LPGC – Fast way to your pesticide analysis!

Throughput is one of the most important parameters in the lab. The more samples we can analyze in a day, the sooner we can get home. Enter Low Pressure GC (LPGC) – this is an invention from our brilliant Jaap de Zeeuw [1-2], where a relatively short analytical column (10 – 15 m) with large ID and thick film (e.g. 0.53 mm and 1.0 µm, respectively) where the flow is restricted with a narrow guard column (e.g. 5 m x 0.18 mm). The restrictor (guard column) allows a normal head pressure at the inlet, while the analytical column is operated under near-vacuum conditions. The low pressure inside the 0.53mm column, shifts the optimum linear velocity about a factor 7 higher, which allows for faster analysis without a total loss of efficiency. The wider ID and thicker film provides also higher capacity, robustness and inertness. In addition, an integrated transfer line adds additional robustness to the method as the absence of phase in the heated transfer line to the MS, helps to reduce background and make the system stabilize faster. Figure 1 shows the LPGC system schematics.

The real question is, how much faster is the analysis? Let’s look at our QuEChERS Performance Mix (#31152), which contains organochlorine, organonitrogen, organophosphorus, and carbamate pesticides commonly used on fruits and vegetables. It’s also a good test of the extraction, cleanup and chromatographic conditions, both GC and LC. Figure 2 shows a conventional GC analysis of this mix.

Figure 2: Traditional GC_MS analysis of QuEChERS performance standard (GC_FS0530). Column: Rxi-5ms, 30 m, 0.25 mm ID, 0.25 µm (cat.# 13423); oven temp: 70 °C (hold 1 min) to 330 °C at 8 °C/min (hold 6.5 min), flow: 1.4 mL/min

Below we have the faster analysis using LPGC (Figure 3):

Figure 3: GC-MS analysis of QuEChERS performance mix using LPGC

The analysis is 4x faster! However, the second analysis starts at a higher temperature, which leads to peak splitting of the first two peaks, methamidophos and dichlorvos. Those are analytes mostly analyzed using LC, so you might not care about the shape. However, if you do, you can simply start the analysis at lower temperature (Figure 4). Methamidophos peak has the best shape at 70 °C, dichlorvos has good peak at temperatures between 60 and 70 °C.

Figure 4: Comparison of early eluting peaks at different initial temperatures

The root cause for the double peaks is a polarity mismatch of the solvent of the sample and the surface of the restriction column. Splitless injection of acetonitrile on a non polar surface can cause droplet formation which can form a multiple injection band. In figure 3 we also see that the first peak is affected. As we do need some solvent condensation for good focusing, the initial oven temperature has a big impact.

But how about helping the peak shape without reducing the initial temperature? We’ll look into using matrix and analyte protectants next time!

Sources:

  1. de Zeeuw, J. Peene, H.-G. Janssen, and X. Lou, J. High Res. Chromatogr. 23, 677-680 (2000).
  2. De Zeeuw, Gas chromatographic device. U.S. Patent #6,301,952 (2001)

2 Responses to “LPGC – Fast way to your pesticide analysis!”

  1. Yuk says:

    Hey Jana,

    For Fig. 4, if the peak distortion was due to a polarity mismatch between ACN and stationary phase, the higher the initial oven temperature the better the peak shape should be. It is strange that the peak shape at 80C was worse than 70C if the peak shape were only affected by the polarity mismatch. It is also strange that only the Methamidophos peak is distorted at 60C while Dichlorvos is fine even though they are practically co-eluting.

    For 80C being worse than 70C, I think it is due to lack of cold trapping as methamidophos and dichlorvos are eluting at 170C ish(i.e. merely 90C higher than the initial oven temp of 80). Initial oven temp does not need to be exactly 20 C below from the boiling point of the solvent, but instead the initial temp can be higher as long as it is ca. 150C lower than the first eluting peak if you don’t want/need solvent recondensation.

    For only affecting Methamidophos, I think there is a secondary interaction with NH2 of Methamidophos, not polarity mismatch. The lower the temperature, the higher the interaction.

    What do you think??

  2. Jana Rousova says:

    Hi Yuk,
    Thank you for your comment! I think you are correct that there is more to it than the solvent trapping and polarity mismatch. Methamidophos is very polar and it is very possible that plays a big role.

Leave a Reply


Restek Domestic Customer Service

Subject

Message

Your Full Name

Your Email

Company Name

Address

Spam Block (Please leave this blank)

all fields required

Thank you

Your message has been sent. We will be in touch shortly.

Message not sent

Sorry, your message could not be sent at this time. Please try again later, or contact Restek or your local Restek representative via phone.

www.restek.com/Contact-Us