Figure 1. Positive ion electron ionization (70 eV) mass spectrum of FC-43.
derstood. Consider the case of calibra- tion by computer in a quadrupole- based gas chromatography (GC)–MS instrument. Data systems for such scanning instruments record data in a time–intensity file under parameters for instrument operation set in the scan function. The time runs from zero at the beginning of the scan to the end of the scan, typically some tenths of seconds later. The measured intensity is the amplitude of the digi- F o r C l i e n t R e v i e w O n l y . A l l R i g h t s R e s e r v e d . A d v a n s t a r C o m m u n i c a t i channel during the scan. The scan function controls electronic parame- ters such as rf/dc values during the scan of the quadrupole mass filter (or current fed to an electromagnetic for a magnetic sector mass analyzer scan). For practical reasons, the scan func- tion starts usually at values that corre- spond to a higher mass and drops to a lower mass. The relationship between the time–intensity values and the mass scale of the recorded mass spectrum is established by mass calibration. A cali- brant compound is introduced into the instrument as a sample of steady concentration. A time–intensity file is recorded for the mass calibrant using the scan function. The data system then determines a mathematical transformation such that its applica- tion to the time–intensity file meas- ured for the calibrant will provide a mass spectrum in which the ions in the mass spectrum of the calibrant are assigned properly. Specifically, for a calibrant such as FC-43 (see following paragraphs), a time at which ions of m/z 69 pass through the mass analyzer o n s I n c . 2 0 0 5 t i z e d d e t e c t o r r e s p o n s e r e c o r d e d w i t h i n e a c h s u c c e s s i v e s h o r t t i m e