摘要:

A calibration strategy for LA-ICP-MS analysis employing aqueous standards having modiÆed absorption coefÆcients F. Boue�-Bigne,a B. J. Masters,b J. S. Crightonc and B. L. Sharp*a aDepartment of Chemistry, Loughborough University, Loughborough, Leicestershire, UK LE11 3TU bLaporte Electronics, Amber Business Centre, Riddings, Alfreton, Derbyshire, UK DE55 4DA cBP Amoco Chemicals, Research and Engineering Centre, Chertsey Road Sunbury-on-Thames, Middlesex, UK TW16 7LL Received 7th July 1999, Accepted 15th September 1999 Laser ablation ICP-MS is a powerful technique for the direct elemental analysis of solids with spatial resolution down to a few microns.However, its range of application is limited by the lack of calibration standards. This paper describes a novel approach to calibration that employs aqueous standards whose absorption coefÆcients are modiÆed, by the addition of a chromophore, to produce the desired ablation yield. Chromophores for the important laser wavelengths at 193, 248 and 266 nm are given.The mechanism of ablation and parametric dependences for the modiÆed aqueous standards were investigated and it was concluded that ablation proceeds by a three step process leading ultimately to nebulisation of the bulk liquid. This is important as such a process should not involve fractionation between elements. Calibration curves produced using the aqueous standards were linear and reproducible, but internal standardisation is required to provide linked calibration for real samples.NIST standard Reference Material 613 Trace Elements in Glass and low density polyethylene were analysed and the results agreed with reference values or those obtained by other techniques. 1 Introduction Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) allows the direct multi-element analysis of solid samples. Qualitative, semi-quantitative, quantitative analyses and isotopic ratio measurements can be performed on a localised spot or the bulk of a sample.The technique is of growing interest in many Æelds, such as the polymer and metallurgical sciences,1±3 the geosciences,4±6 environmental,7 biomedical8 and forensic9 sciences. Although LA-ICP-MS is very attractive, calibration remains a limiting factor when performing quantitative analyses on a wide variety of sample types. No universal method of calibration exists as yet and the ones that are currently used mostly attempt to match the standards to the sample matrix.Different approaches are used to obtain matrix-matched standards. CertiÆed reference materials are commercially available in different types of solid matrices (glass, ceramic, cement, metals, etc.). These reference materials are costly and the range of matrices available does not cover every type of sample (e.g., biological tissue, polymers). Some laboratories prepare their own matrix-matched standards,1,2,7 which are a mixture of an appropriate matrix/binder material and the analytes.Several problems are related to this approach. It is a time-consuming process (in some cases, the sample matrix may not be available, or is not in a pure state). Additionally, these home-made solid standards can have a very heterogeneous distribution of the elements through the matrix substrate. In addition to matrix-matching, internal standards are widely used in LA-ICP-MS for quantitative analyses. The internal standard compensates for the different ablation yields from the sample and the standard and enables the preparation of a calibration graph.It also compensates for the shot-toshot variation and therefore improves the precision of the measurement. Given that the use of an internal standard is required to obtain reliable results, the need for matrixmatching might be regarded as questionable. This paper describes the development of a new method of calibration for quantitative analysis using LA-ICP-MS.It involves the use of aqueous standard solutions whose absorption characteristics are modiÆed by the addition of a chromophore, in order to obtain the desired degree of coupling with the laser energy.10 Preliminary investigations have been carried out on the ablation mechanism of the standard solutions. The new method of calibration was used for the determination of analytes in National Institutes of Standard and Technology (NIST) (Gaithersburg, MD, USA) Standard Reference Material (SRM) 613 Trace Elements in Glass and in Low Density Polyethylene (LDPE). 2 Development and investigation of controlled absorption coefÆcient standard solutions 2.1 Experimental Reagents. The additives used for the modiÆcation of the standard solutions were poly(sodium 4-styrenesulfonate) (Aldrich, Gillingham, Dorset, UK), m-hydroxybenzoic acid (Aldrich), 2-thiobarbituric acid (GPR grade, BDH, Poole, Dorset, UK), 1,10-phenanthroline (ACROS, Fisher ScientiÆc, Loughborough, Leicestershire, UK), nitric acid (Aristar, BDH) and de-ionised water (18 MV).Instruments. Experiments related to the development of the standard solutions took place in two different laboratories (Loughborough University and BP Chemicals, Sunbury-on- Thames); therefore, two instrumental systems are described below. Loughborough University laboratory. An excimer laser, Model EX-742 (Lumonics Ltd., Rugby, UK), was used to ablate the samples. The laser was operated at the KrF transition: 248 nm.All control functions of the laser were performed from a hand-held keypad, which continuously displayed output pulse energy and other relevant operating J. Anal. At. Spectrom., 1999, 14, 1665±1672 1665 This Journal is # The Royal Society of Chemistry 1999parameters of the laser. The laser incorporated a `Stabilase' feature that automatically adjusted the voltage applied to the electrodes to maintain the required output energy.The rectangular shaped laser beam was focused down by a 1 m focal length plano-convex fused silica lens. The long focal length lens was chosen to provide a large depth of focus, which minimises chromatic aberrations, the laser beam having a 1 nm band pass. A mask was positioned after the focal point, in order not to be ablated, to select a section of the beam that provided a Øat energy distribution. A three-element objective was used to image the mask aperture onto the target.This lens combination was optimised to minimise aberrations and to provide high quality imaging at a 15 : 1 reduction. The lenses were coated to minimise reØection losses at 248 nm. Visual control of the ablation was possible with the aid of two systems. The Ærst one combined a retractable mirror, a tube correction lens and an eyepiece, to provide a microscopic view of the target while the laser was off; the second one allowed observation of the ablation of the standard solutions from below the transparent ablation cell, using a mirror positioned at 45� to reØect the image onto a CCD camera. A home-made ablation cell was constructed based upon the design of Arrowsmith and Hughes.11 The ICP-MS used during these studies was a prototype PQ1 instrument (VG Elemental, Winsford, Cheshire, UK).The plasma position, ion optics and gas Øows, were optimised daily to obtain a maximum and stable signal at m/z 115 for a 50 ng ml21 In standard solution. For this particular instrument, a typical count rate of 46106 counts per ppm was achieved.BP Chemicals laboratory. The laser system was originally a Nd:YAG laser (VG laserlab system), emitting at 1064 nm. The original Spectron SL402 (Spectron Laser System Ltd, Rugby, UK) laser was replaced with a Spectron SL282 system to make room for two doubling crystals (DCD and KP*P) in order to obtain emission in the visible (532 nm) and in the UV (266 nm) as well as in the IR. The ICP-MS instrument was a VG Elemental PQ IIz.Chromophore selection. Initial trials to produce a reliable standard solution, using various compounds, established appropriate criteria for the selection of the chromophore. Plasma condition when ablating standard solutions. One of the advantages that laser ablation has over solution nebulisation, as a means of sample introduction, is that l ablation creates a dry plasma. This reduces the amount of oxygen introduced into the plasma and should therefore minimise the occurrence of interfering oxide species.The ablation of solutions was investigated to determine if wet or dry plasma conditions were generated. A solution containing 9.2 g l21 of poly(sodium 4-styrenesulfonate) was ablated at 248 nm to estimate the amount of water being transported to the plasma. This relatively high concentration had previously been determined as being a level that would cause the solution to have an absorption coefÆcient equivalent to that of NIST SRM 613.In this case, the polymer was used without the addition of 2% nitric acid, which reduces its solubility to 4 g l21. Glass tubes of 15 cm length were packed with dried silica gel and sealed at each end with glass wool. The weight of the tubes was accurately measured and recorded. The aerosols generated by laser ablation of a modiÆed solution and conventional nebulisation were allowed to pass through the tubes. The tubes were then re-weighed and the amount of aerosol generated per minute, for each of the sample introduction methods, was recorded.Signal stability. In order to obtain a stable ablation yield, it was thought that either the selected chromophore would have to present a stable absorption at the lasing wavelength or that it would need to be UV stabilised by the use of quenchers. The stability of the chromophores under UV radiation was investigated. The absorption of a modiÆed solution was observed before and after exposure to a mercury pen lamp (l~254 nm). The stability of the ablation yield of the modiÆed standard solution was tested at 266 nm.Optimisation of the use of the modiÆed standard solutions. The inØuence of the laser energy on the ablation yield of standard solutions was studied for different chromophore concentrations. The solutions were ablated at 266 nm; the chromophore used was 2-thiobarbituric acid. Standard solutions were made up, containing 1 mg g21 of Co with 1, 1.8, 2.6 and 3 g l21 of chromophore in 2% nitric acid.Each solution was ablated for 60 s, at different laser energies. Investigation of the ablation mechanism of the standard solutions. To study the ablation mechanism of the modiÆed standards, solutions were prepared having different properties. The following experiments were carried out using the quadrupled frequency Nd:YAG laser (266 nm). The Ærst experiment involved two standard solutions where the additive, 2-thiobarbituric acid, was in solution with two different solvents: H2O and D2O.The two solutions were made up at 3 g l21, giving the same absorption at 266 nm. They were ablated for 1 min; the mass losses after ablation were compared. The mass losses due to the argon Øow above the solutions were monitored to allow the ablated mass to be calculated from the total (evaporationzablation) mass loss. A second experiment involved two different chromophores absorbing at 266 nm: 2-thiobarbituric acid (TBA) and 1,10- phenanthroline (PNT).The two solutions were made up in water to have the same absorption at 266 nm (i.e., 2.13 g l21 of TBA and 1.43 g l21 of PNT). As previously, the mass losses, after 45 s of ablation, were recorded and compared. For all these experiments, each mass loss measurement was repeated ten times in order to apply to them statistical tests to check the validity of the results. Observation and comment on the particular behaviour of mhydroxybenzoic acid as a chromophore in standard solution.It was observed that standard solution freshly made with mhydroxybenzoic acid would turn progressively yellow after a few days. Additionally, the slow kinetics of this reaction were found to be inØuenced by the presence of the analytes in solution, the effect occurring more rapidly with a greater analyte concentration. When ablated with the same energy, the yellow standard solution produced a higher ablation yield than a freshly made solution prepared with the same initial mhydroxybenzoic acid concentration.Determination of the changes that occurred in the modiÆed solution would be indicative of the chemical or physical parameters which are important in the ablation mechanism of the modiÆed standard solutions. The compounds present in the yellow solution were extracted from the acid medium and analysed by MS, 1H NMR and GC-MS. 2.2 Results and discussion Chromophore selection. It would seem logical that an ideal chromophore should be resistant to photodegradation, but as the results presented in the next section show, this is not the critical factor.More importantly the ideal chromophore should have the following characteristics. It should: absorb strongly at the lasing wavelength used for ablation; be soluble in 1±2% nitric acid; not precipitate when in contact with the analytes; and be non-toxic. A suitable chromophore combining these four characteristics was selected for each lasing wavelength.Our efforts were 1666 J. Anal. At. Spectrom., 1999, 14, 1665±1672focused on producing standard solutions for UV laser ablation as this results in a more efÆcient ablation of solid materials than IR ablation and is less prone to fractionation. The chromophores are listed below with their corresponding speciÆc absorptivity and maximum solubility in acid solution. For the 193 nm excimer laser two chromophores were found: (i) nitric acid, e193~3 l g21 cm21, used at 1±2% in solution; and (ii) poly(sodium 4-styrenesulfonate) (see Fig. 1a), e193 ~85 l g21 cm21, maximum solubility in acid solution~4 g l21. For the 248 nm excimer laser: m-hydroxybenzoic acid (see Fig. 1b), e248~30 l g21 cm21, maximum solubility in acid solution~3 g l21. For the 266 nm frequency quadrupled Nd:YAG laser: 2-thiobarbituric acid (see Fig. 1c), e266~80 l g21 cm21, maximum solubility in acid solution~4 g l21. Plasma condition when ablating standard solutions. The amount of aerosol generated by ablation, at 248 nm, of a standard solution containing 9.2 g l21 poly(sodium 4-styrenesulfonate) was 626 mg min21 against 23 550 mg min21 for conventional nebulisation.The plasma was much less loaded with water vapour when ablating the standard solutions. Importantly, it was found that the optimum operating conditions for the standards were similar to those for the solid samples suggesting that the plasma and interface conditions were broadly similar.If this were not the case, the internal standard would have to compensate for differential atomisation/ionisation effects in addition to those related to ablation yield and transport. Identifying such an internal standard for multi-element analysis would be difÆcult, if not impossible. Signal stability. The selected chromophores were found to be unstable under UV irradiation from the mercury pen lamp (257 nm). The absorbance of the standard solutions varied due to the formation of photo-products and mostly resulted in a decrease of the absorbance of the standard solution at the wavelength of interest.For example, Fig. 2 shows the decrease of absorption of a modiÆed solution (6 mg l21 of 2-thiobarbituric acid in 2% nitric acid) exposed to a mercury lamp. However, for poly(sodium 4-styrene sulfonate), a stable absorption was observed at 248 nm after irradiation with UV photons. The apparent stability at 248 and 266 nm was due to the fact that an absorption band with a peak at 224 nm broadened and Øattened on exposure to the UV radiation.Fig. 3 shows the signal stability of a solution containing 3 g l21 of TBA and Fe, Mo and W at 10 ppm in 2% nitric acid ablated at 266 nm. The signal is stable for much longer periods than this, provided that the laser focus is sustained. However, the period shown is relevant to typical acquisition times for calibration purposes. This shows that photo-stability is not critically important.There are two principal reasons for this; Ærst, most of the volume of aqueous standard strongly irradiated by the laser beam is efÆciently ablated and therefore removed, unless the energy threshold value has not been reached; second, the liquid state of the standard offers a renewable surface of fresh solution during the ablation event. Further to these points, making the reservoir volume (typically 1 ml) very much larger than the ablation volume ensures that a large excess of fresh chromophore is always present and also helps to reduce the effects of evaporation, which would otherwise cause a gradual increase in analyte concentration. Thus, the chromophore, used to modify the absorbance of the solution, does not have to be photo-stable to provide stable ablation yield over a long period of time.Given this observation, poly(sodium 4-styrenesulfonate) can also be expected to be an efÆcient chromophore at 193 nm where it has a high speciÆc absorptivity (85 l g21 cm21).Optimisation of the use of the modiÆed standard solutions. A simple aqueous standard solution containing nitric acid does not absorb the UV laser energy unless an ArF excimer laser that emits at 193 nm is used. Very little ablation occurs, if any, and the beam penetrates deeply into the solution and may ablate the containment vessel. The purpose of the chromophore is to improve coupling with the aqueous solution in such a way that the threshold ablation Øuence is reached within the surface layers of the liquid, thereby producing a Æne aerosol that is readily transportable to, and easily processed by, the plasma.The inØuence of chromophore concentration and laser energy on the ablation yield of the standard solutions is shown Fig. 4. The addition of chromophore to the standard solution reduces the threshold Øuence value. This reduction of threshold Øuence has also been observed in experiments involving the doping of solid materials12,13 in order to improve their ablation Fig. 1 Structures of (a) poly(sodium-4-styrenesulfonate), (b) m-hydroxybenzoic acid, and (c) 2-thiobarbituric acid. Fig. 2 Decrease of the absorption of a modiÆed solution (6 mg l21 of TBA in 2% nitric acid) exposed to a mercury lamp. Fig. 3 Stability of the ablation yield of a modiÆed standard solution (3 g l21 of TBA and Fe, Mo andWat 10 ppm) ablated at 266 nm, at the energy optimised for LDPE, and at 10 Hz. J. Anal.At. Spectrom., 1999, 14, 1665±1672 1667at selected wavelengths. In the case of the standard solutions, the addition of the chromophore offers the user the freedom to prepare standards that give a signal of the desired amplitude, when ablated with a Øuence pre-selected to be optimal for the ablation of the sample. The required concentration can be obtained directly from Fig. 4. For the analysis of a sample requiring a low laser Øuence for ablation, a relatively high chromophore concentration will be selected in order to decrease the threshold Øuence value and to obtain an efÆcient ablation (and hence an appropriate analyte count rate) of the standard solution at low energy.For a sample requiring a high laser Øuence to be ablated, a relatively low chromophore concentration will be selected. High laser Øuence was found to produce a violent explosive ablation event yielding large aerosol droplets that were visibly deposited onto the ablation chamber walls. An alternative strategy for determining chromophore concentration when analysing transparent materials is to measure the absorption coefÆcient of the target substrate and to match the solution absorption coefÆcient to this value. This was previously attempted for glass samples8 and surprisingly yielded similar analyte count rates, to within an order of magnitude, for the same analyte concentrations.This approach is of interest for comparing ablation yields, but is unlikely to be used for practical analyses.Investigation of the ablation mechanism of the standard solutions. The results of the H2O±D2O experiment are presented in Table 1. As the masses measured were of the order of 1 mg, a two-tailed t-test was applied to the results to check their validity. It was found that there was a signiÆcant difference between the two means at the 99.9% conÆdence level. The mass loss due to ablation from the aqueous solution was 2.6-fold greater than from the deuterated solution.The process by which the energy absorbed by the chromophore is coupled into the water matrix is complex and could proceed by a variety of routes. However, the direct conversion of several electronvolts of energy into translational motion of a single molecule is unlikely when that molecule is Ærmly embedded in the elastic water matrix. Whether the transfer route is via direct quenching, internal conversion or dissociation, the Ænal step in the energy cascade is most likely to occur through a vibrational process.The experiment with D2O seeks to verify this hypothesis. The physical properties of H2O and D2O are not very different. The boiling point of D2O is 101.4 �C compared with 100 �C for H2O and similarly, the enthalpy of vaporisation is 169.9 kJ mol21 for H2O and 174 kJ mol21 for D2O. These data derive from the fact that the O±H covalent bond is stronger in water than in D2O, which in turn leads to a weaker H bond in H2O compared with the equivalent bond in D2O.However, these properties are insufÆcient to account for the differences found in the experiments. Such differences could only be accounted for if the initial energy transfer from the chromophore to the H2O or D2O matrix is resonantly coupled through a principal or overtone vibrational transition of the solvent. H2O and D2O have signiÆcantly different absorption band patterns in the IR. An example of this highly speciÆc type of energy cascade has been described for the sensitizer [tetrakis(4-sulfonatophenyl) porphine] (TPPS) excited at 532 nm.14 In this case the triplet excited state of TTPS is quenched by O2, which in turn is excited to its singlet state.The measurement of the lifetime of the O2 singlet excited state showed the extent of its coupling with the solvent. This lifetime was found to be shorter when the solution was prepared in H2O than in D2O because H2O has an overtone which directly overlaps with an absorption band in O2.Attempts to maintain a degassed solution in our experimental arrangement proved unsuccessful. The results obtained for the experiment involving two chromophores, TBA and PNT, are shown in Table 2. As previously, a two-tailed t-test shows that there is a signiÆcant difference between these two means at the 99.9% conÆdence level. The solution containing PNT produced an explosive ablation compared to the one with TBA. Four times more solution was removed during the explosive ablation.Two reasons can be advanced to explain the difference found. TBA and PNT were, respectively, at 14.761023 mol l21 and 861023 mol l21 to give the same absorbance at 266 nm. Therefore the laser energy was absorbed by approximately twice the number of TBA molecules compared with PNT. This contributes to a more dispersed partitioning of the laser energy into the TBA solution. Additionally, as shown in Figs. 5a and 5b, TBA molecules present more sites of interaction with the solvent than PNT; therefore, the energy of its excited state is released in a more dispersed way to the solvent.This results in the creation of a Æne aerosol that is readily transportable to the plasma. PNT produces a more localised coupling to the solvent, resulting in the removal of large droplets that are not efÆciently transportable beyond the ablation cell. Contamination of the cell with large water droplets was readily observable under these conditions.The end result of coupling the laser energy to the liquid appears to be a rapid and local increase in temperature and pressure leading to nebulisation of the bulk solution. This is important as such a mechanism should be free of fractionation effects. These experiments underline the fact that the quality of aerosol produced (i.e., the transportable mass) is equally important to the initial ablation yield. Examination of the data in Fig. 6 indicates an approximately Fig. 4 InØuence on the ablation yield of the laser voltage for different concations of 2-thiobarbituric acid, at 266 nm. Table 1 Comparison of ablation yield of standard solutions with difference solvents Chromophore 2-Thiobarbituric acid 2-Thiobarbituric acid Solvent H2O D2O Ar Øow effect Total mass loss Ar Øow effect Total mass loss Average of mass loss/mg 1.5 2.8 1.4 1.9 RSD (%) n~10 12.8 9.2 5.6 9.3 Mass loss due to ablation/mg 1.3 0.5 1668 J. Anal. At.Spectrom., 1999, 14, 1665±1672exponential relationship between ablation yield and chromophore concentration and is suggestive of a process that might be modelled by simple application of Beer's law. This, however, is not the case. A simple form of Beer's law used for LA modelling13 is: For P0wPc: Pc~P0e{kn sLc Ö1Ü or Lc~ 1 kn s ln P0 Pc Ö2Ü where P0 is the incident power, Pc is the threshold power required for ablation to begin, kn s is the absorption coefÆcient of the sample, and Lc is then the depth of material ablated by the pulse.Clearly for constant Pc and Po, increasing kn s requires that Lc is reduced, that is, the volume of material ablated (Lcpr2) (where r is the beam radius) should be reduced as the absorption coefÆcient is raised. That this does not appear to apply here is indicative of other factors being signiÆcant, viz.: (i) this simple analysis assumes that power is absorbed more or less instantaneously before the ablation process begins, which may be true with ps lasers, but not with lasers such as those used here with 15 ns pulse widths; (ii) the power dissipated in the ablated volume, i.e., the power density, DPD~ÖP0{PcÜ=pr2Lc Ö3Ü does rise as the absorption coefÆcient is increased; (iii) there will be attenuation of the beam by coupling to the plasma formed above the liquid surface.The Ærst point to make is that, as shown in Fig. 6, Pc is reduced by increasing the absorption coefÆcient which will improve the yield.Conversely, for a transparent material, Pc increases asymptotically as kn s tends to zero15 so that no ablation occurs. Secondly, because the power dissipated per unit volume is increased, more energy becomes available to convert the sample to a transportable aerosol that will move away from the ablation surface and ultimately reach the plasma. The importance of local transport has recently been demonstrated by Frischknecht et al.16 who showed that exchanging Ar for He, a less dense gas, enhanced ablation yield, although this may also be partly due to modiÆcations of the ablation plasma.Given the relatively long laser pulse it can be anticipated that during the ablation process there will be a moving ablation boundary13 so that the simple absorption model will actually apply in a series of successive time increments with the target being gradually shielded from the beam as material is removed and an ablation plasma develops.Obviously a great deal more work would be required to provide a quantitative model of the ablation yield for such a complex process. Observation and comment on the particular behaviour of mhydroxybenzoic acid as a chromophore in standard solutions. Initial MS analysis of the aged yellow standard solution showed a peak at Mz46, which indicates the presence of a nitrated form of the initial chromophore. Comparative analyses were then performed by 1H NMR and GC-MS between m-hydroxybenzoic acid, a nitrated isomer of mhydroxybenzoic acid, 3-hydroxy-4-nitrobenzoic acid and the unknown yellow mixture.The samples were methylated using BF3±methanol to make the chromatography easier. Both types of analysis conÆrmed that the yellow solution was mainly a mixture of the initial chromophore m-hydroxybenzoic acid and 3-hydroxy-4-nitrobenzoic acid; various ketones and diesters were formed at very low concentrations. 3-Hydroxy-4-nitrobenzoic acid is much less soluble than m-hydroxybenzoic acid in 2% HNO3.The presence of the 3-hydroxy-4-nitrobenzoic acid in the solution may lead, like PNT previously, to a less dispersed release of energy from the excited chromophore to the solvent, producing more localised heating and a higher ablation yield. The nitration of m-hydroxybenzoic acid was not expected, but obviously occurs, and appears to be catalysed by the presence of metal ions. 3 Applications The new method of calibration was applied to the analysis of two samples presenting different matrices: the certiÆed Table 2 Comparison of ablation yield of standard solutions with different chromophores Chromophore 2-Thiobarbituric acid 1,10-Phenanthroline Solvent H2O H2O Ar Øow effect Total mass loss Ar Øow effect Total mass loss Average of mass loss/mg 0.97 1.14 1 1.64 RSD (%) n~10 9.32 4.8 0 9.56 Mass loss due to ablation/mg 0.17 0.64 Fig. 5 Interaction of (a) 2-thiobarbituric acid and (b) 1,10-phenanthroline with water.Fig. 6 Variation of the ablation yield with increasing chromophore concentration. Standard solutions (different chromophore concentrations and 5 ppm Co) ablated at the energy optimised for LDPE, and at 10 Hz. J. Anal. At. Spectrom., 1999, 14, 1665±1672 1669reference material NIST SRM 613, and low density polyethylene (LDPE).17 3.1 Experimental Instruments. The laser ablation analyses were carried out using the excimer laser operating at 248 nm. The analyses of the digested samples were performed with the VG PQ I prototype ICP-MS instrument and with a Varian 300 (Varian Ltd., Walton on Thames, UK) atomic absorption spectrophotometer with graphite furnace Model GTA96. Reagents.Reagents used for the LDPE sample digestion were a borate Øux (Johnson Matthey, Royston, Hertfordshire, UK; 120A: 80% lithium tetraborate, 20% lithium Øuoride), Aristar nitric acid (BDH), tartaric acid (Fisher ScientiÆc). Standard solutions were prepared from 1000 ppm stock reagents (Grade Specpure, Johnson Matthey, and Spectrosol, BDH).Dilution was made with 2% nitric acid solution and 18 MV de-ionised water. The additive used for the modiÆcation of the standard solution was m-hydroxybenzoic acid (Aldrich). The polymeric material was provided by the CSL Food Science Laboratory, Norwich, UK.16 Digestion procedure. As the polymeric material was not a certiÆed reference sample, it was digested to be analysed by ICP-MS and furnace AA. The results were compared to the ones found by LA-ICP-MS and from previous work.17 The digestion procedure used a lithium borate mixture to fuse inorganic samples that were not directly soluble in acidic solution.The sample (150 mg) was previously ashed in a platinum crucible with lid, above a gas burner Øame. The ashes were fused in the molten Øux at 950 �C. The fused solid, when cool, was dissolved in an acidic solution (4% nitric acid±0.5% tartaric acid) and made up to 50 ml to give an overall dilution of 333 : 1.Two replicates were made this way. Standard solutions at 30, 50, 100 and 200 ng g21 of the analytes were made up in the same lithium borate matrix for calibration of the ICP-MS instrument. 9Be, 89Y and 209Bi were used as internal standards. Preparation of trial calibration curves for LA-ICP-MS. Calibration curves were obtained by LA-ICP-MS for each element of interest in the LDPE sample. Their concentration range covered the analyte concentrations present in the sample.ModiÆed standard solutions were prepared with 0, 5, 10, 25, 50, 100 mg g21 of the following elements: Mg, Cr, Zn, Sb and Pb. All standards contained 50 mg g21 of Co, which was used as an internal standard. As the concentration range studied here is relatively high, the amount of chromophore added to the standard solutions had to be small enough in order to obtain analyte signals that would not saturate the detector. For this reason, a concentration of 0.15 g l21 of m-hydroxybenzoic acid was added to the solutions.Standard solutions used for quantitative analysis by LA-ICPMS. A blank solution and a standard solution (7.5 mg g21 of analytes), both containing 0.3 g l21 of m-hydroxybenzoic acid, were prepared. They were both ablated, as well as the LDPE sample and a LDPE blank, known to be free of the analytes of interest. This LDPE blank alwed correction for isobaric interferences due to carbon compounds. The instrument parameters for the calibration and the LDPE analyses by LA-ICP-MS are given in Table 3. 3.2 Results The aqueous calibration solutions were freshly made before use and were stable during a working day. When left for longer periods some precipitation occurred notably with elements such as Pb and Pd. Calibration curves by LA-ICP-MS. The analytes contained in the standard solutions are homogeneously distributed; therefore, unlike solid standards, the modiÆed standard solutions do not require ablation at several sites in order to give a representative response.Each standard solution was continually ablated for 5 min. The acquisition of the ablation signal was preceded and followed by 1 min of non-ablation. This non-ablation blank was subtracted from the ablation signal, before any calculations. The blank solution was treated as a point on the calibration curves. The 5 min ablation acquisition was divided into Æve contiguous portions of 1 min each. The analyte responses were normalised to the cobalt internal standard.The standard deviation was calculated for each point. A weighted regression line was calculated from the calibration data of each element. Errors in the slope and intercept of the regression lines are given in Table 4. All curves have a correlation coefÆcient greater than 0.993. Fig. 7 shows the calibration curve obtained for Mg, which gave the poorest correlation. LDPE analysis. A correction factor (CF) was calculated which allowed correction for differences in sensitivity betwen the standard and the sample. 59Co was the isotope used as internal standard. CF~âÖcpsCo sampleÜ|Öconc:Co stdÜä=âÖcpsCo stdÜ|Öconc:Co sampleÜä (4) Table 3 Laser and ICP-MS parameters Excimer laser parameters– Laser wavelength 248 nm Laser energy 60 mJ Frequency 10 Hz Crater diameter 200 mm Plasma conditions and data acquisition parameters– Rf power 1.25 kW ReØected power v0.5W Argon Øow rate: Coolant 12 l min21 Auxiliary 1 l min21 Carrier 1.1 l min21 Point per peak 1 Dwell time 10 ms Acquisition mode Time resolved analysis (peak jumping) Fig. 7 Calibration curve with the poorest correlation coefÆcient, obtained for 25Mg, from ablation of modiÆed standard solutions. The error bars indicate °3 standard deviations. 1670 J. Anal. At. Spectrom., 1999, 14, 1665±1672The analytes concentrations were then calculated by: conc:elt~âÖcpselt sampleÜ|Öconc:elt stdÜä=âCFxÖcpselt stdÜä Ö5Ü where elt~element, std~standard, CPS~counts per second. The analyte concentrations found by the three different analytical techniques are shown in Table 5a.Data found from previous work are shown in Table 5b. The results obtained by LA-ICP-MS are in good agreement with those found by digestion-ICP-MS, furnace AA and previous work.17 For the LA-ICP-MS analysis, n~6 represents the number of separate sites (200 mm diameter) ablated on the same sample to obtain a representative result for the bulk of the material. The high RSD for Sb indicates its heterogeneity in the LDPE matrix.These data illustrate the difÆculty of making polymer standards18 of known composition and even homogeneity. Previous to this study, a certiÆed NIST SRMglass sample was analysed using the modiÆed standard solutions10 by LAICP- MS. The additive used was poly(sodium 4-styrene sulfonate); the laser used was an excimer laser emitting at 248 nm. The results found for seven isotopes are presented in Table 6.These results demonstrate that modiÆed aqueous standard solutions are suitable for quantitative analysis of solid LDPE and glass material by LA-ICP-MS. 4 Conclusions The data presented have demonstrated that aqueous standards having a modiÆed absorption coefÆcient can be employed in the analysis of solid material by LA-ICP-MS. Samples with different types of matrix, such as glass and low density polyethylene, were successfully analysed. The mechanism of ablation appears to be different from that applying to the solid samples; nevertheless, the use of an internal standard enables practical analysis to be performed, as long as the analytes in the liquid standard and in the solid sample behave equally regarding the internal standard.One of the great advantages of the standard solutions is that simple variation of the additive concentration allows the standards to be ablated at an energy that is optimal for the sample. This characteristic makes their use applicable to a wide variety of sample types. Other practical beneÆts derived from the use of aqueous standard solutions are that they are easier to prepare than solid standards, their elemental composition is readily variable, the distribution of the elements within the standard is homogenous, they offer a renewable surface during the ablation event and they are inexpensive.A future publication will report the extension of this calibration technique to analysis at 266 nm and to other sample types, including stainless steel.References 1 P. J. Fordham, J. W. Gramshaw, L. Castle, H. M. Crews, D. Thompson, S. J. Parry and E. McCurdy, J. Anal. At. Spectrom., 1995, 10, 303. Table 4 Calibration curve statistics for various elements in modiÆed absorption coefÆcient aqueous standards Isotope Linear curve equation Corr. coeff. (R2) Error on slope Error on intercept 25Mg y~158xz50 0.9937 °12 °603 53Cr y~113x270 0.9988 °4 °184 64Zn y~87xz148 0.9993 °2 °110 121Sb y~112xz88 0.9991 °3 °152 208Pb y~383xz900 0.9987 °20 °941 Table 6 Results for analysis of NIST glass Isotope Calculated conc./mg g21 °s NIST certiÆed conc./mg g21 140Ce 36.3 2.6 39a 65Cu 36.5 1.4 37.7a 57Fe 44.7 6.3 51 208Pb Int.std. – 38.57 60Ni 35.2 3.1 38.8 88Sr 77.6 2.0 78.4 238U 35.8 0.4 37.38 aNot certiÆed, given for information only. Table 5a Results for analysis of LDPE DigestionzICP-MS/mg g21 RSD (%) DigestionzAA/mg g21 RSD (%) LA-ICP-MS/mg g21 n~6 RSD (%) 25Mg 57.6 5.6 52.3 6.8 55.5 7.2 53Cr 30.3 8.4 32.1 2.5 34.3 1.6 59Co 28.5 8.2 32.1 2.1 Internal std. – 64Zn 50.8 0.9 48.3 2.3 51.3 17.6 121Sb 62.3 4.8 64.8 1.9 62.6 61.2 208Pb 50.7 1.1 57.7 4.5 52.5 4.4 Table 5b Results for analysis of LDPE from another source17 Nominal concentrations/mg g21 MDa ICP-MS/mg g21 NAA/mg g21 Mg 51.6 61.6 51.7 Cr 51.9 40.8 26.4 Co 49.2 34.6 29.0 Zn 49.3 48.8 39.2 Sb 69.8 74.8 52.0 Pb 62.6 68.3 N/Ab aMD~ microwave digestion. bN/A~not available. J. Anal. At. Spectrom., 1999, 14, 1665±1672 16712 J. T. Westheide, J. S. Becker, R. Jager, H. J. Dietze and J. A. C. Broekaert, J. Anal. At. Spectrom., 1996, 11, 661. 3 A. Raith, R. C. Hutton, I. D. Abell and J. Crighton, J. Anal. At. Spectrom., 1995, 10, 591. 4 S. Chenery and J. M. Cook, J. Anal. At. Spectrom., 1993, 8, 299. 5 A. Moissette, T. J. Shepherd and S. R. Chenery, J. Anal. At. Spectrom., 1996, 11, 177. 6 A. Audetat, D. Gunther and C. A. Heinrich, Science, 1998, 279, 2091. 7 E. Hoffmann, C. Ludke, H. Scholze and H. Stephanowitz, Fresenius' J. Anal. Chem., 1994, 350, 253. 8 B. Masters, Ph.D. Thesis, Loughborough University, Loughborough, 1996. 9 J. Watling, European Winter conference on Plasma Spectrochemistry, 1999, Pau, France. 10 B. Masters and B. L. Sharp, Anal. Commun., 1997, 34, 237. 11 P. Arrowsmith and S. K. Hughes, Appl. Spectrosc., 1988, 42, 1231. 12 R. Srinivasan and B. Braren, Appl. Phys. A, 1988, 45, 289. 13 E. Sutcliffe and R. Srinivasan, J. Appl. Phys., 1986, 60, 3315. 14 M. A. J. Rodgers and P. T. Snowden, J. Am. Chem. Soc., 1982, 104, 5541. 15 T. J. Chuang, H. Hiraoka and A. Modl, Appl. Phys. A, 1988, 45, 277. 16 R. Frischknecht, T. Ulrich, C. A. Heinrich and D. Gunther, European Winter conference on Plasma Spectrochemistry, 1999, Pau, France. 17 P. J. Fordham, W. Gramshaw, L. Castle, H. M. Crews, D. Thompson, S. J. Parry and E. McCurdy, J. Anal. At. Spectrom., 1995, 10, 303. 18 A. M. Dobney, H. Klinkenberg, C. de Koster and A. Mank, European Winter conference on Plasma Spectrochemistry, 1999, Pau, France. Paper 9/905479k 1672 J. Anal. At. Spectrom., 1999, 14, 1665±1672

