Anal. Chem. 1985, 57, 1299-1305
lution of anthracene and phenanthrene was found to be slightly less (0.5-2%) with the cross-linked phase than with the gum. Finally, the temperature stability was higher for the cross-linked phase than €or the gum phase. The crosslinked columns could be used up to 280 "C without significant bleeding or deterioration of the phase film. Registry No. 1-Methylphenanthrene, 832-69-9; 2-methylphenanthrene, 2531-84-2; 3-methylphenanthrene, 832-71-3; 4methylphenanthrene, 832-64-4;9-methylphenanthrene, 883-20-5; 1-methylchrysene, 3351-28-8; 2-methylchrysene, 3351-32-4; 3methylchrysene, 3351-31-3; 4-methylchrysene, 3351-30-2; 5methylchrysene, 3697-24-3; 6-methylchrysene, 1705-85-7; 1hydroxydibenzothiophene, 69747-83-7; 2-hydroxydibenzothiophene, 22439-65-2; 3-hydroxydibenzothiophene,69747-77-9; 4-hydroxydibenzothiophene,24444-75-5; 1-aminophenanthrene, 4176-53-8; 2-aminophenanthrene, 3366-65-2; 3-aminophenanthrene, 1892-54-2; 4-aminophenanthrene, 17423-48-2; 9-aminophenanthrene, 947-73-9; phenanthridine, 229-87-8; benzo[flquinoline, 85-02-9; acridine, 260-94-6; benzo[h]quinoline, 230-27-3; benzo[c]phenanthrene,195-19-7;triphenylene, 217-59-4; benz[a]anthracene, 56-55-3; chrysene, 218-01-9; naphthacene, 92-24-0;nonadecane, 629-92-5;eicosane, 112-95-8;heneicosane, 629-94-7; docosane, 629-97-0; tetracosane, 646-31-1.
LITERATURE CITED Kelker, H. Ber. Bunsenges, Phys. Chem. 1983, 87, 698-703. Witkiewicz, 2 . J. Chromatogr. 1982, 251,311-337. Wlse, S. A,; Bonnett, W. J.; Guenther, F. R.; May, W. E. J. Chromatogr. Scl. 1981, 79, 457-465. Heath, R R.; Jordan, J. R.; Sonnet, P. E.; Tumlinson, J. H. HRC CC, J. High Resolut . Chromatogr . Chromatogr . Commun . 1979, 2, 712-714. Zlelinski, W. L., Jr.; Scanian, R. A,; Miller, M. M. J. Chromatogr. 1981, 209, 87-90.
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Witklewicz, 2.; Rudnicka, I.; Szulc, J.; Dabrowski, R. J. Chromatogr. 1984, 294, 127-137. Finkelmann, H.; Laub, R. J.; Roberts, W. L.; Smlth, C. A. HRC CC, J. Hlgh Resolut. Chromatogr . Chromatogr . Commun . 1980, 7 , 355-356. Finkeimann, H. "Polymer Liquid Crystals"; Ciferri, A,, Krigbaum, W. R., Meyer, R. E., Eds.; Academic Press: New York, 1982; pp 35-62. Finkeimann, H.; Laub, R. J. Chem. Abstr. 1983, 98, 78678. Kong, R. C.; Lee, M. L.;Tominaga, Y.; Pratap, R.; Iwao, M.; Castle, R. N. Anal. Chem. 1982, 54, 1802-1806. Finkeimann, H.; Kock, H.-J.; Rehage, G. Makromol. Chem ., Rapid Commun. 1981, 2 , 317-322. Finkeimann, H.; Laub, R. J.; Roberts, W. L.; Smith, C. A. "Polynuclear Aromatic Hydrocarbons: Physical and Biological Chemistry"; Cooke, M, Dennis, A. J., Fisher, G. L., Eds.; Batteiie Press: Columbus, OH, 1982; pp 275-285. Gray, G. W.; Goodby, J. W. "Smectic Liquid Crystals: Textures and Structures"; Heyden: Philadelphia, PA, 1984. Schroeder, J. P. "Liquid Crystals and Plastic Crystals"; Gray G. W., Winsor, P. A., Eds.; Ellis Horwood, Ltd.: Chichester, England, 1974; VOi 1, pp 356-369 Sojak, L.; Kraus, G.; Ostrovsky, I.; Kralovicova, E.; Krupcik, J. J. Chromatogr. 1981, 206, 475-483. Haky, J. E.; Muschik, G. M. J. Chromatogr. 1981, 214, 161-170. Keiker, H.; Von Schlvizhoffen, E. "Advances in Chromatography"; Giddings, J. C., Keller, R. A,, Eds.; Marcel Dekker: New York, 1968; Vol. 6, pp 247-297. Jones, E. A.; Bradshaw, J. S.; Nishioka, M.; Lee, M. L. J. Org. Chem. 1984, 49, 4947-4951. Richter, E. E.; Kuei, J. C.; Park, N. J.; Crowiey, S.J.; Bradshaw, J. S.; Lee, M. L. HRC CC , J . Hlgh Resolut . Chromatogr. Chromatogr. Common. 1003, 6 , 371-374. Biumstein, A., Ed. "Liquid Crystalline Order in Polymers"; Academic Press: New York, 1978; Chapters 1-3. Later, D. W.; Mcfali, T.; Booth, 0.M.; Lee, M. L.; Tedjamulia, M.; Castle, R. N. fnvlron. Mutagen. 1984, 6 , 497-504.
