A needle has to take considerable force along its axis, as it is forced into through the septum into a vial. Hence in time it becomes weakened, and will eventually bend under the pressure. A clue to this beginning to happen is that the hole formed in the vial caps after injection starts to move away from the centre. As soon as this is observed, the needle should be checked for straightness, and if necessary, replaced.

Needles for autosamplers are inordinately expensive, but even so the cost must be weighed against the cost of an overnight run being wasted if the needle lets go at 2am! Try to resist the temptation to straighten a bent needle. It sometimes works if the needle was not very bent to start with, but like a car which has been straightened after a crash, the next time it will bend really easily.

If using PTFE septa for the vials (white teflon discs), be careful to use only one. they are pressed out of sheets of teflon using a circular cutter, and hence they tend to be dome-shaped, rather than circular. This causes them to stick together, and it is relatively easy to pick up two together, sometimes even three! Together they are very much stronger than one, and offer considerable resistance to the autosampler needle. The user will always deny it, knowing how expensive the needle is, but this is one of the most common causes of a needle bending! If it doubt, check the last vial with a hole in the cap!

The other reason for finding a bent needle is that the X-Y calibration is out. As a consequence, the vial positions are not quite where the autosampler thinks they are, and the needle hits the cap instead of the septum.  X-Y calibration does go out slowly with time, and is one of the things that should be checked when an autosampler is serviced. If the holes in the vial caps are drifting away from the centre but the needle is straight, this is almost certainly the problem. This is a job only for a service engineer or a very competent user.

Replacing a needle is relatively straightforward, although sometimes it is necessary to replace some tubing with the needle (eg Agilent 1050 etc) If there is a needle guide, ensure that this is straight when the new needle is fitted. Autosamplers tend to use reasonable force with the needle, so if the needle is bent, the needle guide could be out of position too.

Reconnect the tubing to the needle, and bear in mind that sometimes this forms part of the flow path after injection, so the connection to the needle may need to be tight. Be careful if the autosampler uses 1/32" steel tubing. This is as fragile as it looks and can easily be damaged. Sometimes it can be afforded a little extra protection by using heat-shrink tubing to coat it. (This is flexible plastic tubing which is larger diameter than the steel tubing. The steel tube is inserted inside the heat shrink tube, which is warmed with a hair dryer. This causes it to shrink, forming a coating over the steel tube, insulating it and offering protecting against abrasion or rough treatment. You can buy short lengths from RS Components, but longer lengths need an electrical wholesaler.)

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