ISSN:0267-9477    

DOI:10.1039/a905479k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Measurement of calcium stable isotope tracers using cool plasma ICP-MS Kristine Y. Patterson,a,b Claude Veillon,a A. David Hill,a Phylis B. Moser-Veillonc and Thomas C. O'Haverb aUSDA, Beltsville Human Nutrition Research Center, Beltsville, MD, 20705, USA bChemistry Department, University of Maryland, College Park, MD, 20742, USA cDepartment of Nutrition and Food Science, University of Maryland, College Park, MD, 20742, USA Received 25th January 1999, Accepted 6th September 1999 A method for the measurement of calcium isotopes (42Ca, 43Ca, and 44Ca) using quadrupole inductively coupled plasma mass spectrometry (ICP-MS) is described.Interferences from polyatomic ions such as 12C16O2 z and 40ArH2 z at the calcium masses are greatly minimized by operating the ICP-MS in the cool plasma mode. Relative standard deviations (RSD) for the 42Ca : 43Ca and 44Ca : 43Ca ratios were found to be about 0.25%. Sample preparation involved using ammonium oxalate at a pH of 8 to separate calcium from samples such as serum, urine, feces, and breast milk.The isotope ratio measurements were used to determine fractional absorption of calcium by a lactating woman after intravenous administration of 42Ca and ingestion of 44Ca. Dietary recommendations for calcium have recently been revised1 based in part on studies of absorption and metabolism in segments of the population such as young women.2±5 These types of calcium utilization studies in human nutrition are often done using dual isotope labeling, requiring simultaneous administration of one isotope orally and another intravenously.This makes it possible to determine the fractional absorption of calcium under speciÆc dietary conditions. Early studies of this type utilized the radioisotopes 45Ca and 47Ca.6 Later, with the availability of enriched stable isotopes of calcium and the analytical methodology to quantify them, it became possible to do the same type of study without the risk of exposure to radiation.This has made it feasible to investigate population groups such as pregnant or lactating women and children. The Ærst nutritional studies with Ca stable isotopes used radiochemical neutron activation analysis (RNAA) to determine the amounts of the stable isotope tracers in feces, plasma and urine with a relative precision of only 1±5%.7 Better analytical precision can be achieved using mass spectrometry. Two mass spectrometric techniques, thermal ionization (TIMS)8,9 and fast atom bombardment (FABMS),10±12 a type of secondary ion mass spectrometry, have been used for more precise Ca isotope determinations. In most reports, the Ca isotope ratio measurements have been made with precisions of about 0.2±0.5% for either technique.Turnlund et al.,13 in a single isotope study with infants, were able to achieve precisions of v0.1% for 46Ca : 48Ca measurements using multiple collectors in a magnetic sector TIMS.The newest technique for isotope ratio measurements is inductively coupled plasma mass spectrometry (ICP-MS). The problem in using this method for calcium is that there are signiÆcant interferences from both doubly charged and polyatomic ions at the same nominal masses as the calcium ions. Stu»rup et al.14 were able to resolve the interferences from the signals for 42Ca, 43Ca, and 44Ca. The 44Ca:43Ca and 42Ca:43Ca ratios were measured with relative standard deviations (RSD) of 0.33 and 0.41%, respectively, using a doublefocusing magnetic sector ICP-MS instrument.With a resolution of 4000 the interferences can be separated from the analyte signals. The unit resolution of quadrupole ICP-MS up to now has made it impossible to use these less expensive and more widely available instruments for Ca isotope measurements. The purpose of this research was to evaluate the use of a quadrupole ICP-MS that had been modiÆed for operation in the cool plasma15 mode for measuring Ca isotopes.When the ICP-MS is operated at low power, the production of doubly charged ions and some polyatomic ions is signiÆcantly reduced, making Ca determination possible. The method was applied to fecal, urine, breast milk, and serum samples from a study of lactating women who had been given enriched 44Ca as an oral dose and enriched 42Ca intravenously. Results of the study will be reported in detail elsewhere. Experimental Instrumentation The ICP-MS measurements were made with a PlasmaQuad IIz (VG Elemental, Winsford, Cheshire, UK) modiÆed to operate in the cool plasma mode.This modiÆcation by the manufacturer involved addition of a grounded metal screen that is inserted between the load coil and torch. In addition, changes were made in the extraction lens electronics to allow for a wider voltage range adjustment and grounding of the interface was enhanced making it possible to use a much lower power plasma while avoiding secondary discharge.A large mechanical vacuum pump on the expansion region (S-option) made it possible to achieve pressures of 1 Torr or less, thereby more than recovering lost sensitivity. Operating the instrument in the cool plasma mode without the additional pump reduced sensitivity by more than an order of magnitude. All measurements were made using a peristaltic pump for the drain (Gilson, Villiers-le-Bel, France), an autosampler (Gilson), a concentric nebulizer (AR35-1-F04, Glass Expansion Pty, Hawthorn, Victoria, Australia) operated as self-aspirating, and a quartz, water-jacketed, Scott-type spray chamber maintained at 4 �C.Details of the instrument settings are given in Table 1. Total calcium determinations of the samples were made by Øame atomic absorption spectrometry (Model 5000, Perkin- Elmer, Norwalk, CT, USA). Details regarding tracer preparation and dosing will be reported elsewhere. J. Anal. At. Spectrom., 1999, 14, 1673±1677 1673 This Journal is # The Royal Society of Chemistry 1999Standard solutions and reagents All samples were diluted for analysis in 0.1 M nitric acid prepared from sub-boiling distilled concentrated nitric acid (Seastar Chemical, Vancouver, British Columbia, Canada) diluted with de-ionized water (18 MV). Working standards were prepared from a calcium stock standard (1000 mg L21, High Purity Chemicals, Charleston, SC, USA) by dilution with 0.1 M nitric acid. The instrument was tuned and parameters were optimized with a 400 mg L21 Ca solution. Calcium was separated from the samples using a saturated solution of high purity ammonium oxalate (99.99z%, Aldrich, Milwaukee, WI, USA) at a pH of #8.Standard Reference Materials were used to verify the accuracy of total Ca determinations (SRM 1549, Dry Milk Powder; SRM 1598, Bovine Serum; SRM 1577b, Bovine Liver; National Institute of Standards and Technology, Gaithersburg, MD, USA). Preparation and administration of Ca isotopes Enriched stable isotopes of calcium, 42Ca (98.78% enriched) and 44Ca (94.10% enriched), were obtained (Oak Ridge National Laboratory, Oak Ridge, TN, USA) as the carbonate salts.A sterile, non-pyrogenic solution of 42Ca was prepared by the Pharmaceutical Development Branch of the NIH Clinical Center. On the dose day, the 42Ca was injected intravenously (iv), and the 44Ca, which had been equilibrated in milk overnight, was ingested with breakfast. The doses were approximately 10.5 mg 44Ca and 2.5 mg 42Ca enriched isotope tracers.The study was approved by the Human Studies Committees of the US Department of Agriculture and the University of Maryland. Before the study began, informed written consent was obtained from the subjects. Samples and sample preparation Urine and fecal collections were made for 2 weeks, plasma or serum and breast milk samples were taken for the Ærst two days and at 1 and 2 weeks post-dose. The urine, milk, and serum samples were stored frozen while the fecal samples were homogenized and lyophilized prior to storage at 220 �C.Urine samples were prepared using a method similar to that described by Turnlund et al.13 A 1 mL volume of urine was pipetted into a small conical tube and centrifuged to remove suspended particles. A portion of the supernatant, 0.8 mL, was transferred into a second tube to which wdded 0.2 mL of a saturated solution of ammonium oxalate. After sitting at room temperature overnight, the samples were centrifuged and the supernatant was discarded.The remaining calcium oxalate precipitate was washed twice with 0.6 mL of de-ionized water and then dissolved in 0.1 M nitric acid. Serum and milk samples (0.2 and 0.05 mL, respectively, this being the minimum amount of sample necessary for the analysis) were weighed into acid-cleaned quartz test-tubes, lyophilized, and dry-ashed overnight at 480 �C. The ash was dissolved in a minimum amount of 0.1 M nitric acid and the calcium separation was completed as described above.Approximately 0.2 g of feces was weighed into acid-cleaned 166100 mm borosilicate tubes. The samples were dry-ashed and then wet-ashed16 to complete the digestion. After dissolution in 10 mL of 0.1 M nitric acid, 0.2 mL of the solution was taken through the oxalate separation described above. Instrumental analysis The samples, containing calcium oxalate, were diluted in 0.1 M nitric acid to a concentration of about 400 mg L21 Ca and the instrument was adjusted to give a count rate of about 3±46105 counts s21 for the 44Ca isotope.Higher count rates were avoided since this could result in gain suppression or `sag'17 and therefore could affect the accuracy of the isotope ratio measurements. The isotope ratios 44Ca : 43Ca and 42Ca : 43Ca were measured. Every four subject samples were bracketed by unenriched samples of the same type and concentration that had also been carried through the calcium separation procedure.This allowed for correction of instrumental mass bias and drift. Procedural blanks were subtracted from each reading but were typically v0.3% of the sample signal. Calculations The amount of stable isotope tracer is determined from the ratio of the tracer isotope to a reference isotope and from the total amount of the element in the sample. The isotope ratio is measured by mass spectrometry, and a second analytical method, such as atomic absorption spectrometry (AAS), is used to determine the total amount of the element.The amount of the tracer can be determined using the following calculations, which are similar to those given by Turnlund et al.18 T~mnzmt Ö1Ü where T is the total amount of the element, mn is the amount of the natural element, and mt is the amount of the stable isotope tracer in mass units. The value for T is determined by an analytical method that is not isotope speciÆc and gives the total amount of the element in mass units such as milligrams.The measured ratio, Ri/x, is equal to the moles of the tracer isotope in the sample divided by the moles of the reference isotope: Ri=x~ mn n .Ainz mt Wt .Ait mn Wn .Axnz mt Wt .Axt Ö2Ü where: Wn~the atomic weight of the natural element, Wt~is the atomic weight of the enriched tracer material, and A is used to designate atomic abundance with the subscripts indicating the isotope and the source of the isotope, viz., i~tracer isotope, x~reference isotope, n~natural element, and t~enriched stable isotope tracer.Rearranging eqn. (1): mn~T{mt Ö3Ü This result can be substituted into eqn. (2) and then solved for the amount of tracer based on the total amount of the element, the abundances of the isotopes, the atomic weights, and the measured ratio mt~ T.Wt.âAin{ÖRi=x .AxnÜä Wn.âÖRi=x .AxtÜ{AitäzWt.âAin{ÖRi=x .AxnÜä Ö4Ü This equation calculates the amount of enriched stable isotope tracer, not the excess amount of the speciÆc isotope enriched in the tracer.The fact that each tracer contains small Table 1 Instrument and data collection parameters Forward power 650W ReØected power v1W Gas Øows Nebulizer 1.04 L min21 Auxiliary 0.50 L min21 Coolant 12.0 L min21 Sample uptake 0.22 mL min21 Data collection Peak hopping Dwell time/sweep 42Ca 0.0307 s 43Ca 0.0612 s 44Ca 0.0307 s Sweeps/reading 490 Number of readings 6 1674 J. Anal. At. Spectrom., 1999, 14, 1673±1677amounts of all the calcium isotopes and is therefore not 100% enriched is taken into account in the calculations. The fractional absorption of the oral calcium dose is determined by the relative amounts of the two tracers in the iv and oral doses, and in the samples.Per cent. fractional absorption (%FA) is calculated as follows: %FA~ 42Caiv 44Caoral | 44Ca 42Ca |100 Ö5Ü where 42Caiv is the iv dose, 44Caoral is the oral dose, and 42Ca and 44Ca are the amounts of the two tracers in the biological sample, all expressed in molar or mass terms.Results and discussion Interferences Under normal plasma conditions, e.g., high temperature plasmas, the signals for nearly all the calcium isotopes have overlap from interfering ions. The most abundant isotope, 40Ca, has an isobaric interference from the most abundant isotope of argon, 40Arz, and titanium isotopes interfere with the minor calcium isotopes, 46Ca and 48Ca. Strontium with isotopes at 84, 86, 87, and 88 u interferes with 42Ca, 43Ca, and 44Ca since strontium has a relatively low second ionization energy and some Sr ions will be doubly charged.There are also polyatomic ions such as 40Ar1H2 z and 12C16O2 z that interfere with 42Ca and 44Ca, respectively. With a nominal resolution of unity, the quadrupole ICP-MS cannot resolve the Ca peaks from the interfering peaks. Stu»rup et al.14 were able to make Ca isotope ratio measurements by using a high resolution, magnetic sector ICP-MS looking at 42Ca, 43Ca, and 44Ca using a resolution of 4000.The polyatomic peaks could be resolved from the analyte peaks, and correction for the effect of the strontium ions was made based on the signal from doubly charged 87Sr at m/z~43.5. Quadrupole ICP-MS instruments can be designed or modiÆed to operate under cool plasma conditions. This changes the polyatomic interferences observed in the spectra, and virtually eliminates interferences from doubly charged ions since the plasma energy is much lower than under normal conditions. Doubly charged strontium ions were not observed under cool plasma conditions. Fig. 1 shows a typical background spectrum for 0.1 M nitric acid under cool plasma conditions at the calcium masses and, overlaid, the spectrum for a 400 mg L21 Ca standard. The low background at 42, 43, and 44 u makes it possible to measure 42Ca : 43Ca and 44Ca : 43Ca isotope ratios while the interference of Ti at 46 and 48 u remains a problem for the minor Ca isotopes, 46Ca and 48Ca. The 48Ca isotope signal appears as a small increase on the background peak at 48 u.One important aspect of the cool plasma is that the sample matrix needs to be removed prior to analysis. Niu and Houk19 have stated that the technique is primarily useful for analyzing `clean' samples such as de-ionized water and mineral acids. Polyatomic interferences from the sample matrix can cause signiÆcant interferences.A frequently used technique for separating calcium from samples prior to TIMS uses alkaline ammonium oxalate at pH 10.8,9,13 At this pH, magnesium coprecipitates as Mg(OH)2. This may not be a problem in TIMS but it results in interferences at 42 and 43 u from 26Mg16Oz and 25Mg16O1Hz, and 26Mg16O1Hz, respectively. Fig. 2 shows the effect on the 42Ca : 43Ca and 44Ca : 43Ca isotope ratios as a result of various amounts of magnesium contamination in a sample solution with a calcium concentration of 400 mg L21.While this represents only about 0.02 and 0.01% of the Mg as MgOz and MgOHz, respectively, it can have a highly signiÆcant effect on the results, especially from samples with low isotope enrichments. The problem can be avoided by making the Ca separation at pH levels of 8 or below. At an ammonium oxalate solution pH of 8.0, contamination of the samples with Mg was not observed. Janghorbani et al.7 have been able to recover Ca as the oxalate from solutions at pH levels as low as 0.7.Non-spectral interferences from matrix components such as sodium chloride were not a factor since these were virtually eliminated in the sample separation. Accuracy, precision and detection limits Inrder to verify the accuracy of the ratio measurements, a solution of enriched 42Ca of known concentration was obtained and the concentration further veriÆed by reverse isotope dilution analysis. Aliquots of calcium standard were spiked with known amounts of the enriched 42Ca solution and then taken through the same separation procedure as the samples.The theoretical and measured ratios were in good agreement, as shown in Fig. 3. The ultimate limitation in the measurement precision of isotope ratios by ICP-MS is counting statistics as described by Poisson statistics. In addition to counting statistics, sources of noise include plasma Øicker from changes in energy transfer to the plasma, and changes in the efÆciency of sample production and transport.20 Some of this noise can be signiÆcantly reduced by making isotope ratio measurements,21 provided that the Fig. 1 Typical background spectrum for the 0.1Mnitric acid obtained using cool plasma conditions. Overlaid is the spectrum for a 400 mg L21 calcium standard in 0.1M nitric acid. Fig. 2 Effect of various concentrations of magnesium on calcium isotope ratios as the result of MgOH and MgO polyatomic interferences at 42 and 43 u. J.Anal. At. Spectrom., 1999, 14, 1673±1677 1675changes occur with a period greater than twice the sweep time for the acquisition. The measurement precision of isotope ratios will be affected by changing conditions that affect one isotope differently from another. Begley and Sharp21 suggest two possible sources of this type of noise: mass bias, and mass calibration imprecision. Ion transmission in the ion optics and quadrupole of ICP-MS instruments varies with mass and can drift over time.This problem was minimized in these analyses by normalizing experimental ratios to standard samples with known isotope ratios, e.g., natural ratios, frequently during each analytical run. The observed drift in the ratio measurement was small, typically less than °0.25% over several hours. The average day-to-day results for the analysis of unenriched samples of various types are given in Table 2. The calcium ratios from unenriched biological materials compare well with those expected for calcium standards, suggesting that the method has little or no systematic bias.Even with optimized conditions, the precision of the ratio measurements has been reported to be 2±3 times the precision that can be attributed to counting statistics alone.21,22 Typical isotope measurement precisions using this method were found to be about 1.5 times that predicted by Poisson statistics. The average %RSD within-run for a sample reading with the instrumental conditions described earlier was about 0.25% for both the 42Ca:43Ca and 44Ca:43Ca isotope ratios.With six readings, the RSD of the mean (relative standard error) of the ratio value is about 0.10% (0.25%/d6). The uncertainty in making the isotope ratio measurements can be used to determine the limit of detection (LOD) for the enriched calcium tracers. The LOD for a tracer is dependent on the amount of natural element present in the sample; the enriched isotope from the tracer must be detected in the presence of the same isotope from the natural element.As a result, the more natural element that is present in a sample, the higher the LOD will be for a tracer in terms of absolute amount. It is more useful to express LOD on a relative basis. The relative amount of tracer present in a sample can be given in terms of % enrichment deÆned as: %enrichment~ R{R R |100 Ö6Ü where R is the natural 42Ca : 43Ca or 44Ca : 43Ca tracer-toreference isotope ratio and R* is the same ratio measured for the sample.When no enriched tracer is present in a sample, R*~R. If the LOD is taken to be the measured ratio plus three times the standard deviation of the mean ratio when no tracer is present, then at the LOD, R*~Rz3sR*, where sR* is the standard deviation of the mean value of R*. Substituting these into eqn. (6) gives: %enrichment~ Rz3sR{R R |100% Rz3sR{R R |100 Ö7Ü or %enrichment~ 3sR R |100~3|%RSDR Ö8Ü Based on the RSD of the mean ratio measurements, the LOD for the two enriched isotope tracers, 42Ca and 44Ca, in a sample is estimated to be 0.3% enrichment.Analysis of biological samples enriched in 42Ca and 44Ca Following administration of the Ca doses, both the oral tracer, 44Ca, and the iv tracer, 42Ca, appeared rapidly in the breast milk. As seen in Fig. 4, the results for a typical subject show the injected tracer being incorporated into the milk by the Ærst feeding post-dose and the oral tracer in the second feeding.Each point on the graph represents one feeding. The cumulative fraction of the doses found in the breast milk for one subject over the Ærst 48 h was about 6.0% of the iv and 1.4% of the oral. The %FA of about 26% found for this subject compares well with the mean value of 30% reported for similar subjects by Kalkwarf et al.5 The two doses do not appear to equilibrate within the Ærst 24 h, suggesting that %FA calculations can better be made on samples collected in the second day or later after dosing.Fig. 5 Fig. 3 Comparison of theoretical and measured ratios of 42Ca and 44Ca in a spiked standard calcium solution. Table 2 The 42Ca : 43Ca and 44Ca : 43Ca ratios in various biological materials corrected for mass bias Material 44 : 43 ratio s 42 : 43 ratio s n Urine 15.4658 0.0242 4.7954 0.0118 20 Serum 15.4500 0.0124 4.7990 0.0096 5 Milk 15.463 0.0212 4.7963 0.0038 6 Feces 15.4649 0.0213 4.7957 0.0082 19 Theoretical 15.4519 4.7926 Fig. 4 Appearance of the 42Ca tracer from the iv dose and the 44Ca tracer from the oral dose in breast milk. Each time point on the graph represents one feeding. 1676 J. Anal. At. Spectrom., 1999, 14, 1673±1677compares %FA calculations for one subject from urine, plasma, and breast milk data for the Ærst and second day after dose. By the second day, the tracers appear to have equilibrated, giving nearly the same result for the %FA regardless of the type of sample analyzed.Quadrupole ICP-MS modiÆed to operate in the cool plasma mode can be used to analyze for stable isotope tracers of calcium. The low background and lack of interferences for the minor isotopes, 42Ca, 43Ca, and 44Ca, make it possible to analyze samples from double-labeling experiments designed to determine fractional absorption. The results of this study compare well with those reported for quadrupole thermal ionization but with smaller sample requirements and less rigorous sample preparation.The need for high resolution ICPMS is avoided by eliminating doubly charged interferences and by reducing polyatomic interferences to insigniÆcant levels. The new generation of ICP-MS instruments include the option of operating in the cool plasma mode. This should improve the availability of analytical instrumentation for calcium stable isotope analyses in metabolic studies. References 1 Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride, Food and Nutrition Board, National Academy of Sciences, Washington, DC, 1997. 2 S. A. Abrams, M. A. Grusak, J. Stuff and K. O. O'Brien, Am. J. Clin. Nutr., 1997, 66, 1172. 3 S. A. Abrams and J. E. Stuff, Am. J. Clin. Nutr., 1994, 60, 739. 4 C. M. Weaver, J. Nutr., 1994, 124, 14185. 5 H. J. Kalkwarf, B. L. Specker, J. E. Heubi, N. E. Vieira and A. L. Yergey, Am. J. Clin. Nutr., 1996, 63, 526. 6 L. Lutwak, Am. J. Clin. Nutr., 1969, 22, 771. 7 M. Janghorbani, A. Sundaresan and V.R. Young, Clin. Chim. Acta, 1981, 113, 281. 8 S. Fairweather-Tait, A. Prentice, K. F. Heumann, L. M. A. Jarjou, D. M. Stirling, S. G. Wharf and J. R. Turnlund, Am. J. Clin. Nutr., 1995, 62, 1188. 9 A. L. Yergey, N. E. Vieira and J. W. Hansen, Anal. Chem., 1980, 52, 181. 10 D. L. Smith, Anal. Chem., 1983, 55, 2391. 11 D. L. Smith, C. Atkin and C. Westenfelder, Clin. Chim. Acta, 1985, 146, 97. 12 X. Jiang and D. L. Smith, Anal. Chem., 1987, 59, 2570. 13 J. R. Turnlund, W. R. Keyes, K. C. Scott and R. A. Ehrenkranz, J. Anal. At. Spectrom., 1993, 18, 983. 14 S. Stu» rup, M. Hansen and C. Molgaard, J. Anal. At. Spectrom., 1997, 12, 919. 15 K. Sakata and K. Kawabata, Spectrochim. Acta, Part B, 1994, 49, 1027. 16 A. D. Hill, K. Y. Patterson, C. Veillon and E. R. Morris, Anal. Chem., 1986, 58, 2340. 17 F. Vanhaecke, G. de Wannemacker, L. Moens, R. Dams, C. Latkoczy, T. Prohaska and G. Stingeder, J. Anal. At. Spectrom., 1998, 13, 567. 18 J. R. Turnlund, M. C. Michel, W. R. Keyes, J. C. King and S. Margen, Am. J. Clin. Nutr., 1982, 35, 1033. 19 H. Niu and R. S. Houk, Spectrochim. Acta, Part B, 1996, 51, 779. 20 M. Carre�, E. Poussel and J.-M. Mermet, J. Anal. At. Spectrom., 1992, 7, 791. 21 I. S. Begley and B. L. Sharp, J. Anal. At. Spectrom., 1994, 9, 171. 22 J. K. Friel, H. P. Longerich and S. E. Jackson, Biol. Trace Elem. Res., 1993, 37, 123. Paper 9/00677J Fig. 5 Fractional absorption of the calcium oral dose on urine, serum, and breast milk. The urine values were from 24 h collection, the serum samples were taken at 24 and 48 h after the dose, and the breast milk was the average from individual feedings taken over 24 h. J. Anal. At. Spectrom., 1999, 14, 1673±1677 16

