Use and evaluation of poly(dithiocarbamate) in electrothermal vaporization inductively coupled plasma atomic emission spectrometry for simultaneous trace analysis of seawater and related biological materials

Spectrochrmica Acta, Vol. 438, Nos 9-11, pp. 1337-1347, Pnnted ,n Great Bntaln.

o%‘-8547/88 Kl3 00 + .oo ,Ct 1988 Pergamon Press plc

1988

Use and evaluation of poly(dithiocarbamate) in electrothermal vaporization inductively coupled plasma atomic emission spectrometry for simultaneous trace analysis of seawater and related biological mate~a~s* W. W. Laboratorium

VAN BERKEL

and F. J. M. J. MAESSEN~

voor Analytische Scheikunde, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018WV Amsterdam. The Netherlands (Recerwd

24 February

1988; in revisedform

16 May 1988)

Abstract-A method is described for the determination of traces of transttion elements in seawater and biological materials. The method comprises sample enrichment by poly(dithio~arbamate) resin and subsequent analysis of the resin by inductively coupled plasma atomic emission spectrometry. The analyte loaded resin is introduced into the plasma through graphite furnace electrothermal vaporization, Special attention has been paid to the matrix effects which occur when analytes and pyrolysis products of the resin enter the plasma stmultaneously. The bulk of the resin matrix can be separated from the analytes by submitting the furnace to a suitable temperature regime. An ashing temperature of 550°C constituted a reasonable compromise with respect to,matrix effects and premature analyte vaporization. The experimental conditions established permit the analysis of up to 20mg of resin. The method was applied to trace analysis of standard reference urine. The concentration values found showed good agreement with the corresponding certified value. 1. INTROIXJCTION

multi-element determination of trace elements, and in particular of transition elements in seawater and biological materials, has strongly increased during the last decade. The inductively coupled plasma (ICP) has been characterized as a sensitive source for atomic emission spectrometry (AES), by which metals can be determined simultaneously over a wide concentration range. However, in seawater and biological materials, the concentration of the elements of interest is often too low to be determined directly by ICP-AES. In addition, the relatively high concentration of the alkaline earth metals present in seawater and biological materials induces severe spectral interferences for several sensitive analysis lines. However, both problems can, in principle, be overcome by the application of enrichment procedures, i.e, procedures aimed at the enhancement of the analyte-matrix ratio of the analysis sample. Enrichment of seawater and biological materials by using chelating agents such as 8-Hydroxyquinoline [l, 21, Chelex-100 [3-S] and Poly(dithiocarbamate) (PDTC) [6-141 has found widespread application. PDTC has the advantage, over chelating agents like S-Hydroxyquinoline and Chelex-100, that it hardly shows any affinity to alkali and alkaline earth metals. PDTC resin has been used for the enrichment of various types of samples including urine [6-91, high purity graphite [lo], coal and shale oil [ 111, bone [12], and serum [13]. For those materials quantitative sequestration was obtained for most of the trace elements investigated, whereas matrix separation was accomplished to such an extent, that the total quantity of alkaline earth metals was reduced with more than seven orders of magnitude [14]. However, the trace elements sequestered by the PDTC resin cannot be recovered by simple elution of the resin with a solution of hydrochloric or nitric acid as is the case with 8-Hydroxyquinoline and Chelex-100. To recover the trace elements from PDTC, the resin has to be mineralized completely. To that end, wet ashing with a mixture of H,O, and HNO, and dry ashing with and without HF have been applied [is]. However, the rather laborious procedure comprising mineralization of the resin followed by analysis of the resulting aqueous solution can be omitted if the analyte loaded resin is directly introduced into the ICP. Recently, several techniques have been described concerning the THE DEMAND for

*This article is dedicated to Prof. C. TH. J. ALKEMADE and is published in a special Honour Issue. ‘Author to whom correspondence should be addressed. 1337

