1382
Anal. Chem. 1985, 57, 1382-1385
Hydride-Generation Atomic Absorption Spectrometry Coupled with Flow Injection Analysis M a n a b u Yamamoto, Makoto Yasuda,' and Yuroku Yamamoto* Department of Chemistry, Faculty of Science, Hiroshima University, Higashisenda, Naka-ku, Hiroshima, 730, J a p a n
A flow injectlon technlque was applied to hydrldegeneratlon atomic absorptlon spectrometry. For the present reactlon, gas segmentation was found to be elfectlve In mlnlmlrlng the broadening of a sample zone without an Increase of noise levels. When 0.5 mL of samples was used, arsenic, antlmony, blsmuth, selenium, and tellurium could be determined wlth the detection llmlts ( S / N = 3) of 0.04-0.3 ng and relatlve standard devlatlons better than 2.5%. About 120 samples could be determlned wlthln an hour. These elements In several NBS SRMs were determlned. Posslbllltles of a dlfferentlal determlnatlon according to the oxldatlon states were exhlblted for arsenic and antlmony In thermal water.
Flow injection analysis (FIA) has been widely used, mainly in the field of colorimetry (1). Recently, it was applied as an automatic sample injection technique for flame atomic absorption spectrometry (AAS) (2-5). Further, it was used as an on-line pretreatment method for AAS t o concentrate by chelate resin (6) or solvent extraction (7) and to remove interferences by ion-exchange (8). Hydride-generation (HG) AAS of bismuth (9) and cold-vapor AAS of mercury (10) were the rare examples of the application of the FIA technique for the system which needs gas-liquid separation to measure the gas phase. In the present study, the FIA technique combined with the gas-segmentation method (11)is applied for the HG-AAS of arsenic, antimony, bismuth, selenium, and tellurium, and its applicability for environmental analysis was exhibited by determining these elements in several NBS SRMs. It was found that the gas-segmentation method is useful for the FIA of the present reaction accompanied with a large amount of gas generation. EXPERIMENTAL SECTION FIA Manifold. A diagram of the FIA manifold is shown in Figure 1, which is constructed with a four- or five-channel peristaltic pump (Atto Co. Ltd, Model SJ-1220), a six-way valve (Nippon Chromatography Co. Ltd., LCV-508-6MP) for sample injection, mixing coil (glass capillary, 2 m, 1.5-mm i.d., 20 turns, volume 3.5 mL), a reaction tube (Teflon tubing, 10 cm, 2-mm i.d., volume 0.3 mL), and a gas-liquid separator (12 cm, 15-mm id., gas-phase volume, 7.5 mL). Apparatus. Nippon Jarrel-Ash Model AA-1 atomic absorption spectrometer was used with hollow cathode lamps (Hamamatsu Photonics Co.) as light sources and an electrically heated quartz cell unit, Nippon Jarrel-Ash Model ASD-100, for atomization. Peak height measurements were done from signals recorded on a strip chart recorder. Wavelength and lamp current were 193.7 nm, 12 mA for As, 217.6 nm, 10 mA for Sb, 223.1 nm, 10 mA for Bi, 196 nm, 20 mA for Se, and 214.1 nm, 10 mA for Te. Reagents. Hydrochloric acid was of special grade reagent for arsenic analysis (Yoneyama Chemical Co., Inc.), and other acids are of special grade reagent for poisonous metal analysis (Kanto I
Present address: AnaIytical Department, Iwakuni-Ohtake
Works, Mitsui PetrochemicalIndustries, Ltd., Waki-cho, Kuga-gun, Yamaguchi, 740, Japan.
