column and vessel, 1.0 ml of 65% m/m nitric acid and 1.0 ml of 30% m/m ensure reproducible chromatographic conditions. Adsorption hydrogen peroxide were added, and the vessels were placed in of coloured components from the tea infusion was visible at the pressure bombs which were closed and heated in the the top of the glass column after injection of the tea infusion.microwave oven. The three bombs were heated at 80% power Adsorption of organic material on the column was also sig- (of 800 W) for 1 min together with a beaker containing 200 ml nalled by the fact that in the absence of the cleaning step, a of water (as an energy buffer). The bombs were then cooled in stable UV (227 nm) baseline was obtained only after 120 min.a water-bath for 10 min. The resulting solutions were pale When sodium hydroxide was injected, it could be seen that yellow. The procedure is almost identical with that described the coloured material moved with the hydroxide through the by Matusiewicz et al.17 The solutions were diluted with water column. The hydroxide injection permitted a new sample (see below) prior to the ICP-MS measurements. Blank experi- injection after 30 min, because a stable baseline was obtained ments were carried out using the same procedure without at that time.tea leaves. For the determination of the amount of metal eluted from the column, 50 ml of the eluate were collected, and diluted 1+1 with water before analysis, to ensure multi-element Preparation of tea infusion determination. The tea bag was fixed to a conical flask with a clip and 200 ml The column was calibrated with 0.5 g l-1 solutions of albu- (at 20 °C) of boiling distilled, de-ionized water was poured min (67000), ovalbumin (43000), chymotrypsinogen A along the side of the flask (not over the tea bag), until the tea (25000), ribonuclease A (13000), aprotinin (6500) and vitamin leaves were fully covered with water (the metal pin in the tea B12 (1355).Blue dextran 2000 (1 g l-1) was used to determine bag was above the water). The magnetic stirrer was started the total exclusion volume and 20 mg l-1 sodium azide to and the tea bag was removed after 5.0 min (optimum infusion determine the total permeation volume of the column.The time; see under Results). Blank experiments were carried out total exclusion and permeation volumes were found to be 6.8 using an empty tea bag but otherwise the same procedure. and 21.4 ml, respectively. A linear relationship was obtained The infusion was centrifuged for 20 min prior to the analysis between the retention time and the logarithm of the molecular by ICP-MS. For the cation-exchange and size-exclusion experi- weight for the range 67000–6500 Da, as shown in Fig. 1. For ments, the infusion was filtered through a 0.45 mm/13 mm vitamin B12 (1355 Da), the retention volume was markedly syringe filter as the injection loop was filled. below the straight line, as can be seen from Fig. 1. In this work, no molecular weight estimates are given below 5000 Da; however, for molecules with retention volumes above that of Cation exchange vitamin B12, the molecular weight is indicated as <1000 Da.The confidence interval for the molecular weight estimates was The flow rate through the cartridge was maintained at 1.0 ml min-1 by means of a peristaltic pump. The strong calculated from the regression line, also taking into account that the observed reproducibility (day-to-day variation) of the cation-exchange cartridges were found to be contaminated with silver, and had to be cleaned thoroughly with 25 ml of retention volumes was 0.1 ml. Thus, a molecular weight of 5000 is given as 4000–6000 (the confidence interval is used 5% m/v nitric acid.Ammonium was then introduced as the counter ion by triplicate injections of 1.04 ml of 1.0 mol l-1 instead of the confidence limits, because of the logarithmic nature of the calibration graph). ammonium acetate; water was used as the carrier liquid. 404 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Calibration and quantitative determination Rh (40 mg l-1) was used as internal standard in all solutions, to correct for changes in the sample uptake rate and plasma conditions. Based on recovery experiments, it was found that the use of external calibration graphs, prepared from standard solutions matrix-matched only with respect to the acid concentration, did not give correct results, owing to matrix effects. Therefore, standard additions calibration graphs were used.These were constructed by standard additions to three of the types of sample solutions studied: decomposed tea leaves, tea infusion, and infusion after SEC.Once established, each graph was used for samples/replicates of a given type. For tea infusion after cation exchange, the ordinary standard additions method Fig. 1 Calibration graph for the estimation of molecular weight for a Superdex 75 HR 10/30 column. Eluent, 0.10 mol l-1 ammonium was used, with additions of standard to each individual repli- acetate of pH 5.5; flow rate, 1.0 ml min-1. Each point represents cate.The standard additions calibration procedure was 2–3 injections. checked by the additions of standards (all 24 elements); the ‘recovery’ of these standards was usually 95–105%. For the decomposed tea leaves, the digest was diluted 1+49 ICP-MS parameters with water before the determination of Mg, Al, Ca, Mn, Cu, Rb, Sr and Ba, whereas a 1+1 dilution was employed for the The ICP parameters were: rf power, 1.00 kW; plasma gas flow other elements. For the tea infusion, Mg, Al, Mn and Rb were rate, 15 l min-1; nebulizer gas flow rate, 0.90 l min-1; auxiliary determined after a 1+49 dilution, whereas a 1+1.5 dilution gas flow rate, 1.0 l min-1; and sample uptake rate, was used for the other elements.The sample was highly diluted 1.0 ml min-1. The mass used for each element, the replicate in the cation-exchange and size-exclusion experiments as a time, dwell time as well as the other MS parameters are given result of the procedure used; for the collected eluates the final in Table 1.As can be seen, different MS parameters were used dilution factors were about 10 and 500, respectively. for the quantitative measurements and chromatographic detec- Blank experiments were carried out for all sample types; the tion, respectively. To ensure an acceptable sensitivity, the contribution from the blank to the total concentration was elements that were detected in the SEC experiments were usually negligible.divided into four groups. Within each group, the replicate time, dwell time and number of sweeps per reading were constant; the values are given in Table 1. Group No. 1 consisted Precision and detection limits of Mg, Al, Ca, Mn, Zn and Rb; group No. 2 contained Cr, Fe and Ni; group No. 3 contained Cu, Sr and Ba; and group For spiked tea infusion and spiked solution of decomposed tea leaves (recovery experiments), the RSDs were below 5% No. 4 was Pb (see Table 1). A separate chromatographic injection was made for each group.with few exceptions. The RSDs were higher for the unspiked Table 1 Mass spectrometry parameters: elemental masses, replicate time and dwell time Quantitative measurements* Chromatographic detection† Element Mass Replicate time/ms Dwell time/ms Replicate time/ms Sweeps per reading Li 7 3750 50 Be 9 1875 25 Mg 24 1875 25 300000 15 Al 27 1875 25 300000 15 Ca 44 3750 50 300000 15 V 51 3750 50 Cr 52 3750 50 700000 35 Mn 55 1875 25 300000 15 Fe 57 5625 75 700000 35 Co 59 3750 50 Ni 60 3750 50 700000 35 Cu 63 7500 100 420000 21 Zn 66 5625 75 300000 15 As 75 1875 25 Se 78 3750 50 Rb 85 1875 25 300000 15 Sr 88 1875 25 420000 21 Rh‡ 103 1875 25 Ag 107 3750 50 Cd 114 1875 25 Sn 118 1875 25 Ba 137 5625 75 420000 21 Tl 205 1875 25 Pb 208 3750 50 2 000000 100 Bi 209 3750 50 * Sweeps per reading: 75, readings per replicate: 1, No.of replicates: 3, points across peak: 1, resolution: normal, scanning mode: peak hop, baseline time: 0 ms, transfer frequency: replicate, polarity: +.† Dwell time: 20 ms, readings per replicate: 1000, No. of replicates: 1, points across peak: 1, resolution: normal, scanning mode: peak hop transient, baseline time: 0 ms, transfer frequency: replicate, polarity: +. ‡ Internal standard. Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 405solutions. For the solutions obtained after cation exchange Elemental Concentrations in Tea Infusion and size exclusion, the RSDs varied considerably, from 1 to The tea bag contained 2.0 g of tea, and 200 ml of water were 40% for the spiked samples, and even more for the unspiked used for infusion; this tea:water ratio is commonly used for samples, because the concentrations were frequently close to making a tea infusion.18 In preliminary experiments, the extrac- the detection limits, owing to the dilution.In the tables tion time was varied between 1 and 10 min, and the metal presenting quantitative results, 95% confidence limits are concentration measured.The metal concentration in the tea always given. The detection limits were determined as three infusion was constant after 5 min of extraction, and this times the standard deviation of a blank solution, which had extraction time was used in subsequent experiments. been carried through the same procedure as that used for The results for 24 elements in tea infusion are given in Table 2. the sample.As for the tea leaves, the major elements are Mg, Al, Ca and Mn. In addition, the concentration of Rb in tea infusion is relatively high. The values given in Table 2 agree well with those RESULTS reported byTakeo3 and Natesan and Ranganathan,6 considering Commercially available tea bags (Lipton Yellow Label) were that different teas were analysed (the concentration of Rb was used as samples, because these are widely used in daily life, not reported). Our values agree less well with those of Ahmed the tea is available in reproducible 2.0 g portions with a et al.;5 however, these workers used a different procedure for precision of 1.5%, and the tea leaves are easily removed from making the tea infusion (5 g of tea in 50 ml of water for 2 min). the infusion when the brewing process has been completed.As can be seen from Table 2, the concentrations of Be, Ag, Cd, Distilled, de-ionized water was used for preparing the tea Sn, Pb and Bi were below the detection limits of the infusion; the pH of the infusion was 5.0.The same pH was procedure used. also obtained when the infusion was prepared with soft tap The calculated extraction efficiency, expressed as per cent. water; however, if the tap water contains 0.5 mmol l-1 hydro- of soluble metal relative to total metal in the tea leaves, is also gencarbonate, the pH will be about 5.5. The pH will also vary given in Table 2. The values agree well with those reported with the type of tea used. previously.3,6 A high extraction efficiency (29–85%) was obtained for Mg, Al, Cr, Mn, Co, Ni, Cu, Zn, Rb and Tl; the highest value was for Rb.A very low extraction efficiency Elemental Concentrations in Tea Leaves (3–6%) was obtained for Ca, V, Fe, Sr and Ba. The difference The results for 24 elements in tea leaves are given in Table 2, in extraction between Mg and the other alkaline earth metal from which it can be seen that the major elements are Mg, Al, ions is very marked; Mg is probably not so strongly bound to Ca and Mn.The most abundant element in tea leaves is K, organic material in the tea leaves. which is normally present at about 20 mg g-1,3–6 but this element was not determined here. The values given in Table 2 are in good agreement with those reported by other workers,3–6 considering that different teas were analysed. The value for Rb Strong Cation Exchange is ten times higher than that previously reported,4 whereas the concentrations of Cd and Pb are lower than those reported The results from the cation-exchange experiments are given in Table 2.The results indicate that Mg, Mn and Rb are present previously.6 For Li, Be, As, Se, Ag, Cd, Sn, Pb and Bi, the concentrations were below the detection limits of the in tea infusion as cationic species (>99.7%), and also that Ca, Zn and Sr are predominantly in cationic form (87–96%). For procedure used. Table 2 Elemental concentrations in tea leaves and tea infusion, extraction efficiency (soluble/total), and fraction not retained by the cationexchange cartridge (non-cationic/soluble). Mean values±95% confidence limits are given; n=3–6 Element Tea leaves/mg g-1 Tea infusion/mg l-1 Soluble/total* (%) Non-cationic/soluble† (%) Li <0.08 0.45±0.06 Be <0.02 <0.2 Mg 2281±23 9400±2000 41±9 <0.3 Al 901±12 3200±200 36±2 75±7 Ca 4502±205 2450±550 5±1 4±7 V 0.18±0.04 0.09±0.02 5±1 Cr 1.5±0.3 8±3 49±24 Mn 730±19 2200±500 29±7 <0.2 Fe 127±10 41±5 3.2±0.5 27±22 Co 0.15±0.05 0.74±0.04 48±16 30±37 Ni 5.5±1.7 38±3 69±22 47±9 Cu 19.5±1.1 76±17 39±9 23±7 Zn 28±2 140±30 49±11 4±7 As <0.1 0.11±0.02 Se <2 0.89±0.09 Rb 68.1±0.9 580±110 85±17 <0.1 Sr 26.5±1.0 14.5±1.5 5±1 13±3 Ag <0.05 <0.04 Cd <0.02 <0.08 Sn <1 <0.2 Ba 25±1 15±7 6±3 46±27 Tl 0.029±0.006 0.22±0.02 76±17 Pb <0.3 <2 Bi <0.1 <1 * Fraction of soluble metal, relative to total metal in the tea leaves.Confidence limits were calculated from the law of propagation of errors (for quotients).† Fraction of non-cationic metal eluted, relative to total soluble metal injected. Confidence limits were calculated from the law of propagation of errors (for quotients). 406 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Table 3 Results from SEC of metal species in tea infusion at pH 5.5. The 95% confidence interval is given for the molecular weight, otherwise the mean value±95% confidence limit is given; n=3–6 Element Retention volume/ml Molecular weight/Da Multiple peaks* (%) Eluted/injected† (%) Mg 17.7 <1000 109±38 Al 14.8 6500–8500 40±4 59±28‡ 16.2 4000–6000 60±4 Ca #25 Mn 20.0 <1000 132±33 Fe 16.1 4000–6000 Ni 16.0 4000–6000 Cu 16.0 4000–6000 Zn 16.1 4000–6000 13±3 20.0 <1000 5±3 #40 82±4 Rb 18.4 <1000 98±22 Sr 6.9 >75000 2±1 97±43‡ 11.2 22000–27 000 1±1 #30 97±1 Ba 6.9 >75 000 63±3 11.1 23000–28 000 37±3 Pb 6.9 >75000 49±5 11.1 23000–28 000 35±4 16.1 4000–6000 9±6 20.0 <1000 7±1 * Relative amount of multiple peaks.† Fraction of metal eluted, relative to amount injected. ‡ Sum of all peaks. the alkaline earth elements Mg, Ca, Sr and Ba, the cationic infusion. The retention volumes of the different metals are also indicated in Fig. 2; all elements co-elute with organic material, fraction decreases with increasing atomic weight of the element. but the retention volumes of the metals vary considerably, The transition metals Fe, Co, Ni and Cu are also mainly in almost from the total exclusion to the total permeation volume.cationic form, but in addition there is a non-cationic fraction In Fig. 3, chromatograms obtained with ICP-MS detection (23–47%). The results show that Al is the metal with the most are shown for Rb, Al and Pb. For Rb, a single narrow peak marked non-cationic behaviour (75%). was obtained; chromatograms similar to that of Rb were obtained for Mg, Mn, Fe, Ni and Cu.The corresponding Size-exclusion Chromatography retention volumes are given in Table 3. Multiple peaks were observed for five elements: two peaks The results are presented in Table 3 and Figs. 2 and 3. In for Al and Ba, three peaks for Zn and Sr, and four peaks for Fig. 2, the chromatogram obtained with UV detection at Pb. The corresponding retention volumes are given in Table 3. 227 nm is shown. Small peaks can be seen at retention volumes The two Al peaks are shown in Fig. 3. The two Ba peaks were of about 7 and 12 ml, and larger peaks at about 16, 17 and similar in appearance to the two Pb peaks in Fig. 3 with the 20 ml, but most of the organic material is apparently eluted at lowest retention volumes. For both Zn and Sr, two small the total permeation volume. Peaks were also observed at baseline-separated peaks were observed at retention volumes higher retention volumes (not shown in Fig. 2), indicating below the total permeation volume. In addition, a third peak processes other than size exclusion.The chromatogram shows was observed for both elements at a retention volume higher that a great variety of organic molecules are present in tea than the total permeation volume; these peaks were large and broad with marked tailing. For Pb, two large and two small peaks were obtained, as can be seen from the chromatogram in Fig. 3. Although the two peaks with the highest retention volumes are fairly small, they were still reproducibly present in the chromatograms.The retention volumes and corresponding molecular weight estimates are summarized in Table 3; for the multiple peaks the relative amounts are also given. The molecular weight estimates given in Table 3 are based on calibration of the column with globular proteins; the shapes of the metal-binding ligands in tea are not known. As can be seen from Table 3, Mg, Mn and Rb were eluted close to the total permeation volume, indicating that these metal ions are either associated with small organic molecules, or not associated with organic material at all.One Zn andone Pb peakwere also observed inthis region, indicating free ions or small molecular weight complexes. From Table 3 it can be seen that for Al, Fe, Ni, Cu, Zn and Pb, a peak was observed at a retention volume of 16.0–16.2 ml; these elements may, therefore, be bound to species with a Fig. 2 Size-exclusion chromatogram of tea infusion, using UV detec- molecular weight in the range 4000–6000 Da.A second Al tion at 227 nm. Lipton Yellow Label tea bag; Superdex 75 HR 10/30 peak was obtained at a retention volume of 14.8 ml, corre- column. Eluent, 0.10 mol l-1 ammonium acetate of pH 5.5; flow rate, sponding to a molecular weight of 6500–8500 Da. For Sr, Ba 1.0 ml min-1. The retention volumes of the different metals are also indicated. and Pb, peaks were observed at a retention volume of Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 407Fig. 3 Size-exclusion chromatogram of tea infusion, using ICP-MS detection at mass 85 for Rb, mass 27 for Al and mass 208 for Pb. Experimental conditions as in Fig. 2. 11.1–11.2 ml, and at 6.9 ml, indicating that these metals may column, and 86% was retained by a 20000 molecular weight cut-off ultrafilter, whereas Powell et al.14 reported that 78% be associated with molecules of the size 22000–28000 Da and of Al passed through a 3000-Da ultrafilter. even larger than 75000 Da.There seems to be no simple relationship between the The fraction of metal eluted from the column, relative to solubility of a metal during tea brewing and the species present the total amount injected, is also given in Table 3, for Mg, Al, in the infusion; however, the major elements Mg, Mn and Rb, Mn, Rb and Sr. The results indicate that Mg, Mn, Rb and Sr which have a high extraction efficiency, are probably extracted were not retained by the column.Only some Al was retained, as Mg2+, Mn2+ and Rb+. as has also been observed by other workers at pH 5.5.13 As If a further characterization of the metal–organic complexes can be seen from Table 3, only the elements present in high present in tea infusions is to be carried out, methods such as concentrations could be determined in the eluate, and even LC–MS and LC–MS–MS could be used to obtain information those only with poor precision, owing to the high dilution of about the organic ligands responsible for the complexation. the sample after injection.Financial support from The Research Council of Norway for DISCUSSION the purchase of most of the instrumental equipment used in SEC and cation-exchange separation based on solid-phase this work is greatly appreciated. extraction cartridges can provide complementary information about the metal species present in tea infusion. The results REFERENCES obtained with the two techniques agree reasonably well if it is 1 Stagg, G.V., and Millin, D. J., J. Sci. Food Agric., 1975, 26, 1439. assumed that the metal-binding organic ligands in a tea 2 Stagg, G. V., Nutr. Bull., 1980, 29, 233. infusion are large polyphenolic compounds. Such compounds 3 Takeo, T., Jpn. Agric. Res. Quart., 1985, 19, 32. occur widely in tea2 and other plants,19,20 and they are 4 Suzuki, S., Matumoto, K., Okada, Y., and Hirai, S., Bunseki probably the Al-binding ligands in tea infusion.15 The poly- Kagaku, 1986, 35, 993. 5 Ahmed, I., Zaidi, S. S. H., and Khan, Z. A., Pak. J. Sci. Ind. Res., meric phenols can form neutral or negatively charged metal 1989, 32, 513. complexes at pH 5,19 which will pass through the cation- 6 Natesan, S., andRanganathan, V., J. Sci. Food Agric., 1990, 51, 125. exchange cartridge. 7 Aluminium in Food and the Environment, eds. Massey, R. C., and The two separation techniques may both disturb the species Taylor, D., Royal Society of Chemistry, Cambridge, 1989.distribution to a certain extent; the metal–organic complexes 8 Metal Ions in Biological Systems, eds. Sigel, H., and Sigel, A., can partly dissociate when passing through the cation-exchange Marcel Dekker, New York, 1988, vol. 24. 9 Aluminium and Health Workshop, Environ. Geochem. Health, or size-exclusion column, and metal complexes with acetate 1990, 12 (1–2). (eluent buffer) may be formed in the SEC experiments. Even 10 Wills, M. R., and Savory, J., in Metal Ions in Biological Systems, so, the ion-exchange results indicate that the elements Mg, Mn eds.Sigel, H., and Sigel, A., Marcel Dekker, New York, 1988, and Rb are present in tea infusion as cations, and the SEC vol. 24, p. 315. experiments show that the species have a low molecular weight. 11 Stewart, W. K., in Aluminium in Food and the Environment, eds. Massey, R. C., and Taylor, D., Royal Society of Chemistry, The species are probably simple inorganic ions; alternatively, Cambridge, 1989, p. 6. the species could be positively charged complexes with small 12 French, P., Gardner, M. J., and Gunn, A. M., Food Chem. organic ligands; however, the latter interpretation is less likely T oxicol., 1989, 27, 495. for the elements in question. 13 Owen, L. M. W., Crews, H. M., and Massey, R. C., Chem. Speciat. Also, Ca, Fe, Co, Ni, Cu, Zn, Sr and Ba appear to be present Bioavail., 1992, 4, 89. mainly in cationic form, but there is in addition a non-cationic 14 Powell, J. J., Greenfield, S. M., Parkes, H. G., Nicholson, J. K., and Thompson, R. P. H., Food Chem. T oxicol., 1993, 31, 449. fraction. For the transition metals Fe, Ni, Cu and Zn, the non- 15 Baxter, M. J., Burrell, J. A., Crews, H. M., and Massey, R. C., in cationic species may be metal–organic complexes in the size Aluminium in Food and the Environment, eds. Massey, R. C., and range 4000–6000 Da, whereas Sr, Ba and Pb are associated Taylor, D., Royal Society of Chemistry, Cambridge, 1989, p. 77. with even larger molecules, in the size range 22000–28000 Da 16 Driscoll, C. T., Int. J. Environ. Anal. Chem., 1984, 16, 267. and >75000 Da. The molecular weight estimates refer to 17 Matusiewicz, H., Sturgeon, R. E., and Berman, S. S., J. Anal. At. calibration against proteins; if polystyrene sulfonates had been Spectrom., 1989, 4, 323. 18 Baltes, W., L ebensmittelchemie, Springer, Berlin, 4th edn., 1995, used for calibration of the column, the molecular weight p. 381. estimates would be one tenth of the values given in Table 3.21 19 Powell, H. K. J., and Rate, A. W., Aust. J. Chem., 1987, 40, 2015. Al is the metal with the most marked non-cationic behaviour 20 Stevenson, F. J., and Vance, G. F., in T he Environmental Chemistry in tea infusion. Two peaks were observed in the chromatogram: of Aluminium, ed. Sposito, G., CRC Press, Boca Raton, FL, the corresponding size ranges were 4000–6000 and 6500– 1989, p. 117. 8500 Da, respectively. Owen et al.13 obtained only a single 21 Zernichow, L., and Lund, W., Anal. Chim. Acta, 1995, 300, 167. peak at pH 5.5, which corresponded to 6100 Da (proteins were used for calibration). Also, other workers have reported that Paper 6/06153B Al is associated with organic material in tea infusion: French Received September 6, 1996 Accepted January 2, 1997 et al.12 found that 95% of Al passed through a cation-exchange 408 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606153b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Automated Continuous-flow Hydride Generation with Inductively Coupled Plasma Mass Spectrometric Detection for the Determination of Trace Amounts of Selenium(iv), and Total Antimony, Arsenic and Germanium in Sea-water SRI JUARI SANTOSA, HIROSHIGE MOKUDAI AND SHIGERU TANAKA* Department of Applied Chemistry, Faculty of Science and T echnology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan An automated continuous-flow hydride generation system was of the HG technique, the batch mode was commonly adopted to overcome the relatively slow reduction rate when metal–acid developed for the determination of trace amounts of SeIV and total Sb, As and Ge in sea-water using ICP-MS detection. reduction systems such as zinc–HCl were used.After the discovery that sodium tetrahydroborate (NaBH4) Since the same responses can be obtained between the Sb species SbV and SbIII, among the As species AsV, AsIII, was a more powerful reducing agent and that the reduction could be carried out in the same aqueous phase, the continu- monomethylarsonic acid and dimethylarsinic acid, and between the Ge species GeIV and monomethylgermanium, a single Sb, ous-flow mode became an alternative and more convenient mode for transporting the generated volatile hydride to a As and Ge species can be used for calibration.A sensitivity enhancement was observed when methanol was used for matrix detection device.12–15 The continuous-flow mode of the HG technique has become even more attactive owing to the relative modification.Matrix modification was also useful for achieving recoveries close to 100% in recovery tests performed ease of automation, which enables the speed of the analytical process to be increased and the analytical precision to be on a sea-water sample. Detection limits of 2.2 ng l-1 for Sb as SbIII, 6.5 ng l-1 for As as AsV, 2.4 ng l-1 for Ge as GeIV and improved. This paper describes an automated continuous-flow HG 0.5 ng l-1 for SeIV were obtained.For replicate determinations in both a standard solution and a real sea-water sample, the technique for use with inductively coupled plasma mass spectrometry (flow HG-ICP-MS). The method is highly sensitive repeatabilities were within 7% (as RSD). The proposed method is rapid (15 samples can be analysed per hour) and sufficiently and enables trace amounts of SeIV as well as total Sb, As and Ge in sea-water to be determined directly.No preconcentration sensitive to allow trace amounts of SeIV and total Sb, As and Ge to be directly determined in surface sea-water samples step is necessary and the method can be use to analyse 15 seawater samples per hour. taken at a high latitude of the North Pacific Ocean from Japan to Canada. Keywords: Automated continuous-flow hydride generation; EXPERIMENTAL antimony; arsenic; germanium; selenium(IV); sea-water; System Configuration inductively coupled plasma mass spectrometry Sea-water sample and reagents (methanol, HCl and NaBH4 solutions) were delivered using an ASA-200 autosampler Since the work of Holak1 on the generation of arsine (AsH3) using zinc metal and hydrochloric acid for the determination (Gilson,Worthington, OH, USA) and peristaltic pumps (Eyela, Tokyo, Japan, Model MP-3) equipped with PharMed (Norton, of As by AAS more than 26 years ago, the generation of a volatile hydride coupled with various detection techniques has Worcester, MA, USA) tubing (0.8 mm id, 4 mm od), respectively. A schematic diagram of the apparatus is given in Fig. 1. become one of the most sensitive analytical methods not only for the determination of As but also for other volatile hydride- The sample from the autosampler is delivered by pump 1 and first mixed with methanol solution delivered by pump 2 forming elements such as Sb, Bi, Ge, Pb, Se, Te and Sn.2–4 The high sensitivity of analytical methods using the hydride gener- through a transfer line of Teflon tubing (1 mm id, 2 mm od).After passing through a 50 cm mixing coil (four turns) made ation (HG) technique for sample introduction can be ascribed to the fact that the HG technique is highly efficient and allows from silicone tubing (2 mm id, 4 mm od), the mixed solution is acidified with HCl and then reduced with NaBH4 solution the preconcentration and separation of the hydride-forming elements from matrix interferences. delivered by pumps 3 and 4, respectively.The transfer line used for delivering the HCl and NaBH4 solutions is the same The high sensitivity and the ability to separate the analyte of interest from matrix interferences make the HG technique as that used for delivering the methanol solution. A 1 m stripping coil (eight turns) made from Teflon tubing (2 mmid, useful for the determination of trace levels of volatile hydrideforming elements in samples with complex matrices such as 4 mm od) facilitates the complete formation of volatile hydrides from the analytes of interest and their stripping from the sea-water.In the HG technique, the volatile hydride may be transferred aqueous phase. In a J-type gas–liquid separator,16 the volatile hydrides are separated from the aqueous phase and swept by in a collection transfer mode (batch mode) or a direct transfer mode (continuous-flow mode). In the batch mode, the hydrides a 300 ml min-1 flow of helium carrier gas into the argon carrier gas stream (0.7 l min-1) of the ICP-MS PMS 2000 are first collected in a cold trap consisting of a U-tube immersed in liquid nitrogen,5,6 or in a closed vessel under pressure7,8 or instrument (Yokogawa Analytical System, Tokyo, Japan) through a warmed Teflon tube (4 mm id, 6 mm od).The length in various absorbing solutions9–11 before their introduction into a detection device. In the early stages of the development of the transfer tube from the gas–liquid separator to the torch Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (409–415) 409Fig. 1 Schematic diagram of the automated continuous-flow hydride generation system for ICP-MS. of the ICP-MS instrument is 1 m. By means of an optical fibre containing the washing-water, its injection needle descends and blocks the infrared light emitted by the fibre sensor. sensor (Model FT-500; Takenaka Denshi Kogyo, Tokyo, Japan), a programmable sequencer (Omron, Kyoto, Japan, Simultaneously, the sequencer activates pump 1 at a flow rate of 3.0 ml min-1 and the injection of washing-water begins.Model Sysmac C20P) is used to control the operation of all peristaltic pumps. After 100 min, the injection needle returns to its original position and the arm of the autosampler moves to the designated vial containing the sample; the injection needle descends Reagents and the sample is then introduced. Twenty minutes after sample introduction, pump 2 is activated at a flow rate of All reagents used were of the highest purity available and of 2.5 ml min-1 and methanol solution merges with the sample at least analytical-reagent grade.De-ionized, distilled water, stream. At 170 s (30 s after the introduction of methanol), the further purified with a Millipore (Bedford, MA, USA) watersequencer activates pumps 3 and 4, both at flow rates of purification system (Milli-Q SP. TOC. reagent water system), 2.0 ml min-1, so that the mixed stream of sample and methanol was used for preparation of all solutions.is successively acidified with HCl and reduced by the NaBH4 An SbV stock solution (1000 mg l-1) was prepared by dissolution. solving 2.222 g of potassium hexahydroxoantimonate(V) Simultaneously with the activation of pumps 3 and 4, the (Wako, Osaka, Japan) in the minimum volume of HCl and spectra of Sb, As, Ge and Se are monitored for up to 70 s. further dilution with purified water to 1 l. An SbIII stock Data acquisition is performed in the last 60 s of the monitoring solution (1000 mg l-1) was prepared by dissolving 2.478 g of step.During this time, the volatile hydrides of the Sb, As, Ge antimonyl potassium tartrate in 1 l of purified water. An AsV and Se species exhibit maximum and stable spectra. After data stock solution (1000 mg l-1) was preparedby dissolving 2.529 g acquisition, the arm of the autosampler moves back to the vial of potassium arsenate (Nakarai Chemical, Kyoto, Japan) in containing the washing-water and the process is repeated. 