1338

W. W.

VAN

BERKELand F. J. M. J. MAESSEN

small quantities of samples into the ICP, by, e.g. direct sample insertion [16, vaporization [22,23]. Sample introduction by means of electrothermal vaporization offers the following distinct advantages: (i) small quantities of sample can be analyzed, (ii) as compared to pneumatic nebulization, a very high sample transport efficiency is obtained and (iii) in the case of solid sample vaporization the bulk of the organic matrix can be separated from the analysis elements by the application of an appropriate temperature regime. Although the limits of determination for ETV-ICP-AES are very attractive [23], significant matrix effects are observed when alkali and alkaline earth metals are present in the sample in major concentrations [24]. Therefore, Azrz et al. [25] concluded that reliable trace analysis of biological materials by ETV-ICP-AES requires the application of standard addition. However, the effect exerted by the matrix constituents of biological materials on the transient ETV-ICP-AES signal seems to be greater for peak height than for peak area measurements. Thus, at least for Pb, HULL and HORLICK [26] found that in contrast to the peak height, peak area was hardly dependent on the NaCl concentration in the sample. The aim of the present study was to explore the analytical performance of ETV-ICP-AES when used in combination with PDTC for the enrichment of seawater and biological materials. BARNES and FODOR [9] examined this method employing a solution of the mineralized resin as the sample. In the present study, however, the analyte loaded resin was directly introduced into the ETV device. Apart from the potential advantages offered by ETV as well as PDTC, it was considered worthwhile to examine PDTC-ETV-ICP-AES because, with this technique, matrix effects (including physical, chemical, and spectral interferences) are independent of the nature of the original sample. introduction

of

171, arc and spark ablation [ 18, 191, laser ablation [ZO,2 13,and electrothermal

2. EXPERIMENTAL 2.1. Instrumentation A Jarrell-Ash AtomComp model 975 ICAP multichannel spectrometer equipped with a monochromator was used throughout this study. Operating parameters and the spectral lines investigated are summarized in Table 1. The data acquisition equipment of the spectrometer was replaced by a 1401 multi-channel intelligent laboratory interface (Cambridge Electronic Design, Cambridge, England), which was connected to the photomultiplier tubes (PMTs) by a current voltage converter. The interface digitized the signals from the PMTs up to a sample speed of 5 x lo4 points SK’, at 12 bit accuracy. Because of the limited linear range of the 1401 interface, an amplifier/filter was placed between the PMTs and the 1401 interface. This home-built instrument consists of an 8-channel system, of which the amplifier has a range of l-1000, and the filter a cut-off frequency of O.l-5OOHz. The operating conditions for the amplifier/filter system are summarized in Table 1. The 1401 interface was controlled by a BBC Master microcomputer. The data obtained were initially stored on disk by the BBC microcomputer and subsequently sent to an HP 1000 F minicomputer, at which the actual data processing took place. A HGA-74 heating device with matching graphite furnaces (Perkin Elmer, Ueberlingen, F.R.G.) was used for pyrolysis of the PDTC resin and analyte vaporization. A gas-tight vaporization system was realized by replacing the windows of the furnace holder by conically shaped nylon caps. These caps, which ended in ribbon gas pipes, were tightly attached to the heating device by O-rings which originally served for fixation of the windows. The furnace was made to form part of the conduit-pipe for providing the inner gas to the ICP by connecting the gas inlet side of the furnace to the flowmeter and its outlet side to the torch. So, the furnace was constantly flushed with argon. The furnace outlet nylon tubing was connected to the inner tube of the torch by means of a ball and socket joint. The total distance of the furnace to the ICP was about 60cm. The quartz sampling boat, with an internal length of 10 mm, and a maximum capacity of40 mg of resin, could easily be introduced into the furnace via the gas inlet side. For reproducible positioning of the boat in the furnace, a calibrated quartz rod was used as a sliding tool. The resin was deposited on the bottom of the furnace by tumbling the sampling boat. 2.2. Reagents and test solutions The poly(dithiocarbamate) (PDTC) resin was synthesized according to the procedure as described by HACKETT and SIGGIA [27]. The reaction time between the molar equivalent ratio 8/l polyethyleneamine M= 1800 (Polyscience Inc., Warrington, PA, U.S.A.) and poly[methylene(polphenyl-

