Solid phase extractions (SPEs) are routinely used in the sample preparation for quantifying analytes in biological fluids such as plasma and urine.most common problems seen with solid-phase extraction (SPE) are poor recovery, reproducibility issues, and sample extracts being insufficiently clean.For this problem we must be find a new technique or a solution that's we can easily rectify it.

The principal problem with AB is that there are some of them that are acids and the other basics so depending on which ones you want to analyze you need to adapt your SPE extractions. Probably right now you are not retaining them in your SPE cartridge. There is plenty of information in the literature so just look for the method that adapts better to your antibiotics of interest

My guess is that you are not retaining your compounds on the SPE column or not eluting them afterwards. You can try changing your SPE sorbent type or you can try changing the solvent with which you are eluting.

I wish you luck!

@Zvonimir eu fiz um teste com 3 tipos de cartuchos strata,agilent e chromaband e tive este cromatograma em anexo como resultado, sendo que o meu analito é detectável em 11,7 min.Já o solvente, estou trabalhando com metanol e pretendo trocar por acetronitrila.

I see...

You can try not only changing the manufacturer of the SPE columns, but also the type of SPE column, that is - the sorbent in it.

And yes, you can try different solvents, sometimes even a mixture of them works.

You can try to see if there are any already published SPE protocols for your analytes.

Good luck!