Field Desorption Mass Spectrometry of Onium Salts Martin C. Sammons, Maurice M. Bursey,' and C. Kenneth White Venable and Kenan Chemical Laboratories, The University of North Carolina, Chapel Hill, NC 275 14
Field desorption mass spectrometry, (FDMS), in which a sample coated on an emitter anode is ionized in a strong electric field and the ion then desorbed by coulombic repulsion between it and the electrode, has been applied to the analysis of inorganic (1,2) and organic salts (3-9). The organic salts investigated so far have been quaternary ammonium salts, and these compounds have been found to give relatively simple spectra in which the cation is the base peak. So few examples have been reported that we wished to extend our earlier studies to demonstrate the practical utilization of this technique for a variety of more or less fragile organic salts, including compounds in which various other atoms such as phosphorus, arsenic, sulfur, and oxygen carry the positive charge. We report the results of this survey here. Three ammonium, six phosphonium, two arsonium, one sulfonium, one sulfoxonium, and two oxonium salts were studied. Their spectra are almost all very straightforward, and in only one case was the cation not the most intense ion. In some instances, it is only the ion in the spectrum. I t is obvious that this technique will be of use in determining cation masses in this type of compound, and the fragmentation observed is in some cases useful for partial structural assignment.
EXPERIMENTAL The instrument used for these studies is an MS-702 mass spectrometer with a source which is similar to that used by Beynon for field ionization mass spectrometry (10). The field desorption emitters were either the high-temperature carbon emitters described by Beckey (11) or the new nickel emitters (12). Cathode voltages were 10 kV with respect to the grounded counter electrode plate. Samples were loaded from aqueous solutions using the emitter dipping technique (13). Several of the phosphonium salts studied were prepared in this laboratory using standard procedures (14, 15). The remainder of the samples were from commercial suppliers. Detection of ions was on Ilford Q-2 photoplates using standard development techniques. The intensities were calibrated using a modified Churchill two-line method (16, 17). The mass range of the instrument under our conditions was about 525. Only ions having an intensity of 5% or greater are mentioned in this paper unless they are part of an isotopic cluster of importance.
RESULTS AND CONCLUSIONS The three ammonium salts tested were tetraethylammonium borohydride, tetra-n- butylammonium iodide, and phenyltrimethylammonium iodide, adsorbed from aqueous solutions. Both carbon and nickel emitters were used for the first salt, only nickel emitters for the last two. At all heating currents between 17 and 30 mA only the cation (C) of the salt and the C 1 peak had an intensity greater than 5% (Table I). No ions were not.ed corresponding to the sum of the masses of the cation and the anion, or of two cations and an anion, as was noted t,o occur previously in some quaternary ammonium salts ( 3 ) . The six phosphonium salts for which spectra were obtained were methyltriphenylphosphonium bromide, ethyltriphenylphosphonium bromide, n-propyltriphenylphosphonium bromide, n- butyltriphenylphosphonium bromide,
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phenyltrimethylphosphonium picrate, and 3,3,5,5-tetraphenyl-3,5-diphosphonia-l-oxacyclohexanedibromide, I. The first five compounds gave simple spectra consisting
fOl H
I
only of the cation and its isotope peak. The sixth, a dication, gave an unusual spectrum in which the dominant species was the C-1 peak, which probably arises by abstraction of the 2-hydrogen by bromide, producing a stable ylide:
+
2Br2C H g )P (*,~,?p2C H g )(
fOl- co)
+ HBr + B r -
(CgHg)2Pt,_,tP(CgH5)2
C H
C H
+
The other peak in this spectrum is a C 17 peak, which may be the hydrated C - 1 peak. Considering the position of the positive charges, we thought it possible that we might find a peak consisting of the dication plus one anion, [C2+ A-1, as seen in other diphosphonium salts (18), but this was not observed. All of these spectra were obtained on nickel emitters, which had been dipped into aqueous solutions. The arsonium salts examined were tetraphenylarsonium chloride and methyltriphenylarsonium iodide. The spectra of these salts adsorbed onto carbon emitters from aqueous solutions were similar in nature in many respects to those reported for other kinds of onium salts. The former compound has the cation again as its base peak. There was evidence that the sample was contaminated with a very small amount of triphenylarsenic, either originally or by pyrolysis, because the cation peak became stronger relative to the molecular ion of this material (mle 306) as the emitter was heated. Spectra were taken at several heating current intervals from 1 2 to 35 mA. Also, a total spectrum was taken from 0-45 mA by slowly increasing the heating current while remaining on one photoplate position. The peaks a t mle 112 and 114 correspond to chlorobenzene, an expected pyrolysis product. Very small peaks (
