Anal. Chem. 1983, 55, 1059-1064
1059
Atomization of Lead in Graphite Furnace Atomic Absorption Spectrometry R.
E. Sturgeorn," D. F. Mitchell, and S. S.
Berman
'
Division of Chemjstty, National Research Council of Canada, Ottawa, Ontario K1A OR6, Canada
A quadrupole Mass spectrometer has been used to investigate, under high vacuum conditions, the gaseous species produced during atomization of lead from an electrothermal atomizer. The amount of lead oxide, detected as a transient molecular Intermediate, Is strongly dependent on the sample ashlng conditions. Reactlon mechanlsms are proposed to account for these observations.
Numerous attempts have been made to delineate the mechanisms of free atom formation in graphite furnace atomic absorption spedrometry (GFAAS). Application of kinetic and thermodynamic considerations to this problem has led to the development of several generalized models and resulted in the classification of the elements studied into groups displaying common atomization characteristics (1-7). However, even a cursory inspection of the literature reveals several notable conflicts in the mechanisms assigned to individual elements by various researchers. Exceptionally diverse atomization mechanisms have been reported for lead, including condensed-phase dissociation of PbO(s) (6),direct evaporation of the reduced metal (2, 4,8, 9), gas-phase dissociation of PbO(g) (7, IO), and dissociation of dimeric lead molecules in the gas phase (4,10, 11). The significant shortcoming of the above mechanistic studies lies with the necessity of identifying precursors to free atoms through implicakion, using kinetic and/or thermodynamic information which can be of limited value. Following an extensive, objective study of both the dynamic and quasi-static methods of obtaining kinetic information from the transient absorbance-time profiles, Frech et al. (12)concluded that it is difficult to extract useful information from energy values obtained !with these methods. Furthermore, conclusions based on thermodynamic arguments lack a sound basis for interpretation siince the primary reaction of importance, that between oxygen and carbon, appears to he kinetically, rather than thermodynamically, controlled (13). Partial pressures of oxygen in the graphite furnace appear to be several orders of magnitude higher than can be explained on the basis of thermodynamic equilibrium (13, 14). Additional analytical techniques are required to provide an unambiguous interpretation of the processes leading to free atom formation. Use of mass spectrometry to detect both atomic and mollecular forms of the vaporizing analyte has recently been shown to be of considerable value in eliminating some of the ambiguities which have arisen in proposed atomization mechanisms (15, 16). The benefits of using GFAAS/MS techniques are significant and unique and therefore offer potential for such mechanistic studies. Both atomic and molecular forms of the vaporized analyte species can be detected with essentially equivalent sensitivity in a rapid sequential fashion. This is a great advantage over techniques relying on molecular absorption spectrometry to detect molecular forms of the analyte during 'NRC No. 21080.
atomization. The poor sensitivity of molecular absorption spectrophotometry necessitates use of large amounts of analyte which, in turn, may alter the reaction conditions and atomization mechanisms within the atomizer (12). Additionally, MS detection allows, in most cases, for unambiguous identification of a species as a result of the characteristically unique isotope ratio exhibited by most elements. This work reports on the use of mass spectrometry to sample the vapor species produced during the atomization of lead from a graphite furnace.
