Reliably Detect Pesticides Down to 10pg With sensitive SIM GC/MS Multiresidue Method

By Amanda Rigdon, Pharmaceutical Innovations Chemist and Rick Lake, Pharmaceutical Market Development Manager

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.

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. ion Qual. ion 1 Qual. ion 2 IS r2 (10-1,000 ppb) methamidophos 5.77 141 95 94 BNB 0.997 dichlorvos 6.02 185 79 109 BNB 0.998 bromonitrobenzene (IS) 7.21 203 201 157 IS - mevinphos 8.26 192 127 109 BNB 0.995 acephate 8.30 136 95 94 BNB 0.982 o-phenylphenol 9.44 170 169 141 BNB 0.997 omethoate 10.23 156 110 109 BNB 0.976 dimethoate 11.77 125 143 93 BNB 0.981 pentachloronitrobenzene (IS) 12.13 295 249 237 IS - diazinon 12.45 179 304 137 PCNB 0.994 chlorothalonil 12.55 266 264 268 PCNB 0.983 vinclozin 13.48 285 198 212 PCNB 0.998 carbaryl 13.65 144 116 115 PCNB 0.996 metalaxyl 13.69 206 160 132 PCNB 0.997 dichlofluanid 14.17 123 167 224 PCNB 0.954 malathion 14.19 173 125 127 PCNB 0.992 thiabendazole I 15.34 201 202 174 PCNB 0.958 captan 15.34 79 119 149 PCNB 0.987 folpet 15.46 260 130 104 PCNB 0.964 imazalil 16.10 215 175 173 PCNB 0.982 myclobutanil 16.34 206 179 150 PCNB 0.973 endrin 16.82 265 279 317 PCNB 0.997 fenhexamid 17.79 177 179 97 PCNB 0.969 4,4?-DDT 17.79 237 235 165 PCNB 0.998 propargite 18.04 173 150 135 PCNB 0.999 triphenylphosphate (IS) 18.09 325 215 326 IS - iprodione 18.47 314 316 187 TPP 0.991 bifenthrin 18.64 181 166 165 TPP 0.998 fenpropathrin 18.82 265 208 181 TPP 0.985 dicofol 18.89 139 251 253 TPP 0.788 permethrin I 20.41 183 165 163 TPP 0.998 permethrin II 20.54 183 163 165 TPP 0.995 deltamethrin 22.87 253 251 181 TPP 0.995 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).

acephate Column: Rxi®-5Sil MS, 30m, 0.25mm ID, 0.25µm (cat.# RE13623), with 5m x 0.25mm ID Rxi® deactivated guard tubing (cat.# RE10029) Sample: 1ppm acephate 1ppm triphenylphosphate (cat.# RE32281) 1ppm 1-bromo-2-nitrobenzene (cat.# RE32279) 1ppm pentachloronitrobenzene (cat.# RE32091) Inj.: 0.5µL splitless (hold 1 min.), PSS Drilled Uniliner® (hole near bottom) inlet liner (cat.# 22989) Inj. temp.: 250°C Carrier gas: helium, constant flow Linear velocity: 32cm/sec. @ 90°C Oven temp.: 90°C (hold 1 min.) to 310°C @ 10°C/min. (hold 5 min.) Det.: MS Transfer line temp.: 300°C Scan range: 50-350amu Ionization: EI Mode: scan GC_EV01036

Figure 2: Use an Rxi®-5Sil MS column to easily resolve a broad range of pesticide chemistries in a single run (500pg).
methamidophos dichlorvos bromonitrobenzene (IS) mevinphos acephate o-phenylphenol omethoate dimethoate pentachloronitrobenzene (IS) diazinon chlorothalonil vinclozolin carbaryl metalaxyl dichlofluanid malathion thiabendazole captan folpet imazalil myclobutanil endrin fenhexamid 4,4?-DDT propargite triphenylphosphate (IS) iprodione bifenthrin fenpropathrin dicofol permethrin I permethrin II deltamethrin Column: Rxi®-5Sil MS, 30m, 0.25mm ID, 0.25µm (cat.# 13623), with 5m x 0.32mm ID Rxi® deactivated guard tubing (cat.# 10039) Sample: 1ppm each compound (custom mixes) 1ppm triphenylphosphate (cat.# 32281 ) 1ppm 1-bromo-2-nitrobenzene (cat.# 32279) 1ppm pentachloronitrobenzene (cat.# 32091) Inj.: 0.5µL splitless (hold 1 min.), PSS Drilled Uniliner® (hole near bottom) inlet liner (cat.# 22989) Inj. temp.: 250°C Carrier gas: helium, constant flow Linear velocity: 32cm/sec. @ 90°C Oven temp.: 90°C (hold 1 min.) to 310°C @ 10°C/min. (hold 5 min.) Det.: MS Transfer line temp.: 300°C Scan range: 50-350amu Ionization: EI Mode: scan GC_EV01034
TIC chromatogram to illustrate unique selectivity and low bleed level.