RECEIVED for review December 3,1984. Accepted February 19, 1985. This work was supported by the National Science Foundation, Grant No. CHE-8314769.
Performance of an Automated Gas Chromatograph-Silica Furnace-Atomic Absorption Spectrometer for the Determination of Alkyllead Compounds Donald S. Forsyth and William D. Marshall* Department of Agricultural Chemistry and Physics, Macdonald College, Ste. Anne de Bellevue, Quebec, Canada H9X 1CO An automated gas chromatograph-atomic absorptlon spectrometer was used to study the decomposltlon-atomization process for alkyllead compounds In a heated quartz tube. Longer term stablllty of the system was achieved with a purglng cycle In whlch the quartz furnace was flushed with large alr flows during solvent elutlon prior to returnlng to normal reduclng atmospheres in hydrogen. The system response was optimized by uslng unlvariate procedures; It was lnsensltlve to changes In atomlzer surface temperatures above 800 OC. No changes In atomlzatlon efflclency for quantltles of lead between 30 pg and 30 ng were observed, suggestlng that excess lead scavenger was present. Although less efficient than hydrogen, other gases also supported the decomposltlon-atomizatlon of the analytes. A major portlon of the lead not atomized in alr or nltrogen was deposlted on the surface of the quartz walls and was revolatllized and atomlzed extremely rapidly If hydrogen was admltted to the furnace. Hydrogen radicals are postulated to mediate both the revolatlllration and atomlzatlon steps.
The use of a silica tube furnace as a convenient atomization
device for the determination of covalent hydrides by atomic absorption spectrometry (AAS) was initially suggested by Chu et al. (1). This detector has rapidly gained wide acceptance (2-4) and has been successfully used by several groups for the gas chromatographic determination of covalent hydrides (5, 6) and a variety of organometallics (7,B) including organoselenium (9, IO), organoarsenicals and -mercurials ( I I ) , organotins (12, 13), and organoleads (14, 15). These customdesigned chromatographic detectors offer a virtually element specific detection as well as an impressive sensitivity, and they are inexpensive. However the mechanism of atomic vapor generation is not well understood, and only a limited consideration has been given to the automation of gas chromatograph-silica furnace systems required for large numbers of environmental samples. Although the term thermal or electrothermal has often been used to describe the atomization process within silica tubes, researchers are becoming increasingly aware that the surface material and composition of the gaseous atmosphere profoundly influence the sensitivity of these devices. Whereas silica furnaces are normally operated a t 800-900 "C for the detection of arsine or selenium hydride, temperatures of 1700-1800 O C are optimal for their atomization from graphite
0003-2700/85/0357-1299$01.50/0 0 1985 American Chemical Society
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Figure 1. Exploded view of silica furnace: A, shaped firebrick upper casing; B, quartz T-tube; C, alloy 875 resistance heating wire; D, hinged aluminum casing; E, type-K thermocouples; F, Swagelok connectors with ceramic inserts; G, shaped firebrick lower casing; H, aluminum mounting bracket; I, machine screws.
tubes (16,17). The highest sensitivities for these analytes have been achieved with extremely fuel-rich hydrogen-oxygen or hydrogen-air flames burning within an unheated quartz atomizer (18). Both oxygen and hydrogen greatly increased sensitivity when added to argon purging gas. Paneth et al. (19)studied the deposition and movement of lead mirrors in silica tubes. They reported a virtually quantitative decomposition of R4Pb (in H2 at 900 "C) and detected two separate quenching reactions which controlled the lifetimes of the alkyl radicals. The first, wall-mediated, decreased with increasing temperature whereas the second became predominent at higher temperatures. Dgdina and Rubegka (20) concluded that selenium hydride was fully atomized within a spatially limited cloud of free radicals (H, OH) in the lower portion of their unheated silica T-furnace and that the decay of the selenium atoms proceeded predominantly on the cell walls. Welz and Melcher (21), studying the fate of arsine in a heated quartz cell, concluded that atomization resulted from collisions with hydrogen radicals. They emphasized the necessity for a well-conditioned surface free from active sites which would catalyze radical recombination. They further noted that the heavier elements within a periodic group have a higher collision efficiency and thus their atomization would be less oxygen- or temperature-dependent. This report describes the design and optimization of an automated GC-AAS system for the determination of alkyllead compounds in environmental samples and considers the mechanism of the decomposition atomization of these analytes in the quartz tube furnace.
