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Mass Spectrometry Forum

the isotopic envelope in order to maxi- mize the intensity of the ion signal in any one ion. One could argue that the ideal mass calibrant would be com- posed only of monoisotopic elements; the compounds in common use come close to meeting this ideal.

Several perfluorinated compounds are in common use as mass calibrants in organic mass spectrometry. Perfluorinated compounds replace all hydrogen atoms in the parent com- pound with fluorine atoms. Examples include perfluorokerosene and perflu- orotributylamine (PFTBA, FC-43, “heptacosa”). We discuss specifically use of the latter calibrant. The struc- t u r e o f F C - 4 3 ( ( n - C 4 F 9 ) 3 N ) i s s u c h

of fluorine, any isotopic contribution to the mass calibrant ion is due to the presence of carbon alone. Even the highest mass ion tabulated in the table (m/z 614) has only 12 carbons, and therefore the abundance of the ion at m/z 615 is expected to be only 13% of the abundance of the ion at m/z 614 (itself observed with a low abundance relative to the base peak at m/z 69). Ion intensity therefore is maximized in one ion rather than diluted across the mass envelope.

Peak Matching Experiments A signature characteristic of the ions derived from the perfluorinated calibra- tion compounds make them useful in

sample — again a relative mass deter- mination against the standard. The separation can be achieved as long as the ion from the calibrant contains no or minimal hydrogen. The exact mass of hydrogen (1.007825) is such that the presence of only a few hydrogen atoms outweighs the negative mass defect contribution of multiple fluo- rine atoms.

We note that PFTBA (FC-43) is a pure compound, but that perfluoro- kerosene is not. Reasonably, one might expect that the exact composition of different “batches” of perfluoro- kerosene produced might vary slightly, with consequential differences in the mass spectrum of the mass calibrant


that starting with the molecular ion, each of the three butyl chains will fragment identically, minimizing the number of different fragment ions in the mass spectrum, and maximizing the intensity of the ion signal at those masses. Finally, these ions are formed with adequate intensity in both the positive and the negative ion mass spectra, and under both electron and chemical ionization conditions. peak matching experiments that record data at higher mass resolution. Higher resolution is used to establish the exact mass of ions derived from organic com- pounds. Each fluorine atom has an exact mass of 18.998405, less than the integral mass of 19. An ion that con- tains multiple fluorine atoms therefore is found usually at an exact mass less than the integral mass. This difference is sometimes termed a negative mass defect, but use of this term should not 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 physics. In contrast, ions derived from organic compounds containing carbon, hydrogen, nitrogen, and oxygen (in rea- sonable proportions) will be found with exact masses that are slightly higher than the integral mass, that is, a “posi- tive mass defect.” standard. The calibration (and tuning) software of the mass spectrometer must accommodate these differences. Additionally, these compounds are synthesized in large batches, which are sufficient to supply the needs of the community for some years, and each batch is shipped with a copy of the electron ionization mass spectrum measured under standard conditions for that particular batch of mass cali- brant. Note that the differences are i o n s I n c . 2 0 0 5 e l e c t r o n i o n i z a t i o n ( E I , 7 0 e V ) m a s s s p e c t r u m o f F C - 4 3 ; t h e h i g h the mass spectrum, and not in the masses of the ions. Figure 1 contains the positive ion e s t m a s s b e c o n f u s e d w i t h t h e s t a n d a r d u s e o f t h e t e r m m a s s d e f e c t i n a t o calibrant ion at m/z 614 was sufficient to cover the mass range accessed most usually in GC–MS. Table I compiles the primary ions in the EI mass spec- trum, listing empirical formulas and the exact masses of the ions. Depending upon the instrument and its mode of operation, the calibration of the instrument can use either the nominal mass or the exact mass to generate the transformation contained in the calibration file. Exact masses listed for these ions are the summa- tions for combinations of 12C and for m i c e x p e c t e d p r i m a r i l y i n t h e d i s t r i b u t i o n o f i n t e n s i t i e s o f t h e i o n s o b s e r v e d i n Other Calibrants Other perfluorinated compounds are in common use as calibrants in GC–MS instruments. The mass spec- trum of perfluorokerosene contains a greater number of ions distributed across the same mass range as FC-43. However, the stability and linearity of modern instruments is such that this larger number of ions usually is not needed for the mass calibration of the scan function. Such ions are useful, Figure 2 illustrates this situation for two ions that are each of integral mass 100. Ions from both the mass calibra- tion compounds and the unknown can be present simultaneously, but still are separated. The known mass of the calibrant ion is used to deduce the exact mass of the ion derived from the 19F. Because there is only one isotope

80 Spectroscopy 20(2)

February 2005


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