摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 267 Automatic Sample Handling and Calibration Methods for Analysis by Flame Atomic Absorption Spectrometry Using Discontinuous Flow Analysis* Brian L. Krieger Graham M. Kimber Mark Selby,? Fraser 0. Smith and Elvan E. Turak Analytical Spectroscopy Group Centre for Instrumental and Developmental Chemistry Queensland University of Technology G. P.O. Box 2434 Brisbane Queensland 400 7 Australia John D. Petty and Russell M. Peachey lonode Pty. L td. P. 0. Box 52 Holland Park Queensland 4 12 I Australia The method of discontinuous flow analysis (DFA) is presented as a rapid automated method of sample handling and analysis in flame atomic absorption spectrometry (FAAS). Three DFA arrangements for combined DFA-FAAS are described.The first provides automatic calibration in approximately 30 s using a single solution the second provides rapid automated standard additions and the third provides an automated version of the sample bracketing procedure. The last two methods allow for concurrent calibration and determination of samples in a single operation lasting around 30 s. This continual re-calibration minimizes the effects of instrument drift and ensures accurate and precise measurement. The DFA-FAAS combination provides a great deal of information that could be used to advantage for quality assurance purposes. However this extra information gathering does not require additional labour on the part of the operator because only one or two standard solutions need to be employed to construct complete analytical calibration curves.In contrast to other automated methods of sample handling the DFA technique uses high-precision cam-driven piston pumps and a valve arrangement to manipulate flowing streams containing reagents diluents standards and samples in a controlled and rapid fashion. Keywords Discontinuous flow analysis; flame atomic absorption spectrometry; automatic calibration stan- dard addition and dilution; sample bracketing Flame atomic absorption spectrometry (FAAS) is widely regarded as an 'aging' analytical method,' an old war horse still fulfilling an essential role in mining agriculture manufac- turing health services environmental studies and a variety of other important applications. However FAAS is fast becoming a non-developing technology; publication of research papers on FAAS has declined and fundamentally new developments are considered to be unlikely.'** The application of new sources such as laser diodes could yet generate a resurgence of scientific interest in FAAS but considerable technological development is necessary before such instruments are commercialized.' We believe that the accepted routine of calibration and sample delivery an area of analytical practice that has changed little since the early days of FAAS,3 still offers considerable scope for fundamental improvement.Methods for automated sample handling and calibration in FAAS reported previously have included exponential dilution chambers435 and various methods of controlled dispersion in flow injection (FI) as reviewed recently by Valcarcel.6 Exponential dilution chambers and FI have also been ~ombined.~.~ Significant recent works on automatic sample presentation and calibration for FAAS are those of Sperling et aL9 for FI methods and Starn and Hieftje" who used a high-performance liquid chromatography (HPLC) pumping arrangement to control the mixing of stan- dards and diluents in a series of discrete ratios. A further new methodology which is described in this paper is to combine FAAS with the new method of discontinuous flow analysis (DFA).''-l3 Conventional FAAS might be described as a 'zero-order' method14 because only a single result is generated for each measurement.As suggested by Sperling et one area for improvement of FAAS is to extend it from a zero-order method to a more information rich first-order methodology.The dilution chamber technique can be considered a first-order * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) York UK June 29-July 4 1993. t To whom correspondence should be addressed. method because the normal outcome is a vector of absorbance values collected during the exponential decay of analyte con- centration according to eqn. (1) rather than the usual single scalar measurement C=Co exp(-ut/v) (1) where C is the concentration at any given instant after introduc- tion of the sample volume C is the initial concentration of the sample u is the flow rate of diluent u is the volume of the chamber and t is time This first-order data vector contains sufficient information to calibrate the instrument response from the initial concentration down to the detection limit.The FI techniques that rely on the controlled dispersion of a plug of sample after injection into a flowing carrier stream are also first-order method^.^,'^.'^ A single transient FI signal contains information on the concentration range from zero to the concentration of the peak maximum. If the dispersion and transport characteristics of the FI system are reproducibly known then a single FI standard injection can be used to construct a complete analytical calibration curve using the so called 'electronic dilution' m e t h ~ d . ' ~ . ' ~ Another FI first-order approach is the 'zone-penetration' technique that disperses the sample within standard and diluent solution^.'^ In the zone-penetration method the sample-to- standard ratio varies across the FI peak permitting optimiz- ation of the sample-to-standard ratio for standard additions ~a1ibration.l~ The sequential injection (SI) analysis technique of Baron et al." is a more recent development of the zone analysis concept where a holding loop is used to store a concentration gradient formed from a single calibrant.A further FI method is based upon zone re-sampling where a standard solution is injected into a first carrier stream and allowed to disperse for a given time t; a zone from the dispersed standard is then re-injected into a second carrier stream and merged together with a sample injected at the same time.19-21 In the zone-resampling technique the concentration of stan- dard added to the sample is controlled by varying At which selects differing zones from the original standard injection.The268 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 zone-resampling technique is a zero-order procedure because after merging of the sample with the standard only a single peak height or area is used in subsequent calculations and because a separate injection of standard and sample is required for each standard In this paper three new first-order methods for sample delivery and calibration for FAAS are presented all based on the new technique of DFA."-l3 The DFA system allows the proportions of samples calibration standards and diluents in a flowing stream to be manipulated rapidly resulting in the measurement of samples and calibration standards concur- rently.Also described in combination with FAAS is how DFA can be used for automation of ( i ) instrument calibration (ii) the 'standard additions' procedure and (iii) a form of the 'sample bracketing' method. Items (ii) and (iii) above include instrument re-calibration each time a new sample is presented for analysis. After the initial optimization of the spectrometer every measurement produces a calibration and a result; the distinction between measurements made for calibration purposes and those made for analytical purposes is no longer necessary. Although each measurement cycle takes a little longer (approximately 30 s) this combination of calibration with measurement means that every cycle produces a result for a determination thus ensuring that productive throughput is not compromised.The DFA-FAAS combination is a first-order method in which rapid manipulation of the analyte concentration in the stream delivered to the spectrometer is achieved by the differential actions of precisely controlled pumps. It is a combination that offers advantages over all previously reported methods for automated calibration and sample handling. A full discussion of these advantages becomes easier if preceded by a detailed description of the technique and is therefore presented at the end of this paper. Experiment a1 Discontinuous Flow Analyser A prototype discontinuous flow analyser (DFA) was used (Ionode Tennyson Queensland Australia). The DFA contains a number of major components that are integral to its oper- ation (Fig.1). The precision machined cam is central to the instrument and its shape dictates the movement of the pistons. Fig. 1 DFA system cut away to illustrate the central drive shaft (A) and encoder disc (B). Also shown are the cam (C) which is split into two sections X and Y the cam follower (D) and piston pumps E-G. While pump E is dispensing pumps F and G are in suction. These pumping modes are reversed in the other half of the cycle (see text). This figure shows only half of the DFA another cam follower is in horizontal opposition to D driven by the other side of the cam A central driveshaft locates both the cam and the encoder disc. The optical encoder disc generates 2500 pulses per revolution and has two functions precise identification of the rotational position of the cam (by counting pulses from an initialization marker) and setting the rate of data acquisition.For each encoder pulse one data point is taken from the detector (in this instance an FAAS instrument) thus for a full 360" rotation 2500 data points are acquired. The rotation speed of the cam is variable between 0.25 and 4 revolutions per minute. The cam is split into two asymmetric parts X and Y (as shown in Fig. l) part X being exchangeable. The profile of X dictates the flow regime for the particular experiment while the profile of Y provides a constant flow regime. In the experiments described below two different X sections were used to produce incremental or gradient flow profiles. The cam is able to drive two sets of horizontally opposed piston pumps (of up to five pistons in each set).Each set moves independently. (Only the left-hand side of the DFA is shown in Fig. 1.) The 360" rotation of the cam is divided into half cycles a measurement half cycle and a refill/dispense-to-waste half-cycle. The piston pumps are poly(methy1 methacrylate) (PMM) with stainless-steel plungers and Viton 0 ring seals. Associated with each piston is a three-way valve (ASCO-Angar Florham Park NJ USA Part No. 368132430) used to direct the discharge or recharge of the pistons. The position of the cam dictates the states of the valves; during the measurement half- cycle the valves are open to the measurement device and during the refill/dispense-to-waste half-cycle the valves allow the piston pumps to refill with reagents or dispense to waste.Poly(tetrafluoroethy1ene) (PTFE) tubing of 1 mm i.d. (Gradko International Winchester UK) is used for all fluid connections between valves reservoirs pistons and the detector. Data acqusition was via a 386-class personal computer (Total Peripherals Brisbane Queensland Australia) operating at a clock speed of 16 MHz and equipped with a PC-74LC data acquisition expansion board (Boston Technology Perth Western Australia Australia). Operation conditions for the analogue-to-digital (A/D) sub-system are shown in Table 1. Signals from two atomic absorption spectrometers were acquired through the chart recorder output and preamplified before A/D conversion via the PC-74LC board. Data acqui- sition software has been written using QuickBasic version 3.5 (Microsoft Redmond CA USA).Experimental conditions and instrumental parameters for the FAAS experiments are listed in Table 2. Atomic Absorption Spectrometry Stock calcium solutions (1000 mg 1-I) were prepared from analytical-reagent (AR) grade calcium carbonate dissolved in AR grade hydrochloric acid and distilled de-ionized water. Table 1 Data acquisition system Resolution 12 bit Non-linearit y Input range Number of analogue inputs Conversion rate G0.75 least significant bit k 5 V (bipolar) 8 differential channels 30 KHz Table 2 Instrumentation and operating parameters Instrumental parameter Varian 1275 Varian AA6 Wavelength/nm 422.7 422.7 Solution uptake rate/ml min-' 6 6 Ca lamp (Varian Techtron)/mA 5 5 Oxidant (air)/l min-l 7 7 Fuel (acetylene)/l min- 2.5 3.5 Slit-width/nm 0.50 0.2JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 269 Fresh working solutions were prepared from this stock solution daily using distilled de-ionized water. The calcium signal was monitored at 422.7 nm and a stoichiometric air-acetylene flame was used throughout. The incremental self-calibration and incremental standard additions methods (Methods 1 and 2 below) were carried out with a Varian 1275 flame atomic absorption spectrometer (Varian Techtron Springvale Victoria Australia). The uptake rate of the nebulizer was controlled with an Alitea (Stockholm Sweden) peristaltic pump using 0.89 mm i.d. poly(viny1 chlor- ide) (PVC) bridged tubing (Gradko International). The uptake was maintained at a rate slightly lower than the normal aspiration rate of the nebulizer.Other operating conditions for the Varian 1275 are shown in Table2. Experiments with the gradient self-calibration method (Method 3 below) were carried out with a Varian Techtron AA6 spectrometer (Varian Techtron) and operating conditions are shown in Table 2. Results and Discussion The DFA combines samples standards diluents and reagents by a process of pre-programmed mixing in the flowing stream under the control of the cam-driven pumps. Thus the analyte concentration in the stream reaching the detector varies in a precise way with time during a cam cycle. The variation of analyte concentration during a cycle is described as either gradient or incremental. In the former case (gradient) analyte concentration increases or decreases linearly with time during the cycle.In the latter case (incremental) the analyte concentration variation with time features several steps and plateaus (Figs. 3 and 4). The experiments described are classified in two ways firstly according to the programme of analyte dilution during the cycle (viz. incremental or gradient) and secondly depending on whether a ‘standard addition’ method is being used. Method 1 Incremental Self-calibration Consider the simple one-piston configuration as schematically illustrated in Fig. 2. Two solutions only are involved one standard and one diluent. The cam design is such that during the measurement portion of the cam cycle an incremental (stepped) concentration profile with four plateaus is generated. The peristaltic pump of the spectrometer ensures that the total flow rate reaching the nebulizer remains constant.However the relative contribution to this constant flow rate from the standard (delivered by the piston pump) and the diluent (aspirated from the reservoir) varies in a stepwise manner during the cycle such that F (constant) = F + Fd (2) where F is the flow rate of the peristaltic pump F is the flow rate of the standard and Fd is the flow rate of the diluent. This is illustrated in Fig. 3(a) plateau 1 represents 100% diluent 0% standard; plateau 2 represents 70% diluent 30% standard; Cam f---. Pe r i st a I t i c . . To FAAS Diluent Discontinuous flow analyser Fig. 2 Single piston DFA-FAAS configuration for Method 1 a con- stant flow rate to the FAAS is governed by the peristaltic pump.However the relative flow rates of standard and diluent reaching the FAAS depends upon the dispensation of standard by the pump which varies as a function of the cam rotation and the complementary uptake of diluent Cam angle -. I Time -+ Concentration - Fig. 3 Illustration of the stepped self-calibration method (Method 1) (a) stepwise increase of analyte concentration during a cycle; (b) resulting change in absorbance during the cycle; and (c) derivation of an analytical working curve from data collected during a single cycle (b) plateau 3 represents 29% diluent 71% standard; and plateau 4 represents 100% standard. Figure 3(b) is an idealized illustration of how these changes in analyte concentration in the nebulizer stream are tracked as changes in absorbance during the cam cycle.Fig. 3(c) shows how knowing the concentration of undiluted standard a calibration curve of absorbance versus concentration can be derived. This can be carried out using only the information gathered during a single cam cycle and with only one standard solution. At present cycle times of the order of 30 s are typical. This can be expected to decrease with further improvement in instrumentation. A plot of the absorbance uersus encoder pulses resulting from an experiment using a single 20 mg 1-’ calcium standard is shown in Fig.4(a) with the resulting calibration plot in Fig. 4(b). The plot is linear with a squared product-moment correlation coefficient (r’) of 0.989. Method 2 Incremental Standard Additions In Method 1 a rapid calibration of the FAAS is achieved.In Method 2 quantification of an unknown can be achieved as part of the DFA cycle. This incremental technique also allows automation of a form of the normally labour-intensive method of standard additions. A schematic representation of the DFA configuration for Method 2 is shown in Fig. 5. The standard is dispensed by the DFA piston pump increasing incrementally during the cycle. The sample and diluent are aspirated into the system from separate containers. Again the rate at which the standard-sample-diluent blend reaches the nebulizer is constant controlled by the speed of the peristaltic pump. The diluent and sample pass into the stream through T-pieces in a fixed proportion to each other. On the highest plateau (Fig.6 )270 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Encoder pulses 0 5 10 15 20 Concentration (ppm) Fig.4 Representative signal traces for Method 1 (a) plot of absorbance versus encoder pulses for a single cycle with a standard solution of 20 mg 1-1 of Ca and (b) the analytical working curve for Ca using data from (a) Per i st a It i c Standard To FAAS Discontinuous flow anal p e r Diluent Sample Fig. 5 Configuration for the incremental standard additions method (Method 2) showing the constant suction provided by the peristaltic pump the piston pump dispensing standard at a rate that varies according to the cam profile the constant uptake of sample and the complementary uptake of diluent the flow rate of the dispensing piston pump is matched to the pumping rate of the nebulizer and consequently the stream at this point ideally consists of standard only.On the lowest plateau the piston pump is stationary and hence the stream at this point contains no standard. The intermediate plateaus represent incremental mixtures of standard with sample-diluent. Fig. 6 is a typical example of the resulting traces. In this case two traces (A and B) have been acquired in sequence using a standard solution of 10 mg 1-l of calcium. Trace A is a diluent only trace in this case the reservoir marked 'sample' in Fig. 5 contained a blank solution. Trace B is a sample plus diluent trace where the sample consisted of a 10 mg 1-l solution of calcium. Trace A is acquired to ascertain a zero absorbance value. It also serves as a check on the validity of the standard additions assumption by allowing the slope of the absorbance 4.0 3 3.6 3.2 4 2.8 0 (5 .- 0 C 9 2.4 1 I 1 1 I I I Encoder pulses 0 200 400 600 800 1000 Fig.6 Plots of absorbance versus encoder counts for replicate pairs of cam cycles using Method 2. In the lower traces A the sample consisted of a Omg 1-' of Ca blank whereas in the upper traces B the sample consisted of a 10 mg 1-' of Ca solution versus concentration plot derived from this trace to be com- pared with the slope of a similar plot based on trace B. The concentration of the unknown is determined by a conventional standard additions calculation. The concen- trations of the additions are related to the known mixing ratios provided by the cam. A regression line is fitted to the absorbance versus addition data and the intercept (hence the unknown concentration) is calculated.Results for synthetic samples (10 mg I-' calcium solutions) using this method show high repeatability [less than 1% relative standard deviation (RSD)] but demonstrate an analyt- ical bias (typically about 2%). This bias appears to be related to the difficulty of exactly matching the flow rates of the peristaltic pump and the piston pump. Method 3 Gradient Self-calibration In this method the cam cycle is divided into two half-cycles encompassing firstly a calibration half-cycle and secondly a sample measurement half-cycle. In the two methods described above the calibration-measurement is performed during a single half-cycle. No analytically useful data are available in the second half-cycle when the piston pump is refilling.However in the method outlined below the full 360" rotation of the cam is used to obtain analytical data by reciprocal action of the piston pumps i.e. when the sample pump discharges other pumps are refilling or expelling and vice versa. In the calibration half-cycle a complete analytical calibration curve is generated. The concentration limits of this calibration plot are defined by two standards of high and low concen- tration. During the sample measurement half-cycle valves are activated to introduce the sample solution to the spectrometer. Fig. 7 shows a typical trace acquired in less than 30s. The two half-cycles include short wash-out phases (Fig. 7). The experimental arrangement for this method is shown schematically in Figs.8 and 9. Understanding of these figures is aided by consideration of the action of the pumps during the calibration half-cycle (Fig. 7). During this half-cycle the composition of stream S5 (the stream arriving at the spec- trometer) and consequently the measured absorbance varies with time according to the simultaneous actions of pumps P1 P2 and P3. The operation of these pumps is described below. Pump PI This is a constant flow-rate dispenser that sets the rate at which the analysis stream reaches the spectrometer. Pump P1 delivers a low concentration standard (stream Sl). Stream S1 is equal in flow rate to stream S5 the stream feeding the nebulizer. The flow rate is somewhat less than the natural aspiration rate of the nebulizer.This is a gradient flow suction pump that with- draws stream S2 at a linearly increasing rate. This is a gradient flow dispensing pump that Pump P2 Pump P3JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 27 1 0.45 1 High -n Wash-out standard 0.38 c 0) a 0.31 5 a 2 0.24 0.17 measurement \ Blank 1 measurement a 0.10 I I I I -1 0 300 600 900 1200 1500 1800 Encoder pulses Fig. 7 Replicate traces of the full cycle for the gradient self-calibration method (Method 3 ) showing the generation of a linear calibration gradient defined by low and high concentration standards followed by measurement of the sample. The calibration and measurement half cycles include short wash-out periods. The top two traces are for a sample solution of 10 mg I-’ of Ca and the bottom two traces are for a sample consisting of a 0 mg 1-l of Ca blank solution.The high and low concentration standards are 20 and 2 mg l-‘ respectively P2 P3 P4 P Fig. 8 Pumping arrangement (top view) for Method 3. Pump P1 is discharging at a constant rate which sets the flow rate to the nebulizer. Pumps P2 and P3 are of equal capacity and both are driven by the same cam frame but in opposite directions. Pump P2 is withdrawing while pump P3 is dispensing. Pumps P2 and P3 have no net effect upon the overall flow rate. During the time that pumps P1 P2 and P3 are refilling or expelling valves (not shown) are activated so that P4 dispenses sample to the FAAS at a constant rate P3 P4 s3 s4 To FAAS s1 PI r 1 I J P2 Fig. 9 Fluid-flow diagram for Method 3.During the first half cycle the piston pumps P1 P2 and P3 act so as to provide a linear gradient in concentration between a standard of low concentration in P1 and a standard of high concentration in P3. Pump P2 acts to withdraw solution at the same rate as it is added by P3 ensuring a constant overall flow rate. At the end of the half cycle valves (not shown) activate to present a sample solution in P4 to the FAAS using the same constant rate as before delivers a high concentration standard (stream S3) at a linearly increasing rate. The actions of pumps P2 (gradient suction) and P3 (gradient dispensing) are complementary. These two pumps are of equal capacity and are driven by the same cam frame (see Fig. 8) but in opposite directions. The additive contribution of P3 to the over-all flow rate is exactly balanced by the subtractive effect of P2 and the total flow rate defined by the constant dispensing rate of P1 remains invariant.Fig. 10(a) and (b) shows the contribution of stream S1 (low standard) and stream S3 (high standard) to the composition of stream S5 during the calibration half-cycle of the rotation of the cam. The result of this simultaneous action of pumps P1 P2 and P3 on the analyte concentration at the nebulizer is shown in Fig. lO(c). At the beginning of the gradient stream S5 is made up entirely of stream S1. The analyte concentration is that of the low standard dispensed by pump P1 (constant flow dispenser). At the completion of the gradient stream S5 is made up entirely of stream S3. The analyte concentration is now that of the high standard dispensed by pump P3.The intermediate variation in composition is linear owing to the linear cam gradient. Thorough mixing before the stream reaches the nebulizer is ensured by a vibrating reed mixerg in the mani- fold (Fig. 9). Fig. 10(d) shows the predicted trace as the spectrometer absorbance measurements track the compositional changes in stream S5 as described by Fig. lO(c). If Beer’s law holds over this concentration range the resulting plot is linear with encoder position and hence with concentration. The calibration half-cycle just described is followed by a sample measurement half-cycle. At the beginning of this phase the valves switch so that the solution now presented to the nebulizer comes only from pump P4.Pump P4 has been refilling during the calibration segment and now dispenses the sample at a constant rate which is equal to the over-all flow rate to the nebulizer during the previous half-cycle. This results in the absorbance plateau region labelled ‘sample measurement’ in Fig. 7. Clearly the absorbance measured on this plateau can be related to the absorbance uersus concentration curve pro- duced in the calibration half-cycle and thus to the determi- nation of the concentration of the unknown. 0 180 360 0 180 360 Cam angie/degrees Cam angle/degrees 1 .o 1 .o 0) c $ 0.5 0.5 8 2 1 500 1000 2 10 20 Encoder pulses Concentration (ppm) Fig. 10 Contribution of the piston pumps to the over-all flow rate during the gradient calibration half cycle in Method 3. (a) proportion of the over-all flow due to the combined effects of pumps P1 and P2 (this is the flow rate at which the low standard is added to the stream); (b) proportion of the over-all flow due to pump P3 (this is the flow rate at which the high standard added to the stream); (c) resulting variation of analyte concentration with encoder pulses; and ( d ) resulting plot of absorbance uersus concentration (assuming adherence to Beer’s law)212 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Both Method 2 and Method 3 have the advantage of providing a calibration with each sample measurement. This continual re-calibration is a potentially rich source of quality assurance data on instrumental drifts and also has the practical result that for actual sample measurements the distinction between short-term and long-term precision is no longer meaningful.Method 3 despite its more complex mechanical arrange- ment has several advantages over Method 2 (stepped standard additions). (a) The resulting trace is readily understandable being similar to the conventional 'multi-point calibration fol- lowed by sample measurement' procedure. (b) Common prob- lems associated with peristaltic pumps namely pulsations and degradation of pump tubing with time are eliminated. The cam-driven piston pumps are highly reproducible over several years of use. (c) The uptake rate of the spectrometer is solely and precisely controlled by a cam-driven piston pump (either pump P1 or pump P4). The difficulty of matching the flow rate from the DFA to nebulizer uptake (as in Method 2) is overcome.( d ) Separate analytical calibration curves consisting of approximately 400 data points are determined for every sample. Because of the abundance of data no assumption of linearity is necessary a linear or higher order regression calculation is applied as statistically appropriate. (e) This close definition of the calibration curve means that the technique known as sample bracketing can be automatically applied to each sample measurement. In a conventional application of the bracketing technique a preliminary measurement establishes the approximate concen- tration of the unknown then standards with concentrations closely bracketing the unknown are prepared and measured and finally the sample absorbance is re-measured.Although the reliability of measurements is improved by this method it is not routinely used because it is clearly very time consuming. With DFA however use of Method 3 means that sample bracketing is automatically effected for all concentrations within the calibration range. A disadvantage of the present gradient calibration arrange- ment is that the sample fills and is dispensed from piston pump P4. This requirement increases wash-out times and carryover between samples and necessitates larger sample volumes than is the case with Methods 1 and 2. It is anticipated that with future improvements in the instrumentation for DFA-FAAS that this problem will be overcome. A replicate set of analyses of a synthetic 10 mg 1-' calcium sample using a 2 mg 1-l calcium solution as the low standard and 20 mg I-' calcium solution as the high standard showed an analytical bias of less than 0.5% and repeatability of better than 0.5% RSD.Comparison With Other Automatic Calibration Methods Of the automatic sample handling and calibration methods described earlier the exponential dilution chamber is the least flexible and extensible option. The FI and DFA methods offer a wide range of sample management options. The primary difference between the FI and DFA methods is that FI relies on controlled dispersion to produce gradients in concentration whereas the DFA technique uses cam-driven piston pumps to deliver precise volumes of samples diluents standards and reagents into a well-mixed stream according to pre-determined flow profiles.The three automatic calibration methods pre- sented are intended to enhance the precision and accuracy of the FAAS method Methods 1 and 2 increment the standard to diluent or standard to sample to diluent ratios over several fixed compositions and the cam profile is cut so as to dwell at each new composition for a period of time long enough to gather statistics at that composition. An added advantage for Method 3 is that because the calibration curve is so finely determined the method provides automatic bracketing of the sample concentration. In conventional FAAS the bracketing method is known to produce favourable precision and accu- racy.3 On the other hand the FI-FAAS combination is subject to limitations in precision as described by T y ~ o n . ~ ~ ~ ~ In comparison with FI techniques there are disadvantages with DFA.Firstly greater care must be exercised in order to minimize the effects of dispersion. Best results with DFA are obtained with laminar flow through short low-volume fluid connections. Secondly the relative complexity and cost of the instrumentation is greater with DFA. The incremental DFA experiments described above (Methods 1 and 2) are similar to those described recently by Starn and Hieftje" using an HPLC pumping arrangement. However the DFA experiments described here demonstrate a number of significant advantages over those of Starn and Hieftje:" (i) fast response time (less than 30 s for an analytical run instead of around 10 min); (ii) no pH dependent pulsation noise has been encountered with the DFA system; (iii) sample introduction is much simpler and far more convenient with the DFA system because there is no need to fill a pump reservoir with sample; and (iv) sample volumes can be as small as a few hundred microlitres with the DFA arrangement.This work was supported by the Commonwealth Department of Employment Education and Training through an Australian Research Council Grant (File No. AE9130183) and an Australian Postgraduate Research Award (Industry). The authors thank Sally Watson for assistance with the artwork and Dennis Sweatman for technical advice. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 References Hieftje G. M. J. Anal. At. Spectrom. 1989 4 115. Sturgeon R. E. J. Anal. At. Spectrom 1992 1 13N. Schrenk W. G. Flame Emission and Absorption Spectrometry VoZume 2 Components and Techniques ed.Dean J. A. and Rains T. C. Marcel Dekker New York 1971 ch. 12. Booher T. R. Elser R. C. and Winefordner J. D. Anal. Chem. 1970 42 1677. Posta J. and Lakatos J. Spectrochim. Acta Part B 1980,35,601. Valcarcel M. Sample Introduction in Atomic Spectroscopy ed. Sneddon J. Elsevier Oxford 1990 ch. 11. Carbonell V. Sanz A. Salvador A. and de la Guardia M. J. Anal. At. Spectrom. 1991 6 233. Beinrohr E. CsCmi P. and Tyson J. F. J. Anal. At. Spectrom. 1991 6 307. Sperling M. Fang Z. and Welz B. Anal. Chem. 1991 63 151. Starn T. K. and Hieftje G. M. J. Anal. At. Spectrom. 1992,7,335. Arnold D. P. Peachey R. M. Petty J. D. and Sweatman D. R. Anal. Chem. 1989 61 2109. Petty J. D. Peachey R. M. and Sweatman D. R. US Pat. 5,080,866 1992. Sweatman D. R. Petty J. D. and Peachey R. M. US Pat. 5,040,898 1992. Sanchez E. and Kowalski B. R. J. Chemom. 1988,2,247. Olsen S. RBiiCka J. and Hansen E. H. Anal. Chim. Acta 1982 136 101. RBiiEka J. and Hansen E. H. Flow Injection Analysis Wiley New York 2nd edn. 1988. Fang Z. Harris J. M. RBBiEka J. and Hansen E. H. Anal. Chem. 1985 57 1457. Baron A. Guzman M. RBiiCka J. and Christian G. D. Analyst 1992 117 1839. Jacintho A. O. Zagatto E. A. G. Bergamin H. Krug F. J. Reis B. F. Bruns R. E. and Kowalski B. R. Anal. Chim. Acta 1981,130 243. Zagatto E. A. G. Jacintho A. O. Bergamin H. Krug F. J. Reis B. F. Bruns R. E. and Arajo M. C. U. Anal. Chim. Acta 1983 145 169. GinC M. F. Krug F. J. Bergamin Filho H. Friere dos Reis B. Zagatto E. A. G. and Bruns R. E. J. Anal. At. Spectrom. 1988 3 673. Tyson J. F. Spectrochim. Acta Rev. 1992 14 169. Tyson J. F. Analyst 1985 110 419. Paper 3/04998A Received August 17 1993 Accepted October 15 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900267

