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Determination of antimony in atmospheric particulate matter by hydride generation and atomic absorption spectrometry
Analytica Chimica Acta, 153 (1983) 33-M Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
DETERMINATION OF ANTIMONY IN ATMOSPHERIC PARTICULATE MATTER BY HYDRIDE GENERATION AND ATOMIC ABSORPTION SPECTROMETRY K. DE DONCKER, R. DUMAREY,
R. DAMS and J. HOSTE*
Institute for Nuclear Sciences, Rijksuniversiteit (Belgium)
Gent, Proeftuinstraat
86, B-9000
Gent
(Received 4th May 1983)
SUMMARY Antimony-124 is used as tracer in the development of a procedure for the wet digestion of atmospheric particulates collected on cellulose filters. The samples are decomposed with sulphuric acid and hydrogen peroxide, followed by hydrofluoric acid to dissolve residual silicates. The yield of the dissolution for antimony is almost 100%. The chemical and instrumental parameters for the hydride generation and subsequent atomic absorption spectrometry were optimized. The precision of the entire procedure is generally better than 10% with a sensitivity for antimony of 1 ng mm3of air. Excellent agreement was found between the results obtained by the proposed method and those from instrumental neutron activation analysis for concentrations of 4-2500 ng m*.
Various techniques are available for the determination of traces of antimony in environmental samples, instrumental neutron activation analysis (i.n.a.a.) and electrothermal atomic absorption spectrometry (a.a.s.) probably being the most accurate methods [l-3]. If hydride generation is combined with a.a.s., the determination of antimony and other volatile hydride-forming elements (e.g., As, Se, Te, Ge, Bi) offers selectivity and sensitivity because most matrix interferences are eliminated. In this technique, the hydride is separated from the digested sample solution after addition of sodium tetrahydroborate and then thermally decomposed in a heated quartz cell placed in the optical path of the spectrometer [4-71. The present paper describes a digestion procedure for antimony in atmospheric particulate materials. This is a prerequisite as losses were encountered with a standard digestion procedure using nitric and perchloric acids [ 81. Tracer experiments with antimony-124 were done, to quantify and subsequently avoid such losses. Additionally, the analytical parameters were optimized. To check for accuracy, results obtained with the technique described are compared with data obtained by i.n.a.a. for air samples with a wide range of antimony concentrations.
0003-2670/83/$03.00
o 1983 Elsevier Science Publishers B.V.
34 EXPERIMENTAL
Apparatus and reagents A Perkin-Elmer model 503 spectrometer was equipped with the mercury/ hydride system MHS-1 and an antimony electrodeless discharge lamp. The output signal was monitored with a Perkin-Elmer Model 56 recorder operated at a range of 10 mV full scale and a 20 mm min-’ chart speed. All reagents were of at least analytical grade: hydrofluoric acid (50%, analytical-grade), hydrogen peroxide (30%, UCB), hydrochloric acid (Suprapur, 30%, Merck), sulfuric acid (95-98%, J. T. Baker), nitric acid (70%, J. T. Baker). Solutions required are: 10% (w/v) hydroxylammonium chloride, 2% (w/v) hydrazinium sulfate, 10% (w/v) sodium iodide with ascorbic acid (1 g 1-l) added for stabilization, 5% (w/v) sodium tetrahydroborate (Aldrich) stabilized with 2% sodium hydroxide. The antimony stock solution (1000 mg 1-l) in 3% (v/v) hydrochloric acid was prepared from antimony trichloride. The lz4Sb tracer had a specific activity of 32 C pg-’ s-l. Optimization of the hydride generation Chemical and instrumental parameters influencing the formation of stibine from antimony, present in particulate matter collected on Whatman 41 cellulose filters, were studied to optimize the analytical method. Valence state. Antimony can be present as Sb(II1) or Sb(V) depending on the oxidizing properties of the solution. As is well known, the valence state plays an important role in the formation of hydrides. The difference in peak shape obtained from Sb(II1) and Sb(V) solutions is illustrated in Fig. 1. Sb(V) decreases the sensitivity by ca. 60%, because of the much faster conversion of Sb(II1) to stibine. The difference in reaction rate increases with increasing pH [9]. This problem may be overcome by preconcentrating the stibine in a cold trap. However, in the MHS-1 arrangement, the stibine is carried continuously to the optical cell during generation. Therefore, any Sb(V) present in the sample solution must be pre-reduced to Sb(II1). Several reducing agents were tested for a solution containing 0.1 pg Sb ml-’ obtained after digestion of an antimony-spiked blank Whatman 41 cellulose filter in a mixture of nitric and perchloric acids. Sodium iodide showed good prereducing properties, when 1 ml of 10% (w/v) NaI per 10 ml of sample solution was used. Addition of sodium tetmhydroborate. In the MHS-1, the tetrahydroborate can be added as a solution or as a pellet. The use of the solution gave the best results. The addition of a fixed volume (2.50 ml) can be done very reproducibly, whereas the production of equal pellets is tedious and timeconsuming. Moreover, the blank values when a pellet (125 mg) was used, were 6-7 ng Sb, whereas the use of the reducing solution gave a blank of only 3.3 f 1 ng. For all further measurements, 2.5 ml of 5% (w/v) sodium tetrahydroborate solution was used for a lo-ml sample. This solution was