1 l of purified water. An AsIII stock solution (1000 mg l-1) was Accordingly, the time needed for the analysis of one sample is prepared by dissolving 1.330 g of arsenic(III ) trioxide in 20 ml 4 min, giving a throughput of 15 samples h-1. For a single of 1 mol l-1 NaOH solution and diluting with purified water analysis, the volumes of washing-water, sample, methanol, HCl to 1 l. Stock solutions of monomethylarsonic acid (MMAA) and NaBH4 solutions needed are 5, 6, 4.2, 2.3 and 2.3 ml, (1000 mg l-1) and dimethylarsinic acid (DMAA) (1000 mg l-1) respectively. were prepared by dissolving 2.455 g of MMAA (Pfalz & Bauer, Waterbury, CT, USA) and 2.047 g of DMAA, respectively, in 1 l of purified water.A commercial standard solution for Operating Conditions for ICP-MS atomic absorption analysis (1000 mg l-1) was used as a stock solution of both GeIV and SeIV. A monomethylgermanium Before the introduction of sample and reagents into the line system, the lens system of the ICP-MS instrument was optim- (MMGe) stock solution was obtained from Yoshiki Sohrin (Department of Chemistry, Kyoto University, Japan).An SeVI ized daily and tuned on masses 76 (40Ar36Ar) and 80 (40Ar40Ar) standard solution (1000 mg l-1) was prepared by dissolving 2.659 g of sodium selenate in 1 l of purified water. Working Table 1 Operating conditions for ICP-MS solutions of SbV, SbIII, AsV, AsIII, MMAA, DMAA, GeIV, Forward rf power 1.3 kW MMGe, SeVI and SeIV were prepared daily by diluting the Argon gas flow rate: respective stock solutions with purified water.Coolant 11 l min-1 A sodium tetrahydroborate solution (1.5% m/v) was pre- Auxiliary 1.1 l min-1 pared daily by dissolving NaBH4 in purified water containing Carrier 0.7 l min-1 0.24% m/v NaOH. Solutions of HCl (3 mol l-1) and methanol 5 mm Torch position (load coil– sampling aperture distance) were prepared daily by diluting 12 mol l-1 HCl and methanol Lenses Tuned on m/z 76 (40Ar36Ar) and (Junsei Chemical, Tokyo, Japan), respectively, with purified m/z 80 (40Ar40Ar) water.Monitored ion (m/z) 72Ge, 75As, 82Se, 121Sb Counting conditions: Dwell time 20 ms (As and Sb) Running Conditions 60 ms (Ge) 80 ms (Se) Samples are first placed in vials in the racks of the autosampler. Number of scans 2 One of the vials is filled with washing-water and an optical Measurement time 70 s fibre sensor is fitted immediately above it.Before sample Data acquisition time 10–70 s introduction, the arm of the autosampler moves to the vial 410 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12so that maximum signals were obtained (Table 1). Coolant, Hydrochloric acid concentration auxiliary and carrier argon gas flow rates were maintained to Although SeIV can be reduced to Se-II by NaBH4 in alkaline ensure the stability of the plasma at a forward rf power of solution,14,17 the generation of its gaseous hydride only occurs 1.3 kW.This rf power is sufficiently stable in both the absence in acidic solution.13,14,17 An acidic solution is also needed to and presence of hydrogen and hydride gases. In order to generate the hydrides of As and Sb13 and Ge.18 Using 1% m/v minimize mass isobaric interferences resulting from argon, NaBH4 solution, the acidity of the sample stream was optim- hydrogen and other species, masses (m/z) 72, 75, 82 and 121 ized by injecting various concentrations of HCl through were chosen for monitoring Ge, As, Se and Sb, respectively.pump 3. It is apparent (Fig. 3) that the profiles of the signal areas of AsV, AsIII, SbV, SbIII and SeIV are similar, i.e., a rapid increase with acid concentration from 0.1 to 4 mol l-1 and RESULTS AND DISCUSSION then relatively constant for acid concentrations of 4–8 mol l-1. Optimization of Analytical Conditions For MMAA, DMAA, GeIV and MMGe, the maximum signal areas were observed at acid concentrations of 4, 1, 0.5 and T etrahydroborate concentration 3 mol l-1, respectively. Although most of the maximum signal In order to facilitate suitable conditions for the simultaneous areas were obtained with 4 mol l-1 HCl, a 3 mol l-1 HCl generation of volatile hydrides of Sb, As, Ge and Se, the solution must be employed for the simultaneous determination concentrations of the HCl and NaBH4 solutions were carefully of SeIV and total As and Ge, so that the same responses among optimized.Using a mixed standard solution of As, Sb, Ge and the As species and between GeIV and MMGe are obtained. Se species (all species were present at a concentration of On the other hand, it seems that SbV is not quantitatively 1 mg l-1 as their elemental forms) from pump 1, the signal reduced to SbH3 by NaBH4. Therefore, to ensure that SbV is areas of the volatile hydrides of these species were examined reduced quantitatively, the pre-reduction of SbV to SbIII is at an HCl concentration of 3 mol l-1 (pump 3) and at NaBH4 necessary.concentrations from 0.2 to 2% m/v (pump 4). As can be seen in Fig. 2, the maximum signal areas of AsV, AsIII, MMAA, SbV, SbIII and SeIV were obtained with 0.5% m/v NaBH4 and Pre-reduction of SbV to SbIII by L-cysteine those of DMAA, GeIV and MMGe with 1% m/v NaBH4. For It is well documented that trivalent and pentavalent Sb and SeVI, no reduction to the hydride occurred. Although most of also As usually show different sensitivities in the HG pro- the maximum signal areas were obtained with 0.5% m/v cess.19–23 As has been found above, with the use of 3 mol l-1 NaBH4, for the purpose of determining SeIV and total As, Ge HCl and 1% m/v of NaBH4, the difference in the sensitivity and Sb, the use of 1% m/v NaBH4 was considered more can be overcome for As species but not for Sb species.The use appropriate because the same responses (signal areas) were of high concentrations of HCl and NaBH4 to overcome the obtained among the As species and between GeIV and MMGe.different sensitivities in the HG of AsV and AsIII has also been For SbV and SbIII, an additional treatment must be performed reported by Narsito and Agterdenbos.24 However, for Sb, the because the same responses were not obtained. Fig. 3 Effect of HCl at a flow rate of 2.0 ml l-1 on the generation of Fig. 2 Effect of NaBH4 at a flow rate of 2.0 ml l-1 on the generation of the volatile hydrides of As, Sb, Ge and Se species.All species are at the volatile hydrides of As, Sb, Ge and Se species using 1% m/v NaBH4 at a flow rate of 2.0 ml l-1. All species are at a concentration a concentration of 1 mg l-1 as their elemental forms and injected at a flow rate of 3.0 ml min-1, and acidified by 3 mol l-1 HCl at a flow of 1 mg l-1 as their elemental forms and injected at a flow rate of 3.0 ml min-1. rate of 2.0 ml l-1. Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 411pre-reduction of SbV to SbIII is the only way to overcome the media has also been reported by Welz and Sucmanova.20 Therefore, the determination of total As is not possible in the difference in their sensitivities. Potassium iodide (KI) has been widely used to reduce SbV presence of L-cysteine. For Se, the reducing property of L-cysteine may be the and AsV to their trivalent oxidation states. It can be used alone25,26 or in combination with ascorbic acid27 and sodium reason for the suppression of the signal area of SeIV.SeIV may be reduced to elemental Se prior to its generation as SeH2. sulfite.28 In recent years, however, the use of L-cysteine as a more powerful and convenient pre-reducing agent than KI has Therefore, the determinatoin of SeIV is best performed in the absence of L-cysteine. been demonstrated.20,29 As a pre-reductant, L-cysteine can rapidly reduce SbV and AsV to their trivalent oxidation states In subsequent experimental work, it was decided that for the determination of total Sb, the 3 mol l-1 HCl containing in weakly acidic media.Moreover, L-cysteine can also act as a signal enhancer and interference reducer, not only for Sb 10% m/v L-cysteine would be used instead of 3 mol l-1 HCl alone and 1% m/v NaBH4. For SeIV, total As and total Ge, and/or As, but also for Ge;20,21,23,29–31 its solution is very stable and it is also able to stabilize AsIII standard solutions.20 their simultaneous determination can easily be performed by using 1% m/v NaBH4 and 3 mol l-1 HCl.These superior properties of L-cysteine over those of KI prompted us to apply L-cysteine in an effort to obtain identical responses for both SbV and SbIII. Various concentrations of L- Matrix modification cysteine were added to 3 mol l-1 HCl and their effect on the HG of As, Sb, Ge and Se species is presented in Fig. 4. On the It is known that the presence of carbon-containing compounds produces a signal enhancement for elements determined by addition of 10% m/v L-cysteine to the HCl solution, the same responses for both SbV and SbIII were obtained, which implies ICP-MS using a nebulizer for sample introduction.32 More recently, the signal enhancement due to carbon-containing that SbV is reduced to SbIII by L-cysteine before its generation as SbH3.In addition, a signal intensity enhancement that has compounds was also observed for elements introduced into the ICP-MS instrument by both a pneumatic nebulizer and been reported by other workers in weakly acidic media was also confirmed by the relatively high concentration of HCl by flow injection hydride generation (FI-HG).33 Considering that the ionization potential of carbon is high used in this study.Although, at low concentrations, L-cysteine enhances the (11.26 eV), the presence of carbon-containing compounds in the plasma would improve the ionization efficiency of poorly signals of GeIV and MMGe to different extents, the same signal areas can be obtained by using more than 5% m/v L-cysteine.ionized elements through an additional electron transfer from these elements to C+ and/or carbon-containing polyatomic On the other hand, L-cysteine causes the signal areas of the As species to be unequal. The signal areas of AsIII and MMAA ions.32,33 It should be noted, however, that signal enhancement due to the presence of carbon-containing compounds is also clearly decrease with increasing L-cysteine concentration.The suppression of the signal of AsIII by L-cysteine in highly acidic common for elements generated as volatile species and detected by methods other than ICP-MS (for example AAS and ICPAES). 34 Therefore, in addition to the modification of the ionization equilibrium in the plasma, the presence of carboncontaining compounds must also modify the physico-chemical properties of the solution (reaction medium), leading to an improvement in the efficiency of volatile species generation.34 With the use of a pneumatic nebulizer for sample introduction, the physico-chemical modification may enhance the efficiency of sample introduction through the production of smaller droplets.33 In this work, methanol, which has been found to give high signal enhancements for As, Se and Te,33 was used for matrix modification. As can be seen in Fig. 5, the injection of methanol solutions up to a concentration of 50% v/v through pump 2 results in signal enhancements for AsV , as a representative As species, and SeIV.For GeIV and SbIII, as representatives of Ge and Sb species, respectively, signal enhancements are only observed on injection of methanol solutions up to concentrations of 5 and 10% v/v, respectively. The maximum enhancement factors are 2.0, 1.9, 1.2 and 1.1 for SeIV, AsV, SbIII and GeIV, respectively. The higher enhancement factors for AsV and SeIV compared with those for SbIII and GeIV are in accordance with the claims of Allain et al.32 and Olivas et al.33 that carbon-containing compounds enhance the signals of elements with ionization energies between 8.5 and 10 eV, such as As (9.82 eV) and Se (9.75 eV), more significantly than those of elements with lower ionization energies.The use of a 10% v/v methanol solution was chosen for matrix modification. In this work, it was found that the amounts of carbon compounds detected in the plasma (monitored as 13C+ and 40Ar13C+) during the determination of SeIV, and total Sb, As and Ge in sea-water or sea-water spiked with SeIV, AsV, SbIII and GeIV, were significantly higher than those detected from Fig. 4 Effect of L-cysteine in 3 mol l-1 HCl at a flow rate of 2.0 ml l-1 water (Milli-Q) or 1 ng l-1 standard solutions of SeIV, AsV, on the signal intensity of the volatile hydrides of As, Sb, Ge and Se SbIII and GeIV (Table 2). The ratios of the 13C+ and 40Ar13C+ species generated by 1% m/v NaBH4 at a flow rate of 2.0 ml l-1.All counts generated from sea-water to those generated from species are at a concentration of 1 mg l-1 as their elemental forms and injected at a flow rate of 3.0 ml min-1. Milli-Q water were 2.29 and 2.97, respectively. For a sea-water 412 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Fig. 5 Effect of 10% v/v methanol at a flow rate of 2.5 ml min-1 on the signal intensity of the volatile hydrides of GeIV, AsV, SeIV and SbIII generated by 1% m/v NaBH4 at a flow rate of 2.0 ml l-1 from a blank and 1 mg l-1 mixed standard solutions.Table 2 Effect of methanol on C and ArC counts for several sample types Signal area (counts) 13C 40Ar13C Sample type 0% (v/v) MeOH 5% (v/v) MeOH 10% (v/v) MeOH 0% (v/v) MeOH 5% (v/v) MeOH 10% (v/v) MeOH (a) Blank (Milli-Q water) 1.01×106 3.85×106 6.43×106 2.36×103 5.61×103 9.78×103 (b) Mixed standard solution* 9.23×105 3.48×106 6.57×106 2.28×103 5.60×103 9.89×103 (c) Sea-water† 2.32×106 6.79×106 7.26×106 7.01×103 1.03×104 1.19×104 (d) Sea-water† spiked with 2.26×106 6.56×106 7.19×106 6.69×103 9.73×103 1.13×104 standard solution‡ (c)/(a) 2.29 1.76 1.13 2.97 1.83 1.21 (d)/(b) 2.45 1.88 1.09 2.93 1.74 1.14 * 1 ng l-1 mixed standard solution of SbIII, AsV, and GeIV and SeIV.† Taken at a sampling site of 48°00N, 172°00W (July 1995). ‡ Sea-water spiked with 1 ng l-1 of SbIII, AsV, GeIV and SeIV. sample spiked with SeIV, AsV, SbIII and GeIV, the 13C+ and similar amounts of carbon and carbon-containing compounds enter the plasma. 40Ar13C+ counts were 2.45 and 2.93 times higher than the 13C+ and 40Ar13C+ counts form the corresponding 1 ng l-1 standard solutions. Analytical Performance The larger amounts of carbon species evolved from seawater samples compared with standard solutions and Milli-Q Recovery water must be caused by the presence of high levels of carbon In previous work, the role of carbon-containing compounds compounds in sea-water. It is known that carbon in sea-water as a signal enhancer was usually observed during the analysis in the form of carbonic acid is in equilibrium with CO2 gas of biological samples.In this work, it was found that carbon- according to the following equation:35 containing compounds in sea-water also act as a signal enhancer. Hence, a recovery test performed on a sea-water 2H+(aq)+CO32-(aq)=H+(aq)+HCO3-(aq) sample, taken at the surface of a sampling site in the north- =H2O(l)+CO2(g) west Pacific Ocean (April 2, 1996) during a cruise of the C/S Skaugran showed that, in the absence of methanol, the recover- During the generation of the hydrides of SeIV and of the Sb, As and Ge species in sea-water, H2 gas is also produced.The ies of SbV, AsV, GeIV and SeIV were all significantly higher than 100% (Table 3). After matrix modification using 10% v/v presence of H2 accelerates and enhances the separation of both the gaseous hydrides and CO2 from the sample solution and methanol, good recoveries were obtained. therefore leads to larger amounts of carbon and carboncontaining ions entering the plasma.Detection limit and precision As shown in Table 2, the difference in the 13C+ and 40Ar13C+ counts obtained from sea-water samples and standard solutions Under the optimum operating condition, the detection limits (3s of the blank) of As as AsV, Se as SeIV, Ge asGeIV and Sb or Milli-Q water can be minimized by the use of methanol.On the introduction of 10% v/v methanol solution from as SbV were 6.5, 0.5, 2.4 and 2.2 ng l-1, respectively (Table 4). The average RSDs from replicate measurements were 2.2, 0.6, pump 2, the ratios of the 13C+ and 40Ar13C+ counts between sea-water samples and Milli-Q water and between spiked sea- 2.4 and 1.4% for AsV, SeIV, GeIV and SbV in a mixed standard solution containing 1000, 50, 20 and 100 ng l-1 of these water samples and mixed standard solutions are close to unity.Hence, the introduction of 10% v/v methanol can modify both elements, respectively. The average RSDs for the determination of AsV, SeIV, GeIV and SbV in the same sea-water sample as sea-water and standard solutions or Milli-Q water so that Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 413Table 3 Effect of methanol on recovery of analytes spiked into a sea-water sample taken in the northwest Pacific Ocean (April 2, 1996) Concentration found/ng l-1 Concentration spiked/ng l-1 In the presence of MeOH (10%) In the absence of MeOH AsV SeIV GeIV SbV Total As SeIV Total Ge Total Sb Total As SeIV Total Ge Total Sb 0 0 0 0 1218 5.7 65.7 263.3 1234 5.7 66.2 268.3 1193 5.4 64.4 269.4 1248 5.5 65.4 269.7 1209 5.6 65.1 265.7 1233 5.6 65.8 268.6 2000 50 50 200 3314 55.8 114.6 470.6 3481 59.8 121.8 479.6 3312 53.6 113.6 473.3 3553 58.6 118.3 479.7 3301 54.1 114.8 468.7 3518 59.9 119.5 483.8 Recovery (%) 105.1±0.6 97.9±1.7 98.2±0.9 102.4±0.9 114.0±1.2 107.7±1.1 108.1±2.3 106.1±1.1 Table 4 Detection limit (3s blank) and precision (RSD) for replicate measurements of analytes in a standard solution and sea-water sample RSD (%) Detection limit*/ Analyte ng l-1 Standard solution† Sea-water‡ As 6.5 2.2 2.1 Se 0.5 0.6 1.8 Ge 2.4 2.4 3.4 Sb 2.2 1.4 6.2 * Five replicate measurements of the blank.† Five replicate measurements of a multi-element standard solution containing 1000, 50, 20 and 100 ng l-1 AsV, SeIV, GeIV and SbV, respectively.‡ Three replicate measurements of a sea-water sample taken in the northwest Pacific Ocean (April 2, 1996). used for the recovery test were 2.1, 1.8, 3.4 and 6.2%, respectively. Application The proposed method was applied to the determination of SeIV and total Sb, As and Ge in samples of surface sea-water from the high latitudes of the North Pacific Ocean taken during a cruise of the C/S Skaugran from Japan to Vancouver (Canada) in the period March 31–April 13, 1996.The samples were filtered on-board the ship through 0.45 mm Millipore filters immediately after sampling using a number of clean sampling facilities and were stored in the dark in poly(propylene) bottles at 0 °C. Analysis was complete within 2 months of sampling. The simultaneous determination of total As and Ge and of SeIV was performed by using 1% m/v NaBH4 and 3 mol l-1 HCl solutions. Total Sb was determined by using 1% m/v Fig. 6 Sampling sites and distribution of SeIV and total As, Sb and NaBH4 and 3 mol l-1 HCl containing 10% m/v L-cysteine.Ge in the surface waters of the north Pacific Ocean during a cruise of the C/S Skaugran (Japan–Canada, March 31–April 14, 1996). As can be seen in Fig. 6, the proposedmethod was sufficiently sensitive for the direct determination of SeIV and total Sb, As and Ge in sea-water, the average concentrations found being of Chemistry, Kyoto University, Japan) for providing the 5.8±3.3 (n=12), 277±28 (n=12), 1323±282 (n=12) and standard of MMGe, to Y.Nojiri (National Institute for 66.8±6.8 (n=12) ng l-1, respectively. Environmental Studies, Japan) for the sea-water samples and The results for SeIV and total Sb and As obtained in this to the captain and crew of the C/S Skaugran during the work were in good agreement with those reported in the sampling cruises. surface waters of various oceanic areas by Measures and Burton,36 Cutter and Bruland37 and Cutter and Cutter38 for REFERENCES SeIV, by Santosa et al.39 and Tanaka and Santosa40 for total As, and by Yamamoto et al.41 and Middelburg et al.42 for total 1 Holak, W., Anal.Chem., 1969, 41, 1712. Sb. However, for Ge, the results in Fig. 6 are slightly higher 2 Nakahara, T., Spectrochim. Acta Rev., 1992, 14, 95. 3 Yan, X.-P., and Ni, Z.-M., Anal. Chim. Acta, 1994, 291, 89. than the reported concentrations of total Ge in the surface 4 van der Jagt, H., and Stuyfzand, P.J., Fresenius’ J. Anal. Chem., water of various oceanic areas.43 1996, 354, 32. 5 Andreae, M. O., Asmode, J.-F., Foster, P., and van’t Dack, L., We gratefully acknowledge The Hitachi Scholarship Anal. Chem., 1981, 53, 1766. Foundation for supporting the work of S.J.S. and The Tokyu 6 Cutter, G. A., Anal. Chim. Acta, 1978, 98, 59. Foundation for providing research support through a Scientific 7 Narasaki, H., and Ikeda, M., Anal. Chem., 1984, 56, 2059. 8 Tsunoda, A., Matsumoto, K., and Fuwa, K., Anal.Sci., 1986, 2, 119. Research Grant.We are also grateful to Y. Sohrin (Department 414 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 129 Shaikh, A. U., and Tallman, D. E., Anal. Chem., 1977, 49, 1093. 30 Le, X.-C., Cullen, W. R., and Reimer, K. J., Anal. Chim. Acta, 1992, 258, 307. 10 Maher, W. A., Analyst, 1983, 108, 305. 31 Welz, B., and S¢§ ucmanova¢¥, M., Analyst, 1993, 118, 1425. 11 Tsalev, D. L., Mandjukov, P. B., and Stratis, J.A., J. Anal. At. 32 Allain, P., Jaunault, L., Mauras, Y., Mermet, J.-M., and Spectrom., 1987, 2, 135. Delaporte, T., Anal. Chem., 1991, 63, 1497. 12 Agterdenbos, J., van Noort, J. P. M., Peters, F. F., and Bax, D., 33 Mun.oz Olivas, R., Que¢¥tel, C. R., and Donard, O. F. X., J. Anal. Spectrochim. Acta, Part B, 1986, 41, 283. At. Spectrom., 1995, 10, 865. 13 Narsito, Agterdenbos, J., and Santosa, S. J., Anal. Chim. Acta, 34 Fernandez de la Campa, M. R., Garcia, E.S., Valdes-Hevia y 1990, 237, 189. Temprano, M. C., Fernandez, B. A., Gayon, J. M. M., and Sanz- 14 Wickstr©ªm, T., Lund, W., and Bye, R., J. Anal. At. Spectrom., Medel, A., Spectrochim. Acta, Part B, 1995, 50, 377. 1995, 10, 803. 35 Libes, S. M., An Introduction to Marine Biogeochemistry, Wiley, 15 Rayman, M. P., Abou-Shakra, F. R., and Ward, N. I., J. Anal. New York, 1992, p. 98. At. Spectrom., 1996, 11, 61. 36 Measures, C. I., and Burton, J. D., Earth Planet. Sci. L ett., 1980, 16 Thompson, M., Pahlavanpour, B., Walton, S. J., and Kirkbright, 46, 385. G. E., Analyst, 1978, 103, 568. 37 Cutter, G. A., and Bruland, K. W., L imnol. Oceanogr., 1984, 17 Wickstr©ªm, T., Lund, W., and Bye, R., J. Anal. At. Spectrom., 29, 1179. 1991, 6, 389. 38 Cutter, G. A., and Cutter, L. S., Mar. Chem., 1995, 49, 295. 18 Ni, Z.-M., and He, B., J. Anal. At. Spectrom., 1995, 10, 747. 39 Santosa, S. J., Wada, S., and Tanaka, S., Appl. Organomet. Chem., 19 Welz, B., and Melcher, M., Spectrochim. Acta, Part B, 1981,36, 439. 1994, 8, 273. 20 Welz, B., and S¢§ ucmanova¢¥, M., Analyst, 1993, 118, 1417. 40 Tanaka, S., and Santosa, S. J., Biogeochemical Processes and 21 Chen, H., Brindle, I. D., and Le, X.-C., Anal. Chem., 1992, 64, 667. Ocean Flux in the Western Pacific, Terre Scientific, Tokyo, 22 Bye, R., T alanta, 1990, 37, 1029. 1995, p. 159. 23 Brindle, I. D., Alarabi, H., Karshman, S., Le, X.-C., Zheng, S., 41 Yamamoto, M., Tanaka, S., and Hashimoto, Y., Appl. Organomet. and Chen, H., Analyst, 1992, 117, 407. Chem., 1992, 6, 351. 24 Narsito, and Agterdenbos, J., Anal. Chim. Acta, 1987, 197, 315. 42 Middelburg, J. J., Hoede, D., van der Sloot, H. A., van der 25 Schramel, P., and Xu, L.-Q., Fresenius¡� J. Anal. Chem., 1991, Weijden, C. H., and Wijkstra, J., Geochim. Cosmochim. Acta, 1988, 340, 41. 52, 2871. 26 Welz, B., and Melcher, M., Analyst, 1984, 109, 573. 43 Lewis, B. L., and Andreae, M. O., Sci. T otal Environ., 1988, 73, 107. 27 Haring, B. J. A., Van Delft, W., and Bom, C. M., Fresenius¡� Z. Anal. Chem., 1982, 310, 217. Paper 6/05545A 28 Terada, K., Matsumoto, K., and Inaba, T., Anal. Chim. Acta, Received August 8, 1996 1984, 158, 207. 29 Brindle, I. D., and Le, X.-C., Anal. Chim. Acta, 1990, 229, 239. Accepted January 2, 1997 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a605545a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Mercury in Biological and Environmental Samples by Inductively Coupled Plasma Mass Spectrometry With the Isotope Dilution Technique JUN YOSHINAGA* AND MASATOSHI MORITA National Institute for Environmental Studies, 16-2 Onogawa, T sukuba, Ibaraki 305, Japan The accurate and precise determination of total mercury (Hg) the same CRMs to compensate for matrix effects arising from the high sodium content of the back-extracted sample.9 in biological and environmental samples by isotope dilutioninductively coupled plasma mass spectrometry (ID-ICP-MS) Campbell et al.10 determined total Hg in a cod muscle CRM by ID-ICP-MS for certification.They found good agreement is described. The precision of Hg isotope ratio (e.g., 200Hg/202Hg) measurements at the 20 ppb level was <0.5%. between the results obtained by ID-ICP-MS and those obtained by other methods including AFS. They also noted The deviation of the measured isotope ratios in a standard Hg solution was <0.5% from the natural ratios. Neither spectral that a stable isotope spike should be added to the sample before overnight digestion at room temperature to ensure interferences nor matrix effects were found to affect the accuracy and precision of the proposed Hg isotope ratio isotope equilibrium, which is followed by a pressurized microwave digestion.analysis. Total Hg concentrations in human hair and sediment CRMs were determined by ID-ICP-MS after 202Hg addition The National Institute for Environmental Studies (NIES), Japan Environment Agency, recently issued a human hair and acid decomposition of the samples.Solvent extraction and back-extraction was used for sediment analysis. The various reference material for Hg speciation.11 During the certification of the total Hg content of this CRM, ID-ICP-MS was employed decomposition methods used for hair samples, i.e., microwave digestion and the Teflon vessel double digestion method, did in this laboratory. The technique was also extended to the determination of total Hg in candidate sediment CRMs which not give any difference in the analytical value.The ID-ICP-MS results were better than those obtained with were prepared at the NIES. This paper describes the determination of Hg in human hair and sediment CRMs by standard additions or internal standardization in terms of accuracy and precision. Analytical results for human hair and ID-ICP-MS, which was considered to be the most accurate and precise analytical method for Hg.sediment CRMs were in good agreement with the certified/ reference values. Keywords: Isotope dilution-inductively coupled plasma mass EXPERIMENTAL spectrometry ; isotope ratio; total mercury; human hair; Instrumentation sediment The ICP-MS instrument used was a Hewlett-Packard (Avondale, PA, USA) HP-4500. Typical operating conditions Mercury (Hg) is commonly determined in environmental and biological samples by AAS or AFS after elemental Hg gener- are summarized in Table 1.The monitored masses were 200 ation and aeration. These analytical techniques are suitable and 202 for sample analyses and other masses (198, 199, 201 for routine analysis because of their high sensitivity and and 204) were also monitored in preliminary experiments on selectivity, reasonable accuracy and precision, and low run- the accuracy and precision of isotope ratio measurements with ning cost. this instrument.The ion lens voltage settings and other param- ICP-MS may be the method of choice for Hg determination eters of the instrument were tuned daily to obtain a 205Tl in certain circumstances, such as in the certification of reference signal of about 100 000 counts s-1 (10 ppb). materials where analytical values from several analytical A pyrolysis-Au amalgamation-AAS (PAAS) system methods with different analytical principles are required. The (MA1-S/MD-1, Nippon Instruments, Tokyo, Japan) was also capability of isotope analysis by ICP-MS offers further suit- used for Hg determination.ability because it permits accurate and precise determination of Hg by the stable isotope dilution (ID) technique. Indeed, ID-ICP-MS has frequently been employed in the certification Table 1 Operating conditions for ICP-MS of element contents in CRMs.1–5 Although ID-ICP-MS has been applied to the determination of a number of elements in Instrument HP-4500 (Hewlett-Packard) Rf power 1.2 kW various matrices, only a few applications have been reported Reflected power <5 W for the determination of Hg.This is surprising because ICP is Plasma gas flow rate 18.0 l min-1 a superior ionization source even for elements with high Nebulizer gas flow rate 1.10 l min-1 ionization potentials (Hg is a typical element), and the isotope Detector voltage -2050 V analysis of Hg, including the ID technique, is an important Sampling cone 0.8 mm diameter (nickel) area of application of ICP-MS.Skimmer cone 0.5 mm diameter (nickel) Sample uptake rate 0.2 ml min-1 (by peristaltic pump) Beauchemin and co-workers6–8 reported the use of multi- Spray chamber Scott type (Pyrex) element ID-ICP-MS, which included Hg as one of the analytes, Nebulizer Concentric type (Pyrex) in the analysis of marine sediment and biological CRMs. The Data acquisition 3 points per mass, 3 s per mass same group also used the ID technique combined with flow Monitored m/z 198, 199, 200, 201, 202, 204 injection for the ICP-MS determination of methylmercury in Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (417–420) 417Reagents One gram of sediment sample, along with an appropriate amount of stable isotope spike, was placed in a 50 ml glass A stock standard Hg solution (1000 mg g-1) was prepared by centrifuge tube and decomposed with nitric acid (4 ml) for dissolving high purity (99.999%) mercury chloride (Soekawa 30 min in a water-bath at 95 °C.Hydrochloric acid (5 ml) was Kagaku, Tokyo, Japan) in 0.3 M nitric acid. Working standard then added and heating was continued for a further 20 min. solutions were prepared daily from the stock solution. The After cooling, 10 ml of a 50% solution of diammonium hydro- stock solution was stored in a Teflon bottle at 4 °C in the dark. gen citrate were added to prevent hydroxide formationfollowed The nitric acid used for sample digestion was of ‘Ultrapure’ by the addition of 10 ml of concentrated ammonia solution.grade (Kanto Chemical, Tokyo, Japan). Hydrochloric acid, Then, 1 ml of a 20% aqueous potassium iodide solution was ammonia solution and 4-methylpentan-2-one (IBMK) were of added. At this point the pH of the solution was 9.5–10.5 as atomic absorption grade from the same manufacturer. High measured by indicator paper. The Hg in the aqueous layer purity potassium iodide (99.9%), atomic absorption grade was extracted into 10 ml of IBMK by mechanically shaking diammonium hydrogen citrate and reagent-grade cysteine were the capped centrifuge tube for 30 min.A portion (2–4 ml) of purchased from Wako Chemical (Osaka, Japan) and were used the IBMK layer was withdrawn into a 10 ml glass test-tube in the solvent extraction of Hg from the acid digest of the fitted with a screw cap and subjected to back-extraction with sediment CRM sample. 