Chemical ionization other reagent gases

In both modes, analyte ionization is due to a gas-phase ion-molecule reaction between analyte molecules and reagent gas ions. The latter are generated from the solvent vapour in the high-pressure ion source by means of energetic electrons, basically similar to the generation of any other reagent gas in a source for chemical ionization. 

In chemical ionization new ionized species are formed when gaseous molecules interact with ions. Chemical ionization may involve the transfer of an electron, proton, or other charged species between the reactants. [8] These reactants are i) the neutral analyte M and ii) ions from a reagent gas. 

It has been observed in this laboratory, as in many others, that compounds, which volatilize from the mass spectrometer direct insertion probe at a certain temperature under electron impact conditions, will often volatilize at a lower temperature (50°C or more) if the analysis is carried out under chemical ionization conditions with reagent gas sweeping around the probe tip and then into the source. For sulfuric acid deposited on a Fluoropore filter, a decrease in volatilization temperature of approximately 70°C has been observed for sulfuric acid when the analysis is carried out in the chemical ionization mode. The elution profile is sharper and better defined than that obtained under electron impact conditions. 

Aqueous solutions of ammonium sulfate and ammonium bisulfate were deposited on Fluoropore filters, placed in the direct insertion probe, and analyzed in the chemical ionization mode (H2O reagent) gas. The samples were heated from 100°C to 330 C at 15 C/minute. No sample ions were observed under these anlaysis conditions, even when several micrograms of ammonium salts were analyzed. The thermal decomposition of ammonium salts of sulfate has been the subject of many studies. (29,30) Some pathways include sulfuric acid production at one stage of the decomposition while others suggest ammonia, SO2 and SO3 are the products. None of these accurately simulate the conditions (temperature, pressure, gas flow) present in our chemical ionization source. However, no sulfuric acid ions (H3SO4+, etc.) were ob-served... 

Hunt et al. [30a] utilized this mixed mode for the first time PPNICI (pulsed positive negative ion chemical ionization mass spectrometry) with quadrupolar instruments for the study of peptide sequences. The reagent gas chosen was a mixture of CH4 and CH3ONO. The resulting plasma was composed of CHj and CHjO (among others) and led to the positive [MH] and negative [M-H]" ions, respectively. 

In chemical ionization, the sample is introduced into the ionization chamber along with 1 or 2 mm Hg of some reagent gas, usually methane. Essentially aU of the electrons ionize methane molecules rather than sample molecules. Once the methane molecules are ionized, a series of ion-molecule collisions yields, among other species, the ions CHs", C2H5, and CsHs . These ions act as strong Lewis acids and can react with sample molecules to produce their corresponding conjugate acids. A sample chemical equation illustrates this behavior ... 

In chemical ionization (Cl) studies the sample is mixed with an excess of reagent gas and this gas is ionized. This initiates ion-molecule reactions, through which the sample itself is eventually ionized and the ions produced are subsequently analysed (Field, 1972). Field ionization (FI) and field desorption (FD) are closely related methods of ionization in FI (Beckey, 1972) the molecule in the gas phase is subjected to a powerful electric field (10 -10 V/cm), and this results in removal of an electron with a much smaller amount of internal energy being supplied, hence fragmentation is reduced. FD (Beckey and Schulten, 1975) differs from other ionization methods in that the sample is ionized without prior vaporization, and hence is an even softer method of ionization. Applications of these three methods to lipid analysis may be found in Games (1978) some of them will also be described in Section 9.6.3. 

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