Chromatography Forum

I´m doing OQ/PV (post anual preventive maintenance) of a Shimadzu Prominence LC. To control the injector module SIL-20A, I inject a Caffeine solution, 6 times. The RSD must be less than 1.0% for areas, and 0.5% for retention times. Areas complies, but rt doesn´t. First the peak shows a time, and then lower and lower and lower and never stabilizes. Mobile phase water:methanol (80:20), low pressure gradient, flow 1.0 mL/min, pumps A and B respectively. I purged well all the sistems, wash the inlet filters and sonicated them with nitric acid 10%.
Anyone knows what could be happening? Could the inlet filters blocked? I´d checked the flow and it´s OK, so does gradient.

You have already checked the "usual suspects" (flow and gradient proportioning). Confirmation would be to look at t0 (if it's repeatable, that confirms that flow is in fact constant) and then run premixed mobile phase in both the A and B pumps (iconfirm that mixing is not the problem).

The only other possibilities are temperature (gradually increasing?) or column problems. I would not expect temperature to make a large difference on a caffeine standard, but you might try a new column and see if that makes any difference.

You should also check for leaks. Maybe you've done that, just wanted to be sure.

I did that. No leaks.

Try longer equilibration times between injections.

Are you actually using a column or some kind of column blank?

I have to use a C18 column, I try a few, and the same problem shows. The sample is diluted in water. And I´d equilibrated it (a lot) before, after and between injections.

then maybe the problem is your column

these days HPLC technicians try not to use a column when they check the instrument
it is simply another factor that could go wrong that is added
you should check using a long capillary tube that creates back pressure and gives the compound some fake retention time

may be, but I´ve tried four different columns, one of them new.

and how is the pump behaving in the PV?

Normally. I think something is partially blocked. Technical service is coming on Monday.

"Estimado" Q. F. Ignacio Viera,

I know what you are saying we suffered a lot with RT unstability some time ago, I discovered leak inside our Prominence, after elimination accpetable retation time variations were verified again.

But, are you sure your pump is working properly?

I hope your Shimadzu Technician can solve it next Monday.

Have nice elutions!!

"Saludos",

Carlos de Souza Teixeira