Electron Impact Ionization Mass Spectrometry Using Field Desorption Activated Emitters as Solid Sample Probes Bernd Soltmann, Charles C. Sweeley, and John F. Holland" Depatfment of Biochemistry, Michigan State University, East Lansing, Michigan 48824
A new method for the analyses of solld samples by mass spectrometvy has been devised. Solublllzed samples are placed on field desorption actlvated emitters and are Ionized by an electron impact beam afler thermal desorption from the wlre. The method has been applled to compounds ranging from organlc crystals to Inorganic salts.
Gaseous samples were used exclusively in the initial application of mass spectrometry to organic compounds. Since many compounds are nonvolatile and cannot be derivatized readily into volatile species, a desire for the analysis of solid samples was a natural consequence of the growth of the method. For this reason, modification of the vacuum system of the mass spectrometer to permit the introduction of samples directly into the ion source has been an area of active interest (1-5) and solid sample inlet systems are now optional accessories in nearly all commercially made mass spectrometers (6). However, these devices bring several problems to the analysis, among which are an uncertainty in the precise moment of volatilization, irregular volatilizatioh processes, instrument-dependent heating rates, sample decomposition prior to ionization, and excessive ion source contamination. Attachment of a dedicated computer system to the mass spectrometer has reduced the severity of some of these disadvantages (7,8). For example, it allows repetitive scanning of the mass spectrometer during the heating interval and thereby removes the need to accurately predict the moment of volatilization. Since the computer can recall scans taken during the peak evaporation time, no valuable data are lost. Nonreproducible heating rates and irregular volatilization are not major problems in this mode. Even with this feature, however, successful routine analysis can be accomplished only with diligent operator attention to the variou details of sample preparation and analysis. Field desorption (FD) mass spectrometry has a well-established potential for the analysis of solid samples with little volatility (9). A variation of the techniques generally used in FD analysis has been developed, using a combined electron impactjfield ionizationjfield desorption source on a Varian MAT CH-5 mass spectrometer. In this procedure, the activated FD emitter with sorbed sample is heated in the normal manner, but without the high voltage that is usually applied to the extraction plate during FD analyses. Instead, the sample is evaporated from the emitter and ionized by an electron beam. The resulting ions are accelerated and focused into the mass spectrometer for analysis. In this mode, the FD emitter is acting as a direct probe sample injector and the resulting mass spectra show characteristic electron impact fragmentation patterns. The optimum emitter current (temperature) for the evaporation of a sample species is called the best emitter temperature (BET), using a nomenclature analogous to the best anode temperature (BAT) of the F D mode. The technique is referred to as EI/D to symbolize electron impact ionization using the field desorption emitter. 1164
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
This technique has been used to calibrate the temperature-current relationship of the activated emitter during heating (10) and to compare BET with BAT for theoretical studies of F D mass spectrometry (11). It is also a superb method of direct probe analysis. This correspondence describes EI/D as a heated direct probe inlet technique and illustrates its utility when used for that purpose. In an analogous application, the FD emitter can be used in a chemical ionization source, where ions are presumed to be produced in the semifluid phase on or near the emitter during thermal desorption (12). This technique, which may be called CI/D, yields ions similar to those produced by classical chemical ionhation mms spectrometry and by field desorption mass spectrometry.
