190
Anal, Chem. 1982, 54, 190-193
Flow Injection Analysis for the Determination of Bismuth by Atomic Absorption Spectrometry with Hydride Generation ove Astrom Depatiment of Analytical Chemistty, Unlversiv of UmeA, S-901 87 Ume A, Sweden
Nonsegmented continuous flow analysis, termed flow injection analysis (FIA), has been modified for the semiautomation of sample introduction in atomic absorption coupled with the hydride technique. A 700-yL bismuth sample Is injected into a continuous flowing stream of hydrochloric acid. Afler reaction with sodium borohydride in a mixing coil the evolved hydride Is swept Into an electrically heated quartz tube by a nitrogen stream and the atomic absorption recorded. The experimental conditions for the FIA system can be chosen in such a way that the chemical interferences will be substantially eliminated. Samples in the range 1-100 ng can be determined at a rate of 180 sampies/h with a detection limit in the subnanogram range.
The hydride generation technique in combination with atomic absorption spectroscopy (AAS) is a commonly used method for the determination of elements like arsenic, selenium, bismuth, tin, antimony, lead, tellurium, and germanium. However, these methods suffer from severe interference effects by numerous cations as well as anions (1-4). Cobalt, nickel, copper, and the precious metals comprise the most serious interfering elements for bismuth according to the literature
(1). The recent development in automated hydride generation has resulted in better systems with regard to precision, sample throughput, and simplicity of operation. However, only small improvements with regard to interference effects have been attained. Therefore, the hydride generation technique still suffers from many disadvantages most of which seem to originate from the poor control over the factors involved in the hydride generation step (3,4). Most of the automated hydride generation systems described in the literature are based on the use of a continuous, air-segmented flow system for performing discrete chemical analysis (the principle of the Auto-Analyzer). It should be mentioned that, for a long time, the opinion has been that air segmentation and the attainment of ‘‘steadystaten signals are essential prerequisites for performing continuous flow analysis. The introduction of flow-injection analysis (FIA) has (5-8), however, brought about new ideas. Flow-injection analysis is based on the use of nonsegmented streams and, contrary to the Auto-Analyzer principle, this technique utilizes the dispersion patterns for analytical purposes. The FIA method constitutes a combination of the following three principles (i) the introduction of a well-defined sample zone into the continuously moving stream, (ii) reproducible timing, and (iii) controlled dispersion. These qualities in combination with unique possibilities of miniaturization offered by such systems imply that the FIA technique has intrinsically greater flexibility than the Technicon-based flow technique. For these reasons the FIA technique appears to be very promising for controlling interference effects in hydride generation systems. Moreover, as will be discussed in this paper, FIA should be useful for gaining fundamental knowledge about the chemical reactions involved. 0003-2700/82/0354-0190$01.25/0
EXPERIMENTAL SECTION Instruments, Equipment, and Manifolds. The flow-injection system was comprised of the following components: an eight-channel peristaltic pump (Gilson, France), a pneumatic air-driven sample injection valve (Altex 201-06) with variable sample volume and a flow meter with a mass flow controller (Brooks 8744 A). An atomic absorption spectrophotometer (in this study three spectrophotometers were used, namely, a Varian-Techtron AA6, a Perkin-Elmer 403, and a Pye Unicam SP W),a stripchart recorder (Vitatron 2001 or Philips PM 8202), and a hollow cathode source, which is a standard Varian-Techtron bismuth hollow lamp, were employed (operating current 8 mA). The manifold described in this paper (Figure 1) has been designed to minimize the dead volumes. All coils in the manifold are made from 0.5 mm i.d. Teflon tubing except for coil a which is made from a Teflon tube of 0.7 mm i.d. Connections between different manifold parts comprised standard liquid chromatography connectors. The gas-liquid separator, which is essentially the same as that described by Vijan et al. (9),is miniaturized as far as possible. To hinder