Perfluorodecalin as a Mass Calibration Standard for Low Resolution Mass Spectra B. S . Middleditch Chemistry Deparfment, University of Glasgow, Glasgow W.2, Scotland
INlow-resolution mass spectrometry, the integral mass of each peak has to be determined beyond doubt. Several automatic mass markers are commercially available, but each is designed for use only with a particular type of instrument. A more widely applicable method of mass calibration invoIves the simultaneous introduction of a reference standard into the spectrometer. The most commonly used reference standards are the perfluoroalkanes ( I ) . However, there is a predominance of ions of low mass in their spectra, and a relatively high partial pressure of standard must be employed if the peaks of higher m/e are to be used for mass calibration. The detector, amplifier, and recorder will then be overloaded at the iow mass end of the spectrum. Other compounds such as heptacosafluorotri-n-butylamine and perfluoroalkyl-s-triazines have been used (2-4) but these suffer from the same disadvantage: the perfluoroalkyl substituents readily undergo cleavage to form fragment ions of low mass. In a search for more suitable markers, the mass spectrum of a bicyclic fluorocarbon, commercial perfluorodecalin, was examined (Table I). Although the ion of m/e 69, which may be ascribed to CF3+,is the base peak, its relative contribution to the total ionization is much less than for the standards previously employed because it cannot be produced by simple fragmentation. The mass range extends conveniently to m/e 462 (molecular ion), and peaks are well distributed throughout the spectrum. At high concentrations a peak is observed at m/e 481, due to an ion-molecule reaction in the ion source. (1) J. H. Beynon, “Mass Spectrometry and its Applications to Organic Chemistry,” Elsevier Publishing Co., Amsterdam, 1960, p 414. (2) F. A. J. M. Leemans and 3. A. McCIoskey, J . Amer. Oil Chem. Soc., 44, 11 (1968). (3) T. Aczel, ANAL.CHEM.,4Q, 1917 (1968). (4) R. H. Wallick, 6.L. Peele, and J. B. Hynes, ibrd.,46, 388 ( 1969).
Several metastable peaks are present, and these are an aid to the rapid location of fragment peaks. Perfluorodecalin is a liquid with sufficient volatility at room temperature to obviate the need for a heated inlet system; it can be introduced from a cold reservoir fitted with a suitable valve and pumping fine. I t is chemically inert and can be pumped away from the spectrometer within seconds. Perfluorodecalin is accordingly a suitable calibration standard for use in mass spectrometry, particularly with low resolution instruments which are extensively used for the mass range m/e 1-500. Perfluorodecalin has been used in this department and elsewhere for several years as a reference for accurate mass measurement by the peak matching method, its advantages in this respect being essentially as outlined above (5).
EXPERIMENTAL The perfluorodecalin was obtained from Ralph Emanuel Ltd., Middlesex, England. The mass spectriim was obtained using an LKB 9000 mass spectrometer under the following conditions: electron energy, 70 eV; ion source temperature, 270 OC; accelerating voltage, 3.5 kV. The sample was introduced from a cold glass reservoir fitted in place of the gas chromatograph column. ACKNOWLEDGMENT The author is indebted to C. J. W. Brooks for encouragement. RECEIVED for review August 8, 1969. Accepted September 5 , 1969. Work partially supported by SRC Grant No. B/SR/ 2398 to C.J. W. Brooks and G.Eglinton. (5) E. Clayton, I.C.I. Mond Division, Runcorn, Cheshire, England, and A. McCormick, A.W.R.E., Aldermaston, Berkshire, England, personal communication, 1969.
Table I. Mass Spectrum of Perfluorodecalin mle 48 1 463 462 444 443 413 412 394 393 381 374 362 356 355 344 343 331 324
2092
Abund. 2 1 13 10 88 2 14 10 98 1 10 3 4 40 7 68 6 1
rn/e 306 305 295 294 293 286 281 274 267 262 256 255 244 243 236 232 231 224
Abund. 2 22 1 39 465 3 7 4 7 5 2 34 20 280 11 1 24 10
m/e 219 217 212 206 205 200 198 194 193 187 186 182 181 179 174 170 169 167
Abund. 7 11 15 4 55 4 1 5 82 1 18 5 1OD 3 11 1 41 65
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
m/e 163 162 156 15.5 150 148 144 143 136 132 131 129 125 124 120 119 118 117
Abund. 3 58 3 57
8 3 4 75 5 33 950 1 1 35 3 12 1 22
m/e 113 112 105 101 100 98 94 93 86 81 79 74 70 69 62 55 50 31
Abund.
1 31 10 3 139 2 4 122 2 8 1 10 12 1000 2 2 11 42