Chemical Co., Inc.). All other reagents were of reagent grade. Distilled deionized NaBH4was used. A 3% NaBH4solution (0.05 M NaOH) was used after filtration through the filter paper, No. 5A (Toyo Roshi Co. Ltd.). Standard solutions (1000 pg/L) for AAS measurement (Wako Pure Chemicals, Ltd.) were used for standard solution of As(III), Sb(III), Bi(III),Se(IV), and Te(1V). Standard solutions (10 wg/L) of As(V) and Sb(V) were prepared from As(II1) and Sb(II1) standard solutions: 5 mL of 1000 pg/L As(II1) or Sb(II1) solution was heated with 2 mL of H2S04and 4 mL of 1% KMnO, at 80 "C for 30 min, and after an excess of KMn0, was decomposed by HzOz,solutions were cooled to the room temperature and diluted to 500 mL with 4 M HC1. Sample Preparation. Low-Alloy Steel (NBS SRM 362). One gram of sample and 20 mL of HN03 (1 + 1) were taken into a Teflon beaker and heated at 120 "C. When the sample was dissolved, the solution was evaporated to 1 mL. After 5 mL of HNO9 and 1mL of HzSO4 were added, the solution was heated to fumes of SOB. Then adding 10 mL of HCl(1 + l),the solution was heated at 50 "C for 30 min and diluted to 100 mL with water. Coal Fly Ash (NBS SRM 1633a). One-tenth of a gram of sample, 15 mL of HF, and 10 mL of HNOBwere taken in a Teflon beaker. After standing for 30 min, 2 mL of 0.3% KMn0, solution was added, and the sample was heated for 20 min at 150 "C. If necessary, excess permanganate solution was added, and then the solution was evaporated to about 10 mL. After addition of 5 mL of HN03, 1 mL of permanganate solution, and 1 mL of HzSO4, the solution was evaporated to fumes of SO3. Then, 10 mL of HCl (1 1) was added, and the solution was heated at 50 "C for 30 min. The solution was diluted to 100 mL with water and filtered through filter paper (No. 5A). Wheat Flour (NBS SRM 1567), Rice Flour (NBS SRM 1568), and Orchard Leaves (NBS SRM 1571). Two grams of the sample and 25 mL of HN03 were added to a Kjeldahl flask. The sample was heated until the evolution of NOz ceased (if necessary, 5-10 mL of HN03 were added and the procedure repeated). One milliliter of HzS04was added and the solution was heated to fumes of SOB. Then 10 d of HC1 (1+ 1)was added and heated for 30 min at 50 "C and diluted to 20-100 mL with water, in order to contain 0.5-20 ng of each element in 1 mL. Standard Conditions. Standard conditions for the determination of the total amount of five elements are given in Table I and standard conditionsfor the selective determination of As(II1) and Sb(II1) in Table 11.
+
R E S U L T S AND DISCUSSION Conditions of the Reduction. Relations between the absorbance and the concentration of Na13H4solution are given in Figure 2. Absorbances were constant when the concentration of NaBH, is higher than 0.25% in the resultant solution. Effects of acidity on the peak heights are given in Figure 3. Among these, higher acidity was necessary for As(V) and Te(1V) than for other elements. The gas flow rate also affects the peak heights of these elements (Figure 4). Maximum absorbances were obtained in the range of 0.2-0.5 L/min except for Bi. The absorbance of Bi decreased with increasing the flow rate as reported by Astrom (9). Under these conditions, atomization temperatures higher than 800 "C for Se, 900 O C for Sb, and 950 OC for As, Bi, and T e are enough to obtain maximum absorptions. In the fol-
0003-2700/85/0357-1382$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
1383
Table I. Analytical Conditions
carrier (water)" NaBH, (3%) HC1 KI segment gas (N2) carrier gas (N,)
As(II1 + V)
Sb(II1+ V)
5 1.5 7 (35%)* 1.5 (50%)c 5 500
10 1.5 7 (17.5%)b 1.5 (8%)' 5 300
" Sample injection volume, 0.5 mL.
flow rate, mL min-' Bi(II1)
Se(1V)
Te(1V)
10 1.5 7 (17.5%)b
10 1.5 7 (17.5%)b
10 1.5 7 (35%)b
5 500
5 200
5
300
Concentration of HC1 in percent. Concentration of KI solution in percent. 5O
Table 11. Analytical Conditions for Selective Determination of As(II1) and Sb(II1)
r
flow rate, mL m i d As(I1I) Sb(II1) 6.5 1.5 7 (PH 5)b 5 500
carrier (water)" NaBH4 soln (3%) buffer soln segment gas (N2) carrier gas (N2)
17 1.5 1.5 (pH 6)c 5 300
Sample injection volume, 0.5 mL. 170 g of citric acid and 445 g of Na2HP04in 1 L of H20. cl10 g of citric acid and 445 g of NaoHPOAin 1 L of H,O.
