Being fairly new to anything comes with some good lessons learned. My recent foray into Solid Phase Microextraction (SPME) has been a lot of fun, but I’d be lying if I said it has not been frustrating, complete with some face-palming moments.
In this series, we are going to take you on a journey, looking at different SPME parameters. Not only do we hope that this series will teach you about the fundamentals of SPME, but also that it can assist you during method development.
To kick off this series, we are going to start with how incubation/extraction Temperature affects compound response. This will cover compounds ranging from very volatile to semi volatile compounds. This topic originally came up during some method development for a residual solvents in cannabis products application (check out that series here!) where we wanted to move from a headspace syringe (HS-Syringe) method to a headspace SPME (HS-SPME) method. Not having the most experience with SPME, it was thought that we could just take our HS-Syringe parameters and use them for the HS-SPME method. And as you may have guessed, we were wrong.
To understand what temperature is best for doing HS-SPME (keep in mind that we are using the SPME Arrow for this study), we set up an experiment where we tested incubation/extraction temperatures from 30°C to 80°C and then compared each compound’s averaged area.
This was also compared to the HS-Syringe, where the incubation temperature was set to 80°C, which is a traditional temperature for this type of analysis.
These results are very interesting. Just looking at the HS-SPME results, having an incubation and extraction temperature of 30°C gives the best response for the hydrocarbons, especially for the hydrocarbons that have higher boiling points. The more polar compounds respond a little differently though. For example,the response increases for methanol and ethanol as the incubation and extraction temperature increase. These samples are in water, so it makes sense that these compounds need Higher Temperatures to partition out of the water and into the headspace. However, for the majority of the compounds (17 out of 22) HS-SPME at 30°C gives better response than at 80°C.
When the HS-Syringe method (80°C, 45 min) is compared to the results for HS-SPME (30°C-80°C, 2 min), the HS-Syringe does give better response for some compounds, but when taking in all of the data, the HS-SPME results are very close and for the majority of the compounds, it gives much better response in a fraction of the time.
So, why does HS-SPME at 30°C give better response than at 80°C for the majority of the compounds? I mean come on, more is always better, right?
Well, as Pawliszyn pointed out in 1995, high temperature can have an adverse affect on the absorption of analytes to the fiber coating. This is due to the decrease of partitioning coefficients, meaning that at higher temperatures, while the analytes will release from the matrix, does not mean that they will absorb to the fiber coating. The analytes are more soluble in the fiber coating at lower temperatures versus higher temperatures and this makes a lot of sense knowing that to desorb the analytes off the fiber and into the gas chromatograph requires the inlet to be at higher temperatures rather than lower.¹
If you have encountered this problem before, when switching from a traditional HS-Syringe method to a HS-SPME method, we know how you feel! Luckily, if you simply adjust your incubation temperature, you should see a nice boost in response for most of your compounds of interest!
It is important to note that the incubation time for all of above results was 120 s and the extraction time was 120 s for the SPME Arrow. Perhaps one could incubate and extract for less time at the higher temperatures. However, clearly higher temperatures were detrimental at the current times. Stay tuned in the future when we look at incubation time.
- Quantitative Extraction Using an Internally Cooled Solid Phase Microextraction Device. Zhouyao. Zhang and Janusz. Pawliszyn Analytical Chemistry 1995 67 (1), 34-43