EXPERIMENTAL Apparatus. Mass spectra were obtained with a Varian MAT CH-5 DF mass spectrometer with a combination EI/FI/FD source. The mass spectrometer is interfaced to a DEC PDP 11/20 computer in which data collection and storage programs are functional. This computer is attached via direct memory access to a DEC PDP 11/40 computer which is operating in a time-shared mode using the multi-task executive program, RSX-11D. The outputs from the mass spectral experiments are obtained from the 11/40 system via a Tektronix 4010 scope terminal and 4610 hardcopy unit. An emitter current programmer (ECP), as described previously (13),was used without modification. This device permits isothermal regulation of the emitter temperature at any point within the range from 50 to 1500 "C and time-dependent linear programming between any two temperatures in this range. The emitter was prepared from 10-pm tungsten wire and was activated in a manner similar to that prescribed for use in standard FD analyses (14-16). The activated emitter can withstand temperatures to >1800 "C (10, 17). Modification of the Source. The EI/FI/FD source must be modified to accommodate the optimal displacement (4 mm) of the heating current contact as shown in Figure 1. This was accomplished by readjusting its position so that firm contact is made when the emitter is in the proper position for EI/D. After this adjustment, whenever the push rod is in position for the normal FD mode, the contact becomes slightly strained; however, no ill effects have been observed. The stop for the push rod movement for the normal FD mode was marked on the outer housing, as shown in Figure 2. A hole was drilled and tapped into the top side of this housing and a screw inserted, with all of the dimensions being selected so that the outer circumference of the screw stops the movement of the push rod 4 mm short of ita normal position. This dimension is critical since it is desirable for the emitter to be as close as possible to the path of the electron ionization beam for maximizing sensitivity and minimizing ion source focusing adjustments. It is essential, however, that the beam not impinge upon the sorbed sample on the emitter itself. Since the insertion or removal of this set screw is the only physical change needed to switch between the FD and EI/D mode, the changeover takes less than 30 s. An additional modification must be made to the electrical circuit of the CH-5DF so that the emitter current power supply will operate when the selector switch is in the position for electron impact. The exact nature of this modification will differ from instrument to instrument; in general, it consists of rewiring the
FILAMENT
CURRENT CONTACT
-
Table I. EI/D Analyses of Sodium Salts of the Halogens
F'LAMENTl I I
I1 I
CURRENT CONTACT
-
TRAP
TRAP
EI/O
FO
Placement of the emitter for field desorption analysis (FD) and for electron impact analysis (EI/D) Flgure 1.
,,
FV VALVE
SET SCREW
BET Melting Boiling mA T , "C point, "Ca point, O c a 60 1150 988 1695 51 920 801 1413 755 1390 43 750 651 1304 38 600 of 1 atm, taken from "CRC HandPhysics", 51st ed.
Current Compound scan, mA NaF 20-70 NaCl 20-64 NaBr 20-60 NaI 20-64 a Values at a pressure book of Chemistry and
r4-l
I O N SOURCE FLANGE
STOP EI/D STOP F 0
t =
U V VALVE
Modification of the Varian combination ion source housing for EI/D mass spectrometry
Figure 2.
m/e
Spectra of sodium salts of the halogens. Illustrated mass spectrum for each compound Is that obtained at its BET Figure 4.
c 50
:0C
15C
250
200
300
350
400
z
"/e
5
10
5
10
15
The mass spectrum of cholesterol (l/gg/FL-dipping method) by EI/D. (6) The mass spectrum of cholesterol (llbg) by direct probe heated inlet Figure 3. (A)
contacts on the function switch to prevent deactivation of the emitter current power supply when the switch is set to the E1 mode. Although this paper describes the minor modification of a Varian EI/FI/FD source, this technique can be applied to any mass spectrometer whose ion source housing can be modified to accommodate the activated emitter and in which some provision for the heating current conductors to pass through the vacuum seal can be made. The electrical current passing through the emitter can be regulated by any well isolated controlled current device similar to the ECP. Sample Application. Sample preparation is identical to that used in FD methods (18-20). Application of the sample to the activated emitter is done by the dipping technique or by applying a drop of solution t o the emitter with a microsyringe. For the most accurate and reproducible studies, the microsyringe method is preferred. Direct Probe Inlet. Solid samples were analyzed with the direct probe inlet of the Varian CH-5 DF. The temperature for these analyses was regulated and programmed by the standard provisions of this attachment.
RESULTS AND DISCUSSION The E1 and EI/D spectra of cholesterol are compared in Figure 3. Both spectra are typical of electron impact-induced ionization and fragmentation of this molecule with the exception that the relative intensities of the M+ and (M 1)' ions are greater in the EI/D spectrum. This may be evidence in support of the proposal by Friedman and co-workers that
+
.5
.