EXPERIMENTAL SECTION Apparatus. The atomizer-mass spectrometer assembly, shown schematically in Figure 1,was mounted in an all-metal, oil diffusion pumped vacuum system. The graphite atomizer was positioned at the end of a two conductor high-vacuum feed through flange (Ceramaseal, 150 A, 5 kV; Lesker Co., Pittsburgh, PA) and consisted of a Varian CRA-63 pyrolytic graphite coated tube and graphite support electrodes. The support electrodes were reduced in length (to =1.2 cm) in order to accommodate the gap between the copper support posts and were inserted into 2 mm deep wells on the inside of these supports. The atomizer tube was then held in place by compressing the support posts together against a tapped machinable glass-ceramic insulator (Macor; Corning Glass, Corning, U.K.). Electrical power to the atomizer was supplied by a commercial Varian CRA-63 power source. As shown in Figure 1, the atomizer was located in the throat of a UTI 10042 (UTHE Technology International, Sunnyvale CA) quadrupole mass analyzer (QMA) with the furnace tube positioned about 2 cm from the electron impact ionization source of the QMA. The quadrupole assembly thus viewed the atomizer tube in an end-on configuration. The temperature-time characteristics of the atomizer surface were obtained by monitoring, through the sample introduction port, the (assumed) black-body emission from the interior wall of the atomizer with calibrated automatic infrared (Thermodot, Model TDGBH, Infrared Industries, Inc., Santa Barbara, CA) and optical (Ircon Inc., Model 1100, Niles IL) pyrometers. Temperatures were recorded under vacuum operating conditions and corrected for transmission losses of the emitted radiation on passage through a Corning 7056 glass observation window. Output from the QMA was displayed on a digital storage oscilloscope and subsequently plotted on a strip-chart recorder for evaluation. Procedure. Sample dosing was carried out at atmospheric pressure without the $MA assembly. A 2.5-pL volume of a 1% (v/v) HNOB solution containing lead, prepared from a serially diluted 1000 pg/mL stock, was injected into the CRA-63 with an adjustable microliter pipet. The sample was air-dried at 380 K and the QMA assembly attached to the vacuum system. Runs were carried out at an average system pressure of 4 x 10P Pa (3 X lo-? torr). An initial heating rate (measured in vacuo) of 1000 K s-l was used for atomization with an upper temperature limit of 2300 K. The effect of various ashing temperatures on the signals was investigated. The QMA was adjusted for a single mass of interest for quantitative work and monitored continuously during atomization. Preliminary scanning over a series of preselected masses was carried out at a scan rate of 20 ms/amu during atomization. This aided in the identification of masses to be individually studied, which were subsequeatly investigated separately as functions of time. Calibration of the QMA at 208 and 224 amu was achieved as follows. A small pellet of high-purity lead was placed into the
0003-2700/83/0355-1059$01.50/0Published 1983 by the American Chemical Society
1060
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983 Vacuum Port Ring
Ionization
Electron Multiplier
/ POWER IN
Furnace Assembly
Filter
Rods
Figure 1. Schematic of GFAAS/MS assembly.
QMA adapter arm. The arm was heated to a temperature sufficient to detect and peak in lead at mass 208, using a high sensitivity setting on the electron multiplier detection system. The QMA was then rapidly cooled and jacketed with dry ice in preparation for a run. It was verified that no detectable lead deposited on the atomizer as a result of such mass calibration procedures. For the measurement of PbO at 224 amu, the DC ramp voltage applied to the filter rods of the QMA was measured at a setting of 208 amu and linearly extrapolated up to the 224 amu position. The effect on the signals of varying the electron energy in the ionization source of the QMA assembly was investigated in order to ascertain the extent of (any) molecular fragmentation within the ion source. Output from the QMA was recorded on a digital storage oscilloscope (Model OS4100, Gould Advance, Essex, England) which was triggered synchronously with the start of the atomization cycle. Data were subsequently transferred to a strip-chart recorder for evaluation. Calibration curves for Pb+ at 208 amu and PbO+ at 224 amu were constructed and the ratio Pb+:PbO+determined. Total time required for sample loading, system evacuation, and sample atomization averaged 1.5 h. Generally, five to six runs could be made per day.
