Anal. Chem. 1987, 59, 2897-2903 (14) Anderson, J. E.; Andres, R. P.; Fenn, J. E. A&. Chem. fhys. 1088, 10, 275. (15) Dun, H.; Mattes, E. L.; Stevenson, D. A. Chem. Phys. 1079, 36, 161. (16) Styrls, D. L. Anal. Chem. 1084, 56, 1070. (17) Vidal-Madjar, C.;. Gonnord, M.-F.; Gulachon, G. A&. Chromotogr. (N.Y.) 1975, 13. (18) Bennet, A. J.; McCarroll, E.; Messmer, R. P. fhys. Rev. B : SolM State 1971, 3, 1397. (19) Katorskl, A.; White, D. J. Chem. fhys. 1984, 4 0 , 3183. (20) Freeman, M. P.; Hagyard, M. J. J. Chem. fhys. 1088, 4 9 , 4020. (21) Gant, P. L.; Yang, K.; Goldstein, M. S.; Freeman, M. I.; Weiss, J. J . fhys. Chem. 1970, 74, 1985. (22) Kailo, D. I n Contact Catalysis, Szabo, 2 . G., Kallo, D., Eds.; Elsevier: New York. 1976; p 336. (23) Farber, M.; Srhrastava. R. D.; Uy, 0. M. J. Chem. SOC. Faraday Trans. 11972, 68, 249. (24) Nakahara, T.; Musha, S. Appl. Spectrosc. 1975, 29, 352. (25) Levln, E. M.; Robblns, C. R.; McMurdle, H. F. Phase Diagrams for Ceramlslsts, 2nd ed.; The Amerlcan Ceramic Society: Columbus, OH, 1969. (26) Altman, R. L. J. fhys. Chem. 1983, 67, 366. (27) Aiper, A. M.; McNally, R. N.; Ribbe, p. G.; Doman, R. c. J. Am. ceram. SOC.1962, 4 5 , 264.
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(28) Chase, M. W.; Curnutt, J. L.; McDonald, R. A,; Syverud, A. N. J. fhys. Chem. Ref. Data 1978, 7, 793. (29) Chupka, W. A.; Berkowitz. J.; Glese, C. F.; Ingram, M. G. J. fhys. Chem. 1958, 62, 611. (30) Sterns, C. A.; Kohl, F. J. J. fhys. Chem. 1073, 77, 136. (31) Kantor, T.; Bezur, L.; Pungor, E.; Winefordner, J. D. Spectrochim. Acta, Part B 1983, 388, 581. (32) Sturgeon, R. E.; Berman, S. S. Anal. Chem. 1983, 55, 190. (33) Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. Hlgh Temp. Sci. 1984, 18, 97. (34) Curtiss, L. A.; Frurip, D. J. Chem. fhys. Left. 1980, 75, 69.
RECEIVED for review December 29, 1986. Accepted August 27,1987. Faculty appointment support for D. A. Redfield from the Northwest College and University Association for Science is gratefully acknowledged. This paper is based on work sponsored by the division of Basic Energy Sciences of the Department of Energy and performed under DOE Contract NO. DE-AC06-76RLO 1830.
Mechanisms Controlling Graphite Furnace Atomization and Stabilization of Beryllium D . L. Styris* Pacific Northwest Laboratory, Richland, Washington 99352
D. A. Redfield Northwest Nazarene College, Nampa, Idaho 83651
The real-time mass spectra of gas-phase specles, produced In a graphlte furnace contalnlng beryllium analyte and a magneslum modlfler, are obtained and used to eiucldate rnechanlsms that control vaporlratlon, atomlzatlon, and stabilization. It Is determlned that free berylllum Is produced from the thermal decomposJtlon of adsorbed monomeric oxide. Beryillum Is lost as polymeric oxides, carbldes, cyanlde, and cyanamide during the atomization phase. Stablilratlon occurs by dehydratlon of the dlhydroxlde, a mechanlsm that is identical wlth that responslbk for stablllratlon of aluminum. Comparlsons are made wlth mechanlsms associated wlth atomlzatlon of aluminum.
