Questions about LC coupled ESI MS

[Crystallee]

Hi. It's my first time coming here. Wonderful Place!
Got several questions when I take a course. Could someone help me? Thanks a lot!

1/ I found when coupling HPLC to ESI, the eluent flow of a HPLC is often split prior to ESI. I think it's because the differences between the flow rates. But what advantages of doing so?

2/ I found several common LC buffers like phosphate buffer are not compatible with ESI. Could someone tell me other buffers in such case? And what about the ion pairing reagents like TFA? Are they OK to couple with ESI?

Thanks a lot

[tom jupille]

1/ I found when coupling HPLC to ESI, the eluent flow of a HPLC is often split prior to ESI. I think it's because the differences between the flow rates. But what advantages of doing so?

ESI has to vaporize the column effluent. A liquid flow of 1 mL/min is equivalent to gas flow over 1L/min. Standard analytical scale columns (4.x mm internal diameter) typically operate at flow rates of 1 - 3 mL/min. For LC-MS, there are two choices to keeping the flow into the interface reasonable:
- use a narrow column at a proportionally lower flow rate (flow rate proportional to the square of the diameter). Many older instruments can not pump reliably at low flow rates
- use a conventional column and instument, and split a portion of the flow into the MS

2/ I found several common LC buffers like phosphate buffer are not compatible with ESI. Could someone tell me other buffers in such case? And what about the ion pairing reagents like TFA? Are they OK to couple with ESI?

In principle, any buffer or additive can be used so long as it is sufficiently volatile to vaporize in the interface (some newer interfaces can be used with non-volatiles, but they will require frequent cleaning). In practice, some buffers affect sample ionization; they can be used, but are not recommended (TFA comes to mind). Most common would be formic acid, ammonium carbonate, etc.