Chromatography Forum

I set up to test some residual solvents with DMSO as my diluent (validated method) and am getting a peak that is overlapping my methanol peak making my run impossible. Even when injecting a number of blanks in a row it's still there. I decided to take my column off and install a retention gap (no stationary phase) column that stuff will just fly through. A did a bunch of injections and bam, it's there. I had a problem like this years ago on a different system (lots of analysts use these GC's for many different tests, though most are DMSO or DMA based) and it turned out to be the sample loop. The way I tested it was by watching the output with the sample valve closed (sample loop is bypassed) and then opening the sample valve on the headspace which puts the loop in-line. I would then see a rise in output which meant contamination. I called Agilent and they had me replace the loop and it fixed it. This time I replaced the sample loop and nothing changed at all.

I'm thinking the only thing it could be is the 6-port valve is contaminated because that's the only thing that's a variable when opening the sample valve besides the sample loop. When I did call Agilent last time I asked if steam cleaning could fix the contamination and they said that rarely works. I tried calling them today but they were closed. I did look the 6-port valve up and it's over $1000 so I don't want to waste the money if there's something else I could try first.

Oh, one more thing. We typically run around 90 oven, 110 loop and 115 transfer line. I cranked all three to 250 and I was getting a lot more response then I was previously. Another test I did was watched the output for a while, which was around 15, and then opened the sample valve. It would jump up to around 250-300 and then settle back down to around 20, constantly. It would sit at 20 forever until I closed the sample valve and then it would drop back down to 15, showing that flushing through the 6-port valve and sample loop was causing static contamination to be sent through the system.

Thanks in advance for any help!

It does sound like the 6-port is bad. You can take it apart to check the rotor and the inner surface of the valve. Just make sure you note the position of the rotor before you remove it (cool your GC down before taking it apart). There could be particulate in the grooves of the rotor (there are three) and I use a toothbrush and MEOH to clean them. If the inner surface of the rotor isn't scratched, you can re-install the cleaned rotor and see what happens. If you can't clean the rotor and the inner surface isn't scratched, you can order a new rotor from Valco Instrument Company and install that to save a lot of $'s. If the inner surface of the valve is scratched, you will have to replace it. It usually costs less to get it from Valco instead of Agilent. Valco is the original manufacturer of the valve. If you have the time, you can send the valve back to Valco to be refurbed. If you do replace the valve, you can get your old valve refurbed at a very nice price and next time the valve goes, you'll have a spare. The valve does have a point where it can't be refurbed, but Valco is very good about letting you know if it's not repairable. Valco also has info on taking the valve apart and cleaning it. Hope this helps.

It does sound like the 6-port is bad. You can take it apart to check the rotor and the inner surface of the valve. Just make sure you note the position of the rotor before you remove it (cool your GC down before taking it apart). There could be particulate in the grooves of the rotor (there are three) and I use a toothbrush and MEOH to clean them. If the inner surface of the rotor isn't scratched, you can re-install the cleaned rotor and see what happens. If you can't clean the rotor and the inner surface isn't scratched, you can order a new rotor from Valco Instrument Company and install that to save a lot of $'s. If the inner surface of the valve is scratched, you will have to replace it. It usually costs less to get it from Valco instead of Agilent. Valco is the original manufacturer of the valve. If you have the time, you can send the valve back to Valco to be refurbed. If you do replace the valve, you can get your old valve refurbed at a very nice price and next time the valve goes, you'll have a spare. The valve does have a point where it can't be refurbed, but Valco is very good about letting you know if it's not repairable. Valco also has info on taking the valve apart and cleaning it. Hope this helps.

Hello cleh,
Do you have Valco's 6-port part #?
Thanks!
Jan

Steam cleaning might not work, but it won't do any harm and is worth a try on that basis before you pull anything apart.

Peter