Anal. Chem. 1999, 71, 1225-1231
Determination of Hydrogen Radicals in Analytical Flames Using Electron Spin Resonance Spectroscopy Applied to Direct Investigations of Flame-Based Atomization Units for Hydride Generation Atomic Absorption Spectrometry Solomon Tesfalidet, Go 1 ran Wikander, and Knut Irgum*
Department of Chemistry, Umeå University, S-901 87 Umeå, Sweden
The atomization mechanism for the low-temperature flame used in hydride generation AAS has been studied by means of electron spin resonance (ESR) spectroscopy. By employing a miniaturized oxygen/hydrogen flame torch, especially constructed to fit in the center of the ESR cavity, it was possible to directly monitor the production of hydrogen radicals in the flame, as well as their consumption upon introduction of arsine into the flame. In this way, using arsenic as a model analyte, it could for the first time be shown in direct experiments that hydride-forming elements are atomized by radical recombination in the flame. The spatial distribution of hydrogen radicals in the quartz tube, whose dimensions are similar to those used for atomizing hydride-forming elements in AAS, was also studied. The principle of directly measuring radicals in miniature flames should be applicable also to other important analytical flame processes, such as native and modified flame ionization and flame photometric detectors for gas chromatography. Hydride generation combined with atomic absorption spectrometry is a widely used technique for the determination of some metalloids of the main groups IV, V, and VI in the periodic table. These elements are first separated from their sample matrixes by volatilization as hydrides in a generation step, with sodium tetrahydroborate(III)-hydrochloric acid being the preferred reduction system. The gaseous reaction products are then purged to an atomizer mounted in the optical path of an atomic spectrometer, where the hydrides are converted to the elemental state of the respective metalloid and subjected to detection by atomic absorption spectrometry. The devices used to accomplish this atomization step include flame-heated quartz tubes,1-3 electrically heated quartz tubes,4,5 flame-in-tube atomizers,6,7 and graphite furnaces.8 * Corresponding author: (e-mail)
[email protected]. (1) Goulden, P. D.; Brooksbank, P. Anal. Chem. 1974, 46, 1431-6. (2) Chu, R. C.; Barron, G. P.; Baumgarner, P. A. W. Anal. Chem. 1972, 44, 1476-9. (3) Siemer, D. D.; Hagemann, L. Anal. Lett. 1975, 8, 323-37. (4) Dedina, J.; Rubeska, I. Spectrochim. Acta 1980, 27, 633-9. (5) Thompson, K. C.; Thomerson D. R. Analyst 1974, 99, 595-601. (6) Godden, R. G.; Thomerson, D. R. Analyst 1980, 105, 1137-56. (7) Welz, B.; Melcher, M. Analyst 1984, 109, 569-72. 10.1021/ac980518+ CCC: $18.00 Published on Web 02/05/1999
© 1999 American Chemical Society
Ever since the establishment of the hydride generation technique,9 reports have been published concerning interferences involved in both the generation10-16 and the atomization17-19 steps. The reaction mechanisms involved in the generation and atomization steps are still not clearly understood, and the present knowledge is mainly based on indirect evidence, sometimes on quite speculative assumptions. The undesirable side reactions that cause the interferences are therefore difficult to understand. Although hydrogen radicals are believed to be involved in both the generation and atomization steps, this paper deals with electron spin resonance (ESR) spectrometry studies of the flame atomization process only; the radical involvement in the liquidphase generation step have also being studied by ESR spectroscopy, but these results will be reported separately.20 Dedina and Rubeska21 have made a serious effort to elucidate the mechanisms involved in the atomization step for hydrogen selenide, using a flame-in-tube atomizer. They found experimentally that an increase of the oxygen flow rate beyond the optimum does not change the signal sensitivity in a way that could be attributed to altered atomization characteristics of selenium. Furthermore, no increase in signal