2–4 ml of 20 mM aqueous cysteine solution, prepared daily, The 202Hg spike was prepared by dissolving 202HgO powder using a vortex mixer for 1 min.The IBMK layer was removed (Oak Ridge National Laboratory, Oak Ridge, TN, USA) in with a Pasteur pipette and the cysteine layer was analysed 1 M nitric acid. The spike solution was stored in a Pyrex glass by ICP-MS. bottle. The accurate 202Hg concentration of the spike solution was determined by a reverse ID technique against an Hg standard solution, prepared as described above, and was found Measurement to be 184 mg g-1.Repeated reverse ID analyses indicated that the concentration did not change during the study period In ID-ICP-MS measurements, the isotope ratio (200Hg/202Hg) (>1 year). of the spiked sample was measured under the conditions shown A 1M hydrobromic acid solution was prepared from a high in Table 1. A blank and a 20 or 50 ppb standard solution of purity reagent (AA-1,000, Tama Chemical, Kawasaki, Japan) natural abundance were measured prior to and after every 3–5 and used for the washing of the sample introduction system samples.The mass discrimination factor was changed when a of the ICP-MS instrument. Millipore purified water was used significant deviation (>0.5%) of the isotope ratio was found for the dilution of standards and decomposed samples. in the standard solution. Such deviation occurred once or twice a day, and was probably due to instrumental temperature- dependent drift of the mass calibration.The altered isotope Samples ratios obtained by the addition of the stable isotope spike were Human hair CRMs from the NIES (NIES CRM No. 5 and 13), BCR (CRM 397) and Shanghai Institute of Nuclear used for the calculation of the Hg concentration in the sample Research (GBW 09101) were used. The sediment CRMs ana- according to the conventional equation.1–10 lysed were NIES CRM No. 2 Pond Sediment, NRCC MESS-1 In the SA method, only 202Hg was monitored using a 3 s and BCSS-1, and NIST (formerly NBS) SRM 1645 River integration of the ion counts.In the IS method, in addition to Sediment. The moisture content of these reference materials 202Hg (3 s integration), 195Pt and 205Tl were also monitored was determined as specified by each organization. NIES candi- using a 1.5 s integration. date CRM No. 12 Marine Sediment for organotin compounds and No. 16 River Sediment for polyaromatic hydrocarbons RESULTS AND DISCUSSION were also analysed.Memory Effect Procedure It is known that the memory effect of Hg increases the blank The hair sample, typically 50 mg, was decomposed by the Teflon vessel double digestion method,12 after adding an counts and worsens the analytical performance of ICP-MS. To reduce the blank counts due to the memory effect, prolonged appropriate amount of the 202Hg spike solution (volume: 100 ml), with 1–1.5 ml of nitric acid. This digestion method is washing of the sample introduction system is often necessary.For example, when a 20 or 50 ppb standard solution was suitable for the determination of volatile elements because the digestion is completed in a doubly closed system. The digest introduced for 3 min, the blank count increased to 10% of the count of the standard solution for a prolonged period when was diluted to 10 g with water in a Pyrex glass test-tube with a screw cap. Microwave digestion of hair samples was also dilute nitric acid was used for washing.It was also found that a pure standard solution produced a greater memory effect tested. The 202Hg spike was added to 50 mg of NIES CRM No. 13 and samples were heated in a microwave oven either than a sample solution of the same Hg concentration. The memory effect severely increased the detection limit from 0.02 immediately afterwards or after overnight digestion at room temperature. The digestion procedure was essentially that ppb under ‘clean’ conditions to 0.2 ppb (3s definition at m/z= 202).It also adversely affected the accuracy and precision of described by Isoyama et al.13 using 1.5 ml of nitric acid. The microwave oven used was for domestic use; the pressurized Hg isotope ratio measurements unless the blank count was measured prior to each sample and standard. digestion vessel, consisting of a Teflon inner vessel (25 ml) and poly(propylene) jacket, was a San-ai Kagaku P-25 (Nagoya, It has been suggested that dilute hydrobromic acid is effective in reducing the memory effect of Hg.15 According to our Japan).A 50 mg amount of unspiked NIES CRM No. 13 was experience, 0.5–1 M hydrobromic acid reduced the blank count more efficiently than nitric, hydrochloric or sulfuric acid or digested by the Teflon vessel double digestion method for Hg determination by ICP-MS with standard additions (SA) and 20 mM cysteine solution. Introduction of 1 M hydrobromic acid for 3 min at high peristaltic pump speed (0.5 rev min-1 or internal standardization (IS) methods.After decomposition and dilution, Hg standard solution was added at levels of 20 0.7 ml min-1) decreased Hg blank counts to a tolerable level (1% of the standard solution) after introduction of a high Hg and 40 ng g-1 or internal standard (Pt and Tl) mixture at a level of 10 ng g-1. standard solution (e.g., 50 ppb) for 3 min. Therefore, 1 M hydrobromic acid washing was employed after analysis of each Sample preparation for sediment CRMs was based on the method of Sanzolone and Chao14 with some modifications.sample and standard. 418 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Table 2 Accuracy and precision of Hg isotope ratio measurements by ICP-MS 198/202 199/202 200/202 201/202 204/202 Day 1 0.334 (0.6)* 0.5655 (0.1) 0.7754 (0.3) 0.4408 (0.2) 0.2290 (0.2) Day 2 0.335 (0.7) 0.5649 (0.4) 0.7746 (0.1) 0.4422 (0.4) NA† Day 3 0.332 (0.8) 0.5689 (0.6) 0.7741 (0.4) 0.4447 (1.2) NA Day 4 NA 0.5674 (0.2) 0.7749 (0.2) 0.4418 (0.3) NA Mean (RSD)‡ 0.334 (0.5) 0.5667 (0.3) 0.7748 (0.1) 0.4424 (0.4) — Theoretical§ 0.334 0.5650 0.7736 0.4414 0.2301 * Mean (RSD, %) of repetitive measurements (n=3–5) in one analysis day.† Not analysed. ‡ Mean (RSD, %) of means of Day 1 to Day 4. § From IUPAC.16 Table 3 Comparison of Hg concentration (mg g-1 dry mass) in NIES CRM No. 13 human hair determined by ID-ICP-MS, SA-ICP-MS and IS-ICP-MS.* n=5 in all instances ID-ICP-MS SA-ICP-MS IS-ICP-MS (Pt) IS-ICP-MS (Tl) Certified value 4.30±0.05 4.25±0.43 4.25±0.03 4.05±0.04 4.42±0.20 * SA-ICP-MS: ICP-MS determination of Hg with standard additions method.IS-ICP-MS: ICP-MS determination of Hg with internal standardization. Accuracy and Precision of Isotope Ratio Analysis of Hg by precision compared with the ID and IS techniques because of instrumental sensitivity drift during the analysis. Moreover, ICP-MS the number of samples required for the SA method is 2–3 Crude values of measuredHg isotope ratios were not consistent times the number required for the ID or IS technique, leading with natural abundance ratios because of the mass discrimi- to poor sample throughput.From this result, it was concluded nation of the instrument. Therefore, mass discrimination was that ID-ICP-MS was the best ICP-MS quantification method corrected daily by the periodic analysis of a Hg standard in terms of precision, accuracy and sample throughput. solution (20 or 50 ppb).The Hg isotope ratios reported here- Table 4 shows a comparison of the Hg concentration in a after are mass discrimination-corrected values. human hair CRM (NIES CRM No. 13) determined by The accuracy and precision of Hg isotope ratio analysis by ID-ICP-MS after three different sample digestion procedures, ICP-MS was examined using a Hg standard solution (20 ppb i.e., microwave digestion after overnight standing, microwave in 0.1 M nitric acid) with natural isotopic composition.The digestion immediately after spike addition, and the Teflon results are shown in Table 2. At this concentration, the 202Hg vessel double digestion method. This comparison was made count was typically about 20000 counts s-1 (total count because Campbell et al.10 reported that, for the ID-ICP-MS >60 000 per 3 s accumulation). Isotope ratios were measured determination of Hg in fish tissue, the sample should be 3–5 times a day for 3–4 d. Thus, the within- and between-day digested overnight at room temperature after the addition of variation in the measured isotope ratios could be examined.the stable isotope spike and nitric acid to the sample before The precision was generally <1% for both within- and microwave heating. They stated that accurate results could not between-day measurements. The deviations from the ‘true’ be obtained if the spike was added to the sample immediately values16 were all<0.5%. From these results it can be concluded before microwave heating, because isotope equilibrium could that ICP-MS measurements of Hg isotope ratios are sufficiently not be established between organically bound Hg in the sample precise and accurate for ID analysis.and the inorganic Hg spike. However, as is evident from In the m/z region of Hg no spectral interferences originating Table 4, the three digestion procedures did not produce any from molecular ions are expected. This was confirmed by the differences in the analytical values for Hg in the hair CRM in observation that the isotope ratios of Hg in digested hair were the present study.It is possible that the present microwave consistent with those in a standard solution. The only exception digestion procedure is more efficient for the destruction of the was 204Hg/202Hg, where an isobaric interference from 204Pb organic matrix and liberation of inorganic Hg from the sample exists. This result indicates that unpredictable spectral inter- matrix than the procedure of Campbell et al.The difference in ferences or other influences from the matrix are negligible in sample matrix (fish tissue in the work of Campbell et al. and isotope ratio measurements of Hg (except for those involving hair in the present study) may also be a cause of inconsistent 204Hg) by ICP-MS. results although methylmercury is the dominant Hg species in both types of sample. Table 5 presents ID-ICP-MS results for human hair CRMs, Determination of Total Hg in Biological CRMs using the Teflon vessel double digestion method.This digestion Table 3 compares the analytical results for NIES CRM No. 13 procedure was chosen because it is an established method and obtained by ID-ICP-MS, SA-ICP-MS and IS-ICP-MS (Tl and Pt). As can be seen, the mean values obtained by ID-ICP-MS, SA-ICP-MS and Pt-IS-ICP-MS were all similar and within Table 4 Comparison of Hg concentration (mg g-1 dry mass) in human the range of uncertainty of the certified value.Internal stan- hair CRM determined by ID-ICP-MS after three different digestion procedures dardization based on Tl gave lower values than the other methods. This might be due to the difference in the extent of Microwave-1* Microwave-2† Electric oven‡ the matrix effect between Hg and Tl. This is probable since 4.31±0.05 (n=3) 4.31±0.14 (n=4) 4.31±0.07 (n=4) the ionization potential of Hg (10.4 eV) is much higher than that of Tl (6.1 eV) and this is an inherent problem of the IS * Microwave-1: Pressurized digestion by microwave irradiation after technique. Although Pt was found to be a better internal addition of stable isotope spike and acid and overnight standing at standard in the present study, probably because its ionization room temperature.† Microwave-2: Pressurized digestion by micro- potential (9.0 eV) is fairly similar to that of Hg, this does not wave irradiation immediately after the addition of stable isotope spike mean that Pt will be the best internal standard for other and acid.‡ Electric oven: Pressurized digestion at 140 °C for 4 h in an electric oven. sample matrices. The present SA-ICP-MS method gave poor Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 419Table 5 Mercury concentrations in human hair CRMs determined Table 6 Mercury concentrations in sediment CRMs by ID-ICP-MS (mg g-1 dry mass) by ID-ICP-MS (mg g-1 dry mass)* Found† Certified Certified/reference Found* value NIES CRM No. 5 4.40±0.08 (n=6) 4.4±0.4 NIES CRM No. 13 4.30±0.06 (n=15) 4.42±0.20 0.987±0.031 (3) 1.1±0.5 NIST SRM 1645 River Sediment BCR CRM 397 11.8±0.2 (n=6) 12.3±0.5 GBW 09101 1.96±0.05 (n=3) 2.16±0.21 NRCC MESS-1 0.174±0.004 (3) 0.171±0.014 NRCC BCSS-1 0.132±0.001 (3) 0.129±0.012 NIES CRM No. 2 1.22±0.02 (3) (1.3)† * Samples (50 mg) were digested with 1–1.5 ml of nitric acid by the Teflon vessel double digestion method at 140 °C for 4 h after addition Pond Sediment NIES candidate CRM 1.10±0.02 (6) 1.22‡ of stable isotope spike.† Mean±standard deviation of repetitive analyses. Number of analyses is indicated in parentheses. No. 12 Marine Sediment NIES candidate CRM 0.866±0.008 (6) 0.95‡ is routinely employed in our laboratory, even though it may No. 16 River Sediment be more time consuming. The ID-ICP-MS results were in * Mean±standard deviation of repetitive analyses. Number of analy- good agreement with the certified values of the CRMs, with ses is indicated in parentheses. † Reference value.‡ Solid sampling- relative standard deviations of 1–2%. This result further pyrolysis-Au amalgamation-AAS. Aqueous Hg standard was used for demonstrates that ID-ICP-MS analysis offers an accurate and calibration. precise determination of Hg in hair samples. back-extraction process. However, the reason for the poor Determination of Total Hg in Sediment CRMs recovery of Hg from NIST SRM 1645 was not pursued further because the accuracy of ID analysis is independent of the The sample decomposition method for geological samples recovery rate if isotope equilibrium is established.reported by Sanzolone and Chao14 was used for sediment analysis. Two major modifications were made to the original method; firstly, a 50% solution of diammonium hydrogen CONCLUSION citrate was added prior to adjusting the sample digest to ID-ICP-MS offers accurate and precise determination of Hg alkaline pH.Without this addition, a dense hydroxide layer in biological and sediment samples. It is more accurate and formed and caused problems in withdrawing an aliquot of the precise than other modes of determination by ICP-MS. The IBMK layer for back-extraction. Secondly, since it is not application of MS to Hg determination has been limited practical to introduce IBMK into an ICP-MS system, back- because the ionization of Hg by conventional ionization extraction from the IBMK layer was employed. The Hg–I sources is not efficient.Therefore, the capability of the isotope complex was very stable in IBMK and quantitative back- analysis of Hg is a unique feature of ICP-MS. This technique extraction was achieved only with 20 mM cysteine solution and is of great value as an alternative method to AAS or AFS not with 1 M nitric or hydrobromic acid. A standard solution particularly in the certification of environmental and biologi- of Hg was pre-treated in a similar way to the sample and the cal CRMs.recovery was examined; the average recovery was 105%. However, since the solubility of IBMK in the aqueous layer is The authors thank K. Takata and C. Komatsu, NIES, for the fairly high, up to 70% suppression in the Hg ion counts from operation of the ICP-MS and PAAS systems. the dissolved IBMK was found after the introduction of several back-extracted samples. Therefore, SA or IS is necessary for REFERENCES the determination of Hg in back-extracted samples if the determination is based on external calibration.The use of a 1 McLaren, J. W., Beauchemin, D., and Berman, S. S., Anal. Chem., 1987, 59, 610. less water-soluble extraction solvent may be recommended in 2 Beauchemin, D., McLaren, J. W., Mykytius, A. P., and Berman, this case. However, the suppression does not affect the accuracy S. S., Anal. Chem., 1987, 59, 778. of Hg isotope ratio measurements because it affects all of the 3 Okamoto, K., Sci.T otal Environ., 1991, 107, 29. Hg isotopes to the same extent and this is certainly the 4 Okamoto, K., Spectrochim. Acta, Part B, 1991, 46, 1615. advantage of the ID technique. 5 Murphy, K. E., and Paulsen, P. J., Fresenius’ J. Anal. Chem., Table 6 shows analytical results for sediment CRMs by 1995, 352, 203. 6 Beauchemin, D., McLaren, J. W., Willie, S. N., and Berman, S. S., ID-ICP-MS. The measured ratio was 200Hg/202Hg. The agree- Anal. Chem., 1988, 60, 687.ment between the found values in NIST SRM 1645 River 7 McLaren, J. W., Beauchemin, D., and Berman, S. S., Spectrochim. Sediment, NRCC MESS-1 andNRCC BCSS-1 and the respect- Acta, Part B, 1988, 43, 413. ive certified values was satisfactory. However, a small deviation 8 Beauchemin, D., McLaren, J. W., and Berman, S. S., J. Anal. At. was found for the analytical values of the NIES candidate Spectrom., 1988, 3, 775. sediment CRMs obtained by ID-ICP-MS from those derived 9 Beauchemin, D., Siu, K. W. M., and Berman, S. S., Anal. Chem., 1988, 60, 2587. from PAAS. The reason for this is not clear. Cooperative 10 Campbell, M. J., Vermeir, G., Dams, R., and Quevauviller, Ph., analytical data on these CRMs are not yet available. J. Anal. At. Spectrom., 1992, 7, 617. As mentioned above, the recovery of Hg from standard 11 Yoshinaga, J., Morita, M., and Okamoto, K., Fresenius’ J. Anal. solutions was close to 100%. However, the recovery was Chem., 1996, in the press. poorer (around 50%) for NIST SRM 1645 River Sediment as 12 Okamoto, K., and Fuwa, K., Anal. Chem., 1984, 56, 1758. estimated from the 200Hg count. PAAS analysis showed that 13 Isoyama, H., Uchida, T., Oguchi, K., Iida, C., and Nakagawa, G., Anal. Sci., 1990, 6, 385. all of the Hg present in this SRM was liberated by nitric acid 14 Sanzolone, R. F., and Chao, T. T., Analyst, 1983, 108, 58. decomposition, employed as described under Experimental, 15 Beary, E. S., NIST, 1996, personal communication. even without the addition of hydrochloric acid. As the 202Hg 16 IUPAC, Pure Appl. Chem., 1991, 63, 991. spike and all of the intrinsic Hg were treated with a mixture of hot nitric and hydrochloric acid for at least 20 min during Paper 6/06171K the decomposition procedure, it is highly probable that during Received September 9, 1996 this process isotope equilibrium was established. Therefore, Accepted November 19, 1996 the poor recovery probably resulted from the extraction or 420 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606171k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Trace Impurities in High-purity Quartz by Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry Using the Slurry Sampling Technique SUSANNE HAUPTKORNa, VILIAM KRIVAN*a, BERTHOLD GERCKENb AND JIRI PAVELb aSektion Analytik und Ho� chstreinigung, Universita�t Ulm, D-89069 Ulm, Germany bNovartis Scientific Services, R-1055.4.02, Postfach, CH-4002 Basel, Switzerland A method has been developed for the determination of 14 ablation17–19 and electrothermal vaporization (ETV).20–22 ETV relevant trace impurities in high-purity quartz based on ETV- seems to be avery promising technique for routine applications, ICP-MS using slurry sampling.The ETV device consisted of a because of the inexpensive equipment, simple handling and the double layer tungsten coil. Experimental conditions were possibility to calibrate with aqueous standards. Furthermore, optimized with respect to the temperature program, the carrier owing to the spatial separation of volatilization (in the vaporgas flow, the i.d.of the aerosol tubing, ICP-MS measurement izer) on the one hand and atomization and ionization (plasma) parameters and internal standardization. Excluding U, on the other, an in situ analyte–matrix separation through calibration had to be carried out by the standard additions sequential volatilization of the sample components is method because of non-spectral matrix interferences. For U, feasible.23,24 simple quantification via calibration curves, recorded with However, when this is not possible, large amounts of matrix aqueous standards, was possible.The observed interferences can reach the plasma simultaneously with the analytes, possibly also aggravated the background evaluation, which seriously leading to matrix interferences and memory effects. The furnace limited the determination of Al and Fe. The method was material may cause additional interferences. Moreover, for applied to the determination of Al, Ba, Co, Cr, Cu, Fe, Li, quadrupole mass spectrometers, the multi-elemental capabili- Mg, Mn, Na, Pb, Sr, U and Zn in two quartz samples of ties for short transient signals (<5 s) are limited, because they different grades of purity. The accuracy of the results was work sequentially, although they are fairly fast.Another critical checked by their comparison with those obtained by aspect is the occurrence of losses during the aerosol transport, independent methods including instrumental neutron activation diminishing sensitivity and reproducibility.25–27 Some workers analysis.The achievable detection limits are between 2 ng g-1 have recommended sodium chloride or NASS-3 seawater ( Li, U) and 70 mg g-1 (Al ). reference material as modifiers for the enhancement of transport efficiency.28–31 However, owing to the possible introduc- Keywords: Inductively coupled plasma mass spectrometry; tion of contaminants, they are not really applicable to the electrothermal vaporization; tungsten coil furnace; slurry sampling; quartz analysis of high-purity microelectronic materials, the less so since the alkali and alkali earth elements, which are the main components of these modifiers, are amongst the most relevant analytes.Industry requirements on the purity of materials for microelec- Mainly on account of their widespread usage in ETAAS, tronic applications, such as quartz, call for analytical methods graphite tubes are the most commonly used vaporizers for allowing accurate determination of trace elemental impurities ETV, e.g., refs.28–33 Nevertheless, graphite vaporizers have a at the ng g-1 level and below.Only a few methods, i.e., neutron number of disadvantages, such as the occurrence of isobaric activation analysis (NAA),1,2 electrothermal atomic absorption interferences by carbon species (e.g., 52Cr and 40Ar12C),34 the spectrometry (ETAAS),3–5 total reflection X-ray fluorescence formation of refractory analyte carbides and the restriction to spectrometry (TXRF)6 and atomic mass spectrometry vaporization temperatures below #2600 °C.Therefore, in (MS),7–11 can provide the detection power necessary for these some cases refractory metal vaporizers are preferable.35–37 applications. In recent years, inductively coupled plasma mass Double layer tungsten coils, as manufactured for halogen spectrometry (ICP-MS)7–9 has developed into one of the most lamps, form simple and inexpensive but nonetheless efficient popular methods for ultratrace analysis.Major advantages of ETV devices, already successfully tested for ETV-ICP- ICP-MS are high instrumental sensitivity, the simplicity of the AES.38–40 They are easily obtained with highly reproducible mass spectra and the possibility of fast multi-element analysis. physical properties, enable high heating rates and temperatures However, these potentials cannot be fully exploited when of up to 3000 °C to be applied even with low cost power ICP-MS is used in combination with conventional nebulization supplies,and only a small piece of quartz apparatus is necessary of solutions, requiring sample digestion.As only sample solu- for mounting of the coil causing only low analyte vapour tions with low salt content can be analysed (usually <0.1%), dilution. Barth and Krivan have already demonstrated its in most cases analyte–matrix separation is also needed.7–9 suitability for the slurry ETV-ICP-AES analysis of silicon Sample dilution and contamination introduced in these carbide.40 additional steps can lead to a considerable increase in the Although the biggest potential of ETV-ICP-MS lies in the achievable LODs.12,13 Moreover, transport efficiencies of the combination with solid sampling or slurry sampling, up to conventional nebulization systems reach only 2–5%.now only a few papers dealing with the analysis of solids have Other sample introduction techniques for ICP-MS include direct insertion of solids,14,15 slurry nebulization,16 laser been published.19,41–49 In none of these works were high-purity Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (421–428) 421materials analysed, although this would be the ultimate field Temperature measurements of the tungsten coil were performed using IR thermometers: the Infrascope Mark 1 (Barnes of application for ETV-ICP-MS. In the present work, a slurry ETV-ICP-MS method has Engineering, Stamford, CT, USA), the Cyclops 52 (Land Infrarot, Leverkusen, Germany) and the Minolta Chromameter been developed for the determination of trace elemental impurities in high-purity quartz.Optimizations of the ICP-MS xy-1 (Minolta, Zu�rich, Switzerland) in the temperature ranges 100–400, 600–3000 and 1500–2900 °C, respectively. and ETV conditions were performed with regard to the applicability to routine analysis. Data Acquisition EXPERIMENTAL As the software employed, issue 2.03a, did not include an Samples and Reagents option for processing transient signals, the given possibilities had to be adapted to ETV measurements. Transient signals The analysed quartz samples were SiO2, -325 mesh, 99.9% were recorded using the scanning mode (qualitative scan pure, Lot No.X8653 from Cerac (Milwaukee, WI, USA), program, ‘mode 1’). This program is designed for recording further denoted as SiO2–1, and Aerosil 200, LOS 7638605 and displaying of mass spectra.However, instead of performing supplied by Ciba-Geigy (Basel, Switzerland), denoted as a scan through different m/z values, the quadrupole was set SiO2–2. Particle sizes were estimated by electron microscopy on a fixed m/z of interest. Therefore, the virtual ‘scan’ performed to be less than 10 and 1 mm for samples SiO2–1 and SiO2–2, by the computer was in reality a single ion monitoring during respectively. the length of time assigned for the scan, and the mass axis in For most elements, standard solutions were prepared by the spectrum has to be considered as a time axis.Scanning dilution of the multi-element stock standard solutions ICP- parameters chosen for these measurements are given in Table 1. Mehrelement-Standardlo�sung IV (1000 mg l-1NO3, Unfortunately, in this mode, only one m/z value can be Merck, Darmstadt, Germany) and Trace Elements I, Ground monitored during one measurement cycle. Consequently, quan- Water and Waste Water Pollution Standard (5–500 mg l-1 in titative multi-element measurements were performed in the 5% HNO3, Perkin-Elmer, Norwalk, CT, USA).Single element ‘peak hopping’ mode (‘mode 2’), which allows fast sequential standards were used for the analyte elements Fe, Al, Li measurement of several isotopes by setting the quadrupole (1000 mg l-1, Merck) and U [1000 mg l-1, prepared by directly on each selected signal maximum (-0.1–+0.1 m/z).dilution of aqueous UO2(NO3 )2 in 1% HNO3], and for the For peak hopping measurements, an exact calibration of the internal standards In (1000 mg l-1, Spex Plasma Standards, quadrupole with respect to the chosen isotopes was carried Spex Industries, Grasbrunn, Germany) and 233U (10g l-1 out, otherwise the signal maximum could not be hit correctly. NIST Certified Reference Material U-233, New Brunswick For this purpose, a so-called ‘marker’ has to be run, in which Laboratory, US Department of Energy, IL, USA).The HNO3 a scan is performed across the m/z ranges of interest. It was used was subboiled from concentrated HNO3 (pro analysi, found preferable to run the marker using continuous sample Merck). For the preparation of slurries and standards, introduction by the nebulizer. The peak hopping parameters de-ionized water was used. for marker and analytical measurement are listed in Table 1. Under optimized conditions, a maximum of four isotopes Instrumentation could be measured during a single run.With mode 2, only signal intensities (counts s-1) are obtained, the recording of A VG PlasmaQuad 1 (VG Elemental at Fisons Instruments, Winsford, UK) ICP-MS instrument was used. The ETV device signal profiles is not possible. For the synchronization of ICP-MS measurements and the was similar to the one described by Barth and Krivan.40 It consisted of a double layer tungsten coil, Type 64655 HLX, ETV temperature program, a connection between the ETV computer and the keyboard of the computer controlling the supplied by Osram (Munich, Germany), connected to a0–24 V, 250 W power