RESULTS AND DISCUSSION Measurements were made at an average system pressure of 4 X Pa. Heating of the CRA-63 produced a pressure pulse within the QMA which reached a maximum of 8 X Pa. This coincided with the highest temperature attained by the atomizer and resulted in maximum outgassing and desorption of material from the tube and support electrodes. The background spectrum which resulted from the heating of the atomizer was very complex, with peaks detected a t almost every mass number up to 230 amu (maximum range studied). No attempt was made to identify any of the species. This background was reduced by over 100-fold in intensity if the support electrodes were “precleaned”by heating them to 3000 K in a Perkin-Elmer HGA-2200 furnace prior to use. Presumably, this distilled out much of the volatile material which would normally be only slowly volatized from the electrodes due to the relatively low temperature attained by them during the atomization cycle. Both P b and PbO species were detected during sample atomization. Possible existence of Pb2(g) could not be ascertained because of the limited mass range of the QMA. Calibration curves based on peak intensity measurements and corrected for “blank” were linear for both Pb+ and PbO+. An “ashing” temperature of 400 K was used for these studies. All sample ashing was conducted in vacuo. The same detector conditions were used for both ions, with the result that their relative intensity scales were directly comparable. The masses of lead used to obtain the Pb+ calibration curve a t 208 amu were not significantly larger than those encountered in many electrothermal atomic absorption determinations of this metal. Although sufficient amplification was available to increase the signals a further 100-fold, the limiting condition imposed on their detection was the existence of a background signal. No background was observed at 208 or 224 amu when a blank or unloaded CRA-63 tube was heated. Introduction of
TIME,
5
Figure 2. Signal-time characteristics of PbO+ and Pb+ (-); perature-time characteristics of atomizer wall (- - -).
tem-
a “blank” 2.5-wL volume of ultrapure water containing 1% ultrapure “OB produced a signal at both 208 and 224 amu. This signal did not have the same temporal characteristics as that of the analyte, but resulted in a raising of the background to produce an intensity a t the analytical peaks equivalent to approximately 0.5 ng of P b at 208 amu and 4 ng of P b (as PbO) at 224 amu. The origin of this effect can only be speculated to be the products of a high-temperature reaction between water and/or acid and the graphite to yield high molecular weight species. Maximum sensitivity was achieved with an ionization source electron energy of 70 eV. Operation of the ionization source at lower voltages will be discussed later. Figure 2 gives the signal-time characteristics for both Pb+ and PbO+ species superimposed on the temperature-time profile of the atomizer. The time constant of the undamped detection system is 0.2 ms and sufficiently fast to accurately follow these transient signals which were obtained on atomization of 3 ng of lead for the Pb+ peak and 10 ng of lead for the PbO+ signal. The sharpness of the PbO+ peak is quite evident as is the fact that it precedes the Pbf signal. This may indicate that PbO(g) may give rise to Pb(g) via a dissociation process following collision with the furnace walls and/or by electron impact fragmentation. In the absence of measured appearance potentials and ionization efficiency curves for both species, one can only speculate as to the actual precursors of these species. PbO(g) may itself be the fragmentation product of a gaseous higher polymer of this oxide (Le., (PbO),) and Pb(g) may be produced from several reactions. These possibilities will be considered in subsequent discussion. Unfortunately, ionization efficiency curves are impossible to obtain when electrothermal atomizers are used as vapor sources for these MS studies with the result that various vapor species can be detected by this technique but the sequence of events leading to their formation can only be inferred from additional experiments, thermochemical considerations and the time-(temperature-)dependentbehavior of the signals. Detection of PbO(g) during atomization is consistent with observations and conclusions drawn by several other researchers (10, 17, 18). Sedykh et al. (10) reported detection (by molecular absorption) of both PbO(g) and Pb2(g)species during atomization of Pb(NO& in a graphite furnace, both species preceding the appearance of Pb(g). It was noted that PbO(g) was produced a t temperatures as low as 940 K. It is not clear whether the lead sample was ashed in this study (10). The large sample masses (3 pg) required for detection of PbO(g) and Pb2(g) in these experiments show that this approach is less satisfactory than MS detection. Frech and Cedergren (17) concluded, from a series of high-temperature equilibrium calculations, that a significant amount of PbO(g) may form during atomization of Pb(N0J2 if the sample is ashed at temperatures less than 890 K, as was done in the present case.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