peak area was noticed when the inlet part of the T-shaped tube, which was used as atomizer, was externally heated from a temperature of 300 to 900 °C. Their conclusion from these indirect observations was that the atomization of selenium hydride was not caused by thermal decomposition, but by free radicals generated in the reaction zone of the diffusion flame. This led to a postulation of the following series of reactions, which are assumed to produce the radicals responsible for atomization: (8) Knudson, E. J.; Christian, G. D. Anal. Lett. 1973, 6, 1039-54. (9) Holak W. Anal. Chem. 1969, 41, 1712-3. (10) Yamamoto, M.; Yamamoto, Y.; Yamashige, T. Analyst 1984, 109, 1461-3. (11) Dedina, J. Anal. Chem. 1982, 54, 2097-102. (12) Åstro ¨m, O. Anal. Chem. 1982, 54, 190-3. (13) Dedina, J. Prog. Anal. At. Spectrosc. 1988, 11, 251-360. (14) Li, Z.; Xiao-quan, S.; Zhe-Ming, Ni. Frezenius’ Z. Anal. Chem. 1988, 32, 64-8. (15) Aggett, J.; Yasuhisa, H. Analyst 1987, 112, 277-82. (16) Arbab-Zavar, M. H.; Howard, A. G. Analyst 1980, 105, 744-50. (17) Smith, A. E. Analyst 1975, 100, 300-6. (18) Welz, B.; Melcher, M. Anal. Chim. Acta 1981, 131, 17-25. (19) Verlinden, M.; Deelstra, H. Fresenius’ Z. Anal. Chem. 1979, 296, 253-8. (20) Tesfalidet, S.; Wikander, G.; Irgum K. Anal. Chem., to be submitted. (21) Dedina, J.; Rubeska, I. Spectrochim. Acta 1980, 35B, 119-28.
Analytical Chemistry, Vol. 71, No. 6, March 15, 1999 1225
H + O2 a OH + O
(1)
AsH3 + H a AsH2 + H2
(7)
O + H2 a OH + H
(2)
AsH2 + H a AsH +H2
(8)
OH + H2 a H2O + H
(3)
AsH + H f As + H2
(9)
The overall reaction for the three stepwise reactions above may be written as follows:
2H2 + O2 a OH + H + H2O
(4)
Dedina and Rubeska21 furthermore assumed that, in the presence of excess hydrogen, only hydroxide and hydrogen radicals are formed in quantities corresponding to the total amount of oxygen, i.e., two radicals for each oxygen molecule. Furthermore, they stated that recombination of radicals is a process considerably slower than their formation, and a concentration well above equilibrium values is therefore present in the postreaction zone. As reaction 3 is fast, a balanced state between hydroxide and hydrogen radicals is readily established, and since the equilibrium constant of reaction 3 is very large (K3 ) 0.21e79645/T), and the hydrogen concentration in the atomizer is much higher than the water concentration, H radicals outnumber OH radicals at least by a few orders of magnitude. According to their hypothesis, using selenium as a model element, the actual atomization mechanism most probably proceeds by two consecutive reactions with prevailing hydrogen radicals:
SeH2 + H a SeH + H2
(5)
SeH + H a Se + H2
(6)
The nature of both of these reactions is highly exothermic; ∆H for the first step (reaction 5) is -189 kJ mol-1, while that for the second step (reaction 6) is -1301 kJ mol-1 (cf. ref 21). Welz and Schubert-Jacobs22 performed a systematic study of the reaction mechanisms involved in their electrically heated quartz tube atomizer. Referring to the atomization mechanism proposed by Dedina and Rubeska,21 they carried out experiments to investigate how the atomization of hydride-forming elements is influenced by the hydrogen gas, which is concomitantly formed during the hydride generation step. They found experimentally that no atomic absorption signal could be obtained when pure arsine was conducted with a stream of argon into a quartz tube heated to 900 °C. When they repeated the same experiment with hydrogen as the carrier gas, they obtained approximately the same peak area sensitivity for arsine gas injected into the tubing that leads to the heated quartz tube as for arsine generated from solution in the ordinary way. According to the authors, this was a clear demonstration of the crucial role of hydrogen in the atomization of arsenic in a heated quartz tube. They furthermore proposed that atomization of arsenic proceeds in a fashion similar to that of selenium hydride, i.e., by collision with “nascent hydrogen”: (22) Welz, B.; Schubert-Jacobs, M. Fresenius’ Z. Anal. Chem. 1986, 324, 8328.
