Anal. Chem. 1988,60,578-582
578
Mechanisms of Lead Vaporization from an Oxygenated Graphite Surface Using Mass Spectrometry and Atomic Absorption Dean A. Bass and J. A. Holcombe* Department of Chemistry, University of Texas,Austin, Texas 78712
The shift in the appearance of gaseous Pb to hlgher temperatures with oxygen In the gas phase or on an oxygenated graphlte surface In graphite furnace atomic absorption spectroscopy (GFAAS) was investlgated by use of mass spectrometry and atomlc absorption. The observed pulse shm of Pb using an oxygenated graphite surface In 1 atm of inert gas indicates that the shift observed in an oxygen-contalnlng sheath gas Is not entirely due to the oxygen In the gas phase. The absence of a shlft with vaporlzation from an oxygenated surface under vacuum Indicates It Is not strickiy a surface process. Mass spectral data show more CO and CO, are evolved from an oxygenated surface than from an unoxygenated surface. Separate GFAAS studies show that >30% CO, Introduced Into the Ar sheath gas produces a shlft equlvalent to that seen off of an oxygenated surface. However, the mass spectral data coupled with dlffuslon calculations suggest that a maxlmum of approxlmateiy 0.84% CO, exlsts within the furnace from the CO, desorbed from the graphlte surface. Unless other sources of CO, can be found, these data indicate that this desorbed CO, may not be responsible for the shift. Pure CO used as a sheath gas in GFAAS produces an early shlft In the Pb absorbance pulse and therefore is not responsible for the late shifts that have been observed. The shift may depend on the resulting quantities of CO and CO, In the furnace and the relative oxidatlve character of the furnace environment.
The analysis of P b is often done with graphite furnace atomic absorption spectroscopy (GFAAS) due to its low detection limits. The sample matrix is often biological, which produces complicating background problems. These problems can be reduced by using a 1000 K thermal pretreatment in a 1700, sheath gas. The O2serves two purposes: it facilitates the combustion of the organic matrix; and it “stabilizes“ the P b (i.e., the P b is vaporized a t much higher temperatures, which allow for a higher thermal pretreatment). These effects of 0, on an aqueous Pb(N03)2absorbance profile are shown in Figure 1. When the P b is vaporized from an oxygenated graphite surface in an Ar sheath gas, the absorbance peak is shifted to a later time or higher temperature. There is also a peak height enhancement, although an enhancement in peak area is not noticed. Vaporization of P b in a 1% O2 in Ar sheath gas gave an even larger shift or later appearance time and also resulted in reduced peak height and area. While these effects are well documented (1-4), there is some debate as to the case of the observed shift. Several mechanisms have been proposed. The first is that the 0, chemisorbed to the graphite directly affects the vaporization of Pb. The effect was first reported by Salmon et al. ( 1 ) who noticed a shift in the P b peak in an oxygen-containing sheath gas and also in an inert sheath gas immediately following heating and cooling with O2 present in the sheath gas. They referred to this as “the first shot back effect”. They
suggested this was a surface-related phenomenon where, under normal conditions, PbO may be reduced at active sites on the graphite surface. When a graphite surface is heated and cooled in oxygen, 0, dissociatively chemisorbs to these active sites and the surface is said to be “oxygenated”. They suggested that this coverage of the active sites inhibits the carbon from reducing the PbO until higher temperatures were reached. This explanation appeared to account for all of the shifts shown in Figure 1. L’vov and Ryabchuk ( 5 ) postulated the existence a gasphase/surface equilibrium where
MO(s,l)
+ M(s,l)
+
‘/202
(1)
An increase in gas phase O2 forces the equilibrium in eq 1to the left, causing a delay in the release of the metal which produces the observed shift for the vaporization of P b in 02-containingsheath gas. L‘vov (6) suggested when there was no O2 in the sheath gas that the late shift was due to the desorption of physisorbed oxygen on the atomizer housing. Byrne et al. (7) suggested a gas phase dissociation where the shift depended on the equilibrium between P b and PbO
Holcombe and Droessler ( 4 ) presented time and spatially resolved atomic absorption (AA) data to gain insight into the problem. They were still left with the “first shot back effect” as the most compelling evidence for a surface phenomenon. They did confirm the greatest peak shift when oxygen was on the surface and in sheath gas. They attributed these results to eq 1 or to the maintenance of the oxygenated surface coverage through the following reaction: cs-0
-
cs
+
co.co2
(3)
U +02
where C, represents active surface sites and C,-0 is an oxygenated surface site. They also showed that a release of O2 from physisorbed oxygen on the atomizer housing was not causing this shift. By inserting a furnace with an oxygenated surface into a housing that had been “cleaned” by a hightemperature atomization in an inert gas, they found that vaporization of P b still produced a shift. They also noted that a mechanism involving eq 2 would explain the shift in appearance temperature but not the shift in the falling edge of the absorbance trace. The gas-phase MO/M reaction also does not explain the effect of the oxygenated surface when there is no O2 in the vapor phase (first shot back effect). In a another paper, Sturgeon and Berman ( 3 ) studied various graphite surfaces. While they were not definite in their conclusions, they acknowledged the probable participation of the oxygenated surface in the vaporization process along with the heterogeneous MO/O, process described in eq 1. The uses of vacuum vaporization and MS have proven useful in the study of GFAAS (8-10). The elimination of gas-phase reactions coupled with the ability to detect mo-
0003-2700/88/0360-0578$01.50/0 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6 , MARCH 15, 1988
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0 327
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i
0 ?6-
D
ffl
n a 0.08-
0
500
lGC0 T i m e {ms)
15SC
2oco
Flgure 1. Results of 0.8 ng of Pb vaporized in GFAAS in (a) an inert
Temperature
(K)
containing 1 % oxygen.