Table I. Pulse Characterization Temperatures
7
appearance peak end
temperature, K vacuum 1 atm Ar Pb' PbO' Pb 1120 680 700 1140 a40 1430 1900 1000 1920
Rowston anld Ottacvay (18)measured the appearance temperatures for lead in a n HGA-74 furnace for samples deposited from aqueous solution and following vacuum deposition of a metal film. The appearance temperature obtained for the metal film was 168 K lower than that for an aqueous solution, suggesting that the metal does not exist in the elemental state prior to atomization when aqueous solutions are injected. The authors conclude that the atomizatiori mechanism involves a thermal or chemicall decomposition of a metal salt or oxide a t the appearance temperature and not before it (18). No sample ashing was used in this (18) study. The pulse clharacterization temperatures for the Pb+ and PbO+ signals are given in Table I. For comparison, the signal-temperature characteristics for lead atomized at atmospheric pressure, as determined by atomic absorption measurements, are also given. These latter data were obtained by using the commercial CRA-63 workhead, an Ar sheath gas, and a heating rate of 900 K s-l during atomization. The appearance and peak temperatures for lead atomized under vacuum conditions are significantly lower than those recorded at atmospheric pressure due to the greater rate of vaporization a t reduced pressures (19, 20). This same observation can be drawn from the data of Styris and Kaye (16) who obtained an appearance temperature of 1500 K for V under high vacuum conditions as compared to an average 2280 K reported at atmospheric pressure ( 2 1 ) . Measured intensities of Pb+ and PbO+ can be converted to partial pressures with the use of the following equation (22):
where K is an instrument constant, I" is the measured ion beam intensity, T is the temperature of the sample source, u is the cross Election for ionization by electron impact, y is the efficiency of the electron multiplier detection system, and AE is the difference iin energy betwee:n that of the ionizing electrons in the ionization source and the appearance potential of the species. Equation 1 can be simplified and used to estimate the ratio of partial pressures of Pb+:PbO+as follows: II
= 0.8-
rPb
I'PbO
It is assumed that the instrument constant is independent of mass (22) and that the source temperature and the factor AE are approximately the same for both species. The latter assumption is justifiable considering that the ionization source voltage is 70 eV and tjhe appearance potentials are 7.3 eV for Pb+ and 9.0 eW for PbO+ (23). Multiplier efficiencies were taken from a calibration curve provided by the manufacturer. Relative ionization cross sections were estimated from the data of Flaim and Ownby (24). The transmission efficiency of the QMA at 208 aind 224 amu was also accounted for. In the absence of sample ashing, the ratio of Pb+/PbO+ was found to be 2.5, based on peak-height measurements, and 9.6, based on relative signal areas. The latter comparison of integrated intensities in more meaningful as it cannot be in-
1061
Table 11. Pb+:PbQ+Ratio sample treatment
at 70 e V
at 28 eV
no ash
9.6 2.0 29 (6)' a Extrapolated from the relative changes noted on ashing when a 70 eV electron energy was used. 650 K ash
fluenced by the kinetics of the atomization process. With the above experimental conditions, about 10% of the lead sample appears to volatilize as PbO. Sample Ashing. Ashing of the sample above 680 K under vacuum resulted in complete vaporization of the lead and no analytical signal. The maximum feasible ashing temperature was 650 K (ramp from 380 K to 650 K over 12 s, no hold). At this temperature about 17% of the lead was lost prior to atomization. The ratio Pb+/PbO+, computed from eq 2 using integrated intensities, was found to be 29. Only 3% of the lead sample appears to volatilize as PbO under these conditions. Ionization Source Energy. Serious errors can be introduced into mass spectrometric results if potential fragmentation reactions are not accounted for (25). In the present case, the ionization source for the $MA was normally operated at 70 eV. As this is well above the appearance potential of PbO+ (9.0 eV, (23)),significant