-0
0.2 0.4
0
Flgure 4.
0.6
0.8
1.0
N 2 flow rate/Lmin" Effect of carrier gas flow rate. Symbols as in Figure 2.
P
A
H
C
S
Ll
I
I N2
HA)
KI(As,Sb)
-
P
WaBH,
o 0.7 0
.
8
u
10 2 0 30 40 50
KI/% Effect of potassium iodide concentration on recovery of As(V) and Sb(V). (0)Peak height of As(V) relative to As(II1); (a)peak height of Sb(V)relative to Sb(II1). Concentrations of KI solution are variable. Other conditions are given in Table I. Flgure 5.
1.0-
E0
2 0.9. r
0.2-
0.8-
0
1
2
3
4
NaBH4 / % Figure 2. Effect of concentration of sodium borohydride solution: (0) As; (0) Sb; ( 0 )Bi; ( 8 )Se; (9)Te (each 10 ppb). Concentration of NaBH, is varlable. Other conditions are glven in Table I. Relative peak heights are the ratios of peak height of variable conditions to that obtained accordlng to the conditions in Table I.
Flgure 8. Effect of gas segmentation on absorption signals (A) with gas segmentation and (6) without gas segmentation; sample, As (10
PPb).
0
2
4
6
8
1
0
HCI /mol.L-' Flgure 3.
Effect of hydrochloric acid concentration. Symbols as in
Figure 2. lowing experiments, the atomization temperature was kept a t 950 "C for all elements. Optimization of the FIA Unit. The hydride-generation reactions are so rapid that sensitivities of these elements were constant, if the length of the hydride-generation tube is longer
than 10 cm. In the present study, the tube length of 10 cm was chosen. Then, the hydrides are separated from the sample solutions in less than 0.1 s after generation. In this short reaction period, most of the metal ions were found not to be reduced to metal, which is preferable to minimize the interferences from transition-metal ions (12). The hydride-generation tube of linear type was better than a coil to minimize a noise due to the generated hydrogen gas (about 30 mL/min). The mixing coil was necessary for the reduction of As(V) and Sb(V) to tervalent states. To obtain the same sensitivity from pentavalent and tervalent species of arsenic and antimony, a coil length of 2 m (a reaction time of 20 s) was enough. Under the condition, As(V) and Sb(V) were recovered quantitatively if the concentration of KI is higher than 4.2% and
1384
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
Table 111. Sensitivity, Detection Limit, Relative Standard Deviation, and Sampling Rate
As(II1, V) Sb(II1,V) Bi(II1) Se(1V) Te(1V) 1% absorpt, ng detection limit,n ng RSD,b % sampling rate, h
0.10
0.12 0.04
0.2
0.7
0.5 100
100
0.17 0.3
0.06 0.05 0.5 120
0.8 120
0.11 0.1 0.8 120
"Estimated as 3 times the standard deviation of the noise. *Estimated from 10 replicate measurements of synthetic samples. Table IV. Permissible Amounts of Diverse Ionsb
ion
As(V)
As(V) Sb(V) Bi(II1) Se(1V) Te(1V) Fe(II1) Co(I1) Ni(I1) Cu(I1)
10 100 100 10 10 000" 10 000" 10 000"
Sb(V) 20
10 oooa
100 1000 10 10 000" 10 000" 10 000" 10 000"
Bi(II1) 100 10
Se(1V)
Te(1V)
10 10 100
10 10 10 000" 10 000" 10 000" 500
10 10000" 10 000" 10 000"
500
50 50 50 10 10 000" 10 000" 10 000" 1000
" Maximum concentration examined in the study. Permissible amounts (weight ratio) correspond to the concentrations that give 10% negative error. As, Sb, Bi, Se, and Te, 10 ppb; sample volume, 0.5 mL. 0.4% in the mixture for respective ions (Figure 5 ) . Effect of Gas Segmentation. One of the characteristics of the FIA in contrast to the conventional continuous flow analysis is the injection of a discreate sample into a continuous carrier flow without gas segmentation, which enabled the rapid and precise determination by using transient signals. However, as shown in Figure 6, the gas-segmentation technique is useful for the present system to enhance the sensitivity and to increase the sampling rate, without a significant increase of the noise levels. This may be specific to the present system, in which gaseous hydride and generated hydrogen (30 mL/min) have to be separated with carrier gas (300 mL/min) from the liquid phase before the atomic absorption measurements. The effect of gas segmentation on the broadening of the sample zone was examined alternatively by colorimetric measurement: that is, 0.5 mL of a concentrated, strongly colored solution of Fe(phen)Q, was injected, feeding water for every line instead of reagents, 2-mL portions of the mixed solution were collected by a fraction collector before the re-
