Mass Spectrometry: Calculations

T he mass spectrometer is a sensitive and specific device that can be used to quantify a wide range of substances. Unfortunately, the sensitivity is not constant, and the signal varies between compounds. Because of the variation, a calibration is needed to transform the mass spectrometric signal to units corresponding to the concentration of the specific analyte. In many of the earliest applications, the calibration simply involved determination of a response factor for a given quantity of analyte. The concentration of an analyte in an unknown could then be calculated from the observed mass spectrometric signal. The obvious difficulty with this calibration is that the response factor changes as a function of instrument operating conditions, and the precision of the analysis is rather low. A considerable improvement in precision can be obtained by using a homologous internal standard,1 but minute differences in extraction efficiency between the analyte and homologous internal standard, differences in the stability of the compounds, and differences in ionization efficiencies still limit the precision of the analysis.2 Use of an isotopically labeled form of the analyte as the internal standard largely eliminates these problems and maximizes the precision of the mass spectrometric analysis.2 Even when an isotopic internal standard is used, the analyst is still faced with the problem of calibration in order to convert the mass spectrometric response to units corresponding to analyte concentration. In this example, however, the mass spectrometer response is a ratio of ion currents at the masses corresponding to the analyte and the internal standard. In the analyses of monoatomic species or small molecules such as CO2 by isotope ratio mass spectrometry, calibration requires little more than the use of a single standard of known isotopic composition to correct for mass spectrometric isotope discrimination. Thereafter, calculations can be made by the principle of mass balance.3’4 Analysis of larger organic compounds does not lend itself to calibration by the above principles. For polyatomic species, the ion current at a given mass is almost never a result of a single isotopic species. Multiple isotopic species and fragments of different elemental composition produce