supply.The power supply was controlled by a mass spectrometer was established, allowing the start of the data acquisition by simulating the operation of the correspond- Sharp PC-7000 computer (Sharp, Osaka, Japan) with software written in GW BASIC. ing key at a signal given by the ETV computer. However, this was only possible in mode 2, whereas in mode 1, the measure- The vaporizer was interfaced to the plasma via quartz tubing with a length of 78 cm and an i.d.of 5 mm. As the quartz ment had to be started manually. Therefore, comparison of the appearance times of signals obtained by ETV-ICP-MS connection is inflexible, the entire ETV set-up had to be fastened to the torch box, enabling it to move together with becomesrather difficult. In both modes, several seconds elapsed between the triggering and the actual commencing of data the torch.In order to prevent oxidation of the tungsten coil during acquisition. This was considered in the temperature program by a time buffer step after the start signal for data acquisition heating, an Ar–H2 mixture [6% H2 , Carbagas (Alphagaz), Basel, Switzerland] was used as carrier gas, the gas flow being and before the beginning of the vaporization step. 800 ml min-1. Lens settings of the mass spectrometer were optimized with the ETV using the 93Nb signal originating from Procedure the Nb impurity vaporized from the tungsten coil at temperatures above 2000 °C.The 93Nb was preferred over a tungsten Sample slurries were prepared by suspending between 5 and 700 mg of quartz in 10 ml of 0.5% HNO3, previously checked isotope, because being in the middle of the m/z range it should give better compromise conditions for multi-element determi- for blank values, in 15 ml polystyrene vessels (Greiner, Frickenhausen, Germany).Slurries with matrix concentrations nations. As optimum torch alignment and plasma conditions did not vary between wet and dry plasmas, optimization below #0.5 g l-1 were prepared by dilution of concentrated stock slurries.3 Homogenization during sampling was of these parameters was performed on 115In using the nebulization system. performed using an ultrasonic probe. The internal standards were added to the suspension media With the experimental set-up described, a fast exchange of nebulizer and ETV unit was possible, which is an important prior to the blank determinations.The concentrations varied between 5 and 20 ng ml-1 for In and between 5 and 25 ng ml-1 feature in routine laboratory work. The ultrasonic probe Labsonic 1510 (B. Braun Melsungen for 233U, depending on the momentary sensitivity of the ICP-MS instrument. For calibration by the standard additions AG, Melsungen, Germany) was used for homogenization of slurries.method, the slurries were spiked twice in sequence with appro- 422 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Table 1 Operating parameters for slurry ETV-ICP-MS ET V— Temperature program: Step Voltage/mV Temperature/°C Time/s Drying 120 Rising slowly 10 90 to 300 15 50 45 Thermal pre-treatment* 1000 1500 118 Time buffer 0 7 (mode 1)/3 (mode 2) Vaporization 12500 2700 5 Cool down 0 Decreasing 5 Clean out 12500 2700 3 Sample volume: 10 ml ICP-MS— Outer plasma gas flow rate 12.5 l min-1 Intermediate gas flow rate 1.0 l min-1 Aerosol carrier gas flow rate (Ar+6% H2) 0.8 l min-1 Forward power 1300 W Reflected power 40 W Data acquisition: Mode 1: recording of signal profiles in ‘qualitative scan’ program Range 100–200 m/z Sweeps 1 Dwell time 10 ms Number of channels 1024 Mode 2: quantitative measurements in ‘peak hopping mode’ Peak hopping sweeps 50 Dwell time 500 ms Dwell time for marker (nebulizer) 10 ms Points per peak 5 * Only for the determination of U.priate amounts of aqueous standard solutions; typically Volatile Cd was chosen as a test element to detect possible 50–250 ng of analyte were added to 10 ml of the slurry. losses of the analyte. Owing to the small mass of the vaporizer, Aqueous standards used for recording the calibration graphs the vaporization enthalpy of the water is not negligible and (typically 5–100 ng ml-1) were prepared by dilution of stock the temperature of the coil is dependent on the amount of standard solutions in 0.5% HNO3 .Standard and slurry ali- water present. Thus, with a fixed voltage setting, the temperaquots (10 ml) were pipetted manually onto the tungsten coil. ture increases near the end of the drying step when most of For each value at least four replicate measurements were the water has already been removed. By gradually reducing performed. In the determination of all elements excluding U, the voltage during drying, analyte losses could be avoided with the standard additions method was used for quantification.a temperature program of acceptable duration. As the vaporiz- The ICP-MS operating parameters and the ETV tempera- ation temperature, 2700 °C was chosen for all elements investiture program are given in Table 1, and the m/z values and gated, although some elements could be vaporized at maximum applicable slurry concentrations in Table 2. significantly lower temperatures (e.g., Pb at 1800 °C).However, at this temperature, complete vaporization of all analytes was ensured and, thus, it could be applied to multi-element determi- RESULTS AND DISCUSSION nations. As only in the determination of U did thermal pre- Optimization of ETV treatment prove to be beneficial, no pyrolysis step was included in the temperature program for all other elements (for a more For the optimization of the drying step, the evaporation of detailed discussion, see below). water was monitored by observing the ArO+ signal at m/z 56.The optimized carrier gas flow for ETV measurements of 800 ml min-1 was approximately higher by 100 ml min-1 than Table 2 m/z values and maximum slurry concentrations used for the the aerosol carrier gas flow rate for nebulization of solutions. determination of 14 elements in quartz This is in accordance with the observations of Becker and Hirner.46 They attributed this phenomenon to an earlier m/z value and relative Maximum slurry appearance of the maximum ion density in a dry plasma Element abundance (%) concentration/g l-1 comparedwith awet plasma.A higher gas flow rate counteracts Al 27 (100) 0.02 this effect by moving the maximum ion density forward to Ba 138 (71.9) 10 the interface. Co 59 (100) 10 Cr 52 (83.8) 10 Three different ids, i.e., 1.1, 2.2 and 5 mm, were tested for Cu 63 (69.1) 20 the quartz connection between the ETV unit and the plasma Fe 56/57 (91.7/2.14) 2.0 torch. They proved to have no significant influence on either Li 7 (92.5) 10 the signal shape or the sensitivity. However, the largest diam- Mg 24 (79) 10 eter (5 mm) was preferred, because the tubes with the smaller Mn 55 (100) 10 diameters tended to be blocked by matrix residues during the Na 23 (100) 10 Pb 208 (99.3) 70 analysis of a slurry.Sr 88 (82.6) 10 In the optimization of the peak hopping parameters, it was U 238 (99.3) 10 essential to adjust the measurement window to the duration Zn 64 (48.9) 20 of the signal to obtain an optimum signal-to-noise ratio.The Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 423width of the window is determined mainly by the number of peak hopping sweeps, whereas the dwell time (measurement time at each point) is less significant. Although, in principle, three points per peak were sufficient, it was found that with five points per peak, better reproducibility was achieved when setting the quadrupole. With the optimized parameters listed in Table 1, a maximum of four isotopes could be determined in one run.One more isotope led to a loss of sensitivity, which could not be regained by a further adjustment of the peak hopping parameters. Several elements, including In, Y, V, Pd and Au, were considered as internal standards for correction of fluctuations of the transport and vaporization processes and for instrumental instability. Preferably, the element used as an internal standard should be close to the m/z value of the analyte elements, have an isotope with a large abundance, and thus give good sensitivity in the MS measurement, and impurities Fig. 2 ETV signal profiles obtained for 63Cu in (A) standard solution (100 pg),(B) quartz slurry (15.4 mg SiO2–1 per vaporization, containing or isobaric matrix interferences from both the samples and the about 26 pg Cu) and (C) slurry spiked with 10 pg Cu in standard standards should not contribute significantly to the signal solution.measured. The indium isotope 115In proved to be most suitable for this purpose. Its m/z value is well in the middle of the RSDs for uranium were improved by between a factor of two range, and thus, it is a good compromise for most elements and ten. determined in the simultaneous multi-element mode. It has a high abundance of 95.6%, gives satisfactory ETV signal profiles (see Fig. 1) and good reproducibility: the RSD of ten replicate Optimization of ETV-ICP-MS for Analysis of Quartz measurements was determined to be around 10% for In spiked to both the 0.5% HNO3 and quartz slurry.Moreover, contrary In Fig. 2, the signal profiles obtained for an aqueous solution, slurry and spiked slurry are given for Cu as a typical example to V and Y, no 115In signal was obtained for sample slurries even at high concentrations of the low purity SiO2–1 sample. for all analyte elements in question. The signals obtained for the slurry show a more pronounced tailing.More or less In the determination of heavy elements such as Pb and Ba, in particular, Au could also be used as an internal standard. pronounced double peaks were also obtained for the other elements. However, quantitative analysis using measurement However, there was no significant improvement compared with In. mode 2 should not be influenced by the signal shape since the signals are integrated. Also, the increase in signal intensity, Using In as the internal standard, the RSD of the ETVICP- MS measurements (n=4) could be improved in 50–60% obtained by adding a spike to the slurry, is evenly distributed over the signal profile (see Cu profiles in Fig. 2; similar of all cases, depending on the analyte, 10–20% of all determined RSD values remained unchanged, whereas 20–30% of the observations were made for the other elements), indicating that the spiked analyte and the analyte contained in the quartz RSDs even increased. Thus, with respect to reproducibility, the use of an internal standard is dispensable, especially when matrix behave similarly in the vaporization process; this is an important prerequisite for calibration via the standard considering that without correction by an internal standard RSDs are usually below 15%.Nevertheless, for the correction additions method. The differences in the appearance times of the ETV-ICP-MS of medium to long-term instrumental sensitivity drifts, an internal standard is still useful.signals, evident from Figs. 1, 2 and 5, were presumably caused by delays in starting the data acquisition by hand. Owing to Uranium constituted a special case inasmuch as it allowed the use of the artificial isotope 233U as a practically ‘ideal’ this uncertainty, it is not possible to obtain and interpret the real appearance times. internal standard, with exactly the same properties and consequently the same behaviour as the analyte. By this means, the As the concentrations of all elements except Na were below the limit of detection in sample SiO2–2, this sample was used to confirm the absence of isobaric interferences from the quartz matrix for the m/z values employed in the analysis (see Table 2).For Na (m/z=23), isobaric interferences by Si-containing species [Si, m/z=28 (92.2%), 29 (4.67%), 30 (3.1%)] can also virtually be excluded. Thus, regarding spectral interferences, it is possible to evaluate the background by measuring the de-ionized water. However, in the analysis of quartz, considerable non-spectral interferences were observed.For most elements determined, the mass response curve (i.e., signal intensity versus sample mass) was non-linear even for fairly low sample masses (see Fig. 3). Moreover, the slopes of the calibration curves obtained for aqueous standards were higher than those obtained by standard additions to the slurry. The signal suppression by the matrix was most pronounced for uranium: a standard spiked to a quartz slurry gave approximately a 20–40 times lower signal intensity than an aqueous standard solution.For this element, a considerable reduction, Fig. 1 ETV signal profiles obtained for 500 pg In in (A) 0.5% HNO3, though not a complete elimination, of the interferences was (B) a slurry of SiO2–1 in 0.5% HNO3 (5 mg SiO2 per vaporization) achieved by a thermal pre-treatment at 1500 °C as part of the and (C) a slurry of SiO2–2 in 0.5% HNO3 (100 mg SiO2 per vaporization).temperature program (see Table 1). By this means, the sensi- 424 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Up to the slurry concentrations given in Table 2, taking these as maxima, the sensitivities obtained for slurries and spiked slurries were the same, within the measurement uncertainties. For U, the use of the isotope 233U as an internal standard makes a correction for the matrix interferences possible (see Fig. 4). In this case, the calibration could be performed using aqueous standard solutions.Owing to their relatively high contents in the tungsten coil, Al and Fe showed high blank values (see the 56Fe signal trace of 0.5% HNO3 in Fig. 5A; the blank signal for 27Al has a similar shape). For this reason, the observed non-spectral matrix interferences aggravate the determination of the background, as the suppressing effect of the matrix cannot be accurately taken into account. An accurate background correction could only be performed with an analyte-free sample.Fig. 3 63Cu signal intensity as a function of the slurry concentration of sample SiO2–1 (A) without and (B) with correction by the internal standard 115In (200 pg). tivity obtained for the quartz slurry could be increased by a factor of about ten compared with the standard temperature program. An increase in the pre-treatment temperature above 1500 °C led to a further increase in sensitivity, but also caused analyte losses and, therefore, could not be applied.For all other elements, the maximum applicable pre-treatment temperatures were not high enough to achieve a similar reduction in the matrix interferences. The suppression of analyte signals in the presence of large amounts of matrix in the plasma is a phenomenon well known in ICP-MS,50 including ETV-ICP-MS analysis of solids, which cannot be completely removed during a pyrolysis step.47,48 The suppression can be caused by changes of the plasma Fig. 4 238U signal intensity as a function of the slurry concentration conditions during vaporization and decomposition of the (SiO2–1) (A) without and (B) with correction by the internal standard matrix aerosol, such as cooling of the plasma and dilution of 233U (250 pg). the analytes in the central channel after expansion of the gases formed. The ionization efficiency as well as the site of the maximum ion density might also be influenced. Interferences of this type have also been attributed to ion repulsion effects,51,52 whereby light analyte elements were usually influenced more strongly than heavy elements. However, in the analysis of quartz by ETV-ICP-MS, the extent of this interference proved to be independent of the mass of the analyte element; while U is most strongly influenced, Pb shows the least matrix interferences.Deposition of oxides originating from the matrix (in this case SiO2) on the interface has also been described as a possible source of signal suppression.51 However, as a standard solution measured immediately after a slurry does not show reduced sensitivity, in the present case, it does not seem to be of relevance.The thermal pre-treatment step employed in the determination of U obviously reduces the amount of matrix reaching the plasma. Presumably, the SiO2 is removed as SiO after reduction by either the tungsten of the coil53 or most probably by the hydrogen mixed with the aerosol carrier gas.54 At higher temperatures the reduction process and the volatilization of the reaction products is enhanced, leading to a further decrease in the matrix interference and thereby to an increased sensitivity for U.As the extent of suppression varies for the individual elements, it cannot be completely corrected for by the internal standard In, although it is subject to the same type of matrix interference as the analytes. Therefore, with the exception of U, calibration was performed by the standard additions method.Even then, accurate results were obtained only for slurry concentrations equal to or rather below the maximum values given in Table 2. They were determined by measuring Fig. 5 ETV signal profiles obtained for 56Fe in (A) 0.5% HNO3, (B) the intensities of the analytes for slurries of different concen- 0.5% HNO3 containing 200 pg Fe and (C) slurry of SiO2–1 (23 mg SiO2 containing about 8.5 ng Fe). trations and the same slurries spiked with aqueous standards.Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 425However, even SiO2–2 is not pure enough with respect to Al In the quartz material SiO2–2, owing to its extremely high purity, the concentrations of all elements except for Na were and Fe for this purpose (see Table 3). Evidently, the relative error made in the background evaluation using de-ionized below the LODs of ETV-ICP-MS. The result obtained for Na is in excellent agreement with the results of the independent water is highest for samples with low analyte concentrations.For high element concentrations, such as in sample SiO2–1, methods, excluding ICP-MS. It is remarkable that the concentrations found by solution ICP-MS for the contamination risk the error resulting from the background evaluation can be neglected (see Table 3 and Fig. 5). elements Mg and Na are between two and three times higher than those obtained by the other methods. It is conceivable that the too high values are due to contaminations introduced Analysis of Samples and Detection Limits during the rather complex analyte–matrix separation procedure described for this method,7 which are difficult to control.In Table 3, the results obtained by slurry ETV-ICP-MS are For the ETV-ICP-MS measurements, on the other hand, the compared with those obtained by independent methods, includelimination of the sample preparation steps and the control of ing INAA, which due to some unique features, normally allows the blank value of the suspension medium prior to slurry a high degree of accuracy to be achieved.In addition, quartz preparation considerably reduce the risk of contamination.12 represents a very suitable matrix for INAA. The concentrations determined by TXRF for Al in both The concentrations determined by slurry ETV-ICP-MS in samples, Co in SiO2–1, and Cr, Cu, Fe and Pb in SiO2–2, sample SiO2–1 are on the whole in good, and in some instances differ considerably from the other results.In these cases, the even in excellent, accordance with those obtained by the other methods are obviously more reliable. independent methods. The mean Co content obtained by ETVDetection limits for 14 elements (Table 4) were calculated ICP-MS is higher by a factor of 1.5 and 2 than the mean as three times the standard deviation of the blank value. Since values determined by INAA and ICP-MS, respectively.the concentrations of all elements except Na were below their However, the uncertainty ranges of ETV-ICP-MS and the LODs in SiO2–2, it was possible to use slurries of this sample INAA results still overlap; for the ICP-MS measurements,7 no with maximum matrix concentrations as blank samples for standard deviations were available. The result determined by determination of the LODs in quartz. The sensitivity of each TXRF is clearly too low. A similar situation exists also with element was determined by standard additions to the same Sr, except that the TXRF result is in better accordance with slurry.This procedure ensures that matrix effects can be fully the results of all other methods excluding ICP-AES, the result taken into account. The concentration of Na was well above of which seems to be too low. The concentration of Pb the LOD in both quartz samples, hence, the standard deviation obtained by ETV-ICP-MS is approximately half those deterof the blank value of the suspension medium was used for mined by ICP-AES and ICP-MS.However, it is confirmed by evaluation of the LOD. The LODs of Al and Fe were estimated TXRF. Unfortunately, INAA is not a suitable method for the determination of this element. using the conditions applied to the analysis of sample SiO2–1 Table 3 Element concentrations determined by slurry ETV-ICP-MS in quartz and comparison with results of independent methods Concentration/mg g-1 This work Independent method Element Sample Slurry ETV-ICP-MS Slurry ETAAS* Solution ETAAS* INAA† ICP-AES† ICP-MS‡ TXRF‡ Al SiO2–1 3200±600 3300±400 2990±150 — 2500±3 — 1700 SiO2–2 <70 1.1±0.2 0.8±0.1 — 1.6±0.1 — 12 Ba SiO2–1 34±7 — — 56±13 26.6±0.3 31 26 SiO2–2 <0.2 — — <0.01 <0.01 <0.025 <0.2 Co SiO2–1 0.8±0.15 — — 0.55±0.1 — 0.4 <0.25 SiO2–2 <0.014 — — 0.0017±0.0001 — — <0.02 Cr SiO2–1 2.1±0.5 3.7±0.6 3.5±0.4 3.0±0.4 2.1±0.1 3 2.2 SiO2–2 <0.02 <0.02 0.007±0.001 0.015±0.004 <0.02 0.02 0.05–1.8 Cu SiO2–1 1.7±0.2 1.62±0.06 1.8±0.1 — 1.60±0.01 0.8 1.63 SiO2–2 <0.05 <0.07 <0.007 — <0.02 <0.03 0.07 Fe SiO2–1 369±18 360±30 390±24 348.0±0.2 233±1 400 306 SiO2–2 <2 0.4±0.1 0.7±0.1 0.5±0.2 0.80±0.01 0.3 0.5–16 Li SiO2–1 1.6±0.1 1.3±0.2 1.7±0.1 — 1.70±0.01 — — SiO2–2 <0.002 <0.012 <0.003 — <0.01 — — Mg SiO2–1 128±15 130±30 150±10 — 147±1 289 — SiO2–2 <0.7 0.21±0.03 0.19±0.01 — 0.30±0.01 0.35 — Mn SiO2–1 14±3 17±4 17.9±0.6 14.0±0.2 10.9±0.1 19 13.3 SiO2–2 <0.03 <0.04 0.012±0.002 <0.02 <0.05 0.029 0.03 Na SiO2–1 73±4 69±9 80±10 79±1 61.6±0.5 198 — SiO2–2 1.1±0.2 1.0±0.2 1.5±0.2 0.8±0.1 1.2±0.1 2.6 — Pb SiO2–1 3.1±0.6 — — — 6.1±0.5 5.5 3.82 SiO2–2 <0.006 — — — <0.01 <0.03 0.025 Sr SiO2–1 29±5 — — 36±5 19.9±0.1 37 27.5 SiO2–2 <0.05 — — <0.8 <0.01 0.003 <0.008 U SiO2–1 0.41±0.03 — — 0.60±0.01 — 0.67 0.50 SiO2–2 <0.002 — — <0.0017 — 0.0002 <0.02 Zn SiO2–1 1.0±0.1 — — <5 1.1±0.1 1 1.1 SiO2–2 <0.4 — — 0.21±0.09 0.10±0.01 <0.25 0.12 * From ref. 3.† From ref. 55. ‡ From ref. 7. 426 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Table 4 LODs obtained for ETV-ICP-MS using maximum slurry should be much less of a problem for matrices that are either concentrations and 10 ml aliquots, and comparison with those for more refractory or more volatile than quartz, and therefore ETAAS are better suited for a simple in situ analyte–matrix separation by sequential vaporization of analytes and matrix. Absolute LOD in quartz/ng g-1 Further improvement of this method can be expected from LOD*/pg Slurry Slurry Solution enhancement of the transport efficiency, which is currently Element ETV-ICP-MS ETV-ICP-MS ETAAS† ETAAS† under investigation. Al 20 70×103 200 500 Ba 5 200 — — Co 3 14 — — REFERENCES Cr 7 20 20 3 Cu 5 50 70 7 1 Franek, M., and Krivan, V., Anal.Chim. Acta, 1993, 274, 317. 2 De Corte, F., De Wisplaere, A., Van den Boer, M., Bossus, D., Fe 30 2000 500 30 Li 0.05 2 12 3 and van Slujs, R., Anal. Chim. Acta, 1991, 254, 127. 3 Hauptkorn, S., and Krivan, V., Spectrochim. Acta, Part B, 1996, Mg 20 700 2 0.4 Mn 2 30 40 4 51, 1197. 4 Phelan, V. J., and Powell, R. J. W., Analyst, 1984, 109, 1269. Na 20 100 7 13 Pb 0.1 6 — — 5 Nakamura, T., Sasagawa, R., and Sato, J., Bunseki Kagaku, 1992, 41, 89. Sr 0.1 50 — — U 0.03 2 — — 6 Reus, U., Spectrochim. Acta, Part B, 1989, 44, 533. 7 Baumann, H., and Pavel, J., Mikrochim. Acta, 1989, III, 423. Zn 5 400 — — 8 Herzner, P., and Heumann., K. G., Anal. Chem., 1992, 64, 2942. 9 Naka, H., and Kurayasu, H., ISIJ Int., 1993, 3, 1252. * In aqueous solution. † From ref. 3. 10 Milton, D. M. P., Hutton, R. C., and Ronan, G. A., Fresenius’ J. Anal. Chem., 1992, 343, 773. 11 Milton, D. M. P., Clark, J., Potter, D., and Hutton, R. C., Anal. Sci., 1991, 7, 1243. (Al, slurry concentration 0.01 g l-1; Fe, slurry concentration 12 Docekal, B., and Krivan, V., J.Anal. At. Spectrom., 1993, 8, 637. 2 g l-1; m/z 57) for which, due to the relatively high contents 13 Friese, K.-Ch., and Krivan, V., Anal. Chem., 1995, 67, 354. of these two elements, the error made in the 14 Hall, G. E. M., Pelchat, J.-C., Boomer, D. W., and Powell, M., J. Anal. At. Spectrom., 1988, 3, 791. background determination was negligible. 15 Karanassios, V., and Horlick, G., Spectrochim. Acta, Part B, 1989, On the whole, the LODs achievable in the analysis of quartz 44, 1361.by slurry ETV-ICP-MS are at the same level as those obtained 16 Mochizuki, T., Sakashita, A., Iwata, H., Ishibashi, Y., and Gunji, for slurry ETAAS.3 Detection limits are, with the exception of N., Fresenius’ J. Anal. Chem., 1991, 339, 889. Al and Fe, in the 1–100 ng g-1 range. The ETV-ICP-MS 17 Imbert, J. L., and Telouk, P., Mikrochim. Acta, 1993, 110, 151. technique developed was even more limited with respect to the 18 Voellkopf, U., Paul, M., and Denoyer, E.R., Fresenius’ J. Anal. Chem., 1992, 342, 917. maximum applicable slurry concentration than the ETAAS 19 Mochizuki, T., Sakashita, A., Iwata, H., Ishibashi, Y., and Gunji, method, because the matrix effects appeared more severe in N., Anal. Sci., 1991, 7, 151. ETV-ICP-MS. Moreover, the absolute LODs of this ETV- 20 Matusiewicz, H., Adv. At. Spectrosc., 1995, 2, 63. ICP-MS technique in aqueous solution, although still between 21 Carey, J.M., and Caruso, J. A., Crit. Rev. Anal. Chem., 1992, 0.03 and 30 pg, are in some instances significantly higher than 23, 397. those reported by other workers with different ETV-ICP-MS 22 Olson, L. K., Vela, N. P., and Caruso, J. A., Spectrochim. Acta, Part B, 1995, 50, 355. set-ups.20 In the present work, the detection capability was 23 Seubert, A., and Meinke, R., Fresenius’ J. Anal. Chem., 1994, found to be mainly limited by the background fluctuation, 348, 510.especially for Al and Fe, which were detectable impurities in 24 Argentine, M. D., and Barnes, R. M., J. Anal. At. Spectrom., 1994, the tungsten coil (see Fig. 5). The fluctuation was obviously 9, 1371. caused by the expansion of the aerosol carrier gas at the 25 Kantor, T., Spectrochim. Acta, Part B., 1988, 43, 1299. beginning of the vaporization step, leading to changes in the 26 Sparks, C. M., Holcombe, J. A., and Pinkston, T. L., Spectrochim. Acta, Part B, 1993, 48, 1607.plasma conditions, visible as a drop in the baseline. This effect 27 Ediger, R. D., and Beres, S. A., Spectrochim. Acta, Part B, 1992, might be less pronounced with graphite furnace ETV systems, 47, 907. as these furnaces cannot usually attain heating rates similar to 28 Hoffmann, E., Lu�dke, C., and Scholze, H., J. Anal. At. Spectrom., the comparatively small tungsten coil. On the other hand, the 1994, 9, 1237. small dimension of the coil made it possible to construct an 29 Hughs, D.M., Chakrabarti, C. L., Goltz, D. M., Gre�goire, D. C., ETV housing of very low dead volume, minimizing dilution of Sturgeon, R. E., and Byrne, J. P., Spectrochim. Acta, Part B, 1995, 50, 425. the analytes. Obviously, this advantage does not offset the 30 Lamoureux, M. M., Gre�goire, D. C., Chakrabarti, C. L., and increased background fluctuation. However, further improve- Goltz, D. M., Anal. Chem., 1994, 66, 3217. ment to the ETV unit might lead to a considerable 31 Sparks, C.M., Holcombe, J. A., and Pinkston, T. L., Appl. improvement in the LODs. Spectrosc., 1996, 50, 86. 32 Lamoureux, M. M., Gre�goire, D. C., Chakrabarti, C. L., and Goltz, D. M., Anal. Chem., 1994, 66, 3208. CONCLUSION 33 Hub, W., and Amphlett, H., Fresenius’ J. Anal. Chem., 1994, 350, 587. Compared with the conventional nebulization of solutions for 34 Gre�goire, D. C., and Sturgeon, R. E., Spectrochim. Acta, Part B, ICP-MS, the slurry technique combined with electrothermal 1993, 48, 1347.vaporization considerably reduces the risk of contamination, 35 Marawi, I., Olson, L. K., Wang, J., and Caruso, J. A., J. Anal. At. is less time consuming, easier to apply and avoids the use of Spectrom., 1995, 10, 7. 36 Tsukahara, R., and Kubota, M., Spectrochim. Acta, Part B, 1990, the highly toxic hydrofluoric acid as a digestion medium. 45, 779. However, the LODs of this technique are not superior to some 37 Shibata, N., Fudagawa, N., and Kubota, M., Anal. Chem., 1991, other methods, such as slurry ETAAS, and in some instances 63, 636. they are even inferior (see the LODs for solution ETAAS 38 Dittrich, K., Fuchs, H., Berndt, H., Broekaert, J. A. C., and in Table 4). Schaldach, G., Fresenius’ J. Anal. Chem., 1990, 336, 303. The strong matrix interference represents the main limitation 39 Dittrich, K., Mohamad, I., Nguyen, H. T., Niebergall, K., Pfeifer, M., and Wennrich, R., Fresenius&rsqu. Chem., 1990, 337, 546. of the technique as applied to the analysis of quartz. This Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 42740 Barth, P., and Krivan, V., J. Anal. At. Spectrom., 1994, 9, 773. 49 Ren, J. M., Rattray, R., Salin, E. D., and Gre�goire, D. C., J. Anal. 41 Moens, L., Verrept, P., Boonen, S., Vanhaecke, F., and Dams, R., At. Spectrom., 1995, 10, 1027. Spectrochim. Acta, Part B, 1995, 50, 463. 50 Evans, E. H., and Giglio, J. J., J. Anal. At. Spectrom., 1993, 8, 1. 42 Gre�goire, D. C., Miller-Ihli, N. J., and Sturgeon, R. E., J. Anal. 51 Gre�goire, D. C., Spectrochim. Acta, Part B, 1987, 42, 895. At. Spectrom., 1994, 9, 605. 52 Beauchemin, D., McLaren, J. W., and Berman, S. S., Spectrochim. 43 Vanhaecke, F., Boonen, S., Moens, L., and Dams, R., J. Anal. At. Acta, Part B, 1987, 42, 467. Spectrom., 1995, 10, 81. 53 Gmelin Handbook of Inorganic Chemistry, T ungsten, System- 44 Wang, J., Carey, J. M., and Caruso, J. A., Spectrochim. Acta, Part No. 54, suppl. vol. A7, Springer-Verlag, Berlin, 8th edn., 1987. B, 1994, 49, 192. 54 Gmelins Handbuch der Anorganischen Chemie, Silicium, System- 45 Darke, S. A., Pickford, C. J., and Tyson, J. F., Anal. Proc., 1989, No. 15, Part B, Verlag Chemie, Weinheim/Bergstrasse, 1959. 26, 379. 55 Fritz, M., and Krivan, V., unpublished results. 46 Becker, S., and Hirner, A. V., Fresenius’ J. Anal. Chem., 1994, 350, 260. Paper 6/06027G 47 Vanhaecke, F., Galba�cs, G., Boonen, S., Moens, L., and Dams, Received September 2, 1996 R., J. Anal. At. Spectrom., 1995, 10, 1047. 48 Boonen, S., Vanhaecke, F., Moens, L., and Dams, R., Spectrochim. Accepted January 6, 1997 Acta, Part B, 1996, 51, 271. 428 Journal of Analytical Atomic Spectrometry, April 1997,

