3.2.2. MS detector selectivity and sensitivity
One of the major objectives of this study was to develop an
instrumental method which would allow identification and quantification
of guaiacol down to its sensory detection and recognition
thresholds. Guaiacol is an extremely potent off flavour and quantifying
it at sensory threshold levels requires special approaches in
terms of sample preparation and instrumental analysis. SPME is
an excellent method of concentrating juice volatiles and the optimal
SPME fibre type and exposure conditions have been previously
reported (Gocmen et al., 2005) and continued with this study. This
leaves detection as the final means to achieve the identification
and quantitation objectives of this study.
There are basically three forms of MS detection: total ion current,
TIC, which collects the responses from all of the masses in
the scan range at any particular time, extracted ion current, EIC,
in which the output from specific masses is pulled out of the TIC
data grid and selected ion monitoring, SIM, which collects the output
at specific masses for selected times (dwell time).
Shown in Fig. 2 are two chromatograms from an orange juice
fortified with 0.5 lg/l guaiacol consisting of a total ion chromatogram
25–300 m/z (lower) and a SIM chromatogram using the combined
signals at m/z 109 and 124 (upper). The retention time of