a too high back-pressure in the line from the gas-liquid separator to the quartz cell, we used a short 1.6 mm i.d. Teflon tube. The dead volume of the system (not including the atomizer) is only about half of the atomizer volume. Experiments with a gas-liquid separator of the stripping type used by Goulden and Brooksbank (10) were also undertaken, but no improvement over the type of gas-liquid separator used by Vijan et al. was found. The tube furnace was constructed from a quartz tube, 6 mm i.d., 170 mm long, fused at the center with a 1.3 mm i.d. quartz tube through which the gases entered. The central part (9 cm) of the tube furnace was wound with 1.4 mm of 0.5 mm Kanthal wire (giving a resistance of approximately 10 0)and isolated with Cerafelt (Johns-Manville). The furnace was surrounded with an aluminum tube. Before measurements with the final atomizer were carried out, experiments were performed on an atomizer with the same physical dimensions but without the protruding ends of the quartz tube. It was established that the signal was much more stable at low gas flows for the longer cell. This was due to the better design with respect to eliminating ignition of the hydrogen at the open ends of the quartz tube. This was also confirmed by Chapman and Dale (11). Furthermore, it was found that the sensitivity was ca. 50% better for the longer quartz tube for the same experimental parameters. The tube furnace was inserted and aligned into the optical pathway of the atomic absorption spectrophotometer. The temperature of the quartz tube was controlled with a variable transformer and measured with a NiCr-Ni thermocouple. The temperature of the quartz tube was found not to be critical provided that the temperature was set above 750 OC. For further experiments a final temperature about 850 O C was chosen. Reagents, Standards, and Procedures. All reagents used were of analytical reagent grade. Water from a Millipore Super-Q water system was used throughout. Sodium borohydride solutions (powder form, Merck) were freshly prepared and made alkaline with sodium hydroxide to inhibit decomposition. The solutions were filtered through a 0.22-pm Millipore filter before use (12). The sample, usually 700 pL,was injected into the hydrochloric acid line and after mixing with sodium borohydride, the solution was sprayed with nitrogen or argon into the gas-liquid separator. For increased mixing between the sample and the hydrochloric acid, the acid line is equipped with a mixing point, i.e., a point 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
191
$
-
00
1.0
0.5
% NaBHq w l v
Effect of sodium borohydride concentration on sensltivity: ( 0 )25 ppb Bi standard; (0)25 ppb Bi in the presence of 500 ppm Co. Coil length (cm): a = 100, b = 10, c = 20. Figure 4.
30
0
10
6
2
1
Pumping speed I arbMary un9S
pumping speed on sensittvii: (0)25 ppb standard (0)25 ppb in the presence of 100 ppm Co. Coil length (cm): a = 100, b and c = 50.
c
OO
Figure 2. Effect of
120
Gss flow ml/min
Flgure 5.
Effect of nitrogen flow rate on sensitivity (50 ppb Bi standard).
.
\
I
\
b!L 0
0
Figure 3.
1
2
L
360
3
0 '
Effect of hydrochloric acid concentration on sensitivity (25
0
ppb Bi standard).
I
20
I
40
1
60
I
80
I
100
Coil length I cm
where the direction of the flow is considerably and suddenly changed. The gaseous hydride is swept into the electrically heated tube furnace and the concentration is measured with the atomic absorption spectrophotometer at 223.1 nm. A transient absorbance is registered by the strip chart recorder. Standard bismuth solutions were always injected before and after each interfering test solution to check for possible changes in the sensitivity. Peak Height vs. Peak Area. In the choice of whether to use the integral or the peak height as a measure of the hydride concentration, information from the literature appears to support the integral. It has been shown that the peak height is dependent upon the oxidation state of the element, due to kinetic effects, but the integral is not (13). However, in the bismuth case the oxidation state problem does not exist. One of the fundamental principles of FIA is that analysis, detection, and registration are fast, simple, and easily performed. For this reason the peak height rather than the integral has been chosen as a measure of the hydride concentration. This also renders the method more widely applicable since many instruments are not equipped with integrating facilities.