action tube, and the absorbance of Fe(phen),C12 was measured a t 510 nm. In this case, the peak height becomes 1.7 times higher and the dispersion changes from 8.71 to 5.29 and the half-width decreased from 17 to 7.0 s by gas segmentation. The half-width for the peak of the colorimetric measurement with gas segmentation is almost the same as that of 6.9 s for the peak by the AAS after hydride generation and gas-liquid separation (as in Figure 6). From these results, it may be concluded that gas segmentation is effective to prevent the zone broadening during the mixing before gas generation and that gas generation and the use of the gas-liquid separator do not cause extra broadening of the sample zone in the present unit. Selective Determination of As(II1) and Sb(II1). In natural water or biological samples, arsenic and antimony are known to be present in different oxidation states. Among these, As and Sb can be determined selectively according to their oxidation states by hydride generation by changing the acidity of sample solution (13). The possibility of the selective determination using the present manifold was examined. In the present system, As(II1) and Sb(II1) could be determined selectively in the ranges of p H 4.5-5.5 and pH 5-7, respectively. As shown in Figure 7, sensitivities of As(II1) and As(V) were the same and the selectivity of As(II1) in the presence of As(V) was satisfactory. The same was found for Sb(II1) and Sb(V). Precisions and Sensitivities. Precisions and sensitivities are given in Table 111. In spite of the small sample volume of the present method, sensitivities expressed by concentration are nearly the same as those of batch or conventional flow methods. Therefore, the amounts that give the 1% absorption in the case of FIA HG AAS decreased and the absolute sensitivities were higher 100 times or so than in the case of the batch or conventional flow method, because of the very small sample volume of the present method. The sensitivity of bismuth of the present study is comparable to that of Astrom (9). Therefore, it may be said that the high sensitivities can be obtained by FIA HG AAS, irrespective of the differences of details of the FI manifold. Such a high sensitivity of the FIA HG AAS is mainly due to the use of the transient signal, because an excess amount of sample is not necessary to obtain a transient signal. Moreover, the very small internal volume of the FIA manifold may be also effective for enhancing the sensitivity by minimizing the dilution of the generated hydride. Interferences of Foreign Ions. Tolerable amounts of foreign ions are given in Table IV. Among these, those of the hydride-forming elements are only about 100 times to analyte ions, but interferences from transition-metal ions are
Table V. Analytical Results of NBS SRMs (ppm)"
sample steel (SRM 362) obsd certified wheat flour (SRM 1567) obsd certified rice flour (SRM 1568) obsd certified orchard leaves (SRM 1571) obsd certified coal fly ash (SRM 1633a) obsd certified (I
As 890 f 20 920 f 50
element Bi
Sb 130 f 2 130 f 10
22 f 1 (20)
Se
Te
11 f 0.7 (12)
10 f 0.9 (11)
0.006 f 0.0003 (0.006)
1.0 f 0.1 1.1 f 0.2
0.39 f 0.03 0.41 f 0.05
0.38 f 0.04 0.4 f 0.1
9.6 f 0.4 10 f 2
3.0 f 0.1 2.9 f 0.3
135 f 5 145 f 15
6.5 f 0.4
0.10 f 0.01 (0.1)
(7)
Values in parentheses: not certified. Average values of five determinations.
