Vials and Caps (Part 2 of 2)
Screw neck vials and screw closures
Screw neck vials N8 – besides crimp neck vials N11 – are among the oldest vials for HPLC and GC. More and more they are replaced by screw neck vials N9, because those are easier to fill due to the wide opening compared to screw neck vials N8. Due to the cap design of the screw closures N8, vials N8 are not – like vials N9 – universally usable on all autosamplers in HPLC and GC notwithstanding the same sizes of 11.6 x 32 mm (OD x height).
A large assortment of vials (clear and amber glass, standard, with label and scale, silanized, micro-vials), inserts (0.1–0.3 mL), caps (numerous colors) and septa (diverse materials) can be found for screw neck vials and closures N9. Besides ready assembled closures (septa are already inserted), bonded closures are available, where the septum is firmly connected with the cap. The advantage is that thick (blunt) HPLC needles cannot push the septum into the vial.
Screw neck vials N 10 are mainly used on Waters®, Shimadzu, PerkinElmer and Jasco instruments. 4 mL screw neck vials N 13 are used in preparative HPLC for large sample volumes with miscellaneous autosamplers.
Especially for headspace GC, headspace screw neck vials and magnetic screw closures N 18 are available. They make the sealing process more convenient, consistent, and safer compared to crimping. The lower septum thickness of 1.5 instead of 3 mm facilitates a better penetration of the needle and the less fragmentation of the septum, being especially important for SPME applications.
Polymagnetic caps are an alternative for magnetic screw caps made from metal. They consist of a polymer with incorporated metal particles and are corrosion-resistant. Moreover, they feature a good grip during screwing due to the ribbing on the cap surface.
For VOC and TOC analyses screw neck vials N 24 (EPA) with screw closures N 24, often called EPA vials (US Environmental Protection Agency), are used. Due to their size (20–60 mL, most frequently used: 40 mL clear glass) mainly bonded closures are applied for a firm fit of the septum. Screw closures with center hole and silicone / PTFE septum are recommended for environmental analysis.
Snap ring vials and closures
Snap ring vials and closures are only offered with the nominal diameter N 11. They feature a quick and convenient sealing method which, however, should only be used in HPLC, as the punctual compacting pressure of the septa against the vial rims by the four pins in the cap doesn’t achieve the same level of tightness as the homogeneously applied pressure through a circular thread or by crimping. The snap ring cap can be used on all common HPLC autosamplers, since the cap design corresponds to that of crimp caps. The 0.3 mL polypropylene snap ring vial is designed for special applications, e.g., in ion chromatography.
Diverse vials are suited as storage vials for powdery or liquid samples. For this purpose, snap cap vials with PE snap caps or screw neck vials with closed top screw closures (without center hole) are used.
Shell vials with PE plugs are an easy and quick to seal vial and closure combination in analysis. They are only suitable in cases of low requirements on tightness, as the plugs have no elastomeric sealing element – mainly they are used for non-critical HPLC analyses.
Autosampler compatibility / VialFinder
The selection of a suitable vial and an appropriate closure depends on the intended application. For sample storage other requirements are valid as for an application in chemical analysis (with an autosampler). The volume of sample determines the necessary useable volume of the vial and the light stability of the sample decides on the usage of clear or amber glass. Thus, the selection is reduced to a few vial sizes and types.
Furthermore, specific dimensions are required for an application with autosamplers, which are determined by inherent specifications of the autosampler. Additionally, the autosampler specifies a need of caps being grippable by the magnet or colored for a photocell. Thick and blunt autosampler needles require the use of soft, sometimes (cross-)slit septa. The following pdf download shows a summary of autosampler compatibility and cross references of vials and closures from common instrument manufacturers with MN products: Download autosampler compatibility
The selection between crimp neck, screw neck, and in some cases snap ring vials is also affected by individual preferences. Generally, the sealing process of screw neck vials compared to crimping is more convenient (no crimping tool), consistent, and safer, because there is no risk of inaccurate crimping results, which occasionally lead to leaky closures and finally to non-reproducible analysis results.
If you already have had a experiences with the usage of a vial type and the corresponding kind of closure, you can easily save money by a supplier change. The VialFinder provides a useful support to find reliably and quickly the equivalent MN products.
Suitable sample vials from the diversity of vials and caps can be easily found by consideration of some details relating to vial size, kind of closure and septum material with respect to the intended application. The search is supported by lists of autosampler compatibilities and cross references, as well as by the VialFinder from SORBTECH.