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1158 ANALYST, JULY 1992, VOL. 117

Table 1Working conditions of the microwave oven used with the different samples studiedSample* Power and time No. of treatments

BCR CRM 062Olive Europoea (Olive Leaves)BCR CRM 060 Lagarosiphon Yajor (Aquatic Plant)BCR CRM 061Platihypnidium Ripariodides (Aquatic Plant)NRCC CRM DORM-1 Dogfish MuscleNRCC CRIPISOLT-1 Dogfish LiverNRCC CRM TORT-1 Lobster HepatopancreasNIST SRM 1572 Citrus LeavesBCR CRM 186 Pig KidneyBCR CRM 184 Bovine Muscle

2minat480 W +4min at 180 W2.5 rnin at 480 W+ 4.5 rnin at 360 W2min at 700 W + 4minat 360W4minat360W+ 10minat180W4minat360W + 10minat 180W4minat360W+ 10minat180W4minat360W+ 10minat180W4 min at 360 W + 10 min a t 180 W4minat360W+ 10minat180W

* BCR = Community Bureau of Reference; NRCC = National Research Council Canada; NIST = National Institute of Standards andTechnology; CRM = certified reference material; and SRM = standard reference material.100

g 75

5 50-C.-ccw

25

2 4 6 8 1 0 1 2PHFig. 1 Extraction of ickel into butan-1-01versus pH using DPTH asextracting reagent. Concentration of nickel in the aqueous phase:2 pg cm-3; volume of aqueous phase: 20 cm3; volume of organicphase: 10cm3Determinationof nickel in biological samplesThe samples were placed in Parr 478 microwave acid digestionbombs. The bombs were cleaned before use with 2 mol dm-3nitric acid for 1 d followed by repeated rinsing with water. Allglassware used was treated in the same way.Samples (about 200 mg) were weighed directly into thedigestion vessels on a digital electronic balance and 4 cm3 ofconcentrated nitric acid were added. The mixture was allowedto stand for 30 min after which 2 cm3 of concentratedhydrochloric acid (12 mol dm-3) were added. The bomb wassealed and placed in the microwave oven with a beaker filledwith 10 cm3 of water. This procedure minimizes the risk ofdamage to the magnetron in that small sample loads can causemagnetron failure, because most of the microwave power isreflected back to the waveguide and magnetron. The workingconditions of the microwave oven are listed in Table 1 .

After digestion, the solutions were evaporated by heating toa small volume (about 0.5 cm3) to eliminate the nitric acid.The pH was adjusted to 3.6 with 1mol dm-3 NaOH solutionand 10 cm3 of buffer plus 2 cm3of 1mol dm-3 NaC104solutionwere added. The solutions were diluted to 250 cm3 withde-ionized water in a calibrated flask. Then, in a 250 cm3separating funnel were placed 125 cm3 of sample solution and15 cm3 of a 0.05% butan-1-01 solution of DPTH. Thefunnelwas shaken vigorously on the mechanical agitator for 5 min,after which the phases were allowed to separate and theorganic phase was collected in order to measure the Niconcentration by ICP-AES at 341.476 nm, as described above.Human urine can be analysed without mineralization. Theurine was acidified with concentrated nitric acid until 1% v/vand stored frozen until required for the determination ofnickel. In 250 cm3 separating funnels were placed 100-150 cm3of acidified urine, which was analysed as described above.In all instances it is convenient to analyse the reagent andbuffer solutions previously in order to test for the presence ofnickel.

100s.;50.-w5 50

25cW

I I I I I I I I I I I I I I I2 4 6 8 10 2 4 6 8 0.5 1.0 1.5 2.0 2.5 3.0[NaC1041/10-2 Shaking time/min [DPTH]/10-3rnol dm-3 mot dm-3

Fig. 2 Influence of experimental variables on the extraction of theNP-DPTH com lex into butan-1-01. ( a ) Influence of perchlorateconcentration; (8 , influence of shaking time; and (c) influence ofreagent concentration (organic phase). Nickel concentration,2 pg ~ m - ~aqueous phase); initial volume of aqueous phase, 20 cm3;initial volume of organic phase, 10 cm3 (total in each instance)Resultsand Discussion