Trace analysis

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1339

Table 1. Expenmental conditions ICP conditions and analysis lines 1.3 16 1.0 1.2

Forward power (k W) Outer gas flow rate (I min- ‘) Intermediate gas flow rate (I min- i) Inner gas flow rate (1 min _ ’ ) Observation height above induction coil (mm)

17

Ionization potential Excitation potential

Wavelength Species Cd cu cu Mn Pb Zn CI

I I II II II I

(V)

(nm)

(V)

I

II

228.802 324.754 224.700 257.610 220.353 213.199 199.362

5.42 3.82 8.23 4.81 7.35 5.79 7.45

8.99 7.72 7.72 7.43 7.42 9.39 11.26

16.90 20.29 20.29 15.64 15.03 17.96 24.38

Data acquisition

interface

and amplifier/filter

systems

For the determination Test elements Sample frequency (Hz) Filter frequency (Hz) Number of points collected/channel

of

Carbon

20 5 400

5 2 600

Electrothermal vaporization device Compromise conditions Cycle

Temperature

Drying Ashing

130 130-550 550 2600

Vaporization Flow rate of the shieldmg

(“C)

Period (s) 20 46 20 12

gas: 0.3 1min- ’

isocyanate)] (Aldrich Chemie, Brussels, Belgium), and carbondisulfide (Merck, Darmstadt, F.R.G.) was kept for 48 h instead of one month [28]. The synthesized resin was dried at 40 “C and subsequently sieved. The 16s250pm fraction was used for all experiments. For the preparation of all the solutions employed, subboiled water was used. Test samples containing traces of Cd, Cu, Mn, Pb, and Zn were prepared from 1 gl-’ BDH Chemical Ltd., (Poole, U.K.) standard solutions. Artificial seawater test samples were prepared from “Suprapur” grade salts, viz. CaCl,.4H,O, KCl, Mg(N0,),.6H,O, and NaCl (Merck). Fot the sequestration experiments with the PDTC resin, 100 ml 0.05 M NH,Ac pure water samples and 100ml 0.05 M NH,Ac artificial seawater samples were spiked with the test elements to a concentration of 1 mgl- ‘. The pH of the samples was adjusted to 5.0 with HNO,. Resin samples, loaded with known quantities of the test elements, were obtained by using 25ml portions of 0.05 M NH,Ac solutions with a pH of 5.0. The concentration of the test elements in these solutions was varied from 0.2 to 200 pgl-r.

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W. W.

VAN BERKEL and F. J. M. J. MAESSEN

2.3. Sequestration and enrichment procedures The sequestration experiments were performed by addition of 1 g of PDTC resin to both the pure water test samples and the artificial seawater test samples. The mixture was shaken for about 24 h in an erlenmeyer flask, and subsequently filtered over a 1.2 pm millipore filter (Millipore, Molsheim, France). In order to establish the extent to which the trace elements were sequestered by the PDTC resin, the filtrate was enriched according to the Chelex-100 procedure as described by VAN BERKEL et al. [S], and subsequently routinely analyzed by ICP-AES. The remaining experiments were carried out with 25 ml test samples, to which 0.5 g resin was added. These mixtures were also shaken for 24 h and filtered, after which the residue was rinsed three times with subboiled water and subsequently thoroughly dried at 60 “C. Prior to analysis the dried resin was ground for about 2min by means of an agate ball mill, and stored in a dry place. A human reference freeze-dried urine sample (Seronorm Trace Elements, Nycomed AS, Oslo, Norway) was reconstituted by addition of 10 ml 0.05 M NH,Ac of pH 5.0. To the reconstituted urine sample 0.2Og PDTC resin was added whereafter the samples were shaken, filtered, and ground as described above. 2.4. Data acquisition Sampling frequency, filter frequency, and the number of data points collected are summarized in Table 1. Peak area and peak height were determined by a non-linear regression method as described by SCHEEREN et al. [29]. This method involves deconvolution of the analytical signal into a number of peaks (or just into a single peak) and the corresponding baseline. Preference was given to peak area measurements, because in ETV-ICP-AES, peak area rather than peak height should be used as a measure for concentration, as was demonstrated by ALVARADO et al. [30].

