Analysis times for semivolatile compounds can limit sample throughput and decrease overall lab productivity. In this technical article, an example of scaling down EPA Method 8270 and using a GC Accelerator kit to reduce cycle time is presented. Significantly faster semivolatiles analysis times were achieved, providing an opportunity for labs to increase sample throughput or process more rush samples.
Scaling Down Your Semivolatiles Column
To achieve the same chromatographic results in less time, the first step is to take the traditional column for semivolatiles analysis, a 30 m long, 0.25 μm ID, 0.25 μm df arylene-type column (e.g., an Rxi-5Sil MS column) and scale it down to a more efficient format that retains the same number of theoretical plates as the original column. A 20 m long, 0.15 mm ID, 0.15 μm df Rxi-5Sil MS column fits these requirements. The proportionally narrower ID and thinner film will improve efficiency (i.e., give narrower peaks) and provide the same selectivity by keeping the phase ratio (β) constant. Higher efficiency results in more theoretical plates per meter of column, so by shortening the column to 20 m, a very similar total number of plates is achieved. These changes result in the same degree of separation that is obtained on the traditional column format.
Scaling Down Method Conditions with the EZGC Method Translator
A scaled-down column requires a scaled-down method to ensure that compound elution temperatures will be the same so that equivalent separations will be achieved in a faster analysis time. Correctly translating the scaled-down column flow-rates and oven temperature ramp rates can be a challenge, but this task is made simple by using Restek’s EZGC method translator. This free, online tool calculates properly translated method conditions based on your original column and method in seconds. Just input your current column dimensions, flow parameters, and oven program, then define the new column format you’d like to use, and the software will do the rest.
With the scaled-down column and translated conditions defined, the new method can be tried in the lab, but, first, you need to review the oven ramp rates and ensure your GC oven can meet them. In this example, the scaled-down oven program has aggressive ramp rates that some GC ovens can’t consistently achieve. Ovens of 120 V would not be able to meet the new scaled-down oven program reliably, which would result in inconsistent retention times for compounds eluting during that ramp. However, this problem can easily be resolved by the use of the GC Accelerator kit.
The GC Accelerator kit was specifically designed for Agilent 6890 and 7890 GCs equipped with mass selective detectors (MSD). The inserts install without interfering with a column in the front inlet position or with the connection to the MSD transfer line. Once in place, the oven inserts occupy oven volume, which reduces the volume of air that the oven needs to heat and cool. This, in turn, permits faster oven ramp rates and quicker oven cooldown times.
In Summary
By applying the principals of column and method scale-down, with the assistance of the GC Accelerator kit to provide the added boost to your oven ramp rates, the analysis of even complex samples can be successfully translated to a much faster method while maintaining the same separation. In the example shown in this study, scaling down EPA Method 8270 resulted in much faster semivolatiles analysis times that will allow labs to increase sample throughput or process more rush samples.
GC Accelerator Oven Insert Kit
for Agilent 6890 and 7890 instruments
- Get the same GC separation in less time—use a GC Accelerator kit and the EZGC method translator to accurately convert methods to a scaled-down column format.
- Scaled-down methods let you speed up analysis time and increase sample throughput without capital investment.
- GC Accelerator kit installs easily without damaging the GC column or interfering with the MS interface.