Hi all,

I am seeing the following issue and need some insight.

In a number of tests I am seeing retention time shifts all peaks(of 0.7 mins, 7 hours into an 11 hour run) with both the standard and sample and during other tests (maybe not related to first issue) a pressure increase.

The mobile phase is: a buffer soln,(10g of citric acid in 1L, pH 7.4)/ACN/Tetrahydrofuran (53/27/20).
Filter the MP with 0.45um nylon filter.
The test is run at ambient temp.

Note:
The system is cleaned (after buffer, 30mins water then 30mins 50/50 ACN/Water).
Column is (C18 5u 4.6 X 250mm), System is a HP Agilent LC system.

Questions:
- Can the temperature of the room affect the retention time?
- Should a buffer m.p be continuously stirred while running the batch of tests (11 hours long)? I think that the retention time change may have to do with the composition of the m.p changing.
- With the pressure increase; do you think the salt could be precipitating? Any good pointers on this?


Thanks in advance for any help. Chris.

To answer your first question YES. Temperature will affect the viscosity of the MP and thus the mass transfer of analytes between SP and MP will be slightly altered. In general if the Rt are decreasing, then one would think that temperature is increasing slightly. Does the Rt shifts correspond to a certain time in the day/night??
To overcome this, try running your column in a column heater, just above ambient and abserve Rt drift.

Are you sparging your MP online with helium? Dissolution of CO2 does result in pH increase from your 7.4, and thus will affect Rt. Test this by measuring the pH of your MP before and after a 11hr run.

I don't like the sound of your cleaning schedule, is this done after every injection, or only at the end of the 11hr run??

I hope that helps you some

Greg

Does your retention time increase? If yes, probably the most common cause would be loss of organics. Theoretically, since the dissolution of salts cause varying and slight expansion, one could expect precipitation at higher pressure. My "feeling" is that you would have to be in a completely different "ball game" of pressures to see this.
You should have true solutions, so stirring to keep things mixed is not indicated.

Does your retention time increase? If yes, then you are probably losing THF from the mobil phase. I've had the same kind of problems with the THF.

As second question: How you measure them volumes to prepare the mobil phase. I advise to you to measure them separately so that with the THF the volumes are not additives and can affect in its form to prepare.

Finally when it uses THF tries not to have to filter the mobiles phases because during the emptiness it will lose an amount of THF and this affected the reproductibility of mobil phase.

Hi again,

Greg, the cleaning schedule is after the 11hour run (sometimes run is as short as 7 hours, depends on # samples).

HW - to answer your question, yes, the retention time does decrease with an increase in room temp.
I observed over the weekend (with other instruments switched off/lights off etc) that the temp dropped, and Rt increased.

Anonymous - yes, I do measure the volumes for the MP separately.

Thanks for tips. In future, I'll be using MP prepared on day of operation only and have a shorter run time, if possible.

Chris

Coincidentally, anon (Feb. 9, 2:46am) posed the same question as mine, no connection to room temp. implied in either, rather, we considered a loss of organic solvent which would produce an increase in rt. From your Feb 10 statement it appears that you are telling us that this is what happened?