Simultaneous Determination of as, CD, Cr and Pb in Aqua Regia

8/12/2019 Simultaneous Determination of as, CD, Cr and Pb in Aqua Regia

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Spectrochimica Acta Part B 57 (2002) 16811688

0584-8547/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S058 4-8 547 0 2 .0 0 1 2 3 - 4

Simultaneous determination of As, Cd, Cr and Pb in aqua regiadigests of soils and sediments using electrothermal atomic

absorption spectrometry and fast furnace programs

Timo Myohanen *, Vaino Mantylahti , Kalevi Koivunen , Rose Matilainena, b b c

Geological Survey of Finland, Geolaboratory, P.O. Box 1237, FIN-70211 Kuopio, Finlanda

Soil Analysis Service Ltd., P.O. Box 500, FIN-50101 Mikkeli, Finlandb

Department of Chemistry, University of Jyvaskyla, P.O. Box 35, FIN-40351 Jyvaskyla, Finlandc

Received 26 February 2002; accepted 31 July 2002

Abstract

A method was developed for the simultaneous multi-element determination of As, Cd, Cr and Pb in aqua regiadigests of soils and sediments using an electrothermal atomic absorption spectrometer with a transversely heatedgraphite atomizer and longitudinal Zeeman-effect background correction. A fast furnace program with no pyrolysis

stage or chemical modifier was used, and the total duration of the time-temperature program of the graphite furnacewas 45 s. The method detection limits calculated from blank samples were 9.5 mg l , 0.18 mg l , 6.2 mg l andy1 y1 y1

2.5 mg l for As, Cd, Cr and Pb, respectively. Sample preparation procedures are briefly described. The accuracyy1

of the method was confirmed with a certified reference soil (recoveries 90110%, within-batch RSD most often lessthan 5% with six replicates) and with 12 sediment samples from an international proficiency test. z-scores werecalculated for the results of the sediment samples with 46 satisfactory and only 2 questionable results. Some analyticalproblems that were encountered during analysis are described. Fast furnace programs were found to be suitable forthe simultaneous determination of the four elements in aqua regia digests of soils and sediments. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Electrothermal atomic absorption spectrometry; Multi-element determination; Fast furnace program; Aqua regiadigestion; As; Cd; Cr; Pb

1. Introduction

The need for accurate and precise analyses oftoxic metals is becoming more and more important,as we learn more about their behavior in theenvironment and in the food chain. The harmful

*Corresponding author. Fax: q358-20-550-3660.E-mail address: [email protected] (T. Myohanen).

effects of many of these metals to man and natureare well known. While the number of elements tobe determined continues to increase, there is alsoa demand for low detection limits. A major sourceof heavy metals in the food chain is the agriculturalsoil, which is a complex and demanding matrixfor chemical analyses. The producers of foodstuffsare more often than ever faced with demands ofclean products, and in many countries national


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1682 T. Myohanen et al. / Spectrochimica Acta Part B 57 (2002) 16811688

limits for heavy metal contents are being set toclassify agricultural soils as uncontaminated orcontaminated. In Finland, new limits of this kind

have recently been proposed w1x.Digestion withaqua regia is a common method

for the determination of trace elements in soilsand sediments. International standard methodsexist for both the digestion of elements soluble inaqua regia w2x and the following determination ofelements in the digests using atomic absorptionspectrometric methodsw3x.This proves that atomicabsorption spectrometry is regarded as a suitabletool for the analyses of heavy metals in soil.Graphite furnace atomic absorption spectrometry

(GFAAS) is often the method of choice when thecontents of the elements to be determined arerelatively low.

Atomic absorption instruments capable of simul-taneous multi-element analysis can significantlyreduce the analytical time w4x. It is also possibleto speed up electrothermal atomic absorption spec-trometric determinations by using fast furnaceprograms. The principles of these rapid determi-nations have been introduced and tested for olderlongitudinally heated graphite furnacesw5x and forthe modern transversely heated furnaces w6x with

references to previous literature on the subject. Cd,Co, Cu and Pb have been determined simultane-ously in a variety of matrices using the sameinstrument as in this study and a furnace programno longer than 30 s w7x. More recent applicationsof fast furnace programs include their use incombination with a permanent chemical modifierw8x and slurry sampling w9x.

