Anal. Chem. 1904, 56, 2721-2723
2721
Determination of Tellurium and Selenium in Atmospheric Aerosol Samples by Graphite Furnace Atomic Absorption Spectrometry Kuen
Y.Chiou a n d Oliver K. Manuel*
Department of Chemistry, University of Missouri, Rolla, Missouri 65401
in a column 1.2 cm i.d. X 10 cm. Reagents. Standard solutions of Te(1V) and Te(V1) were prepared from high-purity chemicals purchased from a commercial source (The British Drug House, Ltd., London, England). The standard solution containing 1000 ppm of Te(IV) was prepared by dissolving 173.7 mg of sodium tellurite (>99% Na2Te03)in 5 mL of 12 M HC1 and diluting to 100 mL with distilled water. The standard solution containing 1000 ppm Te(V1) was prepared by dissolving 186.6 mg of sodium tellurate (>99% NazTeO4)in 5 mL of 12 M HC1 and diluting to 100 mL with distilled water. Standard solutions of Se(IV) were prepared from 99.99% Se02 (Gallard Schlesinger Chemical Mfg. Corp., Clarle Place, NY). A standard solution containing 100 ppm Se(1V) was prepared by dissolving 142 mg of SeOzin 5 mL of 12 M HCl and diluting to 1000 mL with distilled water. Analytical Procedure. Samples were collected on preweighed filters and then weighed again after sample collection. The filter was cut so as to contain approximately 0.1 g of particulate matter. This was digested, first with 25 mL of concentrated HN03with gentle heating and then with 5 mL of 60% HC104 to make a clear solution. After the mixture was filtered through a glass fiber filter, the HC104was expelled from the filtrate by evaporation to near dryness several times after the addition of 1.5-mL portions of concentrated HCl. (Care should be taken during the heating cycles to prevent loss of Se and Te from excess heat.) The final residue was dissolved in 20 mL of 0.05 N HC1, and the solution was placed on top of the column. The Se was eluted with an additional 80 mL of 0.05 N HC1, and then Te was eluted with 150 mL of 0.3 N HCl. The flow rate for elution was -1 mL min-'. The elutents were evaporated to 1-2 mL, and then diluted to a standard volume. Aliquots of these solutions were placed in sampling cups, and automatically transferred to the graphite furnace in 20 X Eppendorf pipets. For Te the peak absorbance was measured at 214.3 nm wavelength using a 3-nm slit, and for Se the peak absorbance was measured at 196.0 nm wavelength using a 2-nm slit width. Nitrogen was used for the furnace purge gas.
A method has been developed for the determlnatton of Te and Se in atmospherlc aerosols by Ion exchange and atomlc absorption (AA) using a graphite furnace atomizer. The method takes advantage of the hlgh sensitivity of AA for Te after removing lnterferlng elements by cation exchange. Up to 3000 kg of Fe3+, Zn2+, Ai3+, Ca2+, K+, etc., did not Interfere with the determlnatlon of 1 pg of Te, but this level of Ag', Hg2+, As3+, Cu2+, and Se4+ gave negatlve errors of 30-80 %. Ion exchange reduced the effects of these foreign lons to < l o % . Recoveries for Te and Se are 92.3% and 92.0%, respectively. The coefficient of varlatlon was about &6% at the 20 ng to 150 ng level of Te and about f4% at the 10 ng to 140 ng level of Se.