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Worth noting is that these equations do not balance if hydrogen radicals are taken as the “nascent hydrogen” source. A redox process must therefore also be involved in order to arrive at As0(g); the alternative route is dimerization of arsenic through As• recombination, which rationalizes the formation of arsenic oligomers (As2 and As4) in the gas phase, as deduced by Welz and Melcher.23 The mechanisms responsible for the interferences that take place in the gas phase have not been investigated as extensively as the liquid-phase interferences and are therefore less well understood. Still, the reports aimed at elucidating the causes and sites of the interference reactions are numerous and contradictory. This may partly be due to the difficulty in differentiating between interferences that take place in the generation step and in the gaseous phase. Verlinden and Deelstra19 studied the effects of arsenic, germanium, tin, bismuth, antimony, tellurium, and mercury on the signal from 100 ng of selenium. With germanium and antimony, a yellowish film of GeSe2, and a grayish film of Sb2Se3, respectively, were deposited on the quartz windows of the atomizer. Results obtained with an open tube (without the quartz windows) showed that the contamination of the windows was a minor source of signal suppression. This verifies that the interference reactions occur in the gas phase. However, the mechanisms of these interferences are not really resolved at present. Experiments of a different nature were performed earlier by Welz and Melcher,23 who placed pieces of Pt foil or small graphite rods or plates at different positions in the heated quartz cell. These experiments showed the signal depression to be strongly dependent on the location of the graphite pieces within the atomizer. No influence on the signal was seen when the gas inlet was allowed to impinge directly on the graphite plate or if the graphite plate was placed exactly in the middle of the heated quartz cell. The signal depression was also negligible when the graphite was placed at the end of the heated quartz cell, close to the windows. A strong signal suppression was, however, observed when the graphite parts were placed about halfway between the gas inlet and the end windows or if a longer graphite rod that protruded further toward the ends was placed in the middle of the heated quartz cell. According to the authors, the fact that the depressing effect is highest when the rods are placed at some distance from the gas inlet suggests that this is the area where most hydrogen radicals are formed and that the signal suppression emanates from a catalytic radical recombination. “Dust or dirt” were noted as forming a catalytic film on the quartz tube surface, increasing the kinetics of the radical recombination and thus reducing the concentration of hydrogen radicals in the atomizer cell. The authors concluded that similar catalytic films are probably produced by most other contaminants that reach the heated quartz cell and are deposited on, or burnt into, the surface. Since free-radical reactions are so frequently hypothesized as playing an important role in the atomization process, a tool for (23) Welz, B.; Melcher, M. Analyst 1983, 108, 213-24.
detecting gas-phase radicals on-line during the atomization step is required to gain a better understanding of the reaction mechanisms involved. The most obvious way would be to measure the radicals directly in the miniature flame. In fact, detection of radicals under partial vacuum in flame gases is since long an established technique in flame chemistry,24,25 where it is practiced by sampling flames burning at atmospheric pressure through orifice probes, drawing the combustion gases rapidly into the partial vacuum in the ESR cavity,24 or by placing an oven sustaining a premixed rarefied flame directly adjacent to the cavity25 so that the flame plume protrudes into the measurement zone. Measurements of radicals directly in a flame burning at atmospheric pressure in the cavity appear to have been published only once before.26 Hydrogen was the only radical detected in this way, and the signals were substantially line-broadened unless the flame was operated under reducing conditions. Despite these restrictions, we still decided to evaluate direct ESR measurements for investigating this analytical flame, since the hydride generation atomizers operate with excess hydrogen. We have consequently designed and built a miniature oxygen/hydrogen flame cell, which can be fitted in the cavity of an ESR spectrometer. This enabled us to study how hydrogen radicals