The aim of this study was to test whether fastfurnace methodology could be applied with simul-taneous determination of four trace elements, As,

Cd, Cr and Pb in a sample matrix that is generallyconsidered difficult because of high acid andchloride content. The experimental part of thiswork was done in the laboratory of Soil AnalysisService in Mikkeli, Finland.

2. Experimental

2.1. Instrumentation

The analyses were carried out using a PerkinElmer Model SIMAA 6000 electrothermal atomic

absorption spectrometer w10x with a transverselyheated graphite atomizer (THGA), longitudinalZeeman-effect background correction and an

AS 72 autosampler device. Hollow cathode lampswere used for Cd, Cr and Pb measurements andan electrodeless discharge lamp was used for thedetermination of As. All lamps were operatedapplying the default currents. End-capped THGAgraphite tubes with an integrated Lvov-type plat-form and pyrolytic graphite coating were usedthroughout. Argon (AGA, Finland) was used asprotective gas.

The analytical wavelengths used were 197.2 nmfor As, 228.8 nm for Cd, 357.9 nm for Cr and

283.3 nm for Pb. The secondary less sensitivewavelength was selected for the determination ofAs instead of the usual 193.7-nm line to overcomeextensive background interference in somesamples.

2.2. Reagents and calibration

Only deionized water was used to prepare solu-tions for the analyses. Hydrochloric acid (32%)and nitric acid (65%, max. 5 ppb Hg) used weresupplied by J.T. Baker. Multi-element standards

for calibration were prepared by progressive dilu-tion of single-element stock solutions (1000 mgl , Reagecon, Ireland). The final dilution of they1

standards was done in a solution containing 7.5%(vyv) HCl and 9.4% (vyv) HNO , and this was3also used as the zero-setting solution for calibra-tion. This was done to match the acid matrix ofthe standards with samples.

The multi-element calibration standard con-tained 200 mg l As, 4.0 mg l Cd, and 120 mgy1 y1

l both Cr and Pb. Five-point calibration curvesy1

were prepared for each element with the autosam-pler by taking different amounts (210 ml) of thestandard. A linear calibration equation with non-zero intercept was used for As and a non-linearequation with zero intercept for the other elements.All measurements were based on integratedabsorbance. A total sample volume of 10 ml wasused, and it was injected in the furnace togetherwith 2 ml of the zero-setting solution as diluent tohelp rinse the autosampler tubing. No matrix mod-ifier was used. Two or more replicates were meas-


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1683T. Myohanen et al. / Spectrochimica Acta Part B 57 (2002) 16811688

Table 1Timetemperature program of the graphite furnace for thesimultaneous determination of As, Cd, Cr and Pb. Sampleinjection temperature 100 8C

Step Temperature Ramp Hold Read Ar flow(8C) (s) (s) (ml min )y1

Dry 120 1 15 250Dry 250 1 15 250Atomize 2200 0 7 X 0Clean-out 2550 1 5 250

ured from each sample solution to obtain a RSDof 6% or less for samples within the calibrationrange.

2.3. Samples and sample preparation

A standard reference soil CMI 7003, Silty ClayLoam with normal analyte levels (Analytika Ltd.,Czech Republic) with certified and relatively lowaqua regia soluble contents for a number of tracemetals was selected as a test material for themethod. The soil was certified by the CzechMetrological Institute w11x. Twelve different sedi-ment samples from an international proficiencytest w12x were also analyzed. The results of the

interlaboratory test, aqua regia soluble trace ele-ment contents of the sediment samples, werealready known w12x, and the samples had alsobeen analyzed previously using the in-house sin-gle-element GFAAS methods. Some in-house ref-erence materials were analyzed as well.

