Method translation and PLOT columns – Analysis of gases on Alumina column
3 Feb 2021One of the tools we can’t live without when modifying/developing a GC method is the EZGC method translator. Changes made to GC method parameters like; inlet/outlet pressure, flow/carrier gas type or even capillary column dimensions will result in different retention times for our analytes and will affect resolution. Method translators calculate new analysis conditions by keeping the analytes’ elution temperatures the same, thus preserving the elution order. Using this tool, the analysis method can be translated to a different column dimension, carrier gas type, linear velocity, for example, with very little time spent on the method development. Method translation works well for the columns with a liquid stationary phase. But how well will it work with PLOT columns?
There are two reasons we thought it would not be accurate for PLOT columns:
- Inaccurate flow control due to flow restriction through the column.
Thick layers of particles are difficult to deposit in an even layer. Uneven coating thickness could affect the column internal diameter/flow (1). - PLOT column chromatography or gas/solid chromatography is based on adsorption/desorption principles – a surface process. Do the same rules work in gas-solid as in gas-liquid chromatography?
To prove the concept, I used an existing method, ASTM D2712 (2), analysis of hydrocarbon impurities in propylene. The method is essential among the propylene producers since propylene is a starting material for many plastics and chemicals. Its purity determines the quality and price. While the analysis is run on a 4-column analyzer, the main analytical column is the Alumina/KCl BOND, and that’s the column I’ll test for this demonstration.
I translated the original method from helium carrier gas to nitrogen and hydrogen using a “Translate” function (Figure 1) – translation to alternative carrier gases. The column length was determined by counting the number of column loops on the cage (3). I was glad the calculated number matched the length of a new column – 50.5m. A faster technique to determine the column length is to measure the column hold-up time (4). However, this approach will not work with Alumina columns because even the lightest analytes, like methane, show some retention. Since I was using FID, there aren’t many other options for unretained compounds.
Figure 1: Translation with the EZGC Method Translator using Alumina/KCl BOND 50m x 0.53mm x 10µm column and helium carrier gas to hydrogen and nitrogen.
Figure 2: Chromatograms using all three carrier gases and translated analysis conditions, respectively.
The obtained chromatograms above (Figure 2) show a similar elution pattern. Additionally, I used the retention times and oven temperature profile to calculate the analytes’ elution temperatures for all three carrier gases. Given that the main principle of the translation is to preserve the elution temperature from one analysis to the other, we should expect minimal deviation between the calculated elution temperatures. (Figure 3, last column). Further, I compared predicted retention times (calculated using speed factor) to the actual retention times (Figure 3). I could not expect better results for both carrier gas and the minor deviations attributed to the estimated column length.
Figure 3: Retention time of the analytes using all three carrier gases, calculated elution temperature, and predicted retention times.
The second parameter investigated was the resolution. Despite the considerable differences between the analytes’ retention times, I noticed little change in the resolution with slightly better numbers when using a nitrogen carrier gas. I plotted the resolution numbers below (Figure 4). Closely eluting compounds, where we strive to maintain resolution, showed a very narrow standard deviation window.
Figure 4: Comparison of resolution numbers obtained using all three carrier gases
Alumina columns and hydrogen carrier gas
We confirmed that with minimal method development (just entering the numbers in the method translator), we could translate the analysis method to any alternative carrier gas. However, some customers have reported unusual results when using alumina columns with hydrogen carrier gas. At higher temperatures, alumina columns may become reactive and catalyze cracking/hydrogenation of the unsaturated hydrocarbons. While I didn’t notice any unusual behavior on the KCl column, I suggest that you check column performance during the method validation step.
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