EXPERIMENTAL SECTION Apparatus. The analytical system consisted of a HewlettPackard Model 5750 gas chromatograph (GC) fitted with an autosampler (Model 7670A), a silica furnace, a Zeiss FMD-3 atomic absorption spectrometer (AAS), and a recording integrator (HP Model 3390A). The silica furnace (Figure 1)consisted of a quartz T-tube (upper furnace tube, 10 cm by 7 mm i.d.; lower T-tube, 6 cm by 4 mm/i.d.) which was heated electrically with 3.6 m of 22-gauge Chrome1 875 resistence wire (4.53 Q m-l, Hoskins Alloys)
Figure 2. Quartz T-tube interface: A, 6.25-mm-0.d. lower tube of furnace; B, 0.64-cm Swagelok nut; C, alumina tube 0.32-cm-0.d. (0.16-mm4.d.) by 10.2 cm long; D, 0.64-cm graphite ferrule; E, 0.640.32-cm Swagelok reducing union; F, H, inlet; G, air inlet; H, 0.32-cm vespeVgraphite ferrule; I, 0.32-cm Swagelok nut; J, 0.32-0.16-mm Swagelok reducing union, K, H, inlet; L, capillary graphite ferrule; M, 0.16-cm Swagelok nut; N, fused silica capillary column.
coiled around the furnace tube, The furnace was encased in shaped firebrick and held in place by a hinged aluminum tube (11 cm by 4 cm id.). This assembly was provided with insulated entry ports for electrical connections and for two type-K thermocouples (placed 1-2 mm from the surface of the quartz tube). The furnace assembly was mounted in an aluminum cradle with machine screws and positioned within the optical beam of the spectrometer with a ball joint affixed below the cradle. The furnace was connected to the GC by a 1-m section of 0.327-mm OV-101 fused silica column which was surrounded by 6.25-mm copper tubing and maintained at 250 O C with heating tape. The AAS was equipped with a Pb hollow cathode lamp (Cathodeon, Ltd.) which was normally operated at the 10-mA setting in conjunction with a deuterium background correction system. The interface of the capillary column transfer line with the lower T-tube of the silica furnace was achieved with two reducing unions and a short section of alumina tubing. Details of this configuration are presented in Figure 2. The capillary column was positioned within a 8.5 cm long by 0.32 cm 0.d. alumina tube by using a 0.32-0.16-cm stainless steel reducing union (Swagelok) which had been modified to accept a 0.16-cm stainless steel tube. The capillary column-alumina tubing assembly was then positioned within the base of the silica furnace by means of a 0.640.32-cm stainless steel reducing union (Swagelok)which had been similarly modified to accept two separate 0.16-cm stainless steel tubes. These steel tubes served as entry ports for makeup gases to the furnace. This configuration allowed for the flow of separate gases between the capillary tubing and the alumina support tube and between the support tube and the inner wall of the silica furnace. Further, it provided a measure of insulation for the portion of the capillary column which was inside the silica furnace and facilitated its positioning relative to the hot upper portion of the furnace. The other end of the transfer line was connected to the GC column with a 0.64-0.16-cm reducing union (Swagelok). The capillary transfer line was looped within the GC oven to facilitate the removal of short sections (1-2 cm) from the furnace end as required. The furnace skin temperature was maintained by a separate temperature controller circuit (mounted within the GC electronic module) which was connected to one of the furnace thermocouples. This circuit was removed in its entirity from a Hall Electrolytic conductivity detector (Model 310, Tracor Northern Inc.), and it maintained the furnace surface temperature within f 2 "C. The second thermocouple in the furnace assembly was connected to a digital readout meter.
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I 6 1120 V K l
Flgure 3. Variable time delay solvent purge circuit: A, solid-state adjustable time delay relay switch; B, momentary contact switch; c, SPST switch; D, neon indicator Ilght; E, air flow solenoid valve (normally closed); F, H, flow solenoid valve (normally open).
The GC electronics module was further modified to accept a timing circuit for purging the furnace with air (500 mL/min) during elution of solvent from the GC column. This timing circuit (Figure 3) consisted of a solid-state time delay relay (Model T2K-00300-461,National Controls Corp.) which was connected to two solenoid valves (Asco No. 8595C53, one normally open and the second normally closed. During automated operation, the circuit was actuated by the autosampler electronics and the relay interrupted the normal flow of reducing gases by closing solenoid one and opening solenoid two, admitting a high flow of air to the furnace. At a preselected time (3-300 s), the relay reversed the position of the solenoid valves, returning the furnace to normal reducing conditions. This purging system eliminated carbonaceous deposits within the optical cell. These deposits resulted from incomplete combustion of solvent at the exits of the furnace and caused a gradual increase in noise with time. The GC was fitted with a 1.8-m,6-mm-diameter, glass column consisting of 10% OV-101 on 80-100-mesh Supelcoport. Optimized operating conditions were as follows: carrier gas, helium, 35 mL min-'; injector temperature, 200 "C; temperature program, isothermal at 50 "C for 1min followed by a linear ramping to 250 "C at 8 min-' and held for 1 min. Furnace operating conditions were 900 "C and hydrogen makeup gas at 50 mL m i d . Reagents and Standards. Alkyllead chlorides (R3PbC1, R2PbC12;R = CH, and C,HS) and alkylphenylleads (R,PhPb, R2Ph2Pb;R = CH,, CzH5)were prepared and purified as previously described (22). Alkylbutylleads (RBBuPb, R2BuzPb;R = CH3, C2H5)were prepared from the corresponding chlorides by the action of butylmagnesium chloride (Morton Thiokol Inc.) in THF (15). The purity of the resulting products was verified chromatographically and the lead content, after oxidation with iodine monochloride, was compared with lead nitrate standards by flame AAS. A stock solution containing Me3BuPb (4.6 X lo4 g/mL), Me2Bu2Pb(4.6 X lo4 g/mL), Et,BuPb (2.5 X lo4 g/mL), and EtzBuzPb (2.5 X lo4 g/mL) in hexane was used for the optimization studies. Dilution of this stock solution with hexane provided working standards. Chromatographicsupport gases were prepurified grade or better.