In the in-house method for digestion with aquaregia (method A)ca. 1.0 g of air-dried and groundsoil is accurately weighed in a digestion tube. WithCMI 7003 and the sediment samples normal air-drying and grinding could be omitted. The dry-

matter content of the samples is determined bydrying a separate portion of the air-dried sampleat 105 8C until constant weight. Two to threemilliliters of deionized water, 7.5 ml HCl and 2.5ml HNO are then introduced in the digestion tube,3and the tubes are allowed to stand covered over-night at room temperature. The digestion(2 h, 1408C) is then performed in a Gerhardt SMA 20 Adigestion apparatus (Gerhardt, Germany). Aftercooling, the digest is filtered into a 100-ml volu-metric flask. The digestion apparatus is rinsed with

deionized water, and the insoluble residue iswashed with 50 ml warm 14% (vyv) nitric acidintroduced in five portions. The digest is diluted

to volume with deionized water.Some portions of CMI 7003 were digested for

comparison by following the digestion proceduregiven in the certificate more strictly. This proce-dure (method B) uses a 3.0-g weighing and con-sequently more acids (22.5 ml HCl and 7.5 mlHNO ). The mixture is shaken manually and3allowed to stand covered for 16 h. The digestiontakes 2 h, and it is performed in the same apparatusas in method A using the same temperature. Theapparatus is rinsed with 20 ml of warm 5% (vyv)

HNO . After filtration into a 100-ml volumetric3flask, the residue is rinsed three times with 5 mlof 5% (vyv) nitric acid and diluted to mark withit.

3. Results and discussion

3.1. Optimization of the THGA timetemperature

program

A timetemperature program that had been usedfor the fast single-element determinations of vari-

ous elements in a THGA furnace in a previouspaper by Li et al. w6x was selected as a startingpoint for the determination. Compared to tradition-al furnace programs the optimization of a fastfurnace program is relatively easy, because thepyrolysis stage and often the matrix modifier aswell can be left out. In a simultaneous determina-tion, the atomization temperature has to be selectedaccording to the least volatile element, in this caseCr, so that all elements are successfully atomized.The drying step needs to be long enough to allow

complete vaporization of the diluent. Injecting thesample solution into a pre-heated furnace furthershortens this stage.

The injection temperature was set at 100 8C assuggested by Li et al. in their paper. Consequently,the drying stage (steps 1 and 2 in Table 1) takesa little longer. The use of an end-capped graphitetube also slows the vaporization. The recommend-ed atomization temperature for Cr is 2300 8Cw13x,but 2200 8C was selected as a compromise forstep 3, because it had been used successfully in


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Table 2Observed characteristic masses(m )for As, Cd, Cr and Pb and0published reference values for end-capped THGA tubes w14x

Element Observed Ref. w14x(pg) (pg)

As 45a 22b

Cd 1.5 1Cr 5.5 4.9Pb 33 21

197.2 nm wavelength used.a

193.7 nm wavelength used, twice the sensitivity of theb

197.2-nm line.

Table 3Method detection limits and limits of quantification for the

simultaneous determination of As, Cd, Cr and Pb, includingdigestion

Element LOD LOQ(mg l )y1 (mg l )y1

As 9.5 23Cd 0.18 0.58Cr 6.2 15Pb 2.5 7.9

Divide by 10 to obtain corresponding values in dry soil orsediment in mg kg , assuming digestion method A. See texty1

for details of the calculation.

the in-house single-element method used previous-ly. The atomization time was extended by 2 sbecause of slower vaporization of Cr in an end-capped tube. The clean-out temperature was raisedas well as the hold time of this stage (step 4),because some memory effects were observed forCr in high concentrations. The total duration ofthe final timetemperature program was 45 s. Thisprogram is shown in Table 1. The characteristicmasses (m ) observed are presented in Table 20with reference values from w14x. The high m0values of the most volatile elements Cd and Pbcan at least in part be explained with the high

atomization temperature used and increased diffu-sional losses. Other m values (As and Cr) are0within "20% of the expected values, taking theless sensitive wavelength for As into account. Still,the characteristic mass of Cr is a bit high as well,which can imply somewhat imperfect atomizationfor this element and the need for even highertemperatures. Ideal atomization conditions cannever be achieved for all elements in a multi-element determination.

3.2. Limits of detection and quantification

Limits of detection (LOD) and quantification(LOQ) were calculated from 24 blank samplesdigested in four batches of six samples on differentdays. Detection limits were calculated from themean and standard deviation s of the blankx0 0samples (all 24 replicates) as follows:

Z ZLODsx q3s (1)0 0

Z ZLOQsx q10s (2)0 0

In this case, the instrumental detection limitsare much lower. The results are method detectionlimits, which include any possible impact from the

digestion or laboratory contamination, and they donot represent the true performance of theinstrument.

