An overview of the chemistry, manufacture, environmental fate and detection of carbofuran - Carbofuran and Wildlife Poisoning: Global Perspectives and Forensic Approaches - page 12

Analytical sensitivity can be increased by specifying certain fragments, i.e., focusing the detector
on specifi c m/z values, or by limiting the scanning interval by reducing it from 100 to 500 down to
150 to 200, for example. When specifi c m/z values are targeted (i.e., 250, 296, 350), the instrument
is set to run in selective ion monitoring or 'SIM' mode. The total ion count (TIC) refers to the sum of
the signal generated by all the ions monitored at any one point. The mass spectrum obtained during
analysis of the compound of interest is generally verifi ed by comparison against a reference mass
spectrum which is often obtained following analysis of the relevant (pure) standard material.
1.5.1.3 Principles of liquid chromatography with mass spectrometry
LC/MS should be viewed as a combination of HPLC and mass spectrometry, in the sense that it
employs a mass spectrometer as a detector (Maurer 2000). This technique is often used when the
analytes are known targets and need to be quantifi ed accurately (Maurer 2000). While older/more
conventional HPLC detection methods may require more complex sample preparations, cleanup
and/or more complex column separations, LC/MS generally requires a simpler cleanup and no deri-
vatisation (Hormazábal, Fosse and Reinham 2006).
In LC/MS, the stream of mobile phase containing the analyte(s) of interest emerges into a com-
partment (an ion-source region before the mass spectrometer) where ionisation occurs by a variety
of ionisation mechanisms and at atmospheric pressure. The ion stream produced is then swept or
drawn under vacuum through a mass 'selector' or 'fi lter'. Known as a quadrupole mass fi lter (which
contains four rods that carry voltage and generate an electromagnetic fi eld within the space/void
between them, see Figure 1.14), this component facilitates the selection of ions by mass before they
arrive at the ion detector.
Compound ionisation occurs in the 'ion-source' region of the mass spectrometer instrument
which operates at atmospheric pressure. Two techniques generally prevail: Electrospray Ionisation
(ESI) and Atmospheric Pressure Chemical Ionisation (APCI), both of which can be operated in posi-
tive and negative ionisation modes (i.e., the detector will be set to monitor positive or negative ions).
Figure 1.14 Quadrupole mass fi lter (photo courtesy of PerkinElmer, Inc., Watham, Massachusetts
(USA))