Perfluorokerosene

Because an increase in resolution causes a decrease in sensitivity, it is best to operate at the lowest resolution commensurate with good results. Some instrument data systems will allow calibration with an external reference material such as perfluorokerosene and then use of a secondary reference material for the internal mass reference. Tetraiodothiophene, vaporized using the solids probe inlet, is recommended as the secondary reference. The accurate masses are 79.9721, 127.9045, 162.9045, 206.8765, 253.8090, 293.7950, 333.7810, 460.6855, and 587.5900. For a higher mass standard, use hexaiodobenzene. Because the mass defect for these internal reference ions are so large, a resolution of 2000 is ample to separate these ions from almost any sample ions encountered in GC/MS. 

Resolution does not affect the accuracy of the individual accurate mass measurements when no separation problem exists. When performing accurate mass measurements on a given component in a mixture, it may be necessary to raise the resolution of the mass spectrometer wherever possible. Atomic composition mass spectrometry (AC-MS) is a powerful technique for chemical structure identification or confirmation, which requires double-focusing magnetic, Fourier-transform ion-cyclotron resonance (FTICR) or else ToF-MS spectrometers, and use of a suitable reference material. The most common reference materials for accurate mass measurements are perfluorokerosene (PFK), perfluorotetrabutylamine (PFTBA) and decafluorotriph-enylphosphine (DFTPP). One of the difficulties of high-mass MS is the lack of suitable calibration standards. Reference inlets to the ion source facilitate exact mass measurement. When appropriately calibrated, ToF mass... 

The first strategy to compensate for mass spectral drift is to tune the instrument. This is typically achieved with the volatile standard, perfluorokerosene, and tuning so that mlz 181 is one-tenth of m/z 69. Unfortunately, this procedure is insufficient to compensate for all the instrumental drift and additional methods are required. 

The exact mass of an ion (4 to 6 decimal points) reliably defines its elemental and isotopic composition, while the method is called high resolution mass spectrometry. The measurements are conducted manually or automatically (computerized). Manual measurements are based on the parallel acquisition of the peak of interest with the closest peak of an ion with the known composition. Any compound with an intense ion peak with m/z value in the region +10% may serve as a marker. The most widespread markers are perfluorokerosene, perfluorotributylamine, and other polyfluorinated compounds. The use of these compounds is based on their volatility, as well as on the fact that fluorine is a monoisotopic element. In the spectra of these compounds intense ion peaks randomly cover all the range between m/z 19 and M+. ... 

Fig. 3.20. Partial 70 eV El mass spectrum of perfluorokerosene, PFK. The peaks are evenly spaced over a wide m/z range. In the low m/z range peaks from residual air do occur.

To ensure accurate mass measurement, spectrometers are calibrated with compounds such as perfluorokerosene (CF3(CF2)nCF3) or perfluorotributylamine ((CF3CF2CF2CF2)3N). In high-resolution spectra, the exact masses of fluorocarbon fragments are slightly lower than those of ions containing C, H, O, N, and S. For high-resolution work, standards should be run with the unknown. 

Mass Calibration The process by which the mass analyzer is calibrated such that a measured and displayed m/z is accurate. Well-characterized calibration compounds are utilized, and measured m/z values for these compounds are compared to theoretical m/z values. Calibrants commonly used include various polymeric species (such as polypropylene glyol, or PPGs poly tyrosine (poly-t)) or fluorinated species (perfluorokerosene or PFK) but can be any compound or mixture (Nal/KI) of compounds properly characterized for MS. 

Figure 15. Computer plot of a partial spectrum of Tetrabromothiophene plus Perfluorokerosene from mass 271 to 420...

The two most widely used reference compounds for El are heptacosafluoro-tributylamine (PFTBA, (C4F9)3N) and perfluorokerosene (PFK, CF3-(CF2)n-CF3). 

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