Chromatography Forum

Dear all,

I see a peak at about 0.8 min when using a 150*4.6 mm column (1 ml/min). According to the theory the dead volume of the column is about 1.5 ml, so no peaks should appear before 1.5 min.

Does anyone know what can cause this peak?

This happened in case of size exclusion or ion-exclusion mechanisms. Repulsion some times happened in ion-exchange and mixed mode chromatography when analyte and stationary phase have the same charge and analytes is not hydrophobic enough to be retained by reverse phase mechanisms (analysis of positively charged molecules on anion-exchange or negatively charged molecules on cation-exchange). If this is the case you need an opposite charge column or need different mobile phase to change ionization state of analytes (or column)
What kind of column you are using and what is your mobile phase?

In size exclusion pores of packing material (silica or polymer) are too small for analyte and interaction does not occur in pores.

The column is a standard C18 column, and the mobile phase is a mix of ionpairing reagents, phosphate buffer and MeOH.

Could be that you are seeing a pressure pulse from the injector.

Hi..
i give you some inputs...probably not the real guilties!

hypothesis 1: something in column that in time elutes
hypothesis 2: something wrong in injection valve connections...control input and outputs of your rheodine

only hypothesis

i hope this will help you
good luck

If its not a pulse: Ion exclusion due to SiO- . What is the pH of the mobile phase?

Or a small hydrophilic ion with the same charge as your ion-pairing reagent.