Optimizing the Analysis of Volatile Organic Compounds

32
Figure 31.
An open split interface or a jet separator
will reduce the gas flow entering a mass
spectrometer vacuum system.
carrier
gas
make-up
gas
purge in
open split interface
jet separator
vacuum
www.restekcorp.com
to MSD
1:10
split
purge out
to
MSD
MS Calibration and Tuning
Calibration allows the correct identification of masses, whereas tuning adjusts the intensity
and peakwidths for masses. The MS is calibrated by adjusting the DC/RF frequency so that
mass axis points are aligned with expected mass fragments of known spectra. Tuning
ensures that target compounds analyzed on the MS will have the same distribution (pattern)
of ions, and peak widths for ions will be narrow enough that adjacent mass peaks will not
overlap. A compound widely used for calibrating and tuning MS systems is perfluorotributylamine
(PFTBA or FC43). Modern instruments introduce PFTBA into the ion source during
the autotune procedure. The instrument software adjusts the MS parameters to match the
known fragmentation pattern for PFTBA. The ion of greatest abundance in the spectrum is
mass 69; the relative abundances of masses 131 and 219 are roughly 50% of that for mass
69 (Figure 32). In analyses of volatiles, mass 502 is less important because its relative abundance
is 1% of the mass 69 value. Low peak heights or a loss of mass 502 generally indicate
a cleanliness problem at the source.
Specific Tuning Requirements: 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 the GC injection port but, alternatively, it can be
purged from a water blank. Abundance criteria for BFB are listed in Table IV.
If tuning with BFB fails under criteria acceptable for PFTBA, decrease the relative abundance
of masses 131 and 219 to 30% of mass 69 by adjusting the entrance lens. If necessary,
slightly decrease the repeller voltage. This procedure targets the ions from mass 173 through
mass 177. A second tuning failure with BFB may dictate recalibration and tuning with
PFTBA. Ion ratios for BFB should be checked every 12 hours. As long as results meet the
specifications in Table IV, no further calibration or tuning is required.
Poor tuning can significantly affect the sensitivity of the MS. Figure 33 shows spectra for a
sample analyzed twice, first after a failing PFTBA tuning with mass 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 32.
Perfluorotributylamine (PFTBA) fragmentation, using a quadrupole mass spectrometer.
69
100
80
60
40
20
0
131
219
50 100 150 200 250 300 350 400 450 500 550 600
Table IV.
US EPA 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 50% of mass 95
175 5-9% of mass 174
176 >95% but