are involved in the flame atomization step by detecting H directly in the flame, under conditions similar to those typically used in a flame-in-tube atomizer. The flame cell, which comprises a fused-silica capillary coaxially mounted in a quartz outer tube, was centered in the ESR cavity and gave stable working conditions so that detection of the generated hydrogen radicals was possible. By using this technique, we have in the present work been able to confirm the presence and involvement of hydrogen radicals in the atomization step. Consumption of hydrogen radicals by hydride-forming elements during the oxygen/hydrogen flame atomization step has been monitored by using arsine as a model hydride. Comparison of used and pristine flame cells are reported, as well as studies aimed at identifying the spatial location of the interference. EXPERIMENTAL SECTION Reagents and Solutions. All chemicals used were of reagent grade and the water was purified using Milli-Q (Millipore, Bedford, MA) equipment. A stock solution of 1.000 g/L As(III) was prepared by dissolving the appropriate amount of ultrapure As2O3 (Ventron-Alfa, Karlsruhe, Germany) in 20 mL of 1 M NaOH and diluting to 1 L with 2 M HCl (p.a.; Merck, Darmstadt, Germany). Weaker solutions were prepared by further dilution with 2 M hydrochloric acid, if not otherwise stated. A 2% (w/v) solution of NaBH4 was prepared by dissolving the appropriate amount of sodium tetrahydroborate(III) (Merck) in 1% (w/v) NaOH. TEM(24) (a) Panfilov, V. N.; Tsvetkov, Yu. D.; Voevodskii, V. V. Kinet. Katal. 1960, 1, 333; Chem. Abstr. 1961, 55, 16160c. (b) Azatyan, V. V.; Panfilov, V. N.; Nalbandyan, A. B. Kinet. Katal. 1961, 2, 295; Chem. Abstr. 1961, 55, 24195g. (c) Azatyan, V. V.; Akopyan, L. A.; Nalbandyan, A. B. Kinet. Katal. 1961, 2, 940-1; Chem. Abstr. 1962, 57, 1567i. (d) Balakhnin, V. P.; Gershenzon, Yu. M.; Kondrat′ev, V. N.; Nalbandyan, A. B. Dokl. Akad. Nauk SSSR 1964, 154, 883-5; Chem. Abstr. 1994, 60, 12668f. (e) Balakhnin, V. P.; Gershenzon, Y.; Nalbandyan, A. B. Dokl Akad. Nauk SSSR 1967, 172, 375-8; Chem. Abstr. 1967, 66, 77923y. A number of additional papers appeared in the Russian literature during the 1960s and early 1970s by the same set of authors, dealing with ESR studies of various rarified flames. (25) Fristrom, R. M. Flame Structure and Processes; Oxford University Press: New York, 1995; p 150 ff. (26) Bennett, J. E.; Mile, B.; Summers, R. Nature (London) 1970, 225, 932-3.
Figure 1. Schematic drawing of the atomization unit comprising the oxygen/hydrogen capillary flame mounted inside the outer tube, which is situated inside the cavity of the ESR spectrometer.
POL (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (Molecular Probes Inc., Eugene, OR) was dissolved in water to a final concentration of 10 mM. Solid diphenylpicrylhydrazyl (DPPH) was used for determination of the spatial sensitivity profile of the ESR cavity. Apparatus. The ESR spectrometer was an ESP 300-E X-band spectrometer (Bruker, Karlsruhe, Germany). The modulation frequency was kept at 100 kHz for all measurements. The modulation amplitude was 0.01 mT in the flame experiments, and always less than 0.5 times the line width of the central peak in the TEMPOL and the DPPH experiments. An atomization unit comprising an oxygen/hydrogen capillary flame inserted in a 7.7mm-i.d. (wall thickness 1.4 mm) tubular quartz cell (see Figure 1 for a schematic drawing) was constructed in our laboratory and utilized for the generation of gas-phase hydrogen radicals. This atomization unit was centered in the ESR cavity, using a sliding mount that allowed the tube to be moved vertically so that the position of the flame inside the ESR cavity could be adjusted for stable detection of the generated hydrogen radicals. The central capillary, which supplied the oxygen (99.995 %; AGA, Sundbyberg, Sweden) to the flame, was a piece of 0.32-mm-i.d. fused-silica tubing. Approximately 30 mm of the external polyimide coating had been removed by burning from the distal end extending into the cell. The hydrogen fuel gas (Grade plus, 99.995 %; AGA) was supplied through the annular channel, either directly from the cylinder or after passage through the hydride generation cell. The capillary was kept coaxially centered in the outer quartz tube by Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
1227