RESULTS AND DISCUSSION Detector Optimization. In a series of optimization experiments, the effects of detector parameters on the system response (a combination of reproducibility and sensitivity) were studied by using univariate optimization procedures. A mixture of four butylated alkyllead salts (Me,BuPb, Me2Bu,Pb, Et3BuPb, and Et,Bu2Pb) was chosen as test analytes rather than the phenylated analogues because they represented a more critical test of the resolving power of the system. A significant deterioration in the performance of the system would be recognized as a decrease in the resolution of the two analogues (Me2Bu2Pband Et3BuPb) with the same
6W
7UO FURNACE
Bw 9W T€MPERATURE ( C )
1000
Flgure 4. Variation in area response (-) and in area to height ratio (. .) as a function of furnace surface operating temperature for 3 ng of Pb (as Me,BuPb) injected into the gas chromatograph. 9
molecular weight. In these studies, the system response was recorded as a function of (a) furnace temperature, (b) position of the transfer line within the base of the furnace assembly, (c) the detector makeup gas flow rate, and (d) the column flow rate. Each of the parameters was separately varied while the other parameters were maintained constant: furnace temperature, 900 "C; column flow rate, 55 mL min-' helium; detector makeup gas, 50 mL min-l hydrogen, and the transfer line (maintained at 250 "C) was positioned directly at the interface of the two silica tubes which comprise the silica furnace. Each data point in these studies represents the average of three replicate injections. The effect of varying the furnace skin temperature (from 600 to 1000 "C) on the peak area and on the peak shape (area-to-height ratio which is the approximate width of the peak at half-height in minutes) is presented in Figure 4. The area response and peak shape for each of the butylates was insensitive to changes in furnace temperature between 800 and 1000 "C (the practical temperature limit of the detector). At lower temperatures, the peak areas were reduced and the peaks were somewhat broadened. The response profile for each of the standards in the mixture was similar; for simplicity, only the results for Me,BuPb are presented. The detector was insensitive to variations in the position of the transfer line within the furnace. The placement of the transfer line at 0,5,10, or 20 mm from the joint between the two tubes which comprise the silica furnace did not influence either the area response or the peak shape for any of the four standards. In these experiments the alumina support tube (Figure 2) was always positioned within the furnace assembly so as to expose a 1-cm section of the capillary transfer line. The flow of hydrogen makeup gas to the detector was found to influence the detector response considerably. While the column flow rate was maintained at 55 mL min-l, the flow rate of hydrogen to the silica furnace (introduced into the furnace assembly via ports K F of Figure 3) was varied from 0 to 150 mL min-'. Only the most concentrated standards resulted
+
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A
Flgure 6. Typical chromatograms of (A) Me,PhPb, Et3PhPb, Me,Ph,Pb, and Et,Ph,Pb and (8) Me,BuPb, Me,Bu,Pb, Et,BuPb, and Et,Bu,Pb standards. Lowest chromatograms represent solvent blanks; middle chromatograms represent 9-12 pg (as Pb) of phenylatyes (A) and 27-31 pg (as Pb) of butylates (B). Upper chromatograms represent 46-58 pg (as Pb) phenylates (A) and 269-307 pg (as Pb) butylates (6).
Figure 5. Variation in area response for 3 ng (-)
.
and for 0.3 ng (- -) of Pb (as Me3BuPb) injected into the gas chromatograph and in area-to-height ratio (. e) (3 ng) as a function of flow rate of H, makeup gas to the furnace.
