I have recently measured 4 samples that didn't contain a certain compound and 3 that did. I measured the 3 samples twice out of the same vial as a duplo measurement but one of them gave a blank signal. I looked for leaks and I saw the autosampler working fine and injecting the right vials so I have no idea why this issue occurred. Is this an issue with my injector? I have attached an image that shows the issue.
Daan Gageldonk, Not enough information provided (no scales or settings, no method provided etc) to suggest a specific cause. Besides aspirating a bit of air (bubble), there are so many possible reasons. Please speak with someone at your school who has practical experience in HPLC analysis using YOUR HPLC system. A few basic things to check:
(1) insure you have enough sample volume (enough = extra volume too) to draw up in the A/I for the two times you report to use each one.
(2) Are the vial caps left loose? They should be. If you inject large volumes with a tight cap or seal, then a vacuum may develop inside the vial preventing any liquid from being drawn up for analysis.
(3) Double check the vial assignments in the actual METHOD. If the vial # (location) is wrong or left blank, then no signal may be seen.
(4) If you use any detector that has a 'Reference Wavelength' software feature turned 'ON', then turn it OFF to prevent invalid data from being collected.
Some time compound is UV active but their wavelength is different from rest of the compounds so that will show poor signal or may be show zero absorbance. one more reason is that, may be compound is not UV active
Shashi Bala: The OP states they are analyzing the same sample multiple times, from the same vial, and sometimes it does not appear. *IOW: Nothing to do with the absence of a chromophore in this case as it showed an UV signal before.
Thank you for your response William. I checked your suggestions and it doesn't seem like any of those were the issue. The frequency of the problem did slowly decline, which was quite strange until it stopped happening all together yesterday afternoon (nearly 300 injections later). Our best guess is that it was a hardware issue that somehow resolved itself, like an inaccurate needle, but we don't really know how it was solved. The tech support of the supplier was even contacted and they had no idea either.
Not quite the resolution we expected but everything seems ok for now.
Daan: Such hardware issues do not "resolve themselves" so it had to have been caused by a failure of some type. If time permits, run instrument PV checks to insure the modules are performing as intended. As no information about the HPLC method used, instrument model and configuration, and/or sample types were provided, it is pointless to speculate.
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