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Scale down for faster GC

20th June 2018


Fig. 1. EZGC method translation software makes it easy to scale down existing methods so users get the same separation in much less time
Fig. 2. Comparison of original method and modelled results based on the scaled down column and translated method. The translated method provides the same elution order and relative peak separation, but the peaks elute in a much shorter analysis time

Scott Grossman and Christopher Rattray explain that fast GC isn’t as simple as just installing a smaller column. To get the same results faster, accurate method translation is essential

It sounds simple: shorter GC analysis times mean higher sample throughput and increased revenue. But, reality can be more complex. Faster analysis can compromise chromatography, leading to coelutions, changes in elution order and other issues that require significant lab time to resolve. If you are happy with the performance of an existing method and just want to get the same results faster, adapting your current method to a scaled-down column is a surprisingly simple means of achieving that objective. The key to successful scale down is modern method translation software, such as the EZGC method translator. This free, online program allows users to accurately translate their method based on a few simple inputs, all in minutes from their desk instead of over days in the lab.

Why does scaling down increase speed?
While it may seem counterintuitive that you can get more done with less column, faster analyses can be obtained by moving to a shorter column with a narrower inner diameter (ID) and thinner film. This is because smaller diameter columns with thinner film produce narrower peaks, which allows analysis times to be compressed without losing resolution. When changing column dimensions, it is important to adjust the inner diameter and film thickness proportionally so the phase ratio stays the same. Because narrower ID, thinner film columns are more efficient, they have more plates per metre and will also need to be shorter in order to provide the same number of total plates as your original column, which is necessary to obtain the same separation. Adjusting column dimensions is the first step in fast GC; however, just scaling down the column won’t give you the same results faster. To get the same separation in less time, you also need to adjust key method parameters to account for the shorter, more efficient column. Fortunately, that’s easy to do using EZGC method translation software.

Making accurate scale down simple
To see how EZGC software simplifies method translation, let’s look at PCB analysis as an example. In this case, we are switching from a 30m, 0.25mm, 0.25 µm column to a 20m, 0.15mm, 0.15 µm column, which is a scaled-down column with the same phase ratio and number of theoretical plates. To obtain the translated method conditions that provide the same chromatographic separation in less time on the new column, simply input the carrier gas, column dimensions, flow rate, outlet pressure, and oven program used in your existing method, as well as the new, scaled-down column dimensions. As shown in Fig. 1, based on these simple inputs, EZGC software provides a full set of new, properly translated conditions that will provide the same separation in significantly less time. This example illustrates a GC-MS analysis with a vacuum at the column outlet, but the software will also work for other detectors by simply switching to an atmospheric pressure outlet setting.

Fig. 2 compares the original method to modelled results using the scaled-down column and newly translated method. In this example, we’re seeing a savings of almost 8 minutes, about 30%, in analysis time. Put simply, this change would allow an instrument to complete a sequence of samples that normally would take 8 hours in a little more than 5.5 hours. In fact, with the final peak eluting at 11.5 minutes under the new set of conditions, thought could be given to changing the end of the method to ramp quicker or end sooner, resulting in an even shorter analysis than the direct translation calculates. If desired, additional optimisation for either analysis speed or system efficiency can be performed using the Pro EZGC chromatogram modeller, a software program that can be used in conjunction with the EZGC method translator for further method refinement. When moving to either a directly translated or further optimised method, make sure to confirm that your GC oven can achieve the new ramp rates. If not, Agilent users can install a GC Accelerator oven insert kit so that ramp rates can reliably be achieved and retention times remain stable.

The last thing to keep in mind if you scale down to faster GC is to consider pairing a split injection technique with the narrower, thinner film column. The much higher efficiency of scaled-down columns will produce narrower, taller peaks, which inherently provide a boost in sensitivity. That gain alone might allow you to still meet detection limits, even for relatively low concentration samples. The value of the split injection is twofold. First, if there were any loading capacity concerns (e.g. a thinner film producing overloaded, fronting peaks), split injection will take care of that. Second, the biggest benefit is that the very rapid transfer of sample onto the column will help keep the bands of molecules tight, and, as a result, the peaks narrow. The slower transfer time of a splitless injection, especially into a relatively wide inlet liner (e.g. a 4mm ID), will cause band broadening, and consequently, peak widening, which might negate some of the efficiency benefits of using a scaled-down column.

Same separation, speedier solution
In conclusion, equivalent chromatographic results can be obtained in much faster analysis times for many applications by moving to a scaled-down column that has the same phase ratio and number of theoretical plates as the original column. EZGC method translation software makes it exceptionally easy to do this on any GC system, and the translated method can be even further optimised if desired. Scaling down an existing method to a faster format is a very effective way to increase lab productivity and revenue.

Scott Grossman and Christopher Rattray are with Restek





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