3. RESULTS AND DISCUSSION 3.1. Sequestration Table 2 lists the results of the sequestration by PDTC resin of the test solutions considered. The poor but nevertheless similar results obtained for the sequestration of manganese from pure water and artificial seawater indicate that, under the enrichment procedure applied, sequestration of transition elements is not affected by alkali or alkaline earth metals. This property of PDTC can, in principle, advantageously be used for the modification of, in particular, environmental samples like seawater and biological materials. A survey of matrix compositions which are typical for seawater, and human body fluids and tissues can be found in Refs [31] and [32], respectively. 3.2. Ashing conditions In order to prevent spattering of the analyte loaded resin upon its abrupt exposure to relatively high temperatures, the resin was, prior to ashing, subjected to a temperature of 130 “C for 20 s. For convenience, that stage of the applied temperature regime is called the drying phase, notwithstanding the fact that the resin had been thoroughly dried before it was introduced into the furnace (see Experimental Section). During the ashing stage, the temperature was ramp-wise raised from 130 “C to the desired final temperature, whereafter the furnace was kept at that temperature for 40s. Subsequently, the charred resin was partly vaporized by step-wise raising of the furnace temperature to the highest value attainable, viz. 2600 “C. This “vaporization” temperature was maintained for 12 s. Table 2. Sequestration (%) of the test elements by PDTC

Test element Cd cu Mn Pb Zn

resin

Subboiled water

Artificial seawater

>99 >99 14 >99 >98

>99 >99 14 >99 >98

Trace analysis of seawater and related biological materials

1341

For simplicity, the adjusted final temperature in the ashing stage is called the “ashing” temperature. In order to prevent the plasma from being extinguished during the ashing stage, the experiments aimed at establishing optimum ashing conditions were performed using a heating rate not exceeding 10 “C s - l. Because of the limited digestion capacity of the plasma for the pyrolysis products of PDTC, a higher heating rate was not feasible, at least not for the eventually chosen resin furnace load of 20mg. Figure 1 shows the net line intensities of the selected analysis lines (cf. Table 1) which were obtained after the application of various ashing temperatures. An adequate discussion of Fig. 1 is facilitated, however, when the following aspects associated with the use of PDTC resin in ETV-ICP-AES are considered first. (i) Matrix effects may originate from processes occurring in the furnace as well as in the plasma. (ii) The bulk of the pyrolysis products enters the plasma during the ashing stage. The analysis elements, however, enter the plasma during the vaporization stage. (iii) The matrix constituents which are in principle capable of influencing the excitation conditions of the plasma during the vaporization stage consist of the partly charred molecular network of the resin which remains in the furnace after completion of the ashing stage. (iv) The composition of the remaining matrix gradually changes with increasing ashing temperature. (v) Both quantity and volatility of the remaining matrix sharply decreases with increasing ashing temperature. This holds for relatively low ashing temperatures in particular. With reference to Fig. 1 it is interesting to observe that the net line intensity of the Pb II, Cd I, and Zn I lines initially sharply increases with increasing ashing temperature. Figure 1 also shows that the extent to which the net line intensity is increased by enhancement of the ashing temperature increases with the “hardness” [33-353 of the spectral lines considered (for ionization and excitation potentials see Table 1). These observations strongly indicate that the effect of the ashing temperature can be explained by the occurrence of matrix effects in the ICP which result from the simultaneous -introduction of analytes and pyrolysis products into the plasma during the vaporization stage. For, the lower the ashing temperature, the larger the quantity of charred resin that is still present in the furnace at the beginning of the vaporization phase. As a consequence, the quantity of pyrolysis products that enters the plasma simultaneously with the analytes during the vaporization stage decreases with an increase of the ashing temperature. An impression of the plasma load which results when various temperature regimes were applied, is presented in the next section. From the point of view of ICP excitation conditions a relatively high ashing temperature should be applied. On the other hand, prevention of analyte losses during the ashing stage urges moderate ashing temperatures. Thus, the curve for Cd in Fig. 1 illustrates the occurrence of premature volatilization at a relatively low temperature. Therefore, in analytical practice a compromise is required. An ashing temperature of 550 “C was considered satisfactory for the simultaneous determination of Cd, Cu, Pb and Zn. Figure 2 shows an emission signal that is representative for these elements.