ISSN:0267-9477    

DOI:10.1039/a606027g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Enzymatic Digestion for the Determination of Trace Elements in Blood Serum by Inductively Coupled Plasma Mass Spectrometry FADI R. ABOU-SHAKRA*, MARGARET P. RAYMAN, NEIL I. WARD, VALERIE HOTTON AND GERALDINE BASTIAN Department of Chemistry, University of Surrey, Guildford, Surrey, UK GU2 5XH A non-specific protease enzyme ( pronase) was used to partial blockages of different parts of the instrument, including the nebulizer, sample injector tube, sampler, skimmer or accel- demonstrate the potential of enzymatic digestion as an alternative sample preparation method for the determination erator cones.In consequence, a treatment step is normally applied to the serum, prior to analysis. Acid digestion is the of trace elements in blood serum by ICP-MS. By measuring the number of peptide bonds in solution, it was found that this most commonly used pre-treatment procedure. It is designed to achieve the complete destruction of organic material (fats, digestion led to a 40% reduction in the level of plasma protein in the samples.Based on the data obtained, it appears that the proteins), thereby preventing the clogging referred to above. There are various conventional procedures such as wet acid majority of these proteins were broken down into smaller polypeptides. Using this digestion technique, a high degree of digestion (with HNO3, H2SO4 or HClO4) in open, pressure or microwave vessels, or dry ashing, which is normally followed instrument stability was achieved during the continuous analysis of blood serum over a 3 h period.Selective by the acid dissolution of the ash. However, these digestion procedures suffer from major disadvantages, being hazardous enhancement of the selenium signal was observed during these analyses. A charge-exchange mechanism between C+ ions and and introducing a high risk of contamination or of elemental losses by volatilization. selenium atoms in the plasma, which leads to the formation of excited Se+ ions, is proposed and is supported by data related Non-specific protease enzymes are capable of breaking down a wide range of proteins into their amino acid components.In to the ionization/excitation energies of the various species involved in this reaction. It was also demonstrated that in this work, we explored the potential of using such enzymes for digesting serum proteins, and the effect that this digestion has order to achieve good accuracy, calibration must be conducted using matrix-matched standards.Finally, the accuracy of the on the stability and accuracy of serum analysis by ICP-MS. As in the study of Christensen and Pedersen,3 the enzyme technique is demonstrated by showing excellent agreement between the experimental results and the certified values for chosen was pronase, which is a protease with a broad speci- ficity, isolated from Streptomyces griseus. The study was con- Seronorm certified serum reference material.ducted by monitoring the levels of selenium, as a model Keywords: Enzymatic digestion; blood serum; inductively element, in serum. Selenium is an essential trace element coupled plasma mass spectrometry ; charge-exchange required for the functioning of a number of different enzymes. interactions It is best known for its role at the active centre of the enzyme glutathione peroxidase, which protects against oxidative stress Trace element levels in blood serum can be used to monitor by scavenging damaging peroxides.Deficiency of selenium has several factors, including dietary habits, the uptake of some been associated with several disease states, including coronary medicinal drugs, exposure to toxic elements and the relation- heart disease and cancers.4,5 Furthermore, selenium is a good ship between trace elements and specific health disorders. model element because a substantial portion of the organic Accordingly, the determination of trace elements in human selenium found in serum is acid resistant, hence complicated serum has been, and still is, the subject of many scientific and digestion procedures are often required in order to measure clinical investigations.Since these studies frequently involve a accurately the levels of selenium in blood serum by ICP-MS.6 large number of samples and the levels to be determined are often extremely low, such determinations require an analytical method which offers high throughput and good sensitivity. Over the last decade, ICP-MS has increasingly been EXPERIMENTAL acknowledged as a rapid multi-element technique which offers Reagents and LaboratoryWare low detection limits.However, despite being heavily documented, problems relating to the analysis of samples containing Nitric acid (69% m/m) used in sample preparation was of Aristar grade from Merck (Poole, Dorset, UK). De-ionized high levels of salts or dissolved solids have not yet been fully resolved.1 water obtained from a Milli-Q purification system (resistivity 18 MV cm) was used throughout.Human serum contains about 90% water, 7% proteins (albumins, globulins and fibrinogen), 2% low molecular mass Standard arsenic,germanium, indium and selenium solutions used were 1000 mg ml-1 standard solutions of arsenic trichlo- organic compounds (metabolites, such as glucose, amino acids or fats) and 1% inorganic substances, which are usually ionic, ride, germanium nitrate, indium nitrate and selenous acid (Merck), respectively.present either as simple salts or bound to plasma proteins.2 When analysing blood serum which has been diluted only Blood serum was prepared from frozen (-10 °C) human serum samples from healthy control individuals at Midhurst 5–10-fold, the high concentrations of salts and proteins in the sample adversely affect the stability of the ICP-MS response Hospital,Surrey, UK. Certified reference serum was lyophilized Seronorm Trace Element Serum from Nycomed Pharma (Oslo, and cause a downward drift in sensitivity, which in turn leads to poor precision and accuracy. This drift is mainly due to Norway).Pronase was obtained from Sigma (Poole, Dorset, Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (429–433) 429UK) in powder form. Bovine serum albumin fraction V (BSA) 0, 2, 5, 10, 15 and 20 ng ml-1, were analysed and the response for each nebulizer was calculated from the ensuing cali- was also obtained from Sigma.All glassware and polypropylene containers were pre- bration curves. The reproducibility of USN–ICP-MS was evaluated by washed, soaked overnight in 10% HNO3 (analytical-reagent grade) and rinsed copiously with de-ionized water. analysis of a 10 ng ml-1 selenium solution spiked with 10 ng ml-1 of Ge, As and In. Sets of 10 readings were repeatedly taken and for each set the reproducibility was calculated as Instrumentation the RSD of this data set.The same solution was used in the investigation of long- The investigations described in this paper were conducted on a Finnigan MAT SOLA ICP-MS instrument (Finnigan MAT, term stability. The samples were introduced into the plasma via the USN and readings were taken at set intervals for a Hemel Hempstead, UK). The samples were introduced into the plasma torch by means of a CETAC U-5000AT ultrasonic period of 3 h. nebulizer (CETAC Technologies, Omaha, NE, USA), and for comparison, by a V-groove nebulizer (Spetec, Erding, Germany).Spectrophotometric Evaluation of the Efficiency of the The ICP-MS operating conditions were as follows: incident Enzymatic Digestion power, 1.4 kW; reflected power, ~0W; nebulizer argon flow In order to evaluate the efficiency of the enzymatic digestion rate, 0.914 l min-1; intermediate argon flow rate, 0.6 l min-1; technique, an estimate of the protein concentration before and coolant argon flow rate, 15 l min-1; peristaltic pump rate, after digestion was made.Ten 1 ml aliquots of serum were 1.35 ml min-1; and detector type, scanning electron multiplier. digested with various amounts of enzyme and diluted 1510 The ultrasonic nebulizer (USN) was operated as follows: with 0.5 M NaOH. The protein concentration in these solutions transducer frequency, 1.4 MHz; main power, 375 W; heating was then determined, as a percentage BSA equivalent, by the temperature, 140 °C; and cooling temperature, 0 °C.biuret method whereby 2 ml of biuret reagent were added to Selenium was measured using the 82Se isotope, the signal 1 ml of each digested serum sample and the solutions were left being corrected for the contribution of 82Kr (abundance at room temperature for a minimum period of 20 min. A 11.6%), by subtraction of the 83Kr signal (abundance 11.5%). purple colour appeared as a result of the reaction between the Signals from the internal standards Ge, As and In were counted –CONH– groups of the protein and copper sulfate.The extent using 74Ge (abundance 36.5%), 75As (abundance 100%) and of this reaction, which is directly related to the concentration 115In (abundance 95.7%). of protein (or peptide linkages) in the samples, was assessed by measuring the absorbance at 555 nm on a UV spectrophoto- Sample Preparation meter. Treatment of the BSA standard solutions in the same way allowed the preparation of a calibration curve of Synthetic selenium solutions at 2, 5, 10, 15 and 20 ng ml-1, absorbance against the percentage of protein in solution, from used in the study of signal stability, were prepared by diluting which the percentage BSA protein equivalent (in terms of the standard 1000 mg ml-1 solution with 1% v/v HNO3 (equiv- intact peptide bonds) remaining in the serum solutions could alent to 0.69% m/v).be read off. Frozen serum samples were left to thaw at room temperature, then pooled and gently mixed to homogenize.Serum solutions were prepared at 5-, 10-, 20- and 50-fold dilution by adding USN-ICP-MS Analysis of Enzymatically Digested Blood 20, 10, 5 and 2 ml of serum, respectively, and 1 ml of Ge, As Serum: Long-term Stability, Reproducibility, Recovery and In solutions (at 1 mg ml-1) to 0.1% v/v HNO3, to give a and Accuracy final volume of 100 ml. In order to assess the stability of the system during the analysis Unless stated otherwise, enzyme digestion was carried out of digested serum, 0.625 ml of pronase was used to digest by adding 50 ml of pronase (at 5 mg ml-1) per millilitre of 12.5 ml of serum.The serum was then spiked with 25 mg of Ge serum and leaving the solution at 37 °C overnight. to a concentration of 100 ng ml-1 and diluted to 250 ml with Biuret reagent was prepared by dissolving 5 g of 0.1% v/v HNO3 (20-fold dilution). A series of 10 readings of CuSO4 5H2O, 12 g of EDTA disodium salt and 2 g of KI in the solution were then taken over a period of 3 h. 400 ml of distilled water and diluting to 1 l with 2.5 M NaOH. To evaluate the reproducibility of the digestion technique, A protein stock standard solution was prepared from BSA 10 1 ml aliquots of serum were separately digested with 50 ml fraction V by dissolving 100 mg of BSA powder in 10 ml of of pronase, spiked to 100 ng ml-1 with Ge internal standard 0.5 M NaOH to give 10 mg ml-1 BSA. Working standard and diluted 20-fold with 0.1% v/v HNO3. The samples were solutions were made from the protein stock standard solution, then analysed and the RSD calculated.each standard of 1 ml containing a different amount of protein. To evaluate the recovery of the method, each of five out of 10 1 ml aliquots of digested serum was spiked with 100 ng of Analysis of Diluted Serum: Effect of Dilution Factor on selenium prior to digestion. The 10 samples were then prepared Long-term Stability for USN and V-groove Nebulizer as normal and analysed.The results were evaluated for different calibration procedures (using synthetic and matrix-matched In order to investigate the dilution factor which would give standards) and different internal standards. satisfactory signal stability, serum solutions, diluted 5-, 10-, Accuracy was tested by multiple analyses of Seronorm serum 20- and 50-fold with 0.1% v/v HNO3 , were analysed repeatedly certified reference material using matrix-matched calibration. over time periods of up to 3 h, using either an ultrasonic or a V-groove nebulizer. RESULTS AND DISCUSSION Analysis of Diluted Serum: Effect of Dilution Factor on Figures of Merit of the USN–ICP-MS System Long-term Stability for USN and V-groove Nebulizer The performance of the USN for synthetic solutions was evaluated in terms of sensitivity (response), reproducibility and Serum, at different dilution factors, was analysed repeatedly over a considerable time period, using both a V-groove nebul- long-term stability.In order to compare the sensitivity of the USN with that of izer and a USN, washing with 0.1% v/v HNO3 between analyses. The data shown in Fig. 1 clearly demonstrate the a V-groove nebulizer, six standard selenium solutions, namely 430 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12counts s-1 when using the ultrasonic nebulizer. This 26-fold improvement in sensitivity is most likely due to an increased sample delivery and a narrower size distribution of the aerosol droplets reaching the plasma.Reproducibility The reproducibility of the results was evaluated as the RSD of 10 readings. This figure was calculated on different occasions on the same analysis day and on different days. The average of 22 determinations of the reproducibility was 2.1% (spread 0.4–5.2%). The use of Ge, As or In as internal standards did not lead to any substantial difference in reproducibility. These observations reflect positively on the short-term stability of the system.L ong-term stability The long-term stability was calculated as the RSD of 12 readings taken at 15 min intervals over a 3 h period. This variable was determined on five different occasions and the average figure was 8.1% (spread 1.5–13%). However, this figure improved substantially when an internal standard was used. Using Ge, for example, the average RSD was 3.8% Fig. 1 Effect of the dilution (DF) on the stability of the signal for (spread 2.0–6.2%).With As and In, on the other hand, the the analysis diluted blood serum by ICP-MS. (a) Using an ultrasonic nebulizer; and (b) using a V-groove nebulizer. figures were 2.7% (1.0–5.2%) and 6.2% (0.6–15%), respectively. These observations clearly highlight the difference between the short- and long-term behaviour of the instrument. effect of sample dilution on the stability of the signal. As the In fact, on all five occasions where the long-term stability was dilution factor increases, the total amount of solids delivered evaluated, a steady downward trend in signal was observed to the plasma is reduced and higher signal stability is achieved.over the 3 h of analysis. On average, the signal dropped by In addition to the deposition of material on the sampler and 15% over this period. Such a decrease in sensitivity could be skimmer cones, possible sources of the observed instability attributed to various factors, including material build-up on include the formation of a yellow deposit inside the injector of the cone and skimmer, and drift in the ion optics/quadrupole the plasma torch.The use of different injector geometries, such transmission efficiency. as those with no constrictions, merely delayed the formation of this deposit. However, once the deposition had started, the deterioration of the signal became very rapid and it often led Selection of appropriate internal standard to the extinction of the ICP, probably as a result of instabilities The reported data also highlight the importance of selecting created by flying debris from this deposit.Such effects are the appropriate internalstandard to correct for such instrumen- more marked with ultrasonic nebulization owing to the higher tal drift. Owing to its proximity to selenium in terms of both sample delivery rate of this nebulizer. mass and ionization energy, As proved to be the best internal Since serum contains high levels of proteins, it is possible standard to use, with the corrected long-term stability figures that these deposits are the by-product of the denaturation of approaching those of the short-term stability.Unfortunately, the proteins in the aerosol as they approach the hot plasma. As is a monoisotopic element which, when analysing chlorine- This theory is supported by the fact that such deposits are not rich matrices (such as blood serum), is subject to a polyatomic observed when analysing acid digested serum, even at lower interference from ArCl+ ions.Hence, using As as an internal dilution factors than those described in this study. If enzymatic standard for measuring selenium in blood serum could lead to digestion were to break down serum proteins in a satisfactory spurious results. The suitability of In and Ge as alternative manner, improved instrument performance would be expected internal standards was investigated. Indium has traditionally without having to resort to higher dilution factors.been used as a universal internal standard. However, problems The difference in the instrument response between 20- and with In were reported when analysing for elements with high 50-fold dilution (approximately 0.35 and 0.14% protein in ionization energies (>850 kJ mol-1) or when the mass of the solution, respectively) when using a USN indicates that there element of interest is very far from 115 u.7,8 The results is a threshold below which protein in solution ceases to deposit obtained by using Ge as an internal standard were much better on the injector tube. Further experimentation showed that this than those obtained with In.Although the ionization energy threshold is not a set figure but a zone corresponding to of Ge (762 kJ mol-1) is much lower than that of selenium between 0.20 and 0.29% protein, in which results become (941 kJ mol-1), the two elements are very close in mass (72.61 irreproducible, with the deposit appearing on some occasions and 78.96, respectively), which could explain the observed but not on others. improved stability.Hence, for the remainder of this study, Ge was used as an internal standard against which all selenium signals were normalized. Figures of Merit of the USN–ICP-MS System Comparison of instrument sensitivity using the ultrasonic and the V-groove nebulizers Spectrophotmetric Evaluation of the Efficiency of the Enzymatic Digestion As expected, substantial enhancement in sensitivity was achieved by switching from a V-groove to an ultrasonic nebul- Owing to the presence of peptide bonds in all proteins, the addition of Cu2+ ions in a moderately alkaline medium to a izer.Originally, the instrument response for 1 ng ml-1 of selenium was 310 counts s-1. This figure increased to 8055 protein-containing solution leads to the formation of a col- Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 431Fig. 3 Long-term stability during the analysis of digested serum by USN–ICP-MS. (×) Without internal standard correction; (+) with Ge as an internal standard. Fig. 2 Effect of pronase E on the number of peptide bonds in serum (measured as percentage BSA equivalent). figure no longer represents the actual protein levels but their equivalent in terms of peptide linkages, such a comparison is not strictly valid. In fact, it appears that the reduction to oured chelate between the Cu2+ ion and the carbonyl oxygen polypeptides has improved the tolerance of the system and (C=O) and amide nitrogen (NH) atoms of the peptide bond.reduced the probability of injector blockage. Despite this The intensity of the purple colour produced is proportional to improved tolerance, rapid system blockage did occur when we the number of peptide bonds which have reacted and therefore attempted to analyse 10-fold diluted digested serum. to the total protein present. Using this characteristic reaction, we measured the number of intact polypeptide bonds (in terms Reproducibility of percentage BSA protein equivalent) in serum samples which were digested with different amounts of pronase and then The reproducibility of the digestion technique was evaluated diluted 10-fold. by the analysis of 10 separately prepared aliquots of the same It can be seen from Fig. 2 that the addition of pronase leads serum sample. The RSD obtained without the use of internal to a substantial decrease (almost 40%) in the number of standard correction was 11.6%.However, internal standard polypeptide bonds in the serum. The tailing off in the graph correction using Ge led to a decrease in this figure to 4.9%. clearly indicates that beyond a certain point, increasing the These results are in line with the expected reproducibility of amount of pronase does not further increase proteolysis. the experimental set-up which was evaluated early in this Literature figures for this protein suggest that it is capable of study.Based on this observation, it is apparent that the breaking down 88% of albumin into its amino acid compo- enzymatic digestion method does not introduce any significant nents.9 This high cleavage capacity is obviously not observed sources of error. Since the digestion technique requires mini- in this study. Although it is possible that 40% of the serum mum sample handling and does not involve any of the likely proteins were reduced to their amino acid components, it is sources of uncertainty such as heating (volatilization or tem- more likely that the effect of pronase was largely limited to perature variation on the hot plate or in the heating block), breaking down the serum proteins into polypeptide chains.A filtration, or centrifugation, this is not surprising. possible explanation for this reduced degree of proteolysis may be the known inhibition of this enzyme by Cu2+ ions,9 which Recovery are contained in serum at a level of approximately 1 mg ml-1.In order to evaluate the recovery of the digestion procedure, 10 1 ml aliquots of a pooled serum sample were digested USN–ICP-MS Analysis of Enzymatically Digested Blood separately. Prior to digestion, five of these aliquots were spiked Serum: Long-term Stability, Reproducibility, Recovery and with 100 ng of Se. The samples were analysed and the recovery Accuracy was calculated using a calibration based on synthetic standards.L ong-term stability The recovery figures with and without Ge correction were 530% and 290%, respectively. In order to evaluate the signal stability of the USN–ICP-MS system during the analysis of a large number of enzymatically digested serum samples, a quantity of serum was digested, Signal enhancement diluted 20-fold and analysed repeatedly over a period of 3 h. Inspection of our results showed that, on average, the signal The analysis protocol was 90 s for sample uptake followed by for Ge in serum decreased by about 55% whereas the sensitivity 270 s of data collection (three readings of 90 s each) and a for selenium increased by a factor of approximately three. The 180 s wash between samples.suppression of the Ge signal is in agreement with the well The data derived from the analyses are plotted in Fig. 3. documented matrix-related effect caused by the high levels of The use of Ge as an internal standard led successfully to a sodium in the samples.10 However, this is clearly not the case decrease in the RSD to a level similar to that obtained during for selenium for which, on the contrary, the sensitivity was the long-term analysis of a synthetic standard.Further, it is enhanced. Since blood serum is rich in carbon, it is possible clear from this plot that when analysing enzymatically digested that the selective enhancement of the selenium signal observed serum, a high degree of signal stability can be maintained over in this study, which is in line with other reports on carbon- a long period of time. Based on the biuret test, the level of containing compounds,11,12 is due to a charge exchange protein in the digested samples (after 20-fold dilution) is about between C+ ions and selenium atoms in the plasma according 0.22%.This figure is above the minimum level required for to the following reaction: optimum performance of the USN–ICP-MS system as determined using non-digested diluted serum.However, since this C++Se�C+Se+* 432 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12According to Niu and Houk,13 charge-exchange reactions are 89±1.7, 89±4.8 and 84±4.3 ng ml-1, which are in excellent agreement with the certified value of 86 ng ml-1. It should be most efficient when the excited state of the product ion is close in energy to that of initial reactant io Therefore, in order for noted, however, that pronase preparations normally contain approximately 20% of calcium acetate for stability.The effect this proposed reaction to take place at any significant level, the first ionization energy of carbon must be close to the of this and other possible contaminants should be investigated before using this digestion method in the determination of energy of excited selenium ions. The ionization energy of selenium is 941 kJ mol-1 whereas according to Moore,14 the other elements.Alternatively, the use of other enzymes or enzyme mixtures of higher purity could be explored. first excited state of Se+, which corresponds to a movement between the ground state of 4p3 4S0 (J=1D) and 4p3 2D0 (J= 1D), is associated with an emission at a wavelength of 759.4 nm. CONCLUSIONS Therefore, the energy required to elevate Se+ to this state is: The use of enzymes for sample digestion offers a means of E=hNAc/l=157 kJ mol-1 decreasing the level of intact protein in serum to an extent that allows for successful routine trace element analysis of this where h is Planck’s constant and NA is Avogadro’s constant.matrix by USN–ICP-MS. This alternative method of sample Hence the energy required to form Se+* is 941+157= digestion is cheap, reproducible and does not require the 1098 kJ mol-1. This figure is extremely close to the first handling of corrosive materials. However, since pronase did ionization energy of C, which is 1086 kJ mol-1.It is therefore not break down all of the protein in the samples, further very likely that this reaction is the reason for the observed investigation of possible matrix-derived enzyme inhibitors is enhancement in the sensitivity of the instrument to Se. By the warranted. same token, the energy required to generate excited As+ ions, which corresponds to a shift between 4p2 3P (J=0) and 4p2 Dr. M. P. Rayman was supported during this study by a 1D (J=2), is 1062 kJ ml-1. This suggests that enhancement Daphne Jackson Memorial Fellowship funded by the will also be observed for As.A repeat of the experiment but Leverhulme Trust, Scotia Pharmaceuticals and the University using 100 ng ml-1 As as an internal standard (this high level of Surrey. V. Hotton and G. Bastian were supported by the was selected to minimize the influence of ArCl+ on the signal) European Union ERASMUS exchange programme. led to an improved recovery of 94%, which we interpret as being due to a similar signal enhancement for both elements.REFERENCES Effect of matrix-matched calibration on recovery 1 Jarvis, K. E., Gray, A. L., and Houk, R. S., Handbook of Inductively Coupled Plasma Mass Spectrometry, Blackie, Glasgow, Based on these observations, we decided to test whether 1992, ch. 5, pp. 148–150. matrix-matched calibration would compensate for the effect of 2 Tortora, G. J., and Grabowski, S. R., Principles of Anatomy and the matrix on the selenium signal.An amount of pooled serum Physiology, Harper Collins College Publishers, New York, 7th was prepared and set aside for this purpose. Digested samples edn., 1987, ch. 19, pp. 568–569. from this pool were spiked with various levels of selenium 3 Christensen, J. M., and Pedersen, L. M., Acta Pharmacol. T oxicol., 1986, 59, 399. prior to the final dilution. The slope of the Se/Ge ratio from 4 Salonen, J. T., Alfthan, G., Huttunen, J. K., Pikkarainen, J., and the matrix-matched calibration was approximately eight-fold Puska, P., L ancet, 1982, ii, 175.greater than that from a synthetic standard solution, and the 5 Willett, W. C., and Stampfer, M. J., Br. Med. J., 1988, 297, 573. recovery calculated using Se/Ge varied between 97 and 104% 6 Rayman, M. P. R., Abou-Shakra, F. R., and Ward, N. I., J. Anal. with an average of 100%. These results clearly highlight how At. Spectrom., 1996, 11, 61. interactions in the plasma can significantly affect the perform- 7 Williams, C. A., Abou-Shakra, F. R., and Ward, N. I., Analyst, 1995, 120, 341. ance of the instrument and point towards the need for careful 8 Vanhoe, H., Dams, R., and Versieck, J., J. Anal. At. Spectrom., consideration of matrix effects on the elements to be analysed 1994, 9, 23. when selecting an appropriate internal standard. The good 9 Product Information Sheet, 0395, P 144, 4, 1743791, Boehringer recovery figures suggest that the enzymatic digestion procedure Mannheim UK, Lewes, East Sussex; www: http://biochem. does not cause any significant loss of selenium from the boehringer-mannheim.com/pack-insert/0165921.pdf. samples. It is imperative, however, that matrix-matched cali- 10 Vandecasteele, C., and Block, C. B., Modern Methods for T race Element Determination, Wiley, New York, 1993, ch. 9, pp. 214–220. bration is adopted if any degree of accuracy is to be achieved 11 Larsen, E. H., and Stu�rup, S., J. Anal At. Spectrom., 1994, 9, 1099. in future selenium analyses. 12 Allain, P., Jaunault, L., Mauras, Y., Mermet, J. M., and Delaporte, T., Anal. Chem., 1991, 63, 1497. 13 Niu, H., and Houk, R. S., Spectrochim. Acta, Part B, 1996, 51, 779. Accuracy 14 Moore, C. E., Atomic Energy L evels, NSRDS-NBS 35, National Bureau of Standards, Washington DC, 1971, vol. II, pp. 150–158. The accuracy of the described procedure was evaluated through the analysis of Seronorm certified serum reference material. Three analyses were conducted on different occasions using Paper 6/07972E Received November 25, 1996 matrix-matched calibration and Ge internal standard correction. The results obtained (average±standard deviation) were Accepted February 10, 1997 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 4

