Negative ion mass spectrometry: The generation of high concentrations of low energy molecular negative ions at high source pressures

Abstract

The effect of the addition of argon and other gases upon the intensities of negative ion species formed in an electron impact source has been investigated. The negative ion current generated for a series of aromatic compounds has been investigated as a function both of sample and argon pressure in the ion source of a ZAB-ZF mass spectrometer. For all compounds studied, a striking enhancement of molecular negative ion current occurred on increasing either the pressure of the sample or of argon. The results are consistent with thermalization of the 50 eV electrons by collisions with neutral molecules in the high pressure ion source and collisional stabilization of the negative ions initially formed. Analytical applications of the technique are discussed.

Negative chemical ionization (NCI) of organic compounds

“…However, a conventional CI source at pressures above ca. 100 Pa shows an absolute intensity loss which is attributed to a scattering of the ion beam (26). Because of these various factors influencing the habitus of an NCI spectrum, it can be difficult to standardize experimental conditions, and drastic differences may be observed if different mass spectrometers are used (see e.g., Sect.…”

“…In early experiments, Ar (4) or N2 (39) have been used; today, probably because of their availability from PCI work, usually 10-100 Pa CH4, i-C4HIO or NH3 are resorted to. Occasionally C2H4 (25), C 0 2 (43), CH4 + He (29), CH4 + Ar (30), CH4 + SF6 (31), Ar + COz (26), and CH2CI2 (32) have been used. Ar, NZ: CH4: no negative ions under CI conditions (31) (50 Pa, 100°C) H-(63%), C-' (18), CH-(8%), CHy'(8%), CH, (3%), (33); (25 Pa, 250°C) C-' (2%), CH-(4%), CH,' (15%), CH, (79%) (34).…”

“…The consequence is an almost linear increase of the electron attachment rate to the sample molecule, and hence of the sensitivity, u p to a maximum which lies depending on the nature of the gas, somewhere between 5 and 30 Pa (22,26,33,44,47,200). At higher gas pressures, the general sensitivity decreases.…”

“…At higher gas pressures, the general sensitivity decreases. This decrease was attributed to scattering of the ion beam (26,44). An effect on the ratio of molecular-to-fragment ions was noted for a series of different compounds only at pressures below ca.…”

“…An effect on the ratio of molecular-to-fragment ions was noted for a series of different compounds only at pressures below ca. 1 Pa where fragment formation starts to increase probably due to increasing EI (26,31,33,47). Again, conflicting results were reported (46) for polychlorodibenzo-p-dioxins were drastically decreasing abundances of M -' and increasing abundances for C1-between ca.…”