If you have been looking at SPME Arrows, because we convinced you of the Arrows’ mechanical robustness, sensitivity, speed, etc., you could find yourself in a decision dilemma. Beyond phase selection, which is the subject of another blog, the various SPME Arrow configurations may leave you stymied. So, it is the intent of the current blog to help remove any of the mystique surrounding the following 3 “major” SPME Arrow configurations:
- 1.1 mm Arrow
- 1.5 mm Wide Sleeve Arrow
- 1.5 mm Arrow
- 250 μm Polydimethylsiloxane (PDMS) (cat.# 27484)
As alluded to earlier, the current blog will focus on the “major” (i.e., 1-3) options. We will address the “minor” (i.e., sub-bullets with phases) options in another blog. First, we will make the choice easier by ignoring option 3, which is a configuration I consider an anomaly to be discussed later. Now that we are down to 2 choices the decision between option 1 (1.1. mm Arrows) and 2 (1.5 mm Wide Sleeve Arrows) becomes quite simple: are you going to conduct direct immersion sampling? If no [i.e., headspace (HS) extractions only], then you should stick with the 1.1 mm Arrows under option 1. If yes [i.e., HS and/or direct immersion (DI) extractions], then you should run with the 1.5 mm Wide Sleeve Arrows under option 2. The 1.5 mm Wide Sleeve Arrows (option 2) were specifically developed for DI sampling. In fact, this version of the Arrow is really just a 1.1 mm Arrow housed in a 1.5 mm septum piercing needle, as shown in the Figure and Table below. To elaborate, option 1a and 2a (above) both have 100 μm of PDMS, because both Arrows have the same phase support tubing diameter and phase diameter, rather the septum piercing needle is different.
So, how do these Wide Sleeve Arrows (option 2) aid in DI sampling? When the fiber phase swells from DI sampling (I said when, not if, because this happens often), the larger 1.5 mm septum piercing needle ensures the phase is not sloughed off and/or damaged when the SPME Arrow support tubing is retracted inside the septum piercing needle. So, why not make the choice even simpler and always use the Wide Sleeve Arrows (option 2). I mean, they are good for both HS and DI sampling, so it is the best of both worlds right? The answer is simple: cost and septum wear. The 1.5 mm Wide Sleeve Arrows cost more than 1.1 mm Arrows, and I would speculate they will put more wear and tear on GC inlet septa and vial septa.
Now what about those 1.5 mm Arrows (option 3) I told you to ignore earlier? Truth is, this 1.5 mm option is only available in PDMS; hence the “anomaly.” They are nice in that there is 2.5x the phase volume of the comparable 1.1 mm PDMS and therefore a potential 2.5x increase in response. However, we have found that the increased phase thickness can have negative impacts on the chromatography, as discussed in our recent manuscript.
Solid phase microextraction (SPME) Arrows; (1) 1.1 mm SPME Arrow; (2) 1.5 mm Wide Sleeve SPME Arrow; and (3) 1.5 mm SPME Arrow. All SPME Arrows composed of the following parts: a color coded screw hub (A), a sealing septum (B), a septum piercing needle (C), a fiber attachment needle (D), and a coated metal fiber (E). d1: Support tubing; d2: septum piercing needle; d3: phase diameter; d4: phase support tubing diameter; l3: phase length; a3: phase area; and v3: phase volume.
One last talking point: We told you there is no free lunch when incorporating the SPME Arrow into your laboratory, because you will have to install a GC Inlet Conversion Kit. It is important to stress that the inlet conversion kits work with all 3 types of SPME Arrow AND you may use the converted inlet with all your standard injection techniques (e.g., liquid syringe, headspace syringe, etc.). So, there is no need to swap back and forth between inlets.