Agilent autosampler carry over
Chromatography Forum: LC Message Board: Agilent autosampler carry over
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By Marcel on Thursday, July 15, 2004 - 01:11 pm:

Dear All,

We are currently setting up an method for the analysis of Mycophenolic acid in plasma by using an LC-MS/MS. We use an API3000 system with an Agilent autosampler, but we are suffering with a huge carry over. We used NH3 to rinse the injection needle and this helps. Only the next problem raised, sometimes (randomly) we see a kind of memory effect. After injection of a high concentration of MPA, followed by several blank samples, sometimes we see an MPA peak in the first blank sample, sometimes in the third etceteras.

Maybe anyone experienced the same problem or maybe anybody has a suggestion how to solve this problem? Could the use of another autosampler (Perkin Elmer 200 series autosampler)be the solution.

Thanks in advance for all your help and efforts.

Marcel van der Linden, Ph.D.

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By Tom Mizukami on Friday, July 16, 2004 - 10:45 am:

Hi Marcel,

Here are things that can help with carryover on the 1100 autosampler:

1) do not overfill the vial, this can force liquid out of the needle hole ad onto the top surface of the needle.

2)use ptfe coated silicone septa. The silicone wipes the needle better during withdrawl

3)clean or replace the needle seat

4)replace the injector rotor, make sure the right rotor is being used, the two slot one.

5)use an uncapped wash vial to clean the exterior of the needle

6)use an injector program to cycle the injection valve or to use multiple wash steps.

Good luck

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By Matt Hurst on Friday, July 23, 2004 - 12:04 am:

Hi Marcel

In addition to getting a blocked needle seat the other big problem we have with 1100 autosamplers is blocked needles.

Any easy way to check is to go into the test functions for the autosampler and execute the option "needle up/valve mainpass" this will allow you to see if solvent is flowing freely through the needle.

Hope this helps

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By Consumer Products Guy on Friday, July 23, 2004 - 08:00 am:

We've only had one blocked needle in years of operation here. What kind of septum caps are you using?

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By henrik on Sunday, August 1, 2004 - 04:33 am:

In new autosamplers the needle is a part of the flowway, and thereby eliminate carryover problems. Sometime you can order a cooted needle and it might help.

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By hplc scientist on Tuesday, August 24, 2004 - 10:55 am:

The Agilent 1100 autosampler does not provide for an external needle wash in the standard model. This is most likely the source of your observed, systematic carryover since the outside of the needle will be contaminated with your sample and that gets into the injector port and if your sample is especially sticky.

Agilent's 'solution' is to sell an external peristaltic pump so that one can wash the outside of the needle...I believe that this is greater contributor to sample carryover than the continuous internal needle wash but it certainly helps.

No one has mentioned this but it is also important to choose the proper needle wash solvent especially if buffers are involved. I don't recommend acetonitrile and I recommend using an aqeous methanolic solution (60:40 MeOH/H2O, for example). Chemistry is very important here in cleaning the outside of the needle.