Discrete Energy Electron Capture Negative Ion Mass Spectrometry

Abstract

Abstract Standard electron capture negative ion mass spectrometry (ECNIMS) has long been known as a sensitive and selective method for the analysis of molecules with positive electron affinities. Applications of this technique for the analysis of environmental contaminants, particularly of halogenated compounds such as polychlorobiphenyls (PCBs), are common. However, the requirement for a moderating gas in the ion source to lower the energy of the electrons into the range required for resonance electron capture generates new experimental variables that are difficult to control. Differences in the moderating gas pressure cause changes in the electron energy distribution and in the resulting ion abundances, whereas small fluctuations in temperature often cause major differences in ion abundance ratios. Even more serious are the uncontrollable reactions between ions, molecules, and radicals within the high‐pressure ion source region that give unpredictable spurious product ions, making spectral interpretation difficult and leading to wide variations of negative ion spectra from different instruments. Because of these difficulties, the advantages in sensitivity and specificity of ECNIMS for analysis of electronegative compounds often have been unrealized. Better results can be obtained by having direct control over the electron energies. Control over electron energies can be achieved using a trochoidal electron monochromator, which allows the operator to tune to or scan a range of electron energies necessary for resonance electron capture. With this device some of the inherent difficulties encountered with ECNIMS can be avoided and reliable spectra for a broader range of compounds becomes possible. The ability to tune to a desired electron energy or to scan over a range of electron energies makes another dimension of analytical information available for identifying compounds. Quadrupole and sector instruments have been equipped with electron monochromators and the results are very promising. A clear advantage of these units is the reproducibility of the data they produce, the potential for three‐dimensional spectra for easier interpretation of the results, and the availability of a new dimension of analytical information. These instruments have not shown the sensitivity inherent in the ECNIMS method. Improvements of about two or three orders of magnitude in sensitivity will be required before these instruments can compete with existing technology for trace analysis of environmental residues or other electron‐capturing compounds. An improvement in the sensitivity of this magnitude is well within reasonable expectations and it should not be long before gas chromatography/electron monochromator mass spectrometry (GC/EMMS) becomes a standard technique for detecting environmental compounds, explosives and substances of abuse.