RESULTS AND DISCUSSION Screening Experiments. In some initial experiments, the results of which are shown in Figures 2-5, the influence of pumping speed, concentrations of hydrochloric acid and sodium tetraborohydride, and the flow rate of nitrogen stripping gas were studied for standard solutions of bismuth with and without interferents. As can be seen from Figure 3, the
Flgure 6. Effect of coil length variation on sensitivity upon injection of a 25 ppb Bi solution in the presence of (filled symbols) and in the absence of 100 ppm Co (open symbols). Coil length (cm): (triangle) a = 100, b = variable, c = 20; (circle)a = 100, b = variable, c =
50.
concentration of hydrochloric acid is not critical above 0.5 M. These results confirm the data obtained by Fernandez (14) and BBdard et al. (15). As is evident from Figures 2 and 4, interference effects can be minimized by using as low a concentration of sodium borohydride as possible in combination with as high a pumping speed as possible. Figure 4 shows that 0.1% of sodium borohydride seems to be a good compromise between the concentration required for quantitative hydride formation and small interference effects. The flow rate of the nitrogen stripping gas was not found to be critical provided it was held in the range 50-100 mL m i d . The Influence of Coil Length. In the screening experiments it was established that the length of coil a is only of minor importance and my be disregarded in the discussion of coil lengths. On the other hand, the lengths of coils b and c were found to be of greatest importance. As is illustrated in Figures 6 and 7, these can be chosen to yield control over the hydride generation reaction even in the presence of potentially interfering substances. Figure 6 shows two pairs of lines with lengths of coil c of 20 cm and 50 cm, respectively. The lower curve in each pair of lines represents a bismuth
192
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
Table 11. Interference upon Analysis of Bismuth (25 ppb Bismuth Standard), % Suppression of Signal ~ length of coil c , cm
Cdl length / cm
Effect of coli length variation on sensitivity upon injection of 25 ppb Bi solutbn In the presence of (0)and In the absence of 1000 ppm Ni (0). Coil length (cm): a = 100, b = 0, c = variable. Sodium borohydride is 0.5% in 0.01 M NaOH. Figure 7.
0 5 10 15 10 ppm Cu 0 100 ppm Cu 0 10 200 pprn Cu 0 10 15 500 ppm Cu 25 1000 ppm Ni 0 5 a Coil a = 100 cm and coil b = 0 cm. Sodium borohydride concentration 0.1% in 0.01 M NaOH. 2.0
-
2 min
Table I. Interference upon Analysis of Bismuth (25 ppb Bismuth Standard)= interfering element, 100 ppm Na, K,Ca, Mg, Al, NO,, PO,, CO,, NH,, SO,, Fe, Cd, Cr Sn Sb As
co Ni, Cu
suppression of signal, % none 5 15 30 80 100 c
a
Conditions are as in Figure 8. Scan
solution containing 100 ppm of cobalt. As shown by the results given in the figure, a shortening of the lengths of coils b and c results in increased sensitivity as well as a smaller influence from the interferent. These effects are greatest for a longer coil c which is evident from the figure when comparing the slopes of the upper two curves and the slopes of the lower two curves. For pure bismuth sample solutions the decrease in sensitivity for longer coil lengths (or reaction times) can be explained by the instability of the bismuth hydride. The decomposition of bismuth hydride is known to occur even at room temperature. Fernandez (14)and Bgdard et al. (15) found for tellurium and bismuth and Chapman (11) for tin, lead, and tellurium that a long collection period resulted in a substantial reduction in sensitivity. As indicated by the upper curve of Figure 6, the reaction rate for the hydride reaction is high enough to be completed even when using coil lengths of b and c less than 20 cm. The upper curve in Figure 7 shows that coil b can even be eliminated without causing much decrease in the signal provided that the coil c is between 20 and 40 cm in length. In order to see any effects from interfering ions with coil b eliminated, we used a stronger interferent than cobalt. The lower curve in Figure 7 was obtained for 25 ppb of bismuth spiked to lo00 ppm with nickel. It can be seen that by reducing the length of coil c complete elimination of the interfering effects from nickel can be achieved. However, this occurs at the cost of a somewhat lower response as well as poorer reproducibility. Interferences. In order to compare the effects of various interferents, conditions far from optimum with regard to coil lengths and concentration of the sodium borohydride had to be chosen. The results are shown in Table I and no noticeable effects were found for the ions Na+, K+, Ca2+, Mg2+,AIS+, NO,, COZ-, NH,+, SO:-, Fe3+,Cd2+,and Cr2+. These results are in agreement with those found for the cations by Smith (1) except for iron, cadmium, and chromium for which he found a suppression of the signal by 10-50%. With regard to anions, no reference has been found to their interference on bismuth, hence there was no basis for comparison. Cobalt, nickel, and copper comprised the most serious interfering
Results of triplicate Injections of Bi standards. From left to right: 10, 25, 30, 40, and 50 ppb Bi. Coil length (cm): a = 100, b = 100, c = 50. Figure 8.
