It is important to choose appropriate vials, caps and septa for your autosampler, and for
your analytical method.
Here’s a few check points:
Vial Size. Most autosampler use the standard 1.8ml vials. However these come in
several flavours. They can be clear or amber, with or without a label, straight sided
or with a narrow tip to enable the last drop to be removed, with screw top, snap on
top or crimp top etc. Other size vials also offer similar options. The vial size must be
big enough to contain enough sample for the injection volume x the number of injections
and a little to spare, but also be small enough to allow the needle to withdraw
the required amount. If the vial is too big, evaporation causes changes in concentration,
and there is a real risk that the needle will suck air. Amber vials are for light-sensitive
samples. The label makes sample identification easier, and the top style is
to suit you. Crimping is quicker if you have a lot of vials, but the tool costs well over£100!
Septa. These can be made of PTFE, rubber or silicone, and the latter two options can
come with a layer of PTFE on one or both sides. PTFE is cheaper, but does not reseal
after piercing. Hence evaporation can occur between duplicate injections if a volatile
sample solvent is used . Be careful to use only one PTFE disc! They are punched out
several hundred layers thick and tend to stick together. If you use two at a time, the
resistance might be enough to bend a tired needle. Silicone is fine for aqueous samples,
but rubber is most commonly used. It reseals, but can leave a slug of rubber in
the needle (known as coring). If this happens it can sometimes be cleared, but spoils
the run. To prevent this, use a septum with a layer of PTFE on top. Rubber is also
soluble in some organic solvents. In this situation it is recommended to use a layer of
PTFE on the bottom surface of the septum. Both problems at once? Us a rubber septum
with PTFE on both sides!
Caps. Caps must clearly be appropriate for the vials. Note that not all screw top vials
use the same screw thread, and sometimes the screw will not stay tight. Mixing
and matching vials and caps may work, but may not so try a sample before buying.
Quality. As with everything this century, there are ultra-cheap versions arriving
every day. There are deals to be had, but be careful to test thoroughly before switching.
The worst problems are vials which break as you tighten the cap, and vials
which have a different thickness of base from batch to batch! The cost saving on the
vials might be needed to replace the needle!
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