0.090 & 0.004 0.08 f 0.01 9.8 f 0.5 10.3 f 0.6
0.010 f 0.003
(0.01)
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
10
2 8
4 6
6 4
8 2
Ib As(lll)/ppb 0 As(V)/ppb
Figure 7. Selectivity on the determination of As(II1) in the presence of As(V). Experimental conditions for As(II1) and As(II1, V) are given in Tables I 1 and I,respectively: (0)As(II1 V); (A)As(II1).
+
Table VI. Selective Determination of Arsenic and Antimony in Thermal Watera (ppb)* As(II1 + V)
As(II1)
Sb(II1 + V)
Sb(II1)
1720 f 20 (I720)'
90.0 f 1.8 (87.5)'
53.5 f 1.2 (54.0)c
1.50 f 0.03 (1.6)'
Onikoube, June 1982, Akita prefecture,Japan. *Averagevalues of 10 determinations. cValues in parentheses are obtained by HG AAS with batch method.
1385
Analytical Results. Analytical results of NBS SRMs are given in Table V, which were in good agreement with certified values. It was confirmed that the standard addition technique is not necessary for the analyses of these elements in the present samples. Comparisons of the results by the FIA technique with those by the batch method (13) for the differential determination of As and Sb in thermal water are given in Table VI. Both results are in good agreement within experimental errors. The standard deviations estimated from the ten measurements of this sample were a little worse than those for synthetic samples (Table IV), though it is well satisfactory for analytical purposes. ACKNOWLEDGMENT We thank A. Ohara for preparing the glassware of the FI unit. Registry No. HzO, 7732-18-5; Bi, 7440-69-9;Se, 7782-49-2; Te, 13494-80-9; As, 7440-38-2; Sb, 7440-36-0. LITERATURE CITED RuZiEka, J.; Hansen, E. H. "Flow Injection Analysis"; Wiley: New York, 1981. Tyson, J. F.; Applten, J. M. H.; Idris, A. B. Analyst (London) 1983, 108, 153.
minimized in the present method. In the study of Astrom (9),interferences of Cu2+and Ni2+ were found to increase with increasing the coil length. Considering that transition-metal ions (Mn+)interfere after being reduced to elemental metal (MO) (12),the surpression of signals with increasing the coil length means that the reaction of Mn+ to Mo or of Mo with the hydride is slow compared with the hydride generation, and then interferences increase with increasing the reaction period by using a long reaction coil. Therefore, in order to minimize the interferences of transition-metal ions, it is desirable to choose the condition not to reduce Mn+to Mo by reducing the reaction time and by separating hydrides from the sample solution as soon as possible. To satisfy such conditions, it may be concluded that the combination of the FIA technique with HG AAS is preferable to the batch method.
Kimura, H.; Oguma, K.; Kuroda, R. Bunsekl Kagaku 1983,32, T79. Zhou, N.; Frech, W.; Lundberg, E. Anal. Chlm. Acta 1983, 153, 23. Tyson, J. F.; Idris, A. B. Ana/yst (London) 1984, 109, 23. Olson, S.; Pessenda, L. C.; RuZiEka, J.; Hansen, E. H. Ana/yst (London) 1983, 108, 905. Kamson, 0. F.; Townshend, A. Anal. Chim. Acta 1983, 155, 253. Nord, L.; Karlberg, B. Anal. Chim. Acta 1983, 145, 151. Astrom, 0. Anal. Chem. 1982,5 4 , 190. De Andrade, J. C.; Pasquini, C.; Baccan, N.; Van Loon, J. C . Spectrochim. Acta, Part B 1983,388, 1329. Skeggs, L. T. Am. J . Clln. Pafhol. 1957,28, 311. Yamamoto, M.; Yamamoto, Y.; Yarnashige, T. Ana/yst (London) 1984, 109, 1461. Yamamoto, M.; Urata, K.; Murashige, K.; Yamamoto, Y. Spectrochim. Acta, Part B 1981,368, 671.
RECEIVED for review October 29,1984.
Accepted February
7,1985. This study was partially supported from the Ministry
of Education, Science, and Culture by grants for aid under Grant 57470071 for M.Y. and 58030061 for Y.Y.