35
I
‘\
+
woo-
c
.a 2
p a x
oxa-
‘\
‘\
‘\
Y,,, ‘\
‘I..,, .‘I--___
02w-
ID
10
Sample
a
volume
‘0
-I M
(ml)
Fig. 1. Effect of the valence state on the absorbance signal (100 ng Sb). Fig. 2. Effect of sample volume on the peak height for 100 ng Sb. Peak height is given as absorbance.
stabilized with 2% (w/v) sodium hydroxide and stored at 4°C. During analyses, the solution was cooled with ice to prevent decomposition of the tetrahydroborate, which led to an irregular introduction of the reagent into the cell, resulting in multiple peaks. Sample volume. Figure 2 illustrates the influence of the sample volume on the peak height. The optimal volume was 10 ml; below 10 ml, incomplete mixing of the reductant and sample gave irreproducible signals. Higher volumes gave broadened peaks caused by the increasing time needed to sweep the stibine out of the sample solution at the argon flow rate of 250 ml min-’ fixed by the manufacturer. Before the measurements, argon at 500 ml min-’ is used to expel oxygen from the system. Cell temperature. The influence of the atomization temperature on the absorbance signal is given in Fig. 3. Below 4OO”C, stibine was not decomposed. Between 600 and 8OO”C, only partial atomization was achieved, whereas above 95O”C, the signal decreased slightly. Therefore 950°C was selected for all further experiments. MEIS- program. The MHS-1 control module permits the choice of four programs which differ only in the timing of the purge and measurement cycle. No significant effects on the absorbance signals were found with the different programs, hence the short program (I) was used (total analysis time 70 s). Background correction. No improvement was found when deuterium background correction was applied, whether for aqueous standard solutions or for digested samples. The optimal operating parameters are summarized in Table 1.
Optimization of the digestion procedure When the standard digestion procedure with nitric and perchloric acids recommended for atmospheric particulate matter collected on filters [8] was
36
Fig. 3. Effect of cell temperature on the peak height (100 ng Sb). Fig. 4. Interferences on the determination
of antimony:
(- - -)
Ni; (-)
Pb; (*em)Cu.
applied, losses occurred ranging from 13 to 54%. This was established by comparing the results to those obtained with i.n.a.a. Accordingly, a twostep radiotracer study with the -y-emitting isotope 124Sb (t1,2 = 60.2 d) was conducted to check the efficiency of other possible digestion mixtures. In the first step, a blank cellulose filter spiked with ‘%b was digested to investigate for losses caused by the possible formation of slightly soluble antimony compounds. In the second step, these experiments were repeated with particulate-loaded filters to check for sorption effects on the undissolved residue of the material. Tmcer-spiked blank filters. An aliquot containing 10 pg of 124Sb was spiked on a Whatman 41 filter (5.5-cm diameter), which was subsequently digested in different mixtures: (A) H2S04-HN03; (B) H2S04-H202; and (C) H2S04-HCl-HNO,. In all cases, the digest was filtered on a Whatman 41 filter. The filtrate was diluted to volume in a lOO-ml volumetric flask with deionized water. After measurement on a NaI scintillation detector, the r-ray activity of the final solution was compared with a diluted undigested aliquot of the original radiotracer solution. In no case were losses encountered. In subsequent experiments, the residual 124Sb activity was measured after hydride generation, for samples digested with mixtures A or B. It was found TABLE 1 Operating parameters Reductant Sample volume Argon flow Cell temperature Hydride program
2.5 ml of 5% NaBH, 10 ml 250 ml min-l 950°C I
Background correction Damping Wavelength Spectral bandwidth Lamp
Off 2 217.6 nm 0.2 nm (slit 3) 8W
31