ISSN:0267-9477    

DOI:10.1039/a900677j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Studies on the quantitative analysis of trace elements in single SiC crystals using laser ablation-ICP-MS{ Erwin Hoffmann,*a Christian Lu»dke,a Jochen Skole,a Heike Stephanowitzb and Gu»nther Wagnerc aInstitut fu»r Spektrochemie und angewandte Spektroskopie (ISAS), Institutsteil Berlin, Rudower Chaussee 5 D-12489 Berlin, Germany bGesellschaft zur Fo»rderung angewandter Optik, Optoelektronik, Quantenoptik und Spektroskopie e.V., Rudower Chaussee 5 D-12489 Berlin, Germany cInstitut fu»r Kristallzu»chtung, Rudower Chaussee 6 D-12489 Berlin, Germany Received 14th June 1999, Accepted 25th August 1999 A technique has been developed for the simultaneous determination of the trace elements Al, Cu, Mn, V, Ti and Fe in single SiC crystals by laser ablation-ICP-MS.The wavelength of 1064 nm of an Nd:YAG laser operating in the `free-running' mode is used for ablation, because in the Q-switched mode the signal-to-noise ratio was too low to obtain the analytical reproducibility required.Sample preparation is simple, as only puriÆcation of the crystal surface with HF is necessary. To prepare calibration standards, multi-element solutions were added to pure SiC powder. The powder was dried, homogenized and pressed to pellets using carbon powder as binding material. The veriÆcation of the calibration was carried out with powdered SiC reference materials. Acceptable recovery rates of between 95 and 110% were obtained. The limits of detection were between 361029 g g21 (V) and 461028 g g21 (Cu).Relative standard deviations measured at trace element concentrations of about 161026 g g21 were less than 10% when the intensities of 10 craters each with 500 laser shots were averaged. Silicon carbide (SiC) forms colourless crystals of hexagonal or rhombic structure. SiC turns glistening-green to blue-black when other atoms are incorporated into the crystal lattice. A number of modiÆcations exist which differ from each other by the structure formed while the crystal is growing.1 SiC crystals promise special applications in the electronics industry because of their semiconductor properties and, for these purposes, they are required to be of very high purity.In order to guarantee the purity of this material, it is necessary to determine quantitatively the concentrations of a number of trace elements, such as Al, V, Cu, Fe, Mn and Ti. Detection limits should not exceed 161027 g g21. A variety of instrumental techniques have been applied to determine trace impurities in SiC powder.2,3 These include instrumental neutron activation analysis (INAA),4 X-ray Øuorescence spectrometry (TXRF),5,6 analysis by ICP-OES4,7 or ICP-MS8 subsequent to sample dissolution without and with separation of the analyte elements from the matrix and direct analysis by slurry atomization ICP-OES,4 slurry atomization ICP-MS9 and laser ablation-ICP-MS.2 Procedures with sample dissolution suffer from high blank signals because, in most cases, fuming sulfuric acid is used.Therefore the detection power for environmental elements such as Al, Fe and Cu is poor.10 Slurry atomization of SiC powders combined with ICP-OES or ICP-MS is more advantageous. However, the technique is limited because powder particles with a diameter larger than 10 mm can be neither transported by a carrier gas nor completely evaporated in an ICP.11 Slurry ICP-OES has detection limits for the elements mentioned above in the range from 361027 g g21 for Ti to 2.161027 g g21 for Al.12 For most elements, the TXRF method has detection limits higher than those of optical atomic spectrometry.13 However, the repeatability of the determinations is quite satisfactory.TXRF is suited for trace element determination in bulk material when matrix separation and element enrichment are possible or when the concentrations of the analytical elements are in the upper 1026 g g21 range. In this case, the technique is advantageous because it is fast and sampling is easy.SiC crystals are extremely difÆcult to dissolve quantitatively for solution analysis. Equally, it is difÆcult to powder SiC crystals to the small particle size and narrow size distribution necessary for slurry sampling. These procedures are not only time consuming, but bear an increased contamination risk. Because of this, a technique is required that is capable of analysing the SiC crystals without using a complicated sample pretreatment. Laser ablation linked to ICP-MS (LA-ICP-MS) is in principle suited to solve the analytical problem of trace element determination in solids such as SiC crystals.14 The aim of this work was to develop a technique for the quantitative analysis of trace elements of analytical interest in single SiC crystals using LA-ICP-MS.It is planned to use the technique for the process control of SiC crystal production. Additionally, the results of this study are intended to demonstrate the potential of LA-ICP-MS as a quantitative analytical technique for the determination of impurity elements in materials such as SiC. Unfortunately, there are no certiÆed SiC reference materials which are suitable for use as calibration standards for trace element determinations.However, a powder of satisfactory purity is available which can be spiked with analyte elements. {Dedicated to Prof. Dr. D. Klockow on the occasion of his 65th birthday. J.Anal. At. Spectrom., 1999, 14, 1679±1684 1679 This Journal is # The Royal Society of Chemistry 1999Experimental Instrumentation An Elan 6000 ICP mass spectrometer (Perkin-Elmer SCIEX Instruments, Toronto, Canada) was used for this work. The system incorporates a 40 MHz free-running radiofrequency generator as the ionization source coupled with an adiabatic plasma sampling interface.15 A Model 320 laser sampler (Perkin-Elmer SCIEX Instruments) was used for sampling into the ICP.The system is based on an Nd:YAG laser, running at 1064 nm. This can be operated in either `freerunning' or `Q-switched' mode. Additionally, equipment for higher harmonic generation (532 nm, 355 nm, 266 nm) and wavelength separation is installed.16 Inside the sample chamber used throughout this study is a small moveable sample stage with a mounting for single SiC crystals. The ablated material was transferred from the laser sampler to the Elan 6000 using 1 m of 5 mm id PVC tubing as used by Gray.17 Preparation of calibration standards To compensate for matrix effects, it is necessary to match the standards to the sample material.Consequently, a puriÆed SiC powder (concentration of the analyte elements lower than the required detection limits mentioned above) was used for the preparation of the calibration standards; multi-element solution standards were added. The wet powder was dried and homogenized in an SiC mortar. Materials in a powdered state cannot be ablated by a laser beam, as the shock wave of the laser pulse scatters the powder particles. It is therefore necessary to immobilize the material, which is possible by use of a binder,14 by pressing to a pellet or by both.The latter procedure was selected in this study. Pellets were prepared by mixing the doped SiC powder with homogeneous carbon powder (weight ratio 3 : 1 respectively) as binding agent, followed by mechanical shaking. Approximately 3 g of the mixture was pressed in a 20 mm diameter pellet disc under a pressure of approximately 8 bar for 5 min.A special sample preparation was not necessary. Handling was carefully carried out under clean bench conditions to protect the sample surface from contamination. Samples and the SiC powder used for the preparation of the standards were received from the Institut fu» r Kristallzu» chtung, 12489 Berlin, Germany. SiC reference materials and reagents were purchased from the following providers: (a) Standard MRC 780-1 from IRSID, B.P. 320, 57214 Maizieres-Le�s-Metz Cedex, France; (b) NIST standard reference material 112b from National Bureau of Standards, Gaithersburg, USA; and (c) SiC F400 from ESK, 52428 Ju» lich, Germany and HF and HNO3 from Merck KgaA, 64293 Darmstadt, Germany. Time-resolved signal studies Initial studies showed that t sensitivity of the LA-ICP-MS instrument was adequate for the determination of the analyte elements over the required concentration range.Therefore, a special optimization of the sensitivity was not necessary. However, the precision of the analytical signals was insufÆcient when the laser was operated in the Q-switched mode as it is usually used. Time-resolved laser ablation signals of the analyte elements were investigated with the aim to improve the reproducibility. Measurements were made at various power settings and laser wavelengths. Figs. 1±3 show signal versus time plots for single craters at m/z (mass to charge ratio) 63 (63Cu) and m/z 65 (65Cu).The sample was an SiC crystal with about 161026 g g21 Cu concentration. Figs. 1 and 2 show the results obtained with 1064 nm pulses for 0.23 J and 161022 J, respectively. For the results in Fig. 3, the fourth harmonic at 266 nm with 1.461023 J pulse energy was used. It was found that the ion number ratio of m/z 63 to m/z 65 differs from the isotope ratio of 63Cu to 65Cu by more than a factor of two, and that all the time-resolved laser ablation signals have a number of ion intensity spikes which result in much higher signal deviations than we are used to determining with solution nebulization.It was found that the intensity spikes at m/z 63 are not correlated to those at m/z 65 and, therefore, they cannot be caused by Cu ions, as also becomes apparent from the comparisons shown in Figs. 1±3. This conØicts with the explanation given in the literature that intensity spikes are a result of unstable plasma conditions which are caused by ablated particles of different diameter passing into the plasma.14 When the laser is operated in the free-running mode, spikes are not observed.In Fig. 4, the signal versus time plot of a signal in the free-running mode is shown (1064 nm, 0.038 J pulse energy, 10 Hz pulse frequency). The ion number ratio of m/z 63 to m/z 65 (2.22°0.06) was found to be in good agreement with the isotope ratio of 63Cu to 65Cu (2.247).The reproducibility was adequate for the trace element determination required. Fig. 1 Ion intensity of 63Cu and 65Cu as a function of the ablation time (laser wavelength, 1064 nm; Q-switched mode; pulse frequency, 10 Hz; pulse energy, 0.23 J). Fig. 2 Ion intensity of 63Cu and 65Cu as a function of the ablation time (laser wavelength, 1064 nm; Q-switched mode; pulse frequency, 10 Hz; pulse energy, 161022 J). Fig. 3 Ion intensity of 63Cu and 65Cu as a function of the ablation time (laser wavelength, 266 nm; Q-switched mode; pulse frequency, 10 Hz; pulse energy, 0.0014 J). 1680 J. Anal. At. Spectrom., 1999, 14, 1679±1684Although it has been reported that ion intensity spikes can interfere with analyte signals, the processes leading to this phenomenon are not well known.18,19 Therefore, further investigations were performed to attempt to understand the signal Øuctuations of LA-ICP-MS using SiC as target material. Fig. 5 shows the distribution of the time-resolved ablation signals at m/z 65 (Q-switched: 1064 nm, 0.18 J, 10 Hz, about 161026 g g21 Cu, 30 s measuring time, 12 measurements; free running: 0.26 J, the other parameters are the same as for the Qswitched mode). The x-axis represents the ranges of the signal height.The following ranges were selected: 10±20; º90±100; 100±200; º900±1000; 1000±2000; º9000±10 000 ions per 40 ms. The number of measured signals in each range was normalized (divided by the number of the Ærst range) so that the graphs of the Q-switched mode and the free-running mode could be shown in the same Ægure.The Q-switched graph has two maxima, indicating two groups of signals, Qs I and Qs II, which must be distinguished. We found that the signals of the Qs II group cause the low reproducibility measured in the initial investigations. In contrast, the reproducibility of the signals of Qs I is high and comparable to that of solution nebulization. The distribution of the signal heights in the freerunning mode is similar to that of the Qs I group as shown in Fig. 5. The two signal ensembles of the Q-switched laser mode were also found when the ion intensities of the other analyte isotopes were measured. The signals of the Qs II group can be easily separated from those of the Qs I group by a statistical test of outlying observation.20 Fig. 6 shows the distribution (in %) of the number of the Qs II signals as a function of the mass to charge ratio.Spikes at mass to charge ratios larger than m/z 85 were not observed. To clarify the origin of the Qs I and Qs II ensembles, ion number ratios were calculated. The ratio of Qs I at m/z 63 to Qs I at m/z 65 was found to be 2.20°0.06, which is near the theoretical value of the isotope ratio of 63Cu to 65Cu. The corresponding ratio of Qs II was 0.75°0.13. We can conclude from this that the Qs II group in Fig. 5 originates from electrically charged particles (clusters) with the same charge to mass ratio as the 65Cu isotope.The time-resolved studies provide a possible explanation of the noisy nature of the laser ablation signals in the Q-switched mode. It can be supposed that particles broken off by the shock of the short laser pulse and transported by the carrier gas into the inductively coupled plasma have such a size that they are not vaporized but only ionized. Then the ionized clusters are transferred into the mass analyser and detected as ions, thus interfering with the analytical signals according to their charge to mass ratios.The ablation process of a free-running pulse is different from the Q-switched mode. It takes more time [Q-switched: (5± 10)61029 s; free-running: more than 161026 s], and consequently thermal processes are dominant during the ablation, as the melted rim of the crater in Fig. 7(a) conÆrms. Obviously, the ablated material is more homogeneously dispersed and is completely vaporized in the inductively coupled plasma.Fig. 7(b) shows a picture of a crater generated by one Qswitched pulse. The crater wall shows the layer structure of the material which was ablated without melting. Instrumental operating conditions As has been described above, it was found that the signals obtained from laser pulses in the free-running mode gave a better reproducibility and reliability of the results for the required analytical elements than the signals from Q-switched laser pulses.Consequently, the free-running mode of laser operation was used for the analytical procedure. The maximum signal-to-noise ratio was obtained if the laser was set at a pulse energy of 0.23 J and sharply focused at the surface of the sample. Although the pulse reproducibility of the laser was good, the amount of material ablated varied considerably from crater to crater. The precision of the analytical results is, therefore, Fig. 4 Ion intensity of 63Cu and 65Cu as a function of the ablation time (laser wavelength, 1064 nm; free-running mode; pulse frequency, 10 Hz; pulse energy, 0.038 J; SiC crystal; about 161026 g g21 Cu).Fig. 5 Number of signals (normalized) as a function of the signal groups at m/z 65: –, Q-switched mode; - - -, free-running mode. Fig. 6 Number of type Qs II signals as a function of the charge to mass ratio (laser wavelength, 1064 nm; pulse frequency, 10 Hz; Q-switched mode; pulse energy, 0.18 J; number of pulses, 300; SiC crystal).J. Anal. At. Spectrom., 1999, 14, 1679±1684 1681greatly improved by use of internal standardization. 29Si as well as 30Si can be used as internal standards for the determination of trace elements in SiC. They are of sufÆciently low abundance (4.7% for 29Si, 3.1% for 30Si) so that the detector is not overloaded. 12C and 13C have to be excluded as internal standards because carbon is used as binder in the calibration standard pellets. Suitable analyte isotopes could be found for the required elements.Only Al has a small background ion interference caused by the 28Si isotope. The interference is a result of abundance sensitivity overlap, because the concentration ratio between Si and Al is extremely high. A summary of the operating conditions and the isotopes selected for the analyte elements is shown in Table 1. Results Calibration, limit of detection, precision Calibration curves with least-squares regression correlation coefÆcients of better than 0.99 were obtained for all the elements analysed.The slope value determines the sensitivity of the element determination. The relative standard deviations of the slope values are listed in Table 2. Additionally, the relative signal standard deviations (analyte concentration of about 161026 g g21) for one crater ablation are given in Table 2. Table 2 also contains the limits of detection (LOD) of the elements of interest. Non-spiked SiC powder was used for the determination of the detection limits.After mixing the SiC with carbon powder (see `Preparation of calibration standards') and pressing to a pellet, we carried out 11 measurements and calculated the detection limits using the 3s criterion for the background deviations of the signals. To verify the calibration, the SiC powder reference materials mentioned above were prepared in the same way as the calibration standards and analysed. The results are shown and compared with the certiÆed values in Table 3.The averaged recovery rates deÆned as the ratio `concentration found' to `concentration given' in per cent are also shown in Table 3. The concentrations found agree acceptably with the certiÆed concentrations. Repeatability Measurement precision and sample homogeneity are two factors affecting the repeatability of the laser ablation technique. Laser ablation creates only small craters with vaporized sample masses in the mg to mg range depending on the pulse energy and pulse number used.The analytical results, therefore, are only representative of the small amounts of material ablated by the laser beam. Consequently, one of the advantages of laser sampling–the capability of spatially resolved analysis–simultaneously turns out to be a disadvantage when bulk representative concentration values are Fig. 7 Laser crater in SiC: (a) free-running mode (1064 nm; single pulse; 0.25 J; crater diameter, about 0.4 mm); (b) Q-switched mode (1064 nm; single pulse; 0.25 J; crater diameter, about 0.4 mm).Table 1 Instrumental and analytical parameters Laser Wavelength 1064 nm Pulse energy 230 mJ Mode Free running Frequency 10 Hz Ablation time per crater 50 s Crater diameter About 200 mm Crater depth About 200 mm Ablated mass About 20 mg Number of craters 10 ICP Electric power 1 kW Transport gas 1 l min21 Dwell time 100 ms Replicates 40 Resolution High Sample Preparation Surface puriÆcation with HF Calibration SiC powder, doped with multi-element solution standards, bound with carbon powder, pressed to pellets (8 bar) Validation SiC reference materials Isotopes 48Ti, 51V, 55Mn, 56Fe, 60Ni, 63Cu, 27Al Internal standard 30Si, 29Si Table 2 Detection limits (based on 3s criterion), relative standard deviation of the slope of the calibration curve (eight measurements at each concentration, seven concentration values for each element), variation of the concentration from crater to crater in a single SiC crystal, relative standard deviation of the single crater ablation Relative standard deviations (%) Concentration variation from crater to crater (%) Isotope Detection limit/mg g21 Slope Single crater 27Al 0.020 1.2 2.8 17 49Ti 0.025 2.9 3.8 25 51V 0.003 1.4 3.2 20 55Mn 0.030 4.1 4.8 30 57Fe 0.020 1.8 3.1 22 63Cu 0.040 1.3 3.0 24 1682 J.Anal. At. Spectrom., 1999, 14, 1679±1684required. To assess the accuracy of the method when applied to real samples, the homogeneity of a typical industrial plant SiC crystal was studied.In the Ærst step, the surface impurity of the sample was studied. SiC crystals grow at such a high temperature that the elements which have to be determined are in the vapour phase during the whole process. After crystallization, the crystal begins to cool down. Condensation on the surface and diffusion into the crystal takes place. Therefore, the surface and outer crystal layers are expected to show increased trace element content.To determine the concentration±depth proÆle, the laser was defocused so that a larger area of the sample was ablated. Then the signals of 10 laser pulses were measured and the element concentrations were determined with the help of the calibration curves. Concentration±depth proÆles of Cu before and after puriÆcation with HF are shown in Fig. 8, indicating that an acid-cleaned surface is necessary to obtain reliable bulk analytical results. Further studies were focused on the question of whether signiÆcant differences can be observed between the signal precision of the one-crater ablation and the repeatability of replicate measurements carried out at different sites (distance between two laser spots, 1 mm) of the sample.The calculation of the correlation coefÆcients showed the statistical independence of the analytical signals for one-crater ablation.21 The relative standard deviations for the signals of 40 measured craters are given in Table 2.It was found that the concentrations of 10 craters have to be averaged to obtain repeatabilities within the 10% level for the bulk analysis of SiC crystals. Conclusion and outlook A rapid technique, involving only puriÆcation of the surface by HF as sample pretreatment, has been developed for the determination of trace elements in single SiC crystals by LAICP- MS. If the laser is operated in the Q-switched mode, the signal deviations of the elements investigated are high, owing to the spiky nature of the ablation signals.However, in the freerunning mode, the relative standard deviations are comparable to those usually obtained by solution methods and adequate for the purpose of this work. Limits of detection are less than 561028 g g21 for all the analytical elements investigated. Our studies have indicated that the low signal-to-noise ratio in the Q-switched mode is due to large ablation particles, which are transported into the plasma by the carrier gas but do not vaporize.In the free-running laser mode, particles introduced into the plasma are small enough to vaporize. It is assumed that the large particles created at the beginning of ablation are vaporized due to absorption of subsequent laser spikes in the free-running mode. However, the free-running mode has two analytical disadvantages: the radiation intensity of the Nd:YAG wavelength is usually too low to generate higher harmonics, and fractional volatilization of analyte elements with low boiling points may take place.To use the advantages of the Q-switched mode, the ablation can be combined with a second laser pulse which arrives at the sample surface with a short time delay. The second pulse vaporizes the large particles which were broken off the sample by the shock wave.22 The second laser beam can be generated by splitting the original beam with a mirror or a prism. A time difference between the arrival of both laser beams at the sample surface can be obtained if the distances which the beams have to run are different.The difference between the beam paths must be about 3 m, which can be realized with the use of several beam reØections at mirrors. The ability to obtain spatial information on element distribution is one of the advantages of the laser system. This aspect can also be important for bulk analysis, which has been demonstrated by investigating the sample homogeneity.Acknowledgements The Ænancial support by the Senatsverwaltung fu» r Wissenschaft, Forschung und Kultur des Landes Berlin and the Bundesministerium fu» r Bildung und Forschung is gratefully acknowledged. H. Stephanowitz also thanks the `Gesellschaft zur Fo»rderung angewandter Optik, Optoelektronik, Quantenelektronik und Spektroskopie e.V.' for a grant. References 1 G. P. Fritz and E. Matern, in Carbosilanes, Springer, Berlin, 1986. 2 H. Nickel and J. A. C. Broekaert, Fresenius' J.Anal. Chem., 1999, 363, 145. 3 J. A. C. Broekaert, R. Brandt, F. Leis, C. Pilger, D. Pollmann, P. Tscho» pel and G. To» lg, J. Anal. At. Spectrom., 1994, 9, 1063. 4 T. Graule, A. von Bohlen, J. A. C. Broekaert, E. Grallath, R. Klockenka»mper, P. Tscho» pel and G. To» lg, Fresenius' J. Anal. Chem., 1989, 335, 637. 5 A. von Bohlen, R. Eller, R. Klockenka»mper and G. To» lg, Anal. Chem., 1987, 59, 2551. 6 M. Franek and V. Krivan, Fresenius' J. Anal. Chem., 1992, 342, 118. 7 G. Zaray, F. Leis, T. Kantor, J. Hassler and G. To» lg, Fresenius' J. Anal. Chem., 1993, 346, 1042. 8 C. Pilger, F. Leis, P. Tscho» pel, J. A. C. Broekaert and G. To» lg, Fresenius' J. Anal. Chem., 1995, 351, 110. 9 F. Kohl, N. Jakubowski, R. Brandt, C. Pilger and J. A. C. Broekaert, Fresenius' J. Anal. Chem., 1997, 359, 317. Table 3 CertiÆed concentrations and measured concentrations with reference to the appropriate reference material MRC 780-1 concentration/mg g21 NIST 112b concentration/mg g21 F400 concentration/mg g21 Isotope Certif.Measured Certif. Measured Certif. Measured Recovery rate averaged (%) 27Al 18 600 18 520°500 4200a 4600°500 1200 1262°100 105°7 49Ti – – 230a 220°15 280 265°19 95°7 51V – – – – 1100 1095°60 100°5 55Mn 290 293°6 – – 34 35°2 102°4 57Fe 13 040 14 500°1000 1300 1450°150 3100 3360°250 110°8 63Cu – – – – 90 94°3 105°6 aGiven but non-certiÆed concentration. Fig. 8 Cu concentration as a function of the number of pulses (depth proÆle) (free-running mode; pulse energy, 0.23 J; wavelength, 1064 nm; SiC crystal; crater diameter, about 0.5 mm; maximum crater depth estimated at 0.2 mm). J. Anal. At. Spectrom., 1999, 14, 1679±1684 168310 T. Graule, PhD Thesis, University of Dortmund, Germany, 1988. 11 B. Raeymackers, T. Graule, J. A. C. Broekaert, F. Adams and P. Tscho» pel, Spectrochim. Acta, Part B, 1988, 43, 923. 12 L. E. Ebdon and M. R. Cave, Analyst, 1982, 107, 172. 13 L. E. Ebdon and J. R. Wilkinson, J. Anal. At. Spectrom., 1987, 2, 325. 14 S. A. Darke, S. E. Long, C. J. Pickford and J. F. Tyson, Fresenius' J. Anal. Chem., 1990, 337, 284. 15 D. J. Douglas and J. B. French, Spectrochim. Acta, Part B, 1986, 41, 197. 16 E. Hoffmann, C. Lu»dke and H. Stephanowitz, Fresenius' J. Anal. Chem., 1996, 355, 900. 17 A. L. Gray, Analyst, 1985, 110, 551. 18 S. T. G. Anderson, R. V. D. Robert and H. N. Farrer, J. Anal. At. Spectrom., 1992, 7, 1195. 19 P. M. Outrige, W. Doherty and D. C. Gregoire, Spectrochim. Acta, Part B, 1996, 51, 1451. 20 U. Graf and H. J. Henning, Mitteilungsblatt Mathematische Statistik, 1952, 4, 1. 21 H. Scholze, E. Hoffmann, C. Lu»dke and A. Platalla, Fresenius' J. Anal. Chem., 1996, 355, 892. 22 J. Uebbing, J. Brust, W. Sdorra, F. Leis and K. Niemax, Appl. Spectrosc., 1991, 45, 1419. Paper 9/04734D 1684 J. Anal. At. Spectrom., 1999, 14, 1679±1684

