25
Atla/. Chem. 1881, 53, 25-29
(4) Hawhalter, J. P.; Rltz, 0. P.; Wallan, D. J.; Dien, K.; M I S , M. D. Appl. Spectrosc. 1080, 3 4 , 144-146. (5) Haushalter, J. P.; Buffett, C.
E.; Morris, M. D. Anal. chem. 1080, 52,
1284-1287.
(13)Tretzel, J.; SchneMer, F. W. Chem. Phys. Lett. 1078, 59,514-518. (14) Kumar, K.; Carey, P. R. J. Chem. Phys. 1075, 63, 3697-3707. (15) Dien, K. F.; M a , M. D., unpublished observatlons. (16) Haushalter, J. P.; Schopfer, L. M.; M s , M. D., unpublished observa-
(8) Lotem, H.; Lynch, R. T., Jr.; Bloembergen, N. phys. Rev. A 1076, 74,
1746-1755. (7) Hudson, B.; Hetherington, W.; Cramer, S.; chebay, I.; Klauminzer, 0. K. Pfm.Net/. Acad. Scl. U . S . A . 1076, 73, 3798-3802.
(8) Lynch, R. T., Jr.; Lotem, H.; Bloembergen, N. J. Chem. phys. 1977, 66, 4250-4251. (9) Carrelra. L. A,; Maguire, T. C.; Malloy, T. B. J . Chem. phys. 1077,
66,2621-2628. (IO) Dutta, P. K.; Spko, T. D. J. Chem. Phys. 1078. 69,3119-3123. (11) Druet, S. A. J.; Taran, J.-P. E.; Bord6. C. J. J. phys. (Orsay, Fr.) 1070, 40, 819-840. (12) Druet, S. A. J.; Taran, J.P. E.; Bord6, C. J. J . phys. (CrsayFr.) 1980, 41, 183-184.
tlons.
(17) Carrelra, L. A.; Qoss, L. P.; Malloy, T. B., Jr. J. Chem. phys. 1078, 69,855-882.
REC-
for review July 23,1980.Accepted October 16,1980. This work was supported in part by the National Science Foundation through Grant CHE 79-15185. The computer used was purchased with support from the National Science Foundation through Grant CHE 78-20065.
Acid-Enhanced Field Desorption Mass Spectrometry of Zwitterions T. Keough” and A. J. DeStefano The Procter & Gamble Company, Miami Valley Laboratorles, P.O. Box 39175, Cincinnati, Ohio 45247
In the field desorption mass spectrometrlc analysis of a wlde varlety of zwltterlons, sensltMty for (M H)’ adduct kms can be slgnlficantly Increased by protonating the zwitterkn to form a quaternary “onlum” salt prior to analysis. Protonation can be effected in one step by simply adding p-toluenesulfonic acld to a solutlon of the zwltterlon. Fleld desorption mass spectra obtained by thls procedure typically exhlbh only a single ion Corresponding to the protonated zwfflerion. The reduced emmer current required for these analyses elhinates the fragment kns or adduct kns (fonned by thermaiy Wllated bimolecular processes) that domlnate the spectra of underlvatlzed zwltterlons. Appllcatlons to zwkterionlc surfactants, amlno aclds, and phospholipids are dlscussed.
