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Anal. Chem. 1981, 53,1306-1307
Thermally Produced Ions in Desorption Mass Spectrometry Sir: In 1976, Holland, Soltmann, and Sweeley (1)published a model for ionization mechanisms in field desorption mass spectrometry (FDMS) which suggested that ions could be formed from a field-independent thermal desorption process. The experimental evidence used to support this model was not completely unambiguous, since the field was required to actually observe the ions. This ambiguity prompted Beckey and Rollgen (2)to counter Holland et al. with another publication and an experiment designed to prove that no field independent ion emission could exist in the temperature ranges used in field desorption. The debate was carried further in two letters to the editor of Organic Muss Spectrometry ( 3 , 4 ) . In order to provide more information, and perhaps to settle the question, we have designed an experiment in which cationized molecules and stable cations from nonvolatile organic salts can be observed without fields of any kind or without any other means of ionization, such as electron beams or reagent gas ions. In other words, ions, and not neutrals, are desorbed by a process which is entirely thermal. We do not conclude from this that the field plays no role in field desorption, but only that among the mechanisms operating in that technique one must also consider the existence of one in which the process is governed almost entirely by the temperature. Since the introduction of field desorption by Beckey in 1969 (5),there have actually been two broad mechanistic questions which have been asked. The first is whether or not field desorption proceeds by the electron tunneling type of mechanism advance for field ionization. This question is prompted by the observation that, while field ionization generally produces the radical ion M+., FD spectra also exhibit the more stable even electron ions MH+, (M Na)+, or stable organic cations from salts, such as quaternary ammonium halides (6-9). While molecular ions are often still observed in field desorption, the existence of the more easily formed ions has led to experiments in which the electric field strengths are relaxed somewhat, in fact below that predicted to be required for electron tunneling. Rollgen and Schulten, for example, demonstrated that (M + Li)+ ions of 1-hexanol could be formed a t field strengths below those necessary for the formation of M+. ions (7). They also demonstrated that cationized species could be observed by using untreated tungsten wires in place of the activated emitters (another form of reducing the field strength) and produced a spectrum of Dglucose in which the (M + Li)+ ion was the base peak (8). Rollgen and co-workers also extended this method to quaternary ammonium salts, resulting in a field desorption spectrum of tetramethylammonium iodide with peaks corresponding to (CH3)4N+and [(CH3)4NI+ (CH3),N]+, using untreated 10-bm tungsten wire (9). The second question is the one addressed by Holland et al. (I) and asks whether the field is necessary at all. The question was raised by their observation that the best emitter temperature (BET) in chemical ionization/desorption methods (CI/D), which used activated emitters, was so very similar to the best anode temperature (BAT) for the same compound in field desorption (10). The experiment they designed involves the observation of the sample desorption profile as the emitter current is scanned across the best anode temperature (1). The high voltage (which produces the field) is turned on and off and apparently has little effect on the rate of desorption of the sample. The ambiguity occurs because the ions can be detected only during the portions of the profile that the high voltage is actually on. While this suggests a
+
process of ion formation and extraction in which there is a completely thermal step, it does not definitively rule out the need for the field in the formation of ions observed in field desorption. In order to resolve this ambiguity, we describe below an experiment in which the samples are thermally desorbed in an electron impact (E11 source, with the electron beam turned off.
EXPERIMENTAL SECTION This work was done by using a Finnigan 4021 quadrupole mass spectrometer and a probe specially designed for thermal ionization. An important aspect of this instrument is that the probe inlet is coaxial with the ion beam exit aperture. The probe itself is described elsewhere in detail (11). The samples were dissolved in water and deposited from solution to cover the entire heated probe tip. The solvent was then evaporated by exposure to the heat from an infrared lamp for a period of 10 min prior to insertion of the probe into the ion source. The glucose spectrum was obtained by using a 1:l mixture of glucose and NaI. The sample was desorbed by passing a current of 1-4 A through the rhenium heater, which was biased between +5 and +15 V with respect to the ion source block. This voltage improved the efficiency of extraction of ions from the source but had little effect on the relative abundance of the ions produced; these could be observed even when the heater was at the same potential as the block. Mass spectra were acquired every 2 s using a Finnigan/INCOS data system, so that the time and temperature dependences could be followed and compared with the desorption of Na+ and K+ which are ubiquitous. Generally spectra could be observed for several minutes. The spectra reported are plotted from single scans, in which the major peaks have absolute intensities in excess of lo6 ions. RESULTS AND DISCUSSION Figure 1 shows the thermal desorption spectrum of glucose (mol wt = 180). The ions observed are cationized species, and Na+ and K+ ions are clearly visible. In addition to the molecular ion species (M + Na)’, peaks corresponding to ions formed by the loss of one, two, or three H20 make up the spectrum. Figure 2 is the thermal desorption spectrum of tetra-nbutylammonium chloride. The “molecular cation” is observed as the base peak. In the electron impact spectra of such compounds, the molecular cation is rarely seen. Instead, the major ions observed result from losses of HX, RX, etc. by thermal decomposition of the bulk sample to produce volatile tertiary amines, which are subsequently ionized by the electron beam. The E1 spectra are then characterized by radical cations of odd mass (12). In contrast, without the electron beam, these radical cations are not observed, and the thermal desorption spectrum is characterized by even mass ions. These most probably arise from a thermal decomposition process which produces product cations on the surface which retain the tetravalently bonded nitrogen and are desorbed intact as ions (13).
