32
MSCalibration andTuning
Calibration allows the correct identification ofmasses, whereas tuning adjusts the intensity
and peakwidths formasses. TheMS is calibrated by adjusting theDC/RF frequency so that
mass axis points are alignedwith expectedmass fragments of known spectra. Tuning
ensures that target compounds analyzed on theMSwill have the same distribution (pattern)
of ions, and peakwidths for ionswill be narrow enough that adjacent mass peakswill not
overlap.A compoundwidely used for calibrating and tuningMS systems is perfluorotributy-
lamine (PFTBA or FC43).Modern instruments introduce PFTBA into the ion source during
the autotune procedure. The instrument software adjusts theMS parameters tomatch the
known fragmentation pattern for PFTBA. The ion of greatest abundance in the spectrum is
mass 69; the relative abundances ofmasses 131 and 219 are roughly 50% of that formass
69 (Figure 32). In analyses of volatiles, mass 502 is less important because its relative abun-
dance is 1% of themass 69 value. Low peak heights or a loss ofmass 502 generally indicate
a cleanliness problem at the source.
Specific TuningRequirements: 4-Bromofluorobenzene:
After the system is calibrated and
tuned, using PFTBA, a 50ng solution of 4-bromofluorobenzene (BFB) is introduced. BFB
usually is introduced by injection through theGC injection port but, alternatively, it can be
purged from awater blank.Abundance criteria for BFB are listed inTable IV.
If tuningwithBFB fails under criteria acceptable for PFTBA, decrease the relative abun-
dance ofmasses 131 and 219 to 30% ofmass 69 by adjusting the entrance lens. If necessary,
slightly decrease the repeller voltage. This procedure targets the ions frommass 173 through
mass 177.A second tuning failurewithBFBmay dictate recalibration and tuningwith
PFTBA. Ion ratios for BFB should be checked every 12 hours.As long as resultsmeet the
specifications inTable IV, no further calibration or tuning is required.
Poor tuning can significantly affect the sensitivity of theMS. Figure 33 shows spectra for a
sample analyzed twice, first after a failing PFTBA tuningwithmass 131 as the base peak
(Figure 33,A), then after a passing tuning (Figure 33, B). The second analysis exhibits a
three-fold increase in sensitivity.
Figure 31.
An open split interface or a jet separator
will reduce the gas flow entering amass
spectrometer vacuum system.
Figure 32.
Perfluorotributylamine (PFTBA) fragmentation, using a quadrupolemass spectrometer.
100
80
60
40
20
0
50 100 150 200 250 300 350 400 450 500 550 600
69
131
219
502
Table IV.
USEPA ion abundance criteria for 4-bromofluorobenzene (BFB).
Mass/Charge Ratio
Relative Abundance Criterion
50
15-50% of mass 95
75
30-80% of mass 95
95
Base peak, 100% relative abundance
96
5-9% of mass 95
173
<2% of mass 174
174
>50% of mass 95
175
5-9% of mass 174
176
>95% but <101% of mass 174
177
5-9% of mass 176
purge out
purge in
toMSD
1:10
split
to
MSD
vacuum
make-up
gas
carrier
gas
jet separator
open split interface
