FormNo. 203-821-249 01/05-REV0
LECO is a registered trademark of LECO Corporation
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Experimental Conditions
Gas Chromatography: Agilent 6890 GC
Column:
20m x 0.18mm x 0.14 µm CLPII (Restek)
Carrier:
Helium at 1mL/min, constant flow
Injection:
1 µL splitless at 250°C, 60 sec. valve
Oven Program:
40°C (1min), 40°/min to 120°, 20°/min to 320°
Total Run Time:
13min
Mass Spectrometry: LECO Pegasus III TOFMS
Ionization:
Electron ionization at 70eV
Source Temperature:
225°C
StoredMass Range:
45 to 550 u
Acquisition rate:
20 spectra/sec.
Data Processing
LECOChromaTOF software with automatic Peak Find andDeconvolution
Extraction andAnalysis of Spinach
The Florida-Modified—California Department of Food and Agriculturemultiresiduemethodwas used
to prepare an extract from frozen spinach purchased at a local grocery store. The spinach extract
was spikedwith pesticides prior to analysis by GC-TOFMS.
Calibration and IonRatio for PesticideAnalysis
With the full mass acquisition that is always available in TOFMS, the choice for identifying
a compound in a sample as a pesticide, and then quantifying that pesticide is usually a
three step process.
1. Does the compoundmeet the pesticide Reference Spectrummatch factor?
2. Does the compound fall within a certain retention time window?
3. Does the compoundmeet the S/N or area threshold set by the user?
However, there is a possibility that amass spectrum of an identified pesticide canmeet the
Reference Spectrum criterion, but still show bias (due to an interfering compound) on amass
selected for quantification. In this case, Ion Ratio can be an excellent QA feature for flagging the
quantification bias.
Ion Ratio is set up in a Calibration Table (Figure 2). After analysis of pesticide standards, Ion
Masses are entered in the Calibration Table and their ratios are calculated from a higher level
standard. The user can define an Ion Ratio Tolerance (%) that will governwhether the analyzed
ratio of a pesticide in a samplewill be flagged as "Passed" or "Failed" (Figure 3). An Ion Ratio
Result marked "Passed" supplements the Reference Spectrummatch, which is the first step in
identifying a peak in the proper retention timewindow as a particular pesticide. "Passed" also
assures quantitative accuracy by indicating that no bias exists onmasses chosen for quantification.