ISSN:0267-9477    

DOI:10.1039/a607972e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Iodine in Food-related Certified Reference Materials Using Wet Ashing and Detection by Inductively Coupled Plasma Mass Spectrometry ERIK H. LARSEN* AND MERETE B. LUDWIGSEN National Food Agency of Denmark Institute of Food Chemistry and Nutrition 19 Mørkhøj Bygade DK-2860 Søborg Denmark Iodine was accurately determined in food-related certified reference materials (CRMs) by wet ashing in closed steel bombs using a mixture of nitric acid and perchloric acid followed by measurement of iodine by ICP-MS at m/z 127. The iodine concentrations determined were 0.15–4.59 mg g-1 (dry mass). Potentially volatile iodine species such as hydrogen iodide were converted during the ashing procedure to nonvolatile species. Hereby memory problems in the ICP-MS due to volatile iodine were prevented.The concentric nebulizer which was used in combination with a low dead-volume cyclonic spray chamber improved the iodine signal intensity and reduced the wash-out time between samples compared with the standard Ryton double-pass spray chamber equipped with a cross flow nebulizer. Iodine contamination from the PTFE liners of the bombs led to blank values at an average of 9 ng per ashing. The limit of detection which was based on three standard deviations of the blank was 30 ng g-1 (dry mass). By adding 3% v/v methanol to the analyte solutions and increasing the plasma power to 1200 W the signal-tonoise ratio and hence the limit of detection was improved by 50% due to the signal enhancement effect by carbon of the incompletely ionized iodine.Keywords Iodine; food ; certified reference materials ; sample introduction ; inductively coupled plasma mass spectrometry; signal enhancement Iodine is an essential element to man and the beneficial effect of iodine-containing sea weed on goitre (swollen thyroid glands due to iodine deficiency) has been known since pre-historic times.1 In the state of iodine deficiency the gland produces too little of the iodine-containing thyroidea hormones. The main source to man of iodine is food and water and for adolescents and adults a recommended dietary intake has been estimated internationally as well as in the Nordic Countries2 at 150 mg d-1. In order to assess the dietary intake of iodine in a population and in population sub-groups at risk of a too low or too high iodine intake accurate data are needed regarding the iodine content of individual food items.Such data are particularly valuable bearing in mind the relatively narrow safety margin of a factor of 2–7 between the maximum tolerable and the essential iodine intake.3 A variety of analytical methods have been used for the determination of iodine in food. Most often a sample dissolution step (wet or dry ashing) has been employed prior to the detection step which often requires that iodine has been converted to and isolated in a particular chemical or physical form amenable to the final detection. Spectrophotometry was used for the determination of iodine based on an iodidecatalyzed reduction of cerium(IV) by arsenic(III ) in a variety of food items.4 NAA has been used after direct irradiation of the sample followed by combustion of the sample in oxygen and absorption of the liberated iodine,5 or by using preconcen- Journal of Analytical Atomic Spectrometry April 1997 Vol.12 (435–439) Wet Digestion The samples were digested in high-pressure steel bombs model DAE II with PTFE liners of 50 ml volume (Berghof Tu�bingen tration or chemical separation prior to detection of the gamma activity of the 131I formed during the activation process.6–8 A related method relies on the chemisorption of radioiodide on gold after decomposition of the sample in oxygen.9 Negative TIMS was used in conjunction with quantification by the isotope dilution technique after isolation of iodine as silver iodide10,11 or after extraction of free iodine into tetrachloromethane.12 Isotope dilution analysis was also used with laser resonance ionization MS after isolation of the analyte as silver iodide.13 A method that aimed at using more simple equipment was based on cathodic stripping voltammetry following oxidative wet ashing.14 Gas chromatography was used for the determination of iodine using derivatization with pentan-3-one following alkaline dry ashing.15 Atomic spectrometric methods used include ICP-AES of generated volatile iodine species from the iodide and iodate analytes7 and ICP-MS for the direct determination of iodine in plasma and in urine.16 Iodine was determined directly by ICP-MS in milk after the addition of base17,18 or after microwave-assisted wet destruction by base.19 Some of the above mentioned analytical techniques particularly those based on radioiodine measurements require highly specialized and therefore relatively inaccessible equipment or involve multi-step procedures which are prone to iodine losses or contamination.In contrast ICP-MS which is becoming an increasingly popular technique in food research and control allows the direct determination of iodine in solution and is a highly sensitive selective and largely interference-free detector for the monoisotopic iodine (127I). Some iodine species e.g. hydrogen iodide are gaseous at room temperature and therefore volatile. The risk of loss of volatile species was reduced by adjusting the iodine-containing analyte solution at alkaline pH and hence suppressing the volatility of hydrogen iodide.17–19 Volatilization of iodine led to severe memory effects which may otherwise cause problems in the sample introduction system of the ICP mass spectrometer instrument.19 Alternatively a wet ashing step that makes use of a strong oxidizing agent such as perchloric acid converted volatile iodine to non-volatile species.11–14 The aim of this paper is to describe the development and evaluation of an analytical method which is based on sample dissolution by wet ashing using a mixture of nitric and perchloric acids in closed steel bombs in combination with ICP mass spectrometer detection.Potential sources of error during sample introduction into the ICP mass spectrometer have been given special attention and the method has been used for the iodine analysis in a range of certified reference materials (CRMs).EXPERIMENTAL 435 Germany). Prior to using new PTFE liners for analytical work the amount of iodine that contaminated the PTFE was reduced by treatment with 4 ml of nitric acid at 160 °C for 4 h. The PTFE liners which were submitted to this procedure were reserved for the analysis of iodine in order to keep the risk of additional external contamination by iodine to a minimum. When not in use the liners were filled with a mixture of 2 ml of nitric acid and 48 ml of water. Instrumentation and Instrumental Settings The iodine content of the diluted wet-ashed residues was determined by a Perkin-Elmer SCIEX ELAN 5000 ICP-MS instrument (Perkin-Elmer SCIEX Concord Ontario Canada).The instrument was run at normal resolution and set to detect the signal intensity at m/z 127 (127I+) in the quantitative and in the graphics data acquisition modes which allowed quanti- fication and recording of the signal intensity versus time respectively. The instrument optimizations included physical positioning of the plasma relative to the mass spectrometer the lens voltage settings rf power and nebulizer gas flow. A 20 ng ml-1 aqueous standard solution of iodine as iodate normally produced a signal of approximately 60000 counts s-1. An AS-90 autosampler with polypropylene sample vials was used in conjunction with a peristaltic pump for the introduction of the sample solutions via two separately tested nebulizer/spray chamber assemblies.Further details of the instrumental settings are given in Table 1. Table 1 Instrumental settings and calibration ICP-MS instrument— Rf power Sampler and skimmer cones Argon flow rates Outer Intermediate Nebulizer Mass-to-charge ratio detected Quantitative mode Dwell time per mass Sweeps per reading Readings per replica Number of replicates Scanning mode Graphics mode (signal intensity versus time) Dwell time per mass Sweeps per reading Readings per replicate Number of replicates Estimated time Scanning mode Sampling system— Autosampler Wash time between samples Read delay Peristaltic pump speed Spray chamber and nebulizer assemblies Calibration— Type Working standard solution Added volumes 436 1000–1300W Platinum 15 l min-1 0.8 l min-1 1 l min-1 (variable) m/z 127 1 3 80 1000 ms Peak hop 1 5 50 ms 1000 278 s Peak hop 120 s 80 s 2.5 ml min-1 (a) Ryton double-pass (Scotttype) with a gem-tipped cross-flow nebulizer (b) Glass cyclonic with a Meinhard (type TR-30-K3) concentric nebulizer Standard additions 10 mg Iml-1 as potassium iodate 50 ml and 100 ml (variable) Journal of Analytical Atomic Spectrometry April 1997 Vol.12 Blank Values and Limit of Detection In spite of thorough cleaning of the utensils by nitric acid prior to the analytical work the iodine concentration level of the blanks significantly exceeded zero.Rigorous testing of each of the possible sources of contamination (the PTFE material pipette tips acids etc.) showed that the PTFE was the only notable source. The iodine blank concentration (average±one Standard Substances and Chemicals An aqueous standard stock solution at 1000 mg ml-1 of iodine was prepared from potassium iodate volumetric standard which contained 99.95–100.05% iodine (Merck Darmstadt Germany). Additionally a commercial 1000±0.5 mg ml-1 iodine standard as sodium iodide in water was acquired from the producer (Teknolab Drøbak Norway). Working standard solutions at 10 mg ml-1 were prepared daily from these stock solutions by dilution with water. Nitric acid pro analysi (Merck) which was sub-boil distilled in an all-quartz apparatus (Hans Ku�rner Rosenheim Germany) and perchloric acid pro analysi were used for the wet ashings.Water (>18 V cm-1) was produced in a Millipore Super-Q apparatus (Millipore Milford MA USA). Samples A variety of CRMs produced by NIST (National Institute of Science and Technology MD USA) and BCR (Community Bureau of Reference Brussels Belgium) certified for total iodine (Table 2) were analysed. The samples were continuously stored in a desiccator at room temperature. The residual water content in the CRMs stored under these conditions was around 2–5%. Procedure Ten to fourteen bombs were taken through the procedure in parallel. Two randomly selected bombs were filled with the digestion acids mixture and taken through the entire procedure to monitor the average and variation of the iodine blank value.Sub-samples of 0.1–0.5 g (dry mass) were weighed into the PTFE liners of the bombs followed by addition of first 3.5 ml of nitric and then 1.5 ml of perchloric acid. After addition of the acids the bomb was immediately closed to prevent the risk of volatilization of iodine and then heated at 160°C for 4 h during the night. After cooling the bomb was opened in a fume hood and the built-up pressure was gently released. Each of the wet-ashed residues was taken to 20 g by water which was added directly to the tared PTFE liner. The volume of this diluted sample residue was then calculated after determination of the density which was (average and standard deviation) 1.115±0.016 g ml-1 (n=24) by weighing of a separate aliquot of the residue.The iodine content of this solution remained non-volatile for at least 5 days. Four millilitres of the solution which was spiked with appropriate volumes of the iodate working standard solution for the purpose of quantification by the standard additions procedure was taken to 10 ml by water in the autosampler vials. This spiked and diluted analyte solution must be analysed on the same day as the iodine may be converted to volatile forms upon storage. Prior to performing quantitative analysis the iodine signal intensities of the diluted sample solutions were recorded versus time in the graphics mode. The shape of the signal profile was used to monitor that the iodine was present as non-volatile species.Following this evaluation the samples were measured in the quantitative mode. For each set of samples two blanks were taken in parallel through the entire procedure. RESULTS AND DISCUSSION Table 2 Quantitative results for iodine in certified reference materials Certified values 0.167±0.024 Reference material (acronym) Hay powder 1.29±0.09 BCR 129 NIST 1570 BCR 150 BCR 186 NIST 1572 NIST 1566a Spinach Milk powder Pig kidney Citrus leaves Oyster tissue Cod muscle 1.84±0.03 4.46±0.42 4.95±0.49 BCR 422 * Indicative value (BCR). standard deviation) was 0.18±0.11 ng ml-1 (n=13) in the final solution for measurement or equivalent to 9 ng per ashing.The LOD based on three standard deviations of the blank is 30 ng g-1 (dry mass) when 0.5 g sample is taken for analysis. This corresponds to an LOD value of approximately 5 ng g-1 for the equivalent amount of fresh sample. New liners which were not previously cleaned led to a contamination level by iodine approximately ten times that found after cleaning. However an attempt to further reduce the blank by other cleaning agents (alkaline detergent) and procedures (sonication) was unsuccessful. Conversion of Volatile Iodine to Non-volatile Species The composition of the acid mixture for the wet ashing was selected with the aim of mineralizing the organic matter of the sample and of converting volatile iodine species e.g.hydrogen iodide in the acidic sample solution to non-volatile species e.g. iodate. This conversion was necessary to prevent the risk of analyte loss and to prevent severe memory problems in the sample introduction system of the ICP-MS detector. Perchloric acid is well suited for such an oxidation at the elevated temperature and pressure which occurs during the bomb ashing. The minimum amount of perchloric acid that was necessary to ensure the oxidation of iodide was about 1.5 ml when used in combination with 3.5 ml of nitric acid for 0.5 g of dry sample. The closed bomb system additionally reduces the risk of loss by evaporation of the analyte during the wet ashing procedure. When using perchloric acid extreme care should be taken to prevent explosions which are likely to occur if concentrated hot perchloric acid is in contact with biological (oxidizable) materials.The use of the closed bomb system however prevents the evaporation of the nitric acid from the acid mixture hence the risk of explosion is reduced to a minimum. When analysing food samples high in fat such as cheese or fatty meat the amount of sample taken for analysis should be reduced to 0.1–0.2 g to prevent excessive pressure build-up due to violent oxidation or ultimately an explosion. The successful conversion of volatile iodine to non-volatile species was indicated by a signal intensity profile (Fig. 1A) recorded in the graphics mode identical to that normally observed in ICP-MS studies of non-volatile elements.In contrast the ICP-MS signal intensity profile showed pronounced fronting and did not reach a steady-state level when volatile iodine species were present in solution (Fig. 1B) and hence quantification became impossible. Furthermore the pronounced tailing of the signal required long wash-out times and increased the risk of analyte carry-over. This unusual signal profile was probably caused by volatile iodine species which adhered to the surfaces of the sample introduction system and tubing etc. In this study the closed high-pressure steel bombs were used to prevent the possible loss of iodine by evaporation during the wet ashing procedure. The use of microwave-assisted wet Journal of Analytical Atomic Spectrometry April 1997 Vol. 12 Sample Introduction When analysing a potentially volatile element like iodine optimum and interference-free sample introduction is of great importance.With the aim of optimizing the iodine signal intensity and minimizing the wa-out time between samples the standard double-pass spray chamber with a gem-tip cross- flow nebulizer was compared with a low-volume glass cyclonic spray chamber equipped with a concentric nebulizer. The ICP-MS signal intensities obtained with these nebulizer-spray chamber assemblies upon aspiration of a 20 ng ml-1 aqueous solution of iodine as iodate (Fig. 2) show that the signal intensity was improved by a factor of two when using the cyclonic spray chamber. With this type of sample introduction device a markedly shorter aspiration time was necessary to arrive at a steady-state signal.Furthermore the iodine signal intensity versus time in Fig. 3 shows that the wash-out time necessary to arrive at the base-line level with the cyclonic Iodine concentration/mg g-1 Other values This study 0.15; 0.17 1.089; 1.166 0.145* 1.09; 1.02 1.15 0.16 1.76; 1.65 4.53; 4.54; 5.07 4.59 Fig. 1 ICP-MS signal intensity versus time of iodine measured in the wet-ashed residue of 100 mg Cod Muscle (BCR CRM 422) diluted to 50 ml with water A measured immediately after dilution and B measured five days after dilution. ashing with the same acid mixture might be feasible.6 However the risk of losing volatile iodine species during venting at elevated pressure in this type of system requires utmost attention and was not tested.437 Fig. 2 ICP-MS signal intensity obtained when using a cyclonic spray chamber with a concentric nebulizer for 20 ng ml-1 of I as iodate A in water; B in 3.5 ml HNO and 1.5 ml HClO diluted to 50 ml by water; C in 3.5 ml HNO 4 3 and 1.5 ml HClO diluted after bomb ashing to 50 ml by water; D in 4 3 3.5 ml HNO and 1.5 ml HClO diluted after bomb ashing of 500 mg potato starch 4 3 to 50 ml by water; and E same as A but using a standard double-pass spray chamber with a cross-flow nebulizer. Fig. 3 ICP-MS signal intensity profiles (tails) for iodine in Cod Muscle CRM (cf. Fig. 1) using A cyclonic spray chamber with a concentric nebulizer; and B double-pass spray chamber with a cross-flow nebulizer.spray chamber is about 40 s in comparison with 100 s when using the double-pass spray chamber. The pulsed noise which was observed with the latter spray chamber (Fig. 3B) may be ascribed to pressure pulsations from the peristaltic pump20 which was used for draining the spray chamber. Even though this pump was also used for draining the cyclonic spray chamber the pulsed noise did not show. The use of the low dead-volume cyclonic spray chamber with the concentric nebulizer is advantageous as it provides better analyte sensitivity without increasing the base-line noise which results in an Journal of Analytical Atomic Spectrometry April 1997 Vol. 12 Signal Enhancement by Methanol The signal intensity of elements with first IE values in the 9–11 eV range (e.g.iodine arsenic and selenium) may be enhanced by introduction of carbon as methanol22 or as other carbon-containing molecules23 into the ICP. In order to optimize this enhancement effect for iodine the S/N for the aqueous and methanol–water (3+97 by volume) solutions of iodate were recorded at m/z 127 at different rf power settings as shown in Fig. 4. The S/N increased only slightly for the aqueous solution when an optimum rf power of 1100W was applied (curve B). However in the 3% methanol solution the S/N was improved by 54% at the 1200 W optimum power setting. At the 1300 W power setting the iodine sensitivity was even greater but the noise increased at the same time and the Fig. 4 ICP-MS S/N-ratio at m/z 127 at different rf power settings for 20 ng ml-1 of I as iodate in A methanol–water mixture (3+97 by volume); and B aqueous solution.The nebulizer gas flow settings for optimum signal intensity (ordinate axis to the right) are given for C methanol–water mixture (3+97 by volume); and D aqueous solution. 438 improved S/N value and hence improved LOD as well as shorter times of analysis. Matrix Effects The first ionization energy (IE) for iodine of 10.3 eV is relatively high compared with most other elements. Consequently the ionization of this element in the argon ICP is incomplete and has been estimated at 29%.21 The sensitivity of iodine obtained with the ICP-MS instrument is however still favourable because iodine is monoisotopic (127I). The sensitivity (Fig.2) obtained for a 20 ng ml-1 solution of iodine as iodate in an acid matrix containing 4.4% nitric acid and 2.0% perchloric acid (curve B) is 74% of that obtained for the same iodate concentration in aqueous solution (curve A). This acid matrix is close in composition to that of the ashed and diluted sample solutions. When the same amount of iodate was subjected to the bomb ashing procedure the sensitivity (curve C) dropped further to 67% of that for the aqueous solution. The sensitivity was exactly the same (curve D) when the iodate was bombashed in the presence of 0.5 g of a biological material (potato starch). The reduction in iodine sensitivity may be caused by a higher liquid viscosity of the acid-containing solutions which reduces the analyte uptake rate.A reduced ionization efficiency of iodine in the chlorine-rich solution may additionally explain the observed effect. The results show that external calibration using an aqueous standard solution or using a matrix-matched acid solution led to inaccurate (erroneously low) results. Therefore the method of standard additions with iodate was used for calibration. resulting S/N decreased slightly (curve A). The nebulizer gas flow necessary for optimum analyte sensitivity varied with changing rf power settings and solvent composition22 (curves C and D). In the presence of 3% methanol in solution the optimum nebulizer gas flow is reduced by approximately 0.05 l min-1. For iodine analyses where an optimum LOD value is required the addition of 3% methanol to the aqueous analyte solution and the use of 1200 W rf power is therefore recommended for the ICP-MS instrument used.Under these analytical conditions nickel sampler and skimmer cones degrade rapidly and the use of platinum cones is recommended.22 CONCLUSIONS An analytical method based on ICP-MS has been presented for the accurate determination of iodine in food-related CRMs. Quantitative Determination of Iodine in Food-related CRMs The performance of the analytical method was tested using CRMs which covered a variety of sample types and iodine concentrations. Prior to running the quantitative analyses all samples were run in the graphics mode to ensure that the ICP-MS signal stabilized at a steady-state intensity within 5–10 s as indicated in Fig.1A. The two additions of iodate for the standard additions calibration procedure were 50–150% and 150–250% respectively in concentration of the expected iodine concentration in the diluted sample solution. Calibration curves constructed this way showing a coefficient of correlation less than 0.999 were not used for quantification as they reflected a deviation from linearity which led to inaccurate results. The slopes of the standard addition calibration curves were close in value for different sample types and the within-day RSD value was 3.4% (n=8). For routine work a larger sample throughput may therefore be obtained by calibrating several unknown samples against one common standard additions calibration curve.When used in the quantitative mode the RSD values of the recorded ICP-MS signal for unknown samples were 0.4–2.9%. The highest values were associated with the lowest measured iodine concentrations. Signal intensity RSD values within this range were used to additionally control the stability of signal intensities during the quantitative measurements. In the case of a non-steady state signal intensity for iodine (cf. Fig. 1B) this RSD value would increase markedly. The quantitative results for iodine in CRMs (Table 2) are in good agreement with the certified or literature values. The proposed method will be used in the near future for an extensive study of iodine in fish and dairy products on the Danish market. Further data on the reproducibility and accuracy will be provided during the course of these investigations.Journal of Analytical Atomic Spectrometry April 1997 Vol. 12 The method fills a gap in the use of ICP-MS for the determination of iodine in solid biological materials. The key to a successful analysis was to ensure that volatile iodine was oxidized to non-volatile species. This was achieved by using perchloric acid and nitric acid at elevated temperature in closed bombs. REFERENCES 1 Orr J. B. and Leitch I. Iodine in nutrition. A review of existing information up to 1927 Special Report No. 123 Medical Research Council London 1929. 2 Nordic Council of Ministers Nordic Nutrition Recommendations Report 198952 Copenhagen 1989. 3 Frey H. Iodine in Risk Evaluation of Essential T race Elements — Essential Versus T oxic L evels of Intake ed.Okarsson A. Nordic Council of Ministers Copenhagen 1995 pp. 119–132. 4 Fischer P. W. L’Abbe� M. R. and Giroux A. J. Assoc. Off. Anal. Chem. 1986 69 687. 5 Dermelj M. Slejkovec Z. Byrne A. R. Stegnar P. Stibilj V. and Rossbach M. Fresenius’ J. Anal. Chem. 1990 338 559. 6 Rao R. R. and Chatt A. Anal. Chem. 1991 63 1298. 7 Rao R. R. and Chatt A. Analyst 1993 118 1247. 8 Langenauer M. Kra�henbu�hl U. and Wyttenbach A. Anal. Chim. Acta 1993 274 253. 9 Mantel M. Analyst 1988 113 973. 10 Heumann K. G. and Schindlmeier W. Fresenius’ Z. Anal. Chem. 1982 312 595. 11 Gramlich J. W. and Murphy T. J. J. Res. Nat. Inst. Stand. T ech. 1989 94 215. 12 Schindlmeier W. and Heumann K. G. Fresenius’ Z. Anal. Chem. 1985 320 745. 13 Fasset J. D. and Murphy T. J. Anal. Chem. 1990 62 386. 14 Holak W. Anal. Chem. 1987 59 2218. 15 Mitsuhashi T. and Kaneda Y. J. Assoc. Off. Anal. Chem. 1990 73 790. 16 Allain P. Mauras Y. Douge� C. Jaunault L. Delaporte T. and Beaugrand C. Analyst 1990 115 813. 17 Baumann H. Fresenius’ J. Anal. Chem. 1990 338 809. 18 Stu�rup S. and Bu�chert A. Fresenius’ J. Anal. Chem. 1996 354 323. 19 Vanhoe H. Van Allemersch F. Versieck J. and Dams R. Analyst 1993 118 1015. 20 Luan S. Pang H. Shum S. C. K. and Houk R. S. J. Anal. At. Spectrom. 1992 7 799. 21 Jarvis K. E. Gray A. L. and Houk R. S. Handbook of Inductively Coupled Plasma Mass Spectrometry Blackie Glasgow 1992. 22 Larsen E. H. and Stu�rup S. J. Anal. At. Spectrom. 1994 9 1099. 23 Allain P. Jaunault L. Mauras Y. Mermet J.-M. and Delaporte T. Anal. Chem. 1991 63 1497. Paper 6/07581I Received November 7 1996 Accepted Janua

ISSN:0267-9477    

DOI:10.1039/a607581i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Removal of Air Interference in Laserinduced Breakdown Spectrometry Monitored by Spatially and Temporally Resolved Charge-coupled Device Measurements M. MILA� N, J. M. VADILLO AND J. J. LASERNA* University of Ma� laga, Faculty of Sciences, Department of Analytical Chemistry, 29071-Ma� laga, Spain Laser-induced breakdown spectrometry is a suitable method yielded a more favorable L/B ratio at high pressures. However, the L/B ratio was improved in air and argon atmospheres at for the direct in-process measurement of materials composition.The emission spectrum from the plasma includes low pressures. Spectroscopic applications of LIBS in air at atmospheric information not only on the analysis area but also on the surrounding atmosphere, mainly lines corresponding to O, N pressure rather than in a vacuum are fostered by a number of practical advantages, including simplicity, flexibility of sample and C if the experiments are being carried out in air at atmospheric pressure.These emission lines could interfere with handling, and low cost due to the lack of maintenance expenses. These advantages become a must if automation and on-line the sample spectrum. Although working under vacuum conditions or the use of controlled atmospheres can be analysis is required. However, several drawbacks limit the use of air in LIBS in comparison with other studied buffer gases. considered to be the best choice, in most practical applications working in air at atmospheric pressure is the common way of In air at high pressure (higher than 100 Torr) the background increases, thereby decreasing the L/B ratio and the spectral analysis.A study was undertaken to evaluate the removal of air interferences in poly(methyl methacrylate) samples by detection.16 Oxidized samples caused by the oxygen present can lead to the emission of molecular bands in the spectra using the temporal and spatial resolution of gated chargecoupled devices, without any sample treatment or alteration of causing severe spectral interferences with the analytes of interest. 13,17 The interest of working in air at atmospheric pressure, the experimental set-up. particularly when in situ analysis is required, necessitated an Keywords: L aser-induced breakdown spectrometry; investigation of the laser-induced plasma emission spectrum atmospheric pressure plasma; poly(methyl methacrylate); from ambient air in an attempt to determine the origin of the space-resolved analysis; time-resolved spectrometry emission in the LIB spectra.18 Although the spatial2,13,19 and temporal behavior of laser produced plasmas has been docu- Laser-induced breakdown spectrometry (LIBS) is based on mented,15,17 no attempt has been made to use simultaneously the optical emission spectral analysis of the plasma resulting these approaches to solve spectral interferences in LIBS.from dielectric breakdown of solid,1,2 liquid,3 gas4 or aerosol5 In this paper, poly(methyl methacrylate) (PMMA) was used samples. The interaction between a laser beam and a sample as a model sample to study spectral interferences by air in is a complex process and is dependent on the characteristics LIBS.Emission from air components can be removed by of both the laser and the target. The shape, size and emission moving the sample away from the laser focal position and spectra of laser-ablated plasmas are largely dependent on the monitoring the spectral pattern of air and sample using a atmospheric surroundings, the gas composition and the press- position-sensitive charge-coupled device (CCD) detector.ure.6–8 LIBS analyte emission taken in different supporting Temporal resolution was also used to improve spectral selecgases provides information on the plasma formation mechan- tivity. Using this approach, optimum conditions for spectral isms, particularly with regard to the role of the buffer gas.6,9,10 detection were established.If these reactions are characterized, pathways for energy transfer among the plasma-producing source, the buffer gas and the EXPERIMENTAL analyte may be discerned and then exploited to optimize sensitivity and detection limits, while minimizing matrix A schematic diagram of the system is shown in Fig. 1. The second harmonic of a pulsed Nd5YAG laser (Continuum, interference. The effect of the surrounding environment on LIBS has Model Surelite I20, wavelength 532 nm, pulse width 5 ns) was used to irradiate commercial PMMA samples.The laser energy been discussed by a number of workers. Grant and Paul11 conducted a study of the LIB spectra produced on a stainless- at the sample was 170 mJ. The laser beam was focused onto the sample surface using a planoconvex lens with a focal length steel target in atmospheres of air, argon, nitrogen and helium at pressures ranging from 0.5 to 760 Torr. The influence of of 100 mmand f-number of 4.Optical collection was performed with a 100 mm focal length biconvex quartz lens. Polymer argon, nitrogen and helium at pressures ranging from 0.001 to 2300 Torr on the emission spectra of aluminum targets has samples were placed approximately 190 mmfrom the collection lens, with the distance from the entrance slit to lens being been reported previously by Owens and Majidi.12 Lee et al.13 studied several metals at pressures over the range 10–760 Torr 200 mm. This results in an optical magnification of about 1.The laser-ablated plasma emission was collected onto the in argon, helium and air atmospheres to determine optimum conditions, and the highest line-to-background (L/B) ratio. entrance slit of a 0.5 m focal length imaging spectrograph (Chromex, Model 500 IS, fitted with three interchange- Optimum results were found for helium because of its high thermal conductivity and ionization potential13,14 and its meta- able gratings of 300, 1200 and 2400 grooves mm-1).The reciprocal linear dispersion of the spectrograph with the stable characteristics.15 Mainly because of lower excitation temperature by the relatively high thermal conductivity, helium 300 grooves mm-1 grating was 1.6 nm mm-1, giving a spectral Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (441–444) 441components and air constituents (essentially N, C and O lines according to its molecular composition). These interferences are not taken into account when the sample plasma emission is much more intense than that of air.However, when the sample signal is weak or its composition is similar to that of air (as occurs in polymeric materials), the air emission lines must be minimized. In this work, PMMA was used as a model compound. Owing to its atomic composition, the LIB spectra of PMMA should exclusively contain emission lines assignable to C and O. In order to assign the exact contribution of PMMA and air to the net signal of both elements, two strategies based on the spatially and temporally resolved Fig. 1 Schematic diagram of the experimental set-up. 1, Nd5YAG capabilities of gateable CCD detectors were examined. laser head; 2, harmonic generator and dichroic lenses; 3, beam splitter; 4, prisms; 5, spectrograph; 6, CCD detector; 7, fast photodiode trigger; 8, focusing lens; 9, collection lens; 10, linear stage; and 11, personal Spatial Resolution computer with frame grabber.The ablation threshold is higher in gases than in solids due to coverage of 120 nm for the detector used. The spectrograph a better coupling of the laser energy. This property was used resolution was 0.07 nm. The spectrograph entrance slit was in this work to avoid spectral interferences without handling 10 mm high by 20 mm wide. the sample but altering the irradiance by changing the focal Detection of the plasma image was conducted with a two- geometry of the experiment. For these purposes, the sample dimensional CCD detector (Stanford Computer Optics, Model was positioned on a linear stage that allowed changes in the 4Quik 05).The CCD consisted of a rectangular matrix of lens-to-sample distance, as shown in Fig. 2. The insets shown 752×512 pixels. The active area is 6×4.5 mm2. Calibration of in Fig. 2 represent the non-dispersed images of the plasma as the detector was conducted with a mercury pen lamp. read by the CCD matrix he spectrograph is placed in Operation of the detector was controlled with 4Spec 1.20 the flat mirror mode.As shown, when the sample was possoftware. The detector was equipped with a micro-channel itioned at the focal point (Fig. 2, left), the image consists of a plate (MCP) image intensifier. The intensified (by variable single plasma that must include contributions of air and voltage VMCP) image of secondary electrons is delivered with PMMA. However, when the sample was displaced 3 mm below extremely high spatial resolution onto a phosphor screen with the focal point (Fig. 2, right), two images corresponding to air a typical efficiency of 10–35%. The MCP shutter is activated breakdown (located exactly at the focal waist) and the sample by application of a negative voltage VPC of 200 V to the photo- plasma appear. When the sample is located above the focal cathode, the duration of which determines the opening time of point, the laser irradiance, which reaches a maximum at the the shutter.MCP integration time (ts), delay time (td) and gain focal point, permits air breakdown and the formation of the were controlled via an RS 232 serial port. first plasma. After this point, the irradiance decreases progress- The model sample used was a 4 mm thick commercial ively. However,the irradiance affecting the sample is sufficiently PMMA sheet. The spectrograph was centered at 397.5 nm in high to allow the growth of the plasma but free of interferences all the experiments.The gain of the MCP was adjusted to the from the surrounding atmosphere. This effect is well illustrated requirements of each experiment to avoid detector saturation. when the spectra corresponding to both situations are The emission signal was corrected by subtracting the dark obtained. In Fig. 3, the spectra corresponding to the mixed signal of the detector, which was separately measured with the plasma (top), and the isolated contributions of PMMA(middle) same ts. In order to reduce statistical errors, ten laser shots and air (bottom) are shown.When the sample is placed at the above the same surface were accumulated after three initial focal volume, the spectrum includes contributions of both air shots to prepare the surface and prevent interferences from and PMMA, and no distinctions between C or O lines surface contaminants. For the spatial analysis the sample was corresponding to the sample emission can be observed. As placed either at the laser focal position ( f=100 mm) or 3 mm shown, only lines at 387.6 (C II), 399.5 (N II), 434.9 (O II ), away from the focal position, totaling a distance of 103 mm from the focusing lens.Under these conditions, the spot diameter of the laser beam was 0.37 cm and the irradiance was approximately 0.5×109 Wcm-2. RESULTS AND DISCUSSION The analysis of solid samples by LIBS is commonly carried out without previous handling or preparation. This fact is cited as an advantage as the vaporization, atomization and ionization processes are carried out in a single step.High laser irradiances are commonly used to improve the atomization process as a large amount of the energy confined during the laser shot is spent in different processes including absorption, reflection, and other laser–plasma and laser–target interactions that are not completely understood. All these facts require that the sample be placed at the laser focal position in order to obtain a better coupling of the optical energy into the solid (with the exception of depth-profiling studies, where the laser energy is decreased to obtain a lower ablation rate).When working in air at atmospheric pressure, optical breakdown of Fig. 2 Details of sample positioning for space-resolved studies. The air may occur at high irradiances; hence, the resulting plasma inset represents in each instance the non-dispersed image of the plasma formed as read by the CCD matrix.emission would include lines corresponding to the sample 442 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12growth, reaching a plateau at about 1500 ns. This result agrees with the large continuum emission of the air plasma. When such a component has been filtered by spatial resolution, the C II signal becomes larger with the integration time (for a period of time dependent on the material and the experimental conditions). However, when the sample is placed at the focal point (bottom), the S/B seems not to be greatly affected by the integration time as the continuum is present from the opening of the shutter.In both instances, when no spatial resolution can be performed, a temporal resolution technique is needed. Temporal Resolution Temporal resolution was also used to discriminate between emission lines corresponding to PMMA or air by studying the temporal behavior of the signal when the sample was positioned at the focal point.It is well known that plasma emission varies with time due to the recombination and relaxation processes that are produced once the laser pulse has ended. For a given laser fluence, the plasma lasts for a time that depends on the analyte and the atmosphere surrounding the sample. In order to study the different behavior between the Fig. 3 LIB spectrum of PMMA at the focal point (top) and contri- emission lines in air and PMMA, the sample was placed at butions of PMMA (center) and air (bottom) when both were physically the focal point and spectra were taken at different delay times isolated by the CCD matrix.Integration time: 1 ms. Delay time: 0 ns. while keeping the integration time constant. As shown in Fig. 5, the effect of recording the spectrum immediately after 444.7 (N II), 461.4 (N II) and 463.1 (N II) nm are clearly the laser shot (top) or delaying the acquisition of the signal by observed. It should be noted that the high background signal 1500 ns (bottom) is critical, showing a comparable effect to prevents good spectral resolution.A drastic change is observed that obtained when spatial resolution was used. A comparison when the laser is defocused. As different signals can be read of both spectra in Fig. 5 with those represented in Fig. 3 (top) independently and simultaneously by the CCD, two spectra and Fig. 3 (bottom) indicates a good agreement with results corresponding to air and PMMA are obtained without the obtained using space removal, and only small differences due need to carry out time-resolved experiments. It is clearly to the temporal behavior of the species are seen.evident that the air contribution to the continuum emission is larger than that of PMMA, exhibiting very broad lines superimposed on the intense emission continuum resulting from CONCLUSION electron–ion recombination, which prevents the correct obser- The LIB spectrum of air was examined and interferences of C vation of the C II lines at 392.1 and 416.3 nm, and the O II and O lines with samples exhibiting a comparable emission line at 467.7 nm.On the other hand, the air spectrum is clearly pattern (in our case PMMA) were discussed. In order to seen including the nitrogen lines at 344.7, 386.3, 386.7, 395.6, remove the air spectrum, without the requirements of working 399.5, 404.3, 424.3, 444.7, 461.4 and 463.1 nm which did not under a controlled atmosphere, two different strategies were appear in the net spectrum.The effect of the spatial resolution on the S/B was followed by monitoring the C II line at 388.7 nm while the integration time was increased. The results are shown in Fig. 4. When the laser was defocused (top), the curve follows an exponential Fig. 4 Variation of S/B with integration time when sample was Fig. 5 Removal of interferences in focus-placed PMMA samples by located at the laser focus (bottom) or defocused (top).Delay time: 0 ns. Signal was taken by using the 388.7 nm C II line, and background time-resolved data acquisition. (Top) Delay: 0 ns. (Bottom) Delay: 1500 ns. was obtained by monitoring the signal at 391.4 nm. Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 4437 Talmi, Y., Sieper, H. P., and Moenke-Blankenburg, L., Anal. followed. On the one hand, the defocusing of the sample Chim. Acta, 1981, 127, 71. produced a physical separation between PMMA and air 8 Piepmeier, E.H., and Osten, D. E., Appl. Spectrosc., 1971, 25, 642. plasmas, which were well resolved by using the spatial reso- 9 Brosda, B., Castell-Mun�oz, R., and Kunze, H.-J., J. Phys. D, 1990, lution capability of a CCD. After analysing both spectra, 23, 735. the masking of C and O emission lines from the PMMA by 10 Kurniawan, H., Kobayashi, T., and Kagawa, K., Appl. Spectrosc., 1992, 46, 581. the air continuum emission was avoided. On the other hand, 11 Grant, K.G., and Paul, G. L., Appl. Spectrosc., 1990, 44, 1349. the different rates of decay of PMMA and air spectra were 12 Owens, M., and Majidi, V., Appl. Spectrosc., 1991, 45, 1463. used to obtain selectively the PMMA emission spectrum by 13 Lee, Y.-I., Thiem, T. L., Kim, G., Teng, Y., and Sneddon, J., Appl. introducing a 1500 ns delay time. The effect is similar to that Spectrosc., 1992, 46, 11. obtained by spatial resolution. 14 Hermann, J., Boulmer-Leborgne, C., Dubreil, B., and Mihailescu, J. N., J. Appl. Phys., 1993, 74, 5. 15 Joseph, M. R., Xu, N., and Majidi, V., Spectrochim. Acta, Part B, 1994, 49, 89. REFERENCES 16 Radziemski, L. J., Loree, T. R., Cremers, D. A., and Hoffman, N. M., Anal. Chem., 1983, 55, 1246. 1 Cabalý�n, L. M., Calvo, N., Ayala, L., and Laserna, J. J., Quý�m. 17 Stoffels, E., van de Weijer, P., and van der Mullen, J., Spectrochim. Anal., 1993, 12, 96. Acta, Part B, 1991, 46, 11. 2 Laserna, J. J., Calvo, N., and Cabalý�n, L. M., Anal. Chim. Acta, 18 Nordstrom, R. J., Appl. Spectrosc., 1995, 49, 1490. 1993, 289, 113. 19 Mao, X. L., Shannon, M. A., Fernandez, A. J., and Russo, R. E., 3 Wachter, J. R., and Cremers, D. A., Appl. Spectrosc., 1987, 41, 1042. Appl. Spectrosc., 1995, 49, 1054. 4 Morris, J. B., Forch, B. E., and Miziolek, A. W., Appl. Spectrosc., 1990, 44, 1040. Paper 6/05763B 5 Ng, K. C., Ayala, N. L., Simeonsson, J. B., and Winefordner, Received August 19, 1996 J. D., Anal. Chim. Acta, 1992, 269, 123. 6 Iida, Y., Spectrochim. Acta, Part B, 1990, 45, 1353. Accepted November 18, 1996 444 Journal of Analytical Atomic Spectrometry, April 1997,