+
Field desorption (FD) mass spectrometry is an effective technique for the analysis of a wide variety of nonvolatile and thermally unstable compounds (I), often providing intact molecular or adduct (e.g., M H+, M + Na+) ions that cannot be observed with other readily available ionization techniques (2). Zwitterionic compounds are biologically and commercially important and are often best characterized by FD. For example, the molecular weights of some thermally unstable amino acids (3) and phospholipids (4) have been directly determined by FD, and we have characterized ammoniocarboxylates (5)and hydroxyammoniocarboxylatea (6)by this technique. Unfortunately, many zwitterionic compounds are thermally unstable a t the emitter temperatures typically required in FD analyses (150-250 “C). The extent of sample pyrolysis on the emitter is strongly dependent upon emitter temperature (5, 6) as are the resulting FD spectra. Hightemperature FD spectra often exhibit little, if any, of the (M + H)+ adduct ion and are dominated by fragment ions and ions formed by intermolecular processes (such as alkyl group transfer). Thus, poor sensitivity and thermal instability of the sample (or the (M + H)+ adduct ion) can severely limit the information content of the FD maas spectra of zwitterionic compounds. We have developed a simple one-step procedure that significantly increases FD sensitivity for the (M H)+ adduct ions of a wide range of zwitterionic compounds and minimizes
+
+
0003-2700/81/0353-0025$01.00/0
the thermal problems just mentioned. This method involves converting the zwitterion to an “onium” salt by adding p toluenesulfonic acid (p-TSA) prior to analysis. The reaction is illustrated in eq 1 for an ammoniocarboxylate. The method (CH3)3N+(CHJbCOO-
+ CH3C6H4SO3H
-
[(CH3)3N(CH2)5COOHI+[CH3CGH,S031(1) is based on the previous observation (7) that the FD mass spectra of quaternary ammonium salts yield an intense ion corresponding to the intact cationic portion of the salt. Our approach has yielded excellent FD mass spectra for zwitterionic carboxylates, phosphates, sulfonates, and sulfates, compounds that differ greatly in the basicity of the anionic portion of the molecule.
EXPERIMENTAL SECTION Instrumentation. All FD mass spectra were obtained by use of a modified Varian/Atlas SM-1B mass spectrometer that has been described previously (5). The original electron impact source was replaced with an FI/FD source similar to that discussed by Beckey and Schulten (8). Source focusing was accomplished manually, with the aid of micromanipulators, by maximizing (at the collector) the molecular ion current generated by field ionization of acetone. The mass scale was manually calibrated with the aid of a Hall-effect probe after analysis of suitable FI/FD reference compounds (9). All spectra were output to a Honeywell Model 1508 Visicorder. Reagents. The amino acids and distearoylphosphatidylcholine (DSL) were obtained from commercial sources and used without further purification. The ammoniocarboxylates were synthesized and purified by methods developed by Laughlin and McGrady (10). The purity of these samples was verified by NMR. The remaining compounds were also synthesized at the Miami Valley Laboratories, and their purity was verified by NMR and TLC. There was no evidence for significant levels of impurities in the FD mass spectra of any of the compounds studied. Procedure. Concentrated solutions of the zwitterion were prepared in suitable solvents such as water, methanol, or a chloroform/methanol mixture. We did not accurately measure the concentration of the zwitterion in these solutions. However, we typically prepare solutions with concentrations of at least 1 pg/pL to ensure adequate sensitivity for analysis on our instrument. Samples were loaded onto high-temperature-activated FD emitters ( I I , 1 2 ) by dipping or by direct syringe loading (13).The FD emitter was then inserted into the ion source and positioned @ 1980 A r w i c a n Chemlcai Society
26
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981 CH3
+I
CloHzlyW&W"-
(CidH)
+
Table I. Summary of the Zwitterions Examined in This Study
PTSA
Amino Acids NH
II
H,NCH~+H(CH,)CH,CO,-(creatine) CH,CH(NH,)CO; {alanine) NH,CO(C+H,),CH(NH,)C02~(glutamine)
-O,CCH(NH,)CH,SSCH,CH($JH~)CO~(cystine)" Ammoniocarboxylates + C,H,,+,N(CH,),(CH,),CO,- (n = 1, 8, 10, 12, 14, 16) Ammoniophosphate CH,OCOC l,H 35
I
CHOCOC,,H,, 150
200
250
300
350
400
450
5W
550
MIZ
Flgwe 1. The dependence of FD results on the relative concentration
I P
CH,OPOC,H~&(CH,),
I
0-
of sample and p-TSA.
