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Comments
To
remove air from the detector, pump degassed mobile phase and put a back-pressure
regulator at the detector outlet, to produce pressure — typically
< 0.7 MPa (6.8 bar, 100 psi) — within the cell. Adjust the injector
so it does not inject air (see Symptom #10).
In isocratic analysis, you can determine if a contaminant is being desorbed from the top of the column instead of coming from the injector by injecting a blank. Do this by connecting tubing between the injector and column, a volume which will make an observable increase in retention time. For example, 100 cm of 0.8 mm (.030 in) OD tubing contains 450 µL, shifting retention about 0.5 minute at 1 mL/min. If the retention times increase, the substances are from the injector or sample. If the retention times do not change, the substances are from the column. Confirm by connecting the tubing between the column and detector. If the retention times now increase, the substances are from the column. The tubing will cause the peaks to be broader, but hopefully the retention times will still be measurable. This test only works with isocratic analysis.
You can confirm that peaks in a gradient run are not associated with the injector by connecting the column directly to the pump (bypassing the injector) and running the gradient.
Non-injector Artifact Peaks
A blank injection can produce artifact peaks that are not really coming from the injector.If the unknown peak occurs instantaneously with the movement of the injector handle (i.e., a retention time of zero minutes), then it is probably due to a pressure disturbance in the detector. This disturbance is caused, in some cases, by the brief flow interruption that occurs each time the handle is turned. Whereas most properly functioning detectors will not respond to such a pressure disturbance, a detector which has become over-sensitive will respond. Oversensitivity is caused by a gas bubble in the cell or a cracked window. If you can produce a detector response by making slight changes in the cell pressure (partially restricting the outlet), this condition probably exists.
If the unknown peak has a retention time greater than zero (quite often it is an unretained peak, i.e., k'=0), then probably a substance is actually being "injected" onto the column. However, the substance is not necessarily coming from the injector. It may be coming from contamination of the column frit or the top of the packing. Such contamination can be desorbed by an injection in which the sample solvent has a composition different from the composition of the mobile phase. It may also be that such contamination can be desorbed by the flow transient caused by handle movement.
Peaks are sometimes observed in gradient analysis with a blank injection. These ghost peaks usually come from the initial or final eluent. Using purer solvents or shortening the equilibration time between runs may eliminate the problem.
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