Vials and Caps (Part 1 of 2)
The Diversity of Vials and Caps
When one is going to purchase a column for HPLC or GC, the first step is usually familiarization with its properties and applications beforehand, to make sure that the separation tasks in the laboratory can be performed. It’s usually a different story in the selection of sample vials – where it is rare that anyone does their homework about the details. Thus, beginners, who have purchased a new analysis instrument, often reorder more expensive vials and caps from the instrument manufacturer. Also experienced chromatographers mostly use the vials, which have always been purchased by the laboratory.
The following article shall give a summary and support for the selection of suitable sample vials from the large range of available vials and caps.
The material “glass” for sample vials
In chromatography generally vials made from the hydrolytic class glass. This type includes borosilicate glasses like Duran®, Pyrex®, Fiolax®, and others. Glasses of this general type are often called neutral glass and have a very good chemical resistance towards acidic and neutral solutions. The relatively low alkali content permits good values for resistance towards alkaline solutions, too.
Materials for closures
There are three different types of closures available:
- Screw top,
- Snap ring.
Different materials are used for each type.
Crimp closures consist predominantly of aluminum. Occasionally of sheet steel, or bi-metal, are used for the magnetic gripper of an autosampler. The shapeable aluminum allows crimping with a high accuracy of fit – especially with manual crimping tools. For crimp caps made of steel (designed as magnetic crimp caps) or bimetal crimp caps electronic high power crimping tools are recommended. If high sample throughput is a requirement, electronic high power crimping tools are required.
Screw closures are mostly made from polypropylene but infrequently are made of phenolic or melamine resins as well as from metal.
Snap ring closures feature an elastic polyethylene polymer.
High demands are made on the septum material, because it is directly in contact with the sample and solvent mixture, the analytes, and the injection needle. According to the needs of temperature resistance, analytical purity, hardness and structure as well as of resealability, septa from different elastomeric materials can be used, which are mostly covered with inert PTFE.
In HPLC the septa often need to have a pre-punctured cross-slit as a penetration aid for thick and blunt needles of autosamplers. Furthermore, the slit avoids a vacuum in the vial during sample removal and thus guarantees constant volumes in case of multiple injections.
Crimp neck vials and crimp closures
Sample vials with a crimp neck for crimping with metal caps are called crimp neck vials. A classification is done by the nominal diameter N at the crimp neck in millimeters. Crimp neck vials N8, N11 and N20 are all commonly used in chemical analysis. This class comprises diverse vial types, which have different useable volumes determined by different sizes and shapes.
Crimp neck vials with crimp closures N8 are also called micro-vials, due to the small useable volumes of 0.1 to 1.2mL. They are used for small sample volumes. Micro-vials with conical bottom are suitable for the smallest volumes.
Crimp neck vials N11 feature diverse types from clear and amber glass. In essence, there are two types of crimp neck vials with small and wide openings. However, vials with wide opening prevail, because they are also advantageous for filling viscous liquids. The useable volumes for standard vials N11 are 1.5 mL. Beside different types of micro-vials (with 15 µL funnel in solid glass bottom, with integrated 0.2 mL insert, conical with a round pedestal glass plate or with conical bottom), diverse inserts with useable volumes of 0.1 to 0.3 mL are offered for the standard vials N 11. Furthermore, silanized vials for polar and pH-sensitive samples, and vials with label and scale for an estimation of filling volume are available.
A corresponding diversity of crimp closures N11 is offered. Additional to silver colored aluminium crimp caps, colored caps for an identification (sample, laboratory, working shift) are available. Magnetic crimp caps N11 are provided for an application with CTC GC PAL autosamplers. Septa are mostly assembled. There are closures with septa for critical applications (analytically pure silicone / PTFE) as well as economical closure versions (natural rubber / TEF, natural rubber / butyl / TEF, Red Rubber / FEP).
Crimp neck vials and closures N13 with 13 mm nominal diameter are seldom used with autosamplers. Due to useable volumes of 1 and 2 mL and an assortment of center tear off caps (tamper-evident closures), they are preferentially used for the storage of pharmaceutical injection solutions.
A diversity of crimp neck vials N20 can be found with different volumes (5–10mL, 20mL, 25–100mL). They are particularly used for headspace GC due to different bottom shapes (flat, rounded), special crimp neck types (flat DIN crimp neck, beveled HS crimp neck) as well as specific dimensions. The flat DIN crimp neck features a stable bearing surface for the septum, which is particularly advantageous for a high internal pressure in the vial. The beveled HS crimp neck featured by vials for instruments from particular manufacturers – Perkin Elmer- in combination with pressure release caps benefits a possible emergency relief before the sample vial could burst. Further crimp closures N20 (center hole caps, center tear off caps, complete tear off caps, bi-metal crimp caps, magnetic crimp caps) with septa of diverse qualities, as well as stoppers of butyl and bromobutyl rubber are available for numerous applications.