Michael P. Balogh

Although I prefer IMMS there may be little problem in most cases determining that 'IMS' means ion mobility mass spectrometry and not imaging mass spectrometry let alone standing as a reference to Dr. Jed Diamond's irritable male syndrome (although I'm sure cases could be made).

My thanks to everyone – I look forward to the comments posted in the margin at http://www.Waters.com/ (Resource Library/Primers). The web version and the hardcopy (due out in early 2009) have clearly benefit from your contributions.

The following list of terms is derived from common usage throughout the industry as an adjunct to the discussions in this primer and includes terms and techniques no longer in common usage.

Abundance: When viewed as similar to absorbance displayed on a UV detector, the vertical increase in signal above background indicates an increased occurrence of that particular ion (when the x axis is calibrated in mass units) or total ions present (when the horizontal axis is calibrated in time or scans). The signal for all ions resulting from the fragmentation of a single analyte or compounds compared to a base peak (the relative abundance of each ion) is used to determine the fit of a fragmented pattern to a library spectrum for positive identification.

Accurate mass: The measured mass value for a compound with an associated error like 5 ppm. Accurate mass also is used commonly to refer to the technique rather than the measured mass. Exact mass is the exact theoretical value for the mass of a compound.

Atmospheric solids analysis probe (ASAP): Based upon work by Horning in the 1970s, this form of sample ionization developed by McEwen and McKay uses a standard atmospheric pressure chemical ionization (APCI) plasma but forms ions by placing the sample in a heated nitrogen stream. The heat volatilizes a surprisingly large number of samples, and ions are formed by charge exchange with metastable ions created by the APCI plasma. Relatively unambiguous identifications can be made of individual compounds from complex mixtures at low levels using accurate mass instruments. See also DART and DESI.

Atmospheric pressure ionization (API): The term used to refer generally to techniques such as electrospray ionization (ESI) and APCI and others that operate at atmospheric pressure.

Atmospheric pressure chemical ionization (APCI): Originally called solvent-mediated electrospray, it is applied successfully more often to neutral molecules that do not ionize easily directly out of solution. APCI provides a current on a sharp pin, positioned in the on-coming aerosol stream, to create a plasma of metastable ions from the solvent itself and transfer the charge from these ions to the analyte as it passes through the plasma. Heating a probe through which the liquid chromatography (LC) or solvent stream passes creates the aerosol.

Atmospheric gas chromatography: Developed by Charles McEwen at DuPont in 2002. Using a heated transfer line, a standard gas chromatography (GC) effluent can be introduced to a standard API (or ESI–APCI) source on a mass spectrometer. This provides an easy and fast changeover from ESI to gas chromatography (GC) for compounds that would be best analyzed by GC. Mode of ionization can be either APCI or atmospheric pressure photoionization (APPI).

Atmospheric pressure photoionization (APPI): Developed in the 1980s but commercialized after 2000 when krypton gas lamps were found to generate sufficient photon energy at 10 eV (approximately) to ionize nonpolar analytes such as PAHs and steroids not typically amenable to ESI and APCI ionization.