Tellurium is a member of the chalocogen elements, which also includes selenium and sulfur. Although many measurements have been reported for selenium and sulfur in environmental samples (1-3), environmental levels of tellurium are very low (Its Clark number is (2 X and we have not been able to find any published reports on tellurium in atmospheric aerosols. Because of low sensitivity, the determination of T e by the colormetric method using Bithmuthiol I1 ( 4 ) , flame atomization atomic absorption spectrometry (5,6), or neutron activation (8and the determination of Se and Te in nickel-base alloys by atomic absorption spectrometry with introduction of the solid sample directly into a furnace (8)are not suitable for atmospheric aerosol sample analysis. Atomic absorption spectrometry in which the T e is introduced by hydrogen telluride generation (9, I O ) and by the standard graphite furnace (11 ) have sufficient sensitivity for aerosol analysis. However, both methods show a significant suppressive effect on the Te signal by other elements, e.g., arsenic, selehium, and copper, which are highly concentrated in atmospheric aerosol samples (9). Tellurium(1V) can be separated from other elements by taking advantage of its quantitative adsorption on a cation exchange resin in equilibrium with a weak hydrochloric acid solution, 0.01 wm. The ion exchange column used to separate Te from interferences contained Amberlite IR-120,50-80 mesh (H+form), packed 0003-2700/84/0356-2721$01.50/0
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R E S U L T S AND DISCUSSION F u r n a c e Parameters. The performance of the graphite furnace for Te and Se determinations was investigated using standard graphite tubes. The peak heights obtained for standard solutions of Te and Se are shown in Figure 1 as a function of charring temperature. From these results, charring temperatures of 500 OC and 350 "C were selected for Te and Se, respectively. The peak responses for T e and Se as a function of atomization temperature are shown in Figure 2. An atomization temperature of 2700 OC was chosen for both elements. I n t e r n a l Gas Flow. Sensitivity vs. internal Nz gas flow through the graphite tube during atomization a t 2700 "C is shown in Figure 3. Lower gas flow settings may be used when optimum sensitivity is desired. If the analytical sensitivity is too great, values of absorbance peaks can be reduced by increasing the internal gas flow. As shown in Figure 3, changes in the internal gas flow produce different effects on each element. For the work reported here, we used N2 at a flow rate of 30 mL min-l. 0 1984 American Chemical Society
2722
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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x h
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(3
w I Y
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200 400
600
800
2a
1000 1200 1400
IC
CHARRING TEMPERATURE PC)
Figure 1. Peak heights of Te and Se as a function of charring temperature.
0 I
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50
100
150
200
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ELUTION VOLUME (mL HCL)
Flgure 4. Peak heights of Te and Se as a function of elution volume, first with 0.05 N HCI and then with 0.3 N HCI for Amberlite IR-120, 50-80 mesh (H' form), in a column 1.2 cm i.d. X 10 cm. Table I. Interference from Foreign Ions
ion added" As(II1) 7 WII) 1800 2000 2200 2400 2600 2800 3000
ATOMIZATION TEMPERATURE ("C)
Figure 2. Peak heights of Te and Se as a function of atomization temperature.
I
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'
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Cu(I1) Mn(I1) Se(1V) Pb(I1) Ni(I1) Cd(I1)
amt added, pg 40 15 10 600 1000 20 500 200 50
ion exchange (separated) % recoveryc Nb 101f5 92f7 105f6 106f7 98f3 102f5 100 f 4 101 f 8 95f6
4 4 4 4 4 4 4 4 4
'
not separated 70 recoveryC Nb 34f4 45f3 82f7 21f5 103f7 56f9 85f6 52 f 10 78f6
4 4 4 4 4 4 4 4 4
"The following ions of major element of aerosols at amounts up to 3000 pg do not influence the Te determination: Al(IV), Fe(III), Mg(II), K(I), Na(I), Ca(I1). b N is the number of replicate determinations. 1 pg of Te was taken.
N, GAS FLOW (rnL mi:')
Figure 3. Peak heights of Te and Se as a function of the internal flow of N2 gas.