that 13% and 82% of the antimony remained in solution when the digest contained nitric acid and peroxide, respectively, because the oxidant removed the iodide needed for reduction of Sb(V) to Sb(II1). These interfering traces of nitric acid or peroxide could be removed with 3 ml of 10% (w/v) hydroxylammonium chloride or 2 ml of 2% (w/v) hydrazinium sulfate on heating for about 20 min. After these additions, the antimony was quantitatively recovered during the hydride generation step (99.0% + 1.2%). Although both modified procedures A and B yielded excellent recoveries, method B was preferred because the digestion was much faster. Tmcer-spiked particulate-loaded filters. When the above digestion technique was applied to filters loaded with atmospheric particulates, negative results (up to 35%) were obtained. In these tests, 10 pg of lz4Sb radiotracer was spiked on a Whatman 41 filter (5.5~cm diameter) loaded with about 300 pg of particulate matter per cm 2. After digestion, the solution was filtered and both residue and filtrate were examined for -y-ray activity. The results are listed in Table 2. The undissolved fraction still contained an important amount of antimony. This was probably caused by adsorption of antimony on the insoluble fraction. For this reason, the procedure was modified by adding 4 ml of hydrofluoric acid after the initial reaction with H2S04/H202 had subsided. Because all traces of hydrofluoric acid must be removed before the addition of the hydrazinium sulfate, 2 ml of sulfuric acid and 2 ml of hydrogen peroxide were added. Following this method, the residue contained less than 2% Sb. However, in some cases, the recovery was not quantitative, probably because of incomplete dissolution of the sample. This could easily be avoided by using larger amounts of acid and peroxide at the beginning of the digestion step, 8 ml and 4 ml, respectively. The hydride generation step, applied to samples digested in this way, yielded complete recoveries of the antimony (99.0% f 0.3%). Recommended procedure Place a filter (Whatman 41, diameter 11 cm) in a loo-ml teflon beaker, containing 8 ml of concentrated sulfuric acid and add dropwise 4 ml of 30% hydrogen peroxide. After the initial reaction has subsided, heat the digest for TABLE 2 Recovery of lz4Sb activity after digestion of spiked particulate-loaded Mixture
lz4Sb activity (%) In filtrate
H,SO,-H,O,-NH,NH,H,S0, H&JO,-H,O,-HF (3:3:4) H,SO,-H,O,-NH,NH,H,SO, H,SO,-H,O,-HF (8:4 :4) H,SO,-H,O,-NH,NH,SO,
filters
(3:2:2) (2:2:3) (2:2:4)
In residue
76-100
6-l 5
9W96
2
10~101
2
38
about 20 min. After cooling, add 4 ml of hydrofluoric acid and evaporate to half the initial volume. Remove traces of hydrofluoric acid by heating after the addition of 2 ml of sulfuric acid and 2 ml of hydrogen peroxide. Finally, add 4 ml 2% (w/v) hydrazinium sulfate to remove all the peroxide. After the digestion, filter the solution on a Whatman 41 filter paper and dilute with 0.1% (w/v) sodium iodide to 50 ml or 100 ml, depending on the antimony content of the digested air samples. Quantify antimony in the final sample solutions by hydride generation, using the parameters given in Table 1. Interferences The interfering effects of Ni%, Cu” and Pb2+ were established by determining the antimony content in the presence of increasing amounts of these ions. The percent enhancement or suppression of the signal, obtained for metal ion/antimony ratios varying from 10m2to 104, is given in Fig. 4. The only significant interference is encountered for Ni2+in 200-fold amounts. RESULTS
AND DISCUSSION
The recommended procedure was used to determine the antimony content in a number of particulate samples collected in the vicinity of a nonferrous plant producing antimony alloys and oxides. The samples were taken by passing daily approximately 400 m3 of air over a ll-cm diameter Whatman 41 filter [lo] . The high-volume samplers were of the LIB type [ 111. Calibration and precision For calibration of the 8.a.s. measurements, two procedures were applied to check for possible systematic errors. When standard solutions in the range 20-100 ng of antimony were measured, the calibration plot of absorbance (A) against ng Sb gave the equation y =x(0.0046 f 0.0001) A ng-’ - (0.0081 + 0.0039) A. When standard additions were made by adding known amounts (25 and 50 ng) of antimony to portions of a digested solution of a particulate sample, the equation of the straight line then obtained was y = x(0.0047 f 0.0003) A ng-’ + (0.1857 + 0.0093) A. The correlation coefficients for these two equations were 0.9990 and 0.9804, respectively. When both methods were applied to the same sample of particulate matter, the agreement was within lo%, as expected from the standard deviation. The result obtained from the calibration plot was (37.0 f 3.2) ng, whereas the result by standard additions was (40.0 f 6.7) ng of antimony. To check further the precision of the technique, several filters were quartered and each part was analyzed. The results (Table 3) show the precision, calculated as the relative standard deviation. These figures include uncertainties arising from inhomogeneity of the filters as well as those from the digestion and measurement procedure.