Flgure 2. Mass spectral signal of 30 ng of Pb ( m l e 208) deposited as its nitrate salt and vaporized off of an unoxygenated surface under vacuum.
lecular species permits the direct determination of products from surface reactions and gives valuable insights into atomization mechanisms. Styris and Kaye (8) first demonstrated the utility of mass spectrometry (MS) in determining vaporization mechanisms. Sturgeon et al. (9) and more recently Bass and Holcombe (10) have used MS to study P b vaporization. In this paper the effects of chemisorbed oxygen on a graphite surface are studied by use of both mass spectrometry and atomic absorption (in vacuo and a t 1atm). CO and C 0 2 observed during vacuum vaporization are also investigated as potential late shifting agents in GFAAS.
RESULTS AND DISCUSSION In a previous work, Bass and Holcombe (10) have shown that the vacuum vaporization of P b deposited as its nitrate salt yields two peaks as shown in Figure 2. These data were obtained by monitoring m / e 208, the major isotope of Pb. A peak for PbO+ ( m / e 224) coincident with the first peak and the absence of a detectable P b AA signal indicated that the first Pb+ peak was from the fragmentation of PbO in the ion source of the MS. The appearance temperature for this peak also coincided with the decomposition temperature of Pb(NO,),
gas, (b) an inert gas off of an oxygenated surface, and (c) an inert gas
EXPERIMENTAL SECTION Apparatus. The system used in this study employed a vacuum system equipped with atomic absorption (AA) and MS and has been described previously (10). Briefly, the apparatus consists of a two-chamber vacuum system, a quadrupole mass analyzer (Uthe Technology International, Model lOOC), a UTI programmable peak selector (PPS),and an Apple 11+ microcomputer. One chamber is kept at low pressures at all times and contains the quadrupole mass analyzer. The other chamber is separated from the analyzing chamber by a gate valve and is brought up and down in pressure for sample introduction. The Apple 11+ controls the timing, heating, and data collection. A special power supply designed in this lab (11)provides a linear temperature ramp (12) for heating the graphite flat. The Apple 11+ also controls the PPS, which is used for data collection and has capabilities of monitoring multiple masses. Temperature measurements were obtained from a photodiode which was calibrated by sighting an optical pyrometer into a cavity drilled in the side of the graphite flat. Temperatures below the output of the photodiode were calculated by using heat transfer parameters of the atomizer system (12). The graphite flats used on the MS/AA system were machined from graphite (grade FE35; Schunk Carbon Technology) with dimensions of 1.5 X 0.76 X 0.17 cm. The flats were purified and pyrolytically coated (Ringsdorff-Werke GmbH). The Pb(N03)z samples consisted of 6 p L of 5 ppm in the MS/AA system and 2 p L of 100 ppb in the conventional GFAAS system. The experiments conducted at atmospheric pressure were done with a CRA-90 using pyrolytically coated tubes (Varian Techtron). The enclosed workhead was designed in this laboratory and has been described previously (13). The optical system that images the furnace on the entrance slit of the monochromator consists of an over-and-under, symmetrical arm mirror configuration. A 0.35-m Czerny-Turnermonochromator (GCA-McPherson EU700, Acton, MA) with 300-pm slit width was used. Reagents. Pb solutions were made from a loo0 ppm Pb stuck solution which was prepared by dissolving solid Pb(NO& in distilled deionized water. The 1% O2 in Ar was purchased premixed and was research grade. The inert gases, CO and COzwere all industrial grades. The typical analysis of this grade COBgave 0.086% O2impurities. COPand Ar were mixed by using rotameters.
Pb(NOJ2
PbO
+ 2NO2(g) + 7 2 0 2
(4)
NO2+and Oz+signals also were detected at this time. It was shown that the second peak originated from the surface reduction
PbO-C,,) * Pb(g)
+ CO,,CO(g)
(5)
where PbO-C,,) represents PbO absorbed on the graphite surface. The mechanism in eq 5 was substantiated by an observed AA signal and a Pb+ MS signal with no other Pbcontaining molecules detected a t this time. It is important to note that both CO and C 0 2 desorbed coincidentally with and in proportion to the Pb. Equation 4 suggests a loss mechanism for P b in the form of PbO. However, at this temperature (575 K) the PbO is a condensable species, and at atmospheric pressures like that found in GFAAS, multiple surface collisions of the PbO would likely result in its recondensation. Thus, this species could be readsorbed and eventually be reduced according to eq 5. This theory was tested by using the thermal pretreatment (i.e. charring) capabilities of the MS/AA system. Figure 3 shows a plot of the area under the second peak of Figure 2 as a function of the pretreatment temperature. This pretreatment heating cycle was 60 s long and was done in both vacuum and 1 atm of N2 in the prechamber. When the P b sample was heated below the decomposition temperature of the nitrate (i.e., -600 K) in either vacuum or 1 atm of N2, no change in the trace in Figure 2 was observed. However, when the pretreatment temperature at 1atm of Nz exceeded that of the Pb(N03)2decomposition temperature, the area of the first peak decreased while the area of the second peak increased. This supports the postulate that the PbO is recondensing on the graphite surface during the nitrate decomposition and then is being reduced to form Pb(g) around 800 K, as shown by the second peak of Figure 2. As the pretreatment temperature was increased beyond 1000 K, the area of the second peak decreased, reflecting the loss of P b
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
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