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There are several ways to decrease analysis time in GC. Reducing analysis time depends on the separations between the peaks of interest. In the case of enough separation, some efficiency can be traded for speed by:
- Using a shorter column length
- Operating the column at a higher flow rate or using flow programming to elute late-eluting compounds.
If the same efficiency is required, options are to:
- Use a faster carrier gas, i.e., hydrogen instead of helium
- Use a shorter capillary with a smaller bore—this produces similar efficiency with shorter run times.
In all of the above situations, analysis time is also limited by the maximum temperature programming that is allowed for the instrument used. A number of instruments employ direct electrical heating to accelerate temperature programming. However, with these systems it is not possible to merely cut out a piece of the column; rather, the whole column has to be replaced, making the system less maintenance-friendly.
Maximum program rate
GC instruments have a maximum program rate that is dependent on oven size, isolation, and oven voltage. In the U.S., 120 V is typically used, which means there is less energy available to heat up more quickly, compared with 220 V, which is standard in many other countries. For faster programming in the United States, a separate higher-voltage instrument is often installed using an additional 208-V power supply.
To obtain reproducible retention times, the maximum program rate should not be exceeded. It is important for the real oven temperature to exactly follow the set oven temperature. Figure 1 shows the consequences of using values that are too high. Components that elute at higher temperatures show high variability in retention time. The actual oven temperature lags behind the set temperature. To compensate for this, oven temperature program rates in the higher temperature segments are significantly lower, which means longer analysis times.
Figure 1 – Impact of oven program rates on reproducibility of analysis time of endosulfan sulfate. Overlay of seven analyses. A solution: reducing oven size
The GC Accelerator (Restek, Bellefonte, PA) (Figure 2) is a modular tool that can be used to reduce the size of the oven. In the laboratory it is sometimes referred to as an “oven pillow.” Once the column is installed, the GC Accelerator parts are connected to build an additional isolating oven wall. This allows faster programming of nearly a factor of 2 in the higher-temperature segments (see Table 1). The GC Accelerator can be used for the Agilent 6890/7890 MS as well as for flame ionization detection (FID) systems. Materials for the accelerator parts can be used up to 450 °C. Due to the reduction in oven size, column cages should not exceed 40 mm in depth.
Figure 2 – GC Accelerator consisting of three isolation boxes to build a new oven wall. Table 1 – Maximum oven temperature program of Agilent 6890/7890 with and without GC Accelerator
Practical, faster GC
Faster GC can be valuable for existing as well as new methods. For methods that need to be made faster and that have exactly the same peak elution order (same chromatogram), it is very important to adjust the oven temperature program. The elution temperatures for the new method need to be the same as for the old method. Free calculators are available to achieve this. The Restek EZGC method translator1 is a simple solution that can be accessed online or downloaded as a Windows compatible program.
The method translator will calculate a new oven temperature program. Significantly higher temperature programming and faster GC for existing applications and instrumentation are possible using the GC Accelerator.
An on-line chromatogram modeler that demonstrates the direct impact of any new conditions, ProEZGC2 allows users to model analytes, change the carrier gas, choose a different column dimension, change column flow or oven temperature, and view results immediately on a computer. The examples shown in Figures 3 and 4 were done using the ProEZGC.
Examples
- Figure 3: Analysis time was significantly faster using a 2× shorter column, and fast programming with up to 73.5 °C/min was possible using the GC Accelerator.
- Figure 4: Flow was increased by a factor of 4, and the oven program required for the same separation was calculated. A program of up to 52 °C/min was required.
Figure 3 – ProEZ
GC model using 2× shorter column. Analysis is nearly a factor of 3 faster due to the fast programming option. Figure 4 – ProEZ
GC model showing the easiest and most cost-effective way to reduce analysis time, i.e., operating at higher flow by applying a higher program rate and using the GC Accelerator. If no loss in efficiency is acceptable, one can choose to use hydrogen as the carrier gas or use a smaller-diameter capillary. A 20 m × 0.15 mm capillary will provide separations similar to a 30 m × 0.25 mm capillary but is much faster. With the GC Accelerator, these modifications will produce enhanced benefits for even faster GC.
3. It is possible to speed up 120-V systems to avoid the need for specific 208-V instrumentation and a separate power supply.3Figure 5 shows the analysis of U.S. EPA 8270 semivolatiles in just 10.5 minutes using a standard 120-V oven with the GC Accelerator.4
Figure 5 – U.S. EPA 8270 semivolatiles in 10.5 minutes using 120-V instrument and GC Accelerator.4 Another common application is ASTM D2887 for simulated distillation. This application using FID can now be run in 8.5 minutes using standard 120-V instrumentation (Figure 6). For this application, the large insert from the GC Accelerator Kit is used (see “3” in Figure 2), It is positioned just behind the front injection port.
Figure 6 – ASTM D2887 accelerated in a standard 120-V oven; column: 5 m × 0.53 mm MXT Simdist, 2.65 µm; carrier: He, 35 mL/min; oven: 40 °C, 35 °C/min → 350 °C; sample: 0.05 µL. Summary
The GC Accelerator is a very easy tool that can be used to decrease analysis times in standard Agilent GC instrumentation. If some separation is traded for speed, analysis times can be reduced by almost a factor of 3 using column dimension and higher flow rates in combination with faster temperature programming.
For similar efficiency, one can use 0.15-mm columns or/and consider hydrogen as the carrier gas. The GC Accelerator has shown to be effective with GC/MS and GC/FID applications.
References
- http://www.restek.com/ezgc-mtfc
- http://www.restek.com/proezgc
- http://www.restek.com/catalog/view/ 52293/23849
- http://www.restek.com/chromatogram/ view/GC_EV1476
Jaap de Zeeuw is with Restek Corp., Weerhaan 9, 4336 KT Middelburg, The Netherlands; tel.: +31 118 623 151; e-mail: [email protected]; www.restek.com. The author thanks Katarina Oden for sharing unique results for high-temperature simulated distillation.