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 273 Development of a Proposed International Standard for Determining Arsenic in Workplace Air Using Hydride Generation Atomic Absorption Spectrometry* Robert D. Foster and Alan M. Howe Occupational Medicine and Hygiene Laboratory Health and Safety Executive Broad Lane Sheffield UK S3 7HQ Proposed Draft International Standard DIS 11 041 described and developed by the International Organisation for Standardisation (ISO) involves sampling air by drawing it though a membrane filter with a paper back- up pad both of which have been treated with sodium carbonate solution. Particulate arsenic compounds and arsenic(ii1) oxide vapour are collected but not arsine. The filter and pad are digested using nitric acid sulfuric acid and hydrogen peroxide.The resultant solution is boiled down to sulfuric acid fumes and then made up to volume with water. Aliquots of the sample solution are prepared for hydride generation by the addition of hydrochloric acid and potassium iodide. Analysis by both continuous flow and flow injection hydride generation atomic absorption spectrometry (HG-AAS) is described. The method exhibits a recovery of >95%. It can be adapted to determine nanogram to milligram levels of collected arsenic making it suitable for testing compliance with the widely differing limit values stipulated in government health legislation of different countries e.g. 0.1 mg m-3 UK Maximum Exposure Limit (MEL) 0.01 mg m-3 US Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit (PEL) and 0.002 mg mP3 US National Institute of Occupational Safety and Health (NIOSH) Short Term Limit Value.The International Standard will supplement existing internationally adopted analytical standards such as NIOSH 7901 which employs electrothermal atomic absorption spectrometry and NIOSH 7900 which employs HG-AAS but which for safety reasons requires the use of a fume cupboard suitable for handling perchloric acid. It should be useful to a wide range of laboratories including those with limited AAS facilities. This paper describes the testing protocol used to validate the method and gives results obtained to date including some obtained by laboratories using the analytical method in an inter-laboratory exercise. Keywords Arsenic and arsenic(ii1) oxide; workplace air sampling; hydride generation; atomic absorption spectrometry; method validation The International Organisation for Standardisation (ISO) is a federation of 92 national standards organizations.The British Standards Institute (BSI) is the member body for the UK. The I S 0 co-ordinates the exchange of information on international and national standards in all fields except those of electrical and electronic engineering liaising with some 450 international organizations in an attempt to encourage consistency of stan- dards. International Standards are developed by nearly 200 I S 0 Technical Committees that are responsible for specific technical areas. Work is carried out through 630 sub- committees. It is estimated that some 20000 engineers scien- tists and administrators participate in the drafting of standards either by technical testing of the procedures or by participation in working group meetings to represent the consolidated views and interests of industry government labour and individual consumers in the standards development process.The number of published International Standards is now approaching 10000 most of which are available in English French and Russian. The format of the Standards is carefully prescribed and wording is kept as simple as possible to ease the task of translation for those reading in other than their first language. In 1986 the Occupational Medicine and Hygiene Laboratory (OMHL) of the UK Health and Safety Executive (HSE) was approached by Sub-committee 2 (SC2) Workplace Atmospheres of Technical Committee 146 (TC146) Air Quality to provide a Convenor for Working Group 2 (WG2) Inorganic Particulate Matter.The OMHL was chosen because of its involvement in the organization of developing analytical methods published in the MDHS Series (Methods for the Determination of Hazardous Substances). These are distributed through HM Stationary Office and are widely used throughout the UK and other parts of the world to monitor the exposure * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1994. of workers to air contaminants in the workplace. Shortly afterwards the Working Group resolved to produce an International Standard for determination of arsenic in work- place air using continuous flow (CF) or flow injection (FI) hydride generation atomic absorption spectrometry (HG- AAS).Much of the practical work required to develop the procedure was subsequently carried out at the OMHL. However the laboratories of similar organizations in other countries were also involved in evaluating the method notably Institut National de Recherche et de Securite (INRS) in France and Instituto Nacional de Seguridad Higiene en el Trabajo (INSHT) in Spain. Occupational exposure to inorganic compounds of arsenic can occur through inhalation ingestion or by absorption through the skin. Chronic exposure usually arises by inhalation and can produce respiratory tract effects notably perforation of the nasal septum skin effects such as aczematous and folicular dermatitis together sometimes with abdominal symp- toms.' The nervous system and liver can also be affected.Inorganic arsenic compounds are recognized as lung and skin carcinogens. Exposure of workers to arsenic can occur during the manufacture and use of arsenic compounds when arsenic oxides are produced during the smelting of metal ores and when metal alloys containing arsenic are heated strongly. Arsenic is alloyed with lead and copper to increase hardness and heat resistance in such processes as the production of lead shot and the manufacture of lead battery grids. Arsenic com- pounds are used in the pottery industry [arsenic(~~~)chloride]; in glass making [arsenic(m)oxide As,O,]; as pigments [cop- per(n)acetoarsenite Cu(COzCH3).3Cu(As02)z]; as insecti- cides (arsenic(m)oxide copper(1r)arsenite [Cu( As02),-H20] copper(I1)acetoarsenite and calcium arsenate [Ca,(AsO,),]}; as fungicides [copper(~~)arsenite]; as herbicides [sodium arsenite (NaAsO,) and calcium arsenate]; as wood preserv- atives (copper and chrome arsenites); in the production of anti-274 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 fouling paints for ships [copper(~~)acetoarsenite]; as a rodenti- cide [arsenic(m)oxide and copper(~i)arsenite]; in the elec- tronics industry in the production of semiconductors (galium arsenide GaAs); and alloyed with selenium to coat photocopier drums.Cancer of the respiratory tract has been reported in excess frequency in workers known to be exposed to dust and fumes containing arsenic whilst engaged in the smelting of metal ores and the production of insecticides. The task of drafting a sampling and analytical method commenced with a literature search.Manufacturers of HG-AAS systems were also approached for information on available instrumentation. A number of methods exist for the measurement of inorganic compounds of arsenic in workplace air. Sampling procedures include those which draw air through untreated membrane filters*5 to collect particulate dust con- taining arsenic and those using sodium carbonate or sodium hydroxide treated membrane filters or without7 treated back-up pads) or treated quartz fibre filters’ in order to collect arsenic(I1I)oxide vapour in addition. These filters are subjected to a wet digestion and the resultant sample solutions are analysed using various instrumental techniques.These include electrothermal AAS5,6 or inductively coupled plasma atomic emission spectrometry ( ICP-AES)4.8 for the direct analysis of the sample solutions and discrete batch HG-AAS.3,7 There was a requirement for a method that employs the techniques of CF or FT-HG-AAS which are increasingly and now pre- dominantly used for the determination of arsenic in preference to discrete batch HG-AAS. There was also a requirement for an effective sample filter digestion procedure for use with HG that does not involve the use of perchloric acid and hence does not require a fume cupboard adapted to scrub out perchloric acid fumes which otherwise present a fire hazard. A working draft was prepared in which the sample digestion procedure was based on US National Institute of Occupational Safety and Health (NIOSH) 7901; with modifications to provide a final solution suitable for HG-AAS rather than electrothermal AAS.After scrutiny and approval by the mem- bers of the Working Group the method was approved as a Committee Draft and a detailed evaluation of sample digestion procedures commenced. Experimental Instrumentation The standard is written assuming that the instrumental tech- nique employed will be HG-AAS. However the method can be used with very little adaptation using HG with atomic fluorescence spectrometry (AFS) or ICP-AES. The two types of HG systems described in the proposed Draft International Standard (DIS) and their expected output are illustrated in Figs. 1-4. Continuous flow systems generate a constant atomic absorption signal and work by pumping a continuous stream of acid test solution and sodium tetrahydroborate solution to a mixing piece.Arsenic in the test solution is reduced to arsine gas which is swept by a stream of argon into a heated silica or quartz absorption cell mounted in the beam of an AA spectrometer. Flow injection systems use a switching valve to inject a discrete volume of test solution into an acid blank stream to produce a transient AA signal. The 197.2 nm arsenic line is used for AA measurements unless the highest sensitivity is required. The 193.7 nm arsenic line is approximately twice as sensitive as the 197.2 nm arsenic line but use of the 197.2 nm line is preferable since the calibration obtained at this wave- length has a greater linear range.The instrumentation used at OMHL in validating the method was a Perkin-Elmer 5100 AA spectrometer using an arsenic hollow cathode lamp without background correction. An electrically heated quartz gas cell was used in conjunction with a Perkin-Elmer FIAS 200 HG system. The system was deliberately de-sensitized by using smaller diameter peristaltic pump tubing than usual (1.14 mm tetr To heated silica or quartz cell mounted in AA wectrometer I F- Gaseous- liquid dm Two channel reactants peristaltic pump a Fig. 1 Schematic diagram of a continuous flow HG system / I I 0 22.50 45.00 Tim e/s Fig. 2 AAS output when valve is operated to change the continuous flow to the mixing piece from acid blank to acidified sample solution; the broken line indicates time at which measurement begins Heated silica or quartz cell ( a ) Autosampler 0000000 mounted in AA spectrometer ....... .. ........... ; ....... ... . . . . . . ........ ............ . . ..;,.:.. .. ; .. :. .. ..- k5i%iEiJ=$ Sample loop . . . . . . Pump 1 FI valve11 I Mixina Diece 1 (&Gas ressure regulator Pu (b) FI Valve functions Fill Injection Fig. 3 Schematic diagram of (a) the FT-HG system; and (b) FI valve functions 0 7.50 Time/s 15.00 Fig.4 AAS output when valve is operated to inject a fixed volume of acidified sample solution into the acid blank flowJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 275 id.) thereby reducing the flow rate of the acid blank stream to approximately 5 ml min-'. The flow rate of the reductant stream was also approximately 5 ml min-'.The system was used in FI mode with a sample loop volume of 200 pl and in simulated CF mode by extending the sample loop of the sample valve to 1500 pl. The latter over-large volume resulted in a stepped change in the AA spectrometer output rather than a peak. Reagents The proposed DIS specifies that reagents used should be of the highest purity consistent with minimum arsenic content. Solutions used in the method include solution for filter treat- ment (1 moll-' sodium carbonate in 5% v/v glycerol solution); blank acid used in HG and in the preparation of samples and standard solutions [dilute hydrochloric acid (1 + l)]; solution used in the preparation of sample and standard solutions for reducing As5+ to As3+ for a more rapid reduction to arsine in the hydride generator (100 g 1-' potassium iodide); solution for use in preparation of sample solutions [dilute sulfuric acid (1 + 9)]; reductant solution for HG [between 2 and 20 g 1-' of sodium tetrahydroborate in 0.1 mol 1-1 sodium hydroxide solution (the concentration used should be in accordance with the requirements of the HG system 2 g 1-' was used for work at the OMHL)].Sample Collection Procedure Particulate arsenic and arsenic compounds and arsenic(II1) oxide vapour are collected by drawing a known volume of air through a sodium carbonate impregnated cellulose ester mem- brane filter and a sodium carbonate impregnated back-up paper pad mounted in a sampler designed to collect the inhalable fraction of airborne particles.In most workplace situations where exposure to arsenic can occur (e.g. in the refining of base metals welding and other hot metal processes) a significant proportion of the arsenic is present in the form of arsenic(1rr)oxide vapour.' This vapour is collected by reac- tion with sodium carbonate on an impregnated cellulose ester membrane filter and impregnated back-up paper pad. As203 + Na2C03+2NaAs02 +C02 A collection efficiency of 1.00 has been reported for similarly prepared filter-pads for laboratory generated particulate arsenic aerosols' and 0.98 for arsenic(I1r)oxide vapour.' Arsenic in the form of arsine passes through the filters and is not collected. Metal arsenides that react with water vapour to yield arsine are similarly not estimated.Procedures The following procedures are modified extracts from the proposed DIS. Filter Preparation Place the cellulose ester membrane filters on a clean poly(tetra- fluoroethylene) (PTFE) sheet or similar inert flat surface in an arsenic-free environment. Establish the volume of sodium carbonate solution required to just wet the entire surface of a filter after it has been allowed to spread for a few minutes. Dispense this volume of sodium carbonate solution onto each filter and allow to dry at room temperature in an arsenic-free environment. Prepare sodium carbonate impregnated back-up pads in a similar fashion. Sample Digestion Place sample filters and back-up pads into beakers. If appro- priate wash dust from the inside of samplers into the beaker using a minimum volume of water.Add 5 ml of concentrated nitric acid and 1 ml of concentrated sulfuric acid to each beaker cover with a watch-glass and heat to approximately 175°C on the hot-plate in a fume cupboard. When the initial vigorous reaction has subsided slide back the watch glasses so that the beakers are only partially covered. Continue to heat each beaker until the solution volume has been reduced to approximately 1 ml and then remove from the hot-plate. Allow the solutions to cool and then carefully add 2ml of hydrogen peroxide to each beaker. Replace the beakers on the hot-plate again covering with the watch-glasses and when the initial vigorous reaction has subsided slide back the watch- glasses so that the beakers are only partially covered. Continue to heat until dense white fumes of sulfur trioxide are evolved (raise the temperature of the hot-plate to 200°C if necessary).If the solution becomes discoloured owing to charring of residual organic material carefully add hydrogen peroxide dropwise until a clear solution is obtained and then evaporate again until the appearance of dense white fumes. Remove the beakers from the hot-plate and allow the solutions to cool. Carefully rinse the watch glass and the sides of each beaker with a small volume of water transfer each solution quantitat- ively into a 10ml one-mark calibrated flask. Filter through a cellulose (paper) filter which has been pre-washed with dilute sulfuric acid and then with water if there is any evidence of undissolved particulate matter. Dilute to the mark with water.Preparation of Solutions for Hydride Generation Prepare blank and sample test solutions for analysis. Transfer an aliquot V ml of the sample digestion solution and (5- V,) ml of dilute sulfuric acid to a 25 ml calibrated flask. Add 12.5 ml of concentrated hydrochloric acid and 2.5 ml of potassium iodide solution and make up to volume with water. Allow 1 h for reduction of pentavalent arsenic to take place before analysis. National occupational exposure limits for arsenic vary considerably and therefore the volume of the aliquot of sample solution used can be varied up to a maximum of 5ml according to the detection limit required. For the lowest detection limit an aliquot volume of 5ml should be used; an aliquot volume of 1 ml is suggested when arsenic in air concentrations are to be compared with a limit value in the region of 0.01 mg m-3 of arsenic; and for limit values in the region of 0.1 mg m-3 of arsenic an aliquot volume of 0.1 ml is suggested.The final test solution matrix is 1 + 1 hydrochloric acid 1 +49 sulfuric acid and 10 g 1 -' potassium iodide. Calibration Prepare calibration solutions from commercially available arsenic standard solutions diluted in the test solution matrix to cover the range 0-50 ng ml-' of arsenic when the 197.2 nm arsenic line is used or 0-25 ngml-' of arsenic when the 193.7 nm arsenic line is used. The upper limit of the working range is dependent upon the performance characteristics of the HG system used and other instrumental factors that affect sensitivity and the linearity of the calibration.In general it is best to work in the linear range of an AA calibration where absorbance is proportional to the concentration of arsenic in solution. A certain amount of curvature can be tolerated. Hydride generation AAS calibrations are more curved than flame AAS calibrations and discretion should be exercised in assessing whether re-calibration over a lower concentration range is necessary. Analysis Set up and operate the HG and AA spectrometer in accordance with the manufacturer's instructions.276 + Vote acceptance IS0 working group 2 -* JOURNAL OF ANALYTrCAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 1 Propose modifications and revise IS0 committee draft i Validation of the Standard The stages in the validation of the method described briefly below are illustrated in the flow diagram given as Fig.5. Test analytical method Filter digestion H y d r ide genera t i o n/AA Testing of the initially proposed method The digestion procedure described in the first Committee Draft was based closely on that in NIOSH 79016 and did not involve sulfuric acid the sample solution was taken to dryness before being re-dissolved in hydrochloric acid. The method was tested both within the OMHL and in the laboratories of similar organizations. Although the procedure could be made to work with care it was found that any lack of thoroughness in boiling off residues of nitric acid led to the formation of iodine from the potassium iodide added to reduce arsenic to the trivalent state prior to HG. The method specified in the first draft used treated mem- brane filters without treated back-up paper pads.A collection efficiency of 94% for arsenic(r1I)oxide in air was reported by Costello et aL9 for membrane filters which were prepared in a similar fashion and this was considered by the Working Group to be adequate for air monitoring purposes. However investigations at INRS indicated that for high levels of arsenic(rr1)oxide in air the use of a treated back-up paper pad with the treated membrane was necessary if an unacceptable level of breakthrough was not to occur. The sampling and digestion methods were therefore modified and incorporated in an improved version of the draft. 4 Method satisfactory? Testing of the mod$ed analytical procedure After preliminary proving at OMHL the method was evalu- ated fully.Analytical recovery was determined using treated Validate method Analytical ruggedness recovery precision etc. Test method for on-site sampling 25 mm filter-pads dosed with arsenic solutions to simulate sampling an atmosphere containing arsenic at the UK 'Maximum Exposure Level (MEL) of 0.1 mg m-3 for 4 h and at 0.1 0.5 2.0 and 5.0 times this limit. The same exercise was :repeated for 37mm filters dosed to simulate the US Occupational Safety and Health Administration Permissible Exposure Limit (PEL) of 0.01 mg m-3. Recovery and precision of measurement at the different dosing levels were determined with the sample solution aliquot volume optimized for measurement of arsenic at the limit value. Measurements were made using both CF-HG and FI-HG techniques.Ruggedness testing was carried out to investigate the degree to which deviations could be made from the recommended procedures and conditions (volumes of reagent hot-plate temperature rates of evaporation of acids etc.) before the method failed to perform satisfactorily. The extent of transition metal interference was also briefly investigated. A number of transition metals mainly those of Groups 8 and 11 cause signal depression in the determination of arsenic by HG-AAS. This interference has been shown to be associated with the reduction of metal ions to the free The most severe interferences are caused by nickel copper and cobalt but with the reagent concentrations used in this method the signal depression caused by 10 pg ml-I of these three metals is less than lo% for a solution containing 10 ng ml- of arsenic.12 Flow injection systems are much less affected by metal interferences than CF systems.r Generate test samples Conduct inter- laboratory exercise Interlaboratory Exercise Laboratories were recruited through the OMHL Workplace Analysis Scheme for Proficiency (WASP) quality assurance scheme from laboratories listed by the National Measurement Process results and statistics Literature search Devise sampling and analytical method for As in workplace air Prepare IS0 committee draft Fig. 5 Preparation of proposed IS0 Standard 11041 Arsenic in Workplace AirJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 277 Accreditation Service (NAMAS) as accredited for occupational hygiene air monitoring and from the customers of instrument manufacturers.The exercise was carried out broadly in accord- ance with I S 0 IS 5725.13 The exercise was not restricted to laboratories using the instrumentation specified in the DIS i.e. CF- or FI-HG-AAS. Some laboratories were asked to adapt the procedures with the minimum modification using HG-AFS HG-ICP-AES or discrete batch (DISCRT) HG-AAS. All laboratories were asked to complete a question- naire to obtain details of their instrumentation and comments on the comprehensibility and ease of use of the DIS. Test samples included filters dosed with known levels of arsenic and those generated from an artificially generated atmosphere of particulate arsenic(II1)oxide. Blank filters were also supplied.Laboratories were asked to analyse the sample solution obtained for each test filter three times and to report blank corrected replicate results. A mean result for each test sample was obtained from the replicate results and from this the percentage of the expected result was calculated. In order that statistics for the accuracy of the method should not be affected by preferential selection of results participants were cautioned that although the levels of the arsenic on the different batches of filters might appear similar they should not assume that they were the same. However in fact four identical samples dosed with arsenic at ‘US’ and ‘UK’ levels were provided to each laboratory. Dosed membrane3lter-paper pads (sets ‘UK’ and ‘US’) The test samples were produced from treated membrane filter-paper pads which were prepared as a number of batches identified by letters.They were dosed with arsenic at levels that simulated sampling at the UK MEL (approximately 50 pg on 25 mm membrane pads) and US PEL (approximately 5 pg on 37mm membrane pads). Two separate mailings of dosed filters at each level were prepared and sent out together with blank filters from the same batches. As far as possible the concentration dosed on the filters of all batches was the same. The volume of arsenic solution spotted onto the filters was measured carefully compensating for small evaporation losses by using the procedure given in BS7653 and determined to a 2 x RSD (relative standard deviation) error of < 1%. The ‘expected’ result was based on the certificated concentration of the standard used to prepare the dosing solution and the accurately determined volume dispensed by the micropipette used for the dosing.No significant difference between the levels spotted onto different batches of filters was noted for either the US batches or the UK sets of treated filter-pads. Inspection of the results obtained by ourselves and from the other laboratories in the exercise justifies the use of these values. The level of arsenic in the blank filters was demonstrated to be very low (<0.02 pg) for all batches. Test air samples from an atmosphere of arsenic(m)oxide (set Test air samples were prepared using cassette-type air samplers containing 25 mm treated membrane filters and paper pads at the laboratory of the National Institute of Occupational Health in Oslo Norway under the direction of Dr.Y. Thomassen. An atmosphere of arsenic(1n)oxide was generated and the air was drawn through 80 samplers simultaneously. It was impossible to control exactly the level of arsenic in the air but the collection of 5-10 pg in each sampler was aimed at. Air drawn through individual samplers was restricted by a critical orifice and the sampling rate was measured carefully. The amount of arsenic in individual samplers should vary with the measured flow rate. A number of the samplers were analysed at OMHL to establish that this was the case and to obtain a factor by which the flow rates through individual samplers could be multiplied in order to calculate ‘expected’ results for arsenic “N W”) collected.Blank filter cassettes were also provided. Several were analysed at OMHL and in all cases the blank was <0.02 pg of arsenic. Field Sampling During the final stages of validation it is intended to test the analytical procedures on samples that have been collected during on-site sampling in workplaces where arsenic is present in the air. Results and Discussion The detailed results of the method validation will be presented in a back-up report.14 The following is a summary of the conclusions so far. Method Performance Bias and recovery Laboratory experiments indicate that the method does not exhibit significant bias. The mean recovery for doped filters in the range 4.8-96 pg of arsenic has been determined to be 100.7% for a sample solution aliquot of 0.1 ml and 98.8% for a sample solution aliquot of 1 ml using CF-HG-AAS; and 99.3% for a sample solution aliquot of 0.1 ml and 102.7% for a sample solution aliquot of 1 ml using FI-HG-AAS.Repeatability The component of the coefficient of variation of the method that arises from analytical variability CV(analysis) is depen- dent upon a number of factors including the volume of the sample solution aliquot used in preparation of the test solution and whether CF- or FI-HG-AAS is used. The CV(ana1ysis) is at a minimum when the concentration of arsenic in the test solution is in the mid-range of the calibration and in laboratory experiments it has been estimated to be about 1% using CF-HG-AAS and about 3% using FI-HG-AAS for measure- ments made at 197.2 nm on test solutions with an arsenic concentration in the range 10-40 ng ml-’.This gives a measure of the repeatability of the method. (The repeatability of an AA method15 at a given level is the closeness of agreement between successive results obtained using the same method on identical material submitted for the test under the same conditions i.e. same operator same equipment same set of reagents same laboratory.) The over-all uncertainty of the method as defined by the Comite European de Normalization ( CEN),I6 has been shown to be within the specification of 30% prescribed by CEN7 for the over-all uncertainty of measurements for comparison with limit values assuming that the coefficient of variation of the method that arises from inter-specimen sampler variability CV(inter) is negligible and that the coefficient of variation of the method that arises from pump flow rate variability CV(flow) is limited to 5%.Detection limit and working range Detection limits for the determination of arsenic are dependent upon the analytical line at which absorbance measurements are made and upon the HG system and AA spectrometer used. However the qualitative and quantitative instrumental detec- tion limits for arsenic defined as three times and ten times the standard deviation of a blank determination have been esti- mated to be approximately 0.3 and 1 ng ml-’ respectively for the 197.2 nm arsenic line.14 For an air sample volume of 960 1 and a sample solution aliquot of 5 ml this corresponds to arsenic in air concentrations of 0.015 and 0.05 pg m-3 respect- ively.The working range of the method is approximately 100 ng to 125 pg of arsenic per sample for absorbance measure-Table 1 Results (%) for IS0 interlaboratory exercise on determining arsenic ‘UK dosed treated 25 mm test filter-pads. Expected result 50.54 pg of As. The significance of the statistics given is explained in the text Laboratory identification number h 4 00 36 CF AAS 3 CF AAS 4 CF AAS 14 CF AAS 18 24 CF CF AAS AAS 26 30 35 37 20 22 1 15 21 29 7 16 CF CF CF FI FI FI DISCRT DISCRT CF CF FI CF AAS AAS AAS AAS AAS AAS AAS AAS AFS AFS ICP ICP 8 CF AAS Hydride generator Instrument Mean result obtained from 3 analyses expressed as a percentage of expected result Filter 1 (%) Filter 2 (YO) Filter 3 (%) Filter 4 (O//.) Mean result for batch of filters (YO) Mean filter blank/pg of As CV of mean results for filters (%) CV of single replicate analysis (Yo) Average CV of three analyses (%) 98.02 100.75 99.60 99.3 1 99.42 0.01 1.13 1.14 1.28 53.59* 106.54 108.36 108.99 107.96 1.9 1.18 1.23 3.19 98.60 101.57 104.37 102.23 101.69 0.50 2.34 2.52 3.32 112.45 105.53 102.89 104.21 106.27 0 4.01 5.14 5.27 106.57 100.91 103.69 104.26 103.86 0.28 2.24 2.32 0.80 102.29 104.68 105.41 106.83 101.81 110.98 102.30 110.08 102.95 108.14 0 0.04 1.61 2.70 1.87 2.88 1.03 2.24 98.48 99.20 102.05 96.87 99.15 6.36 2.18 3.55 2.95 99.86 101.15 99.23 98.58 99.70 0.33 1.10 1.38 0.94 100.13 100.67 100.13 96.65 99.40 1.08 1.86 1.96 2.63 98.08 99.29 100.63 99.32 99.33 0.0 1 1.05 2.20 2.04 71.14 75.75 75.03 70.74 73.16 3.54 3.73 1.35 - 0.4 17.23 18.72 16.57 16.41 17.23 0.043 6.10 6.29 2.24 102.05 33.95* 93.35 121.00 105.47 -0.55 13.41 15.78 9.49 - 94.90 102.30 93.25 94.52 93.25 100.63 80.13 99.15 93.80 0 0 4.13 1.02 1.46 0.88 1.34 - 97.41 100.35 100.38 101.80 99.99 0.45 1.84 1.92 0.85 79.49 72.27 87.21 87.90 81.72 9.01 9.02 0.92 - 92.80 92.81 93.95 92.44 93.00 0.57 0.71 1.43 2.33 * Figure not included in statistical analysis.Table 2 Results for the IS0 interlaboratory exercise for determining arsenic ‘US dosed treated 37 mm test filter-pads. Expected result 5.02 pg of As. The significance of the statistics given is explained in the text Laboratory identification number 36 CF AAS 3 CF AAS 4 CF AAS 8 CF AAS 14 CF AAS 18 CF AAS 24 CF AAS ~~ 26 CF AAS _ _ _ _ _ _ ~ ~ ~ 1 15 DISCRT DISCRT AAS AAS 21 CF AFS 29 CF AFS 7 FI ICP 16 CF ICP 30 CF AAS 35 CF AAS 37 FI AAS 20 FI AAS 22 FI AAS Hydride generator Instrument Mean result obtained from 3 analyses expressed as a percentage of expected result Filter 1 (YO) Filter 2 (YO) Filter 3 (YO) Filter 4 (%) Mean result for batch of filters (Yo) Mean filter blank/pg of As CV of mean results for filters (YO) CV of single replicate analysis (%) Average CV of three analyses (%) 99.60 101.09 100.49 99.63 100.20 0 0.72 0.94 0.73 107.85 110.65 104.46 106.92 107.47 0.22 2.38 2.90 7.13 104.06 106.39 105.52 105.26 105.31 0 0.91 1.64 1.65 104.52 105.99 102.00 103.13 103.91 0 1.66 3.14 3.27 97.54 96.61 96.41 95.87 96.61 0 0.72 0.84 0.72 99.36 101.49 98.43 11 1.97 102.81 0 6.07 6.10 1.27 104.32 104.19 101.53 104.79 103.71 0.13 1.42 1.56 1.78 47.27 95.08 102.99 101.33 99.17 0.63 3.67 6.65 5.19 i02.00 102.13 101.26 100.53 101.48 0 0.73 1.78 1.65 98.21 99.54 98.80 96.61 98.29 0.11 1.26 1.73 1.25 95.88 101.86 99.73 98.87 99.09 -0.01 2.50 3.40 2.82 T i 71 l I .l L . 