-
in a measurable response in the absence of hydrogen, however, a rapid increase in response was observed with low flow rates of hydrogen to a maximum of 25 mL min-l. Thereafter, the response decreased with increases in makeup flows and the peak shape was only marginally improved. Thus the detector response was dependent on the total gas flow through the detector and on the quantity of hydrogen in the gas mixture. The response profile was similar for each of the four analytes. The results for the Me,BuPb are presented in Figure 5. The decrease in response with increased flow rates of makeup gas is to be expected in terms of a decreased residence time of the free atoms within the optical beam of the detection system. That the decrease is not monotonic may reflect changes in the total gas flows resulting from the changing gas mixture. The heat capacities, viscosities and thermal conductivities for the two gases of the mixture are somewhat different. These experiments were repeated with three other concentrations of the standard test mixture (approximately 30 ng or 0.3 ng or 30 pg as P b per injection for each analyte). The responses for 0.3 ng (as Pb) are Me3BuPb are included in Figure 5. The response profile for each analyte, at each of the concentrations, was similar in shape. Specifically the position of optimum sensitivity did not vary with the quantity of lead injected. In a comparison study, the flow rate of hydrogen makeup gas was maintained constant (55 mL min-I), and the effect of column flow rate of helium a t 20, 35, 55, or 77 mL min-l was studied. The sensitivity was greatest at 35 mL min-l whereas the reproducibility increased slightly with increasing column flow rates. At 77 mL m i d , the sensitivity was reduced 29% relative to the maximum; moreover the resolution of the two alkyllead isomers Me2Bu2Pband Et3BuPb was slightly reduced. The reduction in sensitivity when the column flow rate was increased from 35 to 55 to 77 mL min-' exactly paralleled the decreased response observed in the corresponding region of Figure 5. In total, these results suggest that a certain quantity of hydrogen makeup gas must be present in the furnace for maximum atomization of the an-
alyte. Increased total gas flows resulting from higher furnace operating temperatures, from higher hydrogen makeup flow rates, or from higher column flow rates serve only to remove the analyte more rapidly from the optical beam. Although it has been reported that steel or tantalum transfer lines were appreciably less reactive than silica or alumina transfer lines, no indication of decomposition and/or serious deposition of lead within the transfer line has been observed in any of our studies. An uncoated silica capillary transfer line did not result in increased sensitivities relative to the coated transfer line, and varying the operating temperature of the coated transfer line from 200 to 275 O C had no observable effect on the response. It is considered that the very short residence time within the transfer line and the low temperature of the gases entering the furnace do not contribute to decomposition of the alkyllead. A low temperature of the transfer line within the furnace assembly was assured by removing a section of the firebrick around the interface of the transfer line with the furnace. The alumina tube within the interface serves the dual purpose of supporting the capillary column and shielding it from the heat of the furnace. On the basis of optimization studies, standard operating conditions for the system were defined as 50 mL min-' H2 makeup gas to the detector operated at 900 " C and a column flow of 35 mL min-l He. When these optimized conditions were used, a linearity study at both 217 and 283 nm was performed for each of the four alkylbutyllead standards. The response to each of these standards was very similar and varied from each other by less than a factor of 2. The log area response at 283 nm varied linearly as the log of the quantity (30 X 10-l2,X lo-", X lo-'', X g) of alkylbutyllead injected, whereas at 217 nm the response became distinctly curvilinear for the most concentrated standard. This deviation from linearity (based on three replicate injections of each standard) was observed only at 217 nm and only for the most concentrated standard of each of the four butylates. It was concluded that the loss of response a t the highest level was a result of saturation of the photometer electronics rather than a decreased atomization efficiency within the detector cell. The slope of one for the linear portions of these curves confirmed that the detector response was a function only of the decomposition atomization efficiency. Typical responses for a standard mixture of alkyllead phenylates and a mixture of alkyllead butylates are provided in the chromatograms of Figure 6. In both Figure 6A and B, the lower chromatograms represent solvent blanks of hexane, whereas the middle and upper chromatograms of
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
Table I. Relative Detector Response to Me,BuPb in the Presence of Various Makeup Gases makeup gas
re1 area response"
HZ
1
3 "
0.556 f 0.013 0.593 f 0.014 0.329 f 0.029 0.299 f 0.009 0.014 f 0.002 0.004 f 0.0004
Hz, NH, (1~1) NZ CH,
He air
area height ratiob 0.158 f 0.001 0.151 f 0.001 0.149 f 0.001 0.304 f0.005 0.164 f 0.003 0.183 f 0.020 0.191 f 0.005
'Average f 1 standard deviation for three replicate injections. *Approximatelyequal to the peak width at half-height in minutes.
Figure 6A represent 9-12 pg as P b and 46-58 pg as Pb, respectively, of each analyte of a standard mixture of alkyllead phenylates. The middle chromatogram of Figure 6B represents 27-31 pg (as Pb) of a standard mixture of alkyllead butylates and the upper chromatogram represents 269-307 pg (as Pb) of each analyte of the same mixture. The resolution in this last chromatogram remained virtually base line with less than 1% cross-contamination of the central pair of analytes. Lead Atomization Parameters. It was of interest to probe the process whereby alkylleads were decomposed and atomized within a silica tube which was operated 840 "C below the boiling point of elemental lead. The most efficient atomization of methyltriethyllead has been reported to be at approximately 1600 "C by using a graphite tube (23))yet silica tubes operating at much lower temperatures are only 2-to %fold less sensitive (7).As a working hypothesis, it was postulated that hydrogen makeup gas was being atomized within the silica furnace and that the resulting