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LPGC - Fast way to your pesticide analysis!

By
  • Jana Hepner
Tags
  • #Pesticides
  • #Method Optimization
  • #Fused Silica Capillary Columns
  • #Blogs
  • #Food Safety
  • #GC
  • #Low-Pressure GC (LPGC)
  • #MS
  • #Restek Reference Standards
  • #Rxi GC Columns
  • #Product Selection
  • #Reference Standards by Sector
  • #Food & Agriculture
  • #Reference Standards
  • #GC Columns
  • #Columns
  • #Food & Agriculture
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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)

Comments

Mon, Jan 04, 2021

Hello Leon, Thank you for your comment! Yes, you're correct that we aren't comparing apples to apples here. This is not a comparison of similar performance. However, that is not the intention of using the LPGC. The LPGC is a different technique that benefits from creating low pressure inside the analytical column. That allows for using high capacity inert 0.53mm ID columns and requires MS. It is only possible when using wider capillaries with a restriction in the front of the column. The restrictor allows to set a positive inlet pressure to get a flow of 2 mL/min but we have a strong vacuum gradient in the analytical column (0.53mm ID). We are aware that we have a limited number of plates, but that's where the MS (or, better, MS/MS) comes in. I hope that clarifies the low-pressure part. For sure we can do fast separations using narrow-bore columns, but we also have to overcome the challenges using those smaller ID columns if operated in an MS. - We have much lower loadability as the amount of phase per length is much lower. - If we would go for the same efficiency, we probably have to use 7.5m x 0.25mm which you need to operate at 1.4 mL/min. But we can't as our inlet pressure would be negative. If we use higher flow, say 2 mL/min at 6.3 kPa head pressure, we have 102cm/s linear velocity. Now we will still get the challenge to accurately set such low pressures when we use higher (total) flows. You will run into stability issues especially if you want to use a split. We can run this column also faster, say 150 cm/s to trade in some plates for extra speed and have higher inlet pressure. Now we are at a flow of 4 mL/min, and now we also need to look at the vacuum pump, as most pumps do not like >2.5 mL/min. Then we need a turbopump. - Injection bandwidth becomes exponentially more critical with smaller diameter columns, especially if used at high linear velocity. This generates a big challenge for injection speed and/or focusing. Thank you for bringing those interesting points!

Wed, Dec 30, 2020

Hi Sue, We observed that the responses were higher with LPGC. LPGC is also capable of higher volume injection, which would increase the responses further. Thank you for the question! Jana

Mon, Aug 31, 2020

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.

Sue
Wed, Dec 30, 2020

What are the differences in detector response for the LPGC compared with the conventional GC?

Yuk
Fri, Aug 07, 2020

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??