Tips for analysing a drug mix with widely differing concs?

[Ann]

Dear All

We are pretty new to LC-MS (>20yrs HPLC experience). We have some concerns about an upcoming study and wondered whether anyone had any bright ideas

We are about to conduct a drug stability study on a mixture of 4 drugs. We're using an API3000 with an Agilent 1100 HPLC. The concentration of the drugs within the mixture differs widely, because some of the drugs are much more potent than others. With our assay (MS conditions optimised for all the drugs), some drugs are (presumably) ionising with ESI more easily than others so we see much bigger peaks for some drugs than others at equivalent concentrations. Our problem is that the drug (bupivacaine) that produces the biggest peaks is also the drug that will be contained within the mixture at the largest concentration (20-25mg/mL). Another drug (clonidine) which is harder to detect (much smaller peaks) is (inevitably!) very potent and is hence contained within the mix at a greatly reduced concentration (55ug/mL). Our concern is that if we dilute our drug mix down for assay at a concentration where clonidine is easily and reproducibly detected, we will still be injecting masses of bupivacaine on to the system. We have concerns re: ion suppression and carry over (seen bupivacaine carryover at conc of 10ng/mL ) and wondered (short of some kind of extraction to remove bupivicaine from our mix prior to assay!) if anyone had any ideas about how to approach this?

Many thanks

Ann

[H.Thomas]

Hello Ann,

maybe you can use a switch valve before your MS to switch the eluent to waste during elution of the bupivacaine peak?

In another run with a more diluted sample you could quantify the bupivacaine.

[Ann]

Thank you, that's an excellent suggestion...now just need to get hold of a switch valve!

[tima]

You will only have problems with suppression if the components are co-eluting. Or maybe you are injecting directly without using the column and then you would have suppression problems.

If not then make sure you have baseline separation between the two componentsand then suppression is a non issue. You have to think of how the ionization process occurs with nebulization.

As the droplet size is getting smaller and smaller if the component co-elute then the 2 components will be in the same droplet and the higher concentration component is competing with the lower concentration component. Hope this helps explain it.

[alpesh]

you can suppress signal by changing optimised mass parameters slightly for the drug for which you are getting better sensitivity to keep sensitivity for all four molecules same.