Rollgen has also recently reported thermally desorbed tetraalkylammonium ions but observed only the molecular cations for the parent molecule (14). Indeed, we have found that the relative abundances of the decomposition ions can be controlled, and even eliminated, by using lower heating currents. However, we report here a spectrum containing such ions, because of their interesting correspondence with even mass ions observed in other desorption methods. While the production of cationized species and stable molecular cations by thermal desorption suggests that a similar
0003-2700/81/0353-1306$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 8,JULY 1981
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even electron (nonradical) ions; and while the authors argue for an electric perturbation mechanism operating in the fission fragmentation technique, the similarity between that spectrum and the thermal desorption results cannot be overlooked. Again, as in the case of field desorption, we do not suppose that only thermal mechanisms are operative. While cationized species are notable in all of these desorption methods, M+. and MH+ ions are observed in field desorption, radical ions are observed by using lasers (generally at higher power densities), and (M 1)+and (M - 1)-ions are observed by using fission fragmentation induced desorption (20). However, the role of thermal desorption in these methods cannot be ignored, and we expect that further investigation into this mechanism will clarify our understanding of field desorption, laser desorption, and fission fragmentation induced desorption.
+
m /e
Thermal desorption mass spectrum of glucose In a 1:l mixture with NaCI. Figure 1.
LITERATURE CITED
20
40
60
80
IW
I20
140
160
I80
200
220
240
m /e
Figure 2. Thermal desorption mass spectrum of tetra-n-butyiammonium chloride.
mechanism may be operative in field desorption, the absence of radical ions and protonated species in our spectra indicates that it cannot be the only mechanism. We suspect that the field plays a very important role in the formation of these other types of ions. These results can be extended to other desorption methods as well. We have noted that the laser desorption of glycocholic acid (using a COz TEA pulsed laser with a power density of lo6 W/cm and a pulse width of 40 ns) produces only neutral and cationized species and that no M+- or MH+ ions are observed (15,161.Besides producing spectra characteristic of a purely thermal process it is also interesting to note that the laser powers for production of cationized species and neutrals are similar. This may eplain the observation in the experiment by Holland et al. (1) that the sample continues to be desorbed from the emitter during periods in which the field is off. This predominance of cationized species in laser desorption was reported earlier by Kistemaker et al. (17)and by Stoll and Rollgen (18). In addition, Stoll and Rollgen, using a continuous wave COz laser, produced a spectrum of tetra-nbutylammonium iodide, which exhibits the same even mass ions observed in our thermal desorption experiment. An interesting paper by Schueler and Krueger (19) compares spectra of tetrabutylammonium iodide produced by fission fragmentation induced desorption (FFID), laser induced desorption (LID), secondary ion mass spectrometry (SIMS), and field desorption. The spectra all show a strong resemblance to the thermal desorption spectra in their predominance of
(1) Hollend, J. F.; Soitmann, 6.; Sweeley, C. C. Biomed. Mass Spectrom. 1976, 3 , 340. (2) Beckey, H. D.: Rollgen, F. W. Org. Mass Spectrom. 1979, 14, 188. (3) Holland, J. F. Org. Mass Spectrom. 1979, 14, 291. (4) Beckey, H. D. Org. Mass Spectrom. 1970, 14, 292. (5) Beckey, H. D. Int. J . Mass Spectrom. Ion Pbys. 1969, 2 , 500. (6) Schuken, H.R.; Rollgen, F. W. Org. Mass Spectrom. 1075, 10, 649. (7) Rollgen, F. W.; Schuiten, H A . Org. Mass Spectrom. 1075, 10, 660. (8) R6llgen, F. W.; Schulten, H.-R. Z . Naturforsch. A 1975, 30A, 1685. (9) Rollgen, F. W.; Geissman, U.; Helnen, H. J.; Reddy, S. J. Int. J. Mass Spectrom. Ion Phys. 1977, 2 4 , 235. (IO) Sokmann, B.; Sweeley, C. C.; Holland, J. R. Anal. Chem. 1977, 49, 1164. (11) Yergey, A. L.; Vieira, N. E.; Hansen, J. W. Anal. Cham. 1080, 52, 1811. (12) Budzikiewicz, H.; DJerassi,C.; Wllllams, D. H. "Mass Spectrometry of Organic Compounds"; Holden-Day: San Francisco, CA, 1967; p 330. (13) Cotter, R. J.; Yergey, A. L. J. Am. Chem. Soc. 1981, 103, 1596. (14) Stoll, R.; Rollgen, F. W. J . Chem. Soc., Chem. Commun. 1980, 789. (15) Cotter, R. J. Anal. Chem. 1981, 53, 719. (16) Cotter, R. J. Paper presented at the 180th Natlonal Meeting of the American Chemical Soclety, Las Vegas, NV, Aug 1980. (17) Posthumus, M. A.; Kistemaker, P. G.; Meuzelaar, H. C. C.; Noever de Brauw, M. C. Ten Anal. Chem. 1970, 50, 985. (18) Stoll, R.; Rollgen, F. W. Org. Mass Spectrom. 1970, 14, 642. (19) Schueler, 6.; Krueger, F. R. Org. Mass Spectrom. 1079, 74, 439. (20) Becker, 0.; FOrstenau, N.; Knlppelber, W.; Krueger, F. R. Org. Mass Spectrom. 1977, 12, 461.
Robert J. Cotter* Middle Atlantic Mass Spectrometry Facility Department of Pharmacology and Experimental Therapeutics The Johns Hopkins University Baltimore, Maryland 21205 Alfred L. Yergey OSD, IRP National Institute of Child Health and Human Development National Institutes of Health Bethesda, Maryland 20205 RECEIVED for review January 29, 1981. Accepted April 21, 1981. This work was funded in part by grants from the National Institutes of Health, GM-21248, and the National Science Foundation, CHE-7818396, and was conducted at the National Institutes of Health facility in Bethesda, MD.
