however, in peak matching experiments in which the opera- tor would want to use a calibrant ion close in mass to the unknown mass. FC-70 is a compound related to FC-43 de- veloped to provide calibration ions extending to a higher mass range.
A wide range of other compounds has been developed for use in mass spectrometers that used ionization methods other than EI or chemical ionization (CI), and we describe these more in depth in Part III. For EI and CI, triazines (1–3) and fluorinated oils developed for other purposes (as in the Fomblin and Ultramark compounds [4,5]) have been used as mass calibrants that extend the upper limit of the usable mass calibration range. Work continues to develop a calibration compound or mixture of compounds that is as easy to use as FC-43, and exhibits the same desirable char- acteristics, but which allows calibration to a higher mass limit. Fishman et al. (6) suggest that a mixture of fluorinat- ed compounds can be used for calibration in the mass range 100–3000 Da.
We have focused in this column on mass calibration. GC–MS instruments used in certain analytical protocols must be calibrated, of course, but also must meet perform- ance standards. Decafluorotriphenylphosphine (DFTPP) is a compound used for tuning such instruments to meet the performance standards specified in EPA Methods 625, 1625, 8250, and 8270, covering analysis of volatile and semi- volatile compounds in water. This tuning standard was sug- gested in 1975 (7), and improved instrument performance has resulted in ion abundance values that are more accurate but at variance with the standard. As a consequence, an artificial transformation must be applied to the measured data to alter the intensities of the measured ions to bring them into compliance with the standard. Mass calibration is unaffected. Similarly, bromofluorobenzene also is a per- formance standard for mass spectrometric measurements of volatile compounds in water.
tomated mass spectrometer should not result in forgetful- ness of the basics: that mass assignment is a process based on a comparison of relative masses; that the hidden reser- voir inside contains the calibration compound (and eventu- ally needs to be refilled); that ions, whatever their source, are reactive entities; and that calibration needs for different ionization sources result in development of different cali- bration compounds. This last point is the focus of Part III of this series.
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In the usual calibration process, perfluorinated com- pounds used as mass calibrants are allowed to leak into the 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 o n s I n c . 2 0 0 5 s o u r c e f o r c a l i b r a t i o n , a n d t ples, no residual signal is desirable. Both the calibrant com- pound and the sample can be present in the source simulta- neously in peak matching experiments that are used in magnetic sector instruments for exact mass measurements, and in this instance we assume that there is no interaction between the compounds. Ions follow their own rules, of course, and Mosi et al. (8) have shown that several of the ions generated from FC-43 react with polycyclic aromatic hydrocarbons in a quadrupole ion trap mass spectrometer. The relative abundances of the product ions resulting from ion–molecule reactions between the fluorocarbon cations and the hydrocarbons varied with the exact structural iso- mer, allowing their differentiation. h e n a r e p u m p e d a w a y . T o m i n i - m i z e b a c k g r o u n d i n t h e m a s s s p e c t r a m e a s u r e d f o r s a m -
Parting Thought The transparency of the calibration process in a modern au-