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Table I. Positive ions found in the electron ionization

Ion Empirical Formula

CF

3

C

2

F

5

C

3

F

5

C

4

F

9

C

5

F

10

N

C

6

F

12

N

C

7

F

12

N

C

8

F

14

N

C

8

F

16

N

C

9

F

16

N

C

9

F

18

N

C 2 F 4

C 9 F

20

N

C 1 2 F 2 2 N C 1 2 F 2 4 N

Exact Mass (Da)

Integral Mass (Da)

68.995210

69

99.993613

100

118.992016

119

130.992016

131

218.985629

219

263.987100

264

313.983917

314

325.983917

326

375.980743

376

413.977539

414

425.977539

426

463.974335

464

501.971161

502

575.967957

576

613.964722

614

mass spectrum o

f FC-43.

is determined, as well as times for ions at m/z 100, m/z 219, and m/z 614 (ide- ally, for a linear scan function, the time-to-mass conversion also is lin- ear). After calibration, and in the ab- sence on instrumental drift, subse- quent time–intensity taken using the

be linked in the data system to a dif- ferent calibration file.

Perfluorinated Compounds as Mass Calibrants We now consider the nature of the chemical compounds used as mass cali-

“OPERATIONIS A GENERAL TERM THAT ENCOMPASSES A MULTITUDE OF PHYSICAL AND ELECTRICAL FACTORS, MOST OF WHICH ARE TRANSPARENT TO THE CASUAL OPERATOR OF THE INSTRUMENT.

same scan function, and linked to that r n e w O 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 c brants. For use with electron ionization a l i b r a t i o n f i l e ( t h e s l o p e o f t h e l i n e c o n n e c t i n g t i m e t o m a s s ) , i s t r a n s - o r c h e m i c a l i o n i z a t i o n ( b o t h t o p i c s c o v e r e d p r e v i o u s l y i n t h i s c o l u m n ) , t h e

formed into a mass spectrum with the masses assigned correctly. Every data file must be linked to a calibration file, and the data system completes this linkage automatically. Sophisticated mass spectrometer data systems in- clude an automatic function to cali- brate and tune (set thresholds, resolu- tion, and adjust intensity values as required) the mass spectrometer, but the basic process remains the same. The calibration function often is per- formed automatically on start up (or at regular intervals throughout con- tinuous operation), along with other automated tuning functions that opti- mize instrument performance. Note

calibrant typically is a liquid com- pound stored in an internal reservoir adjacent to the ion source. The reser- voir (containing a few milliliters of the calibrant liquid) is heated, and a few millitorr of the calibrant gas passes through a calibrated leak valve into the ionization chamber of the mass spec- trometer as the calibration process be- gins. The ideal calibrant compound is pure and inert, nontoxic, inexpensive, and readily available. As a calibrant, it produces a mass spectrum (using the standard conditions of electron or chemical ionization) that contains a number of abundant, evenly spaced ions distributed across the mass range

that each different scan function must

of interest, each with a limited width to

Circle 59

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