hydrogen radicals were acting as scavengers for metallic lead, the initial decomposition product of the alkyllead analytes. The operating temperature of the furnace was too low to account for any appreciable gas-phase dissociation of hydrogen molecules yet the insensitivity of the detector response to temperature changes (when operated at or above 800 "C) suggested that an excess of scavenger must have been present a t these temperatures. A wall-mediated catalysis of hydrogen atomization was considered to be a more likely source for these scavengers. Other gases which might act as sources of hydrogen atoms were tested for their ability to support the alkyllead decomposition atomization sequence. In these studies, the standard operating conditions for the system were maintained with the exception that different test makeup gases were substituted for hydrogen. The standard mixture of four alkylbutyllead derivatives was used as analytes. The relative detector responses to equimolar quantities of MesBuPb in the presence of ammonia, ammonia-hydrogen (l:l),nitrogen, methane, helium, or air are recorded in Table I. The signal supression for each of the other three components of the standard mixture was very similar to the suppression observed for Me3BuPb. Ammonia and methane were 56% and 30% as effective as hydrogen itself. Because ammonia absorbs strongly a t 217 nm, the detector was operated a t 283 nm for these measurements and comparisons were calculated by using the sensitivity ratio (217vs. 283 nm) observed in the linearity study. Interestingly a 1:l (v/v) mixture of hydrogen and ammonia was not appreciably more effective (59% vs. 56%) at promoting the atomization process than was pure ammonia. Because bond dissociation energies for ammonia, methane, and hydrogen are not appreciably different (460,435,and 432 kJ mol-'), it is considered that side products resulting from the scavenging of lead metal by NH2 or CH, radicals decrease the atomization efficiency. In the absence of a suitable hydrogen source (helium makeup gas), the atomization was reduced to 1.4%. Surprisingly air was even less efficient (0.4%)whereas
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nitrogen was 33% as efficient as hydrogen makeup gas. The peak shape (Table I) for each of the four alkylbutyllead standards was not appreciably changed by any of the different support gases with the exception of nitrogen which resulted in appreciably broadened peaks and a significant decrease in resolution of MezBu2Pbfrom Et3BuPb. That scavengers are present within the hydrogen- or nitrogen-doped atmospheres of the detector was demonstrated by plating lead mirrors inside silica tubes (3.5cm X 6 mm 0.d.) and placing these inserts inside the silica furnace. The lead mirrors were efficiently stripped off the inner surface of these inserts when hydrogen or nitrogen was used as makeup gas. The photometric system was virutally saturated by lead signal at both 217 and 283 nm by using either hydrogen or nitrogen makeup gas at 900 "C. Mirrors which had been air-oxidized were also efficiently volatilized by the hydrogen atmosphere. This suggests that lead oxide would be reduced under these conditions and that the presence of small amounts of oxygen would not cause a loss of analyte response by forming refractory lead oxide. The fate of lead not detected in air or nitrogen detector atmospheres was studied by injecting tetraethyllead (TEL) into the chromatograph when air or nitrogen comprised the makeup gas and then switching to hydrogen makeup gas either 1 or 5 min after the normal retention time for this analyte. For these trials, the chromatograph was operated isothermally a t 100 "C with 55 mL min-l He carrier gas. As soon as hydrogen was admitted into the furnace, a very sharp signal for lead was obtained. When air was the makeup gas, no appreciable signal was detected at the normal retention time for TEL. If the makeup gas was switched to hydrogen 1min later, the resulting signal accounted for 53% of the signal observed for a similar quantity of TEL in a hydrogen atmosphere. When the gases were switched 5 min after the normal retention time of TEL, the recovered lead was unchanged (53%). That the signal was due to lead atomization was confirmed by comparing the signal ratios at 217 and 283 nm and by using blanks in which only solvent was injected. When analogous studies were repeated by using furnace atmospheres of nitrogen, a total of 68% of the lead signal in hydrogen was accounted for by the combination of the signal at the normal retention time for TEL and the signal after switching to hydrogen. The recovery fell to 59% of the total response when the gases were switched after 5 min. Clearly an appreciable portion of the lead which was not atomized when the alkyllead entered the upper furnace was deposited on the walls of the silica tube. These deposits were rapidly revolatilized and atomized as soon as Hz was admitted to the furnace. The resulting extremely sharp signal indicated that this process is very rapid relative to the widths of the chromatographic peaks; thus the detector geometry cannot be contributing appreciably to peak broadening. The width of this signal contrasted the results in nitrogen (Table I) where signal broadening was appreciable, indicating that volatilizationatomization in Nz is appreciably slower. These results also suggest that further increases in sensitivity could be achieved by increasing the chromatographic efficiency of the system. The effects of mixtures of air or N2with H2on the relative response to alkyllead standards were studied in more detail. In these experiments, a constant flow of total makeup gas to the detector was maintained (50 mL min-'); however, the proportion of the Hz in the mixture was reduced in each successive experiment. The results recorded as area response and as area-to-height ratios vs. the percent air or Nz in the mixture are presented in Figure 7. In Figure 7A, the furnace was maintained at 900 "C, whereas in Figure 7B the furnace was maintained at 600 "C; other operating parameters were those defined by the optimization study. For clarity the results
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B
0
25
*
50
15
100
AIR OR N7 IN H 2
-
8
0
25 ' 1IR OR N,50 IN H,
15
100
Figure 7. Variation in area response (-, air; - -, N2) and in area/height ratio (- - -, air; \ \ \, N2)for equal amounts of Me BuPb as a function of the percent of air or nitrogen in the furnace makeup gas. The furnace was maintained at 900 OC (A) or at 600 "C (B).
