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How can analyte protectants and matrix help improve peak shapes?

30 Aug 2020

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.

decorativeFigure 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).

matrix comparison
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.