a silicone rubber septum that was situated at the rear end of the tube and served a dual function as a gas seal. An annular spacer made of PTFE, with one centrally positioned 0.7-mm-diameter and three peripherally placed 1.0-mm-diameter drilled holes, was used to center the end of the capillary that protruded into the ESR spectrometer. This spacer was placed 30 mm below the flame base, not only for thermal protection but also to ascertain that the flow in the combustion zone was reasonably laminar by preventing excessive flow disturbance due to jets emanating from the spacer holes in the annular channel. The temperature, measured 2 mm from the outside wall of the quartz tube in the flame zone, was stable and less than 80 °C at normal working conditions. Investigations on the Spatial Distribution of Hydrogen Radicals in the Flame Cell. The flame cell was mounted centrally in the vertical hole through the ESR cavity and could be slid in the vertical direction to allow studies of spatial distribution of hydrogen radicals in the flame. Such experiments were carried out with 16 mL/min oxygen and 1 L/min hydrogen flows. The positions are reported with respect to the brightest spot of the flame, which was located approximately 1-2 mm above the tip of the capillary. In a comparative experiment, aimed at determining the vertical spatial sensitivity profile of the ESR cavity, a standard sample of solid DPPH was placed at various heights inside the ESR cavity, whereafter the signal from the DPPH sample was repeatedly measured. Investigating the Influence of the Mirror Deposition on the ESR Signal Intensity. A TEMPOL probe was used also to investigate whether the sensitivity was affected by the presence of a mirror deposit. This was done by mounting a sealed glass capillary tube containing 1.9 mL of 10 mM TEMPOL free radical (the height of the liquid zone was approximately 2 mm) inside the outer tube of a used flame cell, where the inner surface was coated by a mirror finish resulting from decomposing generated arsine in the cell for an extended period of time. The flame cell tube was then mounted in the ESR instrument and the mirror covered part was moved in to and out of the ESR cavity, followed by repositioning of the TEMPOL probe to the center of the cavity before each measurement. Hydride Generation. The hydride generation reactor was made in our laboratories, based on the design reported by Dedina.27 The hydrochloric acid and sodium tetrahydroborate(III) solutions were propelled into the generation cell by a peristaltic pump (Minipuls 2, Gilson, Villiers-de-Bel, France). When the effect of arsine on the radical intensity was studied, the hydrochloric acid was replaced by arsenic standards prepared by dilution of stock As(III) solution with 2 M hydrochloric acid. A flow of hydrogen gas, which also acted as fuel for the flame, purged the generated hydrogen gas, and in applicable cases, the arsine generated from the hydride generation reactor, into the ESR flame cell. A schematic drawing of the hydride generation reactor is shown in Figure 2. Acid Washing of the Quartz Tube. The quartz tube was periodically refreshed by brief washing with 40% HF at room temperature, followed by rinsing with water. This was done whenever reduction in signal intensity from hydrogen radicals was (27) Dedina, J. Fresenius’ Z. Anal. Chem. 1986, 323, 771-82.
1228 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
Figure 2. Exploded view of the hydride generation reactor, where sodium tetrahydroborate (III) solution is mixed with hydrochloric acid, with or without the addition of arsenic (III). The arsine is generated in the reaction vessel concomitantly with hydrogen gas. The liquid overflowing the vessel flows down the slanted tube, where additional gas/liquid separation takes place. The hydrogen gas supplied as fuel gas flowed countercurrent to the liquid stream and swept the reaction products from the reactor into the annular channel of the flame cell.
observed relative to the signal from 2 M hydrochloric acid and 2% sodium tetrahydroborate(III). RESULTS AND DISCUSSION ESR Flame Cell Design. When utilized as a capillary cell for generation of hydrogen radicals in the ESR spectrometer, the miniature oxygen/hydrogen flame torch constructed in this work enabled us to study radicals under conditions resembling those in a flame-in-tube atomizer. The cell design was a result of an iterative process, and the final version gave working conditions sufficiently stable to allow detection of the generated hydrogen radicals. A typical ESR spectrum of those species that are generated in the diffusive oxygen/hydrogen flame is displayed in Figure 3. The dominant signal is composed of two narrow lines separated from each other by 50.9 mT. Taking second-order effects into account
Figure 3. Typical ESR spectrum obtained for the species generated in the oxygen/hydrogen flame: hydrogen flow rate 1 L/min; oxygen flow rate 10 mL/min.
