FormNo. 203-821-249 01/05-REV0
LECO is a registered trademark of LECO Corporation
1
Separation Science Application Note
Pegasus
®
IonRatio asQualityAssurance for Pesticide
Analysis byGas Chromatography—
Time-of-FlightMass Spectrometry (GC-TOFMS)
Introduction
Ion Ratio is a ChromaTOF
®
software feature that was originally developed for clients who prior to
their acquisition of a LECO Pegasus Gas Chromatograph (GC)—Time-of-Flight Mass Spectrometer
(TOFMS), weremeasuring polychlorinated biphenyls (PCBs) by GC-Selected Ion Recording (SIR)—
high-resolutionmass spectrometry (HRMS). With SIR, where a full mass spectrum is not available,
to ensure that it was a PCB theywere determining and not an interference, they compared the
ratios of twomolecular ions for each congener (Figure 1). In nature,
35
Cl and
37
Cl exist in a 76%
and 24% proportion, respectively. Exact ion ratios for different chlorination levels can be
calculated, and if SIR determined ratios were outside a percentage range for the expected ratios
(either theoretical or from analyzed standards), then the particular compoundwas not a PCB.
Conversely, if themeasured and expected ratiosmatchedwithin a certain error (and the GC
retention timewas correct), then the compoundwas a PCB. Thismethod is also used in chlorinated
dioxin and furan analysis with SIRHRMS. Even though these clients now had the qualitative power
of a full mass spectrum from a time-of-flightmass spectrometer, they desired Ion Ratio to
supplement PCB verification.
Ion Ratio can be used for pesticide analysis as a Quality Assurance (QA) tool to supplement the full
mass spectrum that is always obtainedwhen doing TOFMS. Most importantly, Ion Ratiomay
illuminate quantificationmass bias from interferences in the casewhere summed ions are used for
quantification. This application note demonstrates Ion Ratio for a group of pesticides analyzed in a
spiked spinach extract.
(mainlib)1,1'-Biphenyl,2,2',3,3',4,5-hexachloro-
30 60 90 120 150 180 210 240 270 300 330 360
0
50
100
36 50
61
73
91
109
127
145
163 180
192
218
253
290
325
360
Cl
Cl
Cl
Cl
Cl
Cl
Figure1.
Mass spectrum of hexachlorobiphenyl. The 360 and 362 ions would be used for selected
ion recording—high-resolutionmass spectrometry. Their proportion, as calculated from
35
Cl and
37
Cl abundances in nature, is approximately 1.25 (360/362).
