Ultimate Purge Union problem

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

6 posts Page 1 of 1
Compared to using the quick swap using the purge Union in GC MS method is resulting in increased retention time, awful peak shape and significant loss of sensitivity. I can not locate any specific guidance on use of the purge Union and would much appreciate any suggestions on why the chromatography has deteriorated and any pointers to manuals on using the purge Union. Thanks.
Which way do you have it set up? Are you using it simply as a No-Vent fitting just before the MS, or for back flushing which has more column after the union?
The past is there to guide us into the future, not to dwell in.
From what I understand the quickswap pretty much is a purged union where the union is at the transferline. The quickswap if fed helium at 4psig from an aux EPC and resistance is proveded by a 17cm .1mm ID capillary through the transferline.
Thanks for your replies.
The set up is for No-vent fitting. The analytical column is installed directly into the union inlet and there is a 2.4 meter length of column from the union outlet to the MS. Is this the correct set up for No-vent or is there too much column length after the union?
Sounds normal. The length of that piece isn't really critical, it can be a meter off without impacting anything.

In our post-column backflush GC-MS/MS system we use 25m x 0.25mm analytical, and 2.5m x 0.18mm deactivated fused silica to the MS. The narrower the last piece, the shorter it can be while maintaining its function. I wouldn't make the diameter difference before and after the union too large.
If the transfer line is too small diameter and too long, you will have more restriction in the last part of the flow path and it can interfere with the calculated flow of the column. Also if the union flow control is leaking and pushing flow into the union during a run it can cause the column flow to be low.

I have a setup with two columns installed going into a Tee fitting just before the MS. The first time I tried it I was using a piece of 0.1 transfer line and two 0.25 columns. I set the active column to 1ml/miin flow and the inactive column to 0.3ml/min flow and the peaks look horrible and the retention time was about double what it should have been. I switched the transfer line to a piece of 0.53 column and everything was perfect. The inlet pressure is set assuming hard vacuum at the end of the analytical column, if there is actually pressure there caused by a restriction then instead of 1ml/min you will have maybe 0.5ml/miin( mine was off about that much). This will cause the peak shape and retention time problems.

The restriction could also be caused by a piece of ferrule stuck in the union, if the column happened to shave off a small flake of ferrule material and it came out into the union after install and turning on flow.
The past is there to guide us into the future, not to dwell in.
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