Negative peak in blank injection at RT of main peak

[Mattias]

I have performed a sequence (isocratic RP method, acetonitrile/acetate buffer pH 5) where I get a negative peak in the blank at the exactly same retention time as the main peak in the samples/standards

The main peak has good retention at about 13 minutes (t0 is about 1 minute). My blank sample is just mobile phase which is also used to dissolve standards and samples.

Detection by UV at 383 nm

It doesn't make any sense? Do you have any idea what is going on?

[tom jupille]

My first guess would be that your mobile phase is contaminated with some of your product while the blank is "clean". In effect, the blank represents a "negative concentration" of analyte relative to the mobile phase.

[Mattias]

Thanks Tom,

That makes sense, although I cannot understand how I managed to contaminate my eluent.

The analyte in question is Aniline Yellow and the concentration range in the samples is very low (few nanograms/ml). Maybe the vapour of the substance was enough to cause this. I will repeat the experiment and be more careful

[vmu]

I get a negative peak in the blank at the exactly same retention time as the main peak in the samples/standards

This may be just a coincidence, but may be not.

Detection by UV at 383 nm

Don't you use a DAD and (unnecessary, wrong) reference wavelength?