fragmentation of PbO molecules may occur to produce Pb. In fact, the observed PbO+ intensity may contain a contribution from the fragmentation of higher polymers of (PbO), in the gas phase (23). A qualitative investigation of fragmentation may be carried out by examining the Pb+/PbO+ intensities at lower electron source energies. In this way the extent of influence of the 70-eV source energy on the results can be evaluated. It must be stressed that only through the measurement of appearance potentials and ionization efficiency curves for each species can a clear distinction be made between primary and fragment ions. The lowest practical operating voltage of the electron impact source was found to be 28 eV. At this potential, the sensitivity of the system dropped by 20-fold for detection of PbO+ and 30-fold for Pb+, necessitating use of 50-ng samples for subsequent intensity measurements. With this electron energy, and in the absence of sample ashing, the ratio of integrated intensities Pb+/PbO+ was 2.0. (In this case the term AE (eq 1)wasi taken into account in using eq 2 since the relative importance of this term increases as electron energy decreases from 70 eV to 28 eV.) This represents a substantial increase in the relative amount of PbO(g) measured. The 70-eV electron energy in the ion source causes significant dissociation of the PbO molecule. Additionally, it was noted that the PbO' signal was significantly broader in time than that obtained with an ionization source energy of 70 eV. It is conceivable that, at still lower electron energies, the relative amount of PbO may increase further. No attempt was made to determine the Pb+/PbO+ ratio by using a 28-eV source energy following sample ashing at 650 K because of the substantial decrease expected in the PbO+ signal. As the sensitivity of the system was already reduced by 20-fold with an electron impact energy of 28 eV, a significant increase in sample size would have been required to detect PbO+ following an ash cycle. Results for this case have therefore been extrapolated from the relative signal change observed on ashing when a 70-eV electron source was used (Le., a decrease in the Pb+/PbO+ ratio of 4.8-fold). A Pb+/PbO+ ratio of 6 was obtained in this manner. Table I1 summarizes the results for the determination of Pb+/PbO+ under the various experimental conditions cited. Atomization of a 150-ng sample resulted in the appearance of double peaks in the Pb+ signal, as shown in Figure 3. This
1062
'
o
o
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
> 6
Figure 3. ng of Pb,
Signal for Pb+ at 208 amu recorded for atomlzation of 150 no sample ashlng.
signal appears to be the result of a convolution of those for PbO+ and Pb+ shown in Figure 2. The earlier Pb+ peak in Figure 3 corresponds to the same point in time as the maximum for PbO+, while the latter Pb+ peak occurs at the same time as the Pb+ maximum shown in Figure 2. It is likely that the earlier Pb+ peak arises due to fragmentation of PbO+ by electron impact. A similar double peak was just detectable as a shoulder on the Pb+ signal during atomization of 50-ng samples. If this is truly the origin of this early time Pb+ peak, it may be possible to select ionization source conditions that result in complete dissociation of PbO(g) and use the resulting Pb+ peak ratios (early-to-late peak intensities) to obtain an estimate of the Pb+/PbO+ ratio rather than using the approximate 28-eV source method outlined earlier. Mass spectrometric studies (23, 26) show that, in the absence of reducing agents, PbO(s) sublimes as an oxide, the equilibrium composition of the vapor phase consisting predominantly of PbO and higher polymers of this molecule (n = 4). Free Pb(g) is also reported to be formed, to a small extent, by direct thermal reduction. It is presumptuous to assume that results and conclusions drawn from mass spectrometric investigations of vaporization processes and equilibrium vapor compositions of bulk materials made with Knudsen cells can be applied to the case of free vaporization of samples from an electrothermal atomizer surface (whether operated at 1 atm or in vacuo). In the latter case, kinetic rather than thermodynamic considerations may determine the vapor composition (cf. ref 27). As well, the highly reducing properties of the graphite substrate will influence reactions at elevated temperatures. Injection into the atomizer of a solution of lead in nitric acid will result in the formation of lead oxide following the dry and ashing cycles. At atmospheric pressure, formation of lead atoms in the gas phase may occur as a result of both evaporation of lead formed during reduction by graphite or thermal decomposition of PbO(s) and also as a result of evaporation and subsequent dissociation of PbO(g). In the present system, operated under vacuum, it is obvious from detection of PbO(g) (Figure 2) that, a t the appearance temperature of Pb(g), elemental lead is not present on the atomizer surface. Thus, thermal dissociation of the oxide according to PbO(s)