39 TABLE 3 Analysis of quartered filters for antimony in air samples with mean (@and relative standard deviation Filter No.
Sb found (ng m-‘) 1
2
3
4
1 2 3 4
27.0 130.0 789.0 2265.0
27.0 147.0 731.0 2455.0
23.0 146.0 634.0 2468.0
32.0 146.0 726.0 2556.0
z
RSD (W)
27.0 142.0 720.0 2436.0
14.0 6.0 8.9 5.0
Accuracy The accuracy of the method was evaluated by treating separate quarters of the same filter by the proposed a.a.s. technique and by instrumental neutron activation analysis. For the latter analysis, a quarter of the filter was pressed to a pellet and irradiated for 7 h at a neutron flux of 1.5 X 1012 n cmT2 s-l. After a decay time of 10-15 days, 124Sb was counted for 15 min on a Ge(Li) detector, using the photopeaks at 602.7 and 1691.0 keV. The standard deviation on this technique varied from 1 to lo%, depending on the antimony concentration. In Fig. 5, the data obtained by the two methods are compared. The agreement is good. The equation of the line is y = x(0.963 f 0.023) + (7.3 ?I 22.4) ng m+, with a correlation coefficient of 0.995. The mean percent difference between the two methods is 10% for the 19 results which vary over a concentration range of three orders of magnitude.
s
102
,’
.z
.’
,/
: Q
. A
/‘.’
,I ,’
10' :
,/
,/’ ,,’
*
,/’ ,’
,’ ,001
d
“.-
lo’
“,.,,,I Inao
ld
“,,,,,I
m3
....JK+
(ngms3)
Fig. 5. Correlation between i.n.a.a. and hydride generation a.a.s.
40
Sensitivity The detection limit, defined as the antimony concentration corresponding to three times the standard deviation of the blank, was calculated to be about 4 ng. It must, however, be emphasized that this value and also the precision depend on the optical quality of the cell. Because of deterioration of its inner walls after repeated use, the precision decreases rapidly. A sensitivity of 4 ng per 2-ml of sample injected into the reaction vessel corresponds to 100 ng in the 50-ml solution of the digest. This is equivalent to 1 ng me3 if at least 100 m3 of air has been filtered, which is normally sufficient for all urban, industrial and normal continental samples. Only at extremely pure background stations would larger volumes of air need to be filtered. This work was supported by the “Interuniversitair Instituut voor Kernwetenschappen” to whom we are sincerely grateful. REFERENCES 1 2 3 4 5 6 7 8
G. C. Kunselman and E. A. Huff, At. Absorpt. Newsl., 15 (1976) 29. B. W. Haynes, At. Absorpt. NewsI., 17 (1978) 49. T. Kamada and Y. Yamamoto, Talanta, 24 (1977) 330. K. C. Thompson and D. R. Thomerson, Analyst, 99 (1974) 595. Y. Yamamoto and T. Kumamaru, Z. Anal. Chem., 281 (1976) 355. W. B. Robbins and J. A. Caruso, Anal. Chem., 51 (1979) 889 A. R. G. Godden and D. R. Thomerson, Analyst, 105 (1980) 1137. Belgisch Instituut voor Normahsatie, NBN T 94-401, Determination of the Lead Mass in Particles Collected by Air Filtration - Atomic Absorption Spectrophotometry, 1977. 9 M. Yamamoto, K. Urata and Y. Yamamoto, Anal. Lett., 14 (1981) 21. 10 R. Dams, R. Vanderborght and F. Adams, Environ. Technol. Lett., 3 (1982) 337. 11 Verein Deutscher Ingenieure, VDI 2463-Blatt 4, VDI Verlag GmbH, Dusseldorf, 1976.
