GC/MS and EPC negative inlet pressures demanded

[MSCHemist]

I just attended another H2 conversion seminar. The topic of ECP's demanding negative inlet pressures came up due to outlet vacuum came up. I don't quite understand the article.

http://www.chromatographyonline.com/lcg ... =&pageID=5

but I understand that because I have a quickswap set at 4psi it won't be an issue but for others I guess it can be.

Doesn't indicating outlet=vacuum under gc parameters deal with it?

[Yama001]

The issue is that the pressure needed to achieve required flow rates will be zero or lower if there is a vacuum at the outlet, helping to suck the carrier through the column. A restriction at the outlet (before the MS) is useful in some cases, though operators would often rather not add that complication.

[Peter Apps]

The issue is that the pressure needed to achieve required flow rates will be zero or lower if there is a vacuum at the outlet, helping to suck the carrier through the column. A restriction at the outlet (before the MS) is useful in some cases, though operators would often rather not add that complication.

Apparently, a restriction at the inlet end is even better, because then the whole column is at reduced pressure, which gives very high optimum flows, and faster separations. It is extra fiddle though, and ordinary EPCs struggle to work out the flows and pressures. Have a look on the Restek web site - Jack Cochran worked out some elegant ways of incorporating inlet restrictors into inlet liners.

Peter

[mckrause]

Only comes into play if you have a high vac source at the outlet, which would be a MS. It's a really, really, really bad idea to convert your carrier to H2 if you're using a MS, unless you're really into replacing multiple parts in your MS on a routine basis.

[MSCHemist]

Yep I use the Agilent Quickswap system which I think they discontinued the kit. It has an insert fed from an auxilary EPC with purge gas and a restrictor that goes through the transfer line.

I am hesitant to recommend it as getting mine to make a seal on the transferline was a nighmare. I was on the phone with Agilent tech 4 times they had me get a new transferline weldment, check the aux EPC. Finally they sent us a new insert and I followed the instructions to tighten it 15 deg past first resistance and it leaked badly again so I kept tightening it a smidge at a time while spraying compressed air at it and looking for m/z 52 in profile mode and after cranking the heck out of it with an oven mitt on I finally got it to form a seal. The siltite ferrules on the columns though haven't given me any problems and they seal nicely.

I is rather handy. I like to switch between a db-5ms for 3-mcpd and a wax column for flavors analysis. It also lets me backflush the column every so often.

If I switch to hydrogen though I'll have to take off the 100 um restrictor and put in a 92 um restictor which I am dreading. Also officially most Agilent reps told me not to use it with hydrogen because it leaks out of the fitting when changing columns but I consider it a minimal and manageable risk.

[MSCHemist]

Restek aslo makes a Quickswap called the No-Vent. It is $200 and has no purge gas. It also uses a restrictor and has an insert with a polyimide ferrule system and it just seals shut when changing the column. Has anyone tried it?