-
1mm from the counterelectrode. A positive potential of 8 kV was applied to the emitter and a negative potential of -7 kV was applied to the counterelectrode so that the total potential difference was 15 kV. If the sample did not provide usable ion current at ambient emitter temperature, the emitter heating current was raised manually from 0 mA in steps of 1-2 mA. We observed the ion current at a beam monitor (located along the flight tube) after each increment in emitter heating current. Typically, it required several minutes to raise the emitter heating current from 0 to -25 mA. Complete mass spectra were recorded at various emitter heating currents when a sufficiently intense ion current was observed at the beam monitor. After completion of the analysis, the emitter surface was cleaned by raising the emitter heating current to 60 mA for -30 s. The relationship between emitter heating current and emitter temperature has been previously studied (14). The emitter temperature was found to vary linearly with emitter heating current over the range of 0 to -25 mk An emitter heating current of 10 mA corresponded to an emitter temperature of -100 "C while an emitter current of 20 mA corresponded to an emitter temperature of -200 O C . We assume that, to a first approximation, a similar relationship between emitter heating current and emitter temperature exists on our system. After recording the normal FD spectrum, we determined the effect of acid (either p-TSA or hydrochloric) on FD behavior by repeating the analysis after adding -20 p L of a 1M solution of acid to -100 p L of the original solution containing the zwitterion. In general, we found no need to accurately measure the relative concentrations of the sample and acid. However, results can be affected if either component is present in large excess. This is illustrated in Figure 1. In this case, the sample of interest was (decyldimethylammonio)hexanoate,C & H , and the acid used was p-TSA. The upper spectrum was obtained with a 10-fold molar excess of C&H. This analysis required a high emitter heating current and the resulting spectrum does not contain a (M+ H)+ adduct ion. It does contain an intense ion formed by intermolecular transfer of a methyl group (5) and a fragment ion generated by decarboxylation of the (M+ H)+ adduct ion. This spectrum is identical with the normal FD mass spectrum obtained without addition of acid. The middle spectrum was obtained from a solution containing approximately equimolar amounts of the sample and acid. This spectrum could be obtained at a lower emitter current than required for the above scan and it exhibits only the (M+ H)+adduct ion. At the reduced emitter heating current there is no evidence for intermolecular methyl transfer or for fragmentation. The bottom spectrum was obtained with a 10-fold molar excess of acid. This spectrum, recorded at 0 mA, is dominated by acid-cluster ions as have been previously observed in the FD mass spectra of organic acids (15). Under these conditions, (3M + H)+ and higher mass clusters dominate the FD
(DSL)
Sulfonates and Sulfates C 2 ~ H ~ 5 ~ ~ C H 3 ~ ~ ~ C 2 H 4 0 ~ ~ c 2 H 4 3s -0
C,,H,~(CH,)*(CH,),SO3C1~H25~(CH3)C3H6S03c12H2~~~cH3)2c3H~s03~ C 1 ~ H ~ ~ N ~ C H ~ ~ 2 ~ C 2 H 4 0 ~ ~ s 0 3 ~
" Failed to show sensitivity enhancement in the presence
of P-TSA.
spectrum of p-TSA. A relatively weak (M+ H)+adduct ion of the sample and a proton-bound dimer of the sample and acid are also evident. Apparently excess acid desorbs at ambient temperature (under the influence of the high electric field) ensuring that the species remaining on the emitter is a 1:l salt of the zwitterion with p-TSA. If during routine application of this technique we observed a spectrum that resembled either the normal FD masa spectrum of the sample or the FD mass spectrum of pTSA, we simply adjusted the concentration of the appropriate component.