2C
20
25
30
25
30
I
SCAN NUMBER Flgure 5. Temperature resolved thermal desorption of sodium salts of halogens from the activated emitter during ECP scanning
vaporization from a thin film on nonreactive surfaces is accompanied by less fragmentation than vaporization from larger masses of compound (21,22). The heating rates for the two methods are significantly different (6.19 mA/min for EI/D and 35 OC/min for EI); however, the temperature at the maximum thermal desorption (BET) is the same as that with conventional direct probe analysis, 110 "C. With similar amounts of sample, the sensitivity of the EI/D method is approximately twice that of the EI. Under the usual conditions of analysis, this would imply that EI/D is approximately 10 times as sensitive as FD. This approximation is in the range of the comparisons that have been observed. The higher temperatures that can be attained when using the activated emitter (up to 1800 "C)allow the analyses of many compounds that could not otherwise be obtained by direct probe E1 methods. Table I illustrates the results of EI/D analyses of sodium salts of several of the halogens. The mass spectra of these inorganic salts, each taken at its BET, are shown in Figure 4. The spectra are very similar to those ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1165
obtained when these compounds are volatilized from a Knudsen Cell (23). As a confirmatory test, m / e 81 of the NaCl analysis was peak matched against a perfluorokerosene ion at m / e 81 (80.995215);in the EI/D mode, the peak was found to be 80.94872, which is 0.32 mmass from the theoretical value for (Na2C1)+. Since only positive ions are detected, the ease with which the monohalogen positive ions are formed follows the sequence of the electronegativity of this series. Figure 5 illustrates the scan (temperature) dependent thermal desorption processes for these salts. These outputs, used to determine BET values, are generated in a manner analogous to the time-dependent mass chromatograms of GC-MS (24). The EI/D method, using sample deployment on an activated emitter and thermal desorption prior to electron impact ionization, was found to be a reliable method for the introduction of solid samples into the mass spectrometer. This new method of analysis requires smaller amounts of sample than the conventional direct probe technique. The efficiency of heating and the high temperatures attainable enable this technique to be used to analyze samples that cannot be done by conventional solid sample devices. The volatilization processes appear more uniform in the EI/D mode, a consequence quite naturally of the more precise and efficient thermal parameters attainable when heating a thin film of the sample on the emitter wire with electronic control of temperature. As a result, the uncertainty of the moment of sample desorption is significantly reduced when repeated analyses of similar samples are performed. These features have encouraged the use of EI/D for the analysis of solid compounds in all cases where solubilization can be effected and the emitter adequately charged with the sample. When combined with an on-line data system running in a cyclic scanning mode, mass chromatography and automatic calculation of qualitative and
quantitative parameters can be combined with the EI/D technique.
LITERATURE CITED (1) C. M. Stevens, Rev. Sci. Instrum., 24, 148 (1953). (2) V. J. Caldecourt, Anal. Chem., 27, 1670 (1955). (3) R. H. Roberts and J. V . Walsh, Rev. Sci. Instrum., 26, 890 (1955). (4) P. De. Mayo and R. I. Reed, Chem. Ind. (London), 1481 (1956). (5) M. W. Echo and T. D. Morgan, Anal. Chem., 29, 1593 (1957). (6) Lab guide, Anal. Chem., 48 (lo),101 (1976). (7) R. A. Hites and K. Biemann, Anal. Chem., 39, 965 (1967). (8) C. C. Sweeley, B. D. Ray, W. I.Wood, J. F. Holland, and M. I. Kirchevsky, Anal. Chem., 42, 1505 (1970). (9) H. D. Beckey, J . Mass Spectrom. Ion Phys., 2, 500 (1969). (10)B. Sotmann, C. C. Sweeley, and J. F. Holland, Int. J . Mass Spectrom. Ion Phys., submltted for publlcation.
(11) J. F. Holland, B. Soltmann, and C. C. Sweeley, Biomed. Mass Spectrom., 3, 340 (1976). (12) D. F. Hunt, J. Shabanowitz, F. K. Botz, and D. A. Brent, Anal. Chem., in press.