ISSN:0267-9477    

DOI:10.1039/a904734d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of trace metals in size fractionated particles from arctic air by electrothermal vaporization inductively coupled plasma mass spectrometry{ Christian Lu»dke,*a Erwin Hoffmann,a Jochen Skolea and Michael Kriewsb aInstitut fu»r Spektrochemie und Angewandte Spektroskopie, Institutsteil Berlin, Albert-Einstein-Strasse 9, 12489 Berlin-Adlershof, Germany bAlfred-Wegener-Institut fu»r Polar- und Meeresforschung, Am Handelshafen 12, 27570 Bremerhaven, Germany Received 12th May 1999, Accepted 27th August 1999 Studies of element composition in small atmospheric particles aid the clariÆcation of processes such as longrange transport, deposition and transformation of particles and quantiÆcation of emission from natural and anthropogenic sources.For this purpose, a highly sensitive method was developed for the trace analysis of atmospheric particles. The particles were sampled and separated according to size, directly on separate small graphite discs arranged behind the jet-nozzles of an eight-stage cascade impactor.To determine the elemental composition of the particles, the ETV-ICP-MS technique was applied. In an appropriately sealed electrothermal vaporizer, linked to an inductively coupled plasma mass spectrometer, the targets were heated and the sample vapour was swept by argon into the plasma. The system described was used for the analysis of long-range transported particles from Arctic air sampled at the German Arctic research station at Spitsbergen, Norway, in spring 1998.For the elements Mn, Fe, Co, Ni, Ag, Cd, Sn, Sb and Pb the trace element content per cubic metre of air was measured as a function of the aerodynamic particle diameter. Air masses of different origin cause characteristic particle distributions at low changes in total dust burden. The relative detection limits for the elements measured in an air volume of 0.275 m3 were determined to be within 0.3±10 pg m23; the overall analytical precision was around 20% for all trace metals.Introduction Both natural and anthropogenic sources emit particles into the atmosphere but the predominant part of metallic trace elements is of anthropogenic origin.1 Potentially toxic metals such as Pb, Cd, Ni, Sb and Tl associated with atmospheric particulate matter are emitted into the atmosphere mostly by burning of fossil fuels, metallurgical plants and smelters and increasingly by waste incineration.2 Ambient atmospheric particles from anthropogenic and natural sources were found to be predominantly oxides, sulfates, chlorides and silicates such as feldspars and black mica.3,4 The greater part of particulates emitted is deposited about the vicinity of the source but, depending on the meteorological conditions and particle size, they may be subjected to long-range transport and will reach remote areas far away from the source regions.Especially in the northern hemisphere where anthropogenic sources are concentrated in Europe, North America and Siberia, the natural cycles are strongly affected by anthropogenic emissions.Well known is the signiÆcant seasonal variation of trace metal levels in the Arctic, where meteorological conditions strongly favour a winter/spring burden. During summer the Arctic is cut off from the pollution sources but in winter and spring the polar front extends to the south over the source areas.5 This leads to a transport of aerosol, at low altitudes, into the Arctic region and causes pollution by anthropogenic mid-latitude emission sources.In this way the `Arctic haze' so termed by Mitchell,6 a dust layer at low altitudes with turbidity as usually encountered in industrial regions, is generated. The examination of this phenomenon, increasing emissions of air polluting substances and the expanding mineral oil extraction industry in the Arctic have led to a need for trace element determinations. In recent studies7±9 on ground-level distributions of particles in arctic aerosol, Æltration and impaction techniques were used for sampling.Hundreds of cubic metres of air were sucked through Ælters for subsequent analysis by particle-induced X-ray emission (PIXE) and instrumental neutron activation analysis (INAA). As, Mn, Sb, Se, V and Zn were measured by INAA, Cr, Ni and Pb by PIXE and Cd in a nitric acid extract by electrothermal atomic absorption spectrometry (ETAAS). The concentrations obtained were in the ng m23 range with an error between 5 and 20%, depending on the element and method.8 To collect size fractionated aerosol samples, a Batelle-type cascade impactor was operated for 5 d and the fractionated aerosol samples were analysed by PIXE.10 The analytical methods mentioned above require time consuming sampling of large air volumes for the determination of the expected very low trace element levels.However, when the time-scale for changes in atmospheric conditions limits the sampling time, a powerful analytical technique is required, combined with a highly efÆcient sampling system if possible.Inductively coupled plasma mass spectrometry (ICP-MS) offers new opportunities for the multi-element determination of trace metals and in combination with electrothermal vaporization (ETV) also for the analysis of solid samples. The Ærst reference to the use of ETV with ICP-MS was by Gray and Date in 1983.11 Several workers have applied this method in practical situations, e.g., for the determination of As and Se in solid materials,12 trace metals in sea-water13 and trace metals in arctic snow.14,15 Coupling ICP-MS with ETV, in which samples are volatilized from a graphite furnace and transported as a dry aerosol into the plasma, allows the direct analysis of solid samples.16±18 Following this idea, a sampling system was {Presented at the European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999.Dedicated to Prof. Dr. Dieter Klockow on the occasion of his 65th birthday. J. Anal. At. Spectrom., 1999, 14, 1685±1690 1685 This Journal is # The Royal Society of Chemistry 1999developed in which airborne particles were impacted on graphite targets and subsequently analyzed by ETV-ICP-MS. The primary aim of this study was to evaluate the newly developed method for sampling particles fractionated according to size, by coupling with the powerful multi-element analysis technique ETV-ICP-MS.As the object of this study the characterization of long-range transported Arctic air particles was chosen. For this study, aerosol sampling was performed at the German Arctic research station in Ny- A lesund (78.9�N, 11.9�E) on Spitsbergen, Norway, in a measurement campaign between March 10 and 30, 1998. Experimental Sample collection Many different sampling devices have been developed for the collection of airborne particulates.19 The method used in our studies was inertial deposition and size fractionation by a cascade impactor.The operating principle of a cascade impactor is well described in the literature.20,21 We used an eight-stage multijet cascade impactor made by Stro» hlein Instruments (Kaarst, Germany). It separates particles at a Øow rate of 36.7 l min21 in eight size classes with cut-off diameters between 0.35 and 16.5 mm. The cut-off particle diameter corresponds to a 50% collection efÆciency and depends not only on the particle size but also on the density and the particle shape.The calculated mass, assuming a spherical particle of density 2 g cm23, ranges between 4.5610214 g (0.35 mm diameter) and 4.761029 g (16.5 mm diameter). Usually, the separation characteristic for cascade impactors is given for spherical particles of unit density. The diameter of actual particles differing from this ideal is expressed as the aerodynamic diameter, dae. Owing to the gas velocity and the resulting Stokes number at a sample volume of 2.2 m3 h21 for our experimental setup, particles larger than 16.5 mm were not collected.Particles smaller than 0.35 mm were collected on a backup Ælter made of quartz- or glass-Æbre. The detailed sampling procedure has been described in previous publications22,23 and only a short summary is given here. Tmpaction plates were newly designed, made of poly(methyl methacrylate) (PMMA), and covered with separate porous graphite impaction targets in such a way that one target is placed under one nozzle.The graphite targets, discs of 6 mm diameter with a central impaction area of 4 mm diameter, 2 mm thick, were manufactured in-house from a rod of pure graphite (RW 003, Ringsdorff-Werke, Bonn, Germany). The chemical characterization of the collected particles requires their sampling on a material which does not interfere with the subsequent analytical procedure. The rough surface of the graphite discs yield a sufÆcient sampling efÆciency and a minimized loss of particles due to elastic collisions.For the measurement campaign, sets of impaction plates were prepared and each plate was equipped with six targets. Well sealed containers as shown in Fig. 1 were used for the transport of the impaction plates. At the scene of sampling the set of impaction plates in the impactor was replaced daily with a new one. To avoid Æeld blanks, replacing was done in a clean bench (US class 100) at the sampling station.During the sampling time of 23 h per day a mean volume of 275 l per target was sampled. Particles impacted on the graphite targets were subsequently analysed in our laboratory in Berlin. ETV-ICP-MS measurements For the multi-element analysis, a Perkin-Elmer SCIEX (Thornhill, ON, Canada) Elan 5000 ICP mass spectrometer was used. As the furnace for the ETV a modiÆed FANES source24,25 manufactured by BBPT Gesellschaft fu» r Physikalisch- technischen Gera»tebau (Berlin, Germany) was used.The anode chamber was replaced by an adapter which accommodates the injection tube of the ICP torch. In this way the electrothermal vaporizer is connected to the torch as closely as possible, avoiding transport losses and non-linear calibration curves.26,27 A scheme of the experimental setup is given in Fig. 2. Two gases are of importance for the performance of the furnace: the stabilization gas and the transport gas.The stabilization gas is an additional gas Øow of 1.2 l h21 argon introduced via the PTFE adapter between the ICP and the furnace. It replaces a mechanical shutter and prevents the Øow of air into the plasma, which causes it to break down, when the lid is lifted or the pivoted arm of the furnace is open. When drying a liquid sample dosed on the target, the part of the stabilization gas streaming through the tube removes the water and other vapours via the dosing hole, supported by the transport gas from the opposite end of the tube.The argon transport gas with a Øow rate of 1 l min21 transports the vaporized material into the ICP. The normalized height of the measured signal for the elements studied as a function of the transport gas Øow through the furnace is given in Fig. 3. The instrumental operating conditions for the Elan 5000 are given in Table 1. Combined with ETV, the Elan instrument was operated in the graphic mode. This application allows one to monitor the intensities of given isotopes as a function of time.The total measuring time was 20 s per heating cycle, triggered by a read signal of the ETV power supply. The intensity±time Æles of each run were evaluated by an in-house developed computer program which permits settings of individual integration limits for each element. The integration times Fig. 1 Set of modiÆed impaction plates. Fig. 2 Scheme of coupling ETV to ICP-MS. 1686 J. Anal. At. Spectrom., 1999, 14, 1685±1690vary between 7 and 10 s and the half-width of the peaks between 1 s (Ag, Cd, Pb) and 3 s (Fe, Ni, Co).Procedure To analyse the impacted particles, the loaded graphite targets were transferred one after the other into a two-part graphite tube container and Æxed there in two slits like a platform. Moving the pivoted arm of the ETV opens the furnace for changing the tube. The two-part graphite tube container which houses the target for ETV was manufactured and coated with pyrolytic graphite to speciÆcation by Ringsdorff-Werke. The closed tube is 28 mm long and has a 6 mm id and 10 mm od in the central part.To minimize the uncertainties in ETV solid sampling measurements, a thoroughly developed concept of the following steps was applied: (1) the ion optics and the plasma position were optimized by nebulization of aqueous standards prior to changeover to ETV; (2) the ETV was mounted behind the injection tube of the ICP torch as closely as possible; (3) the ETV and the power supply were calibrated using a fast pyrometer to operate under computer control at true stabilized temperatures; (4) the graphite targets were cleaned by repeated heating to 2850 �C, according the clean-up programme in Table 2, until stable background signals were obtained; and (5) particle-loaded targets were heated at 2750 �C for 6 s so that more than 90% of the whole signal was measured in the Ærst heating cycle.Mostly in the third consecutive heating cycle the background signal, as determined in advance, will be obtained.For calibration of measurements with the graphite disc, acidic multi-element standard solutions of 0.5, 2.5, 5, 25, 50, 100 and 200 mg l21 were used. The calibration standards for Mn, Fe, Co, Ni, Ag, Cd, Tl and Pb were freshly prepared in 0.029 mol dm21 HNO3 by stepwise dilution of 1 g l21 stock standard solutions from Merck (Darmstadt, Germany). Standards for Sb and Sn were analogously prepared in 0.024 mol l21 HCl.An aliquot of 10 ml of calibration solution was delivered manually to the cleaned disc inside the tube using a micropipette. The heating programme, as given in Table 2, was then started with the lid open at the sample injection port; it was automatically sealed after removing all vapours in the charring step. For all elements studied, linear calibration curves were obtained. The linear regression coefÆcient (r2), the concentration range and the sensitivity for each analyte isotope are given in Table 3.Differences in sensitivity reported in Table 3 are caused by compromise conditions for the furnace heating programme which was needed to determine volatile and nonvolatile elements in a single run. The sensitivity in ETV-ICPMS depends on ionisation energy but much more on element speciÆc properties such as number of isotopes, multiple charging and time dependent thermochemical processes of volatilization, dissociation and ionisation.Detection limits for each element based on 3s blank values are given in Table 4. The higher LODs for Fe and Ni are caused by incomplete cleaning after manufacture resulting in higher blank values for both of these elements. The accuracy of calibration was veriÆed by comparison with other methods and from isoformation by gas-phase digestion described previously.28 The reproducibility of repeated injections of reference solution is near 8% (relative standard deviation).Table 3 presents an example for 10 repeated injections of the 50 mg l21 standard solution. The stability of the instrument was checked daily by measuring the 50 mg l21 calibration point. The relative deviation of the mean intensity for 50 mg l21 varies between 12 and 20% over 10 d for the analyte ions concerned (see Table 3). A correction for residual instabilities of the ETV-ICP-MS Table 1 Instrumental operating conditions for ICP-MS ICP– Rf power 1000 W Plasma gas 14.8 l min21 argon Auxiliary gas 0.8 l min21 argon Nebulizer gas~transport gas 1.0 l min21 argon Sampler and skimmer cone Pt MS– Application Graphics (displays the evolution of a transient signal during an analysis) Number of replicates 80 Dwell time 20 ms Total measuring time 20 s Isotopes measured 55Mn, 57Fe, 59Co, 60Ni, 107Ag, 111Cd, 118Sn, 121Sb, 208Pb Scan mode Peak hopping Signal evaluation Peak integral (calculation performed at external PC) Fig. 3 Normalized signal as a function of the transport gas Øow averaged over all of the isotopes measured.Table 2 Operating programme for ETV Step Temperature/�C Ramp rate/�C s21 Hold time/s Argon transport gas/l min21 Argon stabilization gas/l min21 0 60 20 5 1 1.2 1 150 4 20 1 1.2 2 55a 1 1.2 3 2750 590 4b 1 0c 4 2750 0 6 1 0c 5 15 0 15 0 1.2c The clean-up programme was the same as the operating programme without steps 0 and 1 and with 2850 �C in steps 3 and 4. aData aquisition triggered 12 s after the beginning of the step.bRamp time. cInjection port lid closed. J. Anal. At. Spectrom., 1999, 14, 1685±1690 1687combination, despite the application of a true temperature controlled furnace, is possible by using a well suited internal standard. Measurements were made here without internal standardisation because the precision is sufÆcient to ensure that the differences in the sampled air volumes were determined exactly. Results and discussion Detailed measurements of airborne trace element concentrations at Ny-A lesund were carried out in March 1998.In the following a selected period of four successive days from March 25 to 28 is considered. During this period only weak air movements occurred. The air mass origin was concluded from 5 d backward trajectory analysis (850 mbar, isobaric) calculated by the German Weather Service (DWD) and the Alfred- Wegener-Institute (Potsdam, Germany). The pathways of the air packages which were measured are indicated in Fig. 4. The air masses reaching the station on March 25 (open circles) and March 28 (closed circles) had passed mainly over ice covered areas of the Arctic Ocean. Only on March 26 (solid line) and March 27 (broken line) did the air pass the Siberian coastline and the northernmost part of Siberia. On each day one impactor was operated equipped with eight impaction stages, corresponding to the eight separate size classes (0.35±16.5 mm dae), covered with six graphite targets per stage.From the determined element concentration of each target, the mean stage concentration and standard deviation (n~6) were calculated. Summation over all stages gives a bulk element concentration per day with a standard deviation calculated according to error propagation. Table 5 shows the results. It is obvious that the concentrations show a variability by a factor of 2±3. This cannot be explained by the inhomogeneity of the sampled aerosols. Aerosol sampling on Ælters with three parallel samplers showed a variability between the samplers and the distribution on the Ælters itself in a range of about 5±25% only, depending on the elements which were measured.29,30 The concentrations shown in Table 5 are in very good agreement with data obtained on non-size separated Ælter samples during the same period on a weekly sampling basis.31 Fig. 5 presents the histograms of the percentage related to the bulk element concentration over the eight size classes, (a) for Mn, Fe and Ni and (b) for Ag, Cd and Pb.The elements Co, Sb and Sn, also evaluated, are not shown in Fig. 5 since they follow nearly the same curves. However, as can be seen from Fig. 5, there are signiÆcant differences in elemental size distributions depending on air mass history. The histograms show a signiÆcant change in particle size distribution depending of the path of the air package which was sampled. Fairly clean air from the Arctic Ocean reached Ny A lesund on March 25 and 26 with a typical size distribution for unpolluted longrange transported aerosols.The air masses passing Siberia and the Siberian coast line (March 26 and 27) showed a different size distribution with a shift of the concentration maxima to 3.45 mm aerodynamic diameter on March 27. Table 3 Calibration data Analyte Concentration range/pg Linear regression coefÆcient Sensitivity/ counts pg21 Background counts (mean°s, n~18) Reproducibilitya (100s/mean, n~10) (%) Stabilityb (100s10/mean10) (%) 55Mn 5±2000 0.9958 5220 1418°194 10.0 16 57Fe 50±2000 0.9904 99 4572°835 8.3 18 59Co 5±2000 0.9916 2810 3179°788 7.3 15 60Ni 25±2000 0.9921 538 7719°1525 5.6 14 107Ag 5±2000 0.9924 990 120°28 9.6 12 111Cd 5±2000 0.9919 471 773°99 8.4 17 118Sn 5±2000 0.9998 269 2568°184 – – 121Sb 5±2000 0.9998 93 678°104 – – 208Pb 5±2000 0.9982 1713 1321°195 7.5 20 an repeated injections of 10 ml of the 50 mg l21 standard solution. bRelative deviation of the mean of Æve repeated injections on 10 successive days.Table 4 Limits of detection (LOD) (3s blank, n~18) Analyte Absolute LOD/pg Relative LODa/pg m23 55Mn 1 4 57Fe 25 90 59Co 0.8 3 60Ni 8 30 107Ag 0.08 0.3 111Cd 0.6 2 118Sn 2 7 121Sb 3 10 208Pb 0.3 1 aRelative LOD/pg m23~ absolute LOD=pg 0:275 m3 sample volume Fig. 4 Calculated wind trajectories, March 25 (open circles), March 26 (solid line), March 27 (broken line) and March 28 (closed circles). Table 5 Bulk element concentrations/ng m23 Analyte March 25 March 26 March 27 March 28 Mn 6.8°1.2 2.5°0.2 4.0°0.4 6.5°0.3 Fe 335°46 87°7 170°15 318°12 Ni 58°10 24°7 45°8 25°7 Co 1.8°0.8 0.83°0.24 0.39°0.07 0.38°0.07 Sn 9.5°1.0 8.1°2.2 9.0°0.4 14.8°2.2 Sb 8.0°0.5 8.1°0.7 9.8°0.4 11.8°0.6 Ag 0.2°0.03 0.23°0.09 0.09°0.01 0.08°0.01 Cd 0.47°0.06 9.1°3.7 0.37°0.11 0.36°0.09 Pb 12.4°1.4 14.3°3.4 16.1°2.7 12.9°2.2 1688 J.Anal. At. Spectrom., 1999, 14, 1685±1690The air mass trajectories indicate transport of air polluting substances from northern Russia as the main source region.Sites of heavy industry are located there (Pechora basin, Norilsk area) such as mining industry and metal smelters.32 The estimated high contents of Ni, Fe, Cd, Pb and Sb point to emissions from the large copper±nickel smelters of the Norilsk complex. The very similar shape of the curves for all elements, seen in Fig. 6, indicates a common source. From the results dicussed above, it can be seen that the combination of a welldesigned sampling strategy with subsequent high performance element analyses and backward trajectory calculations leads to detailed information about atmospheric transport processes of pollutants from highly industrialised source regions to remote areas such as the high Arctic.Conclusion and outlook It has been demonstrated that trace elements in size classiÆed atmospheric particles can be determined successfully by ETVICP- MS. The impaction of particles on graphite targets permits their direct analysis without any sample preparation and very good contamination control.The detection power is high enough for all elements studied to allow sampling times of only about 2 h. This newly designed sampling technique in combination with a subsequent high performance analysis technique leads to a much better time resolution for studying short time atmospheric processes, in comparison with wellestablished methods, in a remote area such as polar regions.A remarkable result of this study is the more or less strong variation in the particle size distribution, depending on the meteorological conditions, at a relatively low variation of the bulk element concentration. Further improvements in performance capability can be expected by using truly simultaneous mass spectrometers, e.g., time-of-Øight instruments. In this way, the number of simultaneously measurable isotopes is not limited when fast transient signals are measured.Such instrumentation also promises the availability of accurate isotope ratio information, which can be used for source identiÆcation. Fig. 5 Percentage content of (a) Mn, Fe and Ni and (b) Ag, Cd and Pb versus the aerodynamic diameter. The error bars indicate the standard deviation calculated for six targets per size class. The 100% content of each element between 0.35 and 16.5 mm aerodynamic diameter is given in Table 5. Fig. 6 Measured element content of air versus the aerodynamic diameter, March 27.The error bars indicate an uncertainty of 20%. J. Anal. At. Spectrom., 1999, 14, 1685±1690 1689Acknowledgements The �¡nancial support of the Senatsverwaltung fu¡í r Wissenschaft, Forschung und Kultur des Landes Berlin and the Bundesministerium fu¡í r Bildung und Forschung is gratefully acknowledged.erences 1 J. O. Nriagu, Nature (London), 1989, 338, 47. 2 M. V. Johnston and A. S. Wexler, Anal. Chem.., 1995, 67, 721A. 3 J.Mu¡í ller, J. Aerosol Sci., 1998, 29 (Suppl. 1), S219. 4 J. Kasparian, E. Frejafon, P. Rambaldi, J. Yu, B. Vezin, J. P. Wolf, P. Bitter and P. Viscard, Atmos. Environ., 1998, 32(17), 2957. 5 L. A. Barrie, Atmos. Environ., 1986, 20, 643. 6 M. Mitchell, J. Atmos. Terr. Phys., Suppl., 1956, 195. 7 J. Heintzenberg, H. C. Hansson and H. Lannefors, Tellus, 1981, 33, 162. 8 J. M. Pacyna, B. Ottar, U. Tomza and W. Maenhaut, Atmos. Environ., 1985, 19, 857. 9 B. Ottar and J.M. Pacyna, Geophys. Res. Lett., 1984, 11, 441. 10 B. Ottar, Technical Report 30/86: Air Pollutants in the Arctic, Norwegian Institut for Air Research, Lillestr¢�m, 1986. 11 A. L. Gray and A. R. Date, Analyst, 1983, 108, 1033. 12 S. Boonen, F. Vanhaecke, L. Moens and R. Dams, Spectrochim. Acta, Part B, 1996, 51, 271. 13 G. Chapple and J. P. Byrne, J. Anal. At. Spectrom., 1996, 11, 549. 14 R. E. Sturgeon, S.N. Willie, J. Zheng, A. Kudo and D. C. Gregoire, J. Anal. At. Spectrom., 1993, 8, 1053. 15 M. Kriews and O. Schrems, J. Aerosol Sci., 1998, 29, 735. 16 D. C. Gregoire,M. Lamoureux, C. L. Chakrabarti, S. Al-Maawali and J. P. Byrne, J. Anal. At. Spectrom., 1992, 7, 79. 17 C. M. Sparks, J. Holcombe and T. L. Pinkston, Spectrochim. Acta, Part B, 1993, 48, 1607. 18 M. M. Lamoureux, D. C. Gregoire, C. L. Chakrabarti and D. M. Goltz, Anal. Chem., 1994, 66, 3208¡¾3222. 19 D. Klockow, Fresenius' Z. Anal. Chem., 1987, 326, 5. 20 D. Hochrainer, in Analysis of Airborne Particles by Physical Methods, ed. H. Malissa and J. W. Robinson, CRC Press, West Palm Beach, FL, 1978, p. 7. 21 V. A. Marple and K. Willeke, in Inertial Impactors in Aerosol Measurements, ed. D. A. Lundgren, F. S. Harris, Jr., W. H. Marlow, M. Lippmann, W.-E. Clark and M. D. Durham, University of Florida Press, Gainsville, FL, 1979, p. 90. 22 C. Lu¡ídke, E. Hoffmann, J. Skole and S. Artelt, Fresenius' J. Anal. Chem., 1996, 355, 261. 23 C. Lu¡í dke, E. Hoffmann and J. Skole, Fresenius' J. Anal. Chem., 1997, 359, 399. 24 H. Falk, E. Hoffmann and C. Lu¡í dke, Prog. Anal. Spectrosc., 1988, 11, 417. 25 D. C. Baxter, R. Nichol, D. Littlejohn, C. Lu¡í dke, J. Skole and E. Hoffmann, J. Anal. At. Spectrom., 1992, 7, 727. 26 R. D. Edinger and S. A. Beres, Spectrochim. Acta, Part B, 1992, 47, 907. 27 T. Kantor, Spectrochim. Acta, Part B, 1988, 43, 1299. 28 C. Lu¡ídke, E. Hoffmann and J. Skole, J. Anal. At. Spectrom., 1994, 9, 685. 29 M. Kriews, W. Dannecker, K. Naumann and U. Wa¡í tjen, J. Aerosol Sci., 1988, 7, 1295. 30 U. Wa¡í tjen, M. Kriews and W. Dannecker, Nucl. Instrum. Methods Phys. Res., 1990, 49, 360. 31 M. Kriews and O. Schrems, J. Aerosol Sci., 1998, 29 (Suppl. 1), S685. 32 C. Reimann, H. Nikavaara, P. de Caritat, M. A¡í yra¡ís, V. A. Chekushin and T. E. Finne, in Heavy Metals in the Environment, ed. R.-D. Wilken, U. Fo¡í rstner and A. Kno¡í chel, CEP Consultants, Edinburgh, 1995, vol. 1, pp. 84¡¾87. Paper 9/03815I 1690 J. Anal. At. Spectrom., 1999, 14, 1685¡¾16

ISSN:0267-9477    

DOI:10.1039/a903815i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

The determination of strontium isotope ratios by means of quadrupole-based ICP-mass spectrometry: a geochronological case study Frank Vanhaecke,*a Gu¡ínther De Wannemacker,a Luc Moensa and Jan Hertogenb aLaboratory of Analytical Chemistry, Ghent University, Proeftuinstraat 86, B-9000 Ghent, Belgium bPhysico-chemical Geology, University of Leuven, Celestijnenlaan 200C, B-3001 Leuven, Belgium Received 28th June 1999, Accepted 19th August 1999 Quadrupole-based ICP-mass spectrometry (ICP-QMS) was used for the determination of 87Sr/86Sr isotope ratios in digests of rock samples originating from two magmatic silicate rock formations of the Vosges (the Kagenfels granite and the Nideck rhyolite).The rock formations studied are geographically close to one another and, overall, they show a similar chemical composition. In a preliminary study, the effect of various data acquisition parameters on the isotope ratio precision was systematically studied, permitting optimum conditions to be selected.Rh was used as an internal standard, allowing the blank correction to be made accurately. Cation exchange chromatography was used to avoid isobaric overlap of 87Rbz and 87Srz ion signals to the largest possible extent, while mathematical correction was applied to correct for the remaining interference. The accuracy of the method developed was evaluated by means of isotopic analysis of an oceanic gabbro sample, for which the 87Sr/86Sr isotope ratio was previously characterized by means of thermal ionisation mass spectrometry (TIMS).An excellent agreement between the ICP-QMS and TIMS values was established. On the basis of the isochrons, constructed using the 87Sr/86Sr isotope ratio results and the contents of Rb and Sr (determined by energy-dispersive X-ray �ªuorescence spectrometry) obtained for the Kagenfels and Nideck samples, it could be concluded that for both rock formations, secondary processes (e.g., recrystallisation, high- and low-temperature alteration) have disturbed the Rb¡¾Sr isotopic system.As a consequence, the uncertainties on (i) the initial 87Sr/86Sr isotope ratios and (ii) the ages thus determined are large. Nevertheless, the estimated ages appear to be geologically relevant and provide information on the timing of geological events that affected the rocks several tens of million years after the initial formation. Overall, this case study shows the merits of ICP-QMS for exploratory studies of Sr isotope systematics and geochronology in cases with suf�¡cient variation in the 87Sr/86Sr isotope ratios.Introduction In general, the isotopic composition of the elements is constant in nature.1 However, some exceptions1 exist as a result of (i) mass fractionation,2 (ii) natural radioactivity,3,4 (iii) the interaction of cosmic rays with matter (primarily in the atmosphere)3,4 and (iv) human activity, while (v) deviating isotopic compositions have also been observed for some elements in special classes of meteorites.5 The isotopic composition of Sr in terrestrial material shows variations as a result of the b2-decay of the naturally occurring long-lived radionuclide 87Rb (half-life T1/2~ 48.86109 y) to the stable isotope 87Sr.3,4 As a result, the number of 87Sr atoms actually present after a given time interval t is the sum of those present at the start of the interval (initial 87Sr), plus those produced by the decay of 87Rb during the interval t: 87Srt a87Sri a87RbtOelt ¢§ 1U O1U in which l is the decay constant (~ln 2/T1/2).While it is extremely dif�¡cult to measure the absolute abundance of an isotope, mass spectrometric techniques readily permit accurate and precise determination of isotope ratios. Therefore, it is more convenient to rewrite equation (1) in terms of the 87Sr/86Sr isotope ratio by division through 86Sr (for which the number of atoms present remains unaffected by radioactive decay, i.e., 86Sri~86Srt): O87Sr=86SrUt a O87Sr=86SrUi a O87Rb=86SrUtOelt ¢§ 1U O2U In the geochronological context, the start of the interval can coincide with the formation of a rock or mineral by solidi�¡cation of a silicate melt or with the recrystallisation and isotopic rehomogenisation of an existing rock.Equation (2) is the equation of a straight line in (87Sr/86Sr)¡¾(87Rb/86Sr) coordinates. It is commonly called the `isochron equation', because it expresses the (87Sr/86Sr) ratio of isochronous rocks (same t) that formed from the same, isotopically homogeneous source (same initial ratio), but having different 87Rb/86Sr ratios.3,4 However, this also requires that the system has remained closed with respect to Rb and Sr concentrations during the time interval t.In practice, the age of two or more co-genetic rocks or minerals is derived from the slope of the best �¡t line through the experimentally obtained data points; the intercept yields the initial (87Sr/86Sr) value.Traditionally, thermal ionisation magnetic sector mass spectrometry (TIMS) is used for Rb/Sr geochronology, because most applications require a precision of better than 0.01% relative standard deviation (RSD) on the 87Sr/86Sr ratio. In comparison with TIMS, the isotope ratio precision that can be J. Anal. At. Spectrom., 1999, 14, 1691¡¾1696 1691 This Journal is # The Royal Society of Chemistry 1999obtained with quadrupole-based ICP-mass spectrometry (ICP-QMS) is relatively poor (¢0.1% RSD).6±14 However, for applications for which the ultimate level of precision is not required, ICP-QMS is an attractive alternative to TIMS, owing to its ease of operation (samples can be introduced as aqueous solutions at atmospheric pressure), the widespread availability of ICP-QMS instruments and the much higher sample throughput.Of course, for both ICP-MS and TIMS, the time required for sample preparation exceeds the time required for the actual measurements.Highly favourable cases for ICP-QMS are preliminary dating or isotopic screening studies of rock series in which the expected variation of 87Sr/86Sr ratios exceeds 1% as a result of a combination of relatively great age (w50 million years) and large variations of the Rb/Sr ratios. The present paper consists of two parts. In the Ærst one, the optimisation of the data acquisition parameters for Sr isotope ratio measurements with ICP-QMS is brieØy described.The second part deals with a case study of Rb±Sr dating of two related series of magmatic silicate rocks from the Hercynian (ca. 350 to 250 million years old) Vosges Massif, France. While the application of ICP-MS for Re/Os dating has already been reported in the literature by several authors,5,15,16 to the best of the authors' knowledge this is not the case for Rb/Sr dating, although Chassery et al.17 and Latkoczy et al.18 recently reported on the use of quadrupole-based and sector Æeld ICP-MS, respectively, for the determination of Sr isotope ratios.Experimental Sample preparation A slab weighing 300±500 g was cut with a rock saw with a diamond blade from the interior part of the rock samples collected in the Æeld. Exterior, slightly weathered parts of the slab were removed with a small rock saw. The cleaned slabs were coarsely crushed in a carbon-steel mortar. About 40 g of the homogenised crushed material was ground to Æne powder in a mechanical agate vibrating ball mill.Determination of Rb and Sr concentrations Samples for isotopic measurements were selected on the basis of their Rb and Sr contents, in order to assure a large spread of Rb/Sr ratios. Rb and Sr were determined by energy-dispersive X-ray Øuorescence spectrometry (EDXRF) on `inÆnitely thick' pressed powder pellets. About 3 g of ground silicate powder were pressed into 24 mm diameter pellets, after mixing with a few drops of 2.5% (m/m) aqueous solution of poly(vinyl alcohol) binder.Samples were measured with a Kevex 700 spectrometer (Kevex Instruments Inc., Redwood City, CA, USA), equipped with a Rh-target X-ray tube. Operating conditions were as follows: 40 kV tube voltage, 0.40 mA tube current, Ag secondary target and 30 mm2 SiLi detector collimator. International reference rocks were used as standards. Every sample was measured in duplicate relative to two standards. Full details of the analytical procedure are reported elsewhere.19 Ion-exchange separation of Sr from Rb Self-evidently, for the Sr isotope ratio measurements, the samples of interest had to be taken into solution.Additionally, due to the isobaric overlap of the 87Srz and 87Rbz ion signals, Sr and Rb had to be chemically separated from one another by cation-exchange chromatography prior to the Sr isotope ratio measurements. Approximately 1 g of powdered sample was dissolved in a mixture of 28 M HF and 14 M HNO3 in a PTFE vessel.After evaporation to dryness, the residue was taken up in 10 ml of 2 M HCl. The solution was Æltered through a Whatman 541 Ælter, to retain any gelatinous particles. The Ælter was washed using 5 ml of 2 M HCl. For the cation exchange procedure, a Dowex 50W-X8 column (length, 20 cm; internal diameter, 1 cm) was used (J.T. Baker Chemicals N.V., Deventer, The Netherlands). Na, K, Rb, Mg and the major part of Fe and Ca were eluted from the column using 90 ml of 2 M HCl.Subsequently, Sr was eluted from the column using 40 ml of 8 M HCl. For a gabbroic control sample, a longer column was used because the fairly high Ca content resulted in an accelerated elution of Sr. The Sr-containing fraction was subsequently evaporated to near-dryness and converted into the nitrate form by addition of 1.5 ml of 14 M HNO3 and evaporation to near-dryness. Finally, the residue was taken up in 2.5 ml of Millipore Milli-Q water and diluted appropriately using 0.14 M HNO3.The Sr concentration in the solutions analysed was typically 100±150 mg l21. Sr isotope ratio measurements The instrument used for the Sr isotope ratio measurements is a Perkin Elmer SCIEX ELAN 5000 quadrupole-based ICP-mass spectrometer, in its standard conÆguration (Perkin Elmer, U» berlingen, Germany). A multi-channel peristaltic pump (Minipuls-3), a GemTip cross-Øow nebuliser and a Perkin Elmer Type II spray chamber made of Ryton, drained by the peristaltic pump, were used for sample introduction.This instrument was further equipped with a Perkin Elmer corrosion-resistant torch with standard alumina injector and a Channeltron continuous dynode electron multiplier, operated in the pulse counting mode. Typical operation conditions have been summarised in Table 1. Processing of ICP-QMS data The raw data obtained using the data acquisition parameters shown in Table 2 were processed in the following way. Prior to ratioing, the isotopes of interest were corrected for both (i) signal losses to be attributed to the detector dead time1,20 and (ii) the procedure blank.For the former correction, the detector dead time was determined experimentally on each measuring day according to the procedure described by Russ1 and the value obtained was loaded into the instrument software for automatic correction. In order to improve the reliability of the blank correction, 103Rh was also monitored and used as an internal standard, correcting for potential matrix-induced signal suppression or enhancement, signal drift and instrument instability.The monitoring of 103Rh resulted in a slight deterioration of the isotope ratio precision obtained. It is common knowledge that in ICP-MS, various phenomena cause the relative ion intensities (on a molar basis) to vary as a function of the ion mass. These mass discrimination effects can occur during extraction (nozzle separation effect13), transmission (space charge effects21,22) or detection, and they can amount to several per cent.per mass unit. It is self-evident that for accurate isotope ratio determination, mass discrimina- Table 1 Operation conditions of the Perkin Elmer SCIEX ELAN 5000 ICP-mass spectrometer Rf power 1000W Sampling depth 10 mm from load coil Gas Øow rates Nebulizer gas 0.870 l min21 Auxiliary gas 1.2 l min21 Plasma gas 15 l min21 Sampling cone Nickel, 1.0 mm aperture diameter Skimmer Nickel, 0.75 mm aperture diameter Sample uptake rate 1 ml min21 Lens voltages Tuned for maximum 103Rhz signal intensity 1692 J.Anal. At. Spectrom., 1999, 14, 1691±1696tion has to be appropriately corrected for. In the case of Sr, this correction either (i) involves measurement of an (external) isotope ratio standard with a known isotopic composition or (ii) can be accomplished by monitoring the 86Sr/88Sr isotope ratio, which is constant in nature.23,24 In this work, external standardisation, using NIST SRM 987 (SrCO3) as an isotope ratio standard, was preferred (NIST, Gaithersburg, MD, USA). Comparison of the experimental 87Sr/86Sr isotope ratio result obtained for a Sr standard solution prepared from this reference material and the corresponding certi�¡ed value permitted calculation of a mass discrimination correction factor (CF): CF a O87=86Ucert O87=86Ustd;exp O3U As the experimental result for the 87Sr/86Sr isotope ratio of the standard is the average value of 10 replicate measurements, error propagation, taking into account the standard deviations for both this experimentally determined isotope ratio (sstd,exp) and the corresponding certi�¡ed value (scert), permits the standard deviation on the mass discrimination correction factor (sCF) to be calculated: sCF a CF| AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA sstd;exp O87=86Ustd;exp2 a scert O87=86Ucert 2 s O4U Prior to mass discrimination correction, the results for the samples have also been corrected for the potential bias caused by remaining isobaric overlap of 87Srz and 87Rbz: O87=86Usamp;net a fO87=103Usamp ¢§ O87=103Ubl ¢§ OCF'|0:3857 svp| aO85=103Usamp ¢§ O85=103UblaUg aO86=103Usamp ¢§ O86=103Ubla O5U in which CF'~1z[2(12CF)], assuming that mass discrimination varies linearly as a function of mass, and 0.3857 svp is the 87Rb/85Rb isotope ratio as calculated from IUPAC-tabulated abundances.25 ssamp,net is the standard deviation observed for the results of 10 replicate measurements, calculated according to equation (5).Finally, the `true' isotope ratio for the samples is given by: O87=86Utrue a O87=86Usamp;net|CF O6U and the corresponding standard deviation is given by: strue a AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA sCF CF2 a ssamp;net O87=86Usamp;net2 s |O87=86Utrue O7U On the basis of the ICP-MS (87Sr/86Sr isotope ratios) and EDXRF (Sr and Rb contents) results obtained, the isochrons for both rock formations could be constructed.The content of 87Rb can be calculated from the corresponding Rb elemental content and the isotopic abundance from reference.25 The content of 86Sr, on the other hand, can be calculated from the Sr content and the isotopic abundance (h) of 86Sr, which can be obtained by solving the following set of simultaneous equations (8): hO84U a hO86U a hO87U a hO88U a 1 hO88U hO86U a constant a 82:58 9:86 a 8:375 hO84U hO86U a constant a 0:56 9:86 a 0:057 hO87U hO86U a determined experimentally OICP-MSU a R O8U 8><>: Rearrangement of equation (8) leads to equation (9): hO86U a 1 O9:432 a RU O9U permitting the 87Rb/86Sr ratio to be calculated.Results and discussion Optimisation of data acquisition parameters Preliminary work consisted of a systematic evaluation of the in�ªuence of the data acquisition parameters on the isotope ratio precision and permitted optimum conditions to be selected.For this purpose, the relative standard deviation [RSD(%)] for 10 replicate measurements of the 87Sr/86Sr isotope ratio in a 100 mg L21 Sr standard solution was determined under different conditions. In order to evaluate the in�ªuence of the scanning rate, or more accurately the peak hopping rate, the residence time per acquisition point was varied from 10 to 500 ms (10, 30, 50, 100, 200 and 500 ms) while keeping the total acquisition time per replicate at 2 min.As has already been reported by several research groups, the isotope ratio precision was observed to signi�¡cantly improve with increasing scanning speed:6,9,12,26 forsotope ratio determination, the scanning rate should be suf�¡ciently high to smooth out signal �ªuctuations due to plasma �ªicker and variations in, for example, the sample uptake rate and the nebulisation, ionisation and extraction ef�¡ciencies. Denoyer27 argued that in general, an acquisition time of 30 ms is an optimum compromise between fast hopping through the nuclides monitored and an ef�¡cient use of the total measuring time (high ratio of actual measuring time to mass spectrometer settling time); however, it is interesting to note that in our study a decrease of the acquisition time from 30 to 10 ms resulted in an additional, though small, improvement (by a factor of 1.2 on average) of the isotope ratio precision.Hence, for the actual measurements, a residence time of 10 ms per acquisition point was used.In order to evaluate the in�ªuence of the total measurement time on the isotope ratio precision obtained, the RSD for 10 replicate 87Sr/86Sr measurements was determined using a total measurement time of 5, 15, 30, 60 and 150 s per nuclide monitored and per replicate. As can be expected on the basis of Poisson counting statistics, the isotope ratio precision signi�¡cantly improves when increasing the measurement time and hence the number of counts observed for both nuclides monitored (Fig. 1). In all further work, a compromise setting of 45 s measurement time per nuclide and per replicate was used, enabling a suf�¡cient isotope ratio precision to be obtained while keeping the total acquisition time (for 10 replicates) at an acceptable level. As is to be expected,28 no improvement in the 87Sr/86Sr isotope ratio precision was observed when using 5 instead of 1 acquisition point per spectral peak (in both cases residence time per acquisition point~10 ms), while keeping the total acquisition time per replicate at 2 min.In summary, the aforementioned optimisation experiments Table 2 Data acquisition parameters (after optimization, see text) Data acquisition mode peak hop mode Nuclides monitored 86Sr, 87Sr, 85Rb, 103Rh Number of acquisition points per peak 1 Residence time per acquisition point and per sweep 10 ms Number of sweeps per replicate 3250 Total measurement time per replicate y3 mina Number of replicates 10 aIncluding quadrupole settling time.27 (8) J.Anal. At. Spectrom., 1999, 14, 1691¡¾1696 1693have led to the set of data acquisition parameters shown in Table 2. Finally, the effect of `normalisation' on the isotope ratio precision was evaluated. Although the isotopic composition of Sr varies in nature as a result of the b2-decay of 87Rb to 87Sr, the 86Sr/88Sr isotope ratio is constant and can be used as an `internal standard', correcting for drift or instabilities in the 87Sr/86Sr isotope ratio.For multi-collector ICP-MS instrumentation, successful use of such a normalisation procedure has been reported by Walder and Freedman23 and by Christensen et al.24 As a slight deterioration in the 87Sr/86Sr isotope ratio precision (by a factor of 1.3 on average) was observed when using 86Sr/88Sr as an internal standard, this internal `normalisation' was not used for the actual measurements. As both experiments were carried out using the same residence time per acquisition point and the same total measurement time (only the number of sweeps was varied), this observation can be attributed to a combination of (i) a smaller number of counts observed for each of the nuclides of interest (cf.Poisson counting statistics) and possibly also (ii) a longer time interval between two successive sweeps (monitoring of 3 nuclides instead of 2). Using the optimised conditions, summarised in Table 2, the RSD for 10 replicate measurements of the 87Sr/86Sr isotope ratio in a 100 mg L21 standard solution was observed to be typically 0.1±0.2%.Evaluation of accuracy Before analysing the samples of interest, the accuracy of the procedure developed was evaluated using a rock sample, for which the 87Sr/86Sr isotope ratio was previously characterized by means of multi-collector TIMS by Barling et al.29 This `quality control sample' is an oceanic gabbro drilled from the Mid-Atlantic Ridge.It was subjected to the same sample pretreatment (dissolution and cation exchange chromatography) as the samples. The ICP-MS result obtained, 0.7028 with a standard deviation of 0.0020 (n~10), is in excellent agreement with that obtained by TIMS, 0.702 628 (standard error, 0.000 008). A geochronological case study: granites and rhyolites from the Vosges Massif, France The latest manifestations of Late-Hercynian (i.e., from ca. 320 to 250 million years ago) magmatic activity in the Vosges Massif are the Kagenfels granite and the Nideck rhyolite, both located in the Northern Vosges, France.The Nideck rhyolite is an ignimbrite, i.e., a silicon-rich hot volcanic ash deposited during a powerful volcanic eruption. The Kagenfels granite on the other hand is a plutonic rock formed by slow cooling of a silicon-rich silicate that intruded at rather shallow level in the crust of the earth. The granite is now exposed at the surface due to uplift and erosion of the original cover rocks. The geochemistry and petrology of the two rock formations are currently studied in detail at the University of Leuven and the corresponding results have been30±32 and will be reported on elsewhere.The rocks are very suited for a test case, because several geochronological techniques have already been applied to date the two formations. However, these previous datings have yielded considerably divergent results.33 Hess et al.33 obtained a consistent set of ages for the Kagenfels granite, on the basis of K/Ar and 40Ar/39Ar dating of biotite mineral separates and of Pb-evaporation dating of zircon grains.The formation age of the Kagenfels granite, 331°5 Ma, is apparently 40 Ma older than previously assumed, and older than the biotite 40Ar/39Ar age of 291°4 Ma measured for the Nideck rhyolite. Hence, there is a need to further investigate the reasons for the scattered age data. For the present study, representative samples were selected that showed a very signiÆcant variation of the Rb/Sr ratio.The results obtained for Kagenfels granite and Nideck rhyolite are summarised in Table 3. As can be seen from this table, the typical 87Sr/86Sr isotope ratio precision (expressed as relative standard deviation for 10 replicates) obtained in practice was typically y0.3%, while 2% RSD was used as a realistic estimation of the repeatability of the Rb/Sr results obtained using EDXRF.For one sample, the standard deviation on the 87Sr/86Sr isotope ratio was seen to be atypically high (y1.6%). For this particular sample, the chromatographic separation of Sr from Rb was established not to be very successful, as the Rb concentration in the sample signiÆcantly exceeded that of Sr. Nevertheless, it was decided to take this value into account also, as for isochron construction the statistical weight of an experimental result and its uncertainty are inversely proportional. 34±36 The `Isoplot/Ex' calculation procedure developed by K.R. Ludwig37 was used to analyse and to calculate the parameters of the isochrons. This procedure performs a least squares analysis of the data taking into account the uncertainties of both the X- and Y-parameters. The results are shown in numerical and graphical form in Figs. 2 and 3. Both the Nideck and the Kagenfels data deÆne fairly well correlated linear trends (linear correlation coefÆcients are 0.988 and 0.976, respectively), but the scatter from the best-Æt lines is in both cases larger than what is expected from the instrumental, analytical errors.It thus appears that the Rb±Sr isotopic system has been disturbed by secondary processes (e.g., recrystallisation, high- and low-temperature alteration) since the emplacement. As a consequence, the uncertainty of the estimated initial isotopic ratios and age is quite substantial. Nevertheless, these results can be used. Table 3 Experimentally determined 87Sr/86Sr isotope ratios (ICPQMS) and 87EDXRF).Uncertainties are expressed as standard deviations Kagenfels granite 87Rb/86Sr 87Sr/86Sr Sample 1 5.98 (0.12) 0.7349 (0.0018) Sample 2 19.88 (0.40) 0.8113 (0.0022) Sample 3 38.91 (0.78) 0.9062 (0.0050) Sample 4 57.50 (1.2) 0.8968 (0.0027) Sample 5 97.30 (1.95) 1.1149 (0.0031) Nideck rhyolite 87Rb/86Sr 87Sr/86Sr Sample 1 5.18 (0.10) 0.7263 (0.0020) Sample 2 7.79 (0.16) 0.7323 (0.0019) Sample 3 18.83 (0.38) 0.7607 (0.0021) Sample 4 22.18 (0.44) 0.7910 (0.0035) Sample 5 39.57 (0.79) 0.835 (0.013) Fig. 1 87Sr/86Sr isotope ratio precision [expressed as RSD(%) for n~10] as a function of the measurement time per nuclide. 1694 J. Anal. At. Spectrom., 1999, 14, 1691±1696The calculated, nominal age of 274 Ma (million years) of the Kagenfels is within the range of earlier Rb±Sr whole-rock datings and much younger than the most likely age of formation of 331 Ma.33 The nominal age of 232 Ma derived for the Nideck ignimbrite is deÆnitely too young to correspond to the volcanic eruption age.A geologically reasonable minimum age of formation is about 250 Ma, because the rhyolites are directly overlain by Upper Permian and Lower Triassic sediments. It is interesting that some of the biotite fractions from the Kagenfels granite analysed by Hess et al.33 also yielded apparent K/Ar ages of 260±274 Ma. Hence, it is rather probable that the whole rock Rb±Sr age obtained in the present study reØects a real geological event.A likely process is the partial resetting of the Rb±Sr clock due to alteration of potassic feldspar, a major host phase of Rb and Sr, by hydrothermal Øuids during late-Hercynian uplift and erosion of the Vosges Massif. Fluid circulation is promoted by the development of cracks and faults in uplifted granites due to pressure release. As the Nideck rhyolites were already deposited on the surface at the moment of volcanic eruption, uplift or subsidence should have had a negligible effect on the Rb±Sr system.The young apparent Rb±Sr age of the Nideck rocks (232 Ma) must be attributed to another geological process, such as the recrystallisation and alteration of volcanic glass particles when the permeable volcanic ash became covered by water masses during Late-Permian and Early-Triassic times. Conclusions Although the isotope ratio precision obtained is clearly signiÆcantly poorer than that obtained with TIMS, the results for the granitic and rhyolitic magmatic rock series from the Vosges Massif demonstrate that relevant geochronological data can be derived from ICP-QMS measurements.The case study clearly shows the merits of ICP-MS for exploratory studies of Sr-isotope systematics and geochronology in cases where the expected variation in the 87Sr/86Sr ratio is larger than the analytical precision. The main advantages of quadrupolebased ICP-MS are a high sample throughput, straightforward sample introduction and, especially, the wide availability of ICP-MS instruments in analytical and geochemical laboratories.ICP-MS can be very useful in preliminary studies aiming at a selection of the most interesting specimens out of a larger batch of samples for subsequent TIMS analysis. Another useful application is a quick check of the state of disturbance of the Rb/Sr system in a given rock series. For highly precise isotope ratio determinations, however, ICP-MS cannot compete with TIMS, unless a double focusing magnetic sector ICP-MS instrument, equipped with a multiple collector detection device, is used.23,24,38 References 1 G.P. Russ III, in Applications of Inductively Coupled Plasma Mass Spectrometry, ed. A. R. Date and A. L. Gray, Blackie, Glasgow, UK, 1989, ch. 4, pp. 90±114. 2 T. B. Coplen, J. A. Hopple, S. E. Rieder, H. R. Krouse, J. K. Bo» hlke, R. D. Vocke Jr., K. M. Re�ve�sz, K. J.R. Rosman, A. Lamberty, P. Taylor and P. De Bie¡vre, Pure Appl. Chem., 1999, in the press. 3 G. Faure, Principles of Isotope Geology, John Wiley, New York, USA, 2nd edn., 1986. 4 A. P. Dickin, Radiogenic Isotope Geology, Cambridge University Press, Cambridge, UK, 1995. 5 D. C. Gre�goire, Prog. Anal. Spectrosc., 1989, 12, 433. 6 N. Furuta, J. Anal. At. Spectrom., 1991, 6, 199. 7 M. E. Ketterer, M. J. Peters and P. J. Tisdale, J. Anal. At. Spectrom., 1991, 6, 439. 8 M. E. Ketterer, J.Anal. At. Spectrom., 1992, 7, 1125. 9 S. R. Koirtyohann, Spectrochim. Acta, Part B, 1994, 49, 1305. 10 Q. Xie and R. Kerrich, J. Anal. At. Spectrom., 1995, 10, 99. 11 I. S. Begley and B. L. Sharp, J. Anal. At. Spectrom., 1997, 12, 395. 12 C. R. Que�tel, B. Thomas, O. F. X. Donard and F. E. Grousset, Spectrochim. Acta, Part B, 1997, 52, 177. 13 K. G. Heumann, S. M. Gallus, G. Ra»dlinger and J. Vogl, J. Anal. At. Spectrom., 1998, 13, 1001. 14 F. Vanhaecke, P. Taylor and L.Moens, in ICP Spectrometry and its Applications, ed. S. J. Hill, ShefÆeld Academic Press, ShefÆeld, UK, 1999, ch. 6, pp. 145±207. 15 S. B. Beneteau and J. M. Richardson, At. Spectrosc., 1992, 13, 118. 16 T. Hirata and A. Masuda, Meteoritics, 1992, 27, 568. 17 S. Chassery, F. E. Grousset, G. Lavaux and C. R. Que�tel, Fresenius' J. Anal. Chem., 1998, 360, 230. 18 C. Latkoczy, T. Prohaska, G. Stingeder and M. Teschler-Nicola, J. Anal. At. Spectrom., 1998, 13, 561. 19 L. Vanlerberghe and J.Hertogen, Bull. Soc. Chim. Belg., 1986, 95, 491. 20 F. Vanhaecke, G. De Wannemacker, L. Moens, R. Dams, C. Latkoczy, T. Prohaska and G. Stingeder, J. Anal. At. Spectrom., 1998, 13, 567. 21 G. R. Gillson, D. J. Douglas, J. E. Fulford, K. W. Halligan and S. D. Tanner, Anal. Chem., 1988, 60, 1472. 22 S. D. Tanner, Spectrochim. Acta, Part B, 1992, 47, 809. 23 A. J. Walder and P. A. Freedman, J. Anal. At. Spectrom., 1992, 7, 571. 24 J. N. Christensen, A. N. Halliday, D.-C. Lee and C. M. Hall, Earth Planet. Sci. Lett., 1995, 136, 79. Fig. 3 Rb/Sr isochron for Nideck rhyolite, constructed on the basis of the experimentally determined 87Sr/86Sr isotope ratios (ICP-QMS) and Rb and Sr contents (EDXRF). Fig. 2 Rb/Sr isochron for Kagenfels granite, constructed on the basis of the experimentally determined 87Sr/86Sr isotope ratios (ICP-QMS) and Rb and Sr contents (EDXRF). J. Anal. At. Spectrom., 1999, 14, 1691±1696 169525 K. J. R. Rosman and P. D. P Taylor, J. Anal. At. Spectrom., 1998, 13, 45N. 26 F. Vanhaecke, L. Moens, R. Dams and P. Taylor, Anal. Chem., 1996, 68, 567. 27 E. R. Denoyer, At. Spectrosc., 1994, 15, 7. 28 H. P. Longerich, S. E. Jackson and D. Gu» nther, J. Anal. At. Spectrom., 1996, 11, 899. 29 J. Barling, J. Hertogen and D. Weis, in Proceedings of the Ocean Drilling Program, ScientiÆc Results, ed. J. A. Karson, M. Cannat, D. J. Miller and D. Elthon, Ocean Drilling Program, College Station, TX, USA, 1997, vol. 153, 351. 30 S. Claes, M. Verhaeren and J. Hertogen, Terra Nova, 1995, vol. 7, Abstracts Suppl. 1, p. 301. 31 M. Verhaeren, S. Claes and J. Hertogen, Terra Nova, 1995, vol. 7, Abstracts Suppl. 1, p. 141. 32 J. Mareels and J. Hertogen, unpublished work. 33 J. C. Hess, H. J. Lippolt and B. Kober, Geol. Rundsch., 1995, 84, 568. 34 G. A. McIntyre, C. Brooks, W. Compston and A. Turek, J. Geophys. Res., 1966, 71, 5459. 35 D. York, Earth Planet. Sci. Lett., 1967, 2, 479. 36 D. York, Earth Planet. Sci. Lett., 1969, 5, 320. 37 K. R. Ludwig, Berkeley Geochronology Center, Special Publication No. 1, Rev. November 5, 1998, Berkeley, CA, USA. 38 A. N. Halliday, D.-C. Lee, J. N. Christensen, M. Rehka»mper, W. Yi, X. Luo, C. M. Hall, C. J. Ballentine, T. Pettke and C. Stirling, Geochim. Cosmochim. Acta, 1998, 62, 919. Paper 9/05184H 1696 J. Anal. At. Spectrom., 1999, 14, 1691&p