DETERMINATION OF ANTIMONY IN ATMOSPHERIC PARTICULATE MATTER BY HYDRIDE GENERATION AND ATOMIC ABSORPTION SPECTROMETRY K. DE DONCKER, R. DUMAREY,
R. DAMS and J. HOSTE*
Institute for Nuclear Sciences, Rijksuniversiteit (Belgium)
Gent, Proeftuinstraat
86, B-9000
Gent
(Received 4th May 1983)
SUMMARY Antimony-124 is used as tracer in the development of a procedure for the wet digestion of atmospheric particulates collected on cellulose filters. The samples are decomposed with sulphuric acid and hydrogen peroxide, followed by hydrofluoric acid to dissolve residual silicates. The yield of the dissolution for antimony is almost 100%. The chemical and instrumental parameters for the hydride generation and subsequent atomic absorption spectrometry were optimized. The precision of the entire procedure is generally better than 10% with a sensitivity for antimony of 1 ng mm3of air. Excellent agreement was found between the results obtained by the proposed method and those from instrumental neutron activation analysis for concentrations of 4-2500 ng m*.
Various techniques are available for the determination of traces of antimony in environmental samples, instrumental neutron activation analysis (i.n.a.a.) and electrothermal atomic absorption spectrometry (a.a.s.) probably being the most accurate methods [l-3]. If hydride generation is combined with a.a.s., the determination of antimony and other volatile hydride-forming elements (e.g., As, Se, Te, Ge, Bi) offers selectivity and sensitivity because most matrix interferences are eliminated. In this technique, the hydride is separated from the digested sample solution after addition of sodium tetrahydroborate and then thermally decomposed in a heated quartz cell placed in the optical path of the spectrometer [4-71. The present paper describes a digestion procedure for antimony in atmospheric particulate materials. This is a prerequisite as losses were encountered with a standard digestion procedure using nitric and perchloric acids [ 81. Tracer experiments with antimony-124 were done, to quantify and subsequently avoid such losses. Additionally, the analytical parameters were optimized. To check for accuracy, results obtained with the technique described are compared with data obtained by i.n.a.a. for air samples with a wide range of antimony concentrations.
0003-2670/83/$03.00
o 1983 Elsevier Science Publishers B.V.
34 EXPERIMENTAL
Apparatus and reagents A Perkin-Elmer model 503 spectrometer was equipped with the mercury/ hydride system MHS-1 and an antimony electrodeless discharge lamp. The output signal was monitored with a Perkin-Elmer Model 56 recorder operated at a range of 10 mV full scale and a 20 mm min-’ chart speed. All reagents were of at least analytical grade: hydrofluoric acid (50%, analytical-grade), hydrogen peroxide (30%, UCB), hydrochloric acid (Suprapur, 30%, Merck), sulfuric acid (95-98%, J. T. Baker), nitric acid (70%, J. T. Baker). Solutions required are: 10% (w/v) hydroxylammonium chloride, 2% (w/v) hydrazinium sulfate, 10% (w/v) sodium iodide with ascorbic acid (1 g 1-l) added for stabilization, 5% (w/v) sodium tetrahydroborate (Aldrich) stabilized with 2% sodium hydroxide. The antimony stock solution (1000 mg 1-l) in 3% (v/v) hydrochloric acid was prepared from antimony trichloride. The lz4Sb tracer had a specific activity of 32 C pg-’ s-l. Optimization of the hydride generation Chemical and instrumental parameters influencing the formation of stibine from antimony, present in particulate matter collected on Whatman 41 cellulose filters, were studied to optimize the analytical method. Valence state. Antimony can be present as Sb(II1) or Sb(V) depending on the oxidizing properties of the solution. As is well known, the valence state plays an important role in the formation of hydrides. The difference in peak shape obtained from Sb(II1) and Sb(V) solutions is illustrated in Fig. 1. Sb(V) decreases the sensitivity by ca. 60%, because of the much faster conversion of Sb(II1) to stibine. The difference in reaction rate increases with increasing pH [9]. This problem may be overcome by preconcentrating the stibine in a cold trap. However, in the MHS-1 arrangement, the stibine is carried continuously to the optical cell during generation. Therefore, any Sb(V) present in the sample solution must be pre-reduced to Sb(II1). Several reducing agents were tested for a solution containing 0.1 pg Sb ml-’ obtained after digestion of an antimony-spiked blank Whatman 41 cellulose filter in a mixture of nitric and perchloric acids. Sodium iodide showed good prereducing properties, when 1 ml of 10% (w/v) NaI per 10 ml of sample solution was used. Addition of sodium tetmhydroborate. In the MHS-1, the tetrahydroborate can be added as a solution or as a pellet. The use of the solution gave the best results. The addition of a fixed volume (2.50 ml) can be done very reproducibly, whereas the production of equal pellets is tedious and timeconsuming. Moreover, the blank values when a pellet (125 mg) was used, were 6-7 ng Sb, whereas the use of the reducing solution gave a blank of only 3.3 f 1 ng. For all further measurements, 2.5 ml of 5% (w/v) sodium tetrahydroborate solution was used for a lo-ml sample. This solution was
35
I
‘\
+
woo-
c
.a 2
p a x
oxa-
‘\
‘\
‘\
Y,,, ‘\
‘I..,, .‘I--___
02w-
ID
10
Sample
a
volume
‘0
-I M
(ml)
Fig. 1. Effect of the valence state on the absorbance signal (100 ng Sb). Fig. 2. Effect of sample volume on the peak height for 100 ng Sb. Peak height is given as absorbance.