Atomization of beryllium in graphite furnaces has, for unexplained reasons, received relatively little investigative attention concerning the associated mechanisms. Massen et al. suggest that the oxide is reduced and that the beryllium is vaporized from the solid metal (I). These arguments are based on comparisons of appearance temperatures with the melting point of the monoxide. But, as Massen and Posma demonstrate, such arguments are highly speculative (2). The same investigators (2)also show that the presence of aluminum in the sample increases the appearance temperature for beryllium from about 1500 to 1700 K. Most recently, hightemperature equilibrium calculations by Frech et al. indicate that beryllium should remain nonvolatile up to 1400 K and that relatively large'volumes of water can lead to gaseous dihydroxide formation at temperatures as low as 1200 K (3). The X-ray diffraction experiments by Runnels et al. indicate formation of solid beryllium oxide, which is attributed to
diberylliumcarbide reacting with water during transfer of the furnace to the X-ray diffraction system ( 4 ) . It is proposed by these latter authors that the atomization competes with carbide formation; absorption signals decrease as conditions for carbide formation become more favorable. Existing experimentaldata on any of the above mechanisms is clearly insufficient to justify a conclusion as to which are the responsible mechanisms. Knowledge that is needed in order to determine these mechanisms must include information on real-time formation of intermediate species that are produced in the furnaces; this is, of course, in addition to information on gas phases of the analyte species. It is also useful to make comparisons with more substantiated mechanisms that control atomization of analytes that have similar characteristics. For example, it might be expected that there are similarities between mechanisms that control stabilization of aluminum and beryllium; both are stabilized by magnesium nitrate ( 5 , 6 )and by calcium oxide (7-9). Mechanisms that have been proposed recently for atomization of aluminum (10) may therefore be applicable to atomization of beryllium. This paper presents results from real-time mass spectrometric analysis of gaseous, molecular, and atomic species that are produced in graphite furnaces containing beryllium nitrate. The real-time appearances of these species are used in order to formulate mechanisms that control stabilization and atomization of the beryllium. These mechanisms are discussed and then compared with those for aluminum (10) and with the existing thermal equilibrium calculations for beryllium (3). EXPERIMENTAL SECTION Apparatus. The vacuum system with molecular-beam sam-
pling and combined mass and atomic absorption spectrometric
0003-2700/87/0359-2897$01.50/00 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
Table I. Beryllium Experiment Summary and Implied Reactions Product Modifier Conc. Appear. Temp. ( K) Species m/e Be (ng)/Mg(ng)
Implied Reaction Reaction; (No.)
a. Atomization Heating Phase Be
9
5/0; 51250 20/0
BeCN
35
(CN)z
52
BeNCN
49
2010; 10/500
Be0
25
20/0
2465
’
(Be012
50
20/0 10/500
2590 2180
’
(Be014
100
2010 10/500
2475 2060
,
10/500
Be2C
30
2010 51250
2480 21 90
BeC2
33
Be&
42
2010 10/500 20/0
2355 1990 241 0
Be2
66
20/0 10/500
2465 2050
C4
Thermal Dissoc. of (BeO)niadi
’
%
,
b . Thermal Pretreatment Heating Phase
Be(OD)z
45
MgOD MgODz
43 46
2010 20/7500 0/5000 o/ 10,000
1218
BeOiadi + DzO = Be(0D)z
366 366
capabilities is described in detail in ref 10 and 11. This system provides the means for sampling all of the gas species within a furnace heated in a 1-atm-pressure environment. These experiments use N, (99.997%) for this environment and standard Thermo Jarrel-Ash Corp. (Waltham, MA) pyrolytic coated graphite tube furnaces. The furnace power is provided by a Perkin-Elmer (Uberlingen, FRG) HGA 4000 programmer. A Thermo Jarrel-Ash Video 11 spectrophotometer is used to monitor absorption of the light from a beryllium hollow cathode lamp. Procedure. All beryllium samples are prepared by dissolving Be(NO& (K&K Laboratories, Plainsville, NY)in deionized water (500 mg L-’ Be). Dilutions of 50, 20, 10, and 0.1 mg L-’ are prepared for unmodified samples by appropriate additions of deionized water. Modified samples are prepared by diluting the stock solution with a solution of Mg(N0J2.6H20 (Allied Corp., Morristown, NJ) of appropriate concentration. The samples that are used to investigate the atomization heating phase contain a beryllium-to-magnesium weight ratio of 0.02. This ratio is decreased to 0.002 when the thermal pretreatment heating phase is investigated. Weight ratios that are used for each experiment are indicated in column 3 of Table I. In the former case 2 p L , containing 10 ng and somtimes 20 ng of Be, are pipetted, with a Pyrex capillary, into the furnace. In the latter case 20 ng of
Be is dispensed as a 40-wL D20solution; D20 (Sigma Chemical Co., St. Louis, MO) is used in order to help solve some of the interference problems associated with identification of magnesium hydroxide. This increased quantity of water (D20)is necessary in order to obtain measurable quantities of hydroxides in the thermal pretreatment phase. The furnace pretreatment temperature steps are programmed for 40-s ramp and 20-s hold at 350,370,500, and 1170 K and 5-9 ramp and 10-s hold at 1370K. The atomization step is a 1-s ramp to 2700 K with an 8-s hold. The experiments involving investigations of the thermal pretreatment stage use a modified temperature program. This is done in order to increase vaporization rates at lower temperatures and hence increase gas-phase concentrations to detectable levels. This modification simply replaces the steps that are programmed beyond 370 K with a single step: 1-s ramp to 1670 K and a 9-s hold. Furnace temperature, during the atomization step, is monitored with a Ircon 1100 series optical pyrometer (Ircon, Inc., Niles, IL). The pyrometer output that is developed from the outer, central portion of the furnace surface is calibrated against inner-surface temperatures, which are established by a 0.08-mm-diameter W-5% Re:W-26% Re thermocouple (Omega Engineering, Stamford, CT). Temperature determinations during thermal pretreatment rely solely on outputs
ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
2899
-
---
Without Modifier With Modifier
I
1
Without Modifier With Modifier
I A ' I
I
I
I
1
J
1
I
D Furnace Temperature, K
Figure 1. Composite of temperature profiles of atomic absorption spectroscopy (labeled Be) and mass spectrometry (labeled with superscripts)spectra of free beryllium and beryllium oxides, with (dash line) and without (solid line) magnesium modifier. The ionizer electron energy is 30 eV. The mle values are shown in Table I. The maximum temperature achieved is 2800 K. The Be+ slgnal shown is representattve of that observed wlth and without modifier; the small peaks in this slgnal are attributed to noise.
