Methane negative chemical ionization

Table I. Practical quantitation limits (PQL, ng injected), correlation coefficients (R2 and quadratic regression parameters (a and b) of 21 compounds with electron impact, and positive and negative chemical ionization (methane) Particle Beam mass spectrometry, direct flow injection with full scan mode...

Alternate means of achieving ionization include chemical ionization (Cl), negative chemical ionization (NCI), and fast atom bombardment (FAB). In Cl, a reagent gas like methane is admitted to the ion chamber where it is ionized, producing a cation that undergoes further reactions to produce secondary ions. For example ... 

Representative mass spectral conditions (negative chemical ionization) ion source temperature, 150°C ionizing current, 0.20 mamp electron energy, 70 eV methane reagent gas (source pressure 0.5 to 1 torr). 

The potential of the negative chemical-ionization (n.c.i.) technique for obtaining valuable structural information on very small samples of underivatized oligosaccharides has been demonstrated.200 The n.c.i. spectra of several mono-, di-, tri-, and tetra-saccharides were recorded, using methane as the reagent gas, and direct-probe insertion of the samples into the ion source.201 Fairly intense, molecular ions, M-, were observed in each case. A small number of fragment ions were also observed, but could not be interpreted. Use of dichlorodifluoro-... 

In negative-ion chemical ionization (NCI), a buffer gas such as methane is used to slow down the electrons in the electron beam until some of the electrons have just the right energy to be captured by the analyte molecules. The buffer gas can also help in stabilizing the energetic anions and reduce fragmentation. This is really a physical process and not a true CI process. 

Electron capture negative chemical ionization (ECNI) with methane as reagent gas was used to analyze 17 halogenated diphenylethene, -ethane and -ethanol derivatives such as DDE (150), DDD (151), DDT (152) and dicofol (153)335. Both the ethylenic double bond... 

Duffield, A.M. and Lidgard, R.O. (1986) Analysis of kava resin by gas chromatography and electron impact and methane negative ion chemical ionization mass spectrometry. New trace constituents of kava resin. Biomedical Environmental Mass Spectrometry, 13,621-626. 

All mass spectra were recorded using a Hewlett-Packard 5985B mass spectrometer equipped with a dual electron impact/chemical ionization source including negative ion capability. The source temperature was 125QC, filament emission current was 300 uA, and electron energy was 230 ev. The interface between the GC and MS was a glass-lined stainless steel transfer line. Methane was used both for carrier gas and as the chemical ionization reactant gas. A methane flow of 11.5 mL/min. resulted in an ion source pressure of 0.5 torr. 

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