Reduction of Tellurium(V1). Quantitative recovery of Te by the procedures developed here depend on the conversion of Te(V1) to Te(1V). Earlier studies (12-14) have shown that Te(V1) is not reduced to tellurium hydride with sodium borohydride and that Te(V1) is not adsorbed on the cation exchange resin from a dilute HC1 solution. Since the removal of interference ions by the cation exchange resin is efficient from the 0.05 N HC1 solution but Te(V1) is not retained under these conditions, the reduction of Te(V1) to Te(1V) by HC1 during the drying cycle was studied. Our results indicate that 1-2 mL of HC1 is sufficient to reduce Te(V1). Larger quantities of HC1 result in decreased recovery of tellurium. Separation of Te and Se by Cation Exchange. In 0.05 N HC1, Te(1V) exists predominantly in the form of TeO(OH)+ or Te(OH)3+ and is retained quantitatively on the cation exchange resin (14). As shown in Figure 4,Se is released in the first 100 mL effluent with 0.05 N HC1. Other interfering
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anions would be expected to be released from the cation exchange resin with the Se032-. Te(1V) caught by the cation exchange column is efficiently eluted with 150 mL of 0.3 N HCl. The yield for Te recovery column is about 98%. Most of the interfering cations (Cu, Hg, Ni, etc.) are retained on the cation exchange column when the Te is eluted with 150 mL of 0.3 N HC1. Therefore, the procedure adopted for quantitative separation of Te and Se from interfering ions is as follows. Te(1V) and Se(1V) in 20 mL of 0.05 N HC1 solution is introduced into a column, as described earlier. The 0.05 N HC1 portion of the elutant is used for the determination of Se and the 0.3 N HC1 portion is used for the determination of Te by graphite furnace atomic absorption spectrometry. Effect of Foreign Ions. Reliable analytical measurements of Te in aerosols require its separation from the major constituents of the matrix and from certain other volatile species, e.g., Cu, As, Se, etc. We therefore investigated the effects of various ions on the separation and determination of Te by the proposed method. Table I compares the results obtained when 1pg of Te(1V) was spiked with various foreign ions and then determined by AA with and without ion exchange separation. No interference was observed when many species, e.g., Fe3+,Mg2+,Ca2+,Zn2+, A13+, NO3-, Sod2-,etc. were added to levels of 3000 pg. However, other ions such as Ag+, Hg2+,As3+,Cu2+,Se4+,etc., gave negative errors of 30-80% when they coexist with Te at the levels found in atmospheric aerosol samples. As shown
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table 11. Te in Blanks, Aerosol Samples, and Spiked Samples indigeneous samp1eo tellurium no. vol, m3 amt of Te, ng Nb 1
2
3 4 5
blank 1013 1210 1428 1545
0
302.3 f 3.5 279.0 f 3.0 372.5 f 2.1 323.5 f 4.7
added tellurium amt of Te, ng % recovery Nb
~
4 4 3 2 4
300 200 300 400 400
102 f 2 91f3 93*4 9153 94f2
4 4 3 2
Table 111. Se in Blanks, Aerosol Samples, and Spiked Samples indigeneous samplea selenium no. vol, m3 amt of Se, ng Nb 1 2
3 4 5
blank 125 154 171 185
0
388.6 i 4.2 380.6 f 9.0 359.0 f 5.4 573.5 f 10.3
3 3 3 4 4
~
added selenium amt of Se, ng % recovery Nb 300 400 400 300 500
101 f 1 92f2 94f3 91f4 9lh4
of 92.3% for the Te mixed with aerosols. For Se, the results shown in Table I11 indicate recoveries ranging from 91% to 101%, with an average recovery of about 92.0% for the Se added to aerosols. The coefficient of variation (five replicate samples) was about 6% at the 20 ng to 150 ng level of Te. For Se the coefficient of variation (five replicate samples) was about 4% a t the 10 ng to 140 ng level. The agreement between the method proposed here and the standard addition method was checked and found to be within the usual limit of variation (10%) for elemental analysis.
4
asample no. 1 is a procedural blank for a fiberglass filter analyzed by the procedure employed for those used to collect aerosols. Samples 2, 3, 4, and 5 are for fiberglass filters with aerosols collected for periods of 14-18 July 1983, 4-8 August 1983, 7-9 September 1983, and 3-7 October 1983, respectively. * N is the number of replicate analyses.
3 3 3 4 4
OSample no. 1 is a procedural blank for a fiberglass filter analyzed by the procedure employed for those used to collect aerosols. Samples 2, 3, 4, and 5 are for fiberglass filter with aerosols collected for periods of 19-22 July 1983, 9-12 August 1983, 9-12 September 1983, and 14-18 October 1983, respectively. b N is the number of replicate analyses. in Table I, the ion exchange procedure reduced interference from foreign ions to