"+"
from similar thermocouples,but separate, identically programmed heating cycles are used. The Extrel quadrupole mass analyzer (Extrel Corp., Pittsburgh, PA) is calibrated with argon, carbon dioxide, water, nitrogen, and trichlorotrifluorethane. The electron energy used for ionization is 30 eV. The data recording technique is described in ref 12; a brief description follows. The temporal history of a single mass of interest is obtained from the single-sweep oscilloscope trace of the mass analyzer output; the mass analyzer is operated with the manual mode set to monitor this mass. Single-sweep oscilloscope traces of pyrometer and of atomic absorption spectrometer outputs are produced simultaneously with the mass analyzer signal. The scope sweep is triggered from the "read" output of the furnace power supply and recorded by using a Tektronix C5C oscilloscope camera (Tektronix, Inc., Beaverton, OR).
RESULTS AND DISCUSSION Temperature profiles of spectra obtained during the atomization portion of the heating program are shown in Figures 1 (oxides) and 2 (carbides, cyanide, and cyanamide). The solid line represents results from the aqueous solution (without modifier); ordinate units are identical for each species. The m l e values used in these experiments and the appearance temperatures are shown in columns 2 and 4 of Table I. Spectra from the modified sample are represented by dashed lines. The Be0 and the BezCzare not shown for the latter because respective isobaric interferences from Mg+ and MgO+ prohibit positive identification. The appearance temperatures of those molecular species that are identifiable in the presence of the modifier are 400-600 K less than those associated with the unmodified samples; i.e., the modifier induces a negative temperature shift in the appearance of molecular species. Negative shifts have never been observed in previous mass spectrometric investigations of modifier effects (10, 1 1 ) . A small (-50 K), positive shift is consistently observed for the appearance of free beryllium in the presence of the modifier. The following discussions treat separately the groups of species that are contained in the spectra of Figures 1and 2; each group is discussed under its respective subtitle. Potential mechanisms are presented, and when appropriate data are
DO Furnace Temperature, K
Figure 2. Composite of temperature profiles of atomic absorption spectroscopy (labeled Be) and mass spectrometry (labeled with "+" superscripts) spectra of free beryllium and beryllium carbbs, cyanide, and cyanamide. The modified (magnesium modifier) cases are represented by dash lines. The ionizer electron energies are 30 eV. The m / e values are shown in Table I. The maximum temperature achieved is 2800 K. available, thermodynamic and kinetic evaluations are made concerning these mechanisms; thermodynamic equilibrium is assumed in all cases. Data on adsorbed phases of these species are, of course, not available. But arguments based on knowledge of general adsorption processes are presented. In some cases the statistical mechanics approach to adsorption is used to provide insight concerning reaction feasibility. Oxides and Atomization. The oxide polymers that appear here have been observed by Chupka et al. (13)and by Theard and Hildenbrand (14) in mass spectrometric investigations of equilibrium vapor above solid beryllium monoxide. These authors show the vapor to be primarily composed of Be, 0, (BeO)3,and (BeO)4with smaller quantities of Be0 and other polymeric oxides. Existence of small quantities of Be20, Be30z, and Be604 is reported in ref 13. The temperature profile for (BeO)3is not shown in Figure 1of the present work because unidentified background interference at 75 amu prevents positive identification; monitoring of polymeric oxides that are larger than fourth order is not attempted. The polymers observed here are not artifacts that are due to condensation in the sampling cone. Such condensation does not exist because flow conditions attained in the entrance of this cone, a t operating pressure, are such that few intermolecular collisions occur, and these are predominantly with nitrogen from the operating environment. Similarities between vapor species shown in Figure 1 and the polymers in the equilibrium vapor phase, reported in ref 13, suggest that vaporization in the graphite furnace is primarily a sublimation process. If this is indeed the mechanism involved here, then it must also be responsible for (i) the appearance of Be(g) preceding the appearance of the oxides and (ii) the observed relative intensities of the polymers. From the work in ref 13 and discussions by Spear (15) it is apparent that beryllium monoxide dissociates primarily by the congruent vaporization reaction
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987 1
\
