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A needle has a fairly fine bore, and can easily be blocked. The blockage normally comes from a piece of rubber septum which has broken off as the needle passes through. This is known as coring. There are two things which can be done to prevent or minimise this. The first is to use a PTFE septum. Essentially this is just a plastic disc, which splits as the needle passes through. The only drawback is that if the run time is long, or the sample solvent is volatile, the septum does not re-seal, and so evaporation can occur, giving a higher concentration for the duplicate injection. An alternative is to use a rubber septum which has a layer of PTFE on one or both sides.

There are two other causes of a blocked needle. One is particles in the sample. This is clearly not an acceptable situation for lots of reasons, but assuming that the sample was correctly made up with no undissolved material, particles can arise because of precipitation when the sample is chilled in the autosampler tray, or because the sample is not sufficiently soluble in the eluent. In this latter case, precipitation occurs at the moment of injection, and subsequent samples are then not drawn up because of the blockage.

The second cause is particles in the wash solution. The wash solution should ideally be the same as the eluent at the point of injection, but without the buffer. This ensures that it is totally miscible, does not disturb the dynamic equilibrium in an isocratic system, and has no content which can dry in the needle causing a blockage.

If a needle becomes blocke it must be removed and either cleared or replaced. To clear a blockage, the easiest way is to use needle wire. This is a stiff but very fine wire which can be pushed up the needle to clear the blockage. Remember that it will probably be jammed in, having been pushed in as far as it will go, so push the needle wire in from the other end. You know it came in from the tip of the needle, so it should be able to go out that way too!

Some people claim to clear blocked needles with compressed air. All I will say is to hold on tight to the needle. If someone else in the lab subsequently needs to have it surgically removed, you will find it hard to explain that you didn't do it on purpose!

On some autosamplers, the needle becomes the loop when inserted into the valve, and the sample is thus backflushed into the valve. In this case, if you are able to use the service mode to turn the valve into the Inject position while the needle is not in the valve, and you then turn the pump on, the ensuing back pressure may give a blockage about 2000 reasons to come out. Its a bit brutal, so be careful in case the pressure finds another way out, but it often works!