The observed method detection limits are tabu-lated inTable 3. These values apply to the simul-taneous determination of all four elements. If oneor more elements are left out, the limits maydecrease a bit with decreased instrumental noise.

3.3. Analysis of known test samples

A total of 24 portions of CMI 7003 weredigested in four batches of six samples on differentdays using the in-house digestion method A (seeabove). For comparison, a single batch of sixsamples of the same material was digested usingdigestion method B. Two replicates were digestedfrom each of the 12 sediment samples from theproficiency test using method A, since this wasthe normal laboratory practice for analyses ofheavy metals in soils. Manual dilutions were nec-essary in many cases to bring sample concentra-tions to the calibration range.

The results obtained for CMI 7003 are presentedinTable 4.The results are given for both digestionmethods described above. The uncertainties report-ed are "1 overall standard deviation for bothmethods (ns24 for method A and ns6 formethod B). The results are in quite good agreementwith the certified values. However, some negativebias seems to be evident for Cd, whereas the


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Table 4Determined and certified aqua regia soluble concentrations inreference soil CMI 7003

Element Method A Method B Certified(mg kg )y1 (mg kg )y1 (mg kg )y1

As 11.5"0.9 11.4"0.3 11.6"0.7Cd 0.29"0.03 0.296"0.007 0.32"0.03Cr 47"4 43.5"1.5 42.4"3.6Pb 25.9"1.3 27.2"1.5 25.2"1.1

See text for description of the digestion methods A and B.Uncertainties "1 standard deviation (ns24 for method A andns6 for method B). All values reported in mg kg dryy1

material.

results for Cr tend to be too large. The between-batch variation with method A was significant forAs and Pb at the 95% level of confidence, whenit was examined with a one-way ANOVA. Thiscan be caused by small deviations in the digestionprocedure or deterioration of the graphite tube. ForCd and Cr the variation was not significant.

Digestion with aqua regia is often used to givean estimate about the total concentrations of someelements in soil and sediment samples. However,it is well known that many element fractionsextractable by aqua regiaare not equal to the total

concentrations but much lower. This can be seenfrom the total element contents of reference soilCMI 7003, which were also certified or given forinformation w11x. Approximately 53% of total Cr,69% of total As, 75% of total Pb and 100% oftotal Cd in CMI 7003 are soluble in aqua regiaaccording to the certificate. By the obtained results,both digestion methods A and B that were used(see above), are almost equally effective. A small-er weighing(method A)can be used, if the methoddetection limits obtained with it are low enough

for a specific purpose. Using a smaller weighingalso reduces the concentration of sample-relatedinterfering compounds in sample solution.

The trace metal concentrations found in the 12examined sediment samples are presented graphi-cally in Fig. 1. The interlaboratory median wasobtained from the quarterly reports of the profi-ciency test w12x, and it had been calculated after atwo-stage process of outlier elimination. Theresults obtained with the new method are averagesof two replicate samples. The line of perfect

correlation is shown for comparison in each graph.Linear regression calculations were performed foreach element using the 12 points, and the calcu-

lated slope of the regression line as well as thecorrelation coefficient R are shown in the graphs.2

In general, results from approximately 2030 dif-ferent laboratories were reported for each sample.For Cd, one result obtained with the new methodwas below the calculated LOD, but this point isalso included in Fig. 1. For statistical evaluationof the new results, z-scores were calculated for theresults obtained with the new method (x). Theistatistic z is calculated using the interlaboratorymean and standard deviation s (outliersx

removed): .zs xyxys (3)i

The usual interpretation is that values NzN-2are satisfactory, 2-NzN-3 questionable and NzN)3 unacceptable w15x. The z-scores calculated canbe found in Table 5. Using the limits mentionedabove, only two scores fall into the questionablerange, and none can be considered unacceptable.As can be seen in Fig. 1, very good correlationwas achieved for Cd, Cr and Pb. For As, theresults of the sediment samples tend to be little

low, but thez-scores of As still indicate satisfactoryperformance for all but one sample. The totalelement concentrations of the examined sedimentsamples were unknown.