for Nz, Hz mixtures have been displaced by 5 units on the abscissa in both Figure 7A and B. At 900 "C as the Hzcontent of the mixture was reduced, the signal for the same quantity of alkyllead was also reduced. The signal was suppressed more severely by air than by Nzand the rate of decrease was curvilinear for both gas mixtures. With 25% air in H,, the signal was only reduced by 8%, indicating that diffusion of air into the open ends of the furnace operating under normal conditions would not cause any appreciable signal suppression. Clearly some component of air other than nitrogen severely suppressed the volatilization-atomization sequence. A similar suppression of the selenium signal when air was added to Hz entraining gas in a hydride generator was observed by Dgdina and Rubeska (20). They attributed the decrease in response to a third body interaction which accelerated the reaction of H with 0,. The resulting peroxy radical decayed by combination with H or OH, reforming 0, and Hz or HzO. If air in the entraining gas was replaced by Nzin their system, no signal reduction was observed. Our observations are at odds with several other reports of increased sensitivity to volatile hydrides in the presence of air or oxygen. It was reasoned that relative to other covalent hydride forming elements, the optimum graphite furnace operating temperature for alkylleads is relatively low. If the volatilization-atomization process at 900 "C is quite efficient, an appreciable increase in response would only be evident at lower operating temperatures where the efficiency was reduced. The relative response experiments were repeated at 600 "C. With 25% or 75% of air in Hz,the response remained suppressed relative to the corresponding N,,H2 mixture or to atmospheres of 100% Hz, but at 50% air in H, the signal was increased nearly 3-fold and amounted to 65% of the response observed a t 900 "C under optimum conditions. There appeared to be an instability of the resulting flame evident as a distinct popping sound for the 25% air in H2 at 600 "C but not under any of the other conditions studied at either temperature. This result is consistent with a flame-
induced hydrogen radical formation. It is clear that oxygen plays a complicated and temperature-dependent role in the volatilization-atomization process for lead. It has been suggested (22) that unsaturated oxygen atoms of the silica surface form a complex with molecular hydrogen and that a radical dissociation of the complex results in surface hydroxyls and gaseous hydrogen atoms. At operating temperatures of 900 "C, there is a sufficiently high concentration of hydrogen radicals in the gas phase to efficiently mediate both the revolatilization of metallic lead from the tube surface and the atomization of the lead hydride. At lower temperatures (600 "C), the complex is not sufficiently dissociated to maintain a high concentration of gas-phase H radicals. Because the overall volatilization-atomization process is extremely rapid, it must be concluded that H radicals participate in the atomization of PbH2. Otherwise even a low concentration of H radicals would result in a broadened peak but the total response (area under the peak) should remain unchanged. A reduced response must result from volatilization without atomization. These reactions may be summarized as
+ 2 H F? PbH,(g) P b H 2 + 2H s Pb(atomic) + 2Hz Pb(s)
CONCLUSIONS It has been demonstrated that despite sensitivities in the low picogram range, further increases can be anticipated by improving the chromatographic efficiency of the system. The greatest sensitivities were observed in hydrogen atmospheres although a variety of other gases will also support the decomposition atomization process. Thus it is suggested that hydrogen radicals are beneficial but may not be essential to the detection process. The revolatilization-atomization sequence for metallic lead deposits is very rapid relative to peak widths, forcing the conclusion that hydrogen radicals mediate both the formation and subsequent decomposition of lead hydride.
Anal. Chem. 1985, 57, 1305-1309
ACKNOWLEDGMENT I t is a pleasure to acknowledge many helpful discussions with G. Paquette. Registry No. Me3BuPb, 54964-75-9; MezBuzPb,65151-01-1; Et3BuPb, 64346-32-3;EtzBuzPb,65121-94-0;Me3PhPb, 1904053-0; Et3PhPb, 878-50-2; MezPh2Pb, 42169-20-0; EtzPh2Pb, 4692-79-9: nuartz. 60676-86-0.
LITERATURE CITED Chu, R. C.; Barrons, G. P.; Baumgardner, P. A. W. Anal. Chem. 1972, 4 4 , 1476.
Thompson, K. C.; Thomerson, D. R. Analyst (London) 1974, 99, 595. Godden, R. G.; Thomerson, D. R. Analyst (London) 1980, 105, 1137. Robbins, W.B.; Caruso, J. A. Anal. Chem. 1979, 5 1 , 889A. Fricke, F. L.; Robbins, W. B.; Caruso, J. A. J . Assoc. Off. Anal. Chem. 1978, 6 1 , 1118.
Robbins, W. B.; Caruso, J. A,; Fricke, F. L. Analyst (London) 1979, 104, 35. Van Loon, J. C. Anal. Chem. 1979, 51, 1139A. Fernandez, F. J. At. Absorpf. News/. 1977, 16, 33. Chau, Y. K.; Wong, P. T. S.; Goulden, P. D. Anal. Chem. 1975, 4 7 , 2279.
Van Loon, J. C.; Radziuk, B. Can. J . Spectrosc. 1976, 2 1 , 46.
