Chromatography Forum

The 1100 Agilent HPLC that we use for potency testing started giving an inverse or negative signal when analyzing samples this week. The solvent curve on the chromatograph has the normal shape but has been flipped upside down over the baseline. We have been able to determine that it isn't a mechanical issue and believe that it is related to the detector. Has anyone else experience this problem before?

Sounds as if polarity of the detection had been reversed. This can happen hardware-wise (via reversal of a cable connection) or, more easily, in the software. Look for a switch "polarity" or something similar in the instrument setup.

Switching the polarity to negative in the software flipped the solvent curve back into the positive but now the analyte peaks are negative. [img]IMG-0795.JPG[/img] With that in mind it's clear that the solvent peak is giving an inverse signal but the peaks of interest are behaving correctly.

… then your solvent peak has genuinely changed. Since the solvent peak happens because the absorbance of the injected solvent is different to that of the running buffers, then either the running buffer has changed, or the sample matrix has changed. But do also check that no one has done anything weird with subtracting absorbance at a background wavelength, which can lead to confusions.

Has anyone been able to invert a negative signal during data processing?