as (BeO),(ad) will exhibit desorption behavior that results in negative changes in appearance temperatures with increased quantity of adsorbate; these changes are discussed above. In this case, the Be(g) can appear as a precursor to the gaseous polymer if there is sufficient thermal dissociation of (BeO),(ad) at a temperature that is less than that which corresponds to onset of desorption of the polymer. Further evidence of desorption is contained in data from additional experiments involving surface coverage. That is, negative appearance temperature shifts are also observed when the volume of the sample aqueous solution is changed from 1 to 40 pL; no modifier is used, and the mass of Be is 20 ng. These shifts are the same magnitude as those observed when the modifier is present. Sample spreading is probably not responsible since it would tend toward monolayer-like coverage thus promoting additional active site adsorption. The large water concentration evidently changes the surface topology (18),resulting in adsorption of analyte molecules on weaker sites. The above desorption mechanism is reasonable, based on dissociation energies of free molecules. It should be kept in mind, however, that the magnitudes of dissociation energies of the free molecules are greater, by unknown amounts, than those of respective adsorbate molecules (19);these differences are discussed later. Reference 13 indicates that the free monomer has a lower dissociation energy (106 kcal mol-') than the other gaseous beryllium oxide polymers. It is expected, then, that the monomer will be responsible for free beryllium appearing at the lower temperatures, Le., at temperatures less than those where desorption of (BeO),(ad) occurs. Hence, energies required for polymer desorption must be greater than the 106 kcal mol-' required for dissociation of BeO(g). The importance of the monomer to the kinetics of Be(g) formation can be quantified by noting that thermal dissociation of the free polymer involves the unimolecular reaction
z 'z 1500
2000
2500
3000
Appearance Temperature of (Be0)l. K Figure 3. Sample mass dependence of appearance temperature of beryllium oxide tetramer. Such dependence may be indicative of desorption.
It is also shown by Spear (15) that monomeric and polymeric species are produced by the reaction nBeO(s) = (BeO),(g)
(2)
where 1 In I 4. From the results presented in ref 13 and 15 the partial pressure of (BeO),(g), under equilibrium conditions, should be 21/2orders of magnitude smaller than that of Be(g) at 2000 K. Data from JANAF Thermochemical Tables (16) indicate that this ratio of partial pressures P(BeO),/P(Be) increases monotonically with temperature; e.g., the value of log P(BeO),/P(Be) is -2.3 at 2500 K, the approximate appearance temperature for (BeO),(g), and -1.8 at 4000 K. It is assumed in these calculations that P(Be) and P(0)are equal (13, 15). These pressure ratios indicate that the 2000 K appearance temperature of Be(g) may be too low for existence of a detectable quantity of (BeO), and that this quantity can increase at higher temperatures. Detection of this polymer at a higher temperature is therefore likely and is certainly achieved in this work. It is observed, however, that the ratio P(BeO),/P(Be) is significantly greater in the present work than indicated by the above calculations for reactions 1 and 2. That is to say, at 2700 K, log P(BeO),/P(Be) attains a value of -0.32 as determined from amplitudes in Figure 1 and the (BeO), ionization cross-section (relative to that for Be) of 2.8 given in ref 13. It is concluded that, under equilibrium conditions, reactions 1 and 2 are not responsible for the results of the present study. This conclusion is supported by additional experiments based on the premise that the sublimation process should be relatively independent of quantity of analyte used. When the mass of beryllium contained in a 1-KLsample is varied (10, 20, or 50 ng), the (BeO), appearance temperature is observed to decrease with increasing mass while appearance temperature of Be(g) remains constant. This dependence on mass is shown in Figure 3, where a 5-fold increase in mass results in a 500 K decrease in appearance temperature. This negative temperature shift for (BeO), is not merely a result of larger quantities of vapor products being available; the appearance of Be(g) would then be shifted, and it is not. Hence, the ratio P(BeO),/P(Be), at a given temperature, varies with increased sample quantity; the ratio should remain constant under conditions imposed by reactions 1 and 2. This implies again that, at equilibrium, mechanisms other than congruent vaporization are responsible for the vaporization behavior and Be(g) is not produced as a result of reaction 1. Desorption may be the mechanism involved here. Certainly, the mass dependence exhibited by the (BeO)4appearance temperature (Figure 3) could be indicative of desorption. Increased surface coverage in absorbates commonly decreases the heat of adsorption for a given adsorbate and hence increases the rate of desorption at a given temperature ( 1 7 ) . It might be expected, therefore, that adsorbed polymers such
(BeO),(g)
+
(BeO),-i(g)
+ B e ( d + O(g)
(3)
