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