whether a = lOjO.1 = 100 or CY = 1000/10 = 100. Without being called a separation factor, CY can be used to give as much or as little information as before. If a cannot honestly be designated a separation factor (and not separation index either, which still promises t o o much), what shall it be called? Let it be called what it is, the quotient of distribution coefficients or simply the distribution coefficient quotient. The practice of designating CY the separation factor originated many years ago when this term was first incautiously applied in liquidliquid extraction. I t seems reasonable to conclude that in a system of sound terminology, separation factor should be the designation of a
quantity that will allow separability to be formulated and t o be calculated when the necessary experimental variables are specified. If this postulate is accepted, separation factor should be used t o designate R B I R A , not D A I D B .
E. B. SANDELL School of Chemistry University of Minnesota Minneapolis, Minn. RECEIVED for review November 2, 1967. Accepted January 19,1968.
Elemental Composition Determination by Field-Ion Mass Spectrometry SIR: The unambiguous determination of the elemental composition of a compound is possible if the mass of the molecular ion can be measured with sufficient accuracy ( I ) . A major limitation of the use of electron-impact mass spectrometry for these valuable measurements is that such spectra often show no measurable molecular ion. Field ionization (2-5), whose development is due especially t o Beckey, is reported t o produce relatively abundant molecular ions for all compounds examined (3). N o general study of the application of exact-mass measurement techniques t o field-ion mass spectra has been described; indeed, relatively few low resolution spectra have been reported for nonhydrocarbon compounds (2-5), except for amines (6) and alcohols (7). I n early reports difficulties with low sensitivity, discharges at high voltages, time-consuming replacement of fragile emitters, and limited resolution (8) were discouraging for attempts of exact mass measurements; only one report (9) of this has come t o our attention. Mead and coworkers in a study of sulfur-containing hydrocarbon mixtures report resolution and massmeasuring accuracies well below the performance of their instrument (AEI MS-9) with electron-impact ionization (9). Field ionization has been achieved o n a high-resolution mass spectrometer of lower performance (10). Two recently developed instruments (11, 12), o n the basis of the excellent resolving powers reported, should be capable of mass measurements of accuracies similar t o those reported here. We (1) J. H. Benyon, “Mass Spectrometry and Its Applications to
Organic Chemistry,” Elsevier, Amsterdam, 1960. (2) H.-D. Beckey, H. Knoppel, G. Metzinger, and P. Schulze, “Advances in Mass Spectrometry,” Vol. 3, W. L. Mead, Ed.. The Institute of Petroleum, London, 1966, p. 35, and references
cited therein. (3) H.-D. Beckey, H. Heising, H. Hey, and H. G. Metzinger,
Preprints, International Mass Spectrometry Conference, Berlin, Sept. 1967. (4) G. G. Wanless and G. A. Glock, ANAL.CHEM., 39, 2 (1967). (5) C . Brunnke, Narurforsch. 226, 121 (1967). (6) H.-D. Beckey and G. Wagner, Naturwiss., 2Oa, 169 (1965). (7) H.-D. Beckey and P. Schulze, Ibid.,2la, 214 (1966). (8) C. Brunee, ASTM E-14 Conference on Mass Spectrometry, Dallas, Texas, May, 1966. (9) W. L. Mead, A. R. West, J. S. Halliday, A. Riddoch, and H. Howarth, Ibid.,Denver, Colo., May, 1967. (10) J. H. Beynon, A. E. Fontaine, and B. E. Job, 2.Naturforschg., 21a, 776 (1966). (11) C. Brunnke, G. Kappus, and K.-H. Maurer, 2.Anal. Chem., 232, 17 (1967). (12) H.-D. Beckey, Univ. of Bonn, personal communications, 1967.
report here that accurate mass measurement of field ions of a wide variety of compounds can be made relatively simply in conjunction with such measurements of the electron-impact spectrum. The standard electron-impact ion source of a CEC 21-llOB mass spectrometer was modified (Figure 1) so that a razor blade anode (2, 13) 4.5 mm wide can be inserted between the repellers, with insulation so that a potential of 8-10 kV can be applied between the blade and the ion exit slit (distance 0.25 mm). Detailed drawings will be supplied o n request. The blade can be adjusted externally for distance and angle and removed through a vacuum lock without venting the instrument. The fragile emitter can thus be replaced in a few minutes, a distinct advantage over previous sources. All other operating parameters of the instrument are similar t o those for electron impact; resolution (10 valley definition) of 32,000 by electron impact and 20,000 by field ionization is obtainable. Conversion from field-ion t o electron-impact operation can be achieved in two minutes without interrupting the volatilization of a sample directly in the ion source. Sensitivities are highly dependent on blade condition, but with blade selection the total ionization monitored for most compounds (ca. lo-’ amps./torr) is within an order of magnitude of that observed using a 100 pamp electron current, This sensitivity is >10 times higher than those previously reported (11, 12); however, the reproducibility of our source for quantitative analysis appears t o be much poorer than others ( 2 , 4, 12). The use of photoplate recording obviates most difficulties in exact mass measurements caused by ion current fluctuations. The field-ion spectra of more than 50 different compounds of many types have been obtained t o date; most show abundant molecular ions even when none is observed in the electron-impact spectrum. I n contrast t o the spectra of all other compounds reported (3), CC1, shows a negligible (