RESULTS The zwitterionic compounds studied are listed in Table I. The normal FD mass spectra of many of these compounds exhibit only weak (M + H)+ adduct ions and are dominated by fragment ions or ions formed by intermolecular processes. In the presence of p-TSA, on the other hand, the FD mass spectra of all of the compounds listed (with the exception of cystine) showed enhanced sensitivity for the (M + H)+ adduct ion and minimal fragmentation. I t is not possible with our instrumentation to accurately quantitate the sensitivity enhancement obtained by this technique. However, an indication of the magnitude of the sensitivity enhancement can be obtained from the Visicorder output shown in Figure 2. These data were obtained after syringe loading the emitter with 1 FL of a concentrated solution containing the indicated quaternary ammmoniosulfonatewithout acid (a) and after adding (as discussed above) p-TSA (b). Both scans were obtained from the same solution of the zwitterion with an identical detector sensitivity. Several analyses were performed with and without added p-TSA, and typical results are given in the figure. The spectrum recorded in the presence of p-TSA exhibited a (M H)+ adduct ion and isotope peaks that were significantlymore intense than obse~edin the absence of acid. We also note that, in the presence of p-TSA, the emitter heating current required for the analysis decreased from 25 to 0 mA. We have o b s e ~ e da sizable reduction in the emitter current required for the analysis of most zwitterions after
+
27
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
0-
Y"'*I
40 IU
20 0
+ HI'
I'
I' I " ' I I " I "
1 1 1 1
I I , I l
5b
650
d0
700
800
750
850
MI2
Flgure 4. FD mass spectrum of distearoylphosphatiiine without
addition of
p-TSA
+
Figure 2. Comparison of FD intensities for the (M H)' adduct ion of the indicated quaternary ammoniosuifonate. Spectnm was recorded (a) without addition of p-TSA and (b) after addition of p-TSA.
(upper) and after addition of
p-TSA
(lower).
CH3
+I
Mixture (n = 8,10,12,14,16)
CnH2,+~-N-(CH2)5C02AH3
NH
, + C",l+
1I
IM.
+ " - Wll+
2
m
H2NCNCH2C02H I
Y 1w
225
I 70
80
SO
1W
110
120
130
I 140
150
160
170
250
300
275
328
350
375
425
400
M/Z
m
e 5. FD mass spectwn of an ammonkhemnoate mixture without
addition of
p-TSA
(upper) and after addition of
p-TSA
(lower).
MIZ
Flgure 3. FD mass spectrum of creatine without addition of (upper) and after addition of p-TSA (lower).
p-TSA
addition of p-TSA. However, we are not certain why some compounds desorb at ambient temperature while others require substantial heating. We speculate that chain-length effects may be important. Both of the ammoniosulfonates (Figures 2 and 7) have two long chains attached to the quaternary nitrogen. This may result in a relatively low crystal lattice energy for the corresponding salts. Thus, the ion mobility required for FD might be obtained at a low emitter heating current for these systems. This type of argument has been previously advanced to explain the observed decrease in desorption temperature with increasing chain length for some quaternary ammonium salts (7). Some typical applications of this method of analysis are given in Figures 3-5. The utility of this method for the analysis of thermally unstable amino acids is illustrated by the results obtained from the analysis of creatine, Figure 3. The upper spectrum, obtained in the absence of acid, compares well with a previously published spectrum (2). We observe not only the (M + H)+ adduct ion but also (M + H - HzO)+, (M - HzO)+,and (M - COzH)+. In the presence of p-TSA, the desorption temperature is lowered and the spectrum exhibits only the (M + H)' adduct ion. Similar results have been obtained for alanine and glutamine. We were, however, unable to observe a sensitivity enhancement for cystine.
Figure 4 illustrates results obtained for the analysis of an ammoniophosphate, distearoylphosphatidylcholine. The spectrum obtained in the absence of acid was extremely weak. We did observe an (M H)+ adduct ion, but the spectrum was dominated by the fragment ion formed by loss of choline phosphate. Similar results have been observed in the hightemperature FD mass spectrum of dipalmitoylphosphatidylcholine (4). In the presence of p-TSA, sensitivity is increased, the desorption temperature lowered, and fragmentation completely eliminated. Finally, the utility of this method for the analysis of mixtures of zwitterionic compounds is illustrated by the data in Figure 5. This application involves the qualitative analysis of a five-component equimolar mixture of ammoniohexanoates. The FD spectra obtained in the absence of acid are highly temperature dependent. Depending on the temperature, we observe only (M H - Cod' ions, only (M CH# ions, or, as shown at the top of Figure 5, both (M + H - Cod+ and (M + CH3)+ ions for each component. In the presence of p-TSA (bottom spectrum), an intense series of (M + H)+ adduct ions are the primary species observed.