stabilized with 2% (w/v) sodium hydroxide and stored at 4°C. During analyses, the solution was cooled with ice to prevent decomposition of the tetrahydroborate, which led to an irregular introduction of the reagent into the cell, resulting in multiple peaks. Sample volume. Figure 2 illustrates the influence of the sample volume on the peak height. The optimal volume was 10 ml; below 10 ml, incomplete mixing of the reductant and sample gave irreproducible signals. Higher volumes gave broadened peaks caused by the increasing time needed to sweep the stibine out of the sample solution at the argon flow rate of 250 ml min-’ fixed by the manufacturer. Before the measurements, argon at 500 ml min-’ is used to expel oxygen from the system. Cell temperature. The influence of the atomization temperature on the absorbance signal is given in Fig. 3. Below 4OO”C, stibine was not decomposed. Between 600 and 8OO”C, only partial atomization was achieved, whereas above 95O”C, the signal decreased slightly. Therefore 950°C was selected for all further experiments. MEIS- program. The MHS-1 control module permits the choice of four programs which differ only in the timing of the purge and measurement cycle. No significant effects on the absorbance signals were found with the different programs, hence the short program (I) was used (total analysis time 70 s). Background correction. No improvement was found when deuterium background correction was applied, whether for aqueous standard solutions or for digested samples. The optimal operating parameters are summarized in Table 1.
Optimization of the digestion procedure When the standard digestion procedure with nitric and perchloric acids recommended for atmospheric particulate matter collected on filters [8] was
36
Fig. 3. Effect of cell temperature on the peak height (100 ng Sb). Fig. 4. Interferences on the determination
of antimony:
(- - -)
Ni; (-)
Pb; (*em)Cu.
applied, losses occurred ranging from 13 to 54%. This was established by comparing the results to those obtained with i.n.a.a. Accordingly, a twostep radiotracer study with the -y-emitting isotope 124Sb (t1,2 = 60.2 d) was conducted to check the efficiency of other possible digestion mixtures. In the first step, a blank cellulose filter spiked with ‘%b was digested to investigate for losses caused by the possible formation of slightly soluble antimony compounds. In the second step, these experiments were repeated with particulate-loaded filters to check for sorption effects on the undissolved residue of the material. Tmcer-spiked blank filters. An aliquot containing 10 pg of 124Sb was spiked on a Whatman 41 filter (5.5-cm diameter), which was subsequently digested in different mixtures: (A) H2S04-HN03; (B) H2S04-H202; and (C) H2S04-HCl-HNO,. In all cases, the digest was filtered on a Whatman 41 filter. The filtrate was diluted to volume in a lOO-ml volumetric flask with deionized water. After measurement on a NaI scintillation detector, the r-ray activity of the final solution was compared with a diluted undigested aliquot of the original radiotracer solution. In no case were losses encountered. In subsequent experiments, the residual 124Sb activity was measured after hydride generation, for samples digested with mixtures A or B. It was found TABLE 1 Operating parameters Reductant Sample volume Argon flow Cell temperature Hydride program
2.5 ml of 5% NaBH, 10 ml 250 ml min-l 950°C I
Background correction Damping Wavelength Spectral bandwidth Lamp
Off 2 217.6 nm 0.2 nm (slit 3) 8W
31
that 13% and 82% of the antimony remained in solution when the digest contained nitric acid and peroxide, respectively, because the oxidant removed the iodide needed for reduction of Sb(V) to Sb(II1). These interfering traces of nitric acid or peroxide could be removed with 3 ml of 10% (w/v) hydroxylammonium chloride or 2 ml of 2% (w/v) hydrazinium sulfate on heating for about 20 min. After these additions, the antimony was quantitatively recovered during the hydride generation step (99.0% + 1.2%). Although both modified procedures A and B yielded excellent recoveries, method B was preferred because the digestion was much faster. Tmcer-spiked particulate-loaded filters. When the above digestion technique was applied to filters loaded with atmospheric particulates, negative results (up to 35%) were obtained. In these tests, 10 pg of lz4Sb