elements among those investigated. Injection of a solution spiked to 100 ppm in nickel or copper completely suppressed the bismuth signal. The results given in Table I1 illustrate the efficiency of the present FIA system for minimizing the severe interference effects from copper and nickel (3, 4). Compared with conventional hydride generation systems for bismuth those results corlstitute an essential improvement since about 100 to 1000 times higher concentrations of interferents can be tolerated in the sample. Except for the ideal concentrations of the reagents used in the described FIA system, this improvement is due to the fact that the reaction time is controlled and kept as short as possible thereby favoring the main reaction. The types of interferences may be divided into two classes, those which may be attributed to reactions taking place in the solution and those reactions which occur in the atomizer. It was observed that the solution in the presence of copper was colored yellow more intensively with increasing sodium borohydride concentration. At the same time the signal decreased considerably (1,16). The reduced signal is probably due to coprecipitation of bismuth with free metal or due to the formation of intermetallic compounds or other decomposition products. In these situations it was also found that a brown-yellow precipitate formed in the Teflon tubes. However, on exposure to air for some hours it completely disappeared indicating that it could be a volatile or easily decomposed compound. It was found that when the mixed sample-reagent solution was colored heavily upon injection of an interferent, the signal from a resulting subsequent injection of a pure bismuth solution was reduced significantly. In the worst cases the sensitivity did not return until the cell had been thoroughly cleaned with hydrofluoric acid. This suggests that some volatile compound is formed during the reaction which interacts with the quartz surface. The elimination of interferences can be affected as shown in this work or by complexation or by a combination of both methods.
193
Anal. Chem. 1982, 5 4 , 193-202 0.7 ng
0.03-
Il l
Flgure Q. Detectlon limlt tracings for a I-ppb BI standard. Coll length (cm): a = 100, b = 0, c = 4. Sodium borohydride concentration is 0.25% In 0.01 M sodium hydroxide.
With selenium, e.g., it should be very advantageous to complex the interfering ions with chloride since the optimum conditions for selenium hydride formation occur in regions of high acid concentration.
Sample Throughput, Sensitivity, Detection Limit,and Precision. Figure 8 shows a typical series of triplicate standard injections. The relative standard deviation (n = 6) for a 30 ppb bismuth solution changed from 0.2% to 0.8% when the sample throughput increased from 90 samples/h to 180 samples/h. The sensitivity (1% Abs) was about 0.1 ng for the optimum conditions with respect to the suppression of interference effects. Detection limit, taken as 3X the standard deviation of the noise or approximately three-fifths the peak-to-peak noise which gives about 0.08 ppb from the tracings of Figure 9 (17). For comparison it can be mentioned that the best sensitivity and detection limit f i i e s for bismuth known to the author are those obtained by Hon et al. (18) and Thomson and Thomerson (19). The former obtained 0.4 and
1.5 ng and the latter 0.43 and 0.2 ng for a 1-mL sample, respectively. The detection lilfiit can probably be extended to lower values for the discrete sampling systems by increasing the sampling volume. However, as pointed out by Verlinden et al. (20) the signal at the same time decreases by ca. 50% (Perkin-Elmer MHS-1and MHS- 10, hydride forming element selenium) for a 5-fold increase in sample volume. It is thus rather difficult to obtain a detection limit below 0.1 ppb.