The dissociation energies (AE,)associated with reaction 3 and determined from the data of ref 13 are 106,205,254, and 269 kcal mol-' for n having values of 1, 2, 3, and 4, respectively. If reaction rates for reaction 3 are assumed to be expressed by Arrhenius equations, the rate constants k , involving reaction 3 can be related to the n = 1 rate constant k , by the expression kn/k1 = An/A1 e x p ( G - m n ) / R T
(4)
where Al are the frequency factors, R is the gas constant, and AE, is used in place of activation energy. It is noted that the magnitudes of actual activation energies in eq 4 would be less than those of the dissociation energies used here. This is due to existence of negative activation energies that are related to the association rate constants for reaction 3 (20). These negative energies are small, however, and their influences are negated in eq 4 by the subtraction process occurring in the exponential. The frequency factors for unimolecular processes are shown, by transition state theory, to be inversely proportional to the vibration partition function (21). The order of magnitude of this function is loo-lo', as given in ref 21, so the order of magnitude of the frequency factor ratio in eq 4 is not greater than 10. The upper bound for eq 4 is therefore approximated by the inequality
k , / k , < 10 exp(AE, - AEn)/RT
(5)
For a temperature of 2000 K and for the dissociation energies established in the above discussion, eq 5 yields an upper bound value of the order of lo-''; this value increases to at 2700 K. Indeed, the monomer oxide must be the principal source for free beryllium. Recall, though, that these results apply
ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15,
to the gas-phase unimolecular reaction 3. A discussion of the application of these results to adsorbed species is given below. A reaction similar to reaction 3 but involving adsorbed, instead of gas-phase oxides, results in the same inequality (eq 5), except that in the adsorption case the dissociation energies AE, are assumed to be smaller than the free-state values by the magnitude of the heat of adsorption of the (BeO), molecule. This is the assumption made by Kallo (19);i.e., the sum of energies lost in the bond strengths is equivalent to the newly formed bond strengths of adsorption. Stated in another way, the energy level of the closed reacting system remains unchanged. Now, the rates of desorption of the oxide polymers are indicated by the respective signal rise times, which are observed to be identical in the 2000-2700 K temperature range. Hence, the heats of adsorption are equal, and from the above assumption, so are changes in AE,. The exponential in eq 5 is, therefore, not changed when these oxides are adsorbed. Hence, the results obtained from eq 5 are also applicable to unimolecular reactions involving the adsorbed beryllium oxide polymers. We assume that the ratio of frequency factors in eq 4 is not changed significantly. Hence, the primary source of free beryllium is attributable to thermal dissociation of the monoxide adsorbate, i.e., the reaction BeO(ad)
2000 K
Be(g)
+ O(g)
(6)
Reduction of the adsorbed oxide must also be considered a possible source of free beryllium. It has been shown, in the above discussion, that beryllium oxides in the furnace behave as adsorbates, so reduction of the monomer, for example, might be given by the reaction (BeO)(ad) + C(s) = Be(ad)
+ CO(g)
(7)
A statistical thermodynamic calculation, identical with that used recently by Styris and Redfield (IO),yields a lower bound value for the Gibbs free energy for reaction 7. The calculation assumes that the adsorbed molecule is rigid and moves freely over the surface in a square potential well used to approximate the heat of adsorption. Relative appearance temperatures of desorbed species indicate the sign of the heat of adsorption difference that is contained in the partition functions used to describe the free energy; details can be found in ref 10. For these purposes the moment of inertia is calculated for Be0 by assuming that the atomic spacing in the Be0 is similar to the 1.62-A aluminum-oxygen spacing in AIOz (16). This results in the equilibrium constant K(7) for reaction 7 having an upper bound given by log K(7) < -4.4. Hence, reduction of the adsorbed monomer is not likely. There is, unfortunately, insufficient molecular data available on the polymeric oxides to estimate thermodynamic feasibilities associated with their reduction. The negative temperature shifts that are observed for the oxide polymers when sufficient magnesium is present can be explained by changes that the modifier induces in the activation energy for desorption. The magnesium oxide is predominant, so it “shields” sites where heats of adsorption are greatest. The (BeO), molecules are adsorbed, therefore, on sites having smaller (less negative) heats of adsorption, either on the graphite or on the magnesium oxide crystallites. This is quite unlike the proposed oxidation modification mechanisms that control aluminum atomization during the atomization heating phase (IO). This concept of shielding of the higher heat of adsorption sites is supported by additional experiments in which aluminum, instead of magnesium, is used to “modify” beryllium. The relative concentration of beryllium to modifier (Al) is not changed from that used for the magnesium modifier. The resulting appearance temperatures for (BeO)4(g)and for BezC4(g)are shifted negatively and by the same magnitude as observed when magnesium is
