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Flow considerations for LPGC analysis

17 Oct 2022
 

Recently, while investigating large volume injection using LPGC, we explored flow rate and its impact on sensitivity. Lehotay et al[1] described the flow in the LPGC system as a compromise between the increased optimal velocity in the low-pressure system and the MS detector capability to efficiently pump out the carrier gas. The low-pressure in the analytical column increases the optimal linear velocity by factor of 8; therefore, runs with a flow of 5-8 mL/min would be desirable. However, most MS systems have optimal performance up to 1.5 mL/min.

It is good practice to run a few experiments on your instrument to determine the sweet spot between increased flow and optimal MS performance. Figure 1 shows a typical decrease in peak area and height when the flow is increased from 1.5 mL/min to 3 mL/min.

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Figure 1: Comparison of methyl pirimiphos peak under different flows. SRM: 305 to 180 at 8 collision energy

Using some typical pesticides to measure sensitivity we ran the analysis from 0.75 mL/min to 2.75 mL/min (Figure 2). To be able to compare the different pesticides, the area at 0.75 mL/min was used to normalize the data.

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Figure 2: Comparison of normalized peak area of pesticide residues under different flows.

Figure 2 : The optimal range was determined to be between1 to 2 mL/min. This range is instrument dependent since there are vast differences in MS system capabilities. Our data was collected on Thermo TSQ 8000.

[1] Lehotay SJ, de Zeeuw J, Sapozhnikova Y, Michlig N, Hepner JR, Konschnik JD. There is no time to waste: Low‐pressure gas chromatography– Mass spectrometry is a proven solution for fast, sensitive, and robust GC–MS analysis. LCGC North America. 2020;38(8):457‐466.