Effect of pH on the ExtractionThe influence of pH on the extraction of the NP-DPTHcomplex into butan-1-01 was studied over the range 1.0-12.5.The results (Fig. 1) showed that the extraction is quantitativebetween pH 3.0 and 7.0, and that it takes place to a muchsmaller extent outside this range.Selection of the Buffer SolutionThree buffer solutions were tested: one of pH 3.6 consistingofglycine-HC1 and two of pH 3.8 consisting of potassiumhydrogen phthalate-HC1 and acetic acid-soldium acetate. Allyielded very similar results. Nevertheless, the glycine-HC1buffer was chosen on account of its greater complexing ability,which could be of use in overcoming interferences.Effect of Perchlorate Concentration, Shaking Time andReagent ConcentrationThe extraction of the nickel-DPTH complex into an acidicmedium was investigated by varying the NaC104 concentra-tion up to 6.5 x 10-2 mol dm-3. The extracted fractionincreased on increasing the perchlorate concentration to 3 X10-2 mol dm-3 [Fig. 2 ( a ) ] ;however, further increases in theconcentration, which were obtained by adding 1cm3 of 1moldm-3 NaC104 to an aqueous volume of 20 cm3, had noappreciable effect for subsequent experiments.A study of the influence of the shaking time [Fig. 2 ( b ) ]revealed that the extent of extraction levelled off after 3 min.A shaking time of 5 rnin was selected for subsequentexperiments.Next, the reagent concentration in the organic phase wasvaried while keeping the final volume at 10 cm3. The-resultsobtained [Fig. 2 ( c ) ] showed that the extracted fractionremained constant for DPTH concentrations equal to or

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ANALYST, JULY 1992, VOL. 117 1159greater than 6.83 x 10-4 mol dm-3 (0.03%). A concentrationof 0.05% was used in practice, in order to prevent depletion ofthe reagent by other extractable ions that might be present inthe aqueous medium.Effect of Phase-volume RatioThe volume of the aqueous phase was varied from 10 to 150cm3 (0.05% m/v DPTH); phase volume ratios between 1and15 were tested. Nickel could be quantitatively extracted up toa phase ratio of 8, above which phase separation wasunsatisfactory and the procedure was, therefore, inapplicable.On the other hand, when the volume of the aqueous phasewas increased, the volume of the organic phase decreased,because butan-1-01 is not totally miscible with water; however,the quantitative extraction of nickel could be verified from thecalibration graphs. When a phase rat io of 8 was used (80 cm3of aqueous phase and 10 cm3 of organic phase), onlyapproximately 2.5 ml of organic phase were obtained afterextraction; hence the phase ratio is actually 32. This phaseratio allows the sensitivity of the direct non-extractive methodto be increased by a factor of approximately 32.Determination of Nickel by ICP-AESA wavelength of 341.476 nm was selected as it resulted in thelargest differences between the sample and the noise signals(minimum background equivalent concentration). The nebu-lizer was then optimized: the optimum signal-to-noise ratiowas obtained at a value of 1.9 on an arbitrary scale from0 to 6;the plasma gas flow rate was 12 dm3 min-1; and the auxiliarygas flow rate was 0.5 dm3 min-l. Other experimental variableswere optimized, for which the following values were selected:torch height, first position on an arbitrary scale from 1 o 4 (thefourth position is the ignition position); photomultipliervoltage, 750 V; element time, 360 ms; integration time, 100ms; read delay, 15 s; and spectral range, 1-00nm.

Table 2 Tolerated levels of foreign species in the determination ofNiII*

Species Tolerated ratio( d m )Alkali metal ions, alkaline earth metal ions,Cr1I1,A P , bromide, chloride, fluoride,nitrate, sulfate, phosphate, thiourea,thiosulfate >4OOoMo"', Vv, Selv,ascorbic acid, oxalate 3000As111? ,bll MnII Bill1 2000Hg",Agl,Sblll,Cull 1500Fell, Fell1 1000Zn" 200* Nil1 = 10 ng ~ r n - ~n the aqueous phase.

The limits of detection and determination of the methodwere established according to American Chemical SocietyCommittee of Environmental Improvement definitions.9 Thedetect ion limit thus achieved for an aqueous-to-volume phaseratio of 8 was 0.2 ng cm-3 (aqueous phase) and thedetermination limit was 1.7 ng cm-3 (aqueous phase).Relative standard deviations were 3.4% for 2.5 ng cm-3 and1.8% for 25 ng cm-3 (all concentrations are referred t o theaqueous phase).

Study of InterferencesThe effect of various ions on the determination of nickel by theproposed method was examined under the optimum workingconditions. For this study, different amounts of the ionicspecies tested were added to a 10 ng cm-3 solution of Ni in theaqueous phase, the volume of which was 160 cm3, whereasthat of the organic phase was 20cm3. The starting point was anm/m interferent-to-Ni ratio of 4000; if any interferenceoccurred, the ratio was gradually lowered until the interfer-ence disappeared. The tolerance limits found (Table 2) showthat nickel can be determined in the presence of a variety ofions including most of those that commonly occur with nickelin natural and synthetic samples. Cobalt interferes with thedetermination of nickel up to at least a ratio of 40;hence,another wavelength, 352.454 nm, was chosen to eliminate thisinterference. With this wavelength, the tolerated ratio ofcobalt can be increased to 1000.