73.32 72.32 73.99 72.84 - 0.05 1.39 1.63 0.88 135.26 102.46 49.10* 85.96 97.89 0.04 10.65 10.69 1.64 73.78 72.95 123.79 74.87 87.85 27.47 29.46 12.74 - 1.24 93.51 97.01 92.91 96.91 95.08 0 2.29 1.75 - 101.w 98.74 102.73 100.00 100.62 0 1.67 1.74 0.51 86.96 91.95 86.49 91.15 89.14 0.56 3.15 3.24 1.32 93-08 95.48 88.16 90.75 91.87 0.02 3.41 3.54 2.14 110.58 117.55 107.46 112.57 112.04 -0.12 3.78 4.65 5.70 c1 F % M * Figure not included in statistical analysis. Table 3 Results of IS0 interlaboratory exercise on determining arsenic ‘NW test samplers.Arsenic oxide sampled onto treated 25 mm membrane-pads. The significance of the statistics given is explained in the text Laboratory identification number 36 CF AAS 3 CF AAS 4 CF AAS 8 CF AAS 14 CF AAS 18 CF AAS 24 CF AAS 26 CF AAS 30 CF AAS ~ 37 FI AAS 20 FI AAS 22 FI AAS 1 15 21 DISCRT DISCRT CF AAS AAS AFS 29 CF AFS 7 FI ICP 16 CF ICP 35 CF AAS Hydride generator Instrument Sampler No. 1 - Expected value/pg of As Mean result of 3 analyses/pg of As % of expected result CV of replicate analyses (Yo) Expected value/pg of As Mean result of 3 analyses/pg of As % of expected result CV of replicate analyses (%) Average of expected (YO) Mean filter blank (pg) Sampler No. 2- 6.86 7.24 105.55 0.52 7.11 6.85 96.3 1 13.95 7.1 1 7.3 1 102.85 0.72 7.04 6.90 98.10 1.09 6.93 7.54 108.73 0.33 7.00 5.17 73.88 1.91 6.90 6.61 95.90 1.77 7.18 6.79 94.64 0.47 7.04 7.36 104.59 1.73 6.86 7.03 102.46 0.49 7.07 3.17 44.87 8.84 6.93 7.07 7.21 4.41 5.57 6.79 63.62 78.76 94.14 17.40 2.79 1.11 6.83 5.50 80.62 2.00 7.1 1 4.76 67.02 1.70 7.42 7.75 104.34 3.37 7.11 4.28 60.22 9.41 L a Q P 6.93 7.14 102.97 0.16 104.26 0 7.04 7.48 106.25 0.78 101.28 0.11 7.00 4.83 68.93 2.50 67.98 - 7.04 7.3 1 103.93 0.82 103.39 0 7.11 7.18 100.97 1.19 104.85 6.36 7.07 7.18 101.48 0.99 101.48 0 7.07 5.95 84.13 1.18 79.01 0.13 7.04 6.75 95.92 3.06 95.91 0 7.11 6.70 94.27 1.22 94.46 0 7.11 7.28 102.43 1.98 103.51 0.11 7.28 5.35 73.50 10.14 66.86 - 0.05 6.86 - 7.25 3.73 - 6.84 54.32 - 94.32 9.14 - 0.89 58.97 78.76 94.23 -1.24 0 0 7.1 1 5.75 80.85 0.27 80.74 0.08 7.04 7.66 108.90 4.54 106.62 -0.12 7.18 6.98 97.17 1.52 97.64 0 6.93 6.94 100.12 2.65 101.29 0 7.04 5.96 84.65 0.42 64.76 -0.14 c 0 r QJOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 279 ments made at the 197.2nm arsenic line on test solutions prepared using sample solution aliquots in the range 5-0.1 ml. Interlaboratory Exercise Results The exercise is not at present complete but preliminary returns are encouraging. The results obtained for the analysis of the test filters by 17 laboratories which had provided results and comments on the procedure at the time of the CSI in York in July 1993 are given in Tables 1-3. A further one laboratory found itself unable to carry out the procedure using its equipment. To maintain confidentiality results are tabulated against laboratory identification numbers notified to the laboratories concerned.Results obtained by OMHL using FI-HG-AAS are given against laboratory 36 and using CF-HG-AAS are given against laboratory 37. Three different coefficients of variation (CVs) which have been calculated for the results are given in the tables. 1. CV of mean results. This was calculated from the mean results obtained by triplicate analysis by the laboratory for the test filters of the set. This result gives the best measure of variation which is owing to sample preparation rather than instrumental variability. 2. CVof single replicate analysis. This is an estimate of the CV which would have been obtained for the set had the instrumental analysis been carried out once for each test filter solution rather than three times.3. Average CVfor replicates. For this CVs between replicate analyses of the same test filter sample solutions were calculated then averaged for all the filters of the set. This has been calculated to estimate the variability between determinations which is due to instrumentation rather than to the sample preparation procedure. It is a little premature to carry out a detailed over-all breakdown of statistics. More laboratory results are expected and the origin of some anomalous results needs to be investi- gated to establish whether they are genuine outliers or ‘rogue results’. In any case with so many different instruments involved the value of over-all statistics could be limited. However inspection of results on a laboratory by laboratory basis is fairly instructive.Results obtained using CF- and FI-HG-AAS as specified in the proposed DIS have generally been close to those expected based upon the amounts of arsenic dosed onto the filters and the levels calculated for arsenic(m) oxide collected in the filter cassettes. The only useful statistics are those obtained for CF-HG- AAS since ten of the 19 laboratories reported using this technique. These laboratories also tended to be those determin- ing arsenic routinely on a daily basis and consequently were more experienced than other participants. They consistently obtained results within 10% of the expected result for dosed test filters. The CV has been calculated for each level of dosing in order to obtain a measure of the reproducibility of the method.(The reproducibility of an AA method at a given level1’ is the closeness of agreement between individual results obtained using the same method on identical material submit- ted for the test but under different conditions i.e. different operators different equipment different laboratories different times.) For measurements made at 197.2nm using CF-HG- AAS the reproducibility determined from all mean filter results was found to be 3.7% for 37 mm filter-pads dosed with 5 pg of arsenic and 3.4% for 25 mm filter-pads dosed with 50 pg of arsenic. The analytical recoveries obtained for 5 and 50 mg were 101.9 and 102.9% respectively. Results obtained for samplers were expected to be less consistent since it was anticipated that there would be some variability in the collec- tion of arsenic(II1)oxide in the samplers and because of possible failure to recover all arsenic from the internal surfaces of the samplers.In practice however results were little different to those obtained for dosed filters. A reproducibility of 4.3% and a mean recovery of 100.7% was determined from the results obtained for all samplers analysed using CF-HG-AAS (except those from laboratory 24). It is interesting to note that the method was capable of adaptation by laboratories using the allied techniques of HG-AFS HG-ICP-AES and discrete batch HG-AAS but the numbers of sets of results were too small to yield statistical analysis (only 2-3 for each analytical tech- nique). It is evident that data obtained using these techniques feature more anomalous results than those for CF-HG-AAS.This probably reflects the fact that these laboratories were not in general involved in the day-to-day routine determination of arsenic and were commonly operating their instrumentation in an unfamiliar way. Even so it is clear that it is possible to obtain the expected result for all of these techniques. Overall a wide range of instruments was used by participants in the interlaboratory exercise all of which seem to perform well with the method including systems manufactured by Perkin-Elmer Philips/Unicam PS Analytical and Varian. Close to 100% of the expected values can be obtained using CF- FI- or discrete batch HG-AAS or HG-ICP-AES or HG-AFS. Coefficients of variation for the mean results obtained for a set of filters by a laboratory depend upon the technique used but are commonly in the range of 1-4% whether determined for single replicate analysis or triplicate analysis. The CVs between replicate determination of the same sample solution again depend upon technique but are typically less than 3%.Feedback from participants on the use of the method has been instructive. Most analysts reported finding the method easy to understand and to carry out. Some minor changes to the wording and procedures of standard are indicated however. For instance it is clear that the use of a zero standard in the calibration must be stressed as must the need to include the same concentration of acids in the standards as is present in the sample solutions.Sulfuric acid concentration is particularly influential since although only 2% v/v is present in the sample and standard solutions this causes the AA output signal/arsenic concentration to be significantly lowered. Conclusions Conclusions about the effectiveness of proposed DIS from the investigations carried out so far indicate that fairly small changes need to be made to the current draft of the I S 0 standard before it is in a state to merit publication. Postscript Following the preparation of this paper the drafts of the MDHS and I S 0 standards have been modified slightly so as to no longer require that membrane filters used to collect fume should be treated with sodium carbonate solution. The change has been made because sodium carbonate-treated cellulose ester membrane filters were found to become friable after several weeks storage.Untreated 0.8 pm pore size cellulose ester membrane filters with sodium carbonate-treated paper back-up pads were demonstrated to be effective in collecting > 98% of arsenic when sampling workplace air containing mixed particulate and arsenic(rr1)oxide vapour fume. l4 References 1 Health and Safety Executive Toxicity Review 16 Inorganic Arsenic Compounds HMSO London 1986. 2 World Health Organisation (WHO) or International Agency For Research On Cancer (IARC) JARC Monographs on the Eualuation of the Carcinogenic Risk of Chemicals to Humans Volume 23- Some Metals and Metallic Compounds IARC Lyons 1980. 3 US National Institute for Occupational Safety and Health NIOSH Manual of Analytical Methods Method 7900-Arsenic and Compounds (Revision I 1987) US Government Printing280 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 4 5 6 7 8 9 10 11 Office Washington DC 3rd. edn. 1984 DHHS Publication US National Institute for Occupational Safety and Health NIOSH Manual of Analytical Methods Method 7300-Elements (ICP) US Government Printing Office Washington DC 3rd. edn. 1984 DHHS Publication No. 84-100. US Occupational Safety and Health Administration OSHA Analytical Methods Manual Method I D 105 Inorganic Arsenic in Workplace Air USDOL/OSHA Salt Lake City 2nd. edn. 1991. US National Institute for Occupational Safety and Health NIOSH Manual of Analytical Methods Method 7901 -Arsenic Trioxide US Government Printing Office Washington DC 3rd. edn. 1984 DHHS Publication No. 84-100. Health and Safety Executive MDHS 41 Arsenic and Inorganic Compounds of Arsenic in Air HMSO London 1989. Demange M. Vien I. Hecht G. and Hery M. Mise au point d’une method de prelevement du trioxyde de diarsenic (Development of a method for sampling arsenic trioxide) Institut National de Recherche et de Securite Vandoeuvre 1992 ND 1872-146-92. Costello R. J. Eller P. M. and Delon Hull R. Am. Ind. Hyg. Assoc. J. 1983 44 21. Bax D. Agterdenbos J. Worrell E. and Beneken Kolmer J. Spectrochim. Acta Part B 1988 43 1349. Welz B. and Schubert-Jacobs M. J. Anal. At. Spectrom. 1986 1 23. NO. 84-100. 112 Anderson R. K. Thompson M. and Culbard E. Analyst 1986 111 1143. I3 International Organisation for Standardisation (TSO) International Standard 5725 Precision of Test Methods- Determination of Repeatability and Reproducibility for a Standard Test Method by Inter-laboratory Tests ISO Geneva 2nd. edn. 1986. 14 Foster R. D. and Howe A. M. Development and Validation of a Method for the Determination of Arsenic and Inorganic Compounds of Arsenic in Air (Excluding Arsine) Using Hydride Generation Atomic Absorption Spectrometry Analysis OMHL HSE Sheffield 1993 IR/L/IS/93/05. 15 International Organisation for Standardisation (ISO) International Standard 6955 Precision of Test Methods- Determination of Repeatability and Reproducibility for a Standard Test Method by Inter-laboratory Tests ISO Geneva 1982. 16 ComitC Europten de Normalization (CEN) Workplace Atmospheres-General Requirements for the Performance of Procedures for the Measurement of Chemical Agents CEN Brussels 1992 prEN 482. Paper 3/05445D Received September 10 1993 Accepted October 15 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900273

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 28 1 Progress With the Speciation of Aluminium and Silicon in Serum of Chronic Renal Patients Using Atomic Spectroscopic Techniques* K. Wrobel,t E. Blanco Gonzalez and A. Sanz-MedelS Department of Physical and Analytical chemistry University of Owiedo clJulian Cla weria 8 33006 Oviedo Spain In order to provide further evidence on the possible correlation between aluminium and silicon levels in the serum of renal failure patients and the possibility of the reduction of aluminium bioavailability by the presence of silicon in biological fluids the effects of different factors including storage conditions administration of desferrioxamine (DFO) and kidney transplantation on total aluminium and silicon contents and on their distribution in the same serum samples were examined and compared.Ultramicrofiltration was used for the separation of low molecular weight (LMW) and high molecular weight (HMW) serum fractions and electrother- mal atomic absorption spectrometry (ETAAS) for the determination of both elements. Consistent results were obtained showing that the distribution of aluminium between LMW and HMW serum fractions is a constant factor in the absence of DFO. It was observed that 11 52O/0 of the total aluminium in serum is ultrafiltrable and this value does not seem to be influenced by the total serum elemental concentration storage conditions particular renal pathology of the patients or kidney transplantation. However kidney transplantation induces a ‘clear-up’ of serum aluminium and silicon which is easily observable after a few months.Administration of DFO alters the speciation of aluminium by increasing its relative content in the LMW fraction up to 75&6% of the total element concentration in serum. Conversely distribution of silicon in serum proved to be affected only by the storage conditions. If the sample is stored properly (the pH maintained below 7.8) the ultrafiltrable silicon content results were consistent and reproducible (43ZfI3O/0 of total serum silicon in the LMW fraction was found to be ultrafiltrable). In any case silicon binding to serum proteins must be different to that observed for aluminium (which is mostly bound to transferrin). Moreover the observed distribution of aluminium between LMW and HMW serum fractions was neither related to silicon total concentration nor to the distribution of silicon in serum.Keywords Aluminium; silicon; speciation; ultramicrofiltration; electrothermal atomic absorption spectrometry Strong evidence exists for the toxicity of aluminium to renal failure patients.’ The aluminium body burden is mainly associ- ated with osteodystrophy microcytic anaemia and encepha- l~pathy.’.~ However the mechanism(s) of this toxicity remains ~nknown.~ It is clear that aluminium bioavailability and hence toxicity depends on the physicochemical form of the element as does its transportation within the body fluids and to the tissues where the toxic effects can be ob~erved.~ It has recently been suggested that silicon can reduce the bioavail- ability of aluminium by forming hydroxyaluminosilicates under physiological condition^.^,' Speciation studies of aluminium and silicon together in body fluids are therefore of fundamental importance to provide useful information on the possible interactions between both elements in a physiological environ- ment.The distribution of aluminium between low molecular weight (LMW) and high molecular weight (HMW) serum fractions has been studied in this laboratory using conventional ultrafiltration’ and ultramicrofiltration The dis- tribution of silicon was investigated by ultramicrofiltration and gel filtration.’ Reported results’ indicate that the presence of the main serum proteins (transferrin and albumin) affects the speciation of silicon in model solutions and also in uremic serum samples even if the character of the protein-silicon interaction seems to be rather different (less specific) than that observed for aluminium’ (which is chemically bound to transferrin4).The aim of the present work was to provide further exper- imental evidence of the possible relationship between the distribution of silicon and of aluminium in serum. Therefore the partition of both elements between LMW and HMW ~~~~ ~ ~ * Presented at the XXVIII Colloquium Spectroscopicum t On leave from Institute of Chemistry University of Warsaw $ To whom correspondence should be addressed. Internationale (CSI) York UK June 29-July 4 1993. Bialystok Branch Poland. serum fractions in the same samples was studied using ultramic- rofiltration as a separation technique [chosen for its simplicity small sample size requirements ( 1 ml) and easy control of ~ontamination’~] and electrothermal atomic absorption spectrometry (ETAAS) was used as a specific detector for aluminium and silicon in determinations in serum and in the ultrafiltrable fraction^.'^^'^ The influence of some important factors including sample storage previous administration of desferrioxamine (DFO) to patients and kidney transplantation on silicon and aluminium distribution (speciation) in serum was investigated.The results obtained from all these experi- ments are given and discussed mainly from the point of view of likely (or otherwise) interactions between silicon and alu- minium that could reduce the bioavailability of aluminium (i.e. its LMW fraction).Experimental Apparatus The ETAAS determination of aluminium and silicon concen- trations in the serum samples and their ultrafiltrable fractions was carried out using a Model 3030 Perkin-Elmer atomic absorption spectrometer with a Model HGA-500 graphite furnace equipped with an autosampler Model AS-40 and a PR-100 printer. Ultramicrofiltration experiments were carried out using the Amicon micropartition system (MPS-l) which has been described elsewhere,’ fitted with Amicon YMT membranes (nominal cut-off 30 000 Da). All the instrumentation was housed in a clean room equipped with a filtered laminar air supply to avoid sample contami- nation from aluminium and silicates in dust. Reagents All chemicals used in this work were of analytical-reagent grade. Stock solutions containing 1.000 g 1-’ of silicon and282 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 aluminium were provided by Merck. Intermediate standard solutions of 100mgl-l of the element were prepared by diluting 10ml of the appropriate stock solutions with dilute (1 + 20) nitric acid to 100 ml. Working standard solutions were prepared daily by appropriate dilution of the intermediate solution with Milli-Q water (Millipore). Clinical Samples A serum pool from persons with normal renal function and serum samples from seven patients with end-stage renal failure on regular haemodialysis therapy were provided by the Hospital Central de Asturias in Oviedo Spain. In this latter case samples were always collected before the corresponding dialysis session.During this session DFO (mg kg-' of body mass) was administered and then for each patient 48 h later (before the next dialysis session) the blood was sampled again. Serum samples from 12 renal failure patients initially col- lected before kidney transplantation and then collected again 1 month and 6 months after the surgical operation were provided by the Hospital Universitario 'Marques de Valdecilla' de Santander in Santander Spain. All patients had their kidney function normalized after the first month from the transplantation (urinary creatinine level < 2 g per day). The normal serum pool used as a reference was spiked with aluminium and silicon aqueous standards up to final total elemental contents of 40 pg 1-l and 0.40 mg l-l respectively. For the experiments on the influence of storage conditions and pH of the sample on the partitioning of silicon and aluminium in serum the pH was adjusted with 1 mol I-' sodium hydroxide solution (Merck).Procedures Risks of sample contamination with external aluminium and silicon during sampling transport storage fractionation and determination steps were minimized following the detailed procedures described in previous ~ o r k . ~ ~ ~ * ~ ~ ~ ~ Samples were collected in polystyrene tubes that had been treated with 10% v/v nitric acid for 24 h before use. All components of the MPS-1 ultramicrofiltration system were also soaked for 24 h in 10% v/v nitric acid and rinsed with Milli-Q water before use. The YMT membranes were washed by ultrafiltering twice with 1 ml of 0.1 mol 1-I sodium hydroxide solution and then with ultrapure water until the washings were 'free' from aluminium and silicon (as established by ETAAS).The operating conditions for ETAAS determination of alu- minium and silicon in serum and ultrafiltrates were the same as described previ~usly.'~.~~ Serum samples were diluted with Milli-Q water 1 + 1 and 1 + 3 for total aluminium and silicon determinations respectively. No pre-treatment was necessary for ultrafiltrable serum fractions and in all cases calibration was accomplished by the corresponding standard aqueous solutions of aluminium and silicon. For the separation of LMW and HMW serum fractions 1 ml aliquots of the serum samples were introduced into the ultramicrofiltration cell and then centrifuged (1800g) for 15 min at room temperature.Results and Discussion The procedures described above were applied to an investi- gation of the total aluminium and silicon serum contents in different situations and their distribution (speciation) between the LMW and HMW serum fractions in the same samples under various conditions. The observed influence of serum storage conditions previous administration of DFO to patients and kidney transplantation is presented in the following sections. Influence of Serum Storage Conditions on Ultrafiltrable Serum Aluminium and Silicon Preliminary experimentsg on the distribution of aluminium and silicon were carried out using aged uremic serum samples stored for a long time (more than 3 months) in a refrigerator. It was found that results for the distribution of silicon were rather scattered 12-50% of the total concentration of silicon was present in the ultrafiltrable fraction depending on the sample analysed. Conversely the ultrafiltrable fraction of serum aluminium in the same aged samples ranged from 9 to 13% (mean 11 +2%).The pH was measured in all analysed serum samples and it varied between 7.5 and 8.6. It was observed that the lowest values of silicon in the ultrafiltrable fraction of those serum samples always occurred for pH values above 8. In order to investigate this point further a fresh normal serum pool (adequately spiked to obtain final total concentrations of 40 pg 1-l of aluminium and 0.40 mg I-' of silicon) was prepared and divided into five aliquots which were stored under con- trolled but differing conditions as indicated in Table 1.It was verified that additions of spikes (1-8 pl) did not change the pH of the serum pool (20ml). The distribution of aluminium and silicon in those sera aliquots were then studied following procedures given previously. The results have been summarized in Table 1 which shows that virtually the same relative distri- bution of aluminium was observed for any of the sample storage conditions tested (11+2% of the total serum alu- minium is present in the ultrafiltrate). The distribution of silicon in the fresh and in adequately frozen serum aliquots was virtually the same [ultrafiltrable silicon around 43% of the total concentration of silicon in the sample (Table l)]. However if samples are not frozen the ultrafiltrable fraction of serum silicon decreased to 40% in samples stored at 4°C for 24 h and the level dropped to around 17% for samples stored at room temperature for 24 h (Table 1). It was observed that unfrozen samples at room temperature had elevated pH values (pH>8 Table 1) probably owing to some loss of the C02 content.Finally when the pH of fresh serum samples was artificially increased with dilute sodium hydroxide solution to pH 8.6 a low value of silicon content in the LMW fraction of serum (ultrafiltrable fraction 18_+2%) was also found while the distribution of aluminium remained virtually unchanged as is shown in Table 1. Influence of Administration of DFO on Total and Ultrafiltrable Serum Aluminium and Silicon Levels Desferrioxamine (DFO) is a drug presently used to reduce the total body burden of aluminium in renal failure patients,l4 for aluminium de-toxification. The administration of this drug helps to clear up other trace elements from the body of such patients as has been shown recently for c0ba1t.l~ Therefore the effect of administration of DFO on the total elemental concentrations of silicon and aluminium in serum and on the partioning of these elements between LMW and HMW serum Table1 Influence of sample storage conditions on sample pH and on ultrafiltrable aluminium and silicon in normal spiked serum pool ~ ~~~~ Amount ultrafiltrable (%) Storage condition n pH Aluminium Silicon Stored (- 20°C) Fresh sample 5 7.4-7.6 11.6 1.9 46.5 f 1.8 up to 3 months 5 7.5-7.8 11.9 f 2.3 43.3 1.3 Stored (4°C) 24 h 5 7.6-7.8 11.2_+ 1.4 41.1 k0.3 Stored (25°C) 24 h 5 8.0-8.6 12.6 & 2.3 16.5 1.5 Fresh sample + NaOH* 5 8.6 12.2& 1.8 18.0+ 1.5 MeankSD 11.9L0.5 33.1A14.6 *pH was artificially adjusted to pH = 8.6.