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Reliably Detect Pesticides Down to 10 pg with Sensitive SIM GC-MS Multiresidue Method

By Jason Thomas, Environmental Innovations Chemist

Market demands are increasing for multiresidue pesticide methods that are both sensitive and effective across a broad range of compound chemistries. The Rxi®-5Sil MS column gives you accurate low level results for a wide variety of analytes in a single run.

As labs operate in an extremely competitive market, the demand for more sensitive multiresidue pesticide methods is increasing. A GC/MS method is a logical choice, as this instrument provides a high degree of specificity, yet is relatively inexpensive and easy to operate, compared to LC/MS/MS, high resolution MS, or GC/MS/MS. However, to take full advantage of GC/MS, careful column selection is critical. The column used must be of the proper selectivity to separate compounds that share common spectra, and also exhibit a high degree of inertness and minimal bleed. Here we demonstrate the effectiveness of an Rxi®-5Sil MS column for low level analysis of a wide variety of pesticides differing in volatility, compound class, and degree of activity.

The inertness of the Rxi®-5Sil MS column ensures linear performance down to 10 pg on-column, allowing more accurate low level quantification.

Excellent Response for Difficult Active Compounds

Column inertness, selectivity, and bleed are key considerations and often determine the success or failure of analytical runs. Inertness can be assessed through the behavior of active compounds, which often exhibit disproportionately poor responses at low concentrations. Although the compound list analyzed here contains many compounds with a high degree of activity, low level linearity (10-1,000ng/mL) was established with an r2 value of 0.990 or above for many of these challenging compounds (Table I). In addition, the notoriously problematic compounds of EPA Method 8081, endrin and 4,4′-DDT, were among the least troublesome tested here, attaining values of 0.997 and 0.998, respectively. Note that standards were analyzed for this study and some compounds with r2 values less than 0.990, such as acephate, omethoate, and dicofol, show a more linear response when analyzed in matrix. As shown in Figure 1, the Rxi®-5Sil MS column is also highly inert, producing excellent peak shape even for difficult compounds such as acephate. The linearity, sensitivity, and inertness demonstrated here make the Rxi®-5Sil MS column ideal for more accurate low level quantification of active compounds.

Low Bleed, High Selectivity

Another crucial characteristic for multiresidue pesticide methods is column bleed. Minimizing bleed is critical in preventing interference with target compounds, even in SIM analysis, as some compounds may share ions and have similar bleed spectra. As shown in the TIC chromatogram in Figure 2, the ultra-low bleed of the Rxi®-5Sil MS column allows full scan analysis with minimal interference from column bleed. The Rxi®-5Sil MS column provides excellent separation for the wide range of chemistries tested and the column is also selective enough to easily separate isomers, such as permethrin I and II. Many pesticides have isomers which share common spectra. In these cases, it is imperative to have a column capable of chromatographic separation in order to individually identify these isomers.

In summary, many of the difficulties associated with multiresidue methods are simplified by using the Rxi®-5Sil MS column. Its outstanding inertness, low bleed at high temperatures, and unique selectivity provide a robust capillary column with the sensitivity and longevity needed to address the tough challenges inherent to low level multiresidue pesticide analysis.

Table I  The Rxi®-5Sil MS column provides excellent linearity, and thus more accurate results, for a wide range of pesticide chemistries down to 10pg.

Retention time (min.)Quant. ionQual.
ion 1
Qual.
ion 2
ISr2 (10-1,000 ppb)
methamidophos5.771419594BNB0.997
dichlorvos6.0218579109BNB0.998
bromonitrobenzene (IS)7.21203201157IS-
mevinphos8.26192127109BNB0.995
acephate8.301369594BNB0.982
o-phenylphenol9.44170169141BNB0.997
omethoate10.23156110109BNB0.976
dimethoate11.7712514393BNB0.981
pentachloronitrobenzene (IS)12.13295249237IS-
diazinon12.45179304137PCNB0.994
chlorothalonil12.55266264268PCNB0.983
vinclozin13.48285198212PCNB0.998
carbaryl13.65144116115PCNB0.996
metalaxyl13.69206160132PCNB0.997
dichlofluanid14.17123167224PCNB0.954
malathion14.19173125127PCNB0.992
thiabendazole I15.34201202174PCNB0.958
captan15.3479119149PCNB0.987
folpet15.46260130104PCNB0.964
imazalil16.10215175173PCNB0.982
myclobutanil16.34206179150PCNB0.973
endrin16.82265279317PCNB0.997
fenhexamid17.7917717997PCNB0.969
4,4′-DDT17.79237235165PCNB0.998
propargite18.04173150135PCNB0.999
triphenylphosphate (IS)18.09325215326IS-
iprodione18.47314316187TPP0.991
bifenthrin18.64181166165TPP0.998
fenpropathrin18.82265208181TPP0.985
dicofol18.89139251253TPP0.788
permethrin I20.41183165163TPP0.998
permethrin II20.54183163165TPP0.995
deltamethrin22.87253251181TPP0.995
Standard curve: 10, 25, 75, 150, 500, and 1,000 ng/mL mixed standards, single 1µL injections.

Figure 1  Outstanding peak symmetry for active compounds demonstrates column inertness (500pg).

Peaks
1.Acephate
Acephate Rxi®-5Sil MS
GC_EV01036
ColumnRxi®-5Sil MS, 30 m, 0.25 mm ID, 0.25 µm (cat.# 13623)
using Rxi® Guard Column 5 m, 0.25 mm ID (cat.# 10029)
Sample
Conc.:1 ppm each compound
Injection
Inj. Vol.:0.5 µL splitless (hold 1 min)
Liner:PSS Drilled Uniliner® (hole near bottom) (cat.# 22989)
Inj. Temp.:250 °C
Oven
Oven Temp.:90 °C (hold 1 min) to 310 °C at 10 °C/min (hold 5 min)
Carrier GasHe, constant flow
Linear Velocity:32 cm/sec @ 90 °C
DetectorMS
Mode:Scan
Transfer Line Temp.:300 °C
Ionization Mode:EI
Scan Range:50-350 amu
InstrumentPE Clarus 500 GC & Clarus 500 MS

Figure 2  Use an Rxi®-5Sil MS column to easily resolve a broad range of pesticide chemistries in a single run (500pg).

TIC chromatogram to illustrate unique selectivity and low bleed level.

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