LITERATURE CITED (1) Smith, A. E. Analyst (London) 1975, 700, 300-306. (2) Pierce, F. D.; Lamoreaux, T. C.; Brown, H. R.; Fraser, R. S. Appl. Spectrosc. 1976, 30, 38-42. (3) Pierce, F. D.; Brown, H. R. Anal. Chem. 1976, 48. 693-695. (4) Plerce, F. D.; Brown, H. R. Anal. Chem. 1977, 49, 1417-1422. (5) Ruzlcka, J.; Hansen, E. H. “Flow Injection Analysis”; Wiley-Intersclence: New York, 1981. (6) Betterldge, D. Anal. Chem. 1978, 50, 832 A-846 A. (7) Ranger, C. B. Anal. Chem. 1981, 53, 20 A-32 A. (8) Wolf, R. W.; Stewart, K. K. Anal. Chem. 1979, 57, 1201-1205. (9) Vljan, P. N.; Wood, Q, R. At. Absorpt. Newsl. 1974, 13, 33-37. (IO) Gouiden, P. D.; Brooksbank, P. Anal. Chem. 1974, 46, 1431-1436. (11) Chapman, J. F.; Dale, L. S. Anal. Chim. Acta 1979, 1 7 7 , 137-144. (12) Knechtel, J. R.; Fraser, J. L. Analyst (London) 1978, 703, 104-105. (13) Slemer, D. D.; Koteel, P.; Jariwala, V. Anal. Chem. 1976, 48, 836-840. (14) Fernander, F. J. At. Absorpt. News/. 1973, 12, 83-97. (15) BBdard, M.; Kerbyson, J. D. Can. J . Spectrosc. 1976, 21, 64-68. (18) Verlinden, M.; Deelstra, H. fresenius’ 2. Anal. Chem. 1979, 296, 253-258. (17) IUPAC, Anal. Chem. 1976, 48, 2294-2296. (18) Hon, P. K.; Lau, 0. W.; Cheung, W. C.; Wong, M. D. Anal. Chim. Acta 1980, 775, 355-359. (19) Thompson, K. C.; Thomerson, D. R. Analyst (London) 1974, 99, 595-601. (20) Verlinden, M.; Baart, J.; Deelstra, H. Talanta 1980, 27, 633-639.
RECEIVED for review July 6,1981. Accepted October 19,1981.
Determination of the Composition of Organic Layers Chemically Bonded on Silicon Dioxide Jean-FranGois Erard and Ervin sz. KovBts” Laboratolre de Chimie Technlque de I’Ecole Polytechnlque F M r a l e de Lausanne, 10 15 Lausanne, Switzerland
An analytical method for the determination of the composition of chemlcaily bonded organoslloxy layers Is presented and verified. I t consists of the following steps: (I) dissolution of the surface modified silicon dioxide sample in a solution of hydrogen fluoride in diethyl ether; (11) elimination of the main part of the slllcon tetrafiuorlde formed from the bulk slllcon dioxide, displacing it by nitrogen; (111) analysis of the fiuorosilanes formed quantltativeiy from the organosiioxy substituents or their butyl derivatives, by gas chromatography. The precision of the method Is about *2.0% (95% confidence level) for the surface concentration for samples having about 20 m2 of surface area. The lower limit of applicability is estimated to about 1 m2 samples corresponding to a 5 pmoi silane mixlure but with a precision of 10%.
Little to nothing is known of composite chemically bonded organic layers though they can readily be prepared by treating silicon dioxide preparations with a mixture of silylating agents. 0003-2700/82/0354-0193$01.25/0
This astonishing lack of concern logically implies the lack of an analytical method for the determination of their composition. To remedy this situation, we present a method for the determination of the composition of layers prepared with mixtures of monofunctional organosilanes at the surface of silicon dioxide preparations. The method is essentially an application of Booth’s suggestion for the analysis of silicones: the sample is dissolved in hydrogen fluoride where Si-C bonds are not attacked and the volatile organofluorosilanes are then analyzed (1-3)- The procedure involves several disadvantages, one of them being the handling of anhydrous hydrogen fluoride. This can be avoided by using boron trifluoride instead of hydrogen fluoride, but with both reagents an undesired side reaction, the cleavage of certain types of Si-C bonds, is observed. Especially fragile are aromatic silane bonds which are split even under mild experimental conditions ( 3 , 4 ) . To suppress this reaction we largely profited from literature reports describing the removal of organosilyl protective groups (5-7). These studies show that the reactivity of hydrogen fluoride 0 1982 Amerlcan Chemical Society