ISSN:0267-9477    

DOI:10.1039/a605763b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Comparative Study of Several Nebulizers in Inductively Coupled Plasma Atomic Emission Spectrometry: Low-pressure versus High-pressure Nebulization JUAN MORA, JOSE� L. TODOLI�, ANTONIO CANALS* AND VICENTE HERNANDIS Departamento de Quý�mica Analý�tica, Universidad de Alicante, 03071 Alicante, Spain Five nebulizers for use in ICP-AES were compared. Two of Thus, pneumatic nebulizers are those in which aerosol is generated by the strip and subsequent break-up of the liga- them work at low pressure, a Meinhard and a V-groove nebulizer (VGN), and three at high pressure, a single-bore ments generated from a liquid bulk when it is exposed to a high-velocity gas stream.2,3 Although these nebulizers are very high-pressure pneumatic nebulizer (SBHPPN), a hydraulic high-pressure nebulizer and a thermospray (TN).The common, in particular the concentric type, they suffer from some drawbacks that limit their use as liquid sample introduc- comparison was made using three solvents, water, ethanol and butan-1-ol, using the sample uptake rate (Q l ) as a variable and tion systems (e.g., low analyte transport rates, tendency to become clogged).1 Because of this, nebulizers have been devel- studying its influence on drop size distribution, analyte transport rate and analytical behaviour, i.e., emission intensity oped in which liquid and gas streams interact more efficiently.For some of them, the nebulization principle is not pneumatic, and limits of detection (LODs).The sample introduction system includes a desolvation unit. The Sauter mean diameters whereas for the rest the interaction between the liquid and gas streams has been improved by reducing the cross-sectional of the primary aerosols generated by the high-pressure nebulizers (HPNs) are between 1.5 and 5.8 times lower than area of the gas outlet4–7 and/or the width of the liquid conduction walls.6 Nevertheless, a lower gas section implies a those generated by the low-pressure nebulizers (LPNs), this reduction being more noticeable at high liquid flow rates. In higher gas pressure to keep the gas flow constant.Therefore, gas has to be supplied at a pressure higher than usual, and addition, at high liquid flow rates, HPNs achieve higher analyte transport rates (between 2.4 and 19 times higher), connecting lines and apparatus should withstand this pressure. These requirements increase the cost of the nebulizer and make higher emission signals (up to 1.8 times for methanol and up to 4.5 times for water, using the Mn II 257.610 nm line) and it more difficult to use.Recently, a pneumatic nebulizer which works at high gas lower LODs for nine elements than the LPNs. Among HPNs, the SBHPPN gives rise to the best results at low Q l (i.e., and liquid pressures [single-bore high-pressure pneumatic nebulizer (SBHPPN)] was developed in our laboratories.8 The 0.6 ml min-1), whereas at high Q l (i.e., 1.2 ml min-1) the results are similar for all three HPNs when using methanol SBHPPN affords finer primary aerosols, higher analyte transport rates to the atomization cell, higher sensitivities and lower and butan-1-ol.With water, at high Q l, the TN gives the best results. For all the nebulizers tested, organic solvents limits of detection (LODs) than a conventional pneumatic concentric nebulizer (Meinhard type). So far the SBHPPN has (methanol and butan-1-ol ) provide better results than water, the relative improvement being more important for LPNs been applied in FAAS,8 ICP-AES9 and ICP-MS.10 The so-called thermospray (TN) is a kind of nebuli- (e.g., with VGN at 1.2 ml min-1, the improvement with methanol over water for Mn II is around sixfold) than for zer that offers many advantages, although some limitations, in comparison with pneumatic nebulizers.11,12 This nebulizer can HPNs (e.g., when SBHPPN is used at 1.2 ml min-1 for Mn II this improvement is 4.5-fold).be considered pneumatic in nature, since nebulization takes place by interaction between the liquid stream and a gas Keywords: Inductively coupled plasma atomic emission stream generated through the evaporation of a fraction of the spectrometry ; pneumatic nebulizers; thermospray; hydraulic solvent.13 Recently, the fundamental processes of thermal nebu- nebulizer ; high-pressure nebulization ; drop size distribution ; lization have been studied experimentally.14 As with SBHPPN, analyte transport rate; desolvation the gas and liquid streams emerge from the nebulizer through a single bore.The TN has been widely applied in FAAS,14,15 In recent years, much effort has been dedicated to liquid ICP-AES11,13,16 and ICP-MS.17 The LODs achieved with the sample introduction in atomic spectrometry as a means of thermospray are similar to those obtained with an ultrasonic improving the analytical response. Most of the research has nebulizer.16 been devoted to the development of new and more efficient The hydraulic high-pressure nebulizer (HHPN) is another systems of aerosol generation and transport.1 highly efficient nebulizer which generates the aerosol by making The aerosol generation process (i.e., nebulization) requires a high-speed liquid stream impact against a solid surface (cloud the supply of energy to a liquid bulk by means of a nebulizer.converter). Its analytical behaviour has been tested in FAAS,18 The first step is the generation of some wave-like perturbances ICP-AES19 and ICP-MS,20 and compares favourably with that on the liquid’s surface.The growth and subsequent break-up of a pneumatic concentric nebulizer. of these waves give rise to the droplets of the final liquid Operation of the last three nebulizers (SBHPPN, TN and aerosol.2 The characteristics of these aerosols are very depen- HHPN) requires the use of HPLC pumps and pressuredent on the amount of available energy and on the efficiency resistant transfer lines.For this reason, they will be referred of the energy transfer, whatever the kind of energy employed to as high-pressure nebulizers (HPNs) throughout the rest (kinetic, thermal, acoustic, etc.). of the paper. Accordingly, the conventional pneumatic Usually, nebulizers have been classified on the basis of the nebulizers used in this study will be collectively referred to as low-pressure nebulizers (LPNs). type of energy employed in the break-up of the liquid stream.Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (445–451) 445Table 1 Main dimensions of the nebulizers and working conditions In general terms, the analytical performance of HPNs is superior to that of LPNs. However, HPNs also suffer from Nebulizer — some drawbacks: (i) they are more expensive; (ii) they are MN Meinhard TR-30-A3 more difficult to use; and (iii) they require a desolvation unit Gas outlet cross-sectional area: to avoid the negative effects of a too high solvent load to the 2.83×10-2 mm2 Liquid outlet cross-sectional atomizer.The last problem is more important when working area: 12×10-2 mm2 with organic solvents.9,14 VGN Gas outlet cross-sectional area: So far, most HPN evaluation studies have been carried out 5.66×10-2 mm2 by comparing them one by one with conventional pneumatic TN Outlet cross-sectional area: nebulizers, such as concentric, cross-flow and V-groove 1.27×10-2 mm2 types.9,10,16,19,20 Fraction of solvent vaporized: ~90%14 This study was aimed at evaluating the behaviour in ICPHHPN Outlet cross-sectional area: AES of the three above-mentioned HPNs and two LPNs with 0.31×10-3 mm2 water, methanol and butan-1-ol as solvents.The sample uptake Nebulizer tip to cloud converter rate (Ql) was used as an independent variable and the mean gap: 15mm drop size of the primary aerosol, the analyte transport rate to SBHPPN Outlet cross-sectional area: the atomizer, the emission signal and the LOD were used as 5.92×10-3 mm2 experimental parameters for comparison.To the authors’ Desolvation system — knowledge, this is the first attempt to compare the HPNs with Temperature of the heating 180 each other, working under the same set of experimental unit/°C conditions. Temperatures of the condensation units/°C First step 20 EXPERIMENTAL Second step 0 Five nebulie used, a Meinhard-type pneumatic concen- Plasma operating conditions — tric nebulizer (Meinhard, Santa Ana, CA, USA) (MN) and a Incident power/kW 1.0 V-groove nebulizer (Varian, N.Springvale, Australia) (VGN) Reflected power/W <5 as LPNs, and a laboratory-made TN, an HHPN (Knauer, Integration time/s 0.2 Berlin, Germany) and a laboratory-made SBHPPN as HPNs. Outer gas flow rate/l min-1 16.0 Intermediate gas flow rate/ 1.7 Table 1 shows the most relevant dimensions of each nebulizer. l min-1 The solution was fed at sample uptake rates between 0.6 and Nebulizer – carrier gas flow 0.33 1.2 ml min-1 by means of an Iso-Chrom HPLC pump rate/l min-1 (Spectra-Physics, San Jose, CA, USA) equipped with a pulse Observation height (mm 7 damper (a stainless-steel capillary, 0.50 mm id×1.59 mm above load coil)* od×1.50 m) placed at the outlet of the pump.The nebulizer– Torch Fassel type (4 mm id) Sample uptake rate Variable carrier gas flow was kept constant throughout at 0.33 l min-1 by means of a Model FC260 mass flow controller * Optimized for Mn II line and for all the nebulizers and solvents (Tylan, Torrance, CA, USA).This gas flow was the optimum tested at Qg=0.33 l min-1. in terms of emission signal for all the conditions studied. All the reagents and solvents were of analytical-reagent grade. Table 2 lists the most significant physical properties of Table 2 Physical properties of the solvents (20 °C) the solvents used. Solvent s*/dyn cm-1 g†/cP a‡ w§ All measurements were made in triplicate.Water 70.4 1.00 0.08 1700 Drop size distributions (DSDs) of the primary aerosols were Methanol 22.7 0.60 1.00 693 measured by means of a Model 2600c laser Fraunhofer diffrac- Butan-1-ol 22.8 2.38 0.11 502 tion system (Malvern Instruments, Malvern, Worcestershire, UK). The measurement of the DSDs was made at a down- * Surface tension. stream distance of 11 mm from the nebulizer tip, or from the † Viscosity. back of the cloud converter in the case of the HHPN. In this ‡ Relative volatility, defined as the ratio of liquid volume of solvent to the liquid volume of methanol necessary to saturate a given case, the measurement position was varied vertically so that empty volume.21 the laser beam would intercept the maximum aerosol concen- § Expansion factor, defined as the volume of gas produced by the tration zone.A lens with a focal length of 63 mm was used, evaporation of a unit volume of liquid solvent at its boiling which enabled the system to measure droplets with diameters temperature.14 between 1.2 and 118 mm.The software employed was version B.0D.22 The calculations to transform the energy distribution into size distribution were made using a model-independent temperatures. A Model F3-K thermostated bath (Haake, Karlsruhe, Germany) was used to control the temperature of algorithm that does not preclude any particular distribution function. the second condenser. The desolvation conditions are given in Table 1.The fraction of solvent vaporized inside the TN (Fv) was estimated by using the volume concentration (VC) value of the The analyte transport rate (Wtot) was measured by nebulizing a solution of Mn (100 mg ml-1) and trapping the aerosol at primary aerosol22 as described in a previous paper.14 All the nebulizers were coupled to a desolvation system the exit of the desolvation unit with glass-fibre filters (Type A/E, 47 mm diameter) of 0.3 mm pore size (Gelman, Ann Arbor, which consisted of a heating unit and a condensation unit.Fig. 1 shows a schematic diagram of the desolvation system MI, USA). Collected aerosols were washed out from the filters into calibrated flasks with 1.0% (m/m) hot nitric acid. The used. The first unit was a heating tape (265 W and 15.0 cm length; J. P. Selecta, Barcelona, Spain) coiled around a single- analyte content in each calibrated flask was determined by ICP-AES. One might question whether the analyte is com- pass spray chamber.Temperature was controlled by means of a contact thermometer. The vapour condensation unit featured pletely collected in a filter such as this, placed at the exit of the desolvation unit. Obviously, dry particles smaller than two Liebig condensers (30 cm×1.5 cm id) kept at different 446 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12driven and water cooled, with an operating frequency of 40.68 MHz. The rf power was computer controlled with an automatic matching network.The spectrometer was controlled by an IBM PC. RESULTS AND DISCUSSION Characteristics of the Primary Aerosols Aerosol characteristics determine, to a large extent, transport rate and analytical behaviour in atomic spectrometry. This section describes the influence of the sample uptake rate on the properties of the aerosols generated by the different nebulizers. The nebulizers tested have very different aerosol generation mechanisms. Some of the variables are specific (e.g., the fraction of solvent evaporated for the TN and the nozzle–cloud converter gap for the HHPN).In this study, the specific variables were kept constant for each nebulizer at values close to the optimum. The sample uptake rate is the only variable which is common for all nebulizers. Fig. 1 Schematic diagram of the desolvation system. The Sauter mean diameter (D3,2) was chosen to describe the central tendency of the DSD since it is the most commonly 0.3 mm would pass across the filter.However, this situation used diameter. In addition, other parameters such as aerosol does not apply here, where the aerosol leaving the condensation velocity, cone angle and nebulization yield were observed unit is never completely dry, since owing to the nucleation visually. processes that take place on cooling, the particles do contain liquid. This nucleation effect has two important consequences. Sauter mean diameter of primary drop size distributions First, particles that would be completely dry at the exit of the Fig. 2 shows the variation of D3,2 with Ql for each nebulizer heating unit, and the size of which would be smaller than and solvent.Two well differentiated behaviours appear. First, 0.3 mm at this point, would increase their size in the conden- for TN and HHPN, D3,2 decreases as Ql is increased. This sation unit and be mostly retained by the filter. Second, the effect can be accounted for by the concomitant increase in the filter is wetted when these drops are retained, thus reducing available energy for nebulization.Thus, when working with the pore size. Therefore, the effective pore size would be smaller the TN, an increase in Ql leads to a similar increase in the than the nominal value of 0.3 mm. In addition, it has been nebulization gas flow rate, since the fraction of solvent evapor- checked experimentally, by measuring the DSDs of aerosols ated, Fv, is almost independent of Ql.For instance, when leaving the desolvation system, that the most important frac- nebulizing water under the experimental conditions employed, tion of the aerosol volume is contained in particles between the volumetric gas flow rate increases from 0.92 l min-1 at 1.2 and 10 mm, so that the analyte mass fraction contained in Q1=0.6 ml min-1 to 1.84 l min-1 at Q1=1.2 ml min-1.14 For droplets the diameter of which is smaller than 0.3 mm would the HHPN, a similar increase in Ql multiplies by a factor of be negligible in comparison with the analyte contained in four the kinetic energy available for the nebulization process.droplets larger than 0.3 mm. For the other nebulizers, D3,2 is almost independent of Ql. The experiments on the analytical behaviour in ICP-AES Working with the SBHPPN requires the gas pressure to be were performed with a solution containing 1 mg ml-1 of a total slightly increased as the sample uptake rate is increased, so as of nine elements, prepared from a 1000 mg ml-1 reference to keep gas flow constant.Therefore, the energy of the gas solution (ICP multielemental standard solution IV; Merck, stream also increases, as Ql is increased.8,9 However, this gain Darmstadt, Germany). Table 3 lists the elements, wavelengths in energy is much lower for SBHPPN than for TN and HHPN, and slit widths employed. A Model 2070 ICP-AES instrument so that D3,2 scarcely varies with Ql for the former.For LPNs, made by Baird (Bedford, MA, USA) was used and operated since the energy of the gas stream does not depend on Ql , the under the conditions given in Table 1. An air-vacuum path, kinetic energy per unit mass of liquid decreases as Ql is 1 mfocal length Czerny–Turner monochromator with a grating increased, so that the primary aerosol is slightly coarser. of 1800 grooves mm-1 blazed at 400 nm was employed. The In addition, Fig. 2 shows that the extent of the variation of scanning speed was 400 nm s-1 and the bandpass was 0.01 nm D3,2 with Ql for a given nebulizer is a function of the solvent full width at half-maximum.Detection was effected by means used. Thus, for TN and water, D3,2 decreases from 8.27 to of two photomultipliers, one for the 160–290 and the other for 1.68 mm when Ql is increased from 0.6 to 1.2 ml min-1 (i.e., a the 290–900 nm range. The viewing height was controlled 79.7% decrease). However, with methanol or butan-1-ol, manually.The rf generator was crystal controlled, solid-state D3,2 decreases by about 42%. Further, for TN, at Ql = 1.2 ml min-1, D3,2 is about the same for all three solvents. Table 3 Elements, wavelengths and slit widths used This result can be accounted for by taking into account that, in thermal nebulization, the energy available for nebulization Element line Wavelength/nm Slit width*/nm depends not only on Fv and Ql, but also on the so-called Mn II 257.610 0.2 expansion factor (w) of the solvent.14 Water is the solvent with Ag I 328.068 0.3 the highest w value (Table 2), hence its volumetric gas flow Cd I 228.802 0.1 Co II 228.616 0.2 rate is much higher than that of methanol and butan-1-ol. Cr II 283.563 0.2 This higher energy of water vapour counterbalances the nega- Cu I 324.754 0.4 tive effect of its higher surface tension.14 HHPN gives rise to Fe II 259.940 0.2 similar D3,2 decreases for all three solvents (from 44 to 54%) Zn I 213.856 0.3 when Ql is increased from 0.6 to 1.2 ml min-1.It is also worth Ni II 221.647 0.1 noting that butan-1-ol affords, with the HHPN, higher D3,2 values than water and methanol. This can be explained in * Optimized for each element. Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 447Fig. 2 Variation of the Sauter mean diameter, D3,2, with the sample uptake rate, Ql, for all the nebulizers tested. A, SBHPPN; B, TN; C, HHPN; D, VGN; and E, MN. (a) Water; (b) methanol; and (c) butan-1-ol.Error bars represent the range between the lower and upper values. terms of the high viscosity of butan-1-ol, since in hydraulic a dense cloud moving slowly and carried by the gas stream, so that coalescence losses and gravitational settling will appear. nebulization viscosity contributes to damping the disturbances that appear on the liquid’s surface before and when the liquid In addition, one should take into account that the nebulization yield is not 100% for the HHPN (i.e., not all the liquid flow stream strikes the cloud converter.2 With the pneumatic nebulizers, SBHPPN and LPNs, the role of the viscosity is less is nebulized). A significant fraction (usually 20–50%) of this liquid flow goes to drain at the cloud converter.23 Although significant.Therefore, methanol and butan-1-ol afford lower D3,2 values than water, owing to their lower surface tension the aerosol generated by the HHPN is fine and slow, the analyte transport rate is mainly determined by the aerosol yield.(Table 2).5,9 It is apparent from Fig. 2 that HPNs generate finer aerosols The TN requires some specific comments, since most of the liquid is vaporized (in our case, 90% of the initial liquid than LPNs. Among HPNs, the SBHPPN provides the smallest D3,2 values at low flow rates (e.g., 0.6 ml min-1), whereas the volume is converted into vapour). This causes the analyte concentration to increase with respect to the initial level.TN performs the best at high Ql values (e.g., 1.2 ml min-1). These results can be explained by the energy available for Moreover, as the aerosols leaving the TN are at high temperature, evaporation will take place fairly rapidly. Nevertheless, nebulization in each case. As regards the solvent, Fig. 2 shows that when water is used, at Ql=1.2 ml min-1, the SBHPPN the fact that solvent condensation will be favoured since the solvent vapour expansion and evaporation itself are endo- gives higher D3,2 than the TN, whereas when using methanol or butan-1-ol the D3,2 values obtained with the SBHPPN are thermicprocesses should be taken into account.As evaporation depends on drop size, the smallest droplets will evaporate closer to those of the TN.From these results, it can be concluded that the SBHPPN more quickly than the coarsest droplets, so that an irregular distribution of the analyte into the aerosol is to be expected. summarizes the characteristics of high-pressure nebulization (low D3,2 values) and pneumatic nebulization (finer aerosols for organic solvents than for water).Analyte Transport Rate Fig. 3 shows the influence of Ql on the analyte transport rate, Other dynamic characteristics of the aerosols Wtot. The general trend is that Wtot increases as Ql is increased. This behaviour is inversely related to the variation of the Some other characteristics are also important in order to describe the aerosol (e.g., aerosol velocity, cone angle and aerosol mean drop size as Ql is increased (Fig. 2). Thus, in general, a marked decrease in D3,2 corresponds to a noticeable nebulization yield) since they have a noticeable influence on the transport variables. This section includes a brief description increase in Wtot, whereas slight variations in D3.2 give rise to slight variations in Wtot . In addition to the mean drop size, of these characteristics, based on visual inspection of the aerosols.the nebulization yield must also be taken into consideration, for the HHPN, since an increase in Ql causes the nebulization The aerosol velocity determines, to a large extent, the analyte transport rate. If the aerosol velocity is too high, an important yield to increase (e.g., for water it increases from 24.8 to 46.0% when Ql is increased from 0.6 to 1.2 ml min-1). fraction of the finest droplets will be lost by inertial impact against the bottom walls of the spray chamber.If this velocity As expected from the drop size results (Fig. 2), Fig. 3 shows that under most conditions, HPNs provide higher Wtot values is too low, droplets will be lost by gravitational settling.4 Moreover, when a desolvation unit is used, as in our case, than LPNs. At low sample uptake rates, the SBHPPN gives the highest Wtot values with all the solvents studied, whereas aerosol heating is more efficient if the aerosol velocity is low. With the SBHPPN and the TN, the aerosol velocity is higher with water and butan-1-ol, at high Ql, the TN is the nebulizer that performs the best.For the SBHPPN at 0.6 ml min-1, than with the LPNs, whereas the HHPN generates the slowest aerosols. analyte transport efficiencies are between 21 and 32% for water and methanol, respectively, whereas for LPNs these The cone angle of the aerosol is related to its velocity. The higher the velocity of the aerosol, the sharper is its cone angle. values are 7 and 20%.On the other hand, with the TN at 1.2 ml min-1, the analyte transport efficiencies are between 20 A large cone angle contributes to an increase in the amount of aerosol lost by impact against the side walls of the spray and 25% for water and methanol, respectively, and with the SBHPPN 11 and 30%. These values are between 1.5 and chamber.4 In agreement with their aerosol velocity, the SBHPPN and the TN generate aerosols the cones of which 12.5% for LPNs. As regards the solvents, it can be seen from Fig. 3 that, in are sharper than those corresponding to the LPNs. In the case of the HHPN, aerosol is not conical in shape but is rather like general, organic solvents afford higher Wtot than water.9,14,21 448 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Fig. 3 Variation of the analyte transport rate, Wtot, with the sample uptake rate, Ql, for all the nebulizers tested. A, SBHPPN; B, TN; C, HHPN; D, VGN; and E, MN. (a) Water; (b) methanol; and (c) butan-1-ol.Error bars represent the range between the lower and upper values. Methanol is the solvent which shows the highest Wtot values and HHPN, whereas at Ql=1.2 ml min-1 the improvements in Wtot are more important for LPNs than for HPNs. in all the cases. Table 4 shows the enhancement ofWtot achieved on switching from water to methanol or butan-1-ol, (Wtot)rel. It can be seen that (Wtot)rel decreases when Ql increases for HHPN and TN whereas it increases for the other nebulizers Analytical Behaviour (i.e., pneumatic).This reveals the different aerosol generation Emission intensity mechanisms. For the five nebulizers, methanol provides higher (Wtot)rel Fig. 4 shows the variation of the net emission intensity (Inet) of Mn in ICP-AES versus Ql for each nebulizer using (a) water, values than does butan-1-ol. This is accounted for by the lower D3,2 values obtained with methanol and its higher relative (b) methanol and (c) butan-1-ol as solvents. It can be seen that the variations of Inet are concomitant with those of Wtot volatility (Table 2).As regards the nebulizer, it appears that at a Ql of 0.6 ml min-1 the largest improvements in (Wtot)rel (Fig. 3). Hence, in most cases, HPNs provide higher Inet values than do LPNs. The SBHPPN gives the highest Inet values at caused by switching from water to alcohols correspond to TN Table 4 Analyte transport rates for methanol and butan-1-ol relative to water for the different nebulizers and sample uptake rates tested (Wtot)rel* Ql/ml min-1 Solvent MN VGN TN HHPN SBHPPN 0.6 Methanol 2.89 3.00 3.44 6.18 1.54 0.8 Methanol 4.69 5.33 2.15 2.21 2.33 1.0 Methanol 5.73 6.70 1.74 1.89 2.61 1.2 Methanol 9.09 12.65 1.22 1.31 2.63 0.6 Butan-1-ol 1.35 1.32 1.86 2.10 1.41 0.8 Butan-1-ol 2.38 2.43 1.26 1.17 1.64 1.0 Butan-1-ol 3.89 4.36 0.88 1.16 1.87 1.2 Butan-1-ol 6.32 7.60 0.64 0.78 2.05 * (Wtot )rel=(Wtot)solvent i/(Wtot)water. Fig. 4 Variation of the net emission intensity for manganese, Inet , with the sample uptake rate, Ql, for all the nebulizers tested.A, SBHPPN; B, TN; C, HHPN; D, VGN; and E, MN. (a)Water; (b) methanol; and (c) butan-1-ol. Error bars represent the range betweenthe lower and upper values. Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 449low Ql for all the solvents. However, at higher sample uptake obtained with the three HPNs are similar, perhaps the LODs for water with the TN [Fig. 6(a)] are slightly lower than with rates the relative behaviour of the HPNs depends mainly on the solvent. Thus, the TN gives the highest Inet values for the HHPN and the SBHPPN, and lower than those obtained with LPNs. water [Fig. 4(a)] and butan-1-ol [Fig. 4(c)], and the SBHPPN for methanol [Fig. 4(b)]. As regards the solvent, again in good agreement with the transport results (Fig. 3), the Inet obtained with methanol are CONCLUSIONS higher than those obtained with butan-1-ol and water.As High-pressure nebulization is a good means for liquid sample occurs with Wtot, switching from water to alcohols leads to an introduction in ICP-AES. The analytical figures of merit are increase in Inet that is relatively more important for LPNs usually better than those obtained using the conventional low- than for HPNs. Thus, the Inet values obtained with methanol pressure pneumatic nebulizers. HPNs generate finer aerosols are around six times higher than those for water when MN than do LPNs, thus giving rise to higher Wtot, higher Inet and and VGN are used.In the case of HPNs this factor is up to lower LODs. However, HPNs are more difficult to use and 4.5 for the SBHPPN. more demanding in terms of requirements. The relative behaviour shown by the HPNs depends on the L imits of detection (L ODs) solvent and Ql employed. At low flow rates, the SBHPPN provides the highest sensitivity and lowest LODs, whereas at Figs. 5 and 6 show the LODs for nine elements at Ql values high Ql with water, the TN performs the best.The results of 0.6 ml min-1 (Fig. 5) and 1.2 ml min-1 (Fig. 6) for each obtained with HPNs could be improved by reducing the outlet solvent and nebulizer tested. As expected from the results for section area of the nebulizer tip. Inet (Fig. 4), the LODs follow the order water>butan- Provided that a desolvation system is employed, switching 1-ol>methanol. This behaviour is clearer for 1.2 ml min-1 from water to alcohols improves the drop size distribution of (Fig. 6) than for 0.6 ml min-1 (Fig. 5). As regards the nebulizer the aerosol, the analyte transport and the emission intensity tested, Figs. 5 and 6 show that the improvement in LOD for all the nebulizers tested. introduced by HPNs depends on the value of Ql. Thus, at Ql=0.6 ml min-1 (Fig. 5) the SBHPPN gives the lowest LOD, in agreement with the corresponding Inet values (Fig. 4), especi- The authors thank the DGICyT (Spain) for financial support (Project PB92-0336).ally for water [Fig. 5(a)]. At 1.2 ml min-1 (Fig. 6), the LODs Fig. 5 Limits of detection in ICP-AES with the different nebulizers evaluated, calculated according to the 3sb criterion, sb being the standard deviation obtained from 20 replicates of the blank. (a) Water; (b) methanol; and (c) butan-1-ol. Ql=0.6 ml min-1. 450 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Fig. 6 Limits of detection obtained in ICP-AES with the different nebulizers evaluated, calculated according to the 3sb criterion, sb being the standard deviation obtained from 20 replicates of the blank.(a) Water; (b) methanol; and (c) butan-1-ol. Ql=1.2 ml min-1. 13 Koropchak, J. A., and Winn,D. H., Appl. Spectrosc., 1987, 41, 1311. REFERENCES 14 Mora, J., Canals, A., and Hernandis, V., Spectrochim. Acta, Part 1 Sample Introduction in Atomic Spectroscopy, ed. Sneddon, J., B, 1996, 51, 1535. Elsevier, New York, 1990. 15 Robinson, J. W., and Choi, D. S., Spectrosc. L ett., 1987, 20, 375. 2 Atomization and Sprays, ed. Lefebvre, A. H., Hemisphere, New 16 Vermeiren, K. A., Taylor, P. D. P., and Dams, R., J. Anal. At. York, 1989. Spectrom., 1988, 3, 571. 3 Sharp, B. L., J. Anal. At. Spectrom., 1988, 3, 613. 17 Thomas, C., Jakubowski, N., Stu�wer, D., and Broekaert, J. A. C., 4 Canals, A., Hernandis, V., and Browner, R. F., Spectrochim. Acta, J. Anal. At. Spectrom., 1995, 10, 583. Part B, 1990, 45, 591. 18 Berndt, H., Fresenius’ Z. Anal. Chem., 1988, 331, 321. 5 Canals, A., Hernandis, V., and Browner, R. F., J. Anal. At. 19 Luo, S. K., andBerndt, H., Spectrochim. Acta, Part B, 1994, 49, 485. Spectrom., 1990, 5, 61. 20 Jakubowski, N., Feldmann, I., Stuewer, D., and Berndt, H., 6 Nixon, D. E., Spectrochim. Acta, Part B, 1993, 48, 447. Spectrochim. Acta, Part B, 1992, 47, 119. 7 Olesik, J. W., Kinzer, J. A., and Harkleroad, B., Anal. Chem., 21 Browner, R. F., Canals, A., and Hernandis, V., Spectrochim. Acta, 1994, 66, 2022. Part B, 1992, 47, 659. 8 Todolý�, J. L., Canals, A., and Hernandis, V., Spectrochim. Acta, 22 Instruction Manual, Issue 2.2, Version B.0D, Malvern Instruments, Part B, 1993, 48, 373. Malvern, 1991. 9 Todolý�, J. L., Canals, A., and Hernandis, V., J. Anal. At. Spectrom., 23 Cano, J. M., Todolý�, J. L., Canals, A., and Hernandis, V., paper 1996, 11, 949. presented at the XIV Reunio�n Nacional de Espectroscopý�a, Baeza 10 Hernandis, V., Todolý�, J. L., Canals, A., and Sala, J. V., (Spain), September 1994. Spectrochim. Acta, Part B, 1995, 50, 985. 11 Meyer, G. A., Roeck, J. S., and Vestal, M. L., ICP Inf. Newsl., Paper 6/06781F 1985, 10, 955. Received October 3, 1996 12 Koropchak, J. A., and Veber, M., Crit. Rev. Anal. Chem., 1992, 23, 113. Accepted January 2, 1997 Journal of Analytical Atomic Spectrometry, April 199

