Author: Dragan Krsta
During a recent Introduction to Gas Chromatography workshop at ADKL Labs the topic of sample discrimination came up. Many factors combine to impact sample discrimination and a thorough understanding of the sample and chromatography system are required to address the issue. The workshop concluded with a wonderful set of example data to demonstrate inlet discrimination, which we've shared below.
Most users will be familiar with a version of discrimination. For example, volatile components might be introduced onto the column at a different rate compared to high-boilers. Where there is discrimination it can be hard to quantify (or even see) some components within a sample mixture even though you know they are present. You may miss components completely if they do not make their way onto the column after introduction into the GC inlet. The best way to minimize discrimination is to perform on-column injection. However, this technique presents its own practical limitations and also introduces a lot of non-volatile components onto the column. The vast majority of laboratories stick with the proven, robust vaporizing split/splitless inlet.
In order to demonstrate inlet discrimination during a recent workshop we performed a series of splitless injections using a typical hydrocarbon series. The workshop participants initially struggled to grasp the affect of discrimination. We acquired the below example data while discussing the different parameters and their contribution(s) to sample discrimination.
All parameters were the same for each injection, except the inlet temperature which was varied from 200-300 ⁰C. You can see a comparison of the chromatograms from this experiment below. There is a lack of late-eluting, high-boiling components within the hydrocarbon series when the inlet is at 200 ⁰C (last eluting peak is n-Tetracontane, C40). However, as the temperature of the inlet increases the peak intensity of the high-boiling components increases. In this scenario, the inlet temperature is likely to be causing discrimination of high-boilers by incomplete elution from the needle and also the inlet liner itself.
Adjusting the inlet temperature can help to reduce discrimination in this case, but other factors should also be considered including potential for thermal degradation of other components, inlet pressure, inlet liner design, liner transit time, column position, needle position, injection speed and more. In this recent workshop discrimination had previously been a problem for the attendees and we tailored the content accordingly.
We hope you found this short example helpful, please contact ADKL Labs with any feedback.
Please contact ADKL Labs if you would like to find out more about our training workshops. You can also visit the training page to learn more. We have training workshops from novice to advanced, as well as custom application and method development services.
Note: The views and opinions expressed herein are those of the author and do not necessarily reflect the official position of ADKL Labs.