+
+
+
DISCUSSION The goal of these experiments was to increase FD sensitivity and reproducibility for a wide variety of zwitterions by first converting the zwitterion to a quaternary "onium" salt. This can be accomplished simply by mixing the sample with an acid as long as the acid is strong enough to protonate the anionic
28
ANALYTICAL CHEMISTRY, VOL.
53, NO. 1, JANUARY 1981
CH3
+I
+I
cioH&Ch)aCO
2-
I
loo,
CH3
80.21mA
C*,P
RI
/
Bo. 20
0
20 0
g
100
80 120
;
L
:
?
;
Mi2
20 :
0 230
240
250
280
270
280
280
320
310
320
330
M/Z
W e 8. FD mass spectnm of an ammiohexamate without addition of acid (upper), after addition of HCI (middle), and after addition of p-TSA
(lower).
portion of the zwitterion. p-TSA is a strong acid of relatively low volatility which appears well suited for these experiments. Other strong acids should work as well. To verify that the expected salt was being formed in the strong acid solutions containing ammoniocarboxylates, we compared the infrared spectrum of ClJ-121N+(CH3)2(CH2)6COO(Cl&H) with the infrared spectrum of the solid material recovered from a solution of Cl&H to which had been added p-TSA. The spectrum of C l a H exhibited two intense carboxylate bands a t 1580 and 1380 cm-'. After addition of the acid, the carboxylate bands were quantitatively removed and an intense CO stretching band, characteristic of a COOH group (eq 11, was observed at 1725 cm-'. We infer that this protonation reaction also occurred at least to some extent for zwitterionic phosphates, sulfonates, and sulfates, because of the dramatic change in FD behavior which resulted after addition of the acid to solutions of zwitterions possessing these anionic groups. FD analysis of adenine using 1 N HCl as the solvent has been previously shown (11)to yield increased sensitivity for the (M H)+ adduct ion (by about a factor of 4) relative to FD results obtained with water as the solvent. We therefore assessed the ability of HC1 to enhance FD sensitivity for zwitterions by recording the FD mass spectra of many of the compounds listed in Table I after mixing with approximately equimolar amounts of HC1. Representative results are given in Figures 6 and 7. In Figure 6, the normal FD spectrum of C1&H is compared to spectra obtained after addition of either HC1 or p-TSA. In this case, where we are dealing with a very basic anionic group, sensitivity for the (M H)+ adduct ion is increased and fragmentation is suppressed by addition of either HCl or p-TSA. The results obtained from analysis of a quaternary ammoniosulfonate are compared in Figure 7. The upper spectrum, recorded without addition of an acid, exhibits a weak (M + H)+ adduct ion and an ion formed by attachment of sodium which was apparently present a t low levels in the sample. The spectrum is dominated by fragment ions such as (M H - SO3)+and a series of ions formed by cleavage at the ether linkages. The sulfonate group is much less basic than the carboxylate group and is also leas basic than water, which was used as the solvent for this analysis. Therefore, both HCl and p-TSA initially protonate the solvent preferentially. Upon introduction of the acidified solution into the vacuum system, a volatile acid such as HC1 is evap-
+
+
+
Flgue 7. FD mass spectrum of an ammniosulfonate without additkn of acM (upper), after addition of HCI (middle), and after addition of p T S A (Ioww).