ISSN:0267-9477    

DOI:10.1039/a606781f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Fullerene a Sensitive and Selective Sorbent for the Continuous Preconcentration and Atomic Absorption Determination of Cadmium YANEIRA PETIT DE PEN� Aa MERCEDES GALLEGOb AND MIGUEL VALCA� RCEL*b aDepartment of Chemistry. University of L os Andes Me� rida Venezuela bDepartment of Analytical Chemistry Faculty of Sciences University of Co� rdoba E-14004 Co� rdoba Spain preconcentration is attracting much interest although there seems to be an unnecessary proliferation of reagents used; the most recent advances in this technique as regards the preconcentration of cadmium and other metals in biological and environmental samples by flow injection (FI) for subsequent determination by atomic spectrometry have been reviewed14 and were compiled in a recent monograph.15 A silica gel sorbent loaded with sodium diethyldithiocarbamate (Na-DDC) was used for the preconcentration of cadmium prior to its determination by FAAS; under dynamic conditions large sample volumes (about 200 ml) provided recoveries of 96.2%.16 Fang et al.17 developed an on-line FI system for the preconcentration of cadmium; the metal was precipitated from digested hair and rice with Na-DDC sorbed on a reactor eluted with isobutyl methyl ketone (IBMK) and determined by FAAS.From the foregoing it is obvious that the use of an on-line incorporated sorbent in an analytical system considerably improves the analytical sensitivity and results in a high sampling frequency which justifies current trends towards an increasing use of this preconcentration technique.On the other hand fullerenes have a high analytical potential for metal preconcentration even though only two papers have so far dealt with their performance in this respect. In this work a previously reported simple FI system11 was used to assess the potential of C60 fullerene for preconcentrating cadmium. For this purpose ammonium pyrrolidinedithiocarbamate (APDC) and 8-hydroxyquinoline were used to form neutral chelates of the metal. The effect of various ions on the determination was investigated in order to evaluate the selectivity of the sorbent. A complementary comparative study on the adsorption of the chelates on fullerene and silica RP-C18 phases was also conducted. Finally three biological reference materials were analysed as samples in order to test the performance of the proposed method.Ultratrace levels of cadmium were quantitatively sorbed on a C60 fullerene mini-column to form neutral chelates which were eluted with 200 ml of isobutyl methyl ketone and transferred to an atomic absorption spectrometer. Two chelating reagents viz. ammonium pyrrolidinedithiocarbamate (APDC) and 8-hydroxyquinoline were tested in a simple flow system. The adsorption constant was dramatically increased for the APDC reagent and C60 fullerene and cadmium was selectively separated from co-existing copper lead zinc and iron among other metals. Similar experiments were performed in parallel achieved was 110 and the detection limit was 0.3 ng ml-1 Cd. by using C18-bonded silica as sorbent.The concentration factor For analytical validation cadmium was determined in certified reference biological samples; only the APDC method with C60 fullerene as sorbent provided accurate results. Keywords Fullerene sorbent; preconcentration; chelates ; cadmium determination; atomic absorption spectrometry 60 60 18 Ever since two physicists Huffman and Kra�tschmer devised a method for producing macroscopic amounts of fullerenes in 1990 the properties of new products from these forms of carbon which include new types of polymers improved batteries superconductors catalysts etc. have been predicted but scarcely demonstrated.1 Numerous reviews on the discovery synthesis characterization reactivity and physico-chemical properties of fullerenes and related materials have been published; 2–5 there is even an on-line database of fullerene knowledge, 6 and a recent monograph about this topic has also been published.7 The main problem with fullerenes continues to be the separation of their fraction and purification of its components from the starting soot.Jinno and co-workers8,9 have published about 20 papers on the separation of fullerenes on monomeric and polymeric phases by HPLC. C fullerenebonded trimethylsilylsilica as a stationary phase for the separation of polycyclic aromatic hydrocarbons has been investigated by Jinno’s group,10 who demonstrated one of the possible uses of these new materials. The analytical potential of C60 fullerene as a sorbent material for the preconcentration of metals was first demonstrated by Gallego et al.;11 subsequent experiments with C and C70 fullerenes showed that both sorbents have a high analytical potential for metal preconcentration probably because of their large molecular surface area and volume.Higher sensitivity and selectivity are obtained with neutral chelates than by formation of ion pairs. Fullerenes perform better in metal preconcentration than do conventional solid materials (e.g. C -bonded silica activated carbon and resins).12 Several approaches have been devised for separatinganalytes from matrix elements and preconcentrating the former using a variety of techniques; however ion-exchange and chelating resins are routinely used for this purpose.13 On-line sorbent Journal of Analytical Atomic Spectrometry April 1997 Vol.12 (453–457) Apparatus A Varian (Palo Alto CA USA) 1475 atomic absorption spectrometer equipped with a bead impact system in the burner chamber and deuterium arc background correction was used throughout. The spectrometer output was connected to a Varian 9176 recorder. The hollow cathode lamp for cadmium was operated at 4 mA and the spectrometer was set at228.8 nm with a spectral band-width of 0.7 nm. The acetylene flow rate was 2.0 l min-1 and an air flow rate of 21.5 l min-1 was employed to ensure a clean blue flame. The flow manifold consisted of a Gilson (Villiers-le-Bel France) Minipuls-2 peristaltic pump furnished with poly(vinyl chloride) tubes two Rheodyne (Cotati CA USA) 5041 injection valves and labora- EXPERIMENTAL 453 tory-made sorption mini-columns packed with 80 mg of C60 fullerene or silica RP-C18.The mini-columns were made from PTFE capillaries of 3 mm id and sealed at both ends with small glass-wool plugs to prevent material losses. The minicolumns were initially flushed with 0.1 mol l-1 nitric acid; subsequent use of IBMK as eluent in each operating cycle was sufficient to make them ready for re-use. Reagents and Standard Solutions A 1000 mg l-1 cadmium stock solution was prepared by dissolving 1.000 g of the metal in a small volume of concentrated nitric acid and diluting to 1 l with 1% v/v nitric acid. A 0.1% m/v aqueous solution of APDC (Aldrich Madrid Spain) was prepared; the solution remained stable for at least 3 d.A 0.05% m/v 8-hydroxyquinoline solution (Aldrich) in 5% v/v ethanol was also prepared. IBMK (Probus Madrid Spain) was also used. C60 fullerene (>99.4% Hoechst Frankfurt-am- Main Germany) and polygosyl-bonded silica reversed-phase sorbent with octadecyl functional groups (RP-C18) 60–100 mm particle size (Millipore Madrid Spain) were employed as sorbent materials. Standard solutions (100 ml) containing 0.5–100 ng ml-1 cadmium were all freshly prepared by appropriate dilution of the stock standard solution (1000 mg l-1) in 0.1 mol l-1 nitric acid or at pH 4.5 (adjusted with dilute nitric acid) for the APDC-Cd and 8-hydroxyquinoline-Cd methods respectively. Sample Preparation The reference materials analysed were as follows NIST SRMs 1566a Oyster Tissue and 1577a Bovine Liver and Community Bureau of Reference (BCR) CRM 186 Pig Kidney.All were dried to constant mass by freeze-drying at 6 Pa (0.04 mmHg) for 24 h after which an accurately weighed amount of 1–2 g was digested with 15 ml of 65% nitric acid and 1 ml of 24.5% sulfuric acid in a glass beaker. The mixture was heated at about 200°C on a hot-plate until the sample was completely dissolved and nitrogen dioxide fumes were evolver was allowed to cool for about 2 min and the digestion procedure was repeated (about five times) with multiple additions of 5 ml of nitric acid until a clear solution was obtained and nitrogen dioxide fumes ceased to be evolved. Once cool the solution was transferred quantitatively into a calibrated flask of 50–250 ml capacity and made up to volume with ultrapure (Milli-Q) water.A reagent blank was prepared in parallel. Sub-samples (diluted 2-fold if required) at pH 1 or 4.5 (adjusted with nitric acid) were analysed immediately after preparation by introducing them into the manifold depicted in Fig. 1. On-line Preconcentration–Elution Procedure 2 The FI manifold used for on-line preconcentration and elution is shown in Fig. 1. First [Fig. 1(a)] 6 ml of standard or sample solution containing 0.5–50 ng ml-1 CdII in 0.1 mol l-1 nitric acid (for APDC) or 3–100 ng ml-1 CdII at pH 4.5 (for 8-hydroxyquinoline) were continuously pumped into the system and mixed on-line with the reagent (0.1% APDC or 0.05% 8-hydroxyquinoline in 5% ethanol).The preconcentration time was 2 min. The chelate was adsorbed on the sorbent mini-column (located in the loop of IV1) and the sample matrix sent to waste. During preconcentration an aqueous carrier was pumped to the instrument in order to record the baseline and the loop of IV was filled with eluent (IBMK). In the elution step [Fig. 1(b)] both injection valves were switched simultaneously to pass 200 ml of eluent (injected into the aqueous carrier) through the adsorbed chelate to desorb it and sweep the cadmium to the detector. Peak heights were used as analytical measurements and a blank consisting 454 Journal of Analytical Atomic Spectrometry April 1997 Vol. 12 Fig. 1 FI manifold for the preconcentration/determination of Cd.(a) Preconcentration and (b) elution. Bold lines denote lines relevant in the individual step. IV Injection valve; W waste; IBMK isobutyl methylketone (eluent); C fullerene or RP-C packedcolumn. Reagent 0.1% APDC or 0.05% 8-hydroxyquinoline18(5% ethanol–water). of 200 ml of IBMK injected prior to sample preconcentration was also used (about 0.040 A).To avoid any carry-over between samples the sample tube was filled with the next sample during the elution step. RESULTS AND DISCUSSION Aprevious study showed that the best sensitivityand selectivity of fullerenes as sorbent materials were obtained with neutral chelates.12 In order to test fullerenes for the preconcentration of cadmium two chelating agents were assessed in an FI system similar to that described elsewhere.11 Thus 8-hydroxyquinoline (a classical reagent that precipitates as a lemon-yellow complex with cadmium from dilute acetic acid neutral or ammoniacal solutions and is extractable into nonpolar organic solvents)and APDC (the most common chelating reagent for enrichment of metals in the FAAS technique) were assessed.Dithizone has been used as a chelating reagent for collecting trace amounts of cadmium on an activated carbon column by the FI technique;18 although the sensitivity achieved with this reagent was 20 and 60% higher than that obtained with APDC and 8-hydroxyquinoline respectively the selectivity was not optimum because of the non-specific character of the complexant dithizone. A comparative study of chelate retention (with APDC and 8-hydroxyquinoline) on C60 fullerene and RP-C18 was carried out here in order to select the best conditions for the determination of cadmium.range 0.01–0.3%. Both sorbents provided similar results the absorbance remaining constant above a 0.03% concentration; a 0.1% concentration of APDC in water was chosen. The influence of the 8-hydroxyquinoline concentration was examined over the range 0.01–0.05% (in 5% ethanol–water); the best results were obtained at the highest concentration tested. Because of the insolubility of the reagent in the ethanol–water mixture concentrations above 0.05% required a higher ethanol content which decreased the chelate adsorption and hence the analyticalsignal. A0.05% concentration of 8-hydroxyquinoline was selected for the two sorbent columns.Under the selected conditions the blank had a negligible effect; hence the IBMK injected before the sample served as a suitable blank. FI Preconcentration–Elution Conditions Optimization of the Chelates of APDC and 8-Hydroxyquinoline with Cadmium The effect of the APDC concentration was studied over the Initially an attempt was made to dissolve 8-hydroxyquinoline in dilute ammonia and ethanol; the best results were obtained by dissolving 50 mg of 8-hydroxyquinoline in 100 ml of a 5+95 v/v ethanol–water mixture. The effect of sample pH was examined by introducing 6 ml of a solution containing 10 ng ml-1 (for APDC) or 20 ng ml-1 cadmium (for 8-hydroxyquinoline) at a flow rate of 3.0 ml min-1 into the system for 2 min (Fig.1) the pH being adjusted with dilute nitric acid or ammonia and the reagents (0.1% APDC or 0.05% 8-hydroxyquinoline) being used as carriers. Several organic solvents were tested for elution of the adsorbed chelate from the column viz. IBMK ethanol,acetone and chloroform. The best results (difference between the sample and blank signals) were provided by IBMK where the chelate was most readily soluble and desorbed; in addition the blank The influence of the sample and reagent flow rates the length signal was lower and no dispersion occurred during transfer of the preconcentration coil and the volume of IBMK was to the detector because IBMK is immiscible with water. The studied by using a sample of 10 or 20 ng ml-1 cadmium in the two proposed methods were optimized by using a column APDC and 8-hydroxyquinoline methods respectively.The packed with 80 mg of C60 (1.1 cm×3 mm id) or 80 mg of elution steps was also investigated. Changes in the flow rate effect of the IBMK flow rate in the preconcentration and maximum chelate adsorption was achieved at pH 0.5–5.0 RP-C18 (1.6 cm×3mm id). As can be seen in Fig. 2 the of the sample (6.0 ml) between 1.0 and 3.0 ml min-1 resulted (for C60) and 0.7–2.0 (for RP-C18) for the APDC reagent flow rates through a decreased residence time. Increasing the in very small variations in the signal which decreased at higher and 3.2–6.0 (for C60) and 4.0–5.5 (for RP-C18) for reagent flow rate resulted in concomitant sample dilution and 8-hydroxyquinoline.The optimum pH range was wider for C60 fullerene probably as a consequence of its adsorption hence in decreased atomic signals. Reagent (APDC or constant being greater than that for the RP-C18 sorbent 8-hydroxyquinoline) and sample flow rates of 0.3 and consistent with experimental results obtained for the 3.0 ml min-1 respectively were chosen for the two methods and both columns (C lead–APDC chelate (adsorption constants were 575 and 155 fullerene and RP-C18). The optimum for C60 and C18 respectively).11 On the other hand the 300 cm (0.5 mm id) in all instances; hence a length of 250 cm sorption of the metal chelates is effected through the comlength of the preconcentration 60 coil ranged between 200 and plexing ligands and the results show that APDC forms was used throughout.a cadmium chelate that has a higher affinity for C60 than The volume of eluent (IBMK) played a major role in the for RP-C18 while the 8-hydroxyquinoline chelate exhibits The desorption process was found not to depend on the type chelate elution its effect being studied between 50 and 300 ml. smaller differences with both sorbents. In the APDC method samples were prepared in 0.1 mol l-1 nitric acid. In the of sorbent or chelating reagent used; elution was complete (no 8-hydroxyquinoline method samples were prepared in 0.1 mol carry over) for volumes above 150 ml. At volumes higher than l-1 acetic acid–sodium acetate buffer (pH 4.75); however the 200 ml the atomic signal difference decreased through disperanalytical signal of cadmium decreased by about 30% relative sion of cadmium in the organic solvent; the volume of eluent to samples adjusted to the same pH with dilute nitric acid finally selected was 200 ml.The aqueous flow rate (eluent probably because of the acetate anion at high concentrations carrier) was found to affect peak height which increased up complexing cadmium and hence decreasing the chelate adsorp- to 4.0 ml min-1 owing to the increasing nebulizer efficiency. tion. Consequently samples were adjusted to pH 4.5 with Based on the above results flow variables affect the dilute nitric acid in the 8-hydroxyquinoline method. determination of cadmium similarly with APDC and 8-hydroxyquinoline reagents and with either column; therefore the same flow system can be used in both methods.60 Fig. 2 Effect of pH on Cd absorbance as measured after on-line preconcentration with APDC (#) or 8-hydroxyquinoline ($) on a C fullerene or RP-C18 column. Sample 10 or 20 ng ml-1 Cd for the APDC and 8-hydroxyquinoline methods respectively. Comparison of Preconcentration Methods 18 60 fullerene was used in the present study. Two columns were A comparative study of the chelate retention of APDC and 8-hydroxyquinoline on C60 fullerene and a conventional RP-C sorbent was carried out. In the above-described experiments, 12 we compared the adsorption capacity of C60 and C70 fullerenes; the larger molecular surface area and volume of C70 make this material slightly more effective for metal preconcentration; however because C70 fullerene is very expensive only C packed with 80 mg of C60 fullerene (1.1 cm×3 mm id) or 80 mg of RP-C18 (1.6 cm×3 mm id).Both columns were intercalated sequentially into the FI system depicted in Fig. 1. By using this manifold and optimized chemical and flow variables several calibration graphs for cadmium were constructed using the two methods studied. The figures of merit of the calibration graphs (r=0.998 or 0.999 in all instances) obtained for cadmium (sample volume 6.0 ml) using the APDC and 8-hydroxyquinoline chelates with the two sorbents are summarized in Table 1. The detection limits were calculated as three times the standard deviation of the absorbance obtained for Journal of Analytical Atomic Spectrometry April 1997 Vol. 12 455 Table 1 Figures of merit for the atomic absorption determination of cadmium Range/ng ml-1 0.5–20 Regression equation* A=0.004+0.015x Method APDC–CdII‡ 2–50 3–80 4–100 A=0.002+0.006x A=0.003+0.004x A=0.002+0.003x APDC–CdII§ 8-Hydroxyquinoline–CdII‡ 8-Hydroxyquinoline–CdII§ * A absorbance; x cadmium concentration (ng ml-1 ).Sample volume 6 ml. † Compared with conventional sample introduction of an aqueous solution (A=0.005+1.35×10-4 x). ‡ With C60 fullerene. § With RP-C18. 60 15 injections of 200 ml of IBMK (blank). The precision of the method (expressed as the RSD) was evaluated on 11 samples containing 10 or 20 ng ml-1 cadmium in the APDC and 8-hydroxyquinoline methods respectively. The overall time required for preconcentration (2 min) and elution (a few seconds) of a sample was about 2.5 min; hence the throughput was about 25 samples h-1 (blank processing included).The most interesting conclusions that can be drawn from Table 1 are as follows first the sensitivity (slope of the calibration graph) is higher with APDC than with 8-hydroxyquinoline; second the sensitivity is higher for C fullerene columns in both methods probably because of the higher adsorption capacity of fullerenes.11 Therefore APDC is the better choice for the preconcentration of cadmium on C60 fullerene because it provides a preconcentration factor of 110 for a sample volume of 6 ml which can be increased by using a larger volume. The precision was similar in all instances. 18 Tolerated [metal]5[Cd] ratio 8-Hydroxyquinoline method Dithizone method* Activated carbon 18 60 18 RP-C 200 C 400 RP-C 600 1000 200 500 1000 300 60 500 500 300 500 250 200 800 800 600 800 400 400 600 600 500 400 350 300 150 200 100 150 300 300 400 600 800 500 200 400 400 Interferences The influence of metals that might react with APDC or 8-hydroxyquinoline and replace cadmium in the original chelate was investigated in order to identify potential interferences.Table 2 lists the tolerated ratios of foreign cations to cadmium; the maximum concentration tested was 1000 times that of the analyte. Interferents decreased the cadmium signal in all instances by competition for and consumption of the reagent; hence uncomplexed cadmium was not retained on the column; otherwise if all chelates were formed the volume of IBMK used was inadequate to elute them.As can be seen in Table 2 the selectivity was higher for C60 fullerene as a consequence of its large surface area relative to RP-C18 in addition to its high interstitial volume (which ensured more uniform distribution of the chelate throughout the column and hence readier elution). With the RP-C sorbent (white in colour) the chelate (at high metal concen- Table 2 Tolerated concentrations of foreign cations in the determination of 10 (with APDC) or 20 (with 8-hydroxyquinoline) ng ml-1 cadmium APDC method 60 C 1000 Ion Al3+ Zn2+ 1000 Pb2+ 1000 Mn2+ 1000 1000 Co2+ Cu2+ Fe3+ Ni2+ Sn2+ Hg2+ 1000 1000 1000 800 600 * Data from ref.18. 456 Journal of Analytical Atomic Spectrometry April 1997 Vol. 12 CONCLUSIONS The sensitivity of the proposed methods depends on both the reagent and sorbent (APDC and C sorbent are the best option) while the selectivity is more 60 markedly dependent on the sorbent (C60 is the best alternative). The APDC method with C fullerene as sorbent is clearly superior to existing continuous-flow 60 alternatives using other sorbents or resins in terms of sensitivity and selectivity.15,18 Furthermore most available preconcentration methods have been applied to water samples and few to biological materials owing to the interferences posed by the latter.Thus as shown in this work one should rigorously check for potential interferences in order to avoid unexpected results such as those found here for the Bovine Liver sample. Therefore the APDC reagent in combination with C fullerene as sorbent can probably be applied to highly complex 60 matrices to determine trace levels of cadmium. Enhancement factor† Detection limit/ng ml-1 RSD (%) 1.9 110 45 30 20 0.3 1.1 2.0 2.6 2.1 2.3 2.5 trations) was not adsorbed uniformly on the column; hence its subsequent elution was more difficult. The selectivity of the fullerene column was at least twice that of the RP-C18 column in both methods. The higher selectivity of the APDC method relative to the 8-hydroxyquinoline method can be ascribed to the low sample pH (1.0).In the 8-hydroxyquinoline method some ions (viz. Al3+ Fe3+ Sn2+ Hg2+ and Pb2+) were hydrolysed at pH 4.5 and their hydroxide or basic salts precipitated as a result; however when the sample was mixed with the 8-hydroxyquinoline reagent their corresponding 8-hydroxyquinolinolates were probably formed; hence the ions were tolerated at relatively high levels. For comparative purposes the selectivity of another automated method for the preconcentration of cadmium (pH 2.0) with dithizone (chelating reagent) on an activated carbon column (70 mg sorbent; 2.5 cm×3 mm id) is also included in Table 2.18 This method is scarcely selective owing to the low selectivity of dithizone.Determination of Cadmium in Certified Reference Materials sorbent owing to its low selectivity. Cadmium was determined in three reference materials Oyster Tissue Pig Kidney and Bovine Liver. Although the APDC method was the better alternative for this determination the selectivity achieved with 8-hydroxyquinoline also permitted the determination of cadmium in some samples; hence both methods were employed for comparison. The blank absorbances corresponded to a cadmium concentration of less than 0.5 ng ml-1 (this blank allowed the contribution of cadmium ion in the reagents to the digested sample to be assessed). The analytical results listed in Table 3 are in good agreement with the certified values (within the 95% confidence intervals) for Oyster Tissue and Pig Kidney.However Bovine Liver can only be analysed by the APDC method with C fullerene as sorbent because it is subject to interferences from60iron (content 194 mg g-1) and copper (content 158 mg g-1). As a consequence the concentrations in Bovine Liver obtained with the 8-hydroxyquinoline method were lower particularly with the RP-C18 Table 3 Determination of cadmium in certified reference materials. All values are given in mg g-1 cadmium APDC (C60 ) Certified value 4.15±0.38 Reference material NIST SRM 1566a Oyster Tissue 4.05±0.30 2.82±0.17 0.45±0.05 2.71±0.15 0.44±0.06 BCR CRM 186 Pig Kidney NIST SRM 1577A Bovine Liver The Spanish DGICyT is gratefully acknowledged for financial support (Grant No.PB95–0977). REFERENCES 1 Baum R. M. Chem. Eng. News 1993 November 22 8. 2 Kroto H. W. Allaf A. W. and Balm S. P. Chem. Rev. 1991 91 1213. 3 Special Issue on Buckminsterfullerenes Acc. Chem. Res. 1992 25 (3). 4 Manteca-Diego C. and Moran E. Ann. Quim. 1994 90 143. 5 Lieber Ch. M. and Chia-Chun C. in Preparation of Fullerenes and Fullerene-Based Materials eds. Ehrenreich H. and Spaepen F. Solid State Physics of Fullerenes Academic Press Boston MA 1994. 6 Smalley R. E. T he Almost (but never quite) Complete Buckminsterfullerene Bibliography. (Database available from the author via E-mail BUCKY@-SOL1.LRSM.UPENN.EDU.) 7 Dresselhaus M. S. Dresselhaus G. and Eklund P. C. Science of Fullerenes and Carbon Nanotubes Academic Press San Diego CA 1996.8 Saito Y. Ohta H. Nagashima H. Itoh K. Jinno K. Okamoto Journal of Analytical Atomic Spectrometry April 1997 Vol. 12 Found 60) 8-Hydroxyquinoline (RP-C18) APDC 8-Hydroxyquinoline (RP-C18) (C 4.10±0.39 3.92±0.37 2.78±0.21 0.22±0.13 2.80±0.17 0.38±0.09 3.96±0.34 2.70±0.20 0.32±0.09 M. Chen Y. L. Luehr G. and Archer J. J. L iq. Chromatogr. 1995 18 1987. 9 Kimata K. Hirose T. Moriuchi K. Hosoya K. Araki T. and Tanaka N. Anal. Chem. 1995 67 2556. Jinno K. and Itoh K. J. High Resolut. Chromatogr. 1995 18 569. 1994 66 4074. 1995 67 2524. Spectrom. 1993 8 979. 10 Saito Y. Ohta H. Terasaki H. Katoh Y. Nagashima H. 11 Gallego M. Petit de Pen�a Y. and Valca�rcel M. Anal. Chem. 12 Petit de Pen�a Y. Gallego M. and Valca�rcel M. Anal. Chem. 13 Ebdon L. Fisher A. Handley H. and Jones P. J. Anal. At. 14 Taylor A. Branch S. Crews H. M. Halls D. J. and White M. J. Anal. At. Spectrom. 1996 11 103R. 15 Fang Z. Flow Injection Atomic Absorption Spectrometry Wiley Chichester 1995. 16 Rio-Segade S. Pe�rez-Cid B. and Bendicho C. Fresenius’ J. Anal. Chem. 1995 351 798. 17 Fang Z. Xu S. Dong L. and Li W. T alanta 1994 41 2165. 18 Petit de Pen�a Y. Gallego M. and Valca�rcel M. J. Anal. At. Spectrom. 1994 9 691. Paper 6/06990H Received October 14 1996 Accepted Janua

ISSN:0267-9477    

DOI:10.1039/a606990h

出版商:RSC    

年代:1997

数据来源: RSC