Methane negative chemical ionization compounds

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...

Fig. 6. Methane chemical ionization negative ion mass spectra of pentafluoro benzyl ABA upper with insert of unlabelled material) and dipentafluoro benzyl ODA lower). The compounds were isolated from tomato shoots supplied with 40% -H. >O for six days, then severely wilted. M/z-263 = molecular ion — 1 mass unit after the loss of the pentafluorobenzyl residue +H.M/Z 377 = molecular ion of dipentafluorobenzyl ODA-1 mass unit after the loss of one pentafluorobenzyl residue +H. Ordinate. relative intensity abscissa, mass numbers...

Chemical ionization also deals with the ionization of volatile gas phase samples, and can be applied across-the-board to the same types of samples as electron ionization. Chemical ionization provides abundant molecular ions for those compounds that do not produce a discernible molecular ion by electron ionization. The molecular weight of the sample molecules of interest is reflected in the mass of the protonated molecule (M+H) in positive ion chemical ionization or the mass of the deprotonated molecule (M-H) in negative ion chemical ionization. These positive even-electron ions are formed in an ion/molecule reaction between a gas phase sample molecule M and a strong acid such as CHs" (formed from methane) in which a proton is transferred to the neutral sample molecule to from (M+H). Both electron and chemical ionization can be used in TLC/MS experiments in which the samples within the TLC plate are independently evaporated into the gas phase and swept into the source of the mass spectrometer in a stream of gas. 

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