3.4. Some analytical problems

The most serious problem encountered with thesimultaneous determination of As, Cd, Cr and Pbwas the limited dynamic range of graphite furnaceAAS, a problem pointed out also by Hoenig and

Cilissen w7x. Even many soils and sediments thatare usually considered uncontaminated often con-tain Pb and especially Cr in such large concentra-tions that direct and simultaneous determination ofall the elements by GFAAS without manual dilu-tion is impossible. On the other hand, As and Cdshould often be determined without dilution. TheSIMAA 6000 instrument facilitates the use of threewavelengths of different sensitivity for Pb insimultaneous measurements, but for Cr 357.9 nmis unfortunately the only analytical line available


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Fig. 1. Determined As, Cd, Cr and Pb concentrations and interlaboratory median values for measured sediment samples.

in that kind of measurements. Less sensitive wave-lengths can be used for Cr only in single-element

mode. Non-linear calibration equation was alreadyused for Cd, Cr and Pb to extend the analyticalrange. The calibration ranges were still selected sothat deviation from linearity was not very severe.

Some serious memory effects were observed forCr. Chromium is not a very volatile element, andespecially high concentrations of Cr caused prob-lems. The memory effect is probably caused byformation of chromium carbide. It may be neces-sary to use even higher temperatures and longertimes for atomization and clean-out steps to deter-

mine Cr correctly. This causes more rapid corro-sion of the graphite tubes. One solution might be

to determine Cr in single-element mode using aless sensitive wavelength. Then the rest of thesimultaneously determined elements could bedetermined using much lower atomization andclean-out temperatures and shorter times, since nomemory effects were observed for the more vola-tile As, Cd and Pb.

The most sensitive 193.7-nm As line was firstused. It was soon noted that some samples causedvery large background absorption signals withlevels over 1.0 A, sometimes nearly 1.5 A. At its


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Table 5z-scores for 12 examined sediment samples from a proficiencytest. Seew12xfor the quarterly reports of the test. Questionableresults given in bold figures.

Sample ID As Cd Cr Pb

2001y1 1 y0.10 0.32 0.47 1.252001y1 2 y1.52 0.96 0.77 1.192001y1 3 y0.39 y0.66 y0.23 y0.522001y1 4 y0.22 y0.42 0.50 1.002001y2 1 I2.74 y0.10 y0.10 y0.162001y2 2 y0.34 y0.54 0.67 2.102001y2 3 y0.96 y0.18 0.24 1.462001y2 4 0.24 y0.83 y0.26 y0.022001y3 1 y1.15 0.56 y0.79 0.032001y3 2 y1.76 y0.95 y0.91 y0.362001y3 3 y0.43 y1.03 0.39 y0.75

2001y3 4 y0.45 y1.85 y0.30 y0.81

Average y0.82 y0.39 0.04 0.37

largest value, this background caused a break inthe atomic signal. When the secondary 197.2-nmAs line was used for these samples, the backgroundlevel was much smaller, most often less than 0.5A, and the shape of the atomic absorption signalswas better. When standard graphite tubes wereused instead of end-capped tubes, the backgroundswere smaller, but consequently precision was poor-er at low concentrations and all detection limitsbecame higher. Large background signals at theAs wavelength seemed fairly unreactive to a pyrol-ysis step done with the recommended PdqMg(NO ) -chemical modifier, which is considered3 2the most universal modifier nowadays w16x. Theshape of the atomic absorption signals of As wasclearly better when the modifier was used. A fewexperiments with the reference material CMI 7003and the sediment samples indicated somewhatmore accurate results for As and Cd with the

modifier.

4. Conclusions

A method was developed for the simultaneousdetermination of As, Cd, Cr and Pb in aqua regiadigests of soils and sediments using multi-elementgraphite furnace AAS. This alone shortens theanalytical time needed for GFAAS analysis by afactor of four. The use of fast-furnace methodologyfurther shortens the time required. Fast furnace

programs are also remarkably easy to develop anduse. Satisfactory accuracy and precision was con-firmed by analyzing soil and sediment samples

with known concentrations of trace metals. Quitesevere memory effects were observed for Cr, andthe high temperatures that are needed to removethem can speed up the corrosion of the graphitetubes. Workarounds for this and other analyticalproblems were considered.

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Documents

Simultaneous Determination of as, CD, Cr and Pb in Aqua Regia