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 283 fractions was investigated. To do so ultramicrofiltration experi- ments on seven different uremic serum samples from patients who were undergoing de-toxification therapy with DFO (mg kg-' of body mass) was undertaken. The results observed for all these experiments are summarized in Fig. 1 (a) and (b). As shown in Fig. l(a) 48 h after DFO infusion both total and ultrafiltrable aluminium levels in serum increased abruptly from a mean of 56 k 30 to a mean of 170 + 65 pg 1-l and from 7.3 k 3.1 to 107 & 50 yg l-' respectively.As expected,'v'' the relative amount of aluminium in the ultrafiltrate increased more than the total aluminium indicating that the LMW content of aluminium represents up to 74.9&6.3% of the total serum aluminium concentration in DFO treated sera.','' In the case of silicon the total element serum concentration in the patients was found to be 0.95 k0.65 mg 1-l before administration of DFO and 0.97f0.60 mg 1-l 48 h after administration [Fig. l(b)]. Thus it appears that the total silicon content in serum was not affected by the DFO therapy. Moreover in Fig. l(b) it is shown that the LMW or ultrafiltr- able fractions contained 43.3 f 1.3% of the total serum silicon before and 44.2+4.7% after administration of DFO. Thus the conclusion to be drawn is that distribution of silicon in serum is also unaffected by DFO therapy.It should be mentioned that the results presented in Fig. 1 were obtained for samples stored at -20°C at pH<7.8. In parallel with the above experiments some unfrozen samples (pH > 8) were also tested and lower silicon contents were found in the ultrafiltrable serum fraction (about 20% of total silicon serum concen- tration) supporting previous findings on the effect of age and pH of sera samples (storage conditions). Influence of Kidney Transplantation on Total and Ultrafiltrable Serum Aluminium and Silicon Levels Serum samples were collected from 12 uremic patients before and after kidney transplantation. These samples were analysed for total and ultrafiltrable aluminium and silicon following the previously described procedures in order to evaluate the effect of kidney transplantation on the total content and distribution of silicon and aluminium in serum.The results obtained have been plotted in Fig. 2. As shown by Fig. 2(a) the total serum aluminium content decreased from a mean of 55.4f4.2 before the transplantation to 35.8+2.6pgl-' one month later and to 19.9k9.5 pg1-' six months after the transplantation. Similar decreases were observed for the aluminium content in the ultrafiltrable sera fraction of the patients undergoing operation (mean values 8.1k4.5 4.8k2.6 and 3.2f0.3 pgl-' respectively) [Fig. 2(b)]. Therefore the percentage of ultrafiltrable or LMW aluminium remained virtually unchanged during the whole period of time after transplantation that was investigated (before surgical operation 11.2+ 1.3 12.3 +2.7% after one month from trans- plantation and 12.3 f 1.8% after six months from the transplan- tation) [Fig.2(a)]. Considering the silicon contents it can be seen from Fig. 2(b) that the total concentration in the same serum samples decreased from 1.30k0.60 mg 1-' before the operation to 1.10+0.50 mg 1-1 one month and to 0.73f0.21 mgl-I six months after transplantation. As regards the experiments on silicon distribution unfortunately the serum samples were transported unfrozen from Santander to Oviedo (about a 12 h journey). Therefore the distribution of silicon was probably altered with respect to that actually existing in the fresh sera of the patients. Fairly elevated pH values (pH > 8) were found in all of these samples.This would explain why the relatively ultrafiltrable silicon content was 18.1 4.9 before operation 19.0 f 5.6 one month afterwards and 20.8 & 2.8% six months after operation that is the type of values observed are consist- ent for 'aged' serum samples (see in Table 1 the samples stored for 24 h at 25 "C) Conclusions It has been shown that the total aluminium and silicon contents in serum and the distribution of these elements between LMW and HMW serum fractions are affected in different ways by 200 c I - o 150 I s 0 E 100 00 3 50 n C A Pre-DFO Post-DFO 80 Pre-DFO Post-DFO Fig.1 Influence of administration of DFO on A total and B ultrafiltrable levels of (a) aluminium and (b) silicon in serum; C shows the ultrafiltrable percent of the element.Uremic serum samples 7.4 < pH < 7.6 - 01 a 3 40 a 0 4- 3 20 0 Before After After transplantation 1 month 6 months 50 (bl Before After After transplantation 1 month 6 months Fig.2 Influence of kidney transplantation on A total and B ultrafiltrable levels of (a) aluminium and (b) silicon; C shows the ultrafiltrable percent of the element. Serum pH> 8284 JOURNAL OF ANALYTlCAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 the factors studied here indicating different speciation mechan- isms for both elements. While as expected,8.'0g16 significant increases in the total aluminium serum content were observed after administration of DFO (from 56&30 to 170+65 pg I-' which is in agreement with earlier reports on mobilization of aluminium from body deposits owing to the formation of a stable Al-DFO com- plexs*10*'6) the observed total silicon serum level was not affected by treatment with DFO.After kidney transplantation with normalization of kidney functions the physiological excretion begins and indeed total aluminium and silicon levels in serum samples have been demonstrated to decrease (from 55.4 + 4.2 before the operation to 19.9k9.5 pg1-I six months after the operation for alu- minium and from 1.30&0.60 to 0.73 k0.21 mg 1-' for silicon in sera). The relative distribution of aluminium between LMW and HMW serum fractions in the absence of DFO was found to be a constant factor:" approximately 11+2% of the total aluminium is ultrafiltrable. This partitioning does not seem to be influenced by the storage conditions of the sample the sample ageing the total serum aluminium concentration the silicon level the particular renal pathology of the patient or kidney transplantation.After administration of the DFO both the total concentration of aluminium and the relative content of the element in the ultrafiltrate increased [Fig. l(a)] mainly because of the mobilization of aluminium from body deposits4 and also because of the formation of an ultrafiltrable A1-DFO complex (which has a molecular weight of 587 Da and hence should be present in the ultrafiltrable fraction of the serum) which should be formed slowly from the Al-transferrin c o m p l e ~ . ~ ~ ~ ~ ~ ' ~ Conversely the factor that seems mainly to influence silicon partition between LMW and HMW serum fractions is sample storage conditions.When the serum samples are stored prop- erly the ultrafiltrable silicon content results are consistent and fairly reproducible (ultrafiltrable silicon 43 & 3% of total silicon serum content) in a normal spiked serum pool and also in serum from renal patients before and after the administration of DFO. However the relative silicon content in the ultrafil- trates observed decreases to approximately 20+ 5% of the total silicon concentration if samples were stored without freezing. These observations seem to support earlier sugges- t i o n ~ ~ that silicon-serum protein interactions are less specific than those for aluminium and that they are probably more physical in nature (e.g. owing to adsorption of silicic acid on proteins). Finally in the light of these results and previous experimental evidence the silicon content in serum does not seem to be related to the aluminium content both total aluminium and silicon and their corresponding distribution between the LMW and HMW serum fractions do not seem to be inter-related.As observed with various serum samples containing very different silicon levels,' the ultrafiltrable aluminium fraction remained virtually constant irrespective of the silicon concentration (in other words a change of 'ultrafiltrable aluminium' fraction ,with increasing silicon concentration by forming aluminosil- icates at the physiological pH of was not observed here). The authors thank Dr. J. B. Cannata from the Unidad de Investigacion (Hospital Central de Asturias in Oviedo) and Dr.M. D. Fernandez Gonzalez from the Hospital Universitario 'Marques de Valdecilla' Santander for helpful suggestions and for providing the clinical samples. Financial support for this research from the Comision Interministerial de Ciencia y Tecnologia (CICYT) is gratefully acknowledged. K.W. thanks the Spanish Ministry of Education and Science for the provision of a post-doctorate grant. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Bertholf R. L. Wills M. R. and Savory J. in Handbook on Toxicity of Inorganic Compounds ed. Seiler H . G. Sigel H. and Sigel A. Marcel Dekker New York 1988 pp. 55-64. Bia M. J. Cooper K. Schnall S. Duffy S. Hendler E. Maluche H. and Solomon L. Kidney Int. 1989 36 852. Alfrey A. C. Legendre G.R. and Kaehny W. D. New Engl. J. Med. 1976 294 184. Sanz-Medel A. Fairman B. and Wrobel K. in Element Speciation in Bioinorganic Chemistry ed. Caroli S. Wiley New York in the press. Venturini M. and Berthon G. J. Inorg. Biochem. 1989 37 69. Birchall J . D. Chem. Br. 1990 26 141. Fahal I. H. Yaqoob M. McClelland P. Williams P. S. Roberts N. B. Birchall J. D. Ahmad R. and Bell G. M. poster presented at the 1993 Winter Conference on Plasma Spectrochemistry Granada Spain January 1993. Perez Parajon J. Blanco Gonzalez E. Cannata J. B. and Sanz- Medel A. Trends Elem. Med. 1989 6 41. Wrbbel K. Blanco Gonzalez E. and Sanz-Medel A. J . Anal. At. Spectrom. 1993 8 915. Wrobel K. Blanco Gonzalez E. and Sanz-Medel A. Trends Elem. Med. 1993 10 97. Sanz-Medel A. and Fairman B. Microchim. Acta 1992 109 7 . Sanz-Medel A. Rodriguez Roza R. Gonzalez Alonso R. Nova1 Vallina A. and Cannata J. B. J. Anal. At. Spectrom. 1987,2 177. Perez Parajon J. and Sanz-Medel A. J. Anal. At. Spectrom. 1994 9 111. Cannata J. B. Nefrologia 1992 XXII 295. Perez Parajon J. and Sanz-Medel A. Quim. Anal. 1993 12 30. Day J. P. in Aluminium and Other Trace Elements in Renal Disorders. ed. Taylor D. 1986 Bailliere Tindall London Garcia Alonso J. I. Lopez Garcia A. PCrez Parajbn J. Blanco Gonzalez E. Sanz-Medel A. and Cannata J. B. Clin. Chim. Acta 1990 189 69. pp. 184-192. Paper 31044 74B Received July 27 1993 Accepted October 21 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900281

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 28 5 Determination of Selenium and Arsenic in Mineral Waters With Hydride Generation Atomic Absorption Spectrometry* Marjan Veber Ksenija Cujes and Sergej GomiSCek Department of Chemistry and Chemical Technology University of Ljubljana P. 0.6. 537 Ljubljana SLO-67007 Slovenia A continuous flow hydride generation (HG) technique was applied for the determination of Se and As in highly mineralized mineral waters. The atomization was performed either conventionally in a heated silica tube or in the graphite furnace after collection and thermal decomposition of the hydrides. In most cases the determination of As in real samples was not subject to serious problems. Significant matrix effects were found only for mineral waters with higher Mg Na and sulfate concentrations.With the conventional flow through HG technique reasonably good linearity (r = 0.998) and satisfactory precision [relative standard deviation (RSD)<5%] were obtained in the concentration range from 1 to 50 pg I-'. For the determination of lower concentrations of As and Se in situ preconcentration in the graphite furnace was applied. Within 120 s of hydride introduction for As and 150 s for Se at a sample flow rate 0.5 ml min-' favourable preconcentration factors were obtained that yielded a limit of detection (30,) of 0.02 pg I-'. The precision of this procedure was satisfactory in the concentration range 0.5-10 pg I-' the RSD being 2-16% for both elements under investigation. Keywords Selenium; arsenic; electrothermal atomic absorption spectrometry; hydride generation; precon- centration; mineral water The determination of As and Se in mineral waters is important because of their toxicity additionally Se is also considered to be an important essential element.' The maximum concen- tration levels for drinking water recommended by the World Health Organization (WHO) are 10 pg 1- ' for Se and 50 pg 1- ' for As2 The concentrations in natural waters are usually much lower and therefore in order to obtain reliable quantitative determinations as well as in speciation studies sensitive analyt- ical techniques should be used.Electrothermal atomic absorption spectrometry (ETAAS) is commonly applied for the determination of both As and Se in water sample^.^-^ To eliminate interferences caused by the matrix and to improve the parameters of analytical procedures different preconcentration techniques are usually These techniques are typically time consuming and not very convenient for routine analysis therefore direct approaches are desirable.The hydride generation (HG) techniques have become well established for the routine determination of traces of Se and As in a variety of samples. The continuous HG generation systems have some advantages compared with batch procedures. Such systems especially when they are performed using the flow injection approach have produced very efficient procedures giving excellent precision with low sample and reagent consumption high speed and often fewer The technique also offers good possibilities for automation." In order to improve the sensitivity of the hydride technique to ng 1- ' levels different preconcentration procedures have been introduced.Whereas Van Cleuvenbergen et aL2* used a cold trapping technique which was applied for speciation studies of As in natural waters Tsalev and co-workers21,22 proposed a two-step analytical procedure for the determination of hydride forming elements. The first step was trapping the corresponding hydrides in a cerium(1v)-iodide absorbing solution and the second step direct determination by ETAAS. With this approach which was proposed also for the analysis of mineral waters limits of detection (LODs) in the sub-ng ml-' range were obtained. Recently in situ preconcentration of hydride forming elements in a graphite furnace has also been introduced. The technique which was originally proposed by Drasch et * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1993.has been successfully applied for the determination of hydride forming elements in various matrices at the ng ml-' level. Uthus et ~ 1 . ~ ~ used a similar approach for the determination of As in biological samples Lee25 applied the furnace hydride trapping technique for the determination of picogram amounts of Bi in environmental samples and Andreae26 determined inorganic Te species in natural waters. Sturgeon et al. further improved this technique moreover they have also studied the mechanisms for the sorption and atomization of metallic hydrides in the graphite furnace.27 They have applied a HG graphite furnace technique for the determination of As Sb and Se in sea-water as well as in marine tissues and sediment^.^^-^' Enhancement of the trapping efficiency was achieved with suitable modification of the graphite surface.It was proved that Pd serves as a good absorber for arsenic and selenium hydrides in the graphite furnace therefore the addition of Pd to the graphite tube enables high trapping efficiency to be In the present investigation the continuous flow HG tech- nique was applied for the determination of Se and As in highly mineralized mineral waters. The atomization was performed either conventionally in a heated silica tube or in the graphite furnace after collection and thermal decomposition of the hydrides. Experimental Instrumentation The atomic absorption measurements were performed by using a Perkin-Elmer Model 2280 spectrometer with a Model HGA- 400 graphite furnace and a Model 561 chart recorder was used to follow the analyte absorbances.Arsenic and Se elec- trodeless discharge lamps (Perkin-Elmer) operating at 5 W were used as sources Arsenic and Se absorbances were meas- ured at 193.7 and 196.0nm respectively with the spectral bandpass set at 0.7 nm. The HG system which is shown schematically in Fig. 1 was built in the laboratory. A four channel peristaltic pump Ismatec SA Model MS-reglo with Tygon tubing was employed. Flow rates were controlled either with the pump controls or by the selection of tubings with different diameters. Reactor coil (length = 100 or 75 cm 1 mm i.d.) and gas-liquid separ- ators were made of Pyrex glass (Fig.2). Two types of gas-liquid separators were used for different sample and carrier gas flow rates.286 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 To the atomizer (graphite furnace) NaBH Gas-liquid separator Mixing coil rn To waste Fig. 1 Schematic diagram of the hydride generation system (a) To the quartz cell t 30 I Drain I "r b I inlet To the furnace t u Fig. 2 Gas-liquid separators used for (a) the continuous flow hydride generation system; and (b) in situ preconcentration of hydrides in the graphite furnace. All dimensions in mm Reagents Standard solutions Selenium standard solutions (1.000 g 1-I) were prepared by dissolving of 0.2190 g of Na,SeO ( Se") or 0.1320 g of Na,SeO (Se'') in 20 ml of de-ionized water (Millipore Milli-Q) to which 1.0ml of concentrated HCl was added and diluted to 100 ml with de-ionized water.Arsenic standard solutions (1.000 g 1-I) were prepared by dissolving 0.1320 g of As,03 (As"') or 0.4816 g of As205.5H20 (As') in 2 ml of 20% NaOH solution. The solutions were acidified with HCl to a final pH of 2 and diluted to 100ml with de-ionized water. Working solutions for both elements at lower concentrations were prepared daily by appropriate dilution of stock solutions. Other reagents 1 YO m/v solutions of NaBH (Merck) in 1 YO NaOH a 20% KI solution and a 10% solution of ascorbic acid were prepared by dissolving pro analysi grade reagents (Merck) in de-ionized water. Palladium solution was prepared by dissolving PdCl in HN03 and diluted with de-ionized water.Procedures Sampling SampIes of mineral waters were stored in polyethylene vessels after filtration through 0.45 pm membrane filters and acidified with HCl (pH 2) at 4 "C. Continuous flow determination of As and Se For the determination of As an appropriate volume of sample 115-25 ml) was measured into a 50 ml calibrated flask As was reduced to the As"' state with the addition of 4 ml of concen- trated HC1 1 ml of 1% ascorbic acid and 1 ml of 20% KI solution and the solution was diluted to 50ml. Selenium was reduced to the Se" form by heating the solution for 12 min at 90°C after the addition of 10 ml of HC1 into 15 ml of sample solution. Afterwards the solution was cooled to room tempera- ture diluted to 50 ml and analysed.The measurements were performed with the Ar carrier gas flow rate of 40 1 h-' and reagent and sample flow rates of 10 ml min-l. The reducing reagent used in most of the experi- ments was 1% NaBH,. Determination of As and Se by ETAAS after in situ precon- centration in the graphite furnace Before each measurement the graphite surface was modified with Pd; 50 pl of Pd solution (100 pg 1-I) were injected into the graphite tube and dried. The hydrides were introduced into the graphite tube through the sample introduction opening with a 20 cm long quartz tube (2 mm i.d. 3 mm 0.d.). The tube was narrowed at the end to give a 1 mm 0.d. tip and was connected to the outlet of the gas-liquid separator. During the hydride trapping the graphite tube was heated to 300°C and the internal shield gas flow was stopped or reduced (gas stop and/or mini-flow set-up).Hydride generation was per- formed continuously with a carefully controlled collection time and sample/reagents flow rates. The most commonly applied reagent and sample flow rates were 0.5 mlmin-I and the carrier Ar gas flow rate was set to approximately 5 1 h-'. After the selected time of HG the sample and reagent flows were stopped and after 15-20s purging of the system the quartz tube was removed from the furnace and ETAAS measurements were carried out under the experimental conditions shown in Table 1. Results and Discussion The aim of this investigation was to elaborate a simple and reliable procedure for the determination of trace amounts of Se and As in highly mineralized mineral waters.For these types of water the varying content of major components as well as a great variety of minor components with regard to the occurrence of elements and their concentration levels is characteristic. The content of some major components in the samples of waters investigated is shown in Table 2. The concen- trations of the minor components such as heavy metals are at the low pgl-' level (e.g. Co Cr Ni and Pb 1-10 Cd 0.01 and Cu 1-35 pg 1-l). It is to be expected that such matrices interfere seriously with direct determination by ETAAS of both elements under in~estigation.~ Although the chemical modifier (Pd-Mg nitrate) enables the application of charring tempera- tures up to 1000 "C significant matrix interferences were Table 1 Furnace temperature programmes Operation Step Temperature/"C Time/s' AS- Trapping 1 300 30-300 Analysis 2 450 15 3 2200 4 4 2650 2 Se- Trapping 1 300 30-300 Analysis 2 450 15 3 2400 4 4 2650 2 Internal gas flow rate/ ml min - 0 200 0 200 0 200 0 200JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 G 60 50 2 .= 40 2 30 4- .- c L 0 l5 20 3 n a 1 0 - 287 - - - - - - Table2 Composition of main components in the mineral waters investigated (in mg I-') Component Na+ K+ CaZ + Mg2+ Fe F- a- HCO; so; - A 90-115 20-28 110-180 30-65 7-1 1 0.2-0.4 1-11 1-25 800-1 10 Mineral water B C 1300-1350 1550-1 620 200-220 10-30 250-300 41 0-450 100-110 1050-1150 5-13 4-8 0.7 0.1 140-160 50-90 270-350 2050-2250 4500-4800 7800-8700 D 10-15 2-5 5-60 35 - 0.1 5.4 40 3 30 observed for both elements especially in more highly min- eralized mineral waters samples.The interferences could be reduced only by dilution of samples (e.g. 1 +9) however this approach is not suitable for the determination at trace levels of both elements under consideration. In an effort to avoid these problems and to achieve better sensitivities HG techniques were chosen and investigated. The continuous generation system used in this work was optimized and the appropriate reagent concentrations and sample reagent and carrier gas flow rates were determined and some relevant interferences for reliable analysis were studied. Since both elements are usually present in natural samples at higher oxidation states the reduction to the As"' and Se" state is necessary prior to HG.While successful reduction of As was achieved within a short time after the addition of ascorbic acid and KI the reduction of Se species required only heating with HCl for a longer period. It was established that the same reduction time can be used for different types of samples (Fig. 3). On the basis of the experimental work performed a reduction time of 12min was selected and used in all further experiments. Although the interferences influencing hydride formation have been studied in detail in the past the influence of some cations is investigated here because of the complexity of the composition of mineral waters (mentioned above) and the unfavourable concentration ratios. In Figs. 4 and 5 the ratio of absorption signals ( A A,,) measured from solutions contain- ing different concentrations of metal ions given in Table 3 and those obtained from aqueous standard solutions are presented.A slight decrease in signals (10-15%) for As was observed in the presence of the interferents. For Se a decrease in the signals was observed only for solutions containing higher concen- trations of K+. Among anions the influence of the sulfate anion was noticed (Table4). Since some types of mineral waters can contain higher concentrations of sulfate ions these interferences must be considered. The continuous flow HG system is suitable for the analysis 70 0 2 4 6 8 10 14 Reduction ti me/m i n Fig. 3 Reduction efficiency for mineral waters A B and C 0.40 0.20 0 Solution 1 0 Solution 2 Na K Ca Mg Zn Fe Al Mn Fig.4 Influence of some cations on formation of ASH,. A,= absorbance measured in aqueous solutions A = absorbance measured in the presence of an interferent 1.20 1 .oo 0.80 3 0.60 0.40 0.20 0 Solution 1 OSolution 2 Na K Ca Mg Zn Fe Al Mn SeH,.A,= FL.5 Influence of some cations on formation of - absorbance measured in aqueous solutions A = absorbance measured in the presence of an interferent Table3 Composition of solutions (1 and 2) used for interference studies c/mg 1-' Element Na K Ca Mg Zn Fe Mn A1 1 500 100 100 500 5 0.4 0.1 0.1 2 1500 500 500 1500 50 10 1 1 of samples in the pg1-I concentration range. Their determi- nation is usually not subject to serious problems. To some extent matrix effects were observed for the Mg-Na rich mineral waters and they were reflected in the reduced slope (up to 30%) of the calibration curves.To avoid errors the evaluation of the experimental data should be accomplished with the standard additions technique. The estimated LODs (3a blank signals for a solution with the same matrix components typical of the majority of samples) for this technique were found to be 0.15 pg 1-' under the proposed conditions. Reasonably good linearity (r = 0.998) and satisfactory precision (RSD < 5 YO)288 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 1.00 % 0.80 $ 0.60 9 2 B 3 0.40 0 c - 0.20 Table 4 Influence of SO4'- on the absorption signals of As and Se; the maximum amount of sulfate in the samples is 0.035 mol 1-' - - - - - [~O,~-]/rnol I-' As Se 0.001 0.97 1 .o 0.005 0.96 0.97 0.010 0.95 0.98 0.050 0.80 1 .oo 0.100 0.80 0.98 0.500 0.65 1 .oo *A =absorbance measured in the solution with interferring ion; A = absorbance measured in the aqueous standard solution.were obtained in the concentration range from 1 to 50 pg 1-'. These parameters were not significantly subject to day-to-day variations. For the determination of lower concentrations of As and Se direct preconcentration of As and/or Se in the graphite furnace was examined. Preliminary experiments have shown that the precision of measurements was not satisfactory (RSD being 20-30%) when the same sample and reagent flow rates as in previous experiments were used. Therefore measurements at lower sample and reagent flow rates required modification of the HG system. To reduce dead volumes a different gas-liquid separator [Fig.2(b)] was constructed and the reactor coil was shortened to 75 cm. The selection of the correct carrier gas (Ar) flow rate is vital because it contributes to the transport of the hydrides into the graphite furnace and also influences the mixing of reagents in the reactor coil and therefore it leads to a more efficient HG. As can be seen from the data shown in Fig. 6 the carrier gas flow rate should be maintained between 2 and 5 1 h-l. In this range there is no significant influence of small changes in Ar flow rate on absorption signals while at lower flow rates not only lower sensitivity but also poorer precision is noticeable. A plot of absorbance as a function of sample flow rate is shown in Fig. 7. Under the experimental conditions used this relationship is linear for flow rates up to 1.0ml min-'. In Fig.7 RSD (%) levels for absorbance measurements are also shown which indicate that beyond flow rates of 0.5 ml min-' signal reproducibility deteriorates probably owing to exper- imental difficulties related to the precise time control of the HG. This problem could be overcome with suitable automation of the system. The linear relationship between absorbance and collection which is presented in Fig. 8 was observed for both elements and indicates good trapping efficiency. When all the parameters mentioned above were carefully controlled rectilin- ear dependence of measured absorbance on concentrations of analytes in the solutions was obtained. The correlation O.'O 7 35 W g 0.50 8 0.40 I] 3 0.30 5 $ 0.20 2 Cn C - 0.10 '- z * c- 25 20 - s n - 15 V K 10 5 0 1 2 3 4 5 6 7 8 Ar flow rate/l h-' Fig.6 uersus carrier gas flow rate for 5 pg I-' of Se; collection time 60 s Absorbance signals and relative standard deviation of signals -0 0 0.20 0.40 0.60 0.80 1.00 Flow r a t e h l min-' Fig.7 Dependence of sample flow rate on the absorbance of the analyte for 5 pg I-' of Se; collection time 60 s O'*O I 0.64 3 0.48 n 8 h 5 0.32 Cn c - 0.16 0 60 120 180 240 300 360 420 Time/s Fig.8 Absorbance signals for As and Se versus collection time (concentrations of both elements 1.0 pg I-' sample flow rate 0.5 ml min-l) coefficients were better than 0.995 in the concentration range 0.5-5 pg 1-I. From the slopes of calibration curves the characteristic masses were calculated and they were found to be 22.5 f 1 and 17.5 _+ 1 pg for Se and As respectively. These values are higher than the values reported in the l i t e r a t ~ r e ~ ~ ~ ~ which is probably the result of using less reliable equipment.The values are however comparable to direct ETAAS measurements which were obtained with the same apparatus using platform atomiz- ation with Pd as modifier (22 pg for Se and 16 pg for As). The precision of this procedure under optimum experimental conditions is good the RSD being 2 4 % for both elements under investigation for concentrations 50-fold above the LODs. Even at relatively low sample flow rates (0.5 ml min-') and with a short time of hydride introduction favourable precon- centration factors were obtained. The preconcentration efficiency as well as the reproducibility of parallel runs is illustrated by recorder tracings shown in Fig.9. However it has to be emphasized that although a high temperature cleaning step was performed after each analysis a memory effect was observed after several analyses (Fig. 9 C) therefore additional cleaning of the graphite tube at the maximum temperature could usefully be performed after 10- 15 trapping procedures. The estimated LOD under the experimental con- ditions described (150 s collection time sample flow rate 0.5 ml min-') calculated on the variability of the blank signals ( 3ob) was 0.02 pg I-' for Se while the same value was obtained for As in 120 s. The different collection time which is required to obtain the same values of LOD for both elements is related to the characteristic masses for ETAAS measurements.TheJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 289 1 I - Time Fig. 9 Recorder tracings of the ETAAS measurements; absorption signals after in situ preconcentration of Se ( 5 pg 1-') standard solution A direct determination by ETAAS (20 p1 of sample); B in situ preconcentration (60 s collection of SeH from the same analyte solution sample flow rate 0.5 mlmin-'); and C blank signals (no sample introduction in the furnace) after six collection procedures experimental data also indicate that LODs could be improved with longer collection times and/or with different flow rates of the sample solution and reagents. More attention should nevertheless be paid in this case to the laboratory conditions and the use of purer chemicals since under the proposed conditions the blank values especially in the case of As prevented better figures of merit from being achieved. The magnitude of the absolute blank at a sample flow rate of 0.5 ml min-' and 2 min collection time varied from day-to- day and corresponded to 130-160 pg of As and 75-100 pg of Se in the graphite tube.Some samples of mineral and thermal waters from different regions of Slovenia were analysed with both hydride procedures (Table 5 ) . The evaluation of results was achieved with the standard additions technique (at least three additions to the aliquots of the sample). The slopes of calibration curves obtained for real samples were with few exceptions compar- able to those of the standard solutions for most of the samples investigated.Owing to the lack of certified standards for mineral waters the accuracy of the method was estimated with the analysis of spiked samples in which the concentration levels of As and Se were below the LOD. In most samples examined the recovery test showed good agreement of the results with the calculated values (Table 6). The disagreement in results shown by the sample of thermal water Al(D) suggests that the presence of some other components is important in the generation of selenium hydride. As this sample of thermal water differed from the others in that it contained a higher concentration of sulfide it can be assumed that the presence of sulfide influences the generation of selenium hydride. The analysis of waters with higher sulfide concentrations requires additional investigation.Table 5 Determination of As and Se in some mineral water samples. Results (in pg 1-') are mean of five analyses confidence interval was calculated with t-test P = 0.09 ~~ As 6.0 & 0.6* 6.3 -1 0.6* 3.1 0.4* 2.2 f 0.3" 270 & 8 180-15 20-12 Se < 0.02* 0.10* 0.20* < 0.02* < 0.02* < 0.02 < 0.02" *ETAAS in situ preconcentration. Table6 Recovery test for As and Se in samples by ETAAS in situ preconcentration. Results are averages of three replicates As/pg 1 - ' Se/pg 1 - ' Sample Added Found Added Found V-L (A) 10 9.5f 1 1.0 1.1 f0.2 V-C (A) 10 9.6_+1 1.0 0.95 f 0.2 V3/66 (C) - - 0.5 0.4k0.1 - - 0.5 0.45 kO.1 RgSP (C) A/1 (D) 10 9.6-t 1 20 13k2 B/1 (D) 10 1Ofl 0.5 0.5 f 0.1 Conclusions Continuous HG techniques are suitable for the determination of Se and As in highly mineralized mineral waters. Compared with other techniques e.g.ETAAS HG methods are less subject to interferences. The interferences which do occur can be overcome by applying the standard additions technique. The concentrations of both elements at concentrations greater than 1 pg 1-l can be determined with satisfactory precision by using the conventional approach with atomization in a heated silica tube; the determination of lower levels of As and Se requires more sensitive techniques. In situ preconcentration was found to offer much lower LODs and give reliable results and therefore can be recommended for routine analysis. The financial support of the Ministry of Sciences and Technology of Slovenia is gratefully acknowledged. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 References Maxwell M.H. Kleema C . R. and Narins R. G. Clinical Disorders of Fluid and Electrolyte Metabolism McGraw-Hill New York 1987. Codex Alimentaruis World Health Organization Rome 1988 vol. 12. Stein V. B. Canelli E. and Richards A. H. At. Spectrosc. 1980 1 61. Welz B. Schlemmer G. and Mudakavi J. R. J. Anal. At. Spectrom. 1988 3 695. Bozsai G. Schlemmer G. and Grobenski Z. Talanta 1990 37 545. Mentasti E. Nicolotti A. Porta V. and Sarzanini C. Analyst 1989 114 1113. Van Elteren J. T. Gruter G. J. M. Das H. A. Brinkman U. A. Th. Int. J. Enuiron. Anal. Chem. 1990 43 41. Kamada T. and Yamamoto Y. Talanta 1980 27 473. Hudnik V. and GomiSEek S.Anal. Chim. Acta 1984 157 135. Thompson K. C. and Thomerson D. R. Analyst 1974 99 595. Robbins W. B. and Caruso J. A. Anal. Chem. 1979 51 889. Cutter G. A. Anal. Chim. Acta 1980 98 59. Ebdon L. and Wilkinson J. R. Anal. Chim. Acta 1982 136 191. Welz B. and Schubert-Jacobs M. At. Spectvosc. 1991 12 91. McLaughlin K. Dadgar D. Smyth M. R. and McMaster D. Analyst 1990 115 275. DEdina J. Frech W. Lundberg E. and Cedergren A. J. Anal. At. Spectrom. 1989 4 143. Brindle I. D. Alarabi H. Karshman S. Le X.-c. Zheng S. and Chen H. Analyst 1992 117 407. Flow Injection Atomic Spectroscopy ed. Burguera J. L. Marcel Dekker New York 1989. Narasaki H. J. Anal. At. Spectrom. 1988 3 517. Van Cleuvenbergen R. A. Van Mol W. E. and Adams F. C. J. Anal. At. Spectrom. 1988 3 169. Tsalev D. L. and Mandjukov P. B. Microchem. J. 1987 35 83. Tsalev D. L. Mandjukov P. B. and Stratis J. A. J. Anal. At. Spectrom. 1987 2 135. Drasch G. Meyer L. and Kanert G. Fresenius' 2. Anal. Chem. 1980 304 141. Uthus E. O. Collings M. E. Cornatzer W. E. and Nielson F. H. Anal. Chem. 1981 53 2221. Lee D. S. Anal. Chem. 1982 546 1682. Andreae M. O. Anal. Chem. 1984,56,2064.290 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 27 Sturgeon R. E. Willie S. N. and Berman S. S. Anal. Chem. 1985,57 2311. 44 339. 28 Sturgeon R. E. Willie S. N. and Berman S. S. J. Anal. At. Spectrom. 1986 1 115. 29 Sturgeon R. E. Willie S. N. and Berman S. S. Fresenius’ 2. Anal. Chem. 1986 323 788. 30 Sturgeon R. E. Willie S. N. Sproule G. J. and Berman S. S. J. Anal. At. Spectrom. 1987 2 719. 31 Doidge P. S. Sturman B. T. and Rettberg T. M. J. Anal. At. Spectrom. 1989 4 251. 32 Li Z. Ni Z.-m. and Shan X.-q. Spectrochim Acta Part B 1989 33 An Y. Willie S. N. and Sturgeon R. E. Spectrochirn. Acta Part B 1992 47 1403. Paper 3/048171 Received August 10 1993 Accepted October 10 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900285

