because this mass was high enough to avoid many common interfering
fragments found at lower masses. As shown in the m/z 124
chromatogram in Fig. 3, guaiacol was one of the largest peaks in
the region with few extraneous peaks nearby. Unfortunately, if
the chromatogram is extracted from the TIC chromatogram, the
signal to noise, S/N, ratio is typically only 3. However, if SIM is employed,
S/N can be improved more than 20-fold due to the increased
dwell time at this mass compared to the scanning mode.
Although monitoring a single ion will provide the greatest
selectivity, it cannot detect the presence of interfering impurities
and cannot be used for identification purposes. Monitoring at
two characteristic ions will allow for ion ratioing which can often
be used to detect interfering coeluting components and confirming
identification. For guaiacol, the signals from m/z 109 and 124 are
the best combination to detect the presence of interfering peaks.
As shown in Fig. 1, the m/z 109/124 ratio should be approximately
1.23. If there is a co-eluted impurity containing either of
these ions the ratio will change profoundly. For this study an arbitrary
range of 0.99–1.48 was considered an acceptable validation
for the presence of guaiacol. Identification confirmation was
achieved by matching retention index values from the SIM chromatogram
with those of standard guaiacol. Matches in both mass
ratios and retention index values were required for guaiacol
confirmation.
Also shown in Fig. 3 are three single mass responses acquired in
the SIM mode and the same chromatogram employing all three
masses. These three SIM chromatograms are shown to demonstrate
the relative contribution of each mass to the guaiacol peak