Previously, the online SPE would concentrate about a factor of 10 from direct inject peak areas. Over time I saw a loss in sensitivity, so I changed the SPE cartridges. There is still signficant sensitivity loss, of about 100 X, and now the online SPE is actually less sensitive than direct inject. I double checked direct injection to verify the sensitivity of the instrument and it is the same. Has anyone experienced this before?
If you've demonstrated good sensitivity with the direct injection then the capillary (I presume you're using ESI or APCI) and source are fine so your issue (which sounds like a blockage) must prior to this. Using a standard test solution (e.g. a 1 uM test cmpd solution in 50:50 MeCN:H2O) I would run a number of repeat injections using your standard protocol then compare this to sample being injected/infused into the flow path immdeately prior to the column (via direct injection valve or T-junction). Is there a difference in peak areas between the methods? Flush all the ports of the injection valve (in both directions), replace all the tubing and repeat the above set of injections. Do you see a change? If there's still an issue perhaps there could be a blockage in your sample needle. If you can, dismantle the sample needle assembly and give that a good flush.....repeat the above (or just run a pick-up test).
Dependent on how comfortable you are with setting up switching/Rheodyne valves I personally would completely dismantle the valve to ensure the ports are, and rotor seal is, completely free on any occluding matter!! Good luck!
If your SPE is less sensitive than your direct inject, it is likely to be one of 4 things:
1 Less sample is being applied to your SPE
2 Less sample is being retained by your SPE
3 Less sample is being eluted from your SPE
4 Your ion suppression is increased
Item 4 may change if your analytical column performance has changed. It is easy to check by testing a standard solution without matrix through the SPE
Given that you have changed the SPE, it is not the SPE itself that is causing your loss in sensitivity, so it must be one of the other processes in the method. How much of the hardware is shared by the SPE and direct inject? By looking at the components that are not used by the direct inject system you can narrow down the points of difference. Depending on your system you may be able to switch components around to test them.
If the hardware is all fine and you have excluded a blockage then there may be a change in the chemistry somewhere. Go back to basics and change all your solutions and glassware, check your pH and your SPE part numbers etc
Good luck
Richard
Great advice folks, I really appreciate it. I'm glad to have this as a reference. Funny story, I removed the cartridges to see where it was flowing, and noticed there was no flow from the loop to the cartridge. I didn't realize my loading pump was slightly open so there wasn't enough pressure going from the sample loop to the cartridge. Rookie mistake. Thanks again for the feedback!
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