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 29 1 On-line Microwave Oxidation for the Determination of Organoarsenic Compounds by High-performance Liquid Chromatography-Hydride Generation Atomic Absorption Spectrometry* M. Angeles Lopez-Gonzalvez M. Milagros Gomez Carmen Camara and M. Antonia Palaciost Departamento de Quimica Analitica Facultad de Ciencias Quimicas Universidad Complutense de Madrid 28040 Madrid Spain An on-line high-performance liquid chromatography (HPLC)-microwave oxidation-hydride generation atomic absorption spectrometry coupled system has been developed for the determination of arsenite arsenate dimethylarsinate (DMA) monomethylarsonate (MMA) arsenobetaine and arsenocholine in environmental samples. An anionic cartridge placed before the HPLC anionic column (Hamilton PRP-X1 00) quantitatively retains anionic species such as arsenite arsenate MMA and DMA but not cationic species which are separated and quantitatively determined after microwave-K,S,O decomposition. The anionic species can be separated and determined quantitatively by removing the anionic cartridge from the system and introducing water instead of K,S,O solution.Detection limits of between 0.3 and 0.9 ng were achieved for all species. A conversion efficiency close to 100% was achieved for the species tested working with a microwave oven power of 700 W and 5% K2S208 oxidizing solution. Keywords Arsenic speciation; high-performance liquid chromatography; microwave oxidation; hydride generation; atomic absorption spectrometry Arsenic is widely distributed in the biosphere as a result of its widespread use in industry and agriculture.' Since arsenic species differ greatly in toxicity there is increasing interest in developing analytical procedures to determine quantitatively arsenic compounds in the environment and in biota.Arsenite and arsenate are very toxic whereas dimethylarsinic acid (DMA) monomethylarsonic acid (MMA) and arseno- choline (AsC) are generally less toxic and arsenobetaine (AsB) seems to be n o n - t ~ x i c . ~ ~ ~ The combination of high-performance liquid chromatography (HPLC) and hydride generation atomic absorption spectrometry (HG-AAS) is an important tool for the speciation of these six arsenic species in environmental samples. Nevertheless two problems have not yet been satisfact- orily resolved.Firstly it is difficult to separate quantitatively arsenic species by HPLC and thus chromatographic peak overlapping occurs when certain columns are used. For example arsenite and AsB drift together in the most common anionic and reversed-phase c o l ~ m n s . ~ ~ The coupling of differ- ent columns to separate species more effectively has been reported,&' but the detection limits are unacceptably high. Secondly hydride generation from DMA is less efficient than from inorganic arsenic species and furthermore AsB and AsC do not generate arsine.'-1° Organoarsenic compounds are usually decomposed by photo-oxidation using a combination of UV radiation and K2S208-NaOH when AsB and AsC are to be determined by on-line HG-AAS." Recently papers dealing with on-line analy- sis of liquid samples by microwave digestion with flow injection (FI) AAS and by HG or cold vapour AAS for the determi- nation of Hg As Bi Pb and Sn in urine and waters have been p~blished'~.'~ In a previous paper14 a more efficient HG from organoarsenicals by on-line HPLC-thermo- oxidation-HG-AAS using a powdered graphite oven at 140 "C instead of a UV radiation lamp was reported.The efficiency of conversion of the organoarsenic compounds into arsenate by microwave oven using an FI system has been recently reported by Le et a1." The present work investigates the suitability of an anionic cartridge for separating AsB and arsenite and the feasibility of an on-line HPLC system coupled ~~ ~ * Presented at the XXVIII Colloquium Spectroscopicum Inter- t To whom correspondence should be addressed.nationale (CSI) York UK June 29-July 4 1993. to a microwave oven for the decomposition of organoarsenicals before HG. Experimental Apparatus The hyphenated H PLC-micro wave oxida t ion-HG- AAS apparatus is shown in Fig. 1. Chromatographic module Separation was by HPLC using a Hamilton PRP-X 100 column (25 cm x 4.1 mm i.d.) and a high pressure solvent pump (Waters Model 590). A Waters IC-H anionic cartridge located upstream of the ionic column was used when AsB and AsC species were determined. The anionic cartridge was regenerated after 40 injections for multi-standard solutions and after about 20 injections for samples. For anionic cartridge regeneration 10 ml of 10% NaOH or 10% KOH were forced through the cartridge using a syringe. The solution was removed from the cartridge with 20 ml of Milli-Q water (Millipore).Samples and standard solutions were injected through a six-port Rheodyne type 50 low-pressure sample injection valve fitted with a 100 p1 loop. Microwave oxidation reactor A domestic microwave oven (Balay Model BAHM-111) with a maximum power output of 700 W (variable in nine steps) and an operating frequency of 2450 MHz was used. A loop of poly(tetrafluoroethy1ene) (PTFE) tubing (1.5 m 0.5 mm id.) was placed inside the microwave oven through the ventilation holes. A 250ml beaker filled with water was used to prevent overheating. Hydride generation module The thermo-oxidized effluent was dipped in an ice-bath (Teflon tubing 0.5 m x 0.5 mm i.d.) before HG in order to lower its high temperature and to avoid over-pressure and decompo- sition of sodium tetrahydroborate. The continuous manifold used to generate arsine consisted of PTFE tubing a four-channel peristaltic pump (Gilson HP4),292 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 1 o r 2 Sample I-i Cartridge Microwave oven NaH2P0 Na,HPO ml min-' - U HPLC Pump I K2S208 0.6 ml m i K ' t Ice-bath Gas-I i q u i d TI HCI NaBH separator 1.9 ml min-' 1.9 ml min-' mixing and reaction joints (Teflon tubing 0.5 mm i.d.) and a V-tube gas-liquid separator (Philips). Hydrides were trans- ported by intermediate argon flow to a quartz atomization cell heated by an acetylene-air flame. Atomic absorption spectrometer A Model 2380 Perkin-Elmer atomic absorption spectrometer equipped with an electrodeless discharge lamp operated at 10 W from an external power supply was used.A spectral bandwidth of 0.7nm was selected to isolate the 193.7 nm wavelength. The signals were relayed to a printer and peak height was recorded. Ar Fig. 1 HPLC-microwave oxidation-HG-AAS manifold for arsenic speciation Reagents All reagents used were of analytical-reagent grade. De-ionized water from a Milli-Q system was used throughout. Stock solutions of arsenic compounds (1000rngl-' as As) were prepared by dissolving appropriate amounts of NaAsO (Carlo Erbaj Na,HAsO4.7H2O (Merck) CH,As0,Na26H20 (Carlo Erbaj and (CH3),As0,Na-3H20 (Sigma). The AsB and AsC were reference standards from the Community Bureau of Reference (BCR). The standard stock solutions were stored in glass bottles kept at 4 "C in darkness.Dilute arsenic solutions for analysis were prepared daily. A 5% m/v potassium persulfate (Merck) solution stabilized in 2.5% m/v NaOH (Merck) prepared daily was used as oxidizing solution. A 3% m/v NaBH (Aldrich) solution stabilized in 1.5% m/v NaOH was used as reducing solution. The concentration of HCl was 3 moll-'. An HPLC mobile phase of 17mmoll-' was prepared by mixing solution A [ 1.17 g of NaH,PO,.H,O (Panreac) in 500ml of water] with an appropriate volume of solution B [ 1.20 g of Na,HPO (Panreac) in 500 ml water] to give pH 6.0. A 5 mmol 1-1 mobile phase was prepared similarly. The resulting solutions were filtered through a 0.45 pm membrane filter and de-gassed before use. Sample Pre-treatment Samples were collected and stored in acid-washed polyethylene bottles.Samples not used on the day of collection were stored at 4 "C in darkness. Carbonated waters were de-gassed by sonication for 30 min. Analytical Procedure On-line determination of AsB and AsC Samples and multi-standard solutions (six arsenic species) were injected through the anionic cartridge leading into a 100 pl sample loop. Arsenite arsenate MMA and DMA were retained in the anionic cartridge and AsB and AsC passed through it into the 100 p1 sample loop and entered the chromatographic column. The mobile phase was 5 mmol 1-' phosphate buffer at pH 6.0 and at a flow rate of 1 ml min-'. The column effluent mixed with the oxidizing solution (5% K2S20s in 2.5% NaOH) flowed to the digestion coil in the microwave oven for decomposition.The solution from the microwave oven was cooled in an ice-bath and a T-junction was used to acidify the sample with 3 mol I-' HCl. In a second T-junction the acidified solution was mixed with the 3% NaBH solution. The resulting solution containing volatile arsine flowed to the gas-liquid separator where the liquid phase was drained off and the gas phase entered the quartz atomization cell. The chromatogram peak height signals were recorded. In all subsequent experiments the peak height was the average of at least three injections of each solution. On-line determination of arsenite arsenate M M A and DMA Arsenite arsenate MMA and DMA were determined as described above for AsB and AsC except that the anionic cartridge was removed a mobile phase of 17mmoll-' phos- phate buffer at a flow rate of 2 ml rnin-l was employed and water was introduced instead of persulfate. Under these con- ditions no AsB or AsC signal was observed.Results and Discussion The optimization of the experimental HG conditions is described in a previous paper.' Chromatographic Parameters It has been shown in previous work14 that the arsenite and AsB peaks overlap under all the chromatographic conditions tested. Since AsB and arsenite exhibit different ionic behaviour the arsenite and AsB overlap was prevented by placing anionic cartridges up-stream of the anion-exchange column. Initial experiments in which 100 pl of each arsenic species at 12Opg1-' were injected into the system through anionic cartridges showed that the anionic species arsenite arsenate MMA and DMA are quantitatively retained in the cartridges while the cationic species AsC and AsB (or neutral depending on pHj break through.To optimize the chromatographic AsB and AsC separation in the isocratic mode the phosphate buffer was varied in the ranges 1-20 mmol l-l pH 4-8 and flow rates 0.5-2 ml min-'. Optimum chromatographic resolution was obtained at 5 mmol 1-' pH 6.0 and a flow rate of 1 ml min-'. Fig. 2 shows a typical chromatogram when the six standard species are injected through the anionic cartridge. Only the AsB and AsC broke through the cartridge giving the corresponding signals. Fig. 3 shows the chromatogram for the injection of the six arsenic species onto the HPLC anionic column after removing the anionic cartridge.Only the arsenite DMA MMA and arsenate peaks were obtained because AsB and AsC do not generate arsine in the absence of an oxidizing agent.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 293 0 2.0 4.0 6.0 Tim e/rn i n Fig.2 Chromatographic separation of AsB and AsC using anionic cartridge-HPLC-microwave oxidation-HG-AAS ( 120 ng ml- ' of each six arsenic species were injected injection volume = 100 pl) 0.15 1 0 2.0 4.0 6.0 8.0 Time/m in Fig. 3 of As injection volume 100 pl) using HPLC-heat-HG-AAS Chromatographic separation of arsenic species (120 ng ml- ' Microwave-oxidation Parameters The experimental conditions14 used in on-line AsB and AsC thermo-decomposition were adapted to microwave oxidation which was studied separately for each species using an FI system.The effect of residence time on microwave conversion efficiency at maximum microwave power (700 W) is shown in Fig. 4. Arsenobetane (100 ng ml-') and 100 ng ml-' of AsC 150 A s g 120 - 2. a 0 .- c 9c n $ 60 .- +- a (0 $ 30 E 0 .- 0 C / / I / / I / B 'v 7' 6.0 8.0 10.0 12.0 14.0 16.0 Residence ti rn e/s Fig. 4 species ( 100 ng ml- of As) A arsenate; B AsB; and C AsC Microwave oxidation efficiency uersus residence time of arsenic were injected separately into the chromatographic carrier of phosphate buffer (17 mol l-' pH 6.0 and flow rate 1 ml min- ') and run with 3% persulfate into the microwave oven. Fig. 4 shows that AsC behaves like arsenate i.e. AsC is oxidized to an arsine generator species in a few seconds while AsB is fully converted after 11 s.At a constant microwave power of 700 W and 1.5 m of reaction coil the percentage conversion of both species varies with the concentration of potassium persulfate as shown in Fig. 5. At 1 YO K2S208 no AsB signal was obtained while conversion of AsC was 60% as arsenate. Above 3% of potassium persulfate HG efficiency for both species was 100% as arsenate. In order to identify the decomposition products of the six arsenic species after microwave oxidation at three concen- trations of K2S20s the effluent solution from the microwave oven was collected and injected into the chromatographic column for separation and determination by HPLC-HG-AAS. The results are shown in Table 1. At 1% persulfate solution all organoarsenic species remain in the original form except that AsC is transformed mostly into arsenite and partially into DMA. These results agree with Fig.5 which indicates that AsB does not generate arsine and that the efficiency of AsC conversion is less than 100%. A 3% m/v concentration of K2S208 transforms all compounds into arsine generator species. The transformation of AsB was about 65% to arsenate and 30% to MMA and AsC was converted quantitatively into arsenate. Treatment with 5% K2S208 converts all organic species into arsenate. This concentration was chosen for further experiments. On-line microwave oxidation is as feasible as the previously proposed on-line thermo-oxidation system12 for decomposing organoarsenic compounds into arsine generator species. However the coil length necessary for on-line organoarsenical microwave oxidation is much shorter (1.5 m) than for thermo- oxidation (4.5 m) and microwave power has the advantages of high efficiency fast decomposition and ease of operation.Analytical Performance The retention times detection limits relative standard devi- ations (RSD) and calibration parameters of each species are listed in Table2. At the 120ngml-' level the RSDs were 3-5% (n = 5). These analytical characteristics make the method very promising for application to most environmental samples. Under the conditions given under Analytical Procedure each peak is completely separate. The chromatogram run time is 3.5 min for the determination of AsB and AsC and 8 min for arsenite arsenate MMA and DMA. The different slopes are due to the different peak widths of all species except DMA - 2.100 - C e 0 .- $ 7 5 - 8 0 .- 4- f 50- c a m Fig. 5 Hydride generation efficiency versus concentration of K,S,O (100 ng ml-' of As) A arsenate; B AsB; and C AsC294 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 1 coil length. Products identified by HPLC-microwave oxidation-HG-AAS; results given in YO On-line microwave oxidation products of 100 ng of organoarsenic compounds at three different concentrations of persulfate and 1.5 m Species obtained* Species injected AsC As"' AsB DMA MMA AsV Persulfate concentration 1 3 5 1 3 5 1 3 5 1 3 5 1 3 5 1 3 5 AsV - 98 98 100 100 100 63 95 59 91 71 97 100 100 100 - - - ~ *Note AsC not found. Table 2 Analytical characteristics and calibration parameters for arsenic species; n = 10 ~ ~ _ _ _ _ _ _ _ Species Retention time/s DL*/ng RSD (%) Slope Intercept Correlation coefficient AsC 2.20 0.3 4 0.0090 0.002 0.9994 AsB 3.00 0.4 5 0.0071 0.010 0.998 DMA 2.45 0.9 5 0.0035 0.006 0.992 MMA 3.70 0.4 4 0.0069 0.003 0.9999 AsV 7.20 0.6 4 0.0047 0.001 0.9998 As"' 1.85 0.3 3 0.0099 0.0 13 0.9999 *Detection limit =& f 30.which is only 64% as efficient as an equivalent amount of the other arsenic species. Application The proposed HPLC-microwave oxidation-HG-AAS method was used to determine six arsenic species in mineral water sewage water harbour sea-water synthetic fish extract and sediment extract (Table 3) by calculating their concentration using the respective calibration curve of each species.The spike recovery data for 100ngml-' of each arsenic species in synthetic fish samples are acceptable and confirm the reliability of the proposed method. Total content of As in sewage was determined by ICP-AES by another research institute [Centro de Investigaciones EnergCticas Medioambientales y Tecnologicas (CIEMAT)] and the As found (36+4 mg I-') was similar to the sum of the different species determined by the proposed method. The concentration of AsB found in the harbour sea-water and in the sewage sea-water are unusual. The concentration of AsB in other sea-water samples was lower than the detec- tion limit. Conclusions The proposed on-line HPLC-microwave oxidation-HG-AAS system has been successfully used to determine arsenite arsen- ate MMA DMA AsC and AsB in drinking water sewage and harbour sea-water synthetic fish extract and sediment extract.The novel coupling of anionic cartridges to the HPLC column is of great interest and makes it possible to determine the six arsenic species without the chromatographic overlap- ping of arsenite and AsB. The microwave oxidation system developed is a good alternative to photo-oxidation and thermo-oxidation. The feasibility of thermo-oxidation and microwave oxidation for the decomposition of the organoarsenicals AsB and AsC is Table 3 Speciation of As in waters synthetic fish extract and synthetic sediment extract. Results are expressed as As; nks (n=5) Fish extract*/ Species pg 1-' As"' n.d. 7 AsV 347 f 20 MMA 11Of8 DMA 140+10 AsC 106 + 7 AsB 450 + 30 Recovery in fish (spiked samples %) 110 96 105 108 95 108 Sediment extract*/ n.d.34f 1 18.2f0.8 8.3 k0.3 n.d. n.d. I % I-' Mineral watert/ I-' n.d. 51 + 6 n.d. n.d. n.d. n.d. Sewage water$/ mg 1-' 7.4 f 0.2 n.d. n.d. n.d. n.d. 30f2 Sea-water (harbour&/ n.d. n.d. n.d. n.d. n.d. 47f1 I-' *Provided by the Bureau of Community Reference Material of the European Communities. tCommercially available. $From CIEMAT (Spanish research centre). §From Santander harbour Spain. 1n.d. =Not detected.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 295 similar and the conversion efficiency was close to 100% for the species tested when 5% K2S20 was used. The proposed on-line system is efficient reproducible simple fast cheap and free from cross-contamination. The authors thank Isabel Martin for her collaboration Direccion General de Investigacion Cientifica y Tkcnica (Project PB91-0376) for financial support Max Gormann for revision of the manuscript and BCR for providing reference materials.References 1 Buchet J. P. and Lauwerys R. in Analytical Techniques for Heavy Metals in Biological Fluids ed. Facchetti S. Elsevier Amsterdam 1981. p.75. 2 Lewis R. J. and Tatken R. L. Registry of Toxic Efects of Chemical Substances US Department of Health Education and Welfare Cincinnati OH USA 1978. 3 Marafante E. Vahter M. and Dencker L. Sci. Total Enuiron. 1984 34 223. 4 5 6 7 8 9 10 11 12 13 14 15 Beauchemin D. Siu K. W. M. McLaren J. W. and Berman S . S. J. Anal. At. Spectrom. 1989 4 285. Low G. K. C. Batley G. E. and Buchanan S . J. J. Chromatogr. 1986,368,423. Cullen W. R. and Dodd M. Appl. Organornet. Chem. 1989,3,401. Murer A. J. L. Abildtrup A. Poulsen 0. M. and Christense J. M. Analyst 1992 117 677. Branch S. Bancroft K. C . C. Ebdon L. and O’Neill P. Anal. Proc. 1989 6 73. Shibata Y. and Morita M. Anal. Chem. 1989 61 2116. Rauret G. Rubio R. and Padro A. Fresenius’ J. Anal. Chem. 1991,340 157. Atallah R. M. and Kalman D. A. Talanta 1991 38 167. Tsalev D. L. Sperling M. and Welz B. Analyst 1992 117 1729. Tsalev D. L. Sperling M. and Welz B. Analyst 1992 117 1735. Lopez M. A. Gomez M. M. Palacios M. A. and Camara C. Fresenius’ J . Anal. Chem. 1993 346 643. Le X.-C. Cullen W. R. and Reimer K. J. Appl. Organomet. Chem. 1992 6 161. Paper 3/05883 B Received July 13 1993 Accepted September 29 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900291

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 297 Quality Control of a Recently Developed Analytical Method for the Simultaneous Determination of Methylmercury and Inorganic Mercury in Environmental and Biological Samples* Hgkan Emteborg Negassi Hadgu and Douglas C. Baxter Department of Analytical Chemistry University of Umea S-90 7 8 7 Umea Sweden A recently developed method for the simultaneous determination of methylmercury and inorganic mercury based on extraction butylation capillary gas chromatographic separation and atomic emission detection has been evaluated with respect to analytical quality. A number of reference and candidate reference materials have been analysed within international intercalibration exercises showing good agreement with respect to methylmercury and total mercury.Other small scale laboratory intercomparisons have also been made in order to assess the analytical performance. The materials analysed have a wide range of mercury concentrations from low ng g-' to pg g -'. Finally important characteristics such as artifact formation detector selectivity chromatographic performance and stability of mercury compounds during acid leaching extract ion and der ivatization are discussed. Keywords Mercury speciation; capillary gas chromatography microwave-induced plasma atomic emission spectrometry; environmental fish and biological samples; reference materials Trace metals and persistent organometallic species pose a greater risk to man and the environment than the combined total toxicity of radioactive and organic wastes each year through their accumulation in the food chain.' It has also been recognized for a considerable period of time that organometal- lic compounds are generally much more toxic than their inorganic counterparts owing to biological compatibility where these species penetrate vital barriers such as the blood- brain barrier and cell membranes.This has led to a desire to distinguish between the different forms of specific elements an analytical discipline that is termed speciation and some methods have been developed for the determination of different mercury compounds. The classical methods for mercury speciation are the gas- chromatographic procedure proposed by Westoo' for the determination of methylmercury chloride (MeHgCl) with elec- Iron-capture detection of the halide moiety (GC-ECD) and the method from Magos3 based on a selective reduction of inorganic mercury and total mercury yielding cold mercury vapour (CV) which is detected with atomic absorption spec- trometry3 (AAS). Selective reduction is achieved with SnC1 and SnC1 + CdC1 for inorganic and total mercury respect- ively.The difference between total mercury and inorganic mercury corresponds to the content of methylmercury in the sample. Both methods have been improved by several workers but some drawbacks still persist. These limitations are dis- cussed further below. Newer variants that are now in use include gas chromatography cold vapour atomic fluorescence spectrometry (GC-CVAFS) following ethylation of both Hg2+ and MeHg' with sodium tetraethylb~rate~ and a method analogous to that of Magos involving selective reduction with BrCl+SnCl where bromine monochloride is used as an oxidizing agent for methylmercury prior to addition of the reducing agent SnCl giving results for total mercury.Addition of SnCl alone will yield results for inorganic mercury after which the reduced mercury is amalgamated and thermally desorbed and detected by AFS4 or AAS.' The method discussed here is based on butylation of both Hg2+ and MeHg' with a Grignard reagent after extraction into toluene as diethyldithiocarbamate complexes.6 The butylated species are separated on an open tubular gas- chromatographic column then atomized and excited in an atmospheric pressure microwave induced helium plasma and * Presented at the XXVIII Colloquium Spectroscopicurn Inter- nationale (CSI) York UK June 29-July 4 1993.detected using atomic emission spectrometry at 253.7 nm In the above mentioned methods it is important to dis- tinguish between species-specific and operationally defined methods. The first category is represented by the GC-MIP- AES based procedure and the latter by the method of M a g o ~ . ~ A species-specific method gives a positive identification of a species e.g MeHg through high separation efficiency and high detector selectivity whereas the operationally defined method gives indirect answers since the response in the case of the method proposed by Magos is dependant on which reagent is added to the sample. All operationally defined methods should be compared with species-specific methods as otherwise the analyst is 'blind' in terms of speciation.In order to ensure results of high analytical quality from either category it is essential that certified reference materials (CRMs) or reference materials (RMs) are used. Until recently it was impossible to purchase RMs for methyl- mercury' and there is a need for more materials covering a broad range of concentrations to assess an analytical method in terms of ruggedness accuracy and applicability to various types of biological and environmental samples. It is however impossible to produce a CRM for mercury speciation for every conceivable sample matrix and thus when there are no CRMs or RMs available it is the responsibility of the analyst or laboratory to verify accuracy by means of small scale labora- tory intercomparisons on the matrix of interest or by other measures to assess the analytical quality of the results.It is also important to report on the accuracy of new methods in this field which offer ways to circumvent difficulties by provid- ing either improved chromatography or detector selectivity. In the work described here attempts have been made to verify the analytical quality of mercury speciation results obtained using the GC-MIP-AES method outlined re~ently.~.*>~ Results for speciation of mercury in two Northern pikes (Esox lucius) are also included to demonstrate the applicability of this method to fresh samples. Total mercury in pike samples was determined by CVAAS as described el~ewhere.~ (GC-MIP-AES). Experimental Instrumentation The instrumentation has been discussed in detail pre- v i o ~ s l y ~ ~ * ~ ~ slight alterations being summarized in Table 1.The operating conditions for GC can be found in Table 2.298 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 1 Components of the GC-MIP-AES system Chromatograph Varian 3300 (Solna Sweden) with 15 m fused silica non-polar bonded phase wide-bore capillary column (0.53 mm i.d. 1.5 pm DB-1 J & W Scientific Rancho Cordova CA USA); equipped with a pneumatic automatically controlled four-way valve from Valco TX USA and a Varian 1093 SPI injector with on-column insert Beenakker TM, configuration; torch assembly AHF-Ingenieurburo H. Feuerbacher Tubingen Germany. Details given in refs. 6 and 8 AHF-Ingenieurburo operated with 150 W forward power tuned to give reflected power of <0.5 W polychromator part of 'organic analyser' system MPD 850 AG (Luton Bedfordshire UK).Varian Star integration software for PC use. 20 Hz sampling rate and 1 V max output from photomultiplier tube amplifier read-out Cavity Generator GMW 24-1000 DRL microwave generator Spectrometer 0.75 m Rowland circle direct reading Integrator Table 2 Operating conditions for the gas chromatograph; carrier gas helium flow rate 18 cm3 min-' Column oven Initial column temperature/"C Initial hold time/min Final column temperature/"C Ramp rate/"C min-l Isothermal hold time/min Injector temperature/"C Initial relay Switch delay time/min Final relay Injector Relay 50 1 180 40 1 180 - 1* 2.1 + I t * - 1 =Column effluent vented.t + 1 =Column effluent directed to plasma. Sample Materials Dogfish liver tissue (DOLT-2) and lobster hepatopancreas tissue (TORT-2 presently not certified) were supplied in the form of lyophilized homogeneous powders by the National Research Council of Canada (NRCC Ottawa Canada). Two similarly prepared tuna fish tissue samples (BCR-463 and BCR-464 certification report in preparation) were obtained from the Community Bureau of Reference (BCR Brussels Belgium)." A human whole blood sample (Seronorm 904 Nycomed A/S Oslo Norway) which had previously been analysed,' is also included in the ensuing discussion. Two Northern pikes (wet masses 0.8 and 1.3 kg) were caught in Rickleiin (a stream 50 km north of Umei) and Tavelsjon (a lake 30 km west of Umei).The fish were stored frozen and thawed before analysis. Reference material IAEA MA B3/TM was obtained from the International Atomic Energy Agency (Vienna Austria) and is certified for total mercury only. Sample Preparation The sample preparation procedure has been modified slightly and hence requires description. Samples other than blood and the tissue from pike were dried in a vacuum dessicator at 100mm Hg for 48 h over anhydrous Mg(C104) (Merck Darmstadt Germany)," to determine the moisture content. Separate samples 50-100 mg were accurately weighed into 10 ml glass centrifuge tubes used as supplied. Although sample masses of 200-500mg are generally recommended to avoid problems with inhomogeneity recent results suggest that many RMs exhibit fairly homogeneous trace element distributions even at the milligram mass level.' Thus samples of 50-100 mg were considered to be sufficiently representative of the bulk material as demonstrated by the results given below.Larger amounts of sample would have led to higher reagent blanks and the need for numerous extractions and such practical considerations also dictated the sample masses used. The lyophilized and powdery samples were wetted with 0.5-1.0 ml of saturated NaCl solution (Merck Darmstadt Germany pro analysi quality). Acid leaching to liberate the mercury species was achieved with an addition of 0.1-0.2ml of concentrated sub-boiling distilled hydrochloric acid (orig- inally pro analysi quality from Merck) and shaking for 15 rnin on an automatic shaker (IKA-VIBRAX-VXR Labassco Partille Sweden).Then 2.1-2.2 ml of 1 mol I-' NaOH solution (Eka Nobel Bohus Sweden) was added to neutralize the solution. It had previously been found that the greatest contri- bution to the reagent blank came from the NaOH solution and so purification procedures as reported in ref. 9 were applied. Next 1.5 ml of a pH 9 borate buffer (Merck) were added followed by 1.0 ml of 0.5 moll-' sodium diethyldithio- carbamate (DDTC) solution supplied by Aldrich (Milwaukee WI USA). Subsequently the samples were shaken for 5 min and 1.0 ml of 'distilled in glass quality' toluene (Burdick and Jackson Muskegon MI USA) was added. The sample was shaken for 5 rnin and centrifuged for 5 rnin at 3200g (Wifug centrifuge Bradford UK) after which 0.8 ml of the toluene phase was withdrawn with a Gilson pipette (Villiers-le Bel France) and transferred into another centrifuge tube standing in an ice-water bath.Another 1 ml of toluene was added to the sample and the above procedure was repeated except that 1.0ml was withdrawn the second time. To the combined toluene phases (0.8 + 1.0 ml) 0.3 ml of 2.0 mol dmP3 of n-butylmagnesium chloride in tetrahydrofuran (Aldrich) was added. The reaction was completed within 5 rnin and the excess of Grignard reagent was quenched with 0.5 ml of 0.6 moll-' HC1. Finally the samples were centrifuged and the organic layer containing the butylated mercury species was transferred to 2 ml screw capped glass vials. Sample preparation of the pike samples was performed with an alkaline digestion procedure similar to that described by Cappon and Smith.I3 A sample of 0.5-5.Og of tissue was weighed accurately into a 50ml polycarbonate tube and 10-25 ml of 15% m/v of NaOH (E Nobel) in 0.2% m/v NaCl (Merck) solution were added with agitation.The tube was loosely stoppered and warmed at 50-60°C in a water-bath until the fish tissue was completely dissolved (10-25 min). After cooling 2 ml of the alkaline digest were placed in a 10 ml screw-capped glass centrifuge tube and neutralized with 5-20 drops of concentrated sub-boiling distilled HC1. Next the sample was shaken for 5 rnin on an automatic shaker followed by addition of 1 ml of pH 9 borate buffer (Merck) and 1 ml of 0.5 moll NaDDTC (Aldrich). Extraction and derivatization were then performed as described above.Details of the blood sample preparation procedure can be found in ref. 8. Solutions All solutions were prepared using Milli-Q quality water (Millipore Milford MA USA). Aqueous standards of methyl- mercury chloride (MeHgCl) were diluted from a 158 mg I-' stock solution prepared by dissolving an appropriate amount of the salt (Merck >98% pure) in Milli-Q water. This stock solution was calibratedg against a commercial 1000 mg 1-' Hg standard (as HgC1,) supplied by Analytical Standards AB (Kungsbacka Sweden). Inorganic mercury standards were diluted from the 1000mgl-' standard. The content of inor- ganic mercury was quantified in the MeHgCl standards at a level of 2% using the GC-MIP-AES procedure.