Are you Choosing the Right Cap for your Headspace Vial?

Headspace analysis is particularly sensitive to the quality of the sealing system used to contain the sample. The high pressures and high temperatures in use require high-performance and reproducible seals. Newer analyzers coming on the market continue to push the envelope on these requirements [See, the Shimadzu HS-20, the Agilent 7697A (See Configuration with Tray), and the Thermo Scientific TriPlus for examples of headspace samplers with oven temperatures up to 300°C]. At higher temperatures, high-quality seals are a condition for reproducible analysis free of siloxane contamination.

Why Choose Headspace GC?

Headspace gas chromatography (GC) analysis is a method commonly used with volatile organic compounds, such as blood alcohol content, pharmaceuticals, and food flavoring analyses. The analytes of interest may be found in a complex sample mixture, but the analytes are more volatile than the matrix, so the analytes of interest can be isolated in the gaseous (headspace) portion of the sample vial. By injecting only the gaseous compounds onto the GC column, you can avoid messy chromatograms from unwanted eluting compounds that are contained in the liquid sample. Headspace vials and caps can withstand the pressure buildup of the gas in the vial.
 

Screw-top or Crimp-top?

Screw caps for headspace vials are a recent development, originally intended for magnetic robotic arm autosamplers. They sidestep the problem of crimping on an all-steel cap or a composite steel/aluminum cap. The plated steel caps are reusable and do not require tools to seal or open vials. They have some drawbacks too.

They can only be used for 10mL and 20mL vials. Too much of the internal volume of a 6mL vial is lost to the threaded neck for the cap. Screw cap headspace vials are not suitable for use with mechanical robotic arm autosamplers. The neck of a screw thread vial is a different diameter than that of a crimp top vial, and the screw cap vial lacks the natural ledge formed by the lip of a crimp top vial. That ledge is frequently used by the autosampler to help align and lift the vial.

Experiment to Proof

Today, I’d like to cover a question that I have been receiving from cannabis labs that are doing residual solvents analysis. This blog may not only help cannabis labs but also help labs analyze other matrices looking for residual solvents or very volatile compounds. The question is, “Which type of vial and vial cap combination should we use to do our analysis?”
 

This is referring to the screw-thread caps (18 mm) and crimp caps (20 mm) for 20 mL headspace (HS) vials. So instead of just recommending one or the other based on “feel” or price, we decided to run a little experiment to compare the performance of the two styles. The experiment was designed to analyze a few very volatile compounds and gather data on how well these two types of caps prevent the compounds from escaping the HS vial over the course of a specific period of time. 

Sample preparation included adding propane, butane, and isobutane standards in N, N-Dimethylacetamide (DMA) along with α,α,α-Trifluorotoluene as the ISTD to a 20 mL HS vial. To model the vial cap’s ability to keep these volatile compounds from escaping the vial, we designed a time study. In this study, 14 samples were prepared for each vial type and the samples were split into two sets of 7. Set one (T0) was run immediately after the samples were prepared and set two (T6) was run 6 hours later. It should also be noted that the crimp cap pressure was set to model the tightness of a screw cap. This means that the amount of torque needed to remove a screw cap was similar to that of the torque needed to spin a crimped, crimp cap.
 

The average compound area count from T0 to T6 was compared by calculating the relative percent loss over the 6-hour period of time. The relative percent difference for each HS vial type can be seen in Figure 3.

From the data shown in Figure 3, we can see that the screw caps (blue) did not seal as well from T0 to T6 compared to the crimp caps (red). It is also worth noting that we see very little change in our ISTD over the 6-hour time period for both the screw cap and the crimp cap.
 

After looking at the data and taking everything in, I think it is sufficient to say that if you are working in a lab doing HS analysis of very volatile compounds (gases), you should be using crimp caps. However, even if crimp caps are being used, one still needs to be cautious of the fact that the lighter, more volatile compounds, like the compounds shown here, are still capable of escaping the HS vial. Now, with all this being said, I don’t want to give the impression that screw cap HS vials are worthless. They can definitely be used in HS analysis, especially if the compounds being analyzed are on the less volatile side or your time from sample preparation to analysis is kept to a minimum. As an example take a look at the minimal differences of α,α,α-Trifluorotoluene, so for many customers analyzing the standard list of solvents and volatiles a screw cap may be appropriate.
 

Any More Discussion about Headspace Vial Cap is Welcomed!