+
Pb(1)
+ '/202(g)
(3)
has not taken place. This is consistent with the fact that the equilibrium partial pressure of oxygen would have to be less than atm Pa) for this reaction to proceed spontaneously at 680 K (the appearance temperature for Pb, Table I). This is about 10lO-foldlower than the partial pressure of O2 encountered in a high vacuum atmosphere. Furthermore, it exceeds by 1016-foldthe pressure of O2 in a graphite furnace operating a t atmospheric pressure (13), assuming an ap-
pearance temperature of 1120 K (Table I). Thus, free evaporation of reduced P b is unlikely to account for early formation of Pb(g) and, at low temperatures, PbO(g) will appear in the gas phase as a result of sublimation. Subsequent dissociation of PbO(g) by electron impact fragmentation may account for the early appearance of Pb(g) in the same temperature region as the maximum in the PbO(g) signal (cf. Figure 2). Gas phase dissociation of PbO(g) in this vacuum system is unlikely due to the low collision frequency of PbO(g) with other gas-phase molecules. Impact of PbO(g) with the atomizer wall may result in dissociation of the molecule, but even in this event, the Pb(1) formed must then desorb from the atomizer wall. Vapor pressure considerathns indicate that if the temperature is high enough to detect PbO(g) via sublimation of PbO(s), then Pb(g) should also be detected following its sublimation from the atomizer wall. Such vapor pressure calculations are derived from bulk physical properties, however, and may bear no relationship to behavior of adsorbed species or monolayers for which interaction of the analyte with the graphite substrate may be a primary consideration. At higher temperatures, such as at the peak of the Pb+ signal (1140 K), the equilibrium partial pressure of O2 would have to be ;=W9atm Pa) for reaction 3 to proceed spontaneously. Such requirements are met under the conditions used here (total system pressure Pa) and, as expected, no PbO+(g)is detected at this temperature (cf. Table I and Figure 2), the sample having been transformed to Pb(1). Consequently, existence of PbO(g) is quite brief and results in the sharp, transient signal shown in Figure 2. At intermediate temperatures, it is evident from the above that some thermal dissociation of PbO(s) may occur and give rise to Pb(g) via evaporation of the reduced metal. It may be noted that conclusions can be drawn from thermodynamic calculations such as these provided equilibrium exists and no large activation energies are involved in the reactions (Le., reaction kinetics are rapid). There can be no doubt that the low system pressure used here precludes establishment of gas-phase equilibrium. Nevertheless, thermodynamics should provide a useful basis with which to rule out potential reactions regardless of kinetic considerations. As has been postulated by Byrne (6) and, more recently, L'vov and Ryabchuk (13),initial appearance of Pb(g) may be the result of the thermal dissociation reaction
for which it is assumed that vaporization of the free metal from the atomizer surface is rapid compared to the rate of reduction. Byrne (6) drew a correlation between the measured appearance temperature for (among other elements) P b and the theoretical temperature at which the free energy for reaction 4 satisfied the relationship: AGo/T = R In Kp, where Kp is the calculated equilibrium constant. L'vov and Ryabchuk (13) successfully correlated the appearance temperature for Pb(g) (1000 K) with the temperature at which the absolute rate constant for thermal dissociation of PbO(s) becomes sufficient for detection of this element by GFAAS (i.e., a first-order rate constant k1 ;= s-l (13)). It is proposed (13) that graphite indirectly controls the dissociation reaction by regulating the 02(g)partial pressure within the atomizer. Direct carbon reduction of PbO(s) is not considered nor is the possible existence of PbO(g). The standard free energy change for reaction 4 at 680 K is highly unfavorable (64 kcal/mol (28)). From the relationship AG = AGO
+ RT In Ppbp0,ll2
(5)
where it is assumed that 02(g)exists and does not remain a surface adsorbed species a t this temperature, the reaction remains unfavorable (AG = 19 kcal/mol) even with the con-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
dition that the partial pressures of O2and Pb(g) are less than the system pressure (i.e.,
