Most analysts are more interested in the chemistry of their compounds or their reactions, but it is important not to overlook the chemistry of your sample handling supplies. The chemical composition, as well as other related factors, can have a dramatic effect on the quality of your separations. It also affects the reproducibility of your analysis, efficiency of your automated process, and overall vial performance.
An experiment is a multifactorial process, and assigning the effects of errors or failures from each small element can be a challenge. When a chromatography system is underperforming from a physical issue with a sample vial, cap, or septum, autosampler misalignments, crashes or hangs that cause sequence problems can result. These problems can result in unnecessary downtime, expensive repairs, and even the loss of your precious samples.
The effects of inconsistent or suboptimal chemical composition of these components are less easy to see, but are potentially damaging to your workflow or results.
The ideal chemical composition of a glass vial
Perhaps the key metric in determining the suitability and analytical performance of a vial is the linear COE (coefficient of expansion). COE is a standardized measure of the fractional change in size per degree change in temperature at a constant pressure.
COE is a result of the concentration of Boric Acid in the borosilicate glass used to manufacture a vial, and the chemical resistance properties of the glass. Clear borosilicate glass with a COE of 33 or 51, and amber borosilicate glass with a COE of 51 are known as Type 1. They deliver the best overall vial performance, with:
• Lower pH shifts
• Increased stability above 100 °C
• Greater chemical resistance
It is important to note that the elemental composition of borosilicate glass includes a wide range of metal oxides in addition to Boron. Aluminum, Magnesium, Sodium, Potassium, Iron, Barium, and Titanium can all be incorporated into the recipe in varying amounts. The higher the metal content, the higher the COE. Some manufacturers offer budget vials with a COE in the range 70–71. This is because higher metal concentrations mean that significantly lower amounts of heat are required in the annealing process. It can reduce the cost of manufacturing by up to 75% but will affect vial performance.
It is well proven that 70–71 COE vials are significantly more fragile and less chemically inert. With this type of glass, metals can migrate to the surface of the vial during the annealing process, forming active sites that ‘react’ with your sample. More information about this effect and the full chemical composition of the borosilicate glass that Agilent uses to manufacture its vials can be found here.
The Agilent specification
• The COE of Agilent vials is 32–33±1.5 for clear glass, and 48–56±1.5 for amber glass
• All vials meet ASTM E438 ‘laboratory class glass’ standards
• All vials use Type 1 borosilicate glass.
Choosing the right septa for your work
In the same way that chemical composition underpins the choice of a vial, chemistry is an important factor in choosing the right septa for your work. The best analytical performance will be achieved with a septum in the cap of your vial that protects your sample and is as chemically inert as possible. It should resist leaching or bleeding of materials from the septum into the sample matrix.
Material combinations often used for septa include: PTFE, silicone, red rubber, fluoroelastomers and butyl, for example, with PTFE having by far the widest chemical compatibility. The materials are layered in one of three ways to form a finished septum:
• Single layer – PTFE or red rubber, for single use only
• Bilayer – one layer forms the barrier, the other allows the septa to reseal after injection, usually PTFE and silicone, for repeat injections and sample storage
• Tri-layer – PTFE surrounded on both faces by silicone, improved chemical compatibility, for repeat injections and sample storage.
To limit the impact of siloxane bleed, a phenomenon that can compromise analytical sensitivity and reduce lab productivity, Agilent has developed an industry-leading conditioning process. Siloxane leeches from the silicone layer of the septum and, in untreated septa, levels increase due to multiple injections, elevated temperatures or solvent interaction. Agilent certified septa offer significantly improved vial performance in all areas.
The Agilent specification:
• Proven proprietary compound compositions give excellent chemical compatibility
• Limited siloxane bleed
• Consistent performance regarding the force needed to pierce the septa
More information about the chemical compatibility of various types of Agilent septa can be found here.
If your goal is to improve analytical performance, laboratory productivity, and operational efficiency, Agilent recommends that you stay away from low cost products. For improved vial performance, work with certified 33–51 COE Type 1 borosilicate glass vials. Choose an Agilent certified septum that matches your experimental conditions and sample storage needs perfectly.
keywords: supplies; chemical composition; vials; separations; performance; reproducibility; analysis; sample vial; caps; septum; chromatography; samples; glass vial