Hi to all,
I'm experiencing a too weird fact. I was studying how the matrix of my sample affected ylthe instrumental response of my analyte in LC MS. I use an esi detector, a c18 column. My analyte elutes at more than 10 minutes and the LC method is a gradient. Moreover, i divert LC flow to the waste for the first 10 minutes of the run to prevent MS source contamination from salts and other unwonted analytes. Mobile phases are methanol and water with me oh and ch3coonh4, with pH adjusted with formic acid.
The odd fact is that analyzing the analytes of interest dissolved in water, i have an instrumental response that is 100 fold lower compared to the response of this analyte when is in the final matrix that is NaCl+water.
Why I observe this? How does NaCl affect the instrumental response of the analyte? How is it possible if the analyte elutes too far from the "salt"elution region that is at the beginning of the run? I have excluded the first 10 minutes of the run from MS.
Moreover, I observe this increment only in MS, and not in UV. so it is not a reaction of my analyte with NaCl, but a matrix effect.
Do you have ever observed this? Have you some experiences, or literature to share?
Thank you
It is odd if all these were observed by only injecting analytical grade pure standard. UV response confirmation eliminates the potential prediction regarding the adverse effects of NaCl on your target. Therefore it should be an ion suppression (quenching) drawback. Perform scan analysis instead of SRM to identify if any unpredicted substance arises at the elution frame...Apply this initially by only injecting pure analytical standard and NaCl added analytical standard in comparison...Also, bypass LC system and only once infuse the samples at very low flow (w and w/o NaCl) to compare signal differentiation but also inspect the spectrum variety. Do not forget to clean the ion source after application. Additionally, prepare NaCl concentration in linear concentrations and confirm if any signal decrease is in correlation with increasing NaCl in LC-MS format, not in infusion. Last but not least, your molecule may be fragile in ESI conditions, and in-source fragmentation may be observed when salt is in a reaction pot. I prefer to follow the H adduct in +ESI and certain daughter ions to get a stable response. If amenable you may test the same molecule in -ESI to clarify this is not caused by an adduct-induced ionization issue.
Thanks for answering. So you are saying that adduction to Na could affect the m+h positively in the sense that also the mh+ increases?
Definitely no. I suggest you follow the M+H adduct for SRM acquisition and not select the Na adduct for precursor selection. Na may reduce the MH signal intensity as per your observation. To clarify the NaCl adverse effects on your molecule ionization, additional experiments must be performed. I recommended several approaches above to find out the why ion quenching effect was experienced and during these experiments, I suggest preferring MH precursor if +ESI is in use and your molecule is convenient to form protonated pseudomolecular.
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