(13) J. W. Maine, B. Soltmann, J. F. Holland, N. D. Young, J. N. Gerber, and C. C. Sweeley, Anal. Chem., 48, 427 (1976). (14) H. D. Beckey, A. Heindricks, E. Hilt, M. D. Migahed, H. R. Schulten, and H. U. Winkler, Messtecknlk(Braunschwe@), 79, 196 (1971). (15) H. D. Beckey, S. Bloching, M. D. Migahed, E. Ochterbeck, and H. R. Schulten, Int. J . Mass Spectrom. Ion Phys., 8, 169 (1972). (16) H. R. Schulten and H. D. Beckey, Org. Mass Spectrom., 6, 885 (1972). (17) D. Kunimler and H. R. Schulten, Org. Mass Spectrom., I O , 813 (1975). (18) H. D. Beckey, A. Heindricks, and H. U. Winkler, Int. J . Mass Spectrom. Ion Phys., 3 , 9 (1970). (19) H. R. Schulten, H D. Beckey, A. J. H. Boerboom, and H. I.C. Merizeloar, Anal. Chem., 45, 2358 (1973). (20) K. L. Olson, J. C. Cook, and K. L. Rinehart, Biomed. Mass Spectrom., 1, 358 (1974). (21) R. J. Beuhler, E. Flanigan, L. J. (;reen& and L. Friedman, J . Am. Chem. Soc., 96, 3990 (1974). (22) R. J. Beuhier, E. Flanigan, L. J. Greene, and L. Friedman, Biochemistry, 13, 5060 (1974). (23) R. F. Porter, W. A . Chupka, and M. G. Inghram, J . Chem. Phys., 23, 1347 (1955). (24) R. A. Hites and K. Biemann, Anal. Chem., 42, 855 (1970).
RECEIVED for review February 24, 1977. Accepted April 15, 1977.
Determination of Polyester Prepolymer Oligomers by High Performance Liquid Chromatography Louis M. Zaborsky I1 Process and Product Development, Firestone Synthetic Fibers Company, P. 0. Box 450, Hope well, Virghla 23860
A hlgh performance llquld chromatographic (HPLC) procedure for the determlnatlon of poly(ethy1ene terephthalate) prepolymer oligomers containlng from one to seven terephthalolyl repeat unlts is descrlbed. Adsorption chromatography uslng a Du Pont Zorbax-SIL column wlth chloroform/alcohol eluent (1OO:l by volume) gave the best results. The bls(2hydroxyethy1)terephthalate content determlned by gas chromatographic analysis Is used as the known standard for the HPLC separatlon. Analytical data for two commercial polyester prepolymer samples are included; preclslon and accuracy are within f1.0% for each compound.
The initial reaction in the production of polyethylene terephthalate (PET) is the esterification of terephthalic acid (TA) with ethylene glycol (EG). The actual esterification product consists of bis(2-hydroxyethyl)terephthalate (BHET), the desired compound, unreacted EG and TA, mono(21166
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
hydroxyethyl) terephthalate (MHET), as well as higher molecular weight oligomeric polyesters. The structures of these compounds are given in appendix A. Analytical methods for determining EG, TA, MHET, and BHET by silylation and gas chromatography (GC) have been described previously (I, 2). However, this procedure is capable of determining only -25% of the total prepolymer sample, as the higher molecular weight oligomers are not resolved by GC. Thin-layer chromatography (TLC) methods have been described for characterizing these high molecular weight oligomers (3, 4). However, quantitation of TLC chromatograms is difficult and hard to reproduce. Steric exclusion chromatography (GPC) has been used to determine monomer (BHET), dimer, and trimer in prepolymer samples with rather poor resolution (5). This paper describes a high performance liquid chromatographic (HPLC) procedure capable of quantitatively determining glycol-ended prepolymer oligomers containing from one to seven terephthaloyl repeat units. Quantitation is
