In my last blog, I presented a new technique called low pressure gas chromatography (LPGC, Figure 1). Just to recap, the LPGC system consists of a relatively short analytical column (10 – 15 m) with large ID and thick film (e.g. 0.53 mm and 1.0 µm, respectively) which is restricted with a narrow guard column (e.g. 5 m x 0.18 mm). The restrictor (guard column) allows for maintaining head pressure on the inlet, while the analytical column is under near-vacuum pressure.
Figure 1: LPGC schematics
During my analysis, I’ve run into issues with early eluting peaks. At the initial 80 °C starting temperature the first two compounds, methamidophos and dichlorvos, showed up as distorted, split peaks. I’ve found that the optimal initial temperature was 70 °C, but the question remains: what if the sample is in the matrix? Or what if we use analyte protectants? I’ve decided to investigate so I compared the original (solvent) analysis of the QuEChERS Performance Mix (#31152 ) to the analysis of added analyte protectant (0.1 mg/mL shikimic acid) and finally compared those to the celery matrix (Figure 2).
Figure 2: Comparison of pesticide residues' runs with no matrix or analyte protectant (black trace), celery matrix (red trace) and with analyte protectant (green trace)
Figure 2 shows that the matrix (red trace) distorts the peak shape even further at both tested temperatures. Celery isn’t a very “dirty” matrix (after using dSPE), therefore, it doesn’t act as an analyte protectant for these compounds. On the other hand, the analyte protectant (green trace) helps significantly with the peak shape at both 80°C and 70 °C. At 80 °C, the effect is more pronounced while at 70 °C it helps reduce the tail of methamidophos and narrows the dichlorvos peak. In conclusion, the analyte protectant (shikimic acid) can help with the peak shape at the original temperature, however, it is still preferable to lower the initial temperature to achieve a good solvent trapping.