ISSN:0267-9477    

DOI:10.1039/a905184h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Speciation analysis for iodine in milk by size-exclusion chromatography with inductively coupled plasma mass spectrometric detection (SEC-ICP MS) Luiza Fernandez Sanchez{ and Joanna Szpunar* CNRS EP132, He�lioparc, 2, av. Pr. Angot 64053 Pau-Pyre�ne�es, France. E-mail: joanna.szpunar@univ-pau.fr Received 9th July 1999, Accepted 25th August 1999 A method allowing the determination of iodine species in milk and infant formulas was developed. It was based on the coupling of size-exclusion chromatography (SEC) with on-line selective detection of iodine by ICP MS.Iodine species were quantitatively eluted with 30 mM Tris buffer within 40 min and detected by ICP MS with a detection limit of 1 mg l21 (as I). A systematic study of iodine speciation in milk samples of different animals (cow, goat) and humans, of different geographic origin (several European countries) and in infant formulas from different manufacturers was carried out. Whey obtained after centrifugation of fresh milk or reconstituted milk powders contained more than 95% of the iodine initially present in milk in all the samples investigated with the exception of the infant formulas in which only 15±50% of the total iodine was found in the milk whey.An addition of sodium dodecyl sulfonate (SDS) improved considerably the recovery of iodine from these samples into the milk whey. Iodine was found to be principally present as iodide in all the samples except infant formulas.In the latter, more than half of the iodine was bound to a high molecular (w1000 kDa) species. The sum of all the species recovered from a size-exclusion column accounted for more than 95% of the iodine present in a milk sample. For the determination of total iodine in milk a rapid method based on microwave-assisted digestion of milk with ammonia followed by ICP MS was optimized and validated using CRM 151 Skim Milk Powder. Introduction Iodine is an essential micronutrient to animals and man. It is a constituent of the thyroid hormones, the lack of which causes poor mental and physical development in children, and goiter (enlargement of the thyroid gland) in adults.1 Supplementation with iodine is a common practice.2 The addition of iodine to cow feed in order to enhance production of milk and meat makes milk an important source of iodine.2 In infant nutrition the iodine level in breast milk is known to be affected by the maternal diet whereas infant formula needs to be supplemented with iodine.3 Since excessive intake of iodine can cause toxic goiter (thyrotoxicosis) the supplementary iodine should be strictly limited and controlled by manufacturers and government institutions; a number of methods for precise and accurate quantiÆcation of iodine in food have therefore been developed.4 To date, milk has been analysed for total iodine only without looking into the nature of the species present.Inductively coupled plasma mass spectrometry (ICP MS) has been the usual analytical technique applied,4±10 replacing the classical Sandell and Kolthoff kinetic-catalytic method,11 cumbersome radiochemical neutron activation analysis,12 and tedious GC with electron capture detection (ECD), which requires the conversion of iodine to a 2-iodopentan-3-one derivative.13,14 Despite some reports of the direct analysis of milk diluted with ammonia by ICP MS,10 digestion has always been an integral step of the sample preparation procedure.Tetramethylammonium hydroxide (TMAH),4±6 a TMAH±KOH mixture,9 and diluted ammonia8 have been the most widely used approaches for milk digestion, an alternative being the combustion of a milk sample in an oxygen stream.7 These procedures lead to the conversion of iodine into iodide, which is then determined by ICP MS without any concern for the speciation of the iodine in the original sample. It is well known that the absorption of an endogenous trace element in food by man can be different from that of a supplemented one and, in particular, the absorption of this element by formula-fed children is usually different than that of breast-milk-fed children.This fact renders trace element speciation analysis of milk necessary.15 Most iodine in biological matrices is said to be covalently bound but Bra» tter et al.16 reported that ca. 80% of iodine was present in the breast milk in the form of iodide, in addition to various organic compounds.Studies of speciation of iodine in milk and body Øuids have been practically non-existent because of the lack of a suitable, species-selective methodology, as demonstrated by the recent review papers.17,18 In terms of potential methodology available for speciation analysis for iodine in milk, approaches described for speciation of essential elements (Zn, Cu, Fe, Se),15,16,19,20 toxic elements (Cd)21 and radioactive elements (90Sr, 99Tc, 137Cs and 152Eu)22 in this matrix should be considered.Size-exclusion chromatography has been the most widely used fractionation technique for trace element species present in milk, whereas ICP AES,15,16 ICP MS,16 scintillation radiodetection22 and stripping voltammetry 21 were the most commonly used detection techniques. Since milk is a slurry that cannot be introduced on to the column directly, milk whey was actually analysed. Care should be taken to assure that the composition (in terms of trace elements) of the whey obtained by the centrifugation of the milk matches that present in the original milk sample.The objective of this research was to develop a method able to distinguish and to quantify the different iodine species potentially present in milk. The approach was based on the coupling of SEC and ICP MS following a sample preparation procedure, assuring the complete transfer of iodine-containing {On leave from: Department of Analytical Chemistry, Universidad de Santiago de Compostela, 15706 Santiago, Spain.J. Anal. At. Spectrom., 1999, 14, 1697±1702 1697 This Journal is # The Royal Society of Chemistry 1999species present in milk to the milk-whey fraction. Since problems were encountered during the direct determination of iodine in milk by ICP MS, a rapid open-vessel focussed microwave-assisted digestion was developed for this purpose. Experimental Instrumentation Chromatographic separations were carried out using an HP Model 1100 HPLC pump (Hewlett-Packard, Wilmington, DE, USA) as the sample delivery system.Injections were performed using a Model 7725 injection valve with a 100 ml injection loop (Rheodyne, Cotati, CA, USA). All the connections were made of polyether ether ketone (PEEK) tubing (id 0.17 mm). Analyte species were separated on 106300 mm613 mm Superdex-75 and Superdex-200 columns (Pharmacia Biotech, Uppsala, Sweden) with an exclusion limit of 100 kDa (an effective separation range of 0.5 kDa and 80 kDa) and an exclusion limit of 1300 kDa (an effective separation range of 10 kDa and 600 kDa), respectively. A guard column, TSK PWXL (40 mm63 mm id) (Tosoh Corp., Stuttgart, Germany) was always used.The columns were calibrated (UV detection was used) with the following standards: glutathione (Mr 307), PC2 (Mr 539), PC3 (Mr 771), rabbit liver metallothionein±Cd complex (Mr 6918), cytochrome c (Mr 12 384), bovine albumin (Mr 66 000), and thyroglobulin (Mr 660 000).An ELAN 6000 ICP mass spectrometer (PE-SCIEX, Thornhill, Ontario, Canada) was used as the element-speciÆc detector in HPLC. The column eluate was introduced into the ICP via a cross-Øow nebulizer Ætted in a Ryton spray chamber. For total analyses the samples were fed by means of a Minipuls 3 peristaltic pump (Gilson, Villiers-le-bel, France) that also served for draining the spray chamber. Chromatographic data was processed using the Turbochrom4 software (Perkin-Elmer, Norwalk, CT, USA).All signal quantiÆcations were performed in the peak area mode. A Hitachi Model Himac CS 120GX refrigerated ultracentrifuge (Jouan, Saint Herblain, France) was used for the separation of the milk whey. Lyophilization was carried out using a Model LP3 lyophilizer (Jouan). For the total iodine analysis, samples were digested in a 22 ml open vessel of borosilicate glass Ætted with a 10 censer using a Synthewave S402 microwave digester (2.45 GHz, maximum power 300 W) (Prolabo, Fontenay-sous-Bois, France).Reagents, standards and samples Analytical-grade reagents purchased from Sigma±Aldrich (St. Quentin Fallavier, France) were used throughout unless speciÆed otherwise. 18 MV Milli-Q water (Millipore, Bedford, MA, USA) was used throughout. The Tris±HCl buffer was prepared by dissolving 30 mM of Tris [tris(hydroxymethyl)- aminomethane] in water and adjusting the pH to 7.0 by the addition of hydrochloric acid (1 : 10, v/v).The buffer solution was degassed in an ultrasonic bath before use. Standards containing iodine: thyroglobulin (iodine content approx. 1%), 3,3',5-triiodothyronine sodium salt and potassium iodide were used. The standards were dissolved in water with the exception of triiodothyronine which was dissolved in diluted ammonia and diluted with the chromatographic mobile phase prior to injection. A certiÆed reference material (CRM) 151 Skim Milk Powder (BCR, Brussels, Belgium) with a certiÆed iodine concentration of 5.35°0.14 mg g21 was used to control the accuracy of the total iodine determination.The human milk samples were collected from mothers having delivered at the Hospital Xeral in Santiago, Spain. The samples were collected in polypropylene containers cleaned with 10% HNO3 and immediately frozen (220 �C). Infant formula samples and milk samples of different geographical origin (France, Nactalia Brick; UK, Safeway Long Life Milk; Poland, Mleko Ëaciate; Germany, Haltbare Alpen Milch; Spain, Leche Pascual) were purchased in supermarkets in the appropriate countries.Analytical procedures Sample preparation. Infant formulas and the CRM powder were reconstituted with water according to manufacturers' recommendations. Other milk samples were analysed as received. In order to obtain milk whey, a sample aliquot of a size sufÆcient for the subsequent chromatographic analysis (typically 500 ml) was centrifuged at 50 000 rpm at 4 �C for 15 min (infant formulas and breast milk) or for 60 min (cow and goat milk).The fat (upper layer) and the insoluble residue at the bottom were discarded. The medium fraction was Æltered through a 0.45 mm syringe Ælter prior to analysis for total iodine or prior to chromatography. Freeze-drying was occasionally used for preconcentration of iodine species in milk whey samples; the powder obtained was redissolved in the chromatographic mobile phase. The preconcentration factor obtained was 2- to 9-fold.For experiments with the use of sodium dodecyl sulfonate (SDS), a 500 ml portion of a 4% aqueous solution of the reagent was mixed with a 500 ml aliquot of milk sample in an ultrasonic bath and incubated for 2 h at 37 �C. Enzymolysis was carried out with a mixture (1z2 w/w) of lipase and pronase. An amount of 30 mg of this mixture was added to 5 ml of milk whey diluted twice with the chromatographic mobile phase and incubated for 16 h at 37 �C.Determination of total iodine by ICP MS. For an analysis for the total iodine, a 2 ml aliquot of a milk sample was placed in a reaction tube together with 5 ml of 0.5% v/v ammonia solution and digested in the focussed microwave system at 45 W for 2.5 min. The resulting solution was diluted to 10 ml and fed directly into the ICP. The method of standard additions (at two levels: 100 and 200 mg l21) was applied for the quantiÆcation of the iodine content. Rh was used as the internal standard.The BCR CRM 151 Skim Milk Powder was analysed at least once every day to check the accuracy of the total iodine determination. ICP MS measurement conditions (nebulizer gas Øow, RF power and lens voltages) were optimized daily using a standard built-in software procedure. An aqueous solution of potassium iodide 10 mg l21 was used for sensitivity optimization. Typical examples of the optimum measurement conditions are a nebulizer gas Øow of 1.05 l min21, ICP RF power of 1.1 kW and a lens voltage of 9 V.The same instrumental conditions were employed when ICP MS was used as the chromatographic detector. Speciation analysis of iodine by SEC-ICP MS. The chromatographic mobile phase was the 30 mM Tris±HCl buffer at pH 7.0. The Øow rate was 0.75 ml min21. A sample aliquot of 100 ml was injected. The eluate from the column was fed directly into the ICP. 127I isotope was monitored together with 57Fe, 114Cd, 63Cu, 64Zn and 208Pb. The particular set of elements included the typical toxic and essential metals present in milk.The detailed discussion of speciation of the other elements is beyond the scope of this paper. The dwell time for each isotope was 100 ms and a number of replicates that allowed continuous data acquisition in the peak hopping mode for the duration of the chromatographic run was applied. Typically, 1000 replicates were applied to give a scan duration of 50 min. 1698 J. Anal. At. Spectrom., 1999, 14, 1697±1702Results and discussion Milk cannot be analysed directly by size-exclusion chromatography because it is an emulsion containing solid particles that would clog the inlet Ælter of the column.A prerequisite of a successful speciation analysis is therefore the development of a sample preparation procedure that will allow the quantitative transfer of iodine-containing species present in an original milk sample into a solution that would pass through a 0.2 mm inlet Ælter.For this purpose an approach based on the extraction of iodine-containing species into an aqueous phase (whey) that could be separated from the solid particles (caseine) and fat by (ultra)centrifugation was investigated. The evaluation of the extraction efÆciency was based on the comparison of the iodine concentration in the milk whey with that in the whole milk. Therefore a reliable method for the determination of iodine in these matrices (whey and whole milk) was required. Attempts to determine iodine in milk by direct introduction of samples diluted with ammonia into an ICP MS10 failed.The values obtained for the analysis of the BCR CRM 151 Skim Milk Powder were ca. 60±80% of the certiÆed value. Because the existing digestion methods were judged to be too cumbersome it was decided to develop a rapid open-vessel focussed microwave-assisted digestion method for the determination of the total iodine in milk and milk whey. Optimization of the total iodine determination Three factors were frequently cited in the literature concerning the determination of total iodine in milk by ICP MS: the unsuitability of the commonly used internal standards (In and Rh) in alkaline media because of their hydrolysis and the precipitation of the hydroxides,8 the risk of oxidation of iodide to iodine giving rise to an erroneous ICP MS signal,8 and the need for harsh (prolonged heating at elevated temperatures) conditions for the extraction of iodine from milk.A 3 h extraction at 90 �C with TMAH,4,6 microwave-assisted digestion with ammonia in a pressurized vessel at 700 W (three times),8 or sample combustion in a stream of oxygen7,13 were judged inappropriate for routine analyses in this work. Preliminary experiments indicated that the digestion of a milk sample with diluted ammonia in an open vessel using a focussed microwave Æeld could be the basis of a simple method yielding accurate results.Under the optimized working conditions (0.5% ammonia solution, microwave power of 45 W) a transparent solution that could be fed directly into an ICP MS could be obtained within 2±3 min. Rhodium used as an internal standard did not create any inconvenience, the signal was stable, probably because of the formation of Rh complexes with ammonia. The method of standard additions was evaluated to correct for matrix effects but since the slope of the standard additions curve was similar to that for an external calibration graph it was considered to be rather a preventive measure.The method developed was validated by analysing the BCRCRM151 Skim Milk Powder. The typical precision of Æve measurements realised during a day was ca. 3±4%. The mean of the resultsfferent days was 5.43°0.06 mg g21, in comparison with the certiÆed value of 5.35°0.14 mg g21. Determination of iodine in the whole milk and in the Æltrable milk fraction Three series of samples were investigated: (i) milk samples from different mammals including cow, goat and human breast milk, (ii) cow milk from different European countries including France, Spain, England, Germany and Poland, and (iii) different infant formulas.Such a choice was judged sufÆciently representative to develop a valid methodology for speciation of iodine in milk. The milk samples (natural milk or reconstituted infant formula powder) were subjected to ultracentrifugation and the total iodine concentration in the milk whey [referred to as a Æltrable (0.2 mm) fraction] was compared with the iodine concentration in the initial milk sample.Results are shown in Table 1. The concentrations of iodine in the whole milk are generally between 100 and 200 mg l21, which is a typical level reported earlier in the literature for Denmark9 and Turkey.11 The clearly elevated level in the British sample can be attributed to iodinesupplemented feed or disinfection of teats with iodophores9 or to supplementation with iodide salt.The low level of iodine in the Polish milk sample indicates no supplementation. The concentrations in infant formulas were distinctly lower, with an average value of ca. 50 mg l21. Recovery of iodine into the aqueous (Æltrable) phase. The prerequisite of speciation analysis by SEC-ICP MS, is the presence of all the iodine species potentially present in milk in a 0.2 mm Æltrable solution (whey fraction). The results shown in Table 1 indicate that in all the `natural' milk samples iodine is mostly present in the milk whey with the exception of one (British) sample.On the other hand, the average concentrations of the iodine present in the Æltrable phase in infant formulas are well below 50% of the total iodine present in the whole milk. One preparation showed the recovery as low as 15%. This iodine was found to be present in the solid residue after ultracentrifugation and attempts were made to enhance its recovery in view of investigating the speciation of this iodine by SEC-ICP MS.The approach investigated assumed that the non-extractable iodine was bound to (incorporated in) high-molecular weight compounds abundant in milk. It was based on the incubation of milk with sodium dodecyl sulfonate (SDS), which is a surfactant reagent used in protein chromatography to disrupt the aggregated proteins and to solubilize the proteins (by forming ion pairs) enabling their separation by liquid chromatography. This reagent has been recently successfully employed to improve the recovery of macromolecule bound selenium in yeast samples.23 It was found that the addition of SDS to milk improves the recovery of iodine from the supernatant (whey fraction).In the case of the natural milk samples, this increase was ca. 10±20% but for infant formula samples the amount of iodine recovered in the supernatant was more than twice that in the samples not incubated with SDS.Irrespective of the sample origin, more than 85% of the iodine initially present in milk could thus be recovered in the supernatant after ultracentrifugation, which was then subject to speciation analysis by SEC-ICP MS. Table 1 Total iodine concentration in milk samples of different origin Sample Total iodine concentration/mg l21 Iodide/mg l21 Whole milk Milk whey Milk whey Commercial cow's milk of different geographical origin– France (I) 167°14.6 148.7°13.6 145 France (II) 185.0 163.5 160.0 Spain 190.0 122.5 102.9 Germany 191.5 149.5 137.5 England 625 254 239 Poland 58 51.5 37.1 Milk from other species– Goat milk 326°11.4 293°9.5 279.1 Human milk 108.5°4.0 104.6°6.8 45.8 Infant formulas of different origin– Nidal 47.4°3.3 24.4°5.1 19.8 Nestle� 48.5 18.5 2.6 Milupa 51.2 20.0 3.8 Ordesa 53.1 30.0 1.5 Mead Johnson 21.5 3.76 0.12 Sandoz 32.0 16.5 4.9 J. Anal.At. Spectrom., 1999, 14, 1697±1702 1699Optimization of the SEC-ICP MS conditions for speciation of iodine in milk There is little information on iodine species in milk, which makes the choice of standards to be used for the optimization of chromatographic conditions difÆcult.Iodide should deÆnitely be included since it is often assumed to be the only iodine species in milk; indeed, the iodide concentration in milk is sometimes considered to be a measure of the total iodine present.14,24 Other potentially present species include the thyroid hormones (tetraiodothyronine and triiiodothyronine), reported in milk at concentration levels of between 2 and 12 ng ml21.25,26 Since infant formulas are often produced on the basis of hydrolysed cow's milk proteins, an iodinecontaining protein (thyroglobulin) was included in the array of standards used for the optimization of the chromatographic separation conditions.It should be noted that the presence of this particular protein in milk is unlikely but this was the only iodine-containing protein standard available commercially. In terms of chromatographic techniques size-exclusion was the separation mechanism investigated.The SEC-ICP MS coupling was a method of choice for the determination of iodine speciation in human serum.27,28 Despite the fact that, in theory, the separation should be based on the analyte molecular weight, secondary adsorption and ion-exchange effects make SEC a universal separation technique, as was demonstrated recently for organoselenium16 and organoarsenic compounds.29 SEC has the advantage over other HPLC techniques in terms of a high tolerance to the matrix and the compatibility of the mobile phase with ICP MS.A typical SEC-ICP MS chromatogram of a mixture of standards is shown in Fig. 1. The recovery of thyroglobulin and iodide exceeds 90% whereas the response of the triiodothyronine standard is poor (v10%), probably because of its sorption on the column stationary phase. Note that the elution of iodide is markedly delayed in comparison with the time predicted on the basis of the calibration of the column with the molecular weight standards.Iodide elutes well after the total volume of the column, which means the occurrence of strong non-exclusion interactions. The precision of the SEC-ICP MS analyses was ca. 5% (4.9% for peak area, 5.1% for peak height based on 5 consecutive injections). Quantitative determination of iodide in milk whey by SEC-ICP MS is possible using external calibration with aqueous iodide standards, a good agreement with values obtained by the method of standard additions was obtained.The limit of species-selective determination of iodide (10 times the standard deviation of the blank) was about 1 mg L21. The intensity of the blank was calculated as the average of the intensities (peak height mode) for the replicate measurements within the elution volume of iodide. The standard deviation of the blank is understood as the standard deviation of these intensities.Speciation of iodine in milk SEC-ICP MS chromatographic proÆles of natural milk whey samples and infant formulas of different origin, for which the results of the total iodine determination are given in Table 1, are shown in Fig. 2. Prior to injection on a chromatographic column the samples were preconcentrated by freeze-drying. The preconcentration factor depended on the possibility of the redissolution of the lyophilisate and varied from 2 (cow milk) Fig. 1 SEC-ICP MS chromatogram of iodine-containing standards (10 ng ml21 as I): 1, thyroglobuline; 2, triiodothyrosine; 3, iodide. Column: Superdex-75. Fig. 2 SEC-ICP MS chromatographic proÆles of milk whey samples of different origins. (a) Animal milk: A, goat milk; B, cow milk (Germany); C, cow milk (UK); D, cow milk (France); E, cow milk (Poland); F, cow milk (Spain). (b) Breast milk. (c) Infant formulas: A, Mead Johnson; B, Ordesa; C, Milupa; D, Nestle�.The chromatograms in (a) and (c) were off-set from each other by 56104 cps for the sake of the clarity of presentation. Column: Superdex-75. Peak identiÆcat 1, excluded species (w100 kDa); 2, iodide. 1700 J. Anal. At. Spectrom., 1999, 14, 1697±1702to 9-fold (breast milk). Three basic types of chromatograms can be distinguished. The basic SEC-ICP MS patterns obtained for commercial cow and goat milk samples (Fig. 2a) show one major signal at the elution volume matching that of iodide.The identity of this signal was conÆrmed by spiking the milk sample with a solution of iodide. In addition, a small signal corresponding to a compound excluded from the column can sometimes be seen. This pattern is characteristic for all cow milk samples investigated irrespective of their geographic origin. Human breast milk (Fig. 2b) shows two additional signals: one excluded from the column and one in the middle of the chromatogram; iodide accounts only for ca. 50% of the total iodine present. A completely different pattern is observed for iodine speciation in all but one of the samples of milk formula. Chromatograms of all but one of the samples investigated show a major signal corresponding to an iodine-containing compound excluded from the column. This signal represents 87z6% of the total iodine for Milupa, 73°5% for Ordesa, 65°5% for Mead Johnson, and 84°6% for the Nestle� formula powders. One sample (Nidal) shows a proÆle identical with that of the natural cow samples (a predominating signal from iodide). There are three minor signals in the chromatograms.One, poorly resolved from the major signal on the Superdex-75 column, corresponds to a compound with a molecular mass of ca. 60 kDa and accounts for a few percent of the total iodine present in all the samples except of the Ordesa milk, in which it accounts for ca. 18°2% of the total iodine. Another minor signal corresponds to iodide and represents from 4% (Ordesa) up to 27% (Mead Johnson) of total iodine present. A third minor peak in the middle of the chromatogram is always present but no hypothesis regarding its identity can be put forward.In one case (Nidal) this small peaks elutes at the elution volume of the triiodothyronine standard. Results of the determination of iodide in the milk samples investigated are summarized in Table 1. Attempts were made to obtain an insight into the identity of the excluded compounds by running size-exclusion chromatography on a Superdex-200 column with an exclusion limit of 1300 kDa.Fig. 3 shows that the compound is also excluded from this column. The peak of 60 kDa observed in Fig. 2 is baseline resolved from the excluded compound on the Superdex- 200 column and its molecular weight can be conÆrmed from the calibration curve of the Superdex-200 column. Note that the peak shapes in the chromatograms on a Superdex-200 column are worse than those on a Superdex-75 column and the recovery of iodine from this column was found to be poorer for some samples (especially the recovery of iodide from the Sandoz infant formula).Speciation of iodine in milk samples incubated with SDS The results of the total iodine determination (Table 1) suggested the presence of a substantial water-insoluble fraction of this element in some samples. As indicated above, this iodine can be recovered into the supernatant after incubation of a milk sample with SDS and ultracentrifugation. Fig. 4 compares the chromatograms of the supernatant fraction of the different milk samples without and with the addition of SDS. For natural milk samples (an example of a cow milk sample is shown in Fig. 4a) the patterns of the chromatograms obtained with and without sample incubation with SDS are similar. The iodide peak remains the same whereas there is a small increase in the signal intensity for the compound excluded from the column to account for more iodine extracted.In the case of the infant formula samples (Fig. 4b) the incubation of the reconstituted milk with SDS leads to a marked increase in the intensity of the species excluded from the column. In the absence of SDS, this species apparently stays in the solid residue left over after the centrifugation of the milk whey. The pattern of the chromatogram obtained after the incubation with SDS changes, the concentration of the excluded species being higher than that of iodide.The relative abundance of iodide in the samples incubated with SDS decreases approximately twice (from 100 to 56% for Nidal, from 19 to 10% for Milupa, from 29 to 18% for Mead Johnson). Speciation of iodine in milk after enzymolysis An alternative to the incubation with SDS that can be used to improve the recovery of iodine-containing species from milk Fig. 3 SEC-ICP MS chromatographic proÆles of infant formulas whey samples of different origins obtained on a Superdex-200 column.A, Mead Johnson; B, Ordesa; C, Sandoz; D, Milupa; E, Nestle�. Peak identiÆcation: 1, excluded species (w1300 kDa); 2, ca. 60 KDa compound; 3, iodide. Fig. 4 Effect of the SDS addition on the chromatographic proÆles of different milk samples. Solid line, sample without SDS. Dashed line, sample incubated with SDS as described in Analytical Procedures. (a) Cow milk, (b) infant formula 1, excluded species (w100 kDa); 2, iodide. J. Anal. At. Spectrom., 1999, 14, 1697±1702 1701and to get a deeper insight into the forms of iodine present is the destruction of macromolecular species present in milk by enzymic hydrolysis and monitoring the resulted changes in the speciation of iodine by SEC-ICP MS.A mixture of lipase and protease was investigated for this purpose. These enzymes were found to be effective for the decomposition of milk proteins containing metals (Cu, Zn) for which the elution proÆle was completely changed after enzymolysis, giving rise to a number of low molecular metal-containing species.30 However, this procedure was found unsuitable for iodine species.The enzymic treatment improved the recovery of iodine into the aqueous phase, as in the case of the incubation with SDS, but the SEC-ICP MS chromatograms showed a continuum signal of iodine starting at the exclusion volume and lasting for several minutes (not shown). This signal was followed by a broad iodide signal.Such a chromatogram may suggest that iodine present in the species excluded from the column (Fig. 4) is not covalently bound and is attached to a mixture of macromolecular compounds by coordination bonds or even by less speciÆc interactions. Conclusion Whereas iodide seems to be the major, and practically the only, species of iodine in cow and goat milk, a more complex speciation of this element occurs in human milk and in infant formula, making species-selective analysis necessary to gain a deeper insight in the bioavailability of iodine in baby food.The method developed offers such an approach allowing the discrimination of iodide from a number of other (for the moment unidentiÆed iodine-containing species) and the quantitative determination of a particular species. The major difference between the breast milk and infant formula is the presence in the latter of a macromolecular compound comprising more than 50% of the iodine present in the preparation. Isolation, puriÆcation and characterization of this compound is the prerequisite for drawing conclusions regarding its bio-availability.Acknowledgements LFS acknowledges the research grant of the Caixa Galicia. We thank Profs. P. Bermejo (University of Santiago) and R. Lobinski (CNRS, Pau) for valuable discussions. References 1 E. J. Underwood, Trace Elements in Human and Animal Nutrition, Academic Press, New York, 4th edn., 1997, p. 271. 2 M. Anke, B. Groppel, M. Mu» ller, E.Scholz and K. Kramer, Fresenius' J. Anal. Chem., 1995, 352, 97. 3 M. F. Picciano, Biol. Neonate, 1998, 74, 84. 4 P. A. Fecher, I. Goldman and A. Nangengast, J. Anal. At. Spectrom., 1998, 13, 977. 5 E. Larsen, P. Knuthsen and M. Hansen, J. Anal. At. Spectrom., 1999, 14, 41. 6 G. Radlinger and K. G. Heumann, Anal. Chem., 1998, 70, 2221. 7 Y.Ge�linas, A. Krushevska and R. M. Barnes, Anal. Chem., 1998, 70, 1021. 8 H. Vanhoe, F. Van Allemeersch, J. Versieck and R. Dams, Analyst, 1993, 118, 1015. 9 S. Stu»rup and A. Bu» chert, Fresenius' J. Anal. Chem., 354, 323. 10 H. Baumann, Fresenius' J. Anal. Chem., 1990, 338, 809. 11 G. Go»kmen and G. DagÆh, Analyst, 1995, 120, 2005. 12 R. R. Rao and A. Chatt, Analyst, 1993, 118, 1247. 13 F. Gu, A. A. Marchetti and T. Straume, Analyst, 1997, 122, 535. 14 T. Mitsuhashi and Y. Kaneda, J. Assoc. Off. Anal. Chem., 1990, 73, 790. 15 V. E. Negretti de Bra» tter, S. Recknagel and D. Gawlik, Fresenius' J. Anal. Chem., 1995, 353, 137. 16 P. Bra» tter, I. Navarro Blasco, V. E. Negretti de Bra» tter and A. Raab, Analyst, 1998, 123, 821. 17 J. S. Edmonds and M. Morita, Pure Appl. Chem., 1998, 70, 1567. 18 A. K. Das, R. Chakraborty, M. L. Cervera and M. de la Guardia, Mikrochim. Acta, 1996, 122, 209. 19 Element Speciation in Bio-inorganic Chemistry, ed. S. Caroli, Wiley, Chichester, 1996, pp. 474. 20 B. Michalke, D. C. Muench and P. Schramel, Fresenius' J. Anal. Chem., 1992, 344, 306. 21 B. Michalke and P. Schramel, J. Trace Elem. Electrolytes Health Dis., 1990, 4, 163. 22 P. Gerhart, F. Macasek and P. Rajec, J. Radioanal. Nucl. Chem., 1998, 229, 83. 23 C. Casiot, J. Szpunar, R. Ëobin� ski and M. Potin-Gautier, J. Anal. At. Spectom., 1999, 14, 645. 24 D. Sertl and W. Malone, J. Assoc. Off. Anal. Chem., 1993, 76, 711. 25 B. Mo» ller, I. Bjo»rkhem, O. Falk, O. Lantto and A. Larsson, J. Clin. Endocrinol. Metab., 1983, 56, 30. 26 L. V. Oberkotter, J. Chromatogr., 1989, 487, 445. 27 A. Makarov and J. Szpunar, Analusis, 1998, 26, M44. 28 B. Michalke, P. Schramel and S. Hasse, Mikrochim. Acta, 1996, 122, 67. 29 S. McSheehy and J. Szpunar, unpublished work. 30 L. Fernandez Sanchez and J. Szpunar, unpublished work. Paper 9/05558D 1702 J. Anal. At. Spectrom., 1999, 14, 1697±17

