By now you should know what a SPME Arrow is and that you should be using a SPME Arrow instead of a traditional SPME fiber, because the Arrow is more durable. The same physical dimensions, which make an Arrow more mechanically robust, also afford an increase in sensitivity. Last time, you saw that the phase volumes could increase by approximately 4 to 6x greater on an Arrow. This time, we see how that correlates to an increase in response.
Colton and I took a 100 µm PDMS traditional SPME fiber and a 100 µm PDMS SPME Arrow. We sampled headspace (HS) volatile organic compounds (VOCs) which had been spiked in drinking water at 2.5 ppb, as per method ISO 17943. Everything was equivalent (i.e., equilibration times, extraction times, desorption temperatures, etc…), except for the fibers.
Here is what the respective c-grams look like:
2 minute HS extraction of ISO 17943 VOCs on 100 µm PDMS SPME Arrow and 100 µm PDMS traditional SPME fiber
The following are three take-away messages I would like you to have:
- Obviously, the SPME Arrow’s response is higher than a traditional fiber, which should be of no surprise. It all makes sense that if you increase your phase volume, you increase your volume of target analyte collected, and thereby increase your analytical response.
- It is hard to accurately quantify this by the chromatogram overlay; however, you will have to trust me that the SPME Arrow demonstrated a ~4x increase in response over traditional SPME fibers for most compounds. Low and behold it is right on target with the phase area/volume increase we discussed last time.
- I say “on average” in the previous point, because it is important to note that the increase in response for very volatile compounds like vinyl chloride is ~10x on the SPME Arrow vs the traditional SPME fiber. Whereas, a semi-volatile compound like naphthalene only sees a 2x increase in sensitivity on the SPME Arrow compared to the traditional SPME fiber. This has more to do with partial pressures (i.e., Henry’s law constants) of these compounds and therefore their availability in the headspace being the rate-limiting step. If you have not already done so, take a look back at the chromatogram and you will see what I am talking about.
So now you have the option of a more durable and more sensitive SPME product. What more could you want!? Well, stay tuned for next time when I talk about how you can trade in your Arrow’s increase in sensitivity for speed…
01/10/18 Update: If this is of interest to you, be sure to check out my other post on "SPME Arrow Sensitivity = Speed."