Fig. 1. The relative intensity of Cd (Cl), Cu ( l ), Pb (0) and Zn ( n ) vs the final ashing temperature.

1342

W. W.

VAN

BERKELand

F.

J. M. J.

MAESSEN

Fig. 2. Transient emission profile for 8 mg of PDTC-resin, contaming 5 fig g- ’ of Zn. Experimental conditions, cf. Table 1.

lo

3000

(a)

r---, f

i

;

L!__<_____

k_______]

!

i

k; 0

20

40

60

60

100

120

100

720

Time (s)

1.0 06

T 9

> f E=

06 04 02

00;

20

40

60 60 Time (s)

I

3000

06 2 S 06 r> 5 04 E 02

‘line (s)

Fig. 3. Temperature regime (dashed line) and carbon line intensity (solid line) for ashing temperatures of 450 “C (diagram a), 500 “C (diagram b), 550 “C (diagram c) and 700 “C (diagram d).

Trace analysis

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The transient signals were obtained under the experimental conditions stated in Table 1. Because of its poor sequestration, manganese was rejected as a test analysis element. 3.3. Plasma load and excitation conditions In order to get an impression of the “organic” plasma load in the vaporization stage, the intensity of the C I line was measured continuously for various temperature regimes. The regimes applied encompassed four ashing temperatures. Figure 3 shows the results. The solid curves represent the development of the carbon emission intensity and the dashed curves the corresponding development of the furnace temperature in a complete heating cycle. In order to enable a meaningful discussion of Fig. 3, it must be realized that the C I emission intensity merely provides a rough indication of the plasma loading with organic material. For, owing to organic material, in particular during the ashing stage when the pyrolysis products have a complex composition, the excitation conditions in the ICP may change rapidly with time. Once the resin has been ashed, the conditions become better defined as to both the release of carbon containing vapour and the excitation conditions. This means that the C I emission intensity as a measure for the “organic” plasma load is more appropriate for the vaporization stage than it is for the ashing stage. Comparison of diagrams ad in Fig. 3 then shows that the “organic” plasma load during the vaporization stage decreases dramatically when the ashing temperature is raised from 450 to 550 “C whereas this load hardly decreases further beyond 550 “C. The transient C I emission line intensities measured during the vaporization stage show small but persistent “shoulders”. Since these small peaks were also observed when unloaded furnaces were used, they most probably originate from reactions of the graphite of the furnace with traces of O2 and H,O, which were present in the argon used. The relation between the “organic” plasma load and the excitation conditions of the ICP was examined by measuring the variation of the intensity ratio of the Cu II to Cu 1 line. The intensity ratio may be considered a parameter that is highly sensitive to changes in the excitation conditions [36]. Figure 4 shows the Cu II/Cu 1 intensity ratio vs the ashing temperature and consequently as a function of the “organic” plasma load. When the increase of the Cu II/Cu I ratio with decreasing carbon content is considered, the variation of net line intensity with ashing temperature as depicted in Fig. 1 may be explained by the gradual change of the ICP excitation conditions produced by the pyrolysis products entering the plasma simultaneously with the analysis elements during the vaporization stage. The transient C I line intensities emitted during the vaporization stage were measured by employing the peak deconvolution procedure as described in the experimental section. From the high precision of these measurements (RSD better than 5%) it follows that the “organic” plasma load is very reproducible when a well defined ashing procedure is applied. As a consequence, reproducible excitation conditions during the vaporization stage can be achieved.

025

J 450

500

550

6ocl

650

700

750

800

Ashing temperature (‘Cl Fig. 4. The Cu II to Cu I line intensity

ratlo vs the ashing

temperature.