1987
2901
present; monitoring is attempted only for these two species when the aluminum modifier is present. Such results are expected if the proposed mechanism of shielding is responsible for the negative temperature shifts. Carbides. The carbides, shown in Figure 2, with the possible exception of Be&, are most certainly produced in the furnace. They me not artifacts resulting from the reactions or condensation of Be(g) and C,(g) in the constriction region of the sampling cone. Such artifacts would exhibit identical temperature profiles whether or not modifiers are used, because the Be(g) profile is unchanged by the modifier, and there is no reason to believe that the modifier can provide a carbon sublimation rate increase needed to negatively shift these carbide profiles. It is possible that the monomer may arise from dissociation of the polymer in the ionizer; the polymer and monomer profiles are very similar. It is possible that the signal attributed to Be& may be due, entirely or partially, to the presence of Be203(g)in the furnace. It should be noted, however, that this oxide is not observed in Knudsen cell studies of vapor species over BeO(s). The carbides, like the oxides, appear at temperatures (2400-2500 K) that are higher than those for Be(g), unless aluminum (see preceding paragraph) or magnesium modifiers are used, in which case the carbides appear near the 2000 K appearance temperature of Be(g). These temperatures provide an explanation of why it is that Chupka et al. failed to observe carbides of beryllium in their graphite Knudsen cell experiments; maximum temperatures were limited to 1900 K (22). The similarities between carbide and oxide appearance temperature behavior imply similar mechanisms are involved, i.e., desorption and surface occlusion, as already discussed for the oxides. Formation of the carbides may be due to heterogeneous reactions involving gas-phase beryllium or beryllium oxides with graphite. But, the types of active sites and the desorption energy levels that are available to Be(g) between 2000 and 2800 K are available, whether or not a modifier is present. The profiles that the carbides exhibit at the higher temperatures should, therefore, always be a part of the observed spectrum. This is not what is observed in the data of Figure 2. Hence, the carbides must result from reactions other than Be(g) reacting with graphite. There does appear to be some correlation between the profiles of the gaseous oxides and carbides shown in Figures 1 and 2. Unfortunately, it is not possible to unabmiguously identify BeO(g) in the modified case because of contributions from the Mg isobar. If it is assumed, however, the BeO(g) is shifted similarly to the (BeO),(g), then the carbide formation may be explained by the reaction xBeO(g)
+ ( x + y)C(s) = Be,C,(g) + xCO(g) (8)
Cyanides and Cyanamide. The temperature profiles of the beryllium cyanide spectra (Figure 2) are similar to those of the oxide polymers with and without magnesium modifiers. This can be explained by assuming a homogeneous reaction of the polymers with the cyanogen, i.e. (BeO),(g)
+ n/S(CN),(g)
= nBeCN(g)
+ n/202 (9)
Any cyanogen reaction with Be(g) is evidently of relatively little consequence since the cyanide profile shows no similarity to that of the Be(g). Surprisingly, the beryllium cyanamide (BeNCN) in Figure 2 does not appear to be formed from a homogeneous reaction among the reactants of reaction 8. Its appearance at 1680 K is a precursor to the appearance of all other gaseous species. Hence, the cyanamide must be produced from the heterogeneous reactions (BeO),(ad)
+ n(CN),(g)
-
nBeNCN(g) + ncO(g)
(10)
The reactions 9 and 10 are quite unlike those implied for
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
Table 11. Comparison of Reactions-Beryllium and Aluminum" Analytes Event Reaction Beryllium
Aluminum
Atomization Without Modifier
Thermal decomp. of adsorbed monomeric oxide
Thermal decomp. of solid alumina
With Mg Modifier
Thermal decomp. of adsorbed monomeric oxide
Oxidation of adsorbed a l u m i n u m by magnesium oxide
Thermal decomp. of adsorbed polymeric oxides
Thermal decomp. of solid alumina
Reduction of gaseous monomeric oxide by carbon
Oxidation of adsorbed aluminum by carbon
Cyanide Formation
Reduction of gaseous polymeric oxides by cyanogen
Oxidation of gaseous aluminum by cyanogen
Cyanamide Formation
Reduction of adsorbed polymeric oxides by cyanogen
Reduction of gaseous monomeric oxide by cyanogen
Hydration of the adsorbed monomeric oxide by water
