What could be the reason behind a negative peak mostly arising during the initial stage of sample run (RT 1-10).
Dear Swagat,
It depends on the type of detector
UV/Vis detector: the absorbance of solute is less tan that of the solvent
RID detector: lower refractive index of solute
all the best,
Sami
This is a very fundamental question. Without context, difficult to provide a specific answer. Complete details of the HPLC method used, the sample and a chromatogram would be needed for an accurate answer.
In general, a "negative response" (from whatever type of detector you are using) indicates a change opposite that of the baseline signal. This change in response could be from something simple such as the normal pressure change in the system which occurs each time an injector valve is switched (this 'blip" is normal and will always occur at the Tzero point of the method). It could result from a difference between the mobile phase solution and the injection solvent used (they may have different physical/chemical properties which are detected by the detector. In normal use, the mobile phase and injection solution should be the same, negating this). A more complex reason for a negative peak response might be incorrectly entered data acquisition parameters (e.g. use of a "Reference Signal" software feature to subtract raw data is a rather common one). In all, please contact a local professional chromatographer at your school for help.
Dear Swagat,
that's maybe due to the solvent of your sample is different than your mobile phase system, So the absorbance of sample solvent less than the baseline of your mobile phase that is in case you use a UV detector.
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