W. W. VAN BERKEL and F. J. M. J. MAESSEN

1344

3.4. Quantity of resin in the furnace In general, the precision, accuracy and sensitivity of the PDTC-ETV-ICP-AES

method is improved when the quantity of resin, introduced into the furnace, is raised. For, the use of a relatively large amount leads to small weighing errors, whereas the requirements concerning the uniformity of the analyte distribution on the resin become less stringent. As to the sensitivity, it is a matter of course that the detection limits will decrease with an increasing quantity of resin used for a single analysis. As shown in Fig. 5, for up to 20 mg of resin a linear relationship was observed for the spectral lines of Cd, Cu, and Zn. Greater quantities of resin, e.g. 30mg, resulted inevitably in extinction of the plasma during the ashing stage, at least when treated under the ashing conditions applied. The amount of resin in the furnace may be raised beyond 20 mg without plasma extinction, provided that a more moderate ashing temperature regime is applied. However, since other factors like the limited capacity of the sampling boat-furnace combination employed, prevent the use of quantities significantly greater than 20mg, the latter quantity was considered satisfactory in the present stage of investigation. A mutual comparison of the curves for Cd, Cu, and Zn of Fig. 5, which concern the atomic lines, shows that all measuring points which correspond to a specific quantity of resin are located similarly with respect to each curve. Since the intensities which correspond to a specific quantity of resin have been measured simultaneously, it must be concluded that the results as depicted in Fig. 5 are affected with procedural errors [37]. Additional experiments revealed that the inner gas flow rate (see Table 1) is an extremely critical parameter in PDTC-ETV-ICP-AES. Consequently, the value of this parameter must be adjusted with utmost care. From measurements performed with fixed quantities of resin, covering the range of 4-20mg, the relative standard deviation was calculated to be 6% of the mean for four replicate measurements, irrespective of the quantity of resin analyzed. Therefore, weighing errors and errors due to inhomogeneous analyte loading of the resin are of minor importance compared to the errors which result from an insufficient control of the inner gas flow rate. 3.5. Quantity of the analyte on the resin Figure 6 shows the net line intensity of Cd (diagram a) and Cu (diagram b) vs the amount of analyte on the resin. Since diagram a is also representative for Pb and diagram b for Zn, the results for these elements are not explicitly shown. All calibration curves were measured using 20 mg of PDTC resin. For both Cd and Pb a linear relation was found for the amounts of analyte considered. However, owing to the concentration of Cu and Zn in the blank resin,

Ouantuty of rasm (mg)

Fg.

5. The hne intensity

of Cd (0). Cu (V), Pb (A). and Zn (M) vs the quantity analyzed. Expertmental conditions, cf. Table 1.

of PDTC

resm

Trace analysis

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1345

10' ; =,

m f

1031

i

s

102

1 ;

10'

1 10'

102

103

10'

106

Test element content of the PDTC-resm(ngg”) cul

lo’

lo*

ld

104

lo6

Test element content of the PDTC-resm Ins @‘I Fig. 6. The line intensity of Cd (diagram a) and Cu (diagram b) vs the quantity of the corresponding elements on the PDTC resin. Experimental conditions, cf. Table 1.

as well as to the unavoidable contamination of the analysis sample by these elements, a linear relation between the quantity of analyte and the analytical signal was obtained not before the resin was externally loaded with about 1 pg g- ’ of Cu or Zn. Table 3 lists the minimum quantities (MQs) of the test element which have to be present in the furnace in order to produce significant (3a,,) analytical signals. The MQ-values presented can still further be reduced by optimizing the experimentally variable operating conditions. Thus preliminary experiments, carried out with various inner gas flow rates and observation heights indicated that reduction, of the MQ-values with an order of magnitude is feasible. The limits of determination of the method studied depend apart from the MQ-values, on the attainable concentration factor. The concentration factor is the ratio of the mass of the sample to the mass of the resin which is needed for quantitative sequestration of the analyte from the sample. For a given sample, the quantity of resin needed depends on the capacity of the resin. However, quantitative analyte sequestration can be performed provided the capacity is only partly utilized whereas the allowed extent of capacity utilization depends on Table 3. Mirumum quantities (MQ) to be present in the furnace in order to obtain a significant analytlcal signal (3~~~) when 20mg of PDTC resin is analyzed Minimum Test element Cd cu Pb Zn

quantity

(ng) 0.20 0.050 1.0 0.15

W. W. VAN BERKELand F. J. M. J. MAESSEN

1346 Table

4. Results of the analysis of a standard reference sample. Concentration factor: 50

Analysrs

element

Cd cu Pb Zn *Standard

deviatton

urine

Found value (ngml-‘)

