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LALman wrote:
I am dealing with it here trying to switch 8270/625 analysis to 100ml extractions and LVI, we have to make sure the the whole process is cleaner since any source of contamination makes a 10x larger problem than before. It isn't as simple as extract less, inject more, which is what the accounting people want. When I went from 1uL to 5uL that began to be apparent. But as you say it really came home when I went up to 25uL.
I'm having a problem with spit peaks with this 25uL injection. My solvent peak goes from 1 min to 3.25 min. The void time for my column is 32 seconds. So I am injecting at 60C, purging the inlet at 1 minute, also ramping 60C/sec to 80C. Then holding 80C until 2.75 minutes.
After a few tries its apparent...
Its because my column flow is 2.5mL/min and the pressure pulse at 24psi makes a flow of 5.4mL/min during the pulse. With 1uL and 5uL injections under these conditions, peak splitting was not apparent. But with 25uL the peaks for C18-C25 are splitting. So, I have tried the whole run with the same 24psi pressure pulse and then a flow of 5.4mL/min. This shrank my solvent peak down to 2 minutes so I made appropriate timing adjustments. The peaks are no longer split and are almost exactly 5X the area of the 5uL peaks.
Peak splitting after LVI points towards your solvent boiling dry on your (pre)column in different spots. This can be due to bad wetting behavior of the solvent film on your column, or (too much) solvent being evaporated just too fast to consistently boil down nicely into one spot.
Personally I use solvent vent when working with LVI to reduce the amount of solvent in the inlet down to a couple of µL to avoid this. But if I understand correctly, you work in splitless mode. The drawback of solvent vent is that you have more parameters to optimize (injection time, vent time, vent flow), and you risk losing low boiling point analytes.