(ref 28 Appendix C), we obtain an effective g-value of 2.0036, which means that the center of the symmetry between the two lines is shifted by 2.1 mT to the lower side of the homogeneous magnetic field B in comparison to the g-value of atomic hydrogen, which is 2.002 256 (ref 28 Appendix C). Our value thus differs by less than 0.06% compared to that given in the literature. This signal is characteristic of hydrogen atoms with the free electron confined to an s-orbital (S ) 1/2), having a strong contact interaction with the nucleus of the proton which possesses a nuclear spin of I ) 1/ . 2 The isotropic spin Hamiltonian for this system can be expressed according to the following:
H ˆ ) gβB0Sˆ z + hA0Sˆ zˆIz
(10)
where g is the g-value of the system; β the Bohr magneton, 9.274 × 10-24 J T-1; B0 the magnetic field applied along the z-axis; h the Planck constant, 6.626 × 10-34 J s-1; Sˆ z the electron spin operator; ˆIz the nuclear spin operator; and A0 the hyperfine coupling constant, (in Hz). From a quantum chemical point of view, the s-electron (which is in close contact with the proton) has a probability electron density |ψ(0)|2 equal to 1. Hence, the theoretical coupling constant of atomic hydrogen can be evaluated from the following equation proposed by Fermi:28
A0 ) 8π3/3gβgNβN |ψ(0)|2 ) 1423 MHz ) 50.5 mT (11) where gN is the nuclear g-value of the proton and βN is the nuclear magneton,.051 × 10-27 J T-1. The theoretical value is thus in close agreement with the experimentally observed value of 50.9 mT for H in the present investigation, which also agrees with the findings of Bennett et al.26 and Tkac.29 Distribution of Hydrogen Radicals in the Quartz Tube. The ESR signal from the formed atomic hydrogen species was very stable. Furthermore, we found that the radicals that were produced in the flame were confined to a small region in the cavity. This could be verified by varying the position of the flame capillary in the atomization cell. A plot of the signal vs vertical position of the flame inside the ESR cavity is shown in Figure 4. From this plot it can be seen that the hydrogen radical intensity was highest when the brightest spot of the flame was placed in the center of the cavity. Superimposed on this curve is the spatial sensitivity for a solid sample of DPPH (cf. Experimental Section). The (28) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance Elementary Theory and Practical Applications; McGraw-Hill: New York, 1972; Chapter 3. (29) Tkac, A. Chem. Pap. (Slovac) 1990, 44, 737-768.
Figure 4. Signal intensity as a function of the flame position in the ESR cavity, expressed as vertical departure of the flame center from the center of the ESR cavity, measured in millimeters. The solid line shows the signal when changing the vertical position of the flame in the cavity, whereas the dotted line represents the signal variations obtained when moving solid sample of DPPH vertically in the cavity. Positive height values are above the cavity center.
hydrogen radical intensity decreased when the flame position was displaced from the center by moving the inner quartz capillary either upward or downward, and the distribution of hydrogen radicals appeared to be almost symmetrical around the center of the flame. Part of this apparent symmetry can be ascribed to the variations in spatial sensitivity of the ESR cavity, as shown in Figure 4, but this experiment still demonstrates that the radicals are confined to the flame zone and that their concentration decreases rapidly in the plume. Welz and Schubert-Jacobs22 performed a recovery study by washing the deposited arsenic species from different parts of the quartz tube atomizer, in an attempt to reveal the crucial role of hydrogen radicals in the atomization step. The dissolved arsenic was first reduced to arsine in a hydride generation reactor and collected in a cold trap before sweeping it to the heated quartz tube with an inert gas for atomic spectrometric determination. They found that most of the arsenic was deposited toward the end of the heated zone of the quartz tube atomizer and that very little was found in the hot central part of the tube or at the quartz windows. Our experimental results based on direct measurement of hydrogen radicals provide a verification of their hypothesis that hydrogen radicals are likely to be responsible for preventing deposition of arsenic in the central part of the tube. An important observation worth nothing from the investigation of Welz and Schubert-Jacobs is that the average recovery decreased to 64% when a stream of helium was followed by a stream of hydrogen (30 s later) and to 21% when a stream of only hydrogen was used to carry the arsine from the cold trap to the heated quartz tube. This was aimed to reflect the temperature distribution over the length of the heated quartz tube. Their explanation to this result was that not all of the arsenic species deposited in the quartz tube after thermal decomposition in an inert gas atmosphere can be revolatilized with hydrogen later on. They deduce that an atomization from the condensed phase requires more than just molecular hydrogen. The reactive form of hydrogen molecule, i.e., the hydrogen radical, is apparently Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
1229
Figure 5. Response surface showing the variations in signal from hydrogen radicals when the oxygen and hydrogen gas flow rates in the capillary flame atomization unit were varied simultaneously.