Certified value (ngml-‘)

7.1 +0.3 46_+2 94+4 320+ 10

6.6,1 45* 88* 320*

not reported.

the nature of the analyte [38]. Considering these aspects and also the possibility of the presence of non-analyte transition elements in the sample, the limit of determination attainable with the PDTC-ETV-ICP-AES method, highly depends on the composition of the sample. 3.6. Real sample analysis The applicability of analytical methods can in general be demonstrated in a satisfactory way by employing the method under discussion for the analysis of standard reference materials. However, standard reference materials with certified concentrations of transition elements which are sufficiently low to enable a convincing demonstration of the analytical possibilities of PDTC-ETV-ICP-AES are not available yet. Nevertheless, the method studied was applied to the analysis of a standard reference material (urine) that, at least for an element of special analytical interest (cadmium), contains an attractively low (6.6 ng ml- ‘) concentration. The analytical results are listed in Table 4. The concentration values found showed good agreement with the corresponding certified values. 4. CONCLUSION The results of the study described indicate that PDTC-ETV-ICP-AES is a promising method for the determination of microtraces of transition elements in samples of which the matrix consists predominantly of alkali and alkaline earth metals. A serious limitation of the method is the high concentration of elements of frequent occurrence (e.g. Zn and Cu) in the purest reagents commercially available. Nevertheless, the potentially high detection power of the method justifies a systematic investigation of the optimum resin-to-sample ratio attainable for samples with strongly varying contents of non-analyte transition elements, e.g. seawater and whole blood. Acknowledgements-J.

C. COPPENSis greatly

acknowledged

for his assistance

in this study.

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S. N. Wrllie, R. E. Sturgeon and S. S. Berman, Anal. Chim. Acta 149, 59 (1983). J. A. Buono, R. W. Karin and J. L. Fasching, Anal.Chim. Acta 80, 327 (1975). J. W. Jones, S. G. Capar and T. C. O’Haver, Analyst 107, 353 (1982). R. E. Sturgeon, S. S. Berman, A. Desaulniers and D. S. Russell, Talanta 27, 85 (1980). W. W. van Berkel, A. Overbosch, G. Feenstra and F. J. M. J. Maessen, .I. Anal. Atom. Spectrom. 3,249 (1988). R. M. Barnes, P. Fodor, K. Inagaki and M. Fodor, Spectrochim. Acta 38B, 245 (1983). P. Fodor and R. M. Barnes, Spectrochim. Acta 38B, 229 (1983). Z. Mtanzhr and R. M. Barnes, Spectrochim. Acta 38B, 259 (1983). R. M. Barnes and P. Fodor, Spectrochim. Acta 38B, 1191 (1983). H S. Mahanti and R. M. Barnes, Anal. Chem. 55,403 (1983). H. S. Mahantt and R. M. Barnes, Anal. Chim. Acta 149, 395 (1983). H. S. Mahanti and R. M. Barnes, Anal. Chim. Acta 151, 409 (1983). Z. Mianzhi and R. M. Barnes, Appl.Spectrosc. 38, 635 (1984). R. M. Barnes and J. S. Genna, Anal. Chem. 51, 1065 (1979). R. S. S. Murthy, Z. Horvath and R. M. Barnes, J. Anal. Atom. Spectrom. 1, 269 (1986).

Trace analysis of seawater and related biological materials [16] [17] [18] [19] [20] [Zl] [22] [23]

[24] [25] [26] [27]

[28] [29] [30] [31]

[32] [33] [34]

[35]

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