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 299 Calibration For the purposes of calibration the standard additions method was employed to account for non-quantitative extraction and derivatization of the mercury species. Additions at two concen- tration levels were made to each type of sample material on at least two different occasions prior to the acid leaching step allowing 15 min with shaking for eq~i1ibration.l~ The added concentrations were selected to increase the total levels of mercury by factors of two and three following preliminary screening analyses to determine the approximate concen- trations in the samples. The additions were found to give a linear response with correlation coefficients always above 0.995. Calibration of the pike samples was achieved with external aqueous Hg2+ standards which were extracted and derivatized as described above.Results reported in Table 7 are corrected for recovery. Chromatograms were evaluated using the Varian Star software (Table 1 ) employing peak-area measurements. Reported concentrations in Table 4 have been corrected for determined moisture levels in the lyophilized materials. Results Recoveries of added methylmercury and inorganic mercury from various sample matrices are reported in Table 3. The recoveries were assessed relative to aqueous standard solutions that were processed in the same fashion as the samples i.e by complexometric extraction and butylation. It should be noted that at least for simple aqueous standards essentially complete recovery of mercury species is achieved following a double extraction into tol~ene,'~ and that the derivatization reaction appears to be quantitative." Results for the determination of methylmercury and inor- ganic mercury in several candidate RMs and RMs using the present GC-MIP-AES method are summarized in Table 4 and some representative chromatograms are displayed in Figs.1-4. Total mercury concentrations determined were obtained by summing the results for the two individual species and compared with certified values. Extraction efficiencies of mer- cury from pike tissue are reported for different sample work- up procedures in Table 5. It should be noted that materials other than lyophilized finely dispersed and homogenous samples should be digested with the alkaline digestion pro- cedure prior to extraction and derivatization.To demonstrate the possibility of using acid leaching for sample work-up in fine ground lyophilized materials results are reported in Table6 which indicate that there are no differences between alkaline digestion or acid leaching sample work-up for these ma trices. In Table 7 results for speciation of mercury in Northern pike (Esox lucius) can be found. Note that pikes with a mass of approximately 1 kg are routinely used to monitor mercury levels in aquatic ecosystems. Lakes where the content of total mercury is higher than 1 mg kg-I in pike are deemed unsuit- able for commercial fishing and some 10000 lakes are presently 'blacklisted' in Sweden. Discussion Recoveries In scrutinizing the data given in Tables 3 and 4 it can be seen that better recoveries are generally obtained at lower concen- trations of mercury species in the samples in agreement with previous work.14 Significant differences in recoveries between matrices is also apparent in Table3 even in the case of the two BCR tuna fish samples an effect which was observed by several of the laboratories participating in the certification campaign.1° Unless the extraction efficiency for the work-up procedure is established standard additions to the matrix of interest are still an absolute requirement for calibration pur- Table 3 Recoveries for methylmercury and inorganic mercury values given % & one standard deviation for n = 5 Addition Sample BCR-463 BCR-464 TORT-2 DOLT-2 MeHg 72.5 f 4.1 84.9 f 2.5 100.3 f 12.3 72.1 f 10.8 Hgz+ 76.3 2.2 69.7 f 3.0 90.4 & 6.6 73.3 & 9.0 Table 4 Results for the speciation of mercury in various environmen- tal and biological materials by the GC-MIP-AES method uncertain- ties given as & one standard deviation Material n Certificate value (Certificate value)? (Certificate value)? (Certificate value)? DOLT-2 8 TORT-2 7 BCR-463 5 BCR-464 5 SE-904 5 SE-9047 2 Reference value MeHq Hg2+/1 g- Pg g- 0.740 & 0.03 1 0.693 f0.053 1.1 58 f 0.085 - 0.154+0.009 0.086 fO.010 - -t 3.25 k 0.169 3.04 f 0.244 - (11 labs.) 5.75 f 0.374 5.46 f 0.339 - (12 labs) 3.6k0.4 ng g-' 3.4 ng g-' 0.280 f 0.020 0.403 f 0.003 1.8 k0.5 ng g-' 1.0 ng g-' - - Hgto( P!2 g- 1.90 f 0.09* 1.99 4 0.10 0.240f0.013* -$ 3.24 & 0.16* 2.88 & 0.17 (7 labs.) 5.75 40.37* 5.26k0.21 (7 labs) 5.4f0.6* ng g-' 4.0k0.49 ng g-' *Sum of inorganic and methylmercury.t(Certificate value) = certification report in preparation. $=Data from GC-MIP-AES method will be included in the 9= Expressed as pg g- of MeHg in BCR-463 and BCR-464. 7SE-904 = Data from Brunmark et ~ 1 . ' ~ certification; certified values will be submitted as soon as possible. T 1 50 mV J 1 1 1 1 1 0 1 2 3 4 5 Time/m i n Fig. 1 Chromatogram for 2 pl injection of BCR-464 (49.9 mg) A methylbutylmercury (2.6 min); and B dibutylmercury (4.1 min). Concentrations of different species are presented in Table 4300 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 t a t 0 a v) U 0 1 2 3 4 5 Time/m in Fig. 2 Chromatogram for 2 p1 injection of TORT-2 (103.1 mg) A methylbutylmercury (2.6 min); and B dibutylmercury (4.1 min).Concentrations of different species are presented in Table 4 t a v) C 0 a v) a U B 120 m" A 0 1 2 3 4 5 Time/min Fig. 3 Chromatogram for 2 pl injection of DOLT-2 (102.2 mg) A methylbutylmercury (2.6 min); and B dibutylmercury (4.1 min). Concentrations of different species are presented in Table 4 1 I 1 I 1 1 0 1 2 3 4 5 Time/m in Fig. 4 (4.1 min) peak arising from reagent contamination Chromatogram for 2 pl injection of blank A dibutylmercury Table 5 Extraction efficiencies of mercury from mixed pike tissues for different sample work-up procedures Extraction Sample Leaching digestion efficiency (YO) Pike homogenate Acid leaching HCI + NaCl, 12.2 (mixed tissues) Precipitate* Alkaline digestion (15% NaOH+0.2% NaCl) 22.5 (15% NaOH+0.2% NaCl) 88.0 * Re-digestion of precipitate from acid leaching.Table 6 Comparison of alkali digestion and acid leaching for hom- ogenous finely dispersed samples typical for reference materials Concentration one stan- dard deviation/ng g-l Leaching/ Sample Method digestion MeHg Hg2+ €lgtot [AEA MA Certificate - B3/TM IAEA MA GC-MIP* Acid leaching 420 150 5701 B3/TM IAEA MA GC-MIP Alkaline 411 +9 141 +9 552f401 B3/TM digestion - - 510+70 *Ref. 6. ?Sum of MeHg and Hg2+ (n=3). poses and recovery assessment even when reference materials are available to check the accuracy of the analytical procedure. As indicated in Table 5 very large differences in extraction efficiencies can be found for different sample work-up pro- cedures i.e alkali digestion or acid leaching of pike tissues.To study the efficiency of an acid-leaching procedure it is common to fortify the homogenized fish tissues with a known amount of mercury species and determine the recovery after storage for a few hours. The high percentage recovery values obtained are used as evidence of the acid-leaching efficiency of the method. This type of recovery experiment does not however provide unambiguous evidence for the digestion efficiency because the added mercury species probably adhere to the surface protein thiol groups of the homogenized sample and are easily liberated when the acid is added. However mercury bound within sample particles on a-helix coil proteins are not accessible to acid liberation. Characteristics of the Method A potential disadvantage of the sample preparation method used here is that ethylmercury is converted into inorganic mercury during the complexometric extraction step.This has been discussed previously in a study of mercury speciation in human whole blood' and was observed in the present work when ethylmercury was added to several of the matrices shown in Tables 3 and 4. Although not produced naturally in the environment ethylmercury has been detected by GC-ECD in several reference materials (certified for total mercury only) distributed by the IAEA.I7 Horvat17 speculated that the ethyl- mercury in the IAEA samples could have been produced during the ethanol extraction stage of the analytical procedure analogous to the artifact formation reported by Panaro et a1." These latter workers noted that signals corresponding to the retention time of methylmercury in a GC-ECD system using a packed column 'passivated' with inorganic mercury,lg were obtained when acetone or diethyl ether were injected. This problem became particularly pronounced at higher injector temperatures. Indeed Panaro et al." observed significantly higher methylmercury levels in a variety of marine samples when using GC-ECD than total mercury concentrations deter-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 30 1 Table 7 Speciation of mercury in Northern pike (Esox lucius) Concentration & one standard deviation (n = 3)/ng g- GC-MIP CVAAS Sample Location of catchment Mass/kg MeHg Hg2+ Hgtot Hgtot Pike Pike 236 f 18 1400 f 126* 1363 f 85 Tavelsjon Sweden 1.3 1164&55 Ricklein Sweden 0.8 850 f 40 55 & 10 905 f 170* 858 f 65 *Sum of MeHg and Hg2+ mined by CVAAS.Note that a standard analytical method for methylmercury determinations involves the use of acetone in the sample work-up and GC-ECD.,' The GC-MIP-AES method described here presents a number of features that should preclude any such artifact formation. Firstly no passivation of the chromatographic column is required eliminating the source of inorganic mercury that could participate in unexpected on-column reactions. Secondly toluene is the only organic solvent used in the initial extraction steps for which Panaro et d . l s observed no artifact formation. Thirdly the detection system used is element specific and thus any co-eluting species will in general yield no response. (Only one exception has so far been observed with the present system probably resulting from a spectral inter- ference from phosphorus containing species.') The lack of interfering peaks and excellent chromatographic behaviour of the butylated mercury species is readily apparent in Figs.1-3. As to the inability of the method to differentiate between ethylmercury and inorganic mercury in samples of biological origin the same is true of the aqueous-phase ethylation cryogenic GC-CVAFS technique proposed by Bloom2' and the headspace technique devised by Lansens and Bayens.22 Ethylmercury is thus not amenable to detection in biological samples by any of the modern GC based techniques. The absolute detection limit for the GC-MIP-AES system in its present configuration is 0.4pg of Hg (using the 3s criterion) as either methylmercury or inorganic mercury.Con- centration detection limits depend on the mass of the initial sample volume of toluene used for extraction injection volume and reagent blanks. The long-term stability of the system is excellent during one day the baseline drift is about 5 mV and additionally the response from a single sample remains largely the same from day to day. The stability of the chromatographic system is also very high largely owing to the high capacity of the wide-bore column. Nevertheless following frequent use of the column for the analysis of samples of biological origin involatile organic residues are deposited close to the inlet leading to peak broadening (after > 500 injections) although this problem can easily be overcome by removing the first 40 cm or so of the column at the injector side.Accuracy of the Method As noted above the objective of this work was to verify the accuracy of the proposed GC-MIP-AES based method for the determination of mercury species in environmental and biologi- cal samples. To this end this laboratory has participated in a number of certification programmes (for BCR-463 BCR 464 and TORT-2 yet to be certified) with the objective of producing new RMs. Thus the concentrations of mercury species in these materials were unknown at the start of the exercises. This also applies to DOLT-2 although the results obtained were submit- ted too late to be included in the certification stage. As seen from Table 4 the GC-MIP-AES method has in these intercali- bration exercises given accurate and precise results for marine biological samples.Also included in Table4 is a comparison of results reported previously' with data obtained by Brunmark et for methylmercury and inorganic mercury in human whole blood reference material Seronorm 904. (This material is provided with a reference value for total mercury only.) Brunmark et aZ.I6 determined methylmercury following extrac- tion according to a modified version of the procedure proposed by Westoo2 (as described by Cappon and SmithI3) derivatiz- ation using diazomethane separation by capillary GC and detection by mass spectrometry in the single ion monitoring mode. Inorganic mercury was determined by CVAAS using a version of the method of Magos3 proposed by Velghe et ~ 1 .~ This comparison demonstrates the viability of the GC-MIP- AES method for the determination of mercury species at the low levels typical of human whole blood. Further analytical quality assurance for the determination of mercury species in blood by GC-MIP-AES was obtained in an interlaboratory comparison the results of which were recently reported by Lind et Spiked blood samples were speciated in three laboratories (i) total and inorganic mercury by CVAAS following selective reduction using CdCl + SnCl and SnCl respectively according to a modified version of the method by Magos3 described by Lind et ~ 1 . ; ~ ' ( i i ) methylmer- cury was determined after alkaline digestion aqueous-phase ethylation and cryogenic GC-CVAFS,21 and the total mercury following acid digestion BrCl + SnCl reduction amalga- mation and detection by CVAFS;4 and (iii) methylmercury and inorganic mercury by the present GC-MIP-AES pro- cedure.As shown by the results presented by Lind et all methods performed well and the recoveries of the spiked methylmercury inorganic mercury and total mercury deter- mined by GC-MIP-AES were 96+1 84k2 and 94+8% respectively. Note that for these samples calibration was carried out by the external standard method which could explain the slightly lower recoveries for inorganic mercury. Although these last results represent elevated mercury levels in combination with the data given in Table 4 it seems fairly obvious that the GC-MIP-AES method and associated sample work-up procedure can provide accurate analytical results for methylmercury and inorganic mercury in a variety of matrices over a reasonably extensive concentration range.Conclusions The obtainment of precise and accurate measurements is of fundamental importance in modern society. Quality assurance rules and guidelines (IS0 9000 and EAN 45000) acreditation authorities as well as CRMs have been established to ensure high-quality measurements in vital sectors in society e.g food agriculture environment industrial products and consumer protection. The GC-MIP-AES procedure described provides an accu- rate and precise (Table 4) method for the simultaneous determi- nation of methylmercury and inorganic mercury in environmental and biological samples. This paper demon- strates the viability and accuracy of the proposed method employing only simple wet chemistry and provides highly efficient chromatography (Figs.1-3) coupled with a very sensitive and specific detection system. It would be interesting to perform laboratory intercomparisons between GC-ECD based standardized methods and element specific methods to elucidate whether there is a significant difference in the results as pointed out by other workers."302 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 This work was supported in part by the Swedish Environmental Protection Board and the Centre for Environmental Research in UmeA. References 1 Donard O. and Quevauviller P. Mikrochim. Acta 1992 109 1. 2 Westoo G. Acta Chem. Scand. 1967 21 1790. 3 Magos L. AnaZyst 1971 96 847. 4 Bloom N.and Fitzgerald W. F. Anal. Chim. Acta 1988,208 151. 5 Bloom N. S. and Crecelius E. A. Mar. Chem. 1983 14 49. 6 Bulska E. Baxter D. C. and Frech W. Anal. Chim. Acta 1991 249 545. 7 Byrne A. R. Analyst 1992 117 251. 8 Bulska E. Emteborg H. Baxter D. C. Frech W. Ellingsen D. and Thomassen Y. Analyst 1992 117 657. 9 Emteborg H. Baxter D. C. and Frech W. Analyst 1993 118 1007. 10 Quevaviller Ph. Drabaek I. Muntau H. and Griepink B. Certification Report EUR. Report. CEC Brussels Belgium in preparation. NBS Certijicate of Analysis Standard Reference Material 1566 Oyster Tissue National Bureau of Standards (now National Institute for Standards and Technology) Washington DC 1979. 12 Kurfurst U. Pauwels J. Grobecker K.-H. Stoeppler M. and Muntau H. Fresenius’ J. Anal Chem. 1993 345 112. 11 13 14 15 16 17 18 19 20 21 :22 :23 124 :25 Cappon C. J. and Smith J. C. Anal. Chem. 1977 49 365. Emteborg H. Bulska E. Frech W. and Baxter D. C. J. Anal. At. Spectrom. 1992 7 405. Donard 0. F. X. and Pinel R. in Environmental Analysis using Chromatography Interfaced with Atomic Spectroscopy ed. Harrison R. M. and Rapsomanikis S. Ellis Horwood Chichester Brunmark P. Skarping G. and Schutz A. J. Chrornatgr. 1992 573 35. Horvat M. Water Air Soil Pollut. 1991 56 95. Panaro K. W. Erickson D. and Krull I. S. Analyst 1987 112 1097. O’Reilly J. E. J. Chromatogr. 1982 238 433. Hight S. C. and Corcoran M. T. J. Assoc. Of. Anal. Chem. 1987 70 24. Bloom N. Can. J. Fish. Aquat. Sci. 1989 46 1131. Lansens P. and Bayens W. Anal. Chim. Acta. 1990 228 93. Velghe N. Campe A. and Claeys A. At. Absorpt. Newsl. 1978 17 139. Lind B. Body R. and Friberg L. Fresenius’ J. Anal. Chem. 1993 345 314. Lind B. Friberg L. and Nylander M. J. Trace. Elem. Exp. Med. 1988 1 49. 1989 ch. 7 pp. 189-222. Paper 3/04427K Received July 29 1993 Accepted October 5 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900297

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 303 Determination of Chemical Forms of Mercury in Human Hair by Acid Leaching and Atomic Absorption Spectrometry* Karel Kratzer Petr BeneS and V6ra SpevaCkova Department of Nuclear Chemistry Faculty of Nuclear Science and Physical Engineering Czech Technical University in Prague 115 19 Prague 1 Brehova 7 Czec Republic Dana Kolihova and Jana Zilkova Department of Analytical Chemistry lnstitut of Chemical Technology Technicka 5 766 28 Prague 6 Czech Republic A simple method for the determination of sub-microgram amounts of mercury species in human hair is described. The method is based on the selective leaching of methylmercury from hair with hydrochloric acid followed by determination of the separated mercury species using cold vapour atomic absorption spec- trometry.The results obtained by the proposed method are compared with results obtained by the solvent extraction method. Both methods gave the same results and are suitable for the determination of methylmer- cury and inorganic mercury in hair with natural (10-100 ng g-') or higher contents of mercury with precision higher than 10%. Keywords Mercury species determination; hair; cold vapour atomic absorption spectrometry The levels of naturally-occurring mercury in the environment are generally low. Inorganic mercury (Hgin) can be methylated in the environment the resultant methylmercury (MeHg) is readily taken up by organisms and is released more slowly from organisms than inorganic mercury.',2 Since mercury species are excreted from the body partly by deposition in hair human hair stores information concerning exposure to mercury species chronologically over an extended p e r i ~ d .~ Various methods for the determination of MeHg in environ- mental samples have been developed. Different techniques have been used for separation of MeHg from Hg (ion exchange solvent extraction volatilization distillation).4-10 At present the most frequently used method for the separation is solvent e ~ t r a c t i 0 n . l ~ ' ~ To release mercury species bound in solid samples acid leaching is often u ~ e d ' " ~ . ' ~ ~ ~ occasionally in the presence of copper(r1) salt. However only limited knowledge still exists on the behaviour of individual mercury species in the leaching process and on possible changes in the speciation of mercury during leaching.This also applies for the isolation of mercury from hair. Therefore the leaching of MeHg and Hgin from hair was studied using hydrochloric acid at various concen- trations and the effect of the presence of copper(11) ions on the extraction was investigated. A radiotracer method was used throughout this work. The application of radioactively labelled mercury species greatly facilitates the study of the behaviour of the species in the separation. The labelled species can be easily traced during the separation by measurement of the activity of samples if isotope exchange between the labelled species and other mer- cury species is negligible or slow. The rate and extent of the isotope exchange between MeHg and Hg in aqueous solution is known" and therefore conditions could be selected to maintain a negligible isotopic exchange.The basic problem in the use of the radiotracer method for the proposed purpose was to achieve equal behaviour between the labelled mercury species added to the hair samples and the mercury species naturally present in the hair. It is not easy to prove such behaviour using model experiments practical analyses must be carried out in order to do so. In the present paper a simple and rapid procedure for the separation of MeHg from hair samples is reported and a * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) York UK June 29-July 4 1993. method for the determination of mercury species in hair by cold vapour atomic absorption spectrometry (CVAAS) is pro- posed.The results obtained by this method are compared with the data on MeHg concentration in the same hair determined after alkaline decomposition of hair followed by separation by solvent extraction.16 Apart from the verification of the accuracy of the determination of mercury species by the proposed methods the comparison also helps to solve the problem of the equal behaviour of the labelled and natural mercury species from hair in the separation process. Experiment a1 Apparatus A Radiometric Assembly NV 3102 TESLA with well-type NaI(T1) crystal was used. The atomic absorption spectro- meter was a single purpose instrument TMA-254 TESLA Czechoslovakia capable of determining sub-nanogram amounts of mercury. The solid or liquid sample is placed on a boat then automatically transferred into a combustion furnace where it is burned in a stream of oxygen.The combus- tion products pass through a catalytic furnace where the oxidation is completed. The combustion products are then passed in a stream of oxygen through an amalgamator where mercury is trapped. After heating of the amalgamator to a high temperature the entrapped mercury is released and driven to tandem measuring cells where absorbance is measured. The whole analytical run including all the parameters affecting the sensitivity and reproducibility of the determination are checked and controlled by a microprocessor. Chemicals Hydrochloric acid and sodium hydroxide (Suprapur Merck Germany) were used. Analytical-reagent grade benzene was purified by distillation.Methylmercury chloride (analytical standard Riedel-de Haen Germany) was dissolved in 0.01 moll-' sodium hydroxide to the appropriate concen- tration. A solution of 203Hg(N03)2 in 0.05 mol I-' nitric acid specific activity 250 GBq g-' of Hg (Du Pont de Nemours Germany) was prepared and mercury(11) standard solution (1 g 1 - l ) (Astasol Analytica Czech Republic) was used. All other solutions were prepared using distilled water and analyt- ical-reagent grade reagents. Methylmercury labelled with 203Hg was prepared by the isotope-exchange304 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Hair Samples About 0.5 kg of human hair (a mixture obtained from different persons) was cut to less than 5 mm long pieces using stainless- steel scissors washed according to the procedure recommended by the International Atomic Energy Agency and the World Health Organization17 and homogenized by mixing.A portion of hair was ground down in an agate mill. Radioactively labelled hair was prepared by two alternative procedures Procedure A16 A 20 ml portion of aqueous phase containing 0.01 mol I-' acetate buffer (pH 4.7) 0.001 mol 1-' NaCl and 0.05- 0.1 pg ml-' of Hg as radioactively labelled Hg or MeHg is stirred with 1 g of hair for 1 h. The hair is separated by centrifugation washed twice with 40 ml of distilled water twice with 40 ml of acetone and air dried. Procedure B A 20 ml portion of 2 mol 1-' HC1 containing 0.05-0.1 pg ml- ' of ,03Hgin is shaken on a mechanical shaker with 1 g of hair for 100 h.The hair is separated by centrifugation washed twice with 40 ml of distilled water twice with 40 ml of acetone and air dried. Leaching of Mercury Species For the study of leaching of mercury species from hair using HC1 in the concentration range 0.1-5mol 1-' the following experiments were carried out. After determination of its initial activity (Ao) 10-150 mg of radioactively labelled (with either Me203Hg or ,03Hgin) hair were shaken in a centrifuge tube for appropriate time with 1-2ml of leaching solution which contained hydrochloric acid of the required concentration and in certain cases also 1 moll- ' CuC1 solution acidified by HC1. After centrifugation the activity of the separated aqueous phase (A) was measured and the distribution coefficient (K,) was calculated &-A V' K = - X - A m where V' is the volume of the aqueous phase and m is the mass of hair sample.Determination of Mercury Species by Acid Leaching On the basis of results obtained in the study of leaching the following procedure is recommended (Scheme 1) 90-125 mg of cut hair is shaken with 3.6-5 ml of 2 mol 1-' HCl (the ratio V m is kept at 40 ml g-') on a mechanical shaker for 4 h. The aqueous phase is separated by centrifugation and the mercury content is determined using the TMA-254. From this value the concentration of MeHg in the original hair sample is calculated. The separated hair is washed twice with 5 ml of distilled water and air dried. The content of Hg in hair is determined directly using the TMA-254 (see below). Determination of Mercury Species by Solvent Extraction For verification of the proposed leaching method the extraction procedure16 was modified (Scheme 2) 1.4 ml of 10 mol I-' NaOH is added to 1 g of hair in a centrifuge tube.The tube is kept in a thermostat at 90-95°C for 30min. Then 5ml of distilled water are added to the dissolved sample and the pH is adjusted to 0.5-1.0 using concentrated H2S0,. The sample is cooled to room temperature 1 g of solid KI is added and MeHg is extracted by 30min shaking with 4ml of benzene. After separation of the phases by centrifugation the content of MeHg is determined directly in the organic phase and/or I 1 Hair (90-125 mg)+2 rnol I-' HCI (3.6-5 ml) v/m=40 ml g - ' L 1 I 1 v 1 Shaken for 4 h 1 t I r 1 I I MeHg in aqueous phase I 1 Hair washed with H,O (2 x 5 rnl) I determined by AAS I Air dried 1 1 I Hgi determined by AAS I Scheme 1 Hair (1 g) + 10 rnol I-' NaOH (1.4 ml) heated for 30 min at 90-95 "C H,O added (5 rnl) +concentrated H,SO to pH 0.5-1 Cooling Solid KI ( 1 g) + benzene (4 ml) added I I Shaken for 30 rnin 1 Centrifuged I r 7 1 .MeHg in organic phase (1.5 rnl) determined by AAS I I Organic phase (2 ml) $2 rnol I-' NaOH (2 ml) re-extracted (15 min) IL 1 Centrifuged i___r_l I MeHg in aqueous phase (1.5 rnl) I determined by AAS Scheme 2 after re-extraction into 2 moll-' NaOH using the TMA-254 instrument. Determination of Mercury Using the TMA-254'*-'' A 50-200 pl amount of the solution to be analysed (cf. Schemes 1 and 2) or 10-25 mg of cut hair is introduced into the analyser and thermally treated in a programmed process.The sample is initially dried and then combusted in the stream of oxygen. The mercury vapour is trapped quantitatively on the surface of a gold amalgamator. Mercury preconcentrated in this way is then evaporated into the two measuring cells of the system. Results and Discussion The distribution of labelled mercury species between hair and HCl solution of various concentrations after 4 h of leaching isJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 305 shown in Fig. 1 (cut hair) and Fig. 2 (ground hair). No differ- ences have been found between leaching of mercury species from ground hair labelled by Procedures A and B. However in the case of cut hair the distribution coefficient of Hg depends on the method of spiking used (Fig.1 curves C and E). The presence of Cu2+ in the leaching solution causes a decrease in K of both MeHg and Hgi and makes the separation of these forms less efficient. The dependence of K on time of leaching using 2 moll-' HC1 (Fig. 3) indicates that the equilibrium is reached within approximately 4 h. Only for cut hair labelled with Hg by Procedure A does the quick transfer of the label into the aqueous phase followed by the slow re-uptake by hair occur (Fig. 3 curve C). Similar uptake is observed when non-spiked hair is treated with 2 moll-' HCl containing labelled Hg (curve D). The differences in the behaviour of Hg spiked on cut hair by Procedures A and B suggest that with Procedure A Hg is adsorbed on the surface of hair whereas in Procedure B it is absorbed into hair.On the basis of the above results while respecting the natural level of mercury species in hair and the sensitivity of the analytical method used the leaching with 2 mol I-' HCl for 4 h at the ratio Vrn=40 ml g-' was chosen for separation of mercury species in hair. The applicability of the proposed method was verified by comparison of the results obtained by analysis of the same hair sample using this method (Scheme 1) 1 I x 1 O 4 x103 LY \ I 1x102 - E 10 1 1 x lo-' I 1 I I I 0 1 2 3 4 5 6 IHCll/mol 1-l Fig. 1 Distribution of labelled mercury species between hair and HCl solution after leaching from cut hair for 4 h A MeHg; B MeHg in the presence of 1 mol I-' Cu2+; C Hg prepared by solvent extraction; D Hgin prepared by solvent extraction in the presence of 1 moll-' Cuz+; and E Hg prepared by acid leaching Fig.2 Distribution of labelled mercury species between hair and HCl solution after leaching from ground hair for 4 h A MeHg; By MeHg in the presence of 1 mol I-' Cu2+; C Hgi,; and D Hgin in the presence of 1 moll-' CuZ+ 1 x lo3 r 1 x 102 - E 9 1x10 -- _ - - - - - - - - _ _ - 1 x lo-' lLIZl 0 20 40 60 80 100 Time/h Fig.3 Dependence of K on time of leaching using 2moll-' HCl A MeHg cut hair; B Hg cut hair prepared by solvent extraction; C Hgin cut hair prepared by acid leaching; D uptake of Hg by cut hair; E MeHg ground hair; and F Hgin ground hair and the solvent extraction method (Scheme 2). The results are given in Tables 1 and 2. The results show that consistent values were obtained for the content of methylmercury using both methods.The differences are not statistically significant. Results obtained by direct determination of total mercury in hair are presented in Table 3. Good agreement was reached between these results and the sum of contents of mercury species found by the acid leaching method (Table 1). This confirms that the results obtained by analysis of hair after acid leaching represents the content of Hg,. The good agreement of the data obtained on the speciation of natural mercury in hair using two very different separation methods also suggests that natural mercury species from hair behave in the separation process similarly to radioactively labelled species used in the development of both separation methods. This confirms the validity of basic assumptions of Table 1 Determination of mercury species in hair by AAS using the acid leaching method Mass of hair/ mg 125 107 117 115 111 103 110 112 Mean _+ CI* MeHg found/ ng g-' of Hg 135 124 136 128 128 132 125 120 129 f 5 Hg found/ ng g-' of Hg 283 29 1 287 298 28 5 283 309 293 291 f 8 Total Hg/ ng g-' of Hg 418 415 423 426 413 415 434 413 420 f 12 ~~ * CI = 95% confidence interval.Table2 Determination of MeHg in hair by AAS using the solvent extraction method Mass of hair/ g 0.888 1.032 1.088 1.076 1.024 1.000 1 .om Mean f CI* MeHg found in ng g- of Hg 144 114 113 120 127 123 122 123 10 C6H61phase/ MeHg found in NaOH phase/ ng g-' of Hg 103 121 99 99 118 130 135 115 f 12 * CI = 95 % confidence interval.306 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Table 3 Direct determination of total mercury in hair by TMA-254 Mass of hair/ mg 11.52 15.90 16.12 16.31 15.42 23.46 11.38 17.24 18.03 20.03 Hg found/ ng 5.09 6.96 6.82 6.93 6.67 9.75 4.98 7.42 7.82 8.47 Hg total/ ng 8-l of Hg 441 437 423 424 432 41 5 437 430 43 3 422 Mean k CI* 429 & 6 * CI = 95% confidence interval. radiotracer methods applied in the development of the procedure. The proposed leaching method allows a simple and rapid determination of both MeHg and inorganic Hg mercury in a 100mg sample of human hair. The method is suitable for serial analysis. We thank the International Atomic Energy Agency Vienna Austria for funding this work. References Stary J. and Kratzer K. Int. J. Environ. Anal. Chem. 1980,8 189.Stary J. and Kratzer K. Radiochem. Radioanal. Lett. 1980,437. Bencze K. Fresenius’ J. Anal. Chem. 1990 337 867. May K. Stoeppler K. and Reisinger M. Toxicol. Environ. Chem. 1987,13 153. 