radiotracer was spiked on a Whatman 41 filter (5.5~cm diameter) loaded with about 300 pg of particulate matter per cm 2. After digestion, the solution was filtered and both residue and filtrate were examined for -y-ray activity. The results are listed in Table 2. The undissolved fraction still contained an important amount of antimony. This was probably caused by adsorption of antimony on the insoluble fraction. For this reason, the procedure was modified by adding 4 ml of hydrofluoric acid after the initial reaction with H2S04/H202 had subsided. Because all traces of hydrofluoric acid must be removed before the addition of the hydrazinium sulfate, 2 ml of sulfuric acid and 2 ml of hydrogen peroxide were added. Following this method, the residue contained less than 2% Sb. However, in some cases, the recovery was not quantitative, probably because of incomplete dissolution of the sample. This could easily be avoided by using larger amounts of acid and peroxide at the beginning of the digestion step, 8 ml and 4 ml, respectively. The hydride generation step, applied to samples digested in this way, yielded complete recoveries of the antimony (99.0% f 0.3%). Recommended procedure Place a filter (Whatman 41, diameter 11 cm) in a loo-ml teflon beaker, containing 8 ml of concentrated sulfuric acid and add dropwise 4 ml of 30% hydrogen peroxide. After the initial reaction has subsided, heat the digest for TABLE 2 Recovery of lz4Sb activity after digestion of spiked particulate-loaded Mixture
lz4Sb activity (%) In filtrate
H,SO,-H,O,-NH,NH,H,S0, H&JO,-H,O,-HF (3:3:4) H,SO,-H,O,-NH,NH,H,SO, H,SO,-H,O,-HF (8:4 :4) H,SO,-H,O,-NH,NH,SO,
filters
(3:2:2) (2:2:3) (2:2:4)
In residue
76-100
6-l 5
9W96
2
10~101
2
38
about 20 min. After cooling, add 4 ml of hydrofluoric acid and evaporate to half the initial volume. Remove traces of hydrofluoric acid by heating after the addition of 2 ml of sulfuric acid and 2 ml of hydrogen peroxide. Finally, add 4 ml 2% (w/v) hydrazinium sulfate to remove all the peroxide. After the digestion, filter the solution on a Whatman 41 filter paper and dilute with 0.1% (w/v) sodium iodide to 50 ml or 100 ml, depending on the antimony content of the digested air samples. Quantify antimony in the final sample solutions by hydride generation, using the parameters given in Table 1. Interferences The interfering effects of Ni%, Cu” and Pb2+ were established by determining the antimony content in the presence of increasing amounts of these ions. The percent enhancement or suppression of the signal, obtained for metal ion/antimony ratios varying from 10m2to 104, is given in Fig. 4. The only significant interference is encountered for Ni2+in 200-fold amounts. RESULTS
AND DISCUSSION
The recommended procedure was used to determine the antimony content in a number of particulate samples collected in the vicinity of a nonferrous plant producing antimony alloys and oxides. The samples were taken by passing daily approximately 400 m3 of air over a ll-cm diameter Whatman 41 filter [lo] . The high-volume samplers were of the LIB type [ 111. Calibration and precision For calibration of the 8.a.s. measurements, two procedures were applied to check for possible systematic errors. When standard solutions in the range 20-100 ng of antimony were measured, the calibration plot of absorbance (A) against ng Sb gave the equation y =x(0.0046 f 0.0001) A ng-’ - (0.0081 + 0.0039) A. When standard additions were made by adding known amounts (25 and 50 ng) of antimony to portions of a digested solution of a particulate sample, the equation of the straight line then obtained was y = x(0.0047 f 0.0003) A ng-’ + (0.1857 + 0.0093) A. The correlation coefficients for these two equations were 0.9990 and 0.9804, respectively. When both methods were applied to the same sample of particulate matter, the agreement was within lo%, as expected from the standard deviation. The result obtained from the calibration plot was (37.0 f 3.2) ng, whereas the result by standard additions was (40.0 f 6.7) ng of antimony. To check further the precision of the technique, several filters were quartered and each part was analyzed. The results (Table 3) show the precision, calculated as the relative standard deviation. These figures include uncertainties arising from inhomogeneity of the filters as well as those from the digestion and measurement procedure.
