Explain the problems that may occur during the liquid phase autosampler test

Explain the problems that may occur during the liquid phase autosampler test

The failure of the liquid phase autosampler is easy to find. Sometimes the LC system fails and it is not possible to determine if it is an injector failure. You can manually inject it several times, or use a good liquid phase to automatically inject. Replace the device, if the fault is eliminated, the liquid phase autosampler has a problem.

In addition to the possible failure of the liquid phase autosampler machinery and control system, there may be corresponding problems in operation and methodology. Here, depending on the design characteristics of the liquid phase autosampler, problems may occur during the test. Be explained.

First, the lag volume

The hysteresis volume in a high performance liquid chromatograph refers to the volume of the mobile phase mixer to the column head, including the volume of the connecting tube between the solvent mixer and the mixer to the column head. For low pressure mixing systems, the pump volume is also included. Larger lag volumes may result in poor repeatability of the experiment and difficulty in method migration. In a typical HPLC system, the hysteresis volume is typically between 0.2 mL and 5 mL, depending on the design of the device.

The more important part of the hysteresis volume is the loop volume of the liquid phase autosampler. Only when the volume of the loop is less than 100 μL, its effect on the hysteresis volume can be approximated. This effect will manifest when the loop volume is large. For example, with a 50 μL loop, the hysteresis volume is 300 μL. If a 1 mL loop is used, the hysteresis volume will become 1250 μL. This change directly affects the chromatographic separation results. In the actual separation process, when the column used is fine, the influence of the hysteresis volume must be considered. The solution is to replace the smaller loop and reduce the hysteresis volume.

Second, the sample bottle is too full

If the vial is too full, the sample repeatability may be degraded if the cap is tight and the injection volume is large. The reason is that when the sample in the sample vial is withdrawn, the capped cap does not allow the air to enter in time, causing partial vacuum. Due to this vacuum, the syringe is unable to draw a sufficient amount of sample volume. In extreme cases, even bubbles may appear in the needle for samples of volatile dilute solution, which may affect the normal operation of the analysis.

Some liquid phase autosamplers overcome this problem by adding an empty needle, but this method is not commonly used. A simpler solution is to not overfill the vial. The general loading amount is preferably between 1/2 and 3/4 of the sample bottle.

Third, the adjustment of the depth of the needle

When the sample volume is sufficient, it is not necessary to consider this problem. However, for trace analysis, the adjustment of the depth of the needle will be more prominent. Ideally, the needle can be drilled deeper into the bottom of the vial so that the sample can be used to a greater extent. When the available sample volume is limited, a micro sample vial can be used to increase the relative depth of a given volume of sample. If the needle is too deep, it may be inserted into the bottom of the vial and even cause the tip to become clogged. If the needle depth is insufficient and the sample is only part of the needle tip, the needle will not be able to extract a sufficient amount of sample and some of the air will be replaced.

The above is all about sharing the problems that may occur during the test of the liquid phase autosampler. I hope that it will be helpful for everyone to use this equipment in the future.