formed under the conditions that are met only in the heated zone of the quartz tube. This would mean that any decomposition product deposited more toward the end of the quartz tube cannot be revolatilized independent from the time during which hydrogen is applied. The hydrogen radical distribution in Figure 4 mirrors the distribution of the arsenic deposition which is reported in Figures 5 and 6 of ref 22. Our setup has thus provided the experimental evidence needed to verify that only in the center of the cavity would the intensity of hydrogen radicals be sufficiently high to atomize arsenic according to reactions 7-9. In fact, when the outer tube was slid down and the mirror deposit repositioned close to the burning flame, the deposit rapidly disappeared. This observation cannot be explained by the temperature at the tube surface, which was too low30 to cause significant evaporation of any of the species postulated as forming this deposit.22,23 Consumption of Hydrogen Radicals by Arsine. Since the atomization of hydrides is believed to be brought about by radical reactions, where hydrogen radicals are essential and play an active role, a study of the variation in signal intensity from hydrogen radicals due to introduction of hydrides is appropriate here. First, to be able to monitor changes in hydrogen radical intensity when arsine is introduced into the flame, we had to choose experimental conditions that gave small but easily detectable hydrogen radical signals. A response surface was therefore constructed, where the signal intensity of hydrogen radicals in the center of the cavity was monitored as a function of simultaneous variations of the hydrogen and oxygen flow rates. The response surface obtained is shown in Figure 5. From this plot it became apparent that the minimal detected signal from hydrogen radicals in the flame was obtained for low oxygen flow rates and high hydrogen flow rates. The flow rates for hydrogen (2 L/min) and oxygen (4 mL/min) that gave minimal radical intensity were then chosen for the experiment where arsine was generated and introduced into the system. Comparison of ESR spectra obtained with and without the hydride generation system incorporated revealed that there was (30) Frech, W.; Lundberg, E.; Cedergren, A. Prog. Anal. Atom. Spectrosc. 1985, 8, 257-70.
1230 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
Figure 6. Reduction in the signal from hydrogen radicals in the flame as function of the arsenic(III) concentration introduced into the hydride generation reactor when first increasing and thereafter decreasing the concentration. The dotted lines represent two different cyclic runs (increasing and then decreasing the arsenic concentration) with cleaning of the outer tube between each run.
no significant effect on the monitored intensity of the detected hydrogen radicals in the flame. Pumping sodium tetrahydroborate(III) and hydrochloric acid without arsenic added thus had no influence on the ESR signal. On the other hand, when arsenic was added to the hydrochloric acid, a drastic decline in the monitored signal intensity from hydrogen radicals was observed. According to the mechanism proposed by Welz and SchubertJacobs,22 the atomization of arsenic should proceed through collisions between arsine and hydrogen radicals (cf. eqs 7-9). The monitored decrease is in accordance with their findings and provides the first direct confirmation of the series of circumstantial evidences that led them to the conclusion that hydrogen radicals are consumed during the atomization of hydrides. Another plausible explanation could be that the formed hydrogen radicals are catalytically recombined on the surface of metal particles and/ or decomposition products such as As2 or As2O2 formed in the center of the cavity, a phenomenon observed by Welz and Schubert-Jacobs.22 The relative probability of this phenomenon should however be very low in the center of the cavity where the concentration of hydrogen radicals is high, as demonstrated in the present work. The influence of varying arsenic concentrations in the generation reaction on the monitored signal intensity is shown in Figure 6, where it is evident that the signal intensity decreased when the rate of arsenic introduction was increased. The right part of this figure shows the results when the experiment was repeated, this time decreasing the arsenic concentration, whereby the shape of the curve became almost a mirror of the previous one. We also obtained an almost full recovery of the original signal intensity in the latter experiment, i.e., when the arsenic concentration had returned to zero. Consequently, the signal depression with arsine present cannot have been due to arsenic depositions on the cell surfaces, nor shielding of the flame which could have been suspected because a dark mirror was deposited during the experiment (see below). If decomposition products should have been present in the center of the cavity to any significant degree, we expect that we should have noticed this as a disturbance of the spectrometer setting during the experiments since the dielectric properties of the cavity should change in the presence of metal particles. So
Table 1. Influence of Mirror Deposit on the Inner Surface of the Outer Quartz Tube Wall of the Miniature Torch on the ESR Frequency and Double Integrala mirror
f (GHz)
double integral
none no. 1 no. 2
9.427/9.427 9.426/9.426 9.424/9.424
4.016/4.052 4.037/3.996 4.052/4.016
a Two different outer tubes with mirror deposits were tested, each of them two times. A double integral was first obtained in a region of the tube where no arsenic was deposited, whereafter the outer tube was slid so that the mirror was positioned adjacent to the flame and the double integral was measured.