Determination of Nickel at 352.454 nmThe different instrumental variables for this wavelength wereoptimized. The following values were selected: nebulizer flowrate, 2.0 on an arbitrary scale from 0 to 6; plasma gas flow rate,12 dm3 min-1; auxiliary gas flow rate, 0.5 dm3 min-1; torchheight, first position on an arbitrary scale from 1 to 4;photomultiplier voltage, 900 V; element time, 360 ms;integration time, 40 ms; read delay, 15 s; and spectral range,1.00 nm.The detection limit thus achieved for an aqueous-to-volumephase rat io of 8 at this wavelength was 3.2 ng cm-3 (aqueousphase) and the determination limit was 8.1 ng cm-3 (aqueousphase). Relative standard deviations were 3.9% for 10ng cm-3 and 3.2% for 25 ng cm-3 (all concentrations arereferred to the aqueous phase).

Analysis of Biological MaterialsIn order to ascertain the accuracy of the method, theoptimized procedure was applied to the following certifiedbiological materials: Community Bureau of Reference

Table 3 Application of the proposed method to the determination of nickel in biological samplesSample

BCR CRM 062 Olive Europoea (Olive Leaves)BCR CRM 060Lagarosiphon Major (Aquatic Plant)BCR CRM 061Platihypnidium Ripariodides (Aquatic Plant)NRCC CRM DORM-1 Dogfish MuscleNRCC CRM DOLT-1 Dogfish LiverNRCC CRM TORT-1 Lobster HepatopancreasNIST SRM 1572 Citrus LeavesBCR CRM 186 Pig KidneyBCR CRM 184 Bovine MuscleNIST SRM 2670 Freeze-Dried Urine (Toxic Metals at Normal Levels)NIST SRM 2670 Freeze-Dried Urine (Toxic Metals at Elevated Levels)* Mean of three separate determinations.t Values in pg crn-3.

Ni certifiedpg g-18.040.0420.01.200.262.30.60.420.270.07t0.30t

Ni found*/pgg-17.9 zk 0.438.8f .7430.2k 30.01.40k 0.150.31f .112.6f .10.6 _+ 0.10.39 * 0.03

0.30 f .010.09 * 0.02-l0.28f .Olt

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1160 ANALYST, JULY 1992, VOL. 117(BCR), Certified Reference Materials (CRMs) 062 OliveEuropoea (Olive Leaves); 060 Lagarosiphon Major (AquaticPlant); 061 Platihypnidium Ripariodides (Aquatic Plant); 186Pig Kidney; 184 Bovine Muscle; National Research CouncilCanada (NRCC), CRMs DORM-1 Dogfish Muscle; DOLT-1Dogfish Liver; TORT-1 Lobster Hepatopancreas; NationalInstitute of Standards and Technology (NIST), StandardReference Materials (SRMs) 1572 Citrus Leaves; and 2670Freeze-Dried Urine (Toxic Metals at Normal and ElevatedLevels). The results obtained are given in Table 3, and agreewell with the certified values. The limits of detection found forthe different biological samples dissolved in the aqueous phaseare (in ng (3111-3): CRM 062 Olive Leaves, 0.4; CRM 060Aquatic Plant, 0.2; CRM 061 Aquatic Plant, 0.2; DORM-1Dogfish Muscle, 1.2; DOLT-1 Dogfish Liver, 1.7; TORT-1Lobster Hepatopancreas, 1.1; SRM 1572 Citrus Leaves, 0.5;CRM 186 Pig Kidney, 1.1; CRM 184 Bovine Muscle, 0.9; andSRM 2670 Freeze-Dried Urine, 0.5.We thank the Direccion General de Investigacidn CientificayTkcnica (DGICYT) for supporting this study (Projects PB87-0711 and PB90-0809), and also the Junta de Andalucia.

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ReferencesMiyazaki, A., Kimura, A., Banzho, K., and Umezaki, Y.,Anal. Chim. Acta, 1982, 114,213.Whiteley, R. V., and Merril, R. M., Fresenius Z . Anal . Chem.,1982, 311, 7.Bonilla Abascal, J . R., Garcia de Torres, A., and Can0 Pavon,J. M., Microchem. J. , 1981,26,55.Bustos, A., Sanchez-Rojas, F., Bosch Ojeda, C., Garcia deTorres, A., and Can0 Pavon, J. M., J. Anal. At. Specrrom.,1987, 2,253.Can0 Pavon, J. M., Garcia de Torres, A., and Bosch Ojeda, C.,Analyst , 1985, 110, 1137.Kingston, H. M., and Jassie, L. B., Anal . Chem., 1986,58,261.Aysola, P., Anderson, P., and Langford, C.H., Anal . Chem.,1987,59, 1582.Introduction to Microwave Sample Preparation, eds. Kingston,H. M., and Jassie, L. B., American Chemical Society, Washing-ton, DC, 1988.ACS Committee of Environmental Improvement, Anal. Chem .,1980,52,2242.

Paper I f06438JReceived December 23, I991Accepted February 26, 1992