1305
(11) Chau, Y. K.; Wong, P. T. S. 1976 Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA, Nov 15-19, 1976. (12) Thorburn Burns, D.; Glocking, F.; Harriott, M. Artalyst (London) 1981, 106, 921. (13) Chau, Y. K.; Wong. P. T. S.; Bengert, G. A. Anal. Chem. 1982, 5 4 , 246. (14) Chau, Y. K.; Wong, P. T. S.; Goulden, P. D. Anal. Chlm Acta 1976, 8 5 , 421. ( 1 5 ) Chau, Y. K.; Wong, P. T. S . ; Kramar, 0. Anal. Chim. Acta 1983, 146, 211. (16) Maher, W. A. Anal. Chim. Acta 1981, 126, 157. (17) McDaniel, M.; Shendrikar, A. D.; Reiszner, K. D.; West, P. W. Anal. Chem. 1976, 4 3 , 2240. (18) Siemer, D. D.; Koteel, P. Anal. Chem. 1977, 4 9 , 1096. (19) Paneth, F. A.; Hofeditz, W.; Wunsch, A. J . Chem. Soc 1935, 372. (20) Dgdina, J.; RubeSka, I. Spectrochim. Acta, Part B 1980, 3 5 8 , 119 (21) Welz, B.; Melcher, M. Analyst(London) 1983, 108, 213. (22) Forsyth, D.S.; Marshall, W. D. Anal. Chem. 1983, 5 5 , 2132. (23) Radziuk, B.; Thomassen, Y.; Van Loon J. C ; Chau, Y. K. Anal. Chlm Acta 1979, 105, 255.
RECEIVED for review November 6,1984. Accepted February 11,1985. Financial support was from NSERC, operating grant (WDM), from FCAC, scholarship DSF, and from the Canadian Wildlife Service.
Determination of Phosphate Ion by Gas Chromatography with the Phosphine Generation Technique Shinya Hashimoto, Kitao Fujiwara,* and Keiichiro Fuwa
Department of Chemistry, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
A slmple technlque to generate phosphine from phosphate Ion has been developed for sensltlve determlnatlon of phosphate using sodium borohydride a$ reducing agent. A sample solution containing phosphate was mixed wlth a 6 % sodium borohydrlde solution In a quartz vessel and drled under an incandescent light at 40 O C for 2 h. Phosphine was reproduclbly generated from phosphate by means of heating this vessel at 460 O C . Phosphlne Is selectlvely determined by using a short gas chromatography column and a flame photometrlc detector (FPD). The figures of merlt for the present method are as follows: detection limit, 0.1 ng of P (0.05 ng of P/mL wlth 2-mL samples); dynamlc range, 0.1 ng of P/mL (0.2 ng of P for 2 mL of sample) to 1 mg of P/mL (20 pg of P for 20 pL of sample), relatlve standard deviation In the obtalned slgnal, f3% when measurlng 0.1 mL of 20 ng of P/mL solution. This method Is also appllcable to analyze varlous phosphorus blochemical compounds such as ADP, AMP, IMP, and frans-1,2-dlphenyl-l,2,3,6-fetrahydro-l,2-dlphosphorin 1,2-disulfide.
Phosphorus is one of the important elements to be detected in environmental, biological, and geochemical scientific work, where phosphorus is mainly determined based on the colorimetry of phosphomolybdenum heteropoly blue. Especially, since Murphy and Riley proposed an application of this colorimetric method for determination of soluble phosphate in seawater, it became one of the most popular analytical techniques ( I ) . In spite of much effort of analytical chemists, other analytical methods including atomic absorption spectrometry (2-4), atomic emission spectroscopy (5,6),etc. could not exhibit better figures of merit, e.g., sensitivity and precision, than those of the molybdenum blue method. However,
the phosphorus concentration in some samples (river or lake waters) is too low to be detected even with the molybdenum blue method. In our previous paper, several techniques have been applied for improving the sensitivity of the molybdenum blue method, such as thermal lensing effect (7) and long capillary cell colorimetry (8,9). Although these techniques make it possible to detect parts-per-trillion levels of phosphate ion, some drawbacks still remain in addition to the tedious procedure and complex use of reagents in the process of color development, which is essentially attributed to the molybdenum blue colorimetry: First of all, arsenic and silicate cause interference. Also, since the color development proceeds under 0.3 N sulfuric acid, there is a possibility that the phosphorus detected by this method is not limited to orthophosphate, i.e., release of orthophosphate might occur due to the hydrolysis of phosphate esters if present in the sulfuric acid solution. Also, the detectable dynamic range of absorbance is only 2 or 3 orders of magnitude. Thus, determination of phosphorus by the molybdenum blue method is often troublesome for samples with variations of matrices and a wide range of phosphorus concentration. Matrix effects in spectrochemical analysis are often smaller with gaseous analytes than with liquid as the analytical media. So the technique of reduction of phosphorus in compounds to yield phosphine is a desirable analytical approach. However, the reduction of phosphate ion is difficult because of its high oxidation-reduction potential. The value for a PH03,- couple is -1.12 V (vs. NHE) and that for a H,O-H, couple is -0.828 V (vs. NHE). This means that the reduction of phosphate cannot take place in aqueous solution; it must be done under dry conditions. Romer et al. reduced phosphorus in organic compounds to phosphine by the reaction with hydrogen gas a t 850 "C (10). Nevertheless, phosphate ion (Po43-) was not reduced to phosphine by their method. In our previous work ( l l ) ,phosphine production was at-
0003-2700/85/0357-1305$01.50/0 0 1985 American Chemical Society