ISSN:0267-9477    

DOI:10.1039/a905558d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Vaporization and removal of silica for the direct analysis of geological materials by slurry sampling electrothermal vaporization-inductively coupled plasma-mass spectrometry MuÆda E. Ben Younes,a D. Conrad Gre�goire*b and Chuni L. Chakrabartia aDepartment of Chemistry, Ottawa-Carleton Chemistry Institute, Carleton University, 1125 Colonel By Drive, Ottawa, ON, Canada K1S 5B6 bGeological Survey of Canada, 601 Booth St., Ottawa, ON, Canada K1A 0E8 Received 26th April 1999, Accepted 4th August 1999 Reported is a method for the removal of silica for the direct analysis of solid geological samples high in silica content using slurry sampling electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS).The ETV-ICP-MS vaporization curve for SiO2, sampled as a slurry, is reported for temperatures ranging from 810±2600 �C. This curve showed that silica was completely vaporized at a temperature of 2200 �C. The effect of using HF as a chemical modiÆer to remove silica as the tetraØuoride was studied.It was found that HF could completely remove any Si attributed to silica if sufÆcient modiÆer were added and an adequate reaction time allowed. At reaction hold times that were less than optimal, two Si signals were observed. The Ærst signal, which appears at an earlier time and at a temperature less than 480 �C, is attributed to the volatilization of silicon tetraØuoride. The second signal, which appears at a later time and at a temperature of about 2500 �C, is attributed to the vaporization of residual unreacted SiO2 remaining in the graphite tube, or to the vaporization of a mixture of SiO2 and SiC. 20 ml of 50% HF is effective in completely removing 0.125 mg of SiO2. No adverse effects including corrosive degradation of the graphite tube were observed over the lifetime of the tube, which exceeded 200 Ærings. HydroØuoric acid chemical modiÆer was successful in removing virtually all the silica content in natural silicate standard reference materials.Introduction Many environmental and geological materials contain high concentrations of Si as SiO2 or other more complex silicates. Before analysis of these materials for trace metals can proceed, it is necessary to break up this silicate matrix and to remove it or separate it from the analytes. Slurry sampling combined with ETV-ICP-MS is an attractive approach to the trace analysis of environmental and geological samples.Slurry sampling combines the beneÆts of both solid and liquid sampling.1 When compared to conventional sample preparation techniques, slurry sampling has several important advantages: reduced sample preparation time and lower risk of contamination; the analysis of micro amounts of sample; and full automation. When combined with ETV-ICP-MS, slurry sampling can offer additional advantages. For example, with the ETV system it is possible to use the graphite tube as a chemical reactor.Procedural steps such as the addition of digestion reagents, chemical modiÆers, etc., can all be done on-line automatically. Thus, it may be possible to complete an entire sample preparation sequence within the graphite tube starting with suitably powdered sample material. Bendicho and de Loos-Vollebregt2 and Miller-Ihli3 have reviewed the literature on solid sampling and discussed the beneÆts of ultrasonic slurry sampling, which has been used successfully in graphite furnace atomic absorption spectrometry (GFAAS). Ultrasonic slurry sampling has also been extended to ETV-ICP-MS.4±11 Chemical modiÆers are used to enhance the vaporization properties of both analyte and matrix components.One approach involves converting relatively refractory compounds into more volatile compounds. Halocarbons introduced into plasma gases have been used to improve the vaporization properties of relatively refractory analytes. Kirkbright and Snook12 used 0.1% freon-23 for the determination of B, Mo, Zr, Cr and W,13 and CCl2 and freon-12 to determine trace elements in ceramic powders and SiC.14 Ren and Salin15,16 and Allary et al.17 used freon-12 to enhance the vaporization of carbide forming elements.Matousek et al.18 used Cl2 for the determination of Cr, V, Ti, W, and Zr. PolytetraØuoroethylene was also used as a Øuorinating reagent to promote the vaporization of refractory elements.19±21 Goltz et al.22 used freon-23 to enhance analyte signals for the rare-earth elements in ETV-ICP-MS and Ng and Caruso23 added ammonium chloride (7% m/v) as a means of vaporizing analytes as chlorides.It has been reported in GFAAS studies that samples high in silica content cause serious effects on analyte signals. Bendicho and de Loos-Vollebregt24 reported that when analyzing glass slurries a rapid deterioration in the condition of the graphite tube ensued and a decrease in absorbance signals occurred after 50±100 Ærings.Eams and Matousek25 and Mu» ller-Vogt and Wendl26 attributed the rapid deterioration of the graphite tube when silica was present to the chemical attack of silica on graphite. Bendicho and de Loos-Vollebregt27 analyzed glass samples and reported that this effect can be minimized by using HF, and thus the deterioration effects of the graphite tube could be reduced and the lifetime of the graphite tube extended. For ETV-ICP-MS, microgram quantities of silicate materials vaporized directly into the plasma will result in signiÆcant analyte signal suppression.For this reason, silicate matrices must be eliminated prior to the vaporization step. McIntyre et al.11 used NH4F, NH4F±HF, and HF as Øuorinating reagents for the removal of SiO2. The use of NH4F and NH4F± HF was ineffective in removing silica; however, HF was able to remove much larger quantities of silica and made possible the direct determination of Ra in silicate reference materials.11 Although published separately, the work of McIntyre et al.11 J.Anal. At. Spectrom., 1999, 14, 1703±1708 1703 This Journal is # The Royal Society of Chemistry 1999was completed in this laboratory following the research reported here. The objective of this work was to determine the optimum experimental conditions for the complete removal of silica as silicon dioxide and silica as silicate in geological reference material powders using ultrasonic slurry sampling ETV-ICP-MS.Experimental Instrumentation A Perkin-Elmer SCIEX (Concord, Ontario, Canada) Elan 5000a ICP mass spectrometer equipped with an HGA-600 MS electrothermal vaporizer, a Model AS-60 autosampler, and a USS-100 ultrasonic mixing probe were used. The ultrasonic mixing probe (constructed of high purity Ti) used a mixing time of 30 s at a power level of 12 Wto provide adequate mixing and suspension of slurries. To facilitate larger sample particle sizes, the PFA tip provided with the autosampler was replaced with larger thin-walled PFA capillary tube (id 0.81 mm).Pyrolytic graphite-coated tubes were used throughout. The experimental conditions for both the Elan 5000 and the HGA-600 MS are given in Table 1. A PTFE tube of 80 cm and 6 mm id was used to interface the HGA-600 MS to the plasma torch. Optimization of the plasma and mass spectrometer was accomplished using solution nebulization, prior to switching to ETV mode. No further optimization of the ICP mass spectrometer was required with the exception of small (°50 ml min21) variations in the carrier Ar Øow rate.The operation of the HGA-600 MS was completely computer controlled. During the drying and pyrolysis steps of the temperature program, opposing Øows of argon (300 ml min21) originating from both ends of the graphite tube removed water and other vapours through the dosing hole of the graphite tube. During the high temperature or vaporization step, the dosing hole was sealed by a pneumatically activated graphite probe.Once the graphite tube was sealed, a valve located at one end of the HGA workhead directed the carrier argon gas Øow, originating from the far end of the graphite tube, directly to the argon plasma at a Øow rate of 850 ml min21. Feriments, 30Siz (3.12% abundance) was used to monitor the vaporization of silica. Data in all Ægures and tables are corrected for any background signal present. Standards and reagents High purity argon gas (99.995%, Matheson Gas Products, Ottawa, Ontario, Canada) was used.Solutions were prepared with ultra-pure water obtained from a Milli-Q water puriÆcation system (Millipore Corp., Mississauga, Ontario, Canada). The nitric and hydroØuoric acids used were analytical-reagent grade (Anachemia, Rouses Pint, NY, USA). Finely divided high-purity precipitated silica and high purity silicon carbide were obtained from Spex Industries, Metuchen, NJ, USA. Standard reference materials used were SY-2 syenite rock containing 60.11% SiO2 and BHVO-1, a basalt containing 49.94% SiO2.Preparation of slurries The slurries were prepared by accurately weighing the solid sample (e.g., 0.10 g) and placing it into a 10 ml plastic test-tube. Water was added as the liquid medium. The slurry was mixed using a vortex mixer from which was removed (during agitation) a 1 ml aliquot. If required the 1 ml aliquot was diluted further until a 1 ml aliquot was produced which contained the desired mass to volume ratio of solid to diluent.Results and discussion Freon, used as halogenation agent, has become a popular gasphase matrix modiÆer used to enhance the volatility of refractory elements in ETV-ICP-AES12,15±17 and ETV-ICPMS. 22,28 In most published work, freon has been mixed with the argon gas during the high temperature vaporization step. Freon decomposes at high temperatures to give highly reactive chlorine or Øuorine radicals.These radicals react with sample and matrix components producing relatively volatile analyte chlorides and Øuorides. In this work freon-23 was used as a possible Øuorinating reagent to remove the SiO2 by conversion to volatile SiF4. The results obtained for these experiments indicated that either large quantities of freon or very long (minutes) reaction times are required to complete the removal of silica. Even when using a pyrolysis temperature of 1000 �C, signiÆcant quantities of carbon soot were produced as a result of freon decomposition.If either the freon concentration or the reaction time were increased to levels required to completely remove silica, excess soot was produced causing unwanted plasma effects. Because of this, the use of gas-phase matrix modiÆers was abandoned in favour of using HF. Table 1 Instrument operating conditions and data acquisition parameters for ETV-ICP-MS ICP mass spectrometer– RF power 1100W Coolant Ar Øow rate 15.0 l min21 Auxiliary Ar Øow rate 900 l min21 Carrier Ar Øow rate 850 l min21 Sampler and skimmer Ni HGA-600 MS electrothermal vaporizer– Sample volume 20 ml ModiÆer volume 20 ml Clean up step 1 s ramp, 2650 �C, for 10 s Digestion step 10 s ramp, 60 �C, variable hold time Dry step 5 s ramp, 100 �C for 30 s Pyrolysis step 1 s ramp, 100 �C for 7 s Vaporization step 1 s ramp, 2500 �C for 6 s Data acquisition2 Dwell time 20 ms Scan mode Peak hop transient Points/spectral peak 1 Signal measurement Integrated counts Resolution 0.7 u at 10% peak height Fig. 1 Vaporization curve for 20 ml slurry sample containing 125 mg of SiO2. 1704 J. Anal. At. Spectrom., 1999, 14, 1703±1708Vaporization curve for silicon dioxide As an initial study on the removal of SiO2, a vaporization curve was constructed to determine at which temperature SiO2 vaporization begins and at which temperature the vaporization is complete. The vaporization curve for SiO2 was obtained without using a chemical modiÆer and is shown in Fig. 1. These data were obtained using 20 ml of the SiO2 slurry containing 125 mg of SiO2. The ETV-ICP-MS signal intensity was measured at vaporization temperatures ranging between 810 and 2600 �C. No pyrolysis step was used. The ETV-ICP-MS vaporization curve (Fig. 1) gives an appearance temperature for Si (derived from the vaporization of SiO2) of approximately 1600 �C and reaches a maximum at about 2200 �C. These temperatures coincide with the melting point (1610 �C)29 and the boiling point (2230 �C)29 of SiO2, respectively.A single Si signal was observed over the entire range of vaporization temperatures studied. Clearly, a pyrolysis temperature of 2200 �C cannot be used to remove silica since, at this temperature, many analyte elements are vaporized as well. HydroØuoric acid matrix modiÆer The ETV heating program step during which HF is added and the reaction between sample and HF can best be referred to as the digestion step.The effect of added HF on the removal of SiO2 in ETV-ICP-MS is shown in Fig. 2. For these experiments, a pyrolysis temperature of 100 �C and a vaporization temperature of 2500 �C were used following a digestion step of 60 �C (10 s ramp). The duration of the digestion step (hold time) was varied from 30 s to 300 s. A 20 ml aliquot of 50% hydroØuoric acid was added to 20 ml of the SiO2 slurry sample in the graphite tube. The ETV-ICP-MS signal intensity for Si decreased with increased hold time for the digestion step indicating that most of the silica was removed with a hold time of about 150 s.For digestion step hold times greater than 150 s, little additional SiO2 is lost. This observation is in agreement with the work of McIntyre et al.11 who used HF for the removal of silica from geological materials analysed for Ra using slurry sampling ETV-ICP-MS. The actual Si signals obtained, corresponding to the data given in Fig. 2, are shown in Fig. 3. The Si ETV-ICP-MS signals shown in Figs. 3a and 3b have two distinct signals widely separated in time. The Ærst signal appears at about 1 s Fig. 2 Effect of digestion step hold time on the integrated signal for Si(125 mg) using 20 ml 50% HF. Fig. 3 ETV-ICP-MS signals for 125 mg SiO2 using 20 ml 50% HF at hold time of (a) 30 s, (b) 60 s, (c) 90 s, (d) 120 s, (e) 150 s, (f) 180 s, (g) 300 s. J. Anal. At. Spectrom., 1999, 14, 1703±1708 1705reaching a maximum at about 2 s, and the second smaller signal appears at about 5 s with a maximum at approximately 6 s into the high temperature vaporization step.This suggests that two different Si species are released during the vaporization step and that these two species have very different volatilities. Increasing the hold time above 60 s (Figs. 3c±3g) shows no change in the Si signals with the exception that the second silica signal persists and is somewhat variable indicating that some residual unreacted silica remained in the graphite tube.Origin of Si ETV-ICP-MS signals To investigate the nature of the two Si species observed, the effect of changing the ramp time (time to maximum temperature) or graphite tube heating rate was studied. For these experiments an HF digestion step of 60 �C with a hold time of 30 s, a pyrolysis temperature of 100 �C (7 s hold time) and a vaporization temperature of 2500 �C were used. The effect of changing the graphite tube heating rate during the vaporization step is shown in Fig. 4. The ramp time was varied between 1 s and 10 s giving a heating rate of between 2400 �C s21 and 240 �C s21, respectively. At a ramp time of 1 s, the Ærst signal (Fig. 4a) appeared at 1 s with a signal maximum at about 2 s. The second signal occurred at about 5 s reaching a maximum at 7 s. As the ramp time was increased, there was a large shift in the appearance and signal maximum times of the second signal, and a correspondingly small shift in the signal times for the Ærst signal.The higher the ramp time (slower heating rate) the greater the shift in the second signal. Fig. 4c shows the ETV-ICP-MS signal for Si at a ramp time of 10 s. The appearance time for the Ærst signal was 2 s and for the second signal between 12 and 19 s. The fact that a radical change in the heating rate of the graphite tube results in almost no change in the signal position, height and width of the Ærst Si signal while resulting in a very large change in the shape and size of the second signal suggests that thbut have widely different vaporization characteristics.Since the Ærst, larger, signal appeared at 2 s using a 10 s ramp time, it can be calculated that the appearance temperature of this Si species is about 480 �C. The melting point and boiling point of SiO2 are 1610 and 2230 �C respectively, indicating that the Ærst signal cannot be SiO2 but could be due to SiF4 (bp 286 �C),29 which was formed by the addition of HF as a chemical modiÆer in the digestion step.Both Øuorine and HF are known to intercalate in graphite30 and it is possible that SiF4 is also thermally stabilized through intercalation. From this, it can be surmised that although most of the SiF4 was lost during the reaction and pyrolysis steps, some SiF4 remained and perhaps became intercalated into the graphite tube only to be released later at higher temperatures during the vaporization step.In a separate study, Gre�goire et al.31 reported that small quantities of HCl were retained within the graphite tube by intercalation even after pyrolysis at 400 �C. Fig. 5 shows ETV-ICP-MS signals for Si obtained from the vaporization of (a) SiO2 and (b) SiC, both without modiÆer. A ramp time of 1 s was used for the vaporization step. Fig. 5a shows that the signal for SiO2 occurred between 3.5 and 10 s with the maximum occurring at about 5.9 s, which was coincident with the second Si signal shown in Fig. 4a. Fig. 5b shows that the signal for SiC occurred between 3.5 and 12 s with the maximum at about 7.3 s. This may suggest that the small second signal in Fig. 4a, which appeared at a later time, could be due to unreacted SiO2 remaining following the digestion step and then released at a later time during the vaporization step. It may also be attributed to the release of a mixture of SiO2 and SiC or SiC alone, which may be formed at high temperatures.Optimization of HF matrix modiÆcation for silica removal In Fig. 3g it was shown that, for a digestion step hold time of 300 s, the Ærst larger Si signal essentially disappeared and that only a small decrease in the second Si signal was observed. This indicated that using only 20 ml HF possibly did not convert all of the SiO2 to SiF4. To investigate the effect of increasing the amount of HF on the ETV-ICP-MS signal intensity for Si, the volume of 50% HF used was increased from 20 ml to 70 ml.To ensure an adequate reaction time, a digestion step hold time of 300 s was used. Fig. 6 shows that the second signal (from Fig. 3) appeared at slightly earlier times with increasing amounts of HF and that the second signal was almost completely removed when 70 ml of HF was used. This suggests that there was enough HF and a sufÆcient holding time for the Fig. 4 ETV-ICP-MS signals for 125 mg SiO2 using 20 ml 50% HF at different vaporization ramp times, (a) 1 s, (b) 5 s, (c) 10 s.Hold time at the digestion step was 30 s. Fig. 5 ETV-ICP-MS signals for (a) SiO2 and (b) SiC without using HF as a chemical modiÆer; vaporization step ramp time of 1 s. 1706 J. Anal. At. Spectrom., 1999, 14, 1703±1708SiO2 to react completely with HF to produce SiF4 and that there was also enough time to volatilize SiF4 during the digestion and pyrolysis steps. To obtain the most practical conditions for removing silica, we also determined the reaction step holding time required to eliminate SiO2 when using 70 ml of 50% HF.The results show that the Si signal intensity decreases as the digestion step hold time is increased and essentially disappears at a hold time of 150 s. This result is similar to that obtained when using 20 ml of 50% HF (Fig. 3) with the exception that virtually no second Si signal is apparent. This may mean that the second signal observed earlier was due to the presence of unreacted silica or silica that had escaped contact with the HF.For the sake of brevity, this data is not given here. An important Ænding resulting from this study was that at a hold time of 180 s (using 70 ml 50% HF) the graphite tube failed catastrophically with the end of the graphite tube downstream from the dosing hole (end nearest to plasma) completely breaking off. To determine whether this effect was due to the presence of silica or hydroØuoric acid, the experiment was repeated using only 70 ml of 50% HF and 20 ml H2O.When using only HF, the tube also failed indicating that tube fracture was the result of the corrosive effect of HF on graphite. From this we conclude that using 20 ml of 50% HF and a reaction hold time of 150 s at the digestion step is optimal for the removal of silica while maintaining the integrity of the graphite tube. The small quantities of silica remaining when using 20 ml of HF are not signiÆcant and did not detract from the determination of Ra in silicate materials using slurry sampling ETV-ICP-MS.11 Silica removal from geological reference materials Silica is, of course, the simplest of silicate materials and so it is possible that the removal of Si from more complex silicates may be more difÆcult relative to pure silica.The method developed above was applied to two geological reference materials: syenite SY-2 (60.11% SiO2) and basalt BHVO-1 (49.94% SiO2). Fig. 7 shows the ETV-ICP-MS signals for Si obtained when 0.125 mg BHVO-1 basalt reference material was vaporized (a) without using a chemical modiÆer and (b) using 20 ml of 50% HF as a chemical modiÆer with a reaction time of 150 s.Fig. 7a shows a single large Si signal appearing at about 3.9 s reaching a maximum at about 5.8 s. This signal can be attributed to SiO2. Fig. 7b shows a smaller signal appearing at about 1.6 s with a maximum at about 2.9 s with a very small shoulder appearing at about 4 s.The Ærst signal can be attributed to the release of SiF4 as discussed earlier in this paper. The shoulder may be due to a very small quantity of unreacted silica. This result demonstrates that almost all the silica was removed by using 20 ml of 50% HF (150 s reaction time) and that complex silicate materials can be successfully digested using HF. The results obtained for SY-2 were identical to those reported above for BHVO-1. References 1 S. C. Stephen, D. Littlejohn and J.M. Ottaway, Analyst, 1985, 110, 573. 2 C. Bendicho and M. T. C. de Loos-Vollebregt, J. Anal. At. Spectrom., 1991, 6, 353. 3 N. J. Miller-Ihli, Anal. Chem., 1992, 64, 964A. 4 D. C. Gre�goire, N. J. Miller-Ihli and R. E. Sturgeon, J. Anal. At. Spectrom., 1994, 9, 605. 5 R. W. Fonesca and N. J. Miller-Ihli, Appl. Spectrosc., 1995, 49, 1403. 6 M. Liaw and S. Jiang, J. Anal. At. Spectrom., 1996, 11, 555. 7 N. J. Miller-Ihli, Spectrochim. Acta, Part B, 1996, 51, 1591. 8 S. Hauptkorn, V.Krivan, B. Gercken and J. Pavel, J. Anal. At. Spectrom., 1997, 12, 421. 9 M. Liaw, S. Jiang and Y. Li, Spectrochim. Acta, Part B, 1997, 52, 125. 10 R. W. Fonesca, N. J. Miller-Ihli, C. Sparks, J. A. Holcombe and B. Shaver, Appl. Spectrosc., 1997, 51, 1800. 11 R. St. C. McIntyre, D. C. Gre�goire and C. L. Chakrabarti, Spectroscopy, 1998, 13, 18. 12 G. F. Kirkbright and R. D. Snook, Anal. Chem., 1979, 51, 1938. 13 G. Zaray, T. Kantor, G. Wolff, Z. Zadgorska and H. Nickel, Mikrochim.Acta, 1992, 107, 345. 14 G. Zaray, F. Leis, T. Kantor, J. Hassler and G. Tolg, Fresenius' J. Anal. Chem., 1993, 346, 1042. 15 J. M. Ren and E. D. Salin, Spectrochim. Acta, Part B, 1994, 49, 555. 16 J. M. Ren and E. D. Salin, Spectrochim. Acta, Part B, 1994, 49, 567. 17 J. Allary, G. Hernadez and E. D. Salin, Appl. Spectrosc., 1995, 49, 1796. 18 J. P. Matousek, R. T. Satumba and R. A. Bootes, Spectrochim. Acta, Part B, 1989, 44, 1009. 19 M. Huang, Z. Jiang and Y. Zeng, J. Anal. At. Spectrom., 1991, 6, 221. 20 M. Huang, Z. Jiang and Y. Zeng, Anal. Sci., 1991, 7, 773. 21 Z. Jiang, B. Hu, Y. Qin and Y. Zeng, Microchim. J., 1996, 53, 326. 22 D. M. Goltz, D. C. Gre�goire and C. L. Chakrabarti, Spectrochim. Acta, Part B, 1995, 50, 1365. 23 K. C. Ng and J. A. Caruso, Analyst, 1983, 108, 476. 24 C. Bendicho and M. T. C. de Loos-Vollebregt, Spectrochim. Acta, Part B, 1990, 45, 679. 25 J. C. Eams and J. P. Matousek, Anal. Chem., 1980, 52, 1248. 26 G. Mu» ller-Vogt and W. We, 1981, 53, 651. 27 C. Bendicho and M. T. C. de Loos-Vollebregt, Spectrochim. Acta, Part B, 1990, 45, 695. 28 B. Wanner, P. Richner and B. Magyar, Spectrochim. Acta, Part B, 1996, 51, 817. Fig. 7 ETV-ICP-MS signals for SiO2 from 125 mg of basalt (BHVO-1), (a) without using HF as a chemical modiÆer and (b) using 20 ml of 50% HF chemical modiÆer. Digestion step hold time: 150 s. Fig. 6 Effect of HF volume on the ETV-ICP-MS signals for SiO2, (a) 20 ml, (b) 40 ml, (c) 60 ml, (d) 70 ml of HF. Hold time at the digestion step was 300 s. J. Anal. At. Spectrom., 1999, 14, 1703±1708 170729 Handbook of Chemistry and Physics, ed. D. R. Lide, 74th edn, CRC Press, Cleveland, OH, 1994. 30 W. Slavin, Graphite Furnace AAS: A Source Book, The Perkin Elmer Corporation, Norwalk, CT, 1984, p. 45. 31 D. C. Gre�goire, D. M. Goltz, M. M. Lamoureux and C. L. Chakrabarti, J. Anal. At. Spectrom., 1994, 9, 919. Paper 9/03269J 1708 J. Anal. At. Spectrom., 1999, 14, 1703±17