5 6 7 8 9 10 11 12 13 May K. and Stoeppler K. Fresenius’ 2. Anal. Chem. 1984 317 248. Gage J. C. Analyst 1961 86 457. Westoo G. Acta Chem. Scand. 1968 22 2277. Zelenko V. and Kosta L. Talanta 1973 20 115. Dermelj M. Horvat M. Byrne A. R. and Stegnar P. Chemosphere 1987 16 877. Horvat M. May K. Stoeppler M. and Byrne A. R. Appl. Organometall. Chem. 1988 2 850. Horvat M. Byrne A. R. and May K. Talanta 1990 37 207. Horvat M. Water Air and Soil Pollution 1991 56 95. IAEA-UNEP-FAO-IOC Reference Method for Marine Pollution Studies No 13 Rev. 1. United Nations Environment Programme (UNEP) Regional Seas Programme Activity Centre Geneva. 1992. 14 Stary J. Havlik B. Prasilova J. Kratzer K. and Hanusova J. Radiochem. Radioanal. Letters 1978 35 47. 15 Stary J. and Prasilova J. Radiochem. Radioanal. Lett. 1976 26 193. 16 Kratzer K. Benes P. and Spevackova V. Report on the Second Research Co-ordination Meeting. NAHRES-13 pp. 25-3 1. IAEA Vienna 1992. 17 UNEP-WHO-IAEA Reference Method for Marine Pollution Studies No. 46. United Nations Environment Programme (UNEP) Regional Seas Programme Activity Centre Geneva 1987. 18 Miholova J. Mader P. Szakova J. Slamova A. and SvatoS Z. Fresenius’ J. Anal. Chem. 1993 345 256. 19 HlavaE R. Doleial J. Kolihova D. Sychra V. PietroS L. Valenta S. Puschel P. and Formanek Z. Proceedings of the XIV Colloquium Spectroscopicum Internationale (CSI) Garmisch-Partenkirchen Germany 1985. Paper 3/05092K Received August 23 1993 Accepted November 2 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900303

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 0 307 lntracavity Laser Spectroscopic Method for Determining Trace Amounts of Iodine and Barium in Water and Biological Samples" V. S. Burakov A. V. Isaevich P. Ya. Misakov P. A. Naumenkov and S. N. Raikov Institute of Molecular and Atomic Physics Academy of Sciences of Belarus Minsk 220072 Belarus An intracavity laser spectroscopic method and the necessary instrumentation are described for the direct determination of trace amounts of iodine in water and in biological media. Minimal sample preparation is required for the laser probing of vapours over the surface of the heated liquid in a closed cell. This laser method is also applied to measurements of ultra-low barium contents in water by sample evaporation in a standard graphite furnace electrothermal atomizer.Detection limits of 0.015 mg I-' and 0.2 ng I-' were obtained for iodine and barium respectively which are 2-3 orders of magnitude lower than those obtained using a pulsed dye laser. The precision of the method was 10% for the lowest concentrations measured. The procedure is comparatively simple quick and inexpensive. Keywords Laser spectroscopy; atomic and molecular absorption; electrothermal probe atomization; urine; iodine; barium The determination of low levels of biologically significant elements such as iodine in human tissues water and food is an important problem for the practical care of public health. The prevalence of goitre is endemic in some regions for example in Belarus i.e. the population lives under conditions of natural iodine insufficiency.Along with the determination of iodine in water an important indication of the iodine supply of an organism is the study of the iodine excreted in urine. This permits an assessment of the extent to which goitre is endemic in a particular region to be made as well as providing the possibility of establishing scientifically substan- tiated dosing of iodine-containing preparations used for treat- ing the disease and its prophylaxis. It is also considered necessary to develop new techniques for determining trace amounts of toxic elements among them barium in water and other environmental samples because conventional analytical methods have almost reached their practical limits. It is known that even the smallest amounts of such elements or their compounds have a toxic effects on humans and other organisms The determination of iodine in a water sample by the conventional spectrophotometric methods presents no unsur- mountable problems.For example indicator reactions have been proposed for determining iodine in a water sample using non-coloured organic reagents that form intensively coloured products on reaction with iodine ions in an acidic medium in the presence of hydrogen peroxide.' With spectrophotometry in the visible range an iodine detection limit of 1 pg 1-1 has been achieved for such solutions. However the above method has an essential disadvantage the determination is impeded by the presence in the solution of iron which is widely distributed as well as bromide molybdenum silver and mercury. In real biological media for example in human secretions the spectrometric determination of small amounts of iodine presents a fairly difficult problem because of the intensive light absorption by organic and inorganic products. The physico- chemical and thermal treatment and the preparation of samples to be analysed is not effective moreover it can lead to uncontrollable changes in the concentration of the iodine in the samples especially at low levels.Therefore a colorimetric method is usually used to determine iodine in urine at levels higher than 1 mg 1-'. The application of ionomers with iodide ion-sensitive elec- trodes enables a detection limit for iodine of 0.03 mg 1-1 to be * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) York UK June 29-July 4 1993.attained.2 In real samples however in addition to iodine a number of other ions that impede measurements by ion- selective electrode can be present. In practice there are also chemical methods for measuring iodine in biological media with the same detection limit.3 The disadvantages of these methods are the length of time they take and the necessity of using highly toxic compounds (arsenic brucine etc.). For barium the lowest detection limits attainable are reached by using atomic absorption spectrometry with electrothermal atomization or by inductively coupled plasma mass spec- trometry (ICP-MS). There are a series of modern spectrometers for which detection limits for barium of 0.002-0.04 pg 1- ' can be obtained.However further development of these methods now depends mainly on improvement of service and software which will not lead to noticeable increases in sensitivity. Thus the need for ultra-sensitive laser methods for determi- nations at trace levels is becoming increasingly desirable as the required detection limits for elements decrease. In this paper a laser spectrometric technique is proposed for (i) direct measurements of trace amounts of iodine in water and urine with minimal sample preparation by using the method of intracavity laser spectroscopy (ICLS) with conditions for pro- bing the vapours over the surface of a heated liquid in a closed cell; and (ii) intracavity laser spectral analysis of trace amounts of barium in water by sample evaporation in a standard graphite furnace electrothermal atomizer. The ICLS method is based on the strong dependance of laser radiation intensity on the cavity losses resulting from the introduction into the laser cavity of a layer of a substance having absorption lines (bands) within the active medium gain contour of the laser.- The main advantages of this method are its ultra-high sensitivity to small concentrations of absorb- ing species and high spectral resolution given by the mode structure of the laser radiation.The ICLS method differs favourably from such widely used laser methods as laser- induced fluorescence and from laser-enhanced ionization because of the possibility of simultaneously recording a large number of spectral line shapes in a single experiment. The ICLS system used is similar to that employed in continuum source atomic absorption spe~trometry,~ but the latter was reported to be a few orders of magnitude less sensitive.A detailed description of the proposed ICLS method the basic equations and analytical procedure have been reported Experimental Measurements were carried out using the intracavity laser spectrometer that has been described in detail previ~usly.~~'~308 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 0 It is shown schematically in Fig. 1 and consists of a tunable flash-lamp pumped pulsed (5 ps) dye laser as the primary light source radiating a smooth broadband (10 nm) spectrum in the range 430-700nm; a special cell for liquid samples or a standard graphite furnace atomization source both located in the laser cavity; and a 0.001 nm resolution kchelle spectrograph with an optical multichannel analyser.An hermetic quartz (or glass) cell 270 mm long and 25 mm inner diameter coaxially arranged inside an ohmic heater providing heating of the vapours of up to 100°C and above is used for liquid samples. The temperature inside the cell is controlled to within 1 "C and 10 ml doses of the liquids to be analysed (aqueous solutions and urine) are introduced into the cell. The laser beam passes over the liquid surface along the geometrical axis of the cell. The use of a 5 ps laser makes it possible to provide an effective path length of about 1 km for the laser radiation through the vapours. For the determination of iodine the laser radiated a spectrum in the 580-590 range falling within the region where the visible system of the iodine molecular absorption bands is situated.Iodine in urine is present almost exclusively in the form of inorganic salts mainly potassium iodide. Therefore to prepare standards for calibrating the spectromer aqueous solutions of dried chemically pure potassium iodide were used. A stock solution (1000 mg 1-l) remains unchanged for a long period of time in the absence of light. Working solutions were prepared daily by appropriate dilution of the stock solution with de-ionized triply distilled water. Just before measurements were made the solutions and urine were subjected to the minimum of chemical pre-treatment to decompose the iodide salts using a conventional analytical method a few drops of sulfuric acid and hydrogen peroxide were added directly to the cell.'' A standard electrothermal atomizer' was used in the intra- cavity laser spectrometer for probe atomization.Aqueous samples of BaCl (20 pl) were dried ashed and atomized using a pyrolytic graphite coated graphite tube (28 mm long) with the following programme (i) dry 100 "C 40 s; (ii) ash 300 "C 10 s; (iii) atomize 2600"C 6 s; and (iv) clean 2800"C 3 s. Atomic absorption signals were measured at the barium wave- length of 553.548 nm. The laser radiated a spectrum in the 550-560nm range. The effective path length of the laser radiation through the furnace was about 100 m. Results and Discussion By means of preliminary experiments the most intensive electronic absorption band of molecular iodine with a peak wavelength at about 585 nm was chosen for the measurements. To increase the iodine yield over the surface of the prepared solution the walls and windows of the cell were heated Back mirror output mirror Prism laser I I High uniformly.As the vapour temperature increased there was a noticeable increase in the iodine absorption signal. Strong heating of the cell however led to excessive evaporation of the water the wide absorption bands of which could be superimposed on and hence mask the iodine bands. Therefore the experimentally established optimum vapour temperature for the analysis was 80-90 "C. The dependence of the sensitivity of the determination of iodine in two absorption spectra recording regimes were initially investigated. The first procedure chosen was low resolution (0.04 nm) of the spectrograph when the whole laser spectrum is recorded with wide electronic absorption bands of the evaporated iodine molecules.The second procedure was high spectral resolution (0.001 nm) when a narrow range of the laser spectrum (585.0-585.5 nm) at the peak of the elec- tronic absorption band of iodine is recorded. In the latter the vibrational-rotational line structure of the band is completely resolved [Fig. 2(a)]. With complete resolution of the line absorption spectrum of iodine some gain in sensitivity and accuracy in the determination of trace amounts of iodine is observed. In the first recording regime the observable width of the electronic band is comparable to the width of the laser spectrum. Therefore a change in the iodine content in the laser cavity causes a noticeable transformation of the shape of the laser spectrum which strongly impedes quantitative measurements especially measurement of the background laser intensity beyond the absorption band.In the second method of recording measurements of relative absorption at the narrow vibrational-rotational iodine line centre and beyond it gives much more stable results. Conventionally the relative absorp- tion is expressed as (E -E)/E where Eo = (Eol + E,,)/2 as shown in Fig. 2(b). A calibration graph for iodine was plotted using the high spectral resolution (Fig. 3). The precision (maximum value of the relative standard deviation) of the measurement obtained for ten replicate determinations of iodine in a standard solution was approximately 10% for the lowest measured (0.05 mg 1-l) iodine content.A detection limit derived from 0.1 x the relative I C 585.4 585.2 Q .? 30 a c - Q 10 I I + I J 553.6 553.5 Wavelengthln m Fig. 1 Schematic representation of the intracavity laser spectrometer Fig. 2 Partial laser spectra showing absorption lines of (a) iodine and (b) bariumJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 0 309 0.9 a 0.7 + 2 0.5 a .- + 0.3 K 0.1 I I I I 0.01 0.1 1 10 [Iyrng I-' Fig. 3 Calibration graph for molecular absorption of iodine 5.0 1.5 0) c m fl 8 1.0 -Q m a .- c.' - a 0.5 cc I I I 0.01 1 100 [Bal/pg I-' Fig. 4 Calibration graph for atomic absorption of barium absorbance signal of 0.015 mg 1-l for iodine in solution was achieved. The detection limit for iodine attained in this work can easily be lowered if necessary by 2-3 orders of magnitude by simply replacing the pulsed dye laser of microsecond duration by a continuous wave dye laser (where the effective path length exceeds 1 x lo3 km proportionally increasing the laser pulse duration') all other modules of the intracavity spectrometer remaining unchanged.In this case the detection limit for iodine achieved with the intracavity spectrometer can be compared with the detection limit for iodine using ICP-MS of 0.008 mg 1-'.13 The ICLS method was applied to the measurement of iodine in the urine of patients who had not knowingly taken any iodic compounds in the form of food or medicine. No signifi- cant spectral interferences from the urine matrix were encoun- tered.The organic and mineral components in a urine sample have no effect on the absorption of the laser radiation over the liquid surface by the iodine molecules. In the urine being analysed the iodine contents varied over the range 0.1-1.0 mg I-'. Fig. 2(b) shows the part of the laser radiation spectrum that is transmitted through a furnace in the laser cavity near the barium spectral line. There are no absorption signals in the blank furnace. The calibration graph for barium formed by plotting the relative absorbance versus the concentration in solution is shown on Fig. 4. Fot a relative absorbance not only is the relative magnitude of the absorption signal in the centre of the transmission profile used (by analogy with measurements of the iodine contents) but also the relative width of the atomic absorption line for large barium contents under conditions of total absorption when E=O (points on the graph).The precision of the measurements obtained for ten replicate analyses of barium standard solutions was approximately 10% for the lowest measured (0.7 ng I-') barium content. The accuracy of the ICLS method was determined mainly by the stability of the spectral distribution of the laser radiation intensity. A 20 p1 volume of de-ionized triply distilled water gave an absorption signal at the barium line at a level of 0.5 ng 1-'. The detection limit derived from 0.1 x the relative absorbance signal of 0.2ngl-1 of barium in solution (or 0.004 pg for a 20 pl probe) was achieved. Similar results for intracavity laser spectrometry have been obtained previously for other element^.^^^^ Conclusion In this work the process of intracavity laser spectral determi- nation of trace amounts of iodine in water and urine and barium in water has been optimized.The detection limits for iodine and barium are lower as compared with conventional methods and low iodine contents in real biological samples have been measured directly with minimum preparation of the samples being probed. The proposed method and equipment can be used to validate routine laboratory measurements of low iodine contents in urine and other biological media as well as to determine ultra-low amounts of iodine barium and other elements in real samples without preconcentration. 1 2 3 4 5 6 7 8 9 10 11 12 13 References Kreingold S.U. Sosenkova I. I. Panteleimonova A. A. and Lavrelashvili L. V. Sov. J. Anal. Chem. 1978 33 2168. Econics Ion-Selective Electrodes Econics Moscow Russia. Kienya A. I. Rosental I. A. and Kulchitskaya S. N. Lab. Delo. 1985 9 534. Pachomycheva L. A. Sviridenkov E. A. Suchkov A. F. Titova L. V. and Churilov S . S. Pis'ma Zh. Eksp. Teor. Fiz. 1970 12,60. Peterson N. C. Kurylo M. J. Braun W. Bass A. M. and Keller R. A. J. Opt. SOC. Am. 1971 61 746. Trash R. J. von Weyssenhof H. and Shirk J. S. J. Chem. Phys. 1971 55 4559. Jones B. T. Smith B. W and Winefordner J. D. Anal. Chem. 1989 61 1670. Baev V. N. Belikova T. P. Sviridenkov E. A. and Suchkov A. F. Sov. JETP 1978 74 43. Burakov V. S. Voitovich A. P. Mashko V. V. and Raikov S. N. Lasers-Physics and Applications World Scientific Singapore New Jersey London Hong Kong 1989 p. 39. Burakov V. S. Gvozdev A. A. Kovalev A. Ya. Misakov P. Ya. and Raikov S. N. J. Appl. Spectrosc. (Engl. Transl.) 1989,51 1123. Altgausen A. Ya. Laboratory Clinical investigations Medicine Moscow 1964.. Burakov V. S. Esilevski V. A. Misakov P. Ya. Naumenkov P. A. Pelieva L. A. Pimanov Yu. P. Uzunbadjakov A. S. and Sharnopolski A. I. J. Appl. Spectrosc. (Engl. Transl.) 1985 42 247. Atomic Spectroscopy Detection Limits Perkin-Elmer. Paper 3/03905 F Received July 16 1993 Accepted September 22 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900307

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 311 Particle-induced X-ray Emission Thick-target Analysis of Inorganic Materials in the Determination of Light Elements* J. Perez-Arantegui Departamento de Quimica Analitica Facultad de Ciencias Universidad de Zaragoza 50009 Zaragoza Spain G. Querre Laboratoire de recherche des musees de France Palais du Louvre 75007 Paris France J. R. Castillo Departamento de Quimica Analitica Facultad de Ciencias Universidad de Zaragoza 50009 Zaragoza Spain Particle-induced X-ray emission (PIXE) has been applied to the analysis of inorganic materials to determine some elements with Z<27 Na Mg Al Si K Ca Ti Mn and Fe in thick-target analysis. A PIXE method has been developed for the analysis of geological materials ceramics and pottery.Work has been carried out with an ion beam analytical system using a low particle beam energy. Relative sensitivity detection limits reproducibility and accuracy of the method were calculated based on the analysis of geological standard materials (river sediments argillaceous limestone basalt diorite and granite). Analysis using PIXE offers a number of advantages such as short analysis time multi-elemental and non-destructive determinations and the results are similar to those obtained with other instrumental techniques of analysis. Keywords X-ray specfromefry; particle-induced X-ray emission; thick-target analysis; geological and archaeological materials Although it was first introduced in the 1970s particle-induced X-ray emission (PIXE)' has recently reached a high level of importance as is shown by the number of papers and reviews that have been published in the last few Interesting comparisons with other analytical methods can be found in these publications.One of the main features of PIXE is that it is an X-ray method where excitation is performed by charged heavy particles usually protons. As well as the already established advantages of PIXE for the determination of trace elements' (high sensitivity multi- element method versatility and simple sample preparation good spatial resolution rapid and non-destructive analysis) this method can also give excellent results when quantifying major and minor elements. This is the case even for materials whose matrices consist mainly of light elements (the K X-ray peaks for the light elements are just superimposed on the Bremsstrahlung background produced in the sample and on the background produced by secondary electrons) and for the analysis of thick targets (thicker than the energetic range of the particles). When working with geological samples ceramics or pottery it is precisely the light elements (with 2<27) such as Na Mg Al Si K Ca Ti Mn and Fe which are the major and minor components and therefore the ones that have to be considered when characterizing the matrix of such materials.The principal objective of the present work was to establish a method of analysis that enables the determination of the chemical composition for the major elements (Na Mg Si Al K Ca and Fe) and some minor ones (Ti and Mn) in inorganic materials of the following type geologcal ceramic and archae- ological.Hence geology new materials archaeology and the are important areas of application for PIXE in order to characterize such materials. Experimental Apparatus These studies were carried out with an analytical system of ion beams AGLAE a 6SDH-2 2 MV tandem electrostatic * Presented at the XXVIII Colloquium Spectroscopicurn Inter- nationale (CSI) York UK June 29-July 4 1993. NEC Pelletron accelerator in the Laboratoire de recherche des musees de France (Louvre Paris France).'6917 The ion source was a classical Alphatross r.f. source (National Electrostatics Company). The beam of particles provided by a 45" line reached the sample inside an irradiation chamber under vacuum [l x Torr (1 Torr= 133.322 Pa)] and the detection of X-rays was carried out by an Ortec Si(Li) detector with an Ortec 7800 multichannel analyser.A VME (Versa Module Eurocard) processor system was used for the required automatic operation. Instrumental Parameters and Procedure As the first element to be determined was Na ( Z = l l ) for the determination of the other elements (Na Mg Al Si K Ca Ti Mn and Fe) no filter was placed in front of the detector. Since the bombarded material was an insulator a filament of tungsten was used as an electron gun in order to prevent a background owing to self-charging of the sample. Because of the heterogeneity of the material analysed and of the reduced dimensions of the beam (approximately 1 mm') a sample preparation stage was used in order to give more reliable results.However one of the advantages of PIXE is that it requires only very simple sample preparation. By taking about 100mg of material in a powdered form small pellets were prepared of the standards and samples mixed with an organic binder [Moviol a poly(viny1 alcohol)] and compressed in a steel die under 3 tonne of pressure. These pellets 8 mm diameter and 1 mm thickness were placed in Plexiglass sample holders 4.5 x 4.5 cm (Fig. 1). The target was bombarded with a beam of low energy protons 0.7 MeV with a current of 3 nA and with an integrated charge of 0.4 pC. The analysis of each sample lasted approxi- mately 150 s and the spectrum was registered between 0.8 and 10 keV. The detection angle is 150" and the distance to the detector is 40 mm.Both fitting of the spectra to reference spectra and the calculation of integration of the yield of X-rays over the whole particle range were made using a computer programmed with PIXGRAF and THICK (PIXAN package).I8 The first program takes into account the calibration the parameters of the detector and a background model of iterative peak filing. The312 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 l o o 0 1 I Fig. 1 Plexiglass sample holders; dimension of the holder 0.7 cm second program considers the geometric parameters the characteristics of the incident beam and the composition of the sample under analysis. Analysis of Geological Standards A study of different parameters of analytical interest was made by analysing various geological standards.Five standards were chosen 82GOV1 DR-N (diorite) 84GOV2 GS-N (granite) and 80GOV1 BE-N (basalt) three geostandards from the Centre de Recherches Pktrographiques et Gkochimiques; and two Standard Reference Materials (SRMs) 2704 Buffalo River Sediment (RS) and l c Argillaceous Limestone (AL) from the National Institute of Standards and Technology (NIST). The elements studied were Na Mg Al Si K Ca Ti Mn and Fe. As an example the spectrum given by the standard DR-N can be seen in the Fig. 2. Results and Discussion The relative sensitivity of each element was first calculated in photons ng-’ cm2 pC-’ as shown in the Table 1 together with the minimum detection limit of the signal measured in photons. However it is the calculation of the detection limits which is of greater analytical interest expressed as the concentration of an element in the samples studied.This gives a clearer idea of the sensitivity of the method. If it is considered that if the peak area in the spectrum corresponding to element i is related to its concentration by eqn. (1) and if the instrumental param- eters are maintained constant then there exists a linear relation - 104 u 103 5 102 C C L a) P fn 4- 0 Y- g 10 1 0 2 4 6 8 Energy of X-ray/keV Fig. 2 Spectrum of the DR-N standard (diorite) between the area of the peak and the product of the concen- tration and the integral for element i [eqn. (2)] where Si=area of peak i; Ci=concentration of element i; Q/e = number of incident protons; R = detection solid angle; E = detector efficacity; N = Avogadro’s number; A = relative atomic mass of element i; o= yield of X-ray fluorescence; E= particle energy in the sample; E,=particle energy on reaching the target; o(E) = cross-section probability of ionization; T(E) = transmission of photons from successive depths in the matrix; S ( E ) = stopping power; a = intercept of the calibration line; and I =integral term of eqn. ( 1).si = a + b(CJ) (2) The model proposed by Clayton et a1.I9 was then applied in order to calculate the detection limits from a linear cali- bration with unknown parameters (3) where xd(P,q) =detection limit; oo = parameter fixed by the calibration design for r (number of determinations made from a ‘similar’ source matrix) and n (number of observations); A(P,q) = tabulated values for specified p q and v (degrees of freedom n - 2); p =false positive rate; q = false negative rate; c2 =variance; and b =slope of the line.This approach is used to define the detection limits so that protection against both false positives (reporting an analyte as present when it is not) and false negatives (reporting an analyte as not present when it is) is assured. The method is based upon classical (Neyman-Pearson) statistical theory for hypoth- esis testing it assumes that standard calibration procedures are used to provide estimates of all unknown parameters. The detection limits calculated in this manner for this type of material are 0.91 0.40 1.07 2.18 0.25 0.60 0.11 0.109 and 0.91 (as YO of the element) for Na Mg Al Si K Ca Ti Mn and Fe respectively for r = 1 (number of determinations made from a ‘similar’ source matrix) n = 12 (number of observations) p = 0.1 and q = 0.1 with ten degrees of freedom (n - 2).Although these detection limits would appear high it has to be recognized that they are calculated for working conditions and for a matrix for which it is precisely the major elements that are to be studied and which are the centre of interest for the determination so this is no real disadvantage. In order to check the precision and reliability of the method various determinations were carried out for each of the stan- dards. The results obtained for the diorite (DR-N) for seven determinations can be seen in Table 2. The value proposed for the concentration of each element and the average value obtained together with the standard deviation and the relative standard deviation for each of the elements are all given.The results obtained for the four other standards with respect to the proposed values are also given in Table 2. The chemical compositions are calculated from the linear cali- bration made by determining the product of the concentration and the integral and by taking either a hypothetical integral as would be done in the case of samples of an unknown composition but with a similar matrix (column I) or the known integral for this standard (column 11). From this variation it can be concluded that it is important to analyse various standards together with the unknown samples but Table 1 Parameter Na Mg A1 Si K Ca Ti Mn Fe Relative ~ensitivity/lO-~ 1.2-1.6 0.7-0.8 0.20 0.18 MDL/photons Relative sensitivity and minimum detection limit (MDL) of the s.igna1 photons ng-’ cm2 pC-‘ 0.4-0.5 1.8-2.1 5.6-6.0 7.8-8.3 2.4-2.6 100-150 180-225 300-350 300-375 100-130 100-140 25-40 9-20 8-15JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 313 Table 2 Results (as % of the element) for the analysis of several standards (n=4) DR-N* GS-N BE-N RS AL Element Na Mg Al Si K Ca Ti Mn Fe Cert.7 AV§ sm RSDll Cert. AV SD RSD Cert. AV SD RSD Cert. AV SD RSD Cert. AV SD RSD Cert. AV SD RSD Cert. AV SD RSD Cert. AV SD RSD Cert. AV SD RSD I 2.22 2.40 0.109 4.56 2.65 2.71 0.084 3.10 9.27 9.33 0.362 3.88 24.70 24.62 0.8 13 3.30 1.41 1.30 0.085 6.52 5.04 4.69 0.149 3.18 0.654 0.65 0.030 4.69 0.172 0.179 0.017 9.71 6.78 6.84 0.321 4.69 I 2.80 3.28 0.142 4.33 1.39 1.53 0.156 10.20 7.76 8.22 0.3 18 3.87 30.75 32.04 0.766 2.39 3.84 3.69 0.112 3.03 1.79 1.81 0.058 3.19 0.408 0.43 0.032 7.31 0.0431 - - - 2.64 2.50 0.247 9.88 I 2.36 3.26 0.198 6.06 7.93 6.93 0.103 1.49 5.33 4.81 0.22 1 4.59 17.85 16.69 0.176 1.05 1.15 1.20 0.014 1.19 9.91 10.12 0.147 1.45 1.565 1.61 0.03 1 1.92 0.155 0.138 0.01 3 9.49 8.98 9.05 0.200 2.21 I 0.55f 0.59 0.030 5.06 1.20 1.34 0.106 7.86 6.11 7.26 0.329 4.53 29.08 28.45 0.378 1.33 2.00 2.12 0.023 1.10 2.60 2.82 0.239 8.48 0.457 0.49 0.025 5.02 0.0551 - - - 4.1 1 4.54 0.310 6.83 I 0.015$ - - - 0.25$ 0.25 0.010 4.05 0.69$ 0.25 0.016 6.28 3.20 2.79 0.096 3.44 0.23$ 0.22 0.017 7.76 35.90 37.52 0.03 1 0.08 0.0401 - - - 0.019$ - - - 0.381 0.19 0.026 13.59 * n = 7 .t Cert. = certified value.