Negative chemical ionization source

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). 

Figure 5. Mechanisms for negative ion formation in a chemical ionization source...

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-... 

A method that uses high performance liquid chromatography/ mass spectrometry (HPLC/MS) for the analysis of chlorinated phenoxyacid herbicides is described. During method development different techniques were used to increase both the sensitivity and the specificity of thermospray HPLC/MS for chlorinated acid herbicides. These included the operation of the instrument in the negative chemical ionization (NCI) mode initiated by discharge and the use of a wire-repeller in the ion source for efficient extraction of positive ions. Single quadrupole repeller-induced and multiple quadrupole collision activated dissociation (CAD) experiments were also performed to increase the structural information of the mass spectra. 

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. 

A functionally similar dissociation method, electron transfer dissociation (ETD) was more recently reported (32), specifically for use on QIT instruments, although it is also beginning to be adapted to other mass analyzers. ETD is accomplished by electron transfer to the analyte from a negatively charged species that is produced in a chemical ionization source and directed into the region where the analyte ions are trapped. For peptides and proteins, it produces spectra that... 

Negative chemical ionization (NCI) is a misnomer because only rarely is there an actual chemical reaction involved. Instead, the process requires thermal electrons that are created when a gas in the ion source is used as a buffer to decelerate and... 

Negative chemical ionization is similar to Cl only in so far as the same gases are used in the same ion source, although the pressure is usually... 

Process for the production of ions for an ion storage mass spectrometer, for example, ion trap MS. The ionization does not take place inside of the ion trap analyser instead, ions are formed in an attached ion source and transferred to the ion trap analyser. The special decoupling of ionization and mass analysis in ion trap instruments allows the independent use of GC parameters, the application of negative chemical ionization as well as several MS/MS scan techniques. 

The ions produced in the source are separated in the analyzer according to their mass-to-charge (m/z) ratios. The Thomson unit (Th, named for a famous mass spec-trometrist) corresponds to an m/z ratio of 1 it is widely used in mass spectrometry. In a GC-MS context, we often speak too generally about mass measurement because z = 1 in electron ionization and positive chemical ionization and z = -1 in negative chemical ionization. 

Ion traps with hybrid sources offer particularly interesting possibilities for chemical ionization. For example, they can perform routine negative chemical ionization which is not electron attachment. Let us consider an example that uses methanol as the reagent. In the source, Reactions (4.1) and (4.2) produce the CHjO" anion that can react in the trap with the molecule M according to three reaction pathways proton... 

The third chapter is dedicated to the ionization modes that can be operated in GC-MS electron ionization as well as positive and negative chemical ionization. We explain the principles of operation of internal and external sources and compare the performances achieved by each ionization mode. 

Hunt, D.F., McEwen, C.N. and Harvey, T.M. (1975), Positive and negative chemical ionization mass spectrometry using a Townsend discharge ion source. Anal Chem., 47,1730. 

Negative Ion Chemical Ionization Negative ions are produced under ci conditions by electron capture. Under the higher pressure conditions of the ci ion source, electrons, both primary (those produced by the filament) and secondary (produced during an ionization event), undergo collisions until they reach near-thermal energies. Under these conditions, molecules... 

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