How To Choose the Right Headspace Vial Cap
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. Chrom Tech has a wide selection of headspace vials and caps, so we’ll discuss how to choose a headspace vial and cap.
Choosing Your Headspace Vial
Headspace vials are generally 10- or 20-mL vials in both clear and amber glass options. You’ll notice that headspace vials are available in round- or flat-bottom variations. The type of autosampler (needle goes to the vial vs. vial brought to the needle) typically determines whether a flat- or round-bottomed vial will work best. Also, certain autosamplers require a magnetic cap to transfer the vial, in which case a screw thread vial is standard—and we’ll discuss more below. Alternatively, other customers use a crimp cap vial due to the natural flat edge required by robotic arm autosamplers; plus, crimp caps notoriously offer tight seals for volatile compounds.
Determining Your Headspace Vial Cap
Once you’ve determined what vial you’re going to use for your headspace analysis, you’ll want to find a cap to pair with it. It’s relatively easy to determine if the vial has a screw thread or a crimp top. If there is threading on the vial, you’ll need a cap that can screw on; otherwise, you’ll want to pair the vial with a cap that crimps down with a crimping tool. It’s important to note that screw cap headspace vials are not compatible with mechanical robotic arm autosamplers because they don’t have the natural ledge that autosamplers grip on crimp caps. Likewise, autosamplers that use a magnet (such as the Combipal) will need a magnetic screw cap with an 18mm diameter. Crimp caps are most commonly aluminum but are also available as pressure-release-lined and magnetic-lined options. Crimp caps have a 20mm diameter rather than the 18mm diameter of screw thread caps. Lastly, choose the septa that works best for your application. Septa is available in either PTFE/silicone, molded PTFE/butyl, or high-temp options. Contact our support team if you have any questions about the headspace vial cap options.