$Below the detection limit. 9 AV =average value. ISD =standard deviation. )I RSD = relative standard deviation. these should have as similar a chemical composition as is possible. In Table 2 it can also be observed that sometimes the practical detection limits for some elements (Na Mg A1 and K) seem to be better than predicted. It should be remembered that the concentrations at the detection limit have been calcu- lated at a confidence level of 90% protected against both false positives and false negatives. Hence sometimes lower concen- trations of these elements could be measured but with a poorer accuracy for the low-energy peaks just where the sensitivity and reproducibility are also lower. Conclusions Particle-induced X-ray emission as an analytical method for the determination of major and minor elements (with 2<27) offers a series of advantages simple preparation of the sample; speed the number of samples analysed daily can be very high; multi-elemental character by determining all of the elements between Na and Fe at the same time; a non-destructive method of analysis samples are taken which can be recovered after the analysis; its sensitivity and detection limits under the concentration ranges which are going to be determined; and its reliability accuracy and reproducibility with coefficients of variation similar to those obtained from other instrumental methods of analysis.Therefore PIXE can be successfully applied to the charac- terization of geological materials and indeed other similar inorganic materials such as pottery ancient ceramics or glass.We appreciate the collaboration with the Laboratoire de recherche des musCes de France and especially the cooperation of T. Calligaro J. Salomon F. Saltron and A. Bouquillon. 1 2 3 4 5 6 7 8 9 10 References Johansson S. A. E. and Campbell J. L. PIXE A Novel Technique for Elemental Analysis Wiley Chichester 1988. Proceedings of the International Workshop in Ion Beam Analysis in the Arts and Archaeology Nucl. Instrum. Methods Phys. Res. Sect. B 1986 14 1. Bird J. R. Duerden P. and Wilson D. J. Nucl. Sci. Appl. Sect B 1983 1 357. Houdayer A. Lessard L. and Brissaud I. Nucl. Instrum. Methods Phys. Res. Sect. B 1984 3 412. Maenhaut W. Nucl. Instrum. Methods Phys. Res. Sect. B 1990 49 518. Demortier G. Nucl. Instrum. Methods Phys.Res. Sect. B 1991 54 334. Johansson S. A. E. Analyst 1992 117 259. David D. Surf. Sci. Rep. 1992 16 333. Kristiansson K. and Malmqvist L. Geoexploration 1987 24 517. Burnett D. S. Woolum D. S. Benjamin T. M. Rogers P. S. Z.,314 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Duffy C. J. and Maggiore C. Nucl. Instrum. Methods Phys. Res. Sect. B 1988 35 67. 11 Katsanos A. A. Nucl. Instrum. Methods Phys. Res. Sect. B 1986 14 82. 12 Malmqvist K. G. Nucl. lnstrum. Methods Phys. Res. Sect. B 1986 14 86. 13 Peisach M. Nucl. Instrum. Methods Phys. Res. Sect. B 1986 14 99. 14 Bird J . R. Nucl. Instrum. Methods Phys. Res. Sect. B 1986 14 156. 15 Rye 0. S. and Duerden P. Archaeometry 1982 24 59. 16 Amsel G. Menu M. Moulin J. and Salomon J. Nucl. Instrum. Methods Phys. Res. Sect. B 1990 45 296. 17 Menu M. Calligaro T. Salomon J. Amsel G. and Moulin J. Nucl. Instrum. Methods Phys. Res. Sect. B 1990 45 610. 18 Clayton E. Duerden P. and Cohen D. D. Nucl. Instrum. Methods Phys. Res. Sect. B 1987 22 64. 19 Clayton C. A. Hines J. W. and Elkins P. D. Anal. Chem. 1987 59 2506. Paper 3f03948J Received July 7 1993 Accepted October 12 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900311

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 315 Direct Determination of Lead in Sea-waters by Laser-excited Atomic Fluorescence Spectrometry* Venghout Cheam Josef Lechner Ivan Sekerka and Roland Desrosiers Environment Canada National Water Research Institute Research and Applications Branch Burlington Ontario Canada L7R 4A6 This paper describes a laser-excited atomic fluorescence spectrometric method for direct determination of lead in sea-waters down to femtogram levels. No separation/concentration steps nor chemical modifier were used. A programmable in situ known addition technique was developed and used as an integral part of the method. The technique reduces sample preparation steps and compensates for spectral line drift better than a standards calibration technique.Four sea-water Certified Reference Materials from the National Research Council of Canada were analysed for Pb concentrations which were found to be well within certified ranges. Spike recoveries of 100 _+ 10% were achieved using a Certified Reference Material and an unknown sea-water sample. The practical detection limit was 3 fg of Pb absolute (or 1 ng I-' relative) which to the authors' knowledge is the lowest absolute detection limit ever reported for sea-water analysis. Keywords Laser-excited atomic fluorescence spectrometry; sea-water; lead; known addition; ferntogram detection limit Sea-water is a very complex matrix containing trace levels of lead and other elements (ngl-') which act as interferents. These characteristics form a troublesome but challenging prob- lem for analytical chemists as evidenced in the literature some of which are cited here.'-27 The bulk of the past work relies on means to discard the salt matrix and preconcentrate the metals before their determination using various methodologies.The most common methodology used has been electro- thermal atomic absorption spectrometry ( ETAAS).5,6,9,12,14,16 Other common methodologies include isotope dilution mass spectrometry ( ID-MS),9~"~'5*18~20~27 inductively coupled plasma mass spectrometry (ICP-MS) by standards addition or c a l i b r a t i ~ n ' ~ ' ~ ~ * ~ ~ and anodic stripping voltametry (ASV).8i'4 None of these methods is sensitive enough to detect ng 1-' concentrations of lead in sea-water without time- consuming preconcentration.The most common sample pre-treatment has been the use of off-line separation/concentration techniques. Chelating resins such as Chelex- 1003*6*9,14927 and Chelamine25 have often been used. Another common approach has been the chelation-extraction technique using carbamate5 ammonium pyrrolidin-1-yldithioformate-isobutyl methyl ketone (APDC- IBMK)? dithizone-chloroform," APDC-diethylammonium diethyldithiocarbamate (APDC-DDDC),I4 or APDC-silica gel C18.23 Other off-line separation/concentration techniques use immobilized chelates 8-hydroxyquinoline ( or poly-5-vinyl-8-hydroxyquinoline,4 c~precipitation;~.'~ elec- trochemical deposition on mercury e l e c t r ~ d e ; ~ . ~ ~ evaporation to dryness;' and reductive precipitation." The in situlon-line separation/concentration technique is less commonly used volatile hydrides,16 volatile tetraethyllead,2' and 8-hydro~yquinoline.~~~~~ Most of these techniques are tedious and the various sources of contamination must be closely monitored to achieve a reasonably low blank concentration compared with sample concentration." Table 1 shows that some reported Pb blanks are as high or even higher than sea- water concentrations which can be as low as l ng l-'.(ref. 22). The best blank value reported appears to be 3.7 pg of Pb.24 Although validated analytical methods based on laser- excited atomic fluorescence spectrometry (LEAFS) are few there have been numerous applications of LEAFS to the analyses of real substrates. These include for example river ~ a t e r s ~ ~ - ~ O snow and i ~ e ~ ~ - ~ ~ tap water,35 blood,36 Great 17,19,21,23-25 * Presented at the XXVIII Colloquium Spectroscopicurn Inter- nationale (CSI) York UK June 29-July 4 1993.Lakes w a t e r ~ ~ ~ ~ ~ ore-process solutions,39 biological and vegetation sample^,^@'^ air,43 soil and sediment sample^,^'.^^ and pure metals.40.42,45 Ho wever there has been no method developed for analysis of sea-waters. This paper describes the first LEAFS method for direct determination of femtogram levels of lead in sea-water. No separation/concentration steps nor chemical modifier were necessary. As the sea-water matrix has a wide range of salinity and there is no representative background matrix the stan- dards calibration technique cannot be used with reliable accu- racy. A programmable in situ known addition (standard addition) technique has been developed and used as an integral part of the method. For 3 p1 of sea-water the practical detection limit was 3 fg of Pb absolute (or 1 ng 1-1 relative).Experimental LEAFS The LEAFS instrumentation has been described e l s e ~ h e r e . ~ ~ ? ~ ~ Some key features are given here. The 511 nm line of a copper vapour laser (Metalaser Technologies MLT20) was used to optically pump a Rhodamine 6G dye laser (Laser Photonics). The dye laser output (566 nm) was then frequency-doubled by a second harmonic generator (Autotracker 11 Inrad I) to give the 283 nm ultraviolet light. This light directed through a pierced mirror into a graphite furnace (Perkin-Elmer HGA 2100) was used to excite Pb atoms generated in the furnace.The fluorescent light (406nm) emitted by the excited atoms was collected and measured via a monochromator-photomulti- plier-boxcar system. A 6 kHz repetition rate was used and the peak irradiance of the 283 nm beam in the furnace was about 2 kW cmP2. Sample Handling Sample handling was carried out in a class 100 clean room and in a class 100 laminar flowhood (Microzone Corporation) except for sample injection (25 p1 total liquid volume) which was carried out in a regular laboratory atmosphere. Details of labware cleaning procedure and clean room development have been described earlier.46 Ultrapure chemicals were used. Milli-Q water (Millipore) acidified to 0.2% with ultrapure nitric acid (Seastar) simply referred to as MQW was used as the standards matrix.316 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Table 1 Some reported Pb blank concentrations for various preconcentration techniques Technique Chelation-Extraction Carbamate extraction Di thizone-chloroform Chelamine Chelex- 100 resin Reductive precipitation Tetrahydroborate Immobilized chelate 8-H ydroxyquinoline 8-H ydrox yquinoline Hg film electrode Resin Electrochemical deposition In situion-line concentration I-8-HOQ Methodology ETAAS IDMS ETAAS ICP-MS ETAAS ETAAS ID-ICP-MS DP- ASVt FI-ETAAS Pb blank concentration Ref. 5 24-36 ng 1-' total blank 0.1 ng per extraction 11 1.2 ng absolute 25 0.08 ng column blank 27 1.7 ng absolute 19 <0.8 ng absolute 12 2.99 ng 1-' final column blank* 18 z55 ng I-' 14 3.7 pg absolute 24 *Various blanks ranging from 0.2 to 140 ng 1-'.TDP-ASV = differential pulse anodic stripping voltammetry. Table 2 Results of replicate weighings of 3 and 5 pl of solutions Parameter n 5 p l O f MQW- MeanICLg s/Pg RSD (%) 3 pl of sea-water- n MeaniCLg SiPg RSD (Yo) Run 1 9 4.94 0.2 4.07 9 3.12 0.13 4.21 Run 2 9 5.05 0.11 2.2 9 3.09 0.06 1.82 Run 3 9 5.05 0.16 3.2 9 3.05 0.09 :!.89 Run 4 9 5.02 0.18 3.64 9 3.11 0.09 2.95 Run 5 9 4.99 0.13 2.58 9 3.11 0.09 2.9 Overall 45 5.01 0.16 3.16 45 3.1 0.09 3.05 Programmable Micropipette An electronic micropipette (Rainin Instrument Company motorized microliter pipette Model EP-100) was used to sample microlitre volumes of different liquids or air (as separate segments) into the same tip and to inject the whole into the graphite tube.Tapered pipette tips were found to be necessary for reliable sampling and injection. The tips were acid-soaked rinsed and tested for very low blanks before use. The segmented sampling of 17 pl of MQW (carrier) followed by 3 pl of sea- water 2 p1 of air (spacer) and 5 pl of standard (or MQW) was used. The standard concentration used varied from 5 to 50 ng 1-l. Peak heights were used to calculate Pb concentration in sea-water by the known addition procedure. Results and Discussion The LEAFS optimization has been described e l ~ e w h e r e . ~ ~ . ~ ~ Choice of Purge Gas A brief review of the literature indicates several advantages of adding H to the Ar purge gas in electrothermal techniques. Amos et al.47 observed a 20-fold improvement in sensitivity for the atomic absorption of aluminium when H2 was intro- duced into the Ar carrier gas.They also showed that both spectral and chemical interferences (caused by excess of such interferents as H,PO NaCl KCl MgCl and CaC1,) were considerably reduced when the Ar-H mixture was used for the determination of Pb by atomic absorption spectrometry. Schmid and Krivan4* proved that in 1% NaCl matrix the stability of Pb in the graphite tube improved with the addition of H2 to Ar. For tungsten or other metal atomizers the usual purge gas is Ar mixed with H2 to provide a reducing environ- ment and to prolong the atomizer lifetime.49~50 Goforth and Winefordner4 compared Ar to an Ar-H mixture in their LEAFS study and found that the Ar-H mixture gave better detection limits than Ar for several elements although no comparison was made for Pb.In this study it was found that the Pb fluorescence signals were improved by about 35% if an Ar-H mixture (92-8%) instead of pure Ar was used as the furnace purge gas for our LEAFS system. Furthermore this gas mixture gave better stability in fluorescence signals and a longer tube lifetime ( z 200 firings). Obviously the mixture provides a superior environment than pure Ar for this work probably because of the reducing environment generated by the burning of H2 during the atomization step.47 Furnace Conditions Uncoated graphite tubes were used as they provide an adequate rise time during the atomization period; the pyrolytic graphite coated graphite tubes give too slow a rise time for the Perkin-Elmer HGA 2100.The atomization temperature also depends on tube thickness for example 2100°C for 1 mm and 2300 "C for 1.3 mm thickness. The optimal drying-pyrol- ysis-atomization temperatures and times were as follows 120 "C for 40 s ramped 350 "C for 40 s unramped and 2300 "C for 3 s unramped. The purge gas flow was stopped during atomization. Pipette Accuracy Normal injections of 20 p1 of sample resulted in too much salt matrix in the furnace tube and subsequent signal suppression so a smaller volume was used. Even though the manufacturer's recommended volume range for the micropipette is 10-100 pl a weighing experiment showed that 3 p1 of sea-water can be reliably picked up and dispensed taking the density of sea- water to be 1.025 g ml-'. The same performance was achieved with 5 pl of MQW as shown in Table 2 hence the 3 and 5 pl volumes were taken as true.The weighing experiment wasJOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 317 2.0 Salinity 0 1.2% m 3.5% 1.5 3 c" 1.0 cn 0 .- 0.5 o . . . . . . . . . . . . n . . 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 S a rn ple vo I u rn e/pl Fig. 1 Signal u e r m sample volume for low and high salinity samples repeated for 2 pl of sea-water but the reproducibility was inferior to that obtained with 3 pl. Effect of Water Salinity Two water samples of low and high salinity (12 and 35%0) were used to study the effect of salinity on the fluorescence signal. To provide a constant volume of injected solution a volume of 25 pl (the sum of sea-water and MQW volumes) was chosen for a series of solutions containing an increasing amount of sea-water.Fig. 1 depicts the fluorescence signal versus the volume of sea-water. It shows that for the highly saline water there is a small range of maximum response peaking around 3 pl of sea-water. For the low salinity water the range of maximum response is relatively wide commencing at about 3 pl of sea-water. Since the two salinity values cover the range usually encountered 3 p1 of sea-water was taken as the optimal volume. Sampling Sequence and In Situ Known Addition Analysis The manual injection of 3 pl of sea-water alone into the graphite furnace resulted in very irreproducible responses possibly caused by the irreproducible delivery of such a small amount through the furnace tube sample hole and the irreprod- ucible distribution of solution on the tube itself thus resulting in a variable position of the atomic cloud with respect to the small laser beam.To help attain reproducible sample injection into the graphite tube the inclusion of a carrier (MQW) was necessary. Also as there is no representative background matrix normal standards calibration could not be used and a known addition technique was relied upon which has the advantage of minimizing the effect of instrumental or baseline drifts that the standards calibration technique often encounters. For example the dye laser retuning was clearly less frequent with the known addition technique. To adequately encompass the sample carrier and standard into one containment without premixing the individual solutions a sampling sequence pro- cedure was devised using a programmable micropipette.The technique comprises segmented sample pick-ups and the dis- pensing of the whole into the graphite tube. A number of different sequences of sample pick-ups were tried some of which are (i) pick-up of 5 pl of standard (or MQW) followed by 2 pl of air 3 p1 of sample 2 pl of air and 17 pl of MQW carrier; (ii) sequence (i) followed by an additional step that of picking up 71 pl of air in order to mix the different liquid fractions; (iii) sequence ( i ) in reverse order; (iv) pick-up of 5 pl of standard (or MQW) followed by 2 pl of air 2 yl of sample and 18 pl of MQW carrier; (u) sequence (iv) followed by 73 pl of air to mix the different fractions; and (vi) sequence (iv) in reverse order.The optimal sequence was the pick-up of 17 pl of MQW followed by 3 pl of sea-water 2 pl of air spacer and 5 pl of standard (or MQW); this is referred to as sequence 17-3-2-5 1 2 3 4 5 t t . . t i Or standard) - I Fig. 2 Pick-up and dispense sequence 17-3-2-5 1 pick-up carrier (17 pl MQW); 2 pick-up sea-water (3 p1); 3 pick-up air spacer (2 pl); 4 pick-up standard or MQW (5 pl); and 5 dispense total content of tip. Injected volume (sample + MQW) = 25 pl which is schematically depicted in Fig. 2 showing details of the various steps. The carrier MQW as well as effectively delivering the different segments into the furnace also serves to rinse the pipette tip for next use. A determination consists of analysing the sequence containing 5 pl of MQW followed by the sequence containing 5 pl of standard.A computer program was written and used to calculate sea-water sample concentration and is available on request. The complex sea-water matrix may generate background signals which interfere with the analyte fluorescence signal. Three types of background exist concomitant scatter molecu- lar fluorescence and non-analyte atomic fl~orescence.~~ To determine these backgrounds signals were obtained for sea- water samples at k0.05 nm away from the analytical line and were found to be smaller than the furnace blank observed at the analytical line indicating no background interference. This supports previous that Pb background signals for electrothermal LEAFS are negligible compared with the fluor- escence signal. In situ known addition analyses of MQW yielded 0.42f0.17 ng 1-1 of Pb based on 18 determinations made on two different days.This result supports a previously reported value of <0.9 ng 1-1 (ref. 27). Performance Indicators Fig. 3 shows the typical fluorescence responses for analysis of real samples unspiked and spiked at various concentration levels. The two samples PC-1 and PC-2 with salinity of 31 and 22%0 were collected from the near shore of the Pacific Ocean at Burrard Inlet (North Vancouver) and at Atchinson Point (West Vancouver). The duplicate analyses of the 17 spiked and unspiked samples produced an RSD range of 0.3-8.4% with an average RSD of 2.5%. Calibration curves for this technique are linear over at least three orders of magnitude which is more than adequate for this work.Although the complete linear dynamic range was not obtained as it was unnecessary there is no reason why it should not extend over four or five orders of magnitude. Based on the definition of the International Union of Pure and Applied Chemistry (IUPAC) the detection limit was equated to three times the standard deviation sb of eighteen determinations of the blank (3 yl of MQW) and was found to be 0.5 ng l-l corresponding to 1.5 fg absolute. It is worth noting that replicate analyses of 25 pl of MQW using standards calibration gave a 3s value of 0.6 ng l-' which is practically the same as the 3sb. In principle as the sample matrix effect is cancelled by the known addition technique the effective blank should be the furnace blank; 25 furnace blank responses were taken over a period of several weeks and calculations made318 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 r I - x 03 + Sample Fig. 3 Typical fluorescence signals for spiked and unspiked sea-water samples using the 17-3-2-5 sequence; FB =furnace blank and PC-2 (0) = unspiked sea-water sample from Pacific Ocean (values in parentheses represent amount of Pb in ng 1-'). Boxcar sensitivity was set at 2 V to the left of the vertical line and 1 V to the right of the line. based on concurrent analyses of 16 different sea-water samples giving a 3sb value equal to 0.8 ng1-'. Consequently the detection limit was taken as 1 ng I-' (3 fg absolute). This limit was further confirmed by replicate analyses of a sea-water sample containing 6ngld1 resulting in a 2s value equal to 0.9 ng 1-' which would be the usual definition of a working detection limit.52 The femtogram detection limit for sea-water analysis is to our knowledge the lowest so far achieved (Table 3).Several unspiked and spiked natural samples of various origins and salinities were used for a test of accuracy. Four Certified Reference Materials from the National Research Council of Canada (NASS-4 NASS-3 CASS-3 and SLEW-1) Table 3 Comparison of some recent detection limits were analysed. Table 4 shows that the LEAFS results compare well with the certified values. Also NASS-4 and PC-2 were spiked at 3 different levels and analysed showing adequate recoveries (Table 5). Analysis of a Pacific Ocean Profile Sea-water samples of a vertical profile were collected from a location in the Pacific Ocean between Japan and Hawaii aboard a Russian research vessel.The sampling location was 27" 47' N 174" 59' E and is referred to as AV-HS 10 (Aleksandr Vinogradov Hydro Station No. 10). Sampling details have been provided by Orians and Yang.53 The samples were Methodology DP-ASV* ETAAS-Extraction ETAAS-Chelex ICP-MS$ ICP-MS Flow injection-ETAAS LEAFS Preconcentration procedure Rotating glassy carbon Hg film electrode Chelex- 100 Chelex-100 None APDC-DDDC I-8-HOQ I-8-HOQ Original sample volume/ml 50 250-300 2000 500 1000 5 0.003 Relative detection limit/ng I-' 1 Absolute detection limit/pg 50 5 1.2 0.4 0 4 1.14 1 20T 247 20 5 q 5.7 0.003 Ref. 14 14 14 15 27 24 This work *DP-ASV = differential pulse anodic stripping voltammetry. tAssuming the usual injection volume of 20 pl was used each time.$Standard additions ICP-MS; 3 aliquots of 500 ml of sea-water were used. §Instrumental detection limit =0.03 pg 1-' (ppb) (ref. 53). TAssuming 1 ml of preconcentrated sea-water is aspirated into the ICP mass spectrometer. Table 4 LEAFS results uersus certified values for Pb in certified reference materials Certified Found/ng 1-l No. of Material Salinity (%o) Origin values/ng 1- ' f SD determinations NASS-4 31.3 North Atlantic 13-1-5 11.4 1.5 30 open ocean open ocean nearshore River estuary NASS-3 35.1 North Atlantic 39+6 40.5 f 3.0 12 CASS-2 29.2 North Atlantic 19k6 18.4+ 1.9 12 SLEW-1 11.6 St. Lawrence 28k7 30.2 f 3.0 10JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 319 Table 5 Recoveries (%) of spiked and unspiked natural samples Concentration Recovery No. of Sample Salinity (%o) Origin range/ng 1-' (%I determinations Spiked 31.3 North Atlantic 11-41 102+ 10 15 NASS-4* open ocean Unspiked 21.9 North Pacific 16.4 100 15 PC-2 near shore PC-2* nearshore Spiked 21.9 North Pacific 16-46 102 f 6 12 *Spiked at three different levels of 10 20 and 30 ng 1-l; 4 or 5 replicate analyses at each level. Table6 Pb concentrations of a vertical profile of Pacific Ocean [location AV-HS 10 (27" 47' N 174" 59' E)] Depth/m Concentration/ng 1- ' ICP-MS LEAFS* 25 13.2 14.8 f 1.4 75 13.4 16.8 f 1.1 250 15.1 17.5 f 0.1 500 12.5 16.4f 1.6 lo00 9.1 7.6 f 0.6 2 500 6.6 5.2 f 0.2 ~ ~~ ~ ~~~~~ *Value t- standard deviation of two determinations. analysed by the LEAFS method and the results compare well with those by ICP-MSS3 (Table 6).Recently published data for the Pacific and Atlantic seem to indicate similar Pb levels and profiles between the Oceans and the Great Lakes waters (particularly Lake Superior).38 Conclusion A LEAFS method has been developed for the direct determi- nation of femtogram levels of lead in sea-waters. The in situ known addition technique reduces sample preparation steps and hence possible contamination sources. It also compen- sates better for analytical line and instrumental drifts than the standards calibration technique and is a potential standard procedure using LEAFS for trace metal determination in waters of diverse matrices. The proposed LEAFS method also lends itself to easy automation and robotization. We acknowledge Professor Kristin Orians of the University of British Columbia for kindly providing profiles samples data and information on the ICP-MS method. We also acknowledge Greg Lawson for meticulous sample preparations.Thomas- Louis Tremblay and John Kraft for collecting some sea- water samples. 1 2 3 4 5 6 7 8 9 10 References Young E. G. Smith D. G. and Langille W. M. J. Fish. Res. Board Can. 1959 16 7. Weiss H. V. and Lai M. G. Talanta 1961 8 72. Riley J. P. and Taylor D. Anal. Chim. Acta 1968 40 479. Buono J. A. Karin R. W. and Fashing J. L. Anal. Chim. Acta 1975 80 327. Danielsson L.-G. Magnusson B. and Westerlund S. Anal. Chim. Acta 1978 98 47. Kingston H. M. Barnes I. L. Brady T. J. Rains T. C. and Champ M. A. Anal. Chem.1978 50 2064. Guevremont R. Sturgeon R. and Berman S. Anal. Chim. Acta 1980,115 163. Halliday M. C. Houghton C. and Ottaway J. M. Anal. Chim. Acta 1980 119 67. Sturgeon R. E. Berman S. S. Desaulniers J. A. H. Mykytiuk A. P. McLaren J. W. and Russel D. S. Anal. Chem. 1980 52 1585. Guevremont R. Anal. Chem. 1981 53 911. 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 Shaule B. K. and Patterson C. C. Earth Planet. Sci. Lett. 1981 54 97. Sturgeon R. E. Berman S. S. Willie S. N. and Desaulniers J. A. H. Anal. Chem. 1981 53 2337. Grobenski Z. Lehmann R. Radziuk B. and Voellkopf U. At. Spectrosc. 1984 5 87. Bruland K. W. Coale K. H. Mart L. Mar. Chem. 1985 17 285. McLaren J. W. Mykytiuk A. P. Willie S. N. and Berman S .S. Anal. Chem. 1985 57 2907. Sturgeon R. E. Willie S. N. and Berman S . S. Fresenius' Z . Anal. Chem. 1986 323 788. Akatsuka K. and Atsuya I. Fresenius' 2. Anal. Chem. 1987 329 435. Beauchemin D. McLaren J. W. Mykytiuk A. P. and Berman S . S. J. Anal. At. Spectrom. 1988 3 305. Nakashima S. Sturgeon R. E. Willie S. N. and Berman S . S. Anal. Chim. Acta 1988 207 291. Beauchemin D. and Berman S. S. Anal. Chem. 1989,61 1857. Sturgeon R. E. Willie S. N. and Berman S. S. Anal. Chem. 1989 61 1867. Burton J. D. and Statham P. J. in Heavy Metals in the Marine Enuironment eds. Furness R. W. and Rainbow P. S. CRC Press Boca Raton 1990 ch. 2 pp. 6-25. Liu Z.-s. and Huang S.-d. Anal. Chim. Acta 1992 267 31. Azeredo L. C. Sturgeon R. E. and Curtius A. J. Spectrochim. Acta Part B 1993 48 91.Blain S. Appriou P. and Handel H. Anal. Chim. Acta 1993 272 91. Lan C.-r. Analyst 1993 118 189. Yang L. and Orians K. J. University of British Columbia Vancouver Canada personal communication 1993. Epstein M. S. Bradshaw J. Bayer S. Bower J. Voigtman E. and Winefordner J. D. Appl. Spectrosc. 1980 34 372. Epstein M. S. Bayer S. Bradshaw J. Voigtman E. and Winefordner J. D. Spectrochim. Acta Part B 1980 35 233. Remy B. Verhaeghe I. and Mauchien P. Appl. Spectrosc. 1990 44 1633. Bolshov M. A. Kolosnikov V. G. Rudnev S . N. Boutron C. F. Gorlach U. and Patterson C. C. J. Anal. At. Spectrom. 1992 7 99. Bolshov M. A. Boutron C. F. Ducroz F. M. Gorlach U. Kompanetz 0. N. Rudniev S. N. and Hutch B. Anal. Chim. Acta 1991 251 169. Boutron C. F. Bolshov M.A. Kolosnikov V. G. Patterson C. C. and Barkov N. I. Atmos. Environ. 1990,24A 1797. Bolshov M. A. Boutron C. F. and Zybin A. V. Anal. Chem. 1989,61 1758. Falk H. Paetzold H.-J. Schmidt K. P. and Tilch J. Spectrochim. Acta Part B 1988 43 1101. Omenetto N. Human H. G. C. Cavalli P. and Rossi G. Analyst 1984 109 1067. Cheam V. Lechner J. Sekerka I. Desrosiers R. Nriagu J. and Lawson G. Anal. Chim. Acta 1992 269 129. Cheam V. Lechner J. Desrosiers R. Sekerka I. Nriagu J. and Lawson G. Int. J. Environ. Anal. Chem. 1993 53 13. Bolshov M. A. Zybin A. V. and Smirenkina I. I. Spectrochim. Acta Part B 1981 36 1143. Hendrick M. S. Goliber P. A. and Michel R. G. J . Anal. At. Spectrom. 1986 1 45. Dougherty J. P. Costello J. A. and Michel R. G. Anal. Chem. 1988 60 336. Goforth D.and Winefordner J. D. Anal. Chem. 1986 58 2598. Liang Z. Wei G.-T. Irwin R. L. Walton A. P. and Michel R. G. Anal. Chem. 1990,62 1452.320 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 44 Irwin R. L. Wei G.-T. Butcher D. J. Liang Z. Su E. G. Takahashi J. Walton A. P. and Michel R. G. Spectrochim. Acta Part B 1992 47 1497. 45 Dashin S. A. Mayorov I. A. and Bolshov M. A. Spectrochim. Acta Part B 1993 48 531. 46 Nriagu J. O. Lawson G. Wong H. K. T. and Azcue J. M. J. Great Lakes Res. 1993 19 175. 47 Amos M. D. Bennett P. A. Brodie K. G. Lung P. W. Y. and Matousek J. P. Anal. Chem. 1971 43 211. 48 Schmid W. and Krivan V. Anal. Chem. 1985 57 30. 49 Suzuki M. and Ohta K. Prog. Anal. At. Spectrosc. 1983 6 49. 50 Kinrade J. D. Kostiak W. and McCarthy G. L. in Applications Manual Scintrex AAZ-2 Zeeman Modulated Absorption Spectrophotometer Scintrex Toronto 1986 Technical note TN-3. 51 Butcher D. J. Irwin R. L. Takahashi J. Su G Wei G-T. and Michel R. G. Appl. Spectrosc. 1990 44 1521. 52 Cheam V. Analyst 1992 117 1137. 53 Orians K. J. and Yang L. University of British Columbia Vancouver Canada personal communication 1993. Paper 31038931 Received July 6 1993 Accepted September 17 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900315

出版商:RSC    

年代:1994

数据来源: RSC