And we’re back! Previously, we discussed analyzing residual solvents via HS-GC using a gas-tight syringe. If you missed it, be sure to check it out here! Today, we are talking about a sample extraction technique that is less common, but has great capabilities; known as Solid Phase Microextraction (SPME).
SPME is a sampling technique consisting of a fiber, coated with a sorptive phase. When placed into the headspace of a sample, analytes sorb (adsorb or absorb depending on fiber type) onto the fiber, and are desorbed off the fiber in the GC inlet. We have decided to use the SPME Arrow instead of a traditional SPME fiber for this analysis. The SPME Arrow has a much larger sampling volume, but its best attribute is its durability compared to a traditional fiber (see the figure below). For more information on SPME, be sure to check out Jason Herrington’s blogs here.
When moving from the HS-Syringe method to the HS-SPME method, a couple extraction parameters must be changed. The HS-Syringe performs best under long equilibration times at higher temperatures, whereas the HS-SPME performs best under shorter equilibration times at lower temperatures. We have the data to support this, so check out Chromablography in the future for the proof! These main differences between the HS-Syringe and HS-SPME can be seen in the tables below. The reason for this drastic difference in temperature is if the SPME Arrow is placed in a hot vial, the fibers temperature will rise. The SPME Arrow’s thermally conductive metal core magnifies this problem. Because the fiber is at a hotter than the laboratory atmosphere, the analytes on the fiber will desorb off before the fiber reaches the inlet. Some compounds of interest, like propane and butane, show incredibly low responses using elevated temperatures. By keeping the fiber at a cooler temperature, we are able to keep those analytes on the fiber until it is desorbed in the GC inlet.
The chromatography comparing the HS-Syringe and HS-SPME methods can be seen below.
Notice that the HS-Syringe gives better response for propane, isobutane, and n-butane. You can see the HS-SPME method quickly has excellent recoveries for higher molecular weight compounds. The peaks tail using HS-SPME, but this is a normal result for this type of extraction technology. Overall, things look good on both the HS-Syringe and the HS-SPME Arrow. Stick around for future blogs where we will go into more depth with both techniques!