Doubly-charged ions in the field mass spectra of some organic compounds

International Journal of Mass Spec:rometry and Ion Pizysks Ekevier Publishing Company, Amsterdam - Printed in the Netherlands

Doub~yxharged

92

ions in the field mass spectra of some organic compounds

In the field mass spectrum of naphthalene observed by means of the instrument described by Korostishevsky and Goldenfeld’ there was found a group of peaks near mass number 64, half the molecular mass of naphthalene. Their intensities depended only on the naphthalene vapour pressure and field strength in the whole interval of the temperatures used (300-1400°K). The results were identical when the platinum emitter was replaced by a gold one. These facts allow LIS to consider that these pe&s are related to naphthalene and to reject the idea that their origin is connected with the catalytic processes on the surface of the emitter (see Fig. 1 j. a

c

~’

A..] !

+‘2 mu3

as

/c

130

EC,

E3

Fig 1. h’aphthalene mass-sqecsa. (a) Naphtialene on a platinum emitter; (b) Partially-deuterated naphthalene on a platinum emitter; (c) PartialIy-deuterated naphthalene on a gold emitter.

Thorough study of this part of the naphthalene mass spectrum showed that: a) if the field is sufficiently smali the part of the mass spectrum under investigation is identical to that of the molecular naphthalene ions and consists of “isotopic peaks” with intensities correspondin= to the naturaI abundances of carbon and hydrogen; b) the mass number of each of these ions is equal to half the mass number of the isotopic species of the naphthafene molecule within an accuracy kO.05 m. u. (The mass numbers were established by measuring the magnetic field with the help of nuclear magnetic resonance). The above-mentioned facts ahow us to state that the observed peaks are 3. Mass Sjxczrozneny and Ion Phy.rfcs, 2 (1969) 92-94

SHORT a V=3kV

93

COMMUNICATIONS

b V=4kV

C V=!ikV

< V=s11k’nI

1 ;; fd-

.L1 128

128

r\

128

128

Fig. 2. The infltience of the ionizing fieId on the shape of naphthalene peaks.

caused by doubly-charsed molecuIar ions of naphthalene. Such identification was confirmed by the analysis of naphthalene partially deuteriated in the a-position as shown in Fig. I_ In Fig. 3, field mass spectra of naphthalene are shown at different potentials applied to the emitter. With increase of applied voltage, the peak of singly-charged ions widens on the low-mass side (2b); then in the diffuse part of the peak a second maximum is formed (Zc) and eventually the maximum shifts to Iower mass forming a separate peak (2d). Thus the complete effect is the division of the peak into two parts, one having a sharp outline and constant location on the mass scale and the other a diiuse shape, the location of its maximum deFendinS on the electric field strength. As to the peaks of doubly-charged ions, their shape and location on the mass s&e remain unchanged at all available voltages on the emitter. The widening of a peak in a felt mass spectrum is caused by ionization in the space away from the emitter, for ions formed at some distance from the emitter surface possess smaller energy than these formed on the surface. Therefore the locaTion and shape of a peak in a field mass spectrum contains information about the place of ionization. For our maximal field values (10’ V cm-l) the distance between the “space” and the “surface” peaks corresponds to a distance of about 100 A from the surface. It follows that singly-charged ions of naphthaIene may arise both in the space and at the surface, whereas doubly-charged ones are formed exclusively at the surface Besides naphthalene, doubly
Spectrometry

and Ion Physics,

2 (1969) 92.-94

SHORT

94

C\~MIMUNICA-l-IONS

TABLE 1 RATIOSBETWEEN REXTS OF SXSGLY-

FIELD

APPFtARAXCE

PO-

A.?CC DOUELY-CHARGED

43nzpoKnd

Benzene

Eiaphtbaiene .Anthracene Benzyhmine

ASD

CUR-

IONS

v-; v-

I-/i-

0.55

0.01

3.50 0.45 0.55

0.10 0.50 0.01

are listed_ The 2nd column contitins the ratios Vi,! ? * A of potentials corresponding the appearance of singly- and doubly-charged ions when the recording system has maximum sensitivity. The 3rd column shows +&e observed maximum ratios If */I* of singly- and doubly-charged ion currents. For peaks widening with increase of the &Id (as did the peaks of naphthaIene and anthracene) the valxs of It were determined at the field corresponding to the beginning of the widening. The data about oieEns are not tabulated because of small current vahtes and &sequent large errors in measurements. In addition, Table 1 shows that adding a CH,MH, group to the benzene ring does not change the chxacteristin of double ionization_ So one may assume that doubly-charged ions of benzyIamine are formed by *he removal of z-electrons from the ring. This conclusion is confumed by the presence of the ions (C&&CH2NHt)Hi - which have the pro-per analogue in the mass spectrum of sing&-charged ions.

TItePisarzlzecsky Institute of Physical

Chemistry,

Li-krainian Acaabny of Sciences, Kirt;(USSR)

1 I. 2.

Ko -KY

Received August J. Mass Spcctromet?y

V. A. NKGGENKO 1. v. GOLDENF!XD P. S. DIBROVA

AX= I_ V_ GOLDEXFE, Pdmry T.&m. Ekrperim., 1 (1968) 146.

12th, 1968 and Ion P&sics_

i (1969)

92-94