by water
Dehydration of the hydroxide by magnesium oxide
Dehydration of the hydroxide by magnesium
Oxide Formation Carbide Formation
Thermal Pretreatment Hydroxide Formation Stabilization
Hydration of adsorbed monomeric oxide
Aluminum data from ref 10. cyanide and cyanamide production involving aluminum analytes (IO). Thermal Pretreatment. It is found that, during thermal pretreatment heating phases, the mechanism responsible for modification is similar to that reported in ref 10 for aluminum. For these experiments the furnace temperature is increased from 400 to about 1700 K in 10 s. Sample load is 40 p L of D20 solution containing 20 ng of Be or, if modified, 20 ng of Be and 7500 ng of Mg. Temperature profiles of the resulting spectra are shown in Figure 4. A Be(ODI2 signal ( m / e 45) appears a t 1220 K when the magnesium modifier is absent. The m / e 45 signal does not appear when the modifier is included in the sample. Instead, DMgOD(g) ( m / e 45,46) and Mg(OD),(g) ( m / e 60) appear concurrently near 375 K. The purpose of using D20,in this case, is to minimize maw overlap interferences from C02+ ( m / e 44) signals and to eliminate MgO and MgOH isobaric interferences associated with the Mg isotopes. The same effect, which is reported in ref 10 for aluminum, occurs in both cases because dehydration of the dihydroxide to the oxide becomes possible once the magnesium modifier depletes the water in the furnace by forming HMgOH and magnesium hydroxides (23). This argument is supported by the observation that for beryllium, as for aluminum (IO),the dihydroxide of the analyte is absent in the mass spectra if only a small (1pL) volume of sample solution is loaded into the furnace. This influence of aqueous volume agrees with the results from high-temperature equilibrium calculations made by Frech et ai. ( 3 ) . These same investigations predict that, in the presence of relatively large volumes of water, there can be considerable loss of beryllium by Beformation beginning at 1200 K. They also emphasize that stabilizing agents are likely to minimize the formation of the beryllium hydroxides; this also agrees with findings from the present experiments. GENERAL CONCLUSION It is evident from the above discussion that the overall mechanisms of atomization, with and without the magnesium modifier, are considerably different for beryllium as compared
Furnace Temprature, K
Figure 4. Spectra observed during a thermal pretreatment phase when (a)no modifier is used and (b) mcdlfler is present. Fumace temperature is increased from 400 to 1700 K in 10 s.
to aluminum. The mechanisms responsible for observed aluminum as well as beryllium species are tabulated, for comparison purposes, in Table 11. There is no mechanism, operating during the atomization heating phase, that is common to both beryllium and aluminum and that explains appearances of observed species. It is noted that, when no modifier is used, thermal dissociation of adsorbed and solidphase oxides is responsible for atomization from beryllium and aluminum species, respectively. A single mechanism explains vaporization and stabilization of beryllium and aluminum during the thermal pretreatment phase; i.e., beryllium and aluminum reactions in graphite furnaces are identical at lower temperatures. This is important from a pragmatic view, since it indicates that the lower temperature mechaism may be extended to other analytes that form hydroxides, although there is still far too little data available to draw conclusions
Anal. Chem. 1987, 59, 2903-2908
regarding the extent of applicability. ACKNOWLEDGMENT The authors express their gratitude to D. R. Ells for his invaluable assistance with the apparatus, to N. E. Ballou for his help with the manuscript, and to J. A. Holcombe for valuable suggestions concerning this work. Registry No. BeO, 1304-56-9; Mg, 7439-95-4; graphite, 7782-42-5. LITERATURE CITED (1) Maessen, F. J. M. J.; Balke, J.; Massee, R. Spechochlm. Acta, Part8 1978, 338,311. (2) Maessen. F. J. M. J.; Posma, F. D. Anal. Chem. 1974, 46, 1439. (3) Frech, W.; Lundberg, E.; Cedergren, A. frog. Anal. At. Spechosc. 1985, 8 , 257. (4) Runnels, J. H.; Merryfield, R.; Fisher, H. B. Anal. Chem. 1975, 47, 1258. (5) Mannlng, D. C.; Slavin, W.f Carnrlck, G. R. Spechochlm. Acta, Perf B 1982, 378, 331. (6) Slavin, W.; Carnrick, G. R.; Manning, D. C.; Pruszkowska, E. At. Spectrosc. 1983, 4 , 69. (7) Frech, W.; Cedergren, A.; Cederberg, C.; Vessman, J. Clin. Chem. (Winston-Salem, N . C . ) 1981, 2 8 , 2259. (8) Baxter, D. C.; Frech, W.f Lundberg, E. Analyst (London) 1985, 110, 475. (9) Thompson, K. C.; Gudden, R. G.; Thomerson, D. R. Anal. Chlm. Acta 1975, 74, 289. (10) Styrls, D. L.; Redfield, D. A. Anal. Chem., preceding paper in this Issue.
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(11) Styrls, D. L. Fresenius' 2.Anal. Chem. 1988, 323, 710. (12) Styris, D. L. Anal. Chem. 1984, 56, 1070. e,-.W. A.; Berkowltz, J.; Glese, C. F. J . Chem. Phys. 1959, 30, (13) Chupka, OLI.