far we have not noticed any evidence for this. Hence, from the experimental results obtained so far, it appears plausible to conclude that arsine is indeed atomized by radical recombination, which provides an hitherto lacking experimental verification of postulated hypotheses.21 Influence of the Quartz Cell Surface. The intensities of the monitored ESR signals obtained with used as well as acid-washed (40% HF) quartz tubes were compared. When arsenic concentrations above 50 µg/mL were used, the inner surface of the quartz tube became dark after a while, attaining a mirrorlike finish where the deposition was heaviest. To study the effect of this dark mirror on the signal intensity, the position of the flame was displaced by moving the capillary tube upward through the septum sealing the bottom of the outer quartz tube. The quartz tube was then moved downward by the same length increments in order to reposition the flame in the center of the ESR cavity. The results from this experiment are presented in Table 1, showing that no substantial change in signal intensity was observed when the dark mirror was repositioned to encapsulate the flame. We could also observe a gradual disappearance of the dark mirror when positioned adjacent to the flame. Our results, which are based on direct measurements of hydrogen radicals, reveal, at least when a flamein-tube atomizer is used, that the deposited film can be revolatilized and atomized given that the concentration of hydrogen radicals is sufficiently high. The dark mirror occupies about 20-30 mm of the upper part of the tube starting approximately 20 mm above the position of the flame center. Since the formation of a deposit on the cell wall took place when arsine was introduced in the cell, we conclude
that the deposit consists of arsenic species. The double-integrated signal from TEMPOL in the noncoated region of the cell was 4.034 ( 0.025 (n ) 2; mean ( standard deviation), while in the region of the tube most severely covered by the deposited mirror we obtained TEMPOL signals whose magnitudes were 4.025 ( 0.024 (n ) 4; two measurements each on two different mirrors with removal between). Although these experiments show that the deposit did not cause a signal depression, it is not possible from the present data to confirm if the deposited layer is diamagnetic and being dielectric at the spectrometer frequency of 9.42 GHz. In this context, it is interesting to note that Welz and Melcher23 in their experiments claimed that the deposit is an arsenic oligomer, most probably the dimer As2, or an arsenic oxide. The nature of the deposit is currently under investigation, and the results will be presented in a forthcoming paper.20 CONCLUSIONS In the present work, we have presented a direct method for the detection of hydrogen radicals in an analytical hydrogen/ oxygen flame. By means of the devised experimental setup, it has been possible to monitor the production of hydrogen radicals in the hydride atomization step. It was also possible to monitor the decline and consumption of these radicals when arsenic was introduced to the flame as a model compound for hydride-forming elements. These results provide the direct experimental evidence needed to support the proposed mechanism for atomization of hydrides, i.e., that atomization is brought about by hydrogen radicals as previously postulated by Dedina and Rubeska21 and Welz and Schubert-Jacobs.22 The flame cell developed in the present work might also be used to study other detection mechanisms based on microflames where the involvement of radicals is known or suspected. Candidate techniques for such investigations are the flame detectors used in gas chromatography. ACKNOWLEDGMENT The authors appreciate the constructive discussions with Wolfgang Frech during the preparation of this work and the technical support by Mrs Eva Vikstro¨m. The work was supported by the Swedish Natural Science Research Council through Grant K-KU 8735-312 et seq. Received for review May 12, 1998. Accepted October 20, 1998. AC980518+
Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
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