ISSN:0267-9477    

DOI:10.1039/a903269j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

A longitudinal study of iodine excretion in normal pregnancy determined by inductively coupled plasma mass spectrometry Carl J. Wardley,a Alan Cox,b Cameron McCleodb and Brian W. Morris*a aDepartment of Clinical Chemistry, Northern General Hospital, Herries Road, ShefÆeld, UK S5 7AU. E-mail: brian@clinchem-ngh.demon.co.uk bCentre for Analytical Sciences, University of ShefÆeld, UK Received 28th July 1999, Accepted 6th September 1999 We report here on a method for the determination of urine iodine by ICP-MS.The method proved to be fast, reliable and precise (within batch CVs v2.5%) and to have a low limit of detection (0.000 38 mmol L21). Iodine was determined in a total of 379 urine samples from 86 healthy pregnant women who gave samples at intervals of 4 weeks from 16 weeks pregnancy to 10 weeks post-partum. Fifty-Æve control urine samples were analysed from age-matched non-pregnant females. Iodine excretion (mmol of iodine per mol of creatinine) increased signiÆcantly from 28±40 weeks of pregnancy (pv0.05), returning to non-pregnant control levels by 10 weeks post-partum. This study conÆrms the ability of our ICP-MS method to analyse large numbers of patient samples with the speed and performance acceptable for a routine assay. Iodine deÆciency has been a world-wide health problem for centuries1 manifesting itself, in its most severe form, as endemic cretinism.Globally, it is estimated that one billion people are at risk from iodine deÆciency today with developing countries being the primary areas of concern.Europe, however, is not exempt and a number of countries, e.g., Denmark, The Netherlands and Northern Italy, are amongst those with a low natural iodine supply.2 Iodine deÆciency has been described as the world's most common and preventable endocrine disease and dietary iodine requirements have been studied in many population groups with urine levels in pregnancy of particular interest.3±5 In healthy pregnant women, the regulation of thyroid function depends upon a number of factors that act independently to increase thyroid hormone requirements.(i) A marked increase in the binding capacity of serum due to high circulating levels of thyroid binding globulin (TBG). (ii) Direct stimulation of the thyroid gland by human chorionic gonadotrophin (hCG) acting as a thyrotrophic hormone. (iii) Increased placental de-iodinating activity which may contribute to thyroid hormone metabolism.The combined result of these events is a physiological adaptation of the maternal thyroid gland to pregnancy so long as the availability of iodine remains sufÆcient. Reduced maternal thyroid concentrations during pregnancy can inØuence foetal neurological development but the effects of iodine deÆciency disorders (IDD) occur at all stages of human development, from the foetus through to adulthood. Throughout gestation, a reduced supply of iodine is associated with chronic stimulation of the thyroid gland and increased renal loss of iodine has been suggested as the cause of thyroid enlargement, the so-called `pregnancy goitre', although there are conØicting reports in the available literature.6 Most ingested iodine eventually appears in the urine and urinary iodine concentration is therefore a good marker for iodine deÆciency.The analysis of urinary iodine has been based, historically, on simple colorimetric procedures allowing reasonably rapid determination of small numbers of samples.7 Our aim in the current study was to investigate the basic physiology of iodine excretion during normal pregnancy, using a highly sensitive ICP-MS technique that required minimal sample preparation and manipulation.Experimental Samples Some 86 healthy pregnant women gave a total of 379 urine samples at intervals of four weeks from 16 weeks gestation to term and up to 10 weeks post-partum. Fifty-Æve age-matched females supplied one random urine sample to provide a nonpregnant reference interval for our study.All urine samples were collected into `iodine-free' sterile plastic universal containers (Medical Wire and Equipment Co., Ltd, Corsham, Wiltshire, UK) and stored at 220 �C prior to analysis. Approval for our study was obtained from the North ShefÆeld Hospital Ethics Committee and informed consent was obtained from each volunteer. Iodine analysis Iodine in urine was determined using a Hewlett-Packard HP4500 ICP mass spectrometer (Palo Alto, CA, USA) Ætted with a Babington V-groove nebuliser and glass spray chamber (Scott double pass), together with a CETAC (Omaha, NE, USA) ASX-500 autosampler for ease of sampling (see Table 1 for instrument settings). Potassium iodate (Sigma Aldrich, Poole, Dorset, UK) was used for the preparation of an aqueous stock standard solution (1000 mg L21), subsequently diluted to give a working stock standard of 100 mg L21.We utilised the method of additions calibration for our analysis using matrix-matched working standards covering the calibration range 5±100 mg L21 (5, 10, 15, 20, 50 and 100 mg L21) together with a urine blank solution.Polypropylene tubes were used throughout (Sarstedt, Leicester, UK). Samples were presented to the ICP via the CETAC ASX-500 autosampler and initially aspirated for 40 s at a high pump Table 1 Main ICP operating parameters (HP4500) Forward power 1300 W Plasma gas Øow 16 L min21 Auxiliary gas Øow 1.0 L min21 Nebuliser gas Øow 1.25 L min21 Sampling depth 7.5 mm J.Anal. At. Spectrom., 1999, 14, 1709±1710 1709 This Journal is # The Royal Society of Chemistry 1999speed (0.30 rps y1 mL min21). The system was stabilised for 30 s (pump speed 0.15 rps y0.5 mL min21) prior to analysis. Data acquisition (pump speed 0.15 rps) was based on measurement in the `spectrum' mode (3 points per mass at 1 s per point for 127I) and 5 replicate measurements were used for each solution (total integration time 15 s).A high pump speed of 0.3 rps for 60 s was used for washout prior to the introduction of the next sample. Quality control and patient samples were diluted 1 : 20 with distilled water and values calculated from the calibration curve. The Ænal urine results were expressed as mmol of iodine per mol of creatinine to correct for urine Øow. Creatinine was measured in a Vitros E250 analyser (Ortho Clinical Diagnostics, Rochester, NY, USA) using dry slide technology.Results and discussion Method performance and quality assurance The Hewlett-Packard 4500 ICP-MS is suitable as a fast screening analytical method for the determination of urinary iodine. Our method gave recoveries, at three levels of iodine (2.36, 3.15 and 3.94 mmol L21) of between 101±104%, a detection limit of 0.000 38 mmol L21 and precision (assessed at three levels) ranged from 1.3±2.3% intra-batch to 6.3±12.5% inter-batch (Table 2).Patients There were no signiÆcant differences in urine creatinine excretion between our pregnant and control groups. Log transformed data showed that during the Ærst 24 weeks of pregnancy, iodine excretion did not change signiÆcantly over that seen in our control population (mean value 85.1 mmol iodine per mol creatinine, range 30±245). SigniÆcant increases were seen, however, commencing at 28 weeks (pv0.05), reaching peak values at 40 weeks (mean 120.2 mmol of iodine per mol of creatinine, range 54±268, pv0.05).Iodine levels returned to control levels by 10 weeks post delivery (Fig. 1). The availability of iodine for the maternal thyroid during pregnancy results from a combination of speciÆc factors and is critically reduced by nutritional deÆciency. Although nutritional deÆciency is unlikely to be a factor in our patient group, the conÆrmation of a signiÆcant increase in iodine excretion during the second and third trimesters might suggest a negative iodine balance in the absence of increased intake.Increased iodine excretion might contribute to the increase in thyroid size reported in pregnancy and although (as we have shown) iodine excretion returns to normal post-partum, is interesting to speculate that thyroid changes in pregnancy might pre-dispose susceptible individuals to thyroid disease in later life. The analysis of iodine using the Hewlett-Packard 4500 ICPMS is a fast screening method for urine estimation. The method is reliable and precise, with low limits of detection and high levels of recovery.The availability of this method offers the opportunity for further work in this area, with particular reference to iodine excretion in patients with renal failure, patients using iodine-containing medication, e.g., amiodarone, and in the differential diagnosis of silent thyroiditis and Graves' disease. References 1 C. A. Furnee, F. van der Haan, C. E. West and J. G. A. Hautvast, Am.J. Clin. Nutr., 1994, 59, 1415. 2 J. Brug, M. R. H. Lowik, M. Wedel and J. Odink, Eur. J. Clin. Nutr., 1992, 46, 671. 3 E. J. Silva and S. Silva, J. Clin. Endocrinol. Metab., 1981, 4, 671. 4 P. Caron, M. Hoff, S. Bazzi, A. Dufor, G. Faure, I. Ghandour, P. Lauzu, Y. Lucas, D. Maraval, F. Mignot, P. Ressigeac, F. Vertongen and V. Grange, Thyroid, 1997, 5, 749. 5 K. M. Pederson, P. Laurberg, E. Iverson, P. R. Knudsen, H. E. Gregersen, O. S. Rasmussen, K. R. Larsen, G. M. Eriksen and P. L. Johannesen, J. Clin. Endocrinol. Metab., 1993, 77, 1078. 6 P. P. A. Smyth, M. T. Hetherton, D. F. Smith, M. Radcliff and C. O'Herlihy, J. Clin. Endocrinol. Metab., 1993, 82, 2840. 7 S. Pino, S. L. Fang and L. E. Braverman, Clin. Chem., 1996, 42, 239. Paper 9/06138J Table 2 Intra- and inter-batch quality control data for iodine estimation using the HP4500 ICP-MS Intra-batch Inter-batch Quality control (n~20) Quality control (n~10) Level/mmol L21 0.76 2.22 3.29 0.76 2.22 3.29 Mean 0.72 2.16 3.03 0.72 2.22 3.21 s 0.01 0.05 0.04 0.09 0.14 0.27 RSD (%) 1.4 2.3 1.3 12.5 6.3 8.4 Fig. 1 Iodine excretion in normal pregnancy: mmol of iodine per mol of creatinine. (Values are means of log transformed data.) *~pv0.05. 1710 J. Anal. At. Spectrom., 1999, 14, 1709±1710

ISSN:0267-9477    

DOI:10.1039/a906138j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Elemental X-ray images obtained by grazing-exit electron probe microanalysis (GE-EPMA) Kouichi Tsuji,*{a,b Rik Nullens,a Kazuaki Wagatsumab and Rene� E. Van Griekena aMicro- and Trace Analysis Center Mitac, University of Antwerp (UIA), B-2610 Antwerpen, Belgium bInstitute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba, Sendai, 980-8577, Japan. E-mail: tsuji@imr.tohoku.ac.jp Received 1st July 1999, Accepted 2nd September 1999 A new method, grazing-exit electron probe microanalysis (GE-EPMA), was studied.Only X-rays emitted from the near-surface layer are measured at grazing-exit angles (e.g. v0.5�), whereas, with conventional EPMA, X-rays emitted from deep positions are also measured. Therefore, X-ray spectra with low background are obtained by GE-EPMA. Here, elemental mapping by GE-EPMA is shown for the Ærst time. It was found that surface-sensitive elemental X-ray images were obtained for a thin Au Ælm deposited on a Si wafer. The problems that occur at boundaries of different heights are discussed.Furthermore, it was difÆcult to recognize elemental distributions of Si, S, Ca, Na and Fe for aerosols deposited on a Si wafer in noisy X-ray images when using conventional EPMA; however, clear X-ray images were obtained under grazing-exit conditions. Introduction Electron probe microanalysis (EPMA) is a powerful technique for elemental analysis of small regions, and it is applied abundantly for microanalysis of metals, semiconductor devices, biochemical samples, aerosol particles, etc.1,2 Elemental mapping, which yields two-dimensional information about elemental distributions in samples, is also obtained by scanning electron beams.In this scanning mode, X-ray spectra are measured frequently with an energy-dispersive X-ray detector (EDX) for each measured point. In many cases, elemental X-ray images of near-surface layers (v5 nm) are needed. However, characteristic and Bremsstrahlung X-rays produced in depth are also detected in conventional EPMA.This is the reason why strong continuous X-rays are observed in EPMA spectra. When aerosols and biochemical samples deposited on sample carriers are analyzed, we need only the information from the deposited samples. However, Xrays emitted from the sample carrier are also measured, as shown in Fig. 1(a). The primary electrons penetrate into the sample carrier and are scattered; therefore, characteristic and Bremsstrahlung X-rays are produced from the broadened regions.These unnecessary X-rays inØuence the quality of Xray images. For example, when an Al foil is used as a sample carrier for the analysis of aerosols, strong characteristic and continuous X-rays emitted from the Al foil will obviously disturb the determination of Al and other trace elements in aerosols.3 We have developed a new method: grazing-exit EPMA (GEEPMA). 4 In conventional EPMA, electron-induced X-rays are measured at large take-off (exit) angles, e.g. 40�. In contrast, Xrays are measured at grazing-exit angles of less than 1� in GEEPMA. Under grazing-exit conditions, a phenomenon similar to X-ray total reØection is observed.5 Only the X-rays emitted from near-surface layers of about 5 nm in thickness are detected at grazing-exit angles of less than 0.5�, as shown in Fig. 1(b). Therefore, the X-ray spectra are obtained with low background,6 and the detection limits are also improved.7 In this paper, elemental X-ray images obtained at grazing-exit angles are shown for the Ærst time.A thin Au Ælm and aerosol samples are measured. Aerosols containing toxic elements are dangerous to humans and aerosols also inØuence the global climate.8 In order to understand the type and origin of aerosols, in addition to elemental composition, the elemental analysis of individual aerosol particles is necessary. For this purpose, individual particle analysis by computer-controlled automated EPMA is a promising method.9,10 Experimental The experimental details have been described elsewhere.3,6,7 The experiments were carried out by EPMA (Superprobe-733, JEOL, Tokyo, Japan) with an ultra-thin window EDX (Link Pentafet Model 5373, Oxford Instruments, UK).The EPMA apparatus was operated at an acceleration voltage of 20 kV and at a beam current of 1 nA under a vacuum of less than 261025 Torr. A slit of 0.5 mm in width was attached parallel to the surface of the sample on the front of the EDX.The sensitive area of the EDX used was 30 mm2, of which only an area of 0.566 mm was uncovered in this set-up. The distance between the sample and the EDX was about 100 mm. A brass triangular attachment having an inclination of 45� was placed on the sample holder. The exit angle was controlled by tilting {On leave from Tohoku University. Fig. 1 A simple illustration of the interaction volume of electron beams for a single particle on a Øat sample carrier. The regions observed by conventional EPMA (a) and GE-EPMA (b) are shaded (a). In conventional EPMA, X-rays from both the particle and the sample carrier are detected.In contrast, only the X-rays emitted from the particle are measured by GE-EPMA. J. Anal. At. Spectrom., 1999, 14, 1711±1713 1711 This Journal is # The Royal Society of Chemistry 1999this sample holder. An incident angle of 90� for electron beams is desirable for microanalysis; however, this set-up was difÆcult because the electron source and the EDX were Æxed.Therefore, an incident angle of the electron beam of approximately 45� was used in this work with the triangular attachment. The samples were a thin Au Ælm and aerosol particles. The thin Au Ælm was deposited to a thickness of about 100 nm on a Si wafer by an evaporation method. Aerosol particles were collected on a Si wafer at the campus of the University of Antwerp (UIA), Belgium. Air was sucked with a rotary vacuum pump into a multiple-oriÆce impactor (Berner-type, Hauke, Austria); Ænally, spots (#1 mm in diameter) of deposited atmospheric particles were obtained on a Si substrate.Results and discussion X-ray images of thin Ælm The thin Au Ælm deposited on the Si wafer was intentionally scratched by a needle. The width of the scratch was about 33 mm. X-ray images of AuMa and Si Ka taken at an exit angle of 6� are shown in Fig. 2(a) and (b), respectively. X-rays of Au Ma were practically not observed at the scratched line, where the Au layer was removed.X-rays of Si Ka were also observed on the region covered with the Au Ælm, because the exit angle was large enough to observe Si Ka emitted from the Si wafer under the thin Au layer. The two boundaries of the scratched line are clearly shown in the Au Ma X-ray image [Fig. 2(a)]. However, the right boundary of this line is not very clear in the Si Ka X-ray image [Fig. 2(b)], and the left boundary is even less sharp, in this case where the X-rays were detected from the right side of the sample.This phenomenon was not observed when the sample of the scratched line was placed parallel to the X-ray detector. Similar results have been reported for grazing-exit X-ray Øuorescence analysis.11 This result can be explained by using the simple model shown in Fig. 3. Since X-ray images were taken at a small exit angle of 6�, SiKa X-rays produced under the Au layer near the left boundary can also be detected through the side of the Au layer.In addition, electron beams can excite Si Ka X-rays through the side of the Au layer, because the incident angle of the electron beam is 45� in this work. The opposite phenomenon occurs near the right boundary: the Si Ka X-rays emitted near the boundary are absorbed by the Au layer. This phenomenon occurs when boundaries of different heights are observed at small take-off angles. Fig. 2(c) and (d) shows elemental X-ray images taken at a grazing angle of about 0.8�.In order to calculate the critical angle for detection of characteristic X-rays in GE-EPMA, it is assumed that the characteristic X-rays impinge upon the surface of the substrate at the grazing-incidence angles,12 because the principles of microscopic reversibility and Lorentz reciprocity are applied.5 is estimated by the following equation: h&1:65=E|ÖZr=AÜ1=2 Ö1Ü where E(keV) is the energy of characteristic X-rays, Z is the atomic number, A is the atomic weight, and r (g cm23) is the density of the substrate.13 For Au Ma (2.1 keV) from a Au layer (19.3 g cm23), h is estimated to be 2.2�.Since the exit angle (0.8�) for the X-ray image of Au Ma [Fig. 2(c)] was less than this critical angle (2.2�), the AuMa intensities decreased in comparison with those in Fig. 2(a). As shown in Fig. 2(d), Si Ka X-rays are practically not observed at regions covered with the Au Ælm.This indicates that surface-sensitive elemental X-ray images can be obtained by GE-EPMA. In the Si Ka Xray image in Fig. 2(d), a similar problem to that in Fig. 2(b) is found. The left boundary of the scratched line is obscure, and Si Ka X-rays emitted from the Si wafer covered with the Au layer near this boundary are also detected. This result can be explained by using the model in Fig. 3 again. In the grazing-exit arrangement, the absorption of Si Ka X-rays in the Au upper layer increases because of the longer path.Therefore, the right boundary of the scratched line in the Si Ka X-ray image is clearer than the left boundary, as shown in Fig. 2(d). This problem at the boundary should require attention when X-ray images are observed at grazing-exit angles. X-ray images of aerosols Fig. 4(a) shows a secondary electron image of aerosols of 2± 10 mm sizes. For this sample, elemental X-ray images of Si Ka, S Ka, CaKa, NaKa and Fe Ka were taken at different exit angles.The X-ray images taken at 8� are shown in Fig. 4(b)± (f). As shown in Fig. 4(b), since aerosols were collected on the Si wafer, Si Ka X-rays emitted from the Si wafer are too strong to allow observation of the Si Ka X-rays from the aerosols. Xray images of other elements are also not clear. White dots appear at the points where the corresponding elements do not exist. This is due to high background intensities caused by Bremsstrahlung X-rays in the Si wafer.The elemental X-ray images taken at a grazing-exit angle of 0.5� are shown in Fig. 4(g)±(k). The X-ray image of Si Ka for aerosols is clearly found, as shown in Fig. 4(g). The critical angle of Si Ka (1.74 keV) from a Si (2.2 g cm23) wafer is approximately 1.0�. Hence, Si Ka X-rays emitted from the surface of the Si wafer are drastically decreased at exit angles of less than 1.0�. This is the reason why clear X-ray images of aerosols were obtained at an exit angle of 0.5�. Similarly, X-ray Fig. 2 Elemental X-ray images of AuMa [(a), (c)] and Si Ka [(b), (d)] for a Au thin layer deposited on a Si wafer.Part of the Au layer was removed by scratching it. The exit angles are 6� [(a), (b)] and 0.8� [(c), (d)]. X-rays were detected from the right side of these pictures. X-ray images were measured at 2006140 pixels for a total measuring time of about 1 h at each exit angle at an area of 95665 mm. Fig. 3 Model of a Au thin Ælm on a Si wafer to explain the Si Ka X-ray image near the boundary.Si Ka X-rays produced under the Au layer near the boundary are detected through the side of the Au layer as shown in the left side, while the Si Ka X-rays emitted from the left of the boundary are absorbed by the Au layer as shown in the right side. 1712 J. Anal. At. Spectrom., 1999, 14, 1711±1713images of S Ka, CaKa, NaKa and Fe Ka are clearly obtained, as shown in Fig. 4(h)±(k). From these elemental X-ray images, SiO2 particles (in the bottom left and in the upper right), a CaSO4 particle (in the bottom right) and Fe-rich particles (in the center) are identiÆed in Fig. 4(a). Conclusions Elemental X-ray images obtained by GE-EPMA were demonstrated. The X-rays emitted from the depth of the carrier cannot be detected under grazing-exit conditions because of refraction effects at the vacuum±sample interface;4,6,7 therefore, X-ray spectra with low background are measured at each measured point. As a result, clear X-ray images are obtained with low noise levels.GE-EPMA is useful especially for the observation of particles deposited on a Øat sample support. Absorption in the particle itself has to be considered for quantitative analysis of large particles.6 The problem of X-ray imaging for near boundaries of different heights was pointed out. Acknowledgement One of the authors (K. Tsuji) was Ænancially supported by Japan Society for the Promotion of Science (JSPS) and by a Grant-in-Aid (11650828) from the Ministry of Education, Science, Sports and Culture.Part of this work was supported by the Belgium OfÆce for ScientiÆc, Technical and Cultural Affairs under contract MN/10/01. The authors thank Dr. Z. Spolnik and Professor J. Zhang for the sample preparation of aerosols, and also thank Dr. J. Injuk for useful suggestions. References 1 S. J. B. Reed, Electron Microprobe Analysis, Cambridge University Press, Cambridge, UK, 1993. 2 P. Duncumb, J. Anal. At.Spectrom., 1999, 14, 357. 3 C. Ro, J. Osa�n and R. Van Grieken, Anal. Chem., 1999, 71, 1521. 4 K. Tsuji, K. Wagatsuma, R. Nullens and R. Van Grieken, Anal. Chem., 1999, 71, 2497. 5 R. S. Becker, J. A. Golovchenko and J. R. Patel, Phys. Rev. Lett., 1983, 50, 153. 6 K. Tsuji, Z. Spolnik, K. Wagatsuma, R. Nullens, J. Zhang and R. Van Grieken, Spectrochim. Acta, Part B, 1999, 54, 1251. 7 K. Tsuji, Z. Spolnik, K. Wagatsuma, R. Nullens and R. Van Grieken, Mikrochim. Acta, in press. 8 Atmospheric Particles, ed. R. M. Harrison and R. E. Van Grieken, Wiley, Chichester, 1998. 9 H. Van Malderen, S. Hoornaert and R. Van Grieken, Environ. Sci. Technol., 1996, 30, 489. 10 W. Jambers and R. Van Grieken, Environ. Sci. Technol., 1997, 31, 1525. 11 T. Noma and A. Iida, Rev. Sci. Instrum., 1994, 65, 837. 12 L. G. Parratt, Phys. Rev., 1954, 95, 359. 13 R. Klockenka»mper, Total-reØection X-Ray Fluorescence Analysis, Wiley, New York, 1997. Paper 9/05301H Fig. 4 Secondary electron image (a) of aerosol particles deposited on a Si wafer, and elemental X-ray images of Si Ka [(b), (g)], S Ka [(c), (h)], Ca Ka [(d), (i)], Na Ka [(e), (j)] and Fe Ka [(f), (k)]. The exit angle was 8� [(b)±(f)] and 0.5� [(g)±(k)]. X-ray images were measured at 3006200 pixels for a total measuring time of about 1h at each exit angle at an area of 65645 mm. J. Anal. At. Spectrom., 1999,