orated along with the solvent and the zwitterion is not converted to the quaternary ammonium salt. Therefore, the spectrum recorded in the presence of HC1 (middle scan) is very similar to that obtained without addition of an acid. p-TSA, which is much less volatile than HCl, is not removed upon introduction into the spectrometer and can protonate the zwitterion once the solvent has been evaporated. The resulting FD spectrum (bottom scan) shows enhanced sensitivity and no fragmentation. For sulfonates and sulfates, spectra recorded in the presence of p-TSA exhibited more intense (M H)+ adduct ions and less fragmentation than spectra recorded in the presence of HC1. Finally, we compared p-TSA addition to alkali ion attachment (16). Alkali ion attachment is known to yield excellent FD sensitivity for polar nonvolatile molecules such as carbohydrates (17). Although we were able to effect sodium ion attachment to ammoniocarboxylates, we did not observe a significant sensitivity enhancement by this technique. Furthermore, fragmentation was not suppressed because of the high emitter heating current (>20 mA) required to effect alkali ion attachment (16). For zwitterions, spectra recorded in the presence of p-TSA exhibited much less fragmentation and much more sample-related ion current than were observed with alkali ion attachment.
+
ACKNOWLEDGMENT We gratefully acknowledge the contributions of J. D. Pryne, who recorded many of the FD spectra, E. P. Gosselink and C. R. Degenhardt, who synthesized and purified many of the compounds examined in this study, and R. A. Sanders, R. G. Laughlin, and R. P. Oertel for their valuable suggestions throughout the course of this work.
LITERATURE CITED Beckey, H. D. "Rindples of FW Ionization and Field DesOrptkn Mass Spectrometry", 1st ed.; Permagon Press: Oxford, England, 1977. Fales, H. M.;MUM, 0. W. A.; Winkler, H. U.; Beckey, H. D.; Damlco, J. N.; Banon, R. Anal. Chem. 1975, 47, 207-219. Wlnkler, H. U.; Beckey, H. D. Ofg. Mess Spectfom. 1972, 6. 655-660. Wood, 0. W.: Lau, P. Y.; Subba Rao. 0. N. B k d . Mess Spectrom. 1976, 3, 172-176 and references therdn. Sanders, R. A.; DeStefano, A. J.; Keough, T. Org. Mess Spectrom. 1980, 15, 348-350. Keough, T.; DeStefano, A. J.; Sanders, R. A. olg. Mess Spectr0m. 1980, 15, 351-354. Velth, H. J. Org. Mess Spectroom. 1976. 1 1 , 629-633 and refwenoes
thereln.
2s
Anal. Chem. 1981, 53, 29-33 (8)
(9)
Schulten, H.R.; Beckey, H. D. Ofg, Mess Spectrom. 1973. 861-867. McEwen, C. N.; Bollnskl, H. G. Biomed. Mass Speckom. 1975, 2, 112-114.
(10) (11)
Laughiin, R. G.; Mcaady. J., unpublished work. Schulten, H.R.; Beckey, H. D. Org. Mass Spectrom.
(12)
High-temperature-actbated emitters were purchased from MMwest AnalyUcai Consukants, Inc., Champaign, IL. Linden, H. 6.; Winkler, H. U.; Beckey, H. D. J. phys. E . 1977, 70,
(13)
885-895.
1972, 6,
657-660.
D.; Schulten, H.R. Org. Mess Speckom. 1975, 70, 8 13-8 18. (15) Wood, G. W.; Oldenbug, E. J.; Lau, P. Y.; Wade, D. L. Can. J . Chem. 1978, 56, 1372-1377, (16) Roellgen, F. W.; Schulten, H A . Org. Mess Speckom. 1975, 70, (14)
Kuemmler,
660-668. (17) Rome, J. C.; Puo. G. Org. Mess Spectfom. 1977, 12, 26-32.
RECEIVEDfor review July 7,1980. Accepted October 2,1980.
Analysis of Styrene Polymers by ,Mass Spectrometry with Filament-Heated Evaporation Harold R. Udseth' and Lewis Friedman Department of Chemistry, Brookhaven National Laboratoty, Upton, New York 11973
A mass spectrometric study of moderate-sized polystyrene (nominal mol wt 2100) is reported. The polystyrene was evaporated from a probe filament heated at 1000 OC/s under both electron-impact (EI) and chemicai-lonlzatlon (CI)conditions. Under E1 conditions extenslve fragmentation and depoiymerizatlon were observed but nmers out to n = 11 were detected. Under CI conditions, nmers up to n = 27 were detected, and spectra that approximately reproduced the oligomer distribution were obtained. Temperature measurements and activation energies of evaporation rates for many of the ionic species were also measured.