39 TABLE 3 Analysis of quartered filters for antimony in air samples with mean (@and relative standard deviation Filter No.
Sb found (ng m-‘) 1
2
3
4
1 2 3 4
27.0 130.0 789.0 2265.0
27.0 147.0 731.0 2455.0
23.0 146.0 634.0 2468.0
32.0 146.0 726.0 2556.0
z
RSD (W)
27.0 142.0 720.0 2436.0
14.0 6.0 8.9 5.0
Accuracy The accuracy of the method was evaluated by treating separate quarters of the same filter by the proposed a.a.s. technique and by instrumental neutron activation analysis. For the latter analysis, a quarter of the filter was pressed to a pellet and irradiated for 7 h at a neutron flux of 1.5 X 1012 n cmT2 s-l. After a decay time of 10-15 days, 124Sb was counted for 15 min on a Ge(Li) detector, using the photopeaks at 602.7 and 1691.0 keV. The standard deviation on this technique varied from 1 to lo%, depending on the antimony concentration. In Fig. 5, the data obtained by the two methods are compared. The agreement is good. The equation of the line is y = x(0.963 f 0.023) + (7.3 ?I 22.4) ng m+, with a correlation coefficient of 0.995. The mean percent difference between the two methods is 10% for the 19 results which vary over a concentration range of three orders of magnitude.
s
102
,’
.z
.’
,/
: Q
. A
/‘.’
,I ,’
10' :
,/
,/’ ,,’
*
,/’ ,’
,’ ,001
d
“.-
lo’
“,.,,,I Inao
ld
“,,,,,I
m3
....JK+
(ngms3)
Fig. 5. Correlation between i.n.a.a. and hydride generation a.a.s.
40
Sensitivity The detection limit, defined as the antimony concentration corresponding to three times the standard deviation of the blank, was calculated to be about 4 ng. It must, however, be emphasized that this value and also the precision depend on the optical quality of the cell. Because of deterioration of its inner walls after repeated use, the precision decreases rapidly. A sensitivity of 4 ng per 2-ml of sample injected into the reaction vessel corresponds to 100 ng in the 50-ml solution of the digest. This is equivalent to 1 ng me3 if at least 100 m3 of air has been filtered, which is normally sufficient for all urban, industrial and normal continental samples. Only at extremely pure background stations would larger volumes of air need to be filtered. This work was supported by the “Interuniversitair Instituut voor Kernwetenschappen” to whom we are sincerely grateful. REFERENCES 1 2 3 4 5 6 7 8
G. C. Kunselman and E. A. Huff, At. Absorpt. Newsl., 15 (1976) 29. B. W. Haynes, At. Absorpt. NewsI., 17 (1978) 49. T. Kamada and Y. Yamamoto, Talanta, 24 (1977) 330. K. C. Thompson and D. R. Thomerson, Analyst, 99 (1974) 595. Y. Yamamoto and T. Kumamaru, Z. Anal. Chem., 281 (1976) 355. W. B. Robbins and J. A. Caruso, Anal. Chem., 51 (1979) 889 A. R. G. Godden and D. R. Thomerson, Analyst, 105 (1980) 1137. Belgisch Instituut voor Normahsatie, NBN T 94-401, Determination of the Lead Mass in Particles Collected by Air Filtration - Atomic Absorption Spectrophotometry, 1977. 9 M. Yamamoto, K. Urata and Y. Yamamoto, Anal. Lett., 14 (1981) 21. 10 R. Dams, R. Vanderborght and F. Adams, Environ. Technol. Lett., 3 (1982) 337. 11 Verein Deutscher Ingenieure, VDI 2463-Blatt 4, VDI Verlag GmbH, Dusseldorf, 1976.