(14) Theard, L. P.; Hlldenbrand, D. L. J . Chem. Phys. 1964, 41, 3416. (15) Spear, K. E. I n Treetlse on SolM State Chemlstry, Hannay, N. B., Ed.; Plenum: New York, 1976; Vol. 4, pp 176-177. (16) Chase, M. W.; Curnutt, J. L.;McDonald, R. A.; Syverud, A. N. JANAF Thermochernlcal Tables, 1978 Supplement, Amerlcan Chemical So clety and Amerlcan Institute of Physlcs: New York, 1978. (17) Wedler, G. Chemisorption : An Experimental Approach ; Butterworths: Boston, MA, 1976. (18) Fejes, R. Contact Catalysis; Szabo, Z.,Kallo, D., Ed.; Elsevler: New York, 1976; Vol. 1, p 167. Kallo, D., Ed.; Elsevier: New (19) Kallo, D. I n Contact Catalysls; Szabo, Z., York, 1976; p 336. (20) Hammes. G. G. frlnclples of Chemical Kinetics; Academic: New York, 1978, p 151. (21) Moore, J. W.; Pearson, R. G. Kinetics and Mechanism, 3rd ed.; Wlley: New York, 1981; pp 163-173. (22) Chupka, W. A.; Berkowitz, J.; Glese, C. F.; Inghram, M. G. J . Phys. Chem. 1958, 62, 611. (23) Curtlss, L. A.; Frurlp, D. J. Chem. Phys. Left. 1980, 75, 69.
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RECEIVED for review February 9,1987. Accepted August 27, 1987. Faculty appointment support, from the Northwest College and University Association for Science, for D. A. Redfield is gratefully acknowledged. This paper is based on work sponsored by the Office of Basic Energy Sciences of the Department of Energy and performed under DOE Contract NO. DE-AC06-76RLO 1830.
High-Temperature, High-Sensitivity Pyrolysis Field Ionization Mass Spectrometry Hans-Rolf Schulten*
Fachhochschule Fresenius, Department of Trace Analysis, Dambachtal20, 0-6200 Wiesbaden, Federal Republic of Germany Norbert Simmleit and Rolf Muller
Institut Fresenius, Chemical and Biological Laboratories, 0-6204 Taunusstein 4, Federal Republic of Germany
An Improved experknental setup for tlmaresolved In-source pyrolysls (Py) field lonlzatlon (FI) mass spectrometry (MS) Is dedbed. SmaH samples (10-400 p.g) are placed In quartz crucibles, Inserted vla a modlfled dlrect lntroductlon system, and then heated quad-llnearly from 50 to 800 O C with rates programmed from 0.2 to 10 OC s-'. Complex blomaterlals, such as spruce needles or coals, and synthetic polymers (e.g. polyethylene, Teflon) have been Investigated by the new Py-FI mass spectrometrk devlce In the mass range from m / z 70 to m / z 2100. Thls device shows Improved sensltlvity for the productlon of F I Ions above m / z 250; an augmentatlon of the total Ion count by a factor of approxlmately 10 Is observed. Furthermore, it Is now posslble to lnvestlgate thermally stable synthetic polymers, whkh also produce abundant high m a s F I Ions. The mean varlatlon coefflclent calculated from the Intensltles of the F I slgnals with 5 1 0 0 % relatlve abundance Is 10 % Thus, the highly reproducible Py-FI mass specfra can be evaluated by pattern recognltlon techniques and provlde a large amount of unlque, speciflc lnformatlon about the composition and structure of complex materials.
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In recent years pyrolysis (Py) mass spectrometry (MS) has become an important tool to investigate complex materials 0003-2700/87/0359-2903$01.50/0
(1). In almost all studies low-voltage electron impact (EI) MS is used for analysis of the samples. The E1 ions obtained are a combination of molecular and fragment ions of the pyrolyzates. A number of difficulties are encountered: (1)it may not be possible to distinguish which ions arise from fragments and which from molecular species, (2) the fragmentation will vary considerably depending on the instrumental conditions, and (3) normally only low mass ions (less than m / z 300) are observed. This is mainly due to the electronic excitation energy of the ionization process and the subsequent mass spectroscopic fragmentation but is also due to the thermal effects of flash pyrolysis by Curie point. The igormation content of Py mass spectra can be markedly imQroved by soft ionization modes such as field ionization (FI) (2) and chemical ionization (3). FI-MS normally yields almost exclusively molecular ions of the pyrolyzates, even in the mass range up to m / z 500 and above. The methodology and applications of temperature-programmed, time-resolved Py-FI to natural and synthetic substances up to 1984 have been reviewed ( 2 , 4 ) . Since that time there have been further Py-FI studies of complex biomaterials including proteins ( 5 ) , foodstuffs (6),tobacco (7-9), beech leaves ( I O ) , spruce needles ( I I ) , lignins,whole soil (12,13),soil horizons (14,15),terrestrial and aquatic humic substances (16), coals (I7-21), and synthetic polymers (22,23). These studies have demonstrated 0 1987 American Chemical Society