Low molecular weight oligomers of polystyrene have been extensively studied by use of mass spectrometric techniques. Early attempts were made to identify complex involatile and thermally unstable materials by pyrolysis mass spectrometry (I) using simple heating techniques. This approach was continued by using laser heating and time-of-flight mass spectrometry (2), for rapid analysis of degradation products. More recently field desorption (FD) ionization techniques have been applied to the characterization of polystyrene oligomers separated by liquid chromatography (3). Matsuo (4) et al. have reported FD mass spectra of polystyrene samples with molecular weight distributions peaked at values of 600,2200,4000, and 8500. The FD ion source technique provides a valuable approach to the solution of the problem of obtaining m m spectra from thermally unstable involatile molecules. Recent work in chemical ionization ion sources with direct insertion sample probes and with FD emitters has shown that volatility enhancement of complex molecular systems can be achieved by interaction of the chemical ionization plasma and the sample on the probe or emitter surface (5-9). This volatility enhancement has been attributed to heterogeneous ion molecule reactions which give rise to species on the probe surface with lower activation energies of sublimation or desorption. Quantitative differences in activation energies of desorption of species responsible for the E1 and CI mass spectra of Pb(NOS)*have been observed (10). The purpose of this report is to present the result9 of a study of the mass spectra of samples of styrene polymer (mol w t 21001, obtained by rapid heating at rates of approximately loo0 O C / s . Smooth rhenium ribbon surface direct insertion 0003-2700/81/0353-0029$01.00/0
probes were used in both electron-impact and chemical-ionization sources. Rapid heating studies were made with measurement of both sample probe temperature and mass spectra. Polystyrene was selected as a relatively fragile involatile molecular system, capable of undergoing competitive thermal depolymerization during the desorption process. Quantitative determination of temperature dependences of rates of desorption were made to investigate the effect of rapid heating in the presence and absence of the CI plasma on competitive rates of polymer desorption.
EXPERIMENTAL SECTION Evaporation studies were carried out in both a chemical ionization and an electron impact ion source and in each case the products were extracted and analyzed by a computer-controlled quadrupole mass spectrometer. A DEC PDP 8/E computer was used to control the Extranuclear Laboratories Inc. quadrupole power supplies which operated an Extranuclear Laboratories Inc. 3/8 in. diameter quadrupale rod mass analyzer assembly. Studies below 1200 amu were made with an Extranuclear Laboratories Inc. power supply operating at a frequency of approximately 1.7 MHz. A special low-frequency (292 IrHz)supply capable of mass analysis in our experiments up to 70 OOO m u was used for the mass range of lOO0-4Wl amu. Details of the computer-controlled mass analysis and data acquisition system have been previously presented (10). Samples were evaporated from a rhenium ribbon filament that was heated at a rate of approximately loo0 O C / s . The spectra were taken at scanning rates of from 2 to 11amu/ms and scanned in steps ranging in width from 0.28 to 10 amu. The CI spectra were taken with argon and methane as reagent gases et a pressure of 0.1 torr. Temperature measurements of rates of sample evaporations were made by using the resistance of the rhenium as a thermometer. The probe filament was heated with a constant current rapid heating pulser. The voltage drop across the filament was measured and recorded in the computer during the course of the evaporation with a 5-m voltage to frequency converter. Accurate values for the change in resistance of the filament and correspondingly the change in temperature could thus be recorded for each 1.2-ms time period of the sample evaporation. The precision of sample probe temperature measurement is estimated at f3 K. The sample probe surface is not uniformly heated, with a considerable temperature gradient from the center to ends supported on heavy wire leads. Consequently, the absolute temperature of sample desorption is uncertain by a value larger than the precision of measurement. This uncertainty is estimated to be less than 25 K. Measurements of slopes of plota of log relative ion intensity vs. reciprocal absolute temperature depend on differences in temperature rather than absolute measurements 0 1980 American Chemlcal Society
