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Mass Spectrometry Forum

Masses in Mass Spectrometry: Perfluors and More

Part II of this four-part series on mass calibration looks at the chemical compounds used as mass calibrants in mass spectrometry.

Kenneth L. Busch

C Certifying Performance A mass spectrometer is a user-calibrated instrument in which the masses of the ions are determined as relative val- ues (think of a double-pan analytical balance). As a conse- quence, instrument operation (that is, the particular scan function used) must be certified to provide correct values for the measurement of ion masses. This certification is provided through a process of calibration, which involves examination of the mass spectrum of a mass calibrant. The mass calibrant is a compound for which masses and intensi- ties of ions within its mass spectrum are known. Just as some of us remember using that double-pan analytical bal- ance to compare unknown masses against certified known masses, a calibration of the instrument is performed to demonstrate that the operation of the mass spectrometer, under the specified set of operational parameters, produces a mass spectrum of the calibrant with masses of the ions properly assigned. orrections belong up front. Part I of this column series included a description of mass scales and units, ex- horting readers and users to pay attention to basics. An essential part of basics is good proofreading, a skill that seems to have deserted this author in preparing Part I. Table II in Part I (Spectroscopy 19[11], 32–34 [2004]) lists the exact mass for 13C incorrectly as 13.993355. I considered assigning the blame for such an egregious error to electronic voting machines (it was, after all, an election year, and an election month), or fat fingers on the keyboard, but instead I admit to sloppiness and apologize for the error. The correct value for the exact mass of 13C is 13.003355. My thanks to the readers who notified me of the error and demanded a recount. This second installment in the series deals with the chem- ical compounds used as mass calibrants in mass spectrome- try. A preview of this topic appeared in Mass Spectrometry Forum in June 1994. While the underlying need for mass calibration in mass spectrometers has not changed, the expansion of more specialized instruments into higher mass ranges, the use of diverse and different ionization methods, and the specific mass accuracy needs of new communities now accessing mass spectral data, all combine to support a more detailed review of mass calibration, including tradi- 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 t i o n a l u s e i n l o w e r - m a s s r a n g e high-throughput biomic systems (Parts III and IV). “Operation” is a general term that encompasses a multi- tude of physical and electrical factors. Most of these factors are transparent to the casual operator of the instrument; taken together, the set of parameters constitutes the scan function. As long as the parameters do not change (as in i n s t r u m e n t s ( P a r t I I ) , a n d e x t e n d i n g t o n e w i o n i z a t i o n m e t h o d s a n d h i g h e r - m a s s , i n s t r u m e n t a l d r i f t ) , c a l i b r a t i o n o f t h masses are assigned correctly. The calibration file contains the mathematical transformation required to assign masses properly to the relative data measured by the instrument. Computer-assisted calibration in modern instruments often also involves instrument tuning such that measured intensi- ties of calibrant-derived ions (or a secondary performance factor compound) fall within a predetermined range. This aspect of tuning is an issue separate from determination of the mass scale, and will be discussed separately in a later series of columns that deals with ion intensities. e i n s t r u m e n t c o n n e c t s t h e s c a n f u n c t i o n w i t h a c a l i b r a t i o n f i l e , a n d t h e m e a s u r e d Kenneth L. Busch is old enough to have 1) used a double- pan analytical balance in chemistry laboratory, and remembers the lecture about draconian consequences should his too-sudden release damage the knife edge, 2) ordered FC-43 directly from that Florida company, and knew where the name came from, and 3) manually tuned the instrument using a scope to set the proper peak shape of the 502 and 614 ions. Contact him at: wyverners@yahoo.com. What Calibration Entails Individual steps in the calibration process should be un-

76 Spectroscopy 20(2)

February 2005


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