ISSN:0267-9477    

DOI:10.1039/a905301h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Rapid determination of Cu, Fe, Mg, Mn and Zn in wood pulp by direct sample insertion-inductively coupled plasma-optical emission spectrometry using a pyrolytically coated graphite sample probe Michael E. Rybak,a Panos Hatsis,b Kevin Thurbideb and Eric D. Salina aDepartment of Chemistry, McGill University, 801 Sherbrooke St. W., Montre�al, Que�bec, Canada H3A 2K6 bPulp and Paper Research Centre, McGill University, 342 University St., Montre�al, Que�bec, Canada H3A 2A7 Received 2nd March 1999, Accepted 23rd August 1999 A rapid method for screening wood pulp samples by direct sample insertion-inductively coupled plasma-optical emission spectrometry (DSI-ICP-OES) is described.Solid wood pulp samples were introduced directly into an inductively coupled plasma, using a pyrolytically coated graphite DSI sample probe, after in situ chemical treatment with HCl and NaF. Drying and ashing steps were performed by inductively heating the sample probe in the ICP coil prior to plasma ignition.The analysis time of the method from sample acquisition to analysis was of the order of several minutes per sample, as compared to several hours when conventional dissolution methods are used. Agreement with reference values for wood pulp samples ranged from 3.4±16% (absolute) for high-concentration analytes (Mg, Mn) and 1.7±50% (absolute) for low-concentration ones (Cu, Fe, Zn) using external standards. Precision ranged from 6±50% RSD and was highly dependent on the element and pulp sample studied.Absolute detection limits for the method were of the range of 50±1000 pg, translating into relative detection limits of 20±400 ppb based on a 2.5 mg pulp sample. The merits of using DSI-ICP-OES for the direct analysis of wood pulps, and of using a pyrolytically coated graphite probe for this type of application, are discussed. Introduction The detrimental environmental ramiÆcations from bleaching wood pulps with chlorinated reagents have recently led to increased regulatory pressure to use a totally chlorine free (TCF) bleaching process in the pulp and paper industry.1 In TCF bleaching, hydrogen peroxide (H2O2), by means of its alkaline reacting species (HOO2) and decomposition intermediates (HO? and O2 2?), is used to delignify and brighten the pulp. The decomposition of H2O2 is integral to the deligniÆcation and bleaching processes, but it must be carefully controlled in order to accomplish TCF bleaching efÆciently.While the hydroperoxy anion (HOO2) is primarily responsible for the brightening of the pulp, the hydroxide (HO?) and superoxide (O2 2?) radicals account for much of its deligniÆcation. These radicals, however, only show marginal selectivity towards lignin over cellulose, and destruction of the cellulose results in a lower yield and a weaker pulp. To further complicate matters, certain transition metal species (e.g., MnO2, Mn2z, Cu2z and Fe2z) are known to accelerate H2O2 decomposition, whereas other species (e.g., Mg2 z, SiO3 22) will inhibit this acceleration. 2 The aforementioned metal species are commonly present in wood, and because of their inØuence on hydrogen peroxide degradation, there exists an ideal metal content proÆle for effective TCF bleaching.3 The metal proÆle of the pulp sample may be adjusted either by chelation of the metals with ethylenediaminetetraacetate (EDTA) or diethylenetriaminepentaacetate (DTPA), or by washing the pulp at a low pH (1.5± 3.0) followed by replenishment of the magnesium ion.4 With the metal content proÆle of the pulp having such a great inØuence on the TCF bleaching process, and the adjustment of this proÆle a common practice in the paper industry, there exists a need for a means by which the levels of metals present in the pulp can be determined with reasonable speed and accuracy.Magnesium and manganese, the two most important elements in terms of their inØuence on the TCF bleaching process, are found in relatively high concentration in pulps.Typical concentration ranges are 200±400 ppm for Mg and 50± 250 ppm for Mn in Canadian kraft (chemically treated) pulps. Other elements that occur in lower concentrations include Cu and Zn (0±10 ppm) and Fe (20±100 ppm). The desired metal proÆle for a pulp destined for TCF bleaching is such that the Mg level is maintained at least within the natural range expected, and that the concentration of the transition metals is reduced to its lowest level possible (of the order of 1 ppm or less).Considering the range of metal concentrations expected to occur naturally, and the thresholds for these metals deemed acceptable for TCF bleaching, the desired techniques for determining these analytes in wood pulp should have at least semi-quantitative capabilities, with quantitative results for the most inØuential elements being desirable. The current method used by the paper industry for the determination of these metals in pulp samples involves dry or wet ashing of the sample followed by hot-plate digestion in HCl, and subsequent analysis by Øame atomic absorption spectrometry (FAAS).5,6 The use of inductively coupled plasma-optical emission spectrometry (ICP-OES) in the above methods in place of FAAS is now a common practice in the pulp and paper industry, primarily because of the multielement capabilities of ICP-OES.Although they are commonplace for preparing solid samples for analysis, hot-plate digestions have several inherent disadvantages: volatile element losses; contamination of the sample from air, contact with the sample vessel, or reagents required for sample digestion; and unacceptably long sample dissolution times.An expeditious alternative to dissolution of the solid pulp sample would be the direct analysis of the solid itself. Many solid samples have been successfully determined by ICP-OES by taking advantage of thermal sample introduction techniques such as electrothermal vaporizaton (ETV)7,8 or direct sample insertion (DSI).9±12 In J.Anal. At. Spectrom., 1999, 14, 1715±1722 1715 This Journal is # The Royal Society of Chemistry 1999many of these cases solids were introduced into the ICP by ETV or DSI with minimal a priori sample treatment and any necessary sample treatment was performed in situ. Of these two techniques, the open design of the DSI lends itself most conveniently to rapid replacement of the sample holder, and the addition of both solid samples and liquid reagents.Consequently, it was decided that the direct analysis of pulp samples would be approached using this technique. In its most conventional conÆguration, DSI entails the axial elevation of a sample directly into the center channel of the annular plasma discharge by means of a sample carrying probe. The intrinsic beneÆts of DSI are obvious: 100% of the sample is introduced into the excitation source, and cup-shaped sample probes facilitate the introduction of various solids and liquids, as well as the addition of reagents for in situ chemical sample treatment.Physical sample treatment steps, such as drying and pyrolysis, can also be performed either by proximate positioning of the sample underneath the plasma, or by induction heating in the ICP load coil prior to ignition of the plasma.13 Graphite cup DSI, like ETV, is hampered by the formation of refractory carbides, which prove difÆcult to volatilize, by sample intercalation into the pores and interstices of the graphite, which results in poor reproducibility in the volatilization event, and by the susceptibility of graphite to chemical attack. Intercalation can be minimized and a resistance to chemical attack can be imparted to a graphite surface by depositing a highly ordered layer of pyrolytic graphite.14 Recently, a means of depositing a pyrolytic graphite coating on the interior of a graphite DSI cup in the ICP was developed.15 Promising improvements in signal reproducibility and sensitivity were observed, but the performance of the new coated probe had yet to be evaluated in terms of its resistance to chemical attack and usefulness for routine analysispe of this study was two-fold: to evaluate the use of DSI as an expeditious means of screening wood pulps for metals that inØuence the efÆciency of the TCF bleaching process; and to evaluate the performance of pyrolytically coated graphite DSI probes for performing routine analyses with extensive in situ chemical treatment. Experimental Pyrolytically coated graphite probes and DSI apparatus Hollow-stemmed, long undercut graphite cup sample probes were machined in-house from J@ high density graphite electrodes (S-8 HD, Bay Carbon, Bay City, MI, USA) on a benchtop lathe (Emco Compact 5, Emco Maier, Columbus, OH, USA) according to the dimensions indicated in Fig. 1(a). The interior of the cup portion of the DSI probe was then pyrolytically coated with graphite directly in the plasma15 by means of a vapor phase deposition procedure, depicted schematically in Fig. 1(b). In brief, a 10% (v/v) mixture of methane in argon was directed through the hollow stem of the probe toward the walls of the cup interior as the cup portion of the probe was positioned in a 2 kW argon plasma. The methane undergoes gas-phase pyrolysis, large aromatic molecules are generated by dehydrogenation, and collision of these macromolecules with the substrate results in a pyrolytic graphite deposit.Details of the experimental conditions used in the coating process appear in Table 1, and Æne points pertaining to the pyrolytic coating procedure have been previously described.13 A stepper motor controlled direct sample insertion device (DSID)13 was used to elevate the DSI sample probes axially into a 27.12 MHz inductively coupled plasma source with an automatching network (HFP-2500 and AMN-2500E respectively, Plasma Therm, Inc., St.Petersburg, FL, USA). Optical emission signals collected from the ICP were imaged to the entrance slit of a Rowland circle-type polychromator (Model 750, Thermo Jarrell Ash, Franklin, MA, USA) equipped with a galvanically driven quartz refractor plate in the incident light Fig. 1 Pyrolytically coated direct sample insertion (DSI) probe: (a) dimensions of probe used; (b) depiction of the pyrolytic coating process.Table 1 Experimental summary Pyrolytic coating process– Plasma forward power 2 kW ReØected power 0±8 W Plasma gas Øow rate 16 l min21 Auxiliary gas Øow rate 2 l min21 Coating gas Øow rate 500 ml min21 Coating time 20 min Insertion depth 0 mm ATOLCa Sampling and sample pretreatment– Sample mass 1±4 mg of dried pulp Chemical treatment 20 ml of conc. HCl 10 ml of 10% (m/v) NaF External standards 20 ml of mixed element standard: Cu: 0.25±1.25 ppm Fe: 1±5 ppm Mg: 25±125 ppm Mn: 10±50 ppm Zn: 0.5±2.5 ppm Inductive drying forward power 50 W (y150 �C) Inductive drying reØected power 8±10 W Drying time 90 s Inductive pyrolysis forward power 150 W (y550 �C) Inductive pyrolysis reØected power 50±55W Pyrolysis time 90 s Drying/pyrolysis probe position 0 mm ATOLC Direct sample insertion analysis– Plasma forward power 2 kW ReØected power 0±5 W Plasma gas Øow rate 16 l min21 Auxiliary gas Øow rate 1.8 l min21 Insertion depth 0 mm ATOLC Viewing height 20 m ATOLC Insertion time 30 s Exposure time 40 ms per position Number of exposures per traceb 300 Galvanometer settle time 3 ms Wavelengths monitored 324.8 nm (Cu I)c 259.9 nm (Fe II) 279.6 nm (Mg II) 293.3 nm (Mn II) 213.9 nm (Zn I) aATOLC: Above top of load coil.bIncludes both on-peak and off-peak exposures. cOrigin of emission lines are denoted as I (ground state) or II (singly ionized). 1716 J. Anal. At. Spectrom., 1999, 14, 1715±1722path for off-peak background correction and capable of highspeed signal processing suitable for transient signals16 (Trulogic Systems, Mississauga, ON, Canada).Signals were interpreted using Grams/32 (Galactic Industries, Salem, NH, USA). Samples, standards and reagents Wood pulp samples used throughout this study were obtained from the Pulp and Paper Research Institute of Canada (Paprican, Pointe-Claire, QC, Canada). The unavailability of a certiÆed reference material for wood pulp necessitated the use of pulp samples that had been analyzed using a standardized method (Canadian Pulp and Paper Association Standard Method G.34P with ICP-OES, analyses performed by Paprican) for this study.Two pulp samples representative of the most common types encountered in routine wood pulp analysis were selected: a kraft pulp (brownstock) [Fig. 2(a)], in which alkaline attack is used to chemically fragment the lignin molecules of the wood chips (by cooking the chips in a solution of NaOH and Na2S at approximately 175 �C), and a thermomechanical pulp (TMP) [Fig. 2(b)], which is generated by pressurized steam pretreatment of wood chips followed by mechanical shredding and deÆbering of the chips by means of a rotary-disk reÆner.17 These same samples are currently being developed and characterized by Paprican for use as industry standard reference materials. Mixed element standard solutions were prepared by serial dilution of both multi-element and single-element standards (High-Purity Standards, Charleston, SC, USA, and Fisher ScientiÆc, Nepean, ON, Canada, respectively) with 0.5% trace metal grade HNO3 (Instra-Analyzed, J.T. Baker, Phillipsburgh, NJ, USA) in distilled, deionized water (Milli-Q water system, Millipore Corp., Bedford, MA, USA). Trace metal grade HCl (Instra-Analyzed) and ACS grade NaF (Baker- Analyzed, J. T. Baker) were used for in situ treatment of the pulp samples. Procedure An accurately determined mass of dried wood pulp in the range of 1±4 mg was deposited directly into a graphite DSI cup on a 0.01 mg readable balance.The probe was then mounted on the DSID, and 20 ml of concentrated HCl and 10 ml of a 10% (m/v) solution of NaF were added to the cup [Fig. 3(a)]. The DSI cup was then positioned such that the top of the cup was even with the top of the highest turn of the ICP load coil, i.e., 0 mm above the top of the load coil (ATOLC). Forward power was then applied to the ICP load coil at settings of 50 W for 120 s to dry the sample and 150 W for 120 s to ash the sample by inductively heating the graphite DSI probe [Fig. 3(b)]. These power settings have been determined to correspond to probe temperatures of approximately 150 �C and 550 �C, respectively. Although more conservative drying and pyrolysis times of the order of 30±60 s would have been sufÆcient for each respective task, longer times were employed as a conservative approach. The sample probe was then retracted below the ICP coil and the plasma was ignited and set to a forward power of 2.0 kW [Fig. 3(c)]. The probe was then elevated by the stepper motor until it was just below the plasma discharge (220 mm ATOLC) and stopped there for 2 s prior to insertion so that the plasma could recover from disruption in the Ar Øow caused by the initial probe elevation. Finally, the probe was inserted into the plasma [Fig. 3(d)] for 30 s at a position of 0 mm ATOLC, retracted and cooled.Signal acquisition was started upon the arrival of the probe at the stabilization position. Details regarding other instrumental settings appear in Table 1. Unless explicitly stated otherwise, it can be assumed that the procedure described above was used throughout the study. External standardization was used to determine the concentration of the metals present in the pulp samples. Calibration curves were constructed from signals acquired using aqueous multi-element standards. The standard solutions were analyzed in a manner identical to that described above for the pulp samples, including drying and pyrolysis steps, except that 20 ml of standard were deposited in the sample probe in place of the pulp sample. Fig. 2 Pulp samples used in this study: (a) kraft pulp (brownstock); (b) thermomechanical pulp (TMP). Fig. 3 Schematic depiction of the DSI procedure: (a) 10 ml of 10% (m/v) NaF and 20 ml of conc. HCl are added to a pulp sample of known mass; (b) inductive drying (50W forward power) and pyrolysis (150 W) ated sample; (c) retraction of the sample probe and ignition of the 2.0 kW ICP; (d) insertion of the sample.J. Anal. At. Spectrom., 1999, 14, 1715±1722 1717Results and discussion Method development In developing the procedure used for the direct analysis of wood pulps by DSI, the wood pulp was at Ærst analyzed using no chemical pretreatment, and drying and pyrolysis steps were performed only out of necessity so as to prevent plasma overloading.Although a transient signal could be obtained by this simple procedure it was found to be highly irreproducible (when corrected for sample mass) and the signals would evolve over a relatively long period of time, often on the order of 5±15 s for most elements. Since HCl is used in the pulp and paper industry standard solution dissolution methods5,6 to treat the pulp sample after ashing, the addition of an aliquot of concentrated HCl was incorporated into the DSI procedure. No appreciable improvement in signal was observed when HCl was added to the DSI cup with the deposited pulp sample after the drying and ashing steps. When HCl addition preceded the desolvation and pyrolysis events, however, the time over which the signals would evolve was diminished, although the signals were still irregular in appearance and poor in terms of reproducibility.Sodium Øuoride, as well as other halidecontaining solids and gases, are known to act as halogenating agents, forming relatively volatile metal±halide compounds. Agents such as these have been used to improve the volatilization of analytes in DSI from graphite probes, especially for refractory carbide and oxide forming elements.18 In experiments in the present study in which NaF was added, improved analyte volatilization was observed when addition was incorporated prior to drying and ashing of the pulp sample.The best results in terms of signal appearance and reproducibility, however, were achieved when HCl and NaF were added together before sample drying and pyrolysis.Fig. 4 shows the signals obtained from a brownstock sample inserted using a pyrolytically coated DSI probe when 20 ml of concentrated HCl and 10 ml of 10% (m/v) NaF are added prior to drying and ashing. Most analytes were completely vaporized in less than 5 s, with extremely volatile elements such as Zn being completely volatilized in 2 s.Iron, however, was very difÆcult to volatilize, with analyte still being vaporized from the probe 25 s after insertion into the plasma (Fig. 4). This is not a fault of the analysis procedure, but rather a shortcoming of DSI as an analytical technique. The longer time needed to vaporize Fe results simply because the temperature at which Fe is volatilized is relatively high in comparison to the other elements studied, and the maximum temperature that the sample probe can attain upon insertion, close to 2000 �C, was insufÆcient for rapid volatilization. The maximum temperature that a DSI probe can reach upon insertion can be increased by using a mixed-gas plasma, such as an oxygen±argon plasma,19 but the use of such conditions will result in consumption of the sample probe. When solid samples are analyzed by DSI, the reproducible deposition of a given mass of solid sample for each assay is often not a practicality, thus necessitating the incorporation of an acceptable sample mass range for insertion analysis.In this study, a range of 1±4 mg was established as suitable for the mass of pulp that could be used in DSI. The rationale behind this directive takes the following points into consideration. Since sample mass was being determined on a balance readable to 0.01 mg, the use of samples less than 1 mg would greatly compromise the precision to which the pulp mass could be determined and thus degrade the overall precision of the technique.The metals of interest present in wood pulp occur often at concentrations of 1 ppm or higher, meaning that the detection limit of the technique used would have to be at least 1 ng absolute if a 1 mg pulp sample was used. This is well above the detection limits that have been previously demonstrated with the instrumentation used in this study.13 With at least 1 mg of pulp sample, transient signals were visually discernible from the background emission for all analytes of interest in both the brownstock (Fig. 4) and TMP samples. For the mass range of 1±4 mg, a linear response of signal area as a function of sample mass was observed for the analytes of interest. Fig. 5 shows this behavior for Mg and Mn, the highest-concentration analytes in both pulp samples. Although larger sample masses could be determined with greater precision, non-linearity in the analyte transient signal area as a function of pulp mass was observed at higher sample masses (w8 mg).Probe comparison Fig. 6 shows the signals obtained with both pyrolytically coated and uncoated DSI probes for a Øuffed softwood TMP sample. As expected, the pyrolytically coated probe yielded sharp transient signals with little to no multiple peaking as compared to the uncoated probe. Since the uncoated probe surface is thinner, highly irregular and more porous in comparison to the pyrolytically coated surface, it was more Fig. 4 Typical signals for a kraft pulp (brownstock) sample treated with 10 ml of 10% (m/v) NaF and 20 ml of concentrated HCl using a pyrolytically coated DSI probe.Sample mass is approximately 1.5 mg. Fig. 5 Analyte transient signal area as a function of pulp sample mass (brownstock) (z, Mn; 6, Mg). 1718 J. Anal. At. Spectrom., 1999, 14, 1715±1722susceptible to factors such as intercalation and preferential volatilization due to spatial temperature disparities, both of which lead to multiple peaking.Table 2 compares the signal reproducibility between a pyrolytically coated probe and an uncoated probe for the analysis of the same Øuffed softwood TMP. Surprisingly, when both probes were relatively new (less than 25 sample insertions) there was a marginal difference between the % RSD values of the integrated signals for most of the elements studied. This is in stark contrast to what had been observed with pyrolytically coated DSI probes previously.15 In explaining this, the origin of the primary analyte volatilization event upon sample insertion must be considered.Wood pulp is a highly polymerized, Æbrous material, and when a pulp sample was dried and pyrolyzed, the sample was reduced to a carbonaceous residue with a high surface area. After pyrolysis, the metals initially present in the pulp sample were most likely still in contact with this carbon residue and were vaporized from the surface upon insertion into the plasma. The result was an initial vaporization event that was inØuenced greatly by the sample residue, which was identical in both cases.While this may explain the similarities in signal reproducibility, vaporization from the carbon residue alone does not account for the marked difference in signal appearance in Fig. 6. The difference in signal appearance probably arises from secondary recondensation and revaporization events on the probe surface subsequent to vaporization from the carbonaceous pulp residue.Although recondensation±revaporization after the initial vaporization event will inØuence the appearance of the peak shape, the initial vaporization event will be the predominant inØuence in signal reproducibility. While the observed signal reproducibility was virtually identical for both pyrolytically coated and uncoated probes that were relatively new, the precision achieved with the latter degraded rapidly after 25±50 insertions to the point where the signal RSD was on the order of 100±200%.Visual inspection of the probe revealed that the cup portion of the probe was being oxidized and was disintegrating with repeated use. It is obvious that damage to and disintegration of the sample probe will degrade signal reproducibility due to sample losses, intercalation and the like, and that the highly crystalline, ordered, non-porous surface of the pyrolytically coated probe will demonstrate ameliorated resistance to oxidate attack. Less obvious was the source of the oxidative attack.Several potential sources of oxidation exist. Firstly, it has been documented that Na, as well as other alkali and alkaline earth elements, have a catalytic effect on the rate of oxidation from graphite.14 As an example, 20±40 ppm of Na, K, V or Cu has been shown to increase the rate of dry oxidation of graphite by upwards of 6-fold. Since O2 and CO2 both support the oxidation of graphite, and both were expelled from the pulp sample during the pyrolysis step, a catalytic oxidative effect from the Na deposited as NaF was probably occurring.Additionally, the reaction of Øuorine with graphite could be taking place, resulting in the loss of graphite from the probe due to the formation of various Øuorocarbon species, such as CF4 and C2F6, which are known to occur at temperatures less than 700 �C.20 Limit of detection (LOD) Determining the limit of detection (LOD) for the method was not trivial due to the unavailability of a true Æeld blank, i.e., a blank consisting of a matrix representative of the sample being analyzed (a wood pulp sample in this case).21 Although a blank was used in the external standardization curve, it was an aqueous sample, and the use of its standard deviation for the purposes of estimating the LOD would be inappropriate.A better approach would be the duplicate analysis of samples with analyte concentrations 10±30 times the expected detection limit, and then using the observed standard deviation in the LOD estimation.Owing to the limitations of the samples available, this approach was also not possible. As a compromise, the off-peak transient signals used for background correction were taken from the pulp samples analyzed, corrected for their respective offset from zero along the ordinate axis, and integrated. The standard deviation from these integrated signals was then incorporated into the LOD deÆnition as recommended by IUPAC (eqn. 1): cL à 3sB=m Ö1Ü where cL is the concentration LOD, sB is the standard deviation in the blank signal, and m is the slope of the calibration curve. The calculated LODs using this practice appear in Table 3. For the values reported in Table 3, it is Fig. 6 InØuence of the pyrolytic coating on analyte volatilization for a Øuffed softwood thermomechanical pulp (---, uncoated probe; – pyrolytically coated probe). Table 2 Signal reproducibility comparison (Øuffed softwood TMP) RSD (n~10) (%) Element Uncoated probe (v25 insertions) Pyrolytically coated probe Cu 23 23 Fe 11.5 13 Mg 6.3 6.0 Mn 14 4.1 Zn 10.3 10.8 J.Anal. At. Spectrom., 1999, 14, 1715±1722 1719important to mention that the off-peak signals were not corrected for sample mass, and that the standard deviations were pooled from the two different samples analyzed (n~10 for each sample of brownstock and TMP). This was done because the external calibration curve was assumed to be valid for all pulp samples irrespective of sample type or mass.By comparison these values are, with the exception of Mg, approximately one order of magnitude higher than those obtained when the standard deviation of the integrated aqueous blank signal was used in eqn. 1 (Mg was approximately 102 higher). Care should be taken in extracting a practical quantitation limit (PQL) from these LOD values (normally 3±10 times the method detection limit), as they are not based on the standard deviation of low-concentration samples.The PQL will as a result most likely be somewhat higher than expected. Precision and accuracy Table 4 presents a comparison of the precision and accuracy for the analysis of the two pulp samples studied using DSI (with external standards) and the industry standard method (CPPA Method G.34P). In terms of precision, the wet oxidation method almost always faired better than DSI. With aqueous solutions DSI has been shown capable of achieving precision that rivals that of solution nebulization. Percentage RSD values, typically of less than 5%, and in extraordinary cases less than 1%, have been realized.22 Irreproducibility in sample deposition, insertion and volatilization is the primary inØuence in the observed precision. However, when analyzing a solid directly by DSI the precision is often much worse, as the previously mentioned factors are now more inØuential, and new factors such as inter-sample heterogeneity and sample size variability further compound the imprecision of the technique.Evidence of the inØuence of sample homogeneity on precision can be seen when the % RSD values obtained for the TMP sample in Table 4 are compared with those obtained for a similar TMP sample that had been mechanically Øuffed (Table 3). Although low-concentration elements (Cu and Zn) show no appreciable improvement in precision, for highconcentration elements (Mg and Mn) the % RSD is reduced by approximately an order of magnitude when the sample is Øuffed.It is also important to note that the physical constraints of DSI in terms of sample size dictated the use of an extremely small sample (on the order of 1±4 mg). Although this mass of pulp was more than sufÆcient to generate detectable signals for the analytes of interest, slight errors in accurately determining such a minute sample mass can greatly inØuence the observed precision. When the DSI precision was compared between the two pulp types studied, the reproducibility of the brownstock was generally better than the TMP sample, with RSD values ranging from 7±20% versus 16±44%, respectively (Table 4).A similar trend in reproducibility was also observed with the wet oxidation values, suggesting that an inherent difference that exists between the two pulps plays a role. The difference is that in the kraft pulping process (brownstock), the lignin is dissolved away chemically, leaving cellulose and hemicellulose in the form of intact Æbers, whereas thermomechanical pulping (TMP) yields a distribution of shortened Æbers resultant from a mechanical shredding and deÆbering process.17 It should be noted, however, that the kraft process, in practice, also chemically degrades a certain amount of the cellulose and hemicellulose Æbers, whereas the thermomechanical pulping leaves most of the cellulose and hemicellulose intact.Metals in wood such as Ca, K, and Mg are often partially bound to the carboxyl groups present in the cellulose and hemicellulose, and heavy metals such as Fe and Mn are often chelated by wood constituents.23 The chemical damage from the conditions experienced in the kraft process makes the analytes easier to liberate from the ashed pulp in both the wet oxidation and DSI analyses, thus yielding the improved precision in the brownstock values.When the accuracy (using the CPPA Method G.34P values as a reference) was compared for the pulps, a trend opposite to that observed with the precision values was evident.For Mg and Mn, the two elements present in the highest concentrations, agreement with the TMP values was 5.0 and 4.4%, respectively, as compared to 15 and 16% for the brownstock sample (Table 4). The discrepancy in accuracy can be attributed in part to sampling of the pulp for analysis. The TMP sample, upon air drying, took the form of rather large, Æbrous pieces [Fig. 2(b)] that had to be physically separated into smaller pieces with plastic tweezers in order to Æt into the DSI sample probe, and this action probably assisted in homogenizing the TMP sample.By comparison, the brownstock sample [Fig. 2(a)] was not as Æbrous, but occurred in a variety of sizes, the smaller of which were suitable for deposition into the DSI cup. Sampling was consequently favored towards these smaller pieces, as the larger pieces of the brownstock pulp proved difÆcult to manually separate into smaller ones.The same trend was true of Fe, although the agreement between the values was somewhat poorer (29% versus 44% for TMP and brownstock, respectively). In both pulp samples, the value for Fe concentration as determined by DSI was always lower than at determined by the wet oxidation method, due most likely to the incomplete volatilization of Fe from the sample probe as described earlier (Fig. 4). The concentrations reported for Cu and Zn were quite low in comparison to the previous three elements, with Cu being close to the detection limit of the wet oxidation technique.Consequently, the % RSD was relatively high by comparison and no deÆnite correlation between sample and % RSD could be obtained. Notwithstanding Fe, the error in the determination of the elements does not appear to be biased low or high of the reference value. This is reinforced by the fact that the average error for all of the determinations is only 4% (Table 4).Standard additions were performed on both the brownstock and TMP samples, but no apparent beneÆt was seen in adopting this calibration approach for the pulp analysis. In addition to being more laborious in terms of the number of samples that had to be run, the obtained precision and accuracy were most often poorer than obtained with external standards. This is attributable to the way that the reproducibility of the pulp signals inØuences the determined analyte concentration differently in external standards and standard additions. With standard additions, the uncertainty in the slope and intercept of the calibration curve was considerably higher than that in the external standards curve, a consequence of the fact that the standard additions curve was generated from pulp samples, as opposed to liquid standards in the external calibration case.Furthermore, the curve itself is used in standard additions to determine the analyte concentration (by determining the point of intercept with the abscissa), whereas analyte concentration is Table 3 Limit of detection (LOD) for pulp analysis LOD Element Absolute/ng Relative (ppb)a Cu 0.052 21 Fe 0.94 375 Mg 6.7 2700 Mn 0.56 225 Zn 0.20 80 aBased on a 2.5mg pulp sample (median of mass range used in this study). 1720 J. Anal. At. Spectrom., 1999, 14, 1715±1722determined by interpolation of signals from a more precise curve in external standards. Conclusions The respective merits of rapidly analyzing wood pulps for various metals by DSI-ICP-OES and using a pyrolytically coated graphite cup DSI probe for this type of routine analysis have been demonstrated. The pulp and paper industry has a persistent need for on-line and extremely rapid off-line methods of monitoring various process parameters so as to prevent unnecessary delays in production and the production of offgrade products.24 The use of DSI for rapidly determining the trace metals proÆle of wood pulps prior to TCF bleaching proves excellent in fulÆlling this mandate.Considering the short times needed to dry, ash, and insert the sample into the plasma, a raw pulp sample analysis by DSI can be completed in 5 min from pulp sample procurement. The expeditious nature of the DSI analysis compares extremely well relative to the several hours needed to digest pulp using industry standard wet oxidation methods.5,6 Although precision and accuracy do suffer somewhat when a solid sample is analyzed directly by DSI, quantitative results were obtained for process-inØuential metals in wood pulp samples (Mg and Mn), and semi-quantitative results were realized for low-concentration, but still process-inØuential metals (Cu, Fe, and Zn).With the exception of the values obtained for Fe, there appears to be little bias in the error of the determination (average error of 4%). It is common for industry pulp samples to have typical metal concentrations that vary (or will be varied by means of chemical treatment) in concentration over several orders of magnitude.With this reality considered, the demonstrated accuracy and precision of DSI-ICP-OES appears more than adequate for the purposes of a rapid screening technique. It is important, however, to consider that these results are indeed preliminary, as they are based on the replicate analysis of only two, albeit well characterized, samples (one of each wood pulp type).The replicate analysis of more samples of each pulp type covering the expected concentration range for the analytes of interest would give a more comprehensive picture statistically in terms of the expected precision and accuracy of the technique. Precision can be improved by homogenizing the pulp sample prior to analysis, e.g., by mechanical ØufÆng (compare % RSD for Øuffed TMP in Table 2 with TMP in Table 4), and by using a larger sample. Although DSI and ETV have physical constraints that limit the size of sample that can be deposited, higher capacity thermal sample introduction techniques, such as induction heating vaporization (IHV),25 are quite capable of handling larger samples.The direct analysis of wood pulps using a technique such as IHV should be explored in the future. The pyrolytically coated graphite DSI probe demonstrated a greatly enhanced resistance to oxidative and chemical attack, resulting in a longer useful lifetime than an uncoated graphite probe.Although an improvement in precision was not observed for the pulp samples, greater signal sensitivity and shorter signal evolution times were observed when a pyrolytically coated probe was used. Acknowledgements The authors wish to thank the Analytical Services Division of the Pulp and Paper Research Institute of Canada for the generous provision of wood pulp samples and analyses for this study. For scholarship support, MER would like to gratefully acknowledge the Ænancial support of the Province of Que�bec through Fonds pour la Formation des Chercheurs et l'Aide a¡ la Recherche (FCAR), and PH gratefully thanks the Natural Sciences and Engineering Research Council of Canada (NSERC).Authors MER and EDS gratefully acknowledge funding from NSERC through an NSERC Operating Grant. References 1 B. Van Lierop, N. Liebergott and M. Faubert, J. Pulp Pap. Sci., 1994, 20, J193. 2 J. D. Sinkey and N. S. Thompson, Pap. Puu, 1974, 5, 473. 3 J. Prasakis, M. Sain and C. Daneault, TAPPI J., 1996, 79, 161. 4 J. Bouchard, H. M. Nugent and R. M. Berry, Preprints CPPA International Pulp Bleaching Conference, 1994, 33. 5 Standard Method G.34P, Canadian Pulp and Paper Association. 6 Standard Method T 266 om-88, Technical Association of the Pulp and Paper Industry (TAPPI). 7 D. R. Hull and G. Horlick, Spectrochim. Acta, Part B, 1984, 38, 843. 8 I. Atsuya, T. Itoh and T. Kurotaki, Spectrochim. Acta, Part B, 1991, 46, 103. 9 A. Lorber and Z. Goldbart, Analyst, 1985, 110, 155. 10 E. D. Salin, C. V. Monasterios and A. M. Jones, Anal. Chem., 1986, 58, 780. 11 W. E. Pettit and G. Horlick, Spectrochim. Acta, Part B, 1986, 41, 699. 12 G. Zaray, J. A. C. Broekaert and F. Leis, Spectrochim. Acta, Part B, 1988, 43, 241. 13 C. D. Skinner and E. D. Salin, J. Anal. At. Spectrom., 1997, 12, 725. 14 W. Huettner and C. Busche, Fresenius Z. Anal. Chem., 1986, 323, 674. 15 M. E. Rybak and E. D. Salin, J. Anal. At. Spectrom., 1998, 13, 707. 16 G. Le�ge¡re and P. Burgener, ICP Inf. Newsl., 1987, 13, 521. 17 G. A. Smook, Handbook for Pulp and Paper Technologists, Angus Wilde, Vancouver, 2nd edn., pp. 36±45. 18 V. Karanassios, M. Abdullah and G. Horlick, Spectrochim. Acta, Part B, 1990, 45, 119. Table 4 Determination of Cu, Fe, Mg, Mn, and Zn in pulp samples Direct sample insertion (n~7) CPPA method G.34P (Paprican) (n~3) Pulp sample Element Concentration (ppm) RSD (%) Concentration (ppm) RSD (%) Error (%) Brownstock Cu 1.27 11 0.84 7.39 51 Fe 10.7 20 19.4 12.5 245 Mg 593 13 515 1.9 15 Mn 93.2 6.7 80.3 0.99 16 Zn 11.8 20 11.6 1.9 1.7 TMP Cu 1.64 18 1.10 40 49 Fe 40 44 56.7 1.6 229 Mg 201 39 212 15 25.2 Mn 91 41 88 15 3.4 Zn 11.8 16 14.0 7.7 216 Average error 4.1 J. Anal. At. Spectrom., 1999, 14, 1715±1722 172119 X. R. Liu and G. Horlick, J. Anal. At. Spectrom., 1994, 9, 833. 20 W. Ru» dorff and G. Ru» dorff, Z. Anorg. Allg. Chem., 1947, 253, 281. 21 Analytical Methods Committee, Analyst, 1987, 112, 199. 22 R. L. A. Sing and E. D. Salin, Anal. Chem., 1989, 61, 163. 23 E. Sjo» stro»m, Wood Chemistry: Fundamentals and Applions, Academic Press, San Diego, CA, USA, 2nd edn., p. 107. 24 B. B. Sithole�, Anal. Chem., 1995, 67, 87R. 25 D. M. Goltz, C. D. Skinner and E. D. Salin, Spectrochim. Acta, Part B, 1998, 53, 1139. Paper 9/01694E 1722 J. Anal. At. Spectrom., 1999, 14, 1715±17

ISSN:0267-9477    

DOI:10.1039/a901694e

出版商:RSC    

年代:1999

数据来源: RSC