Chromatography Forum

Running a solatek 72/velocity with a 7890 GC 5975 MS, using a vocarb 3000 trap in the P&T system. Running 8260 method. For the past couple of months we have had major trouble with naphthalene. Now its to the point that every couple of days napthalene drops out and will not pass when we run a 5 point curve. So we keep changing traps. Change trap the napthalene falls right back into place. We thought maybe it was just a paticular lot #, but no, so then we tried changing our method some (ex. longer desorb), that made napthalene more stable but through off some of our early eluters ( ex. MTBE). Any other ideas please.

C0408,

Please post your conditions. Transfer line temp, desorb temp, desorb flow (split flow), etc.... Are you using moisture control? At the moment, I would expect a cold spot somewhere or an improper temperature may be affecting your naphthalene.

Best regards.

AICMM,

Conditions:

Transfer Line Temp 150
Sample mount 90
Purge ready temp 40
Purge Flow 40ml/min
Dry purge temp 20
Dry purge flow 100ml/min
Desorb temp 250
Desorb time 1.50 min
Desorb flow 300ml/min
Split flow 32ml/min

We are using moisture control. We also noticed this morning that our surrogates are now starting to drop out as well, could this also be indicative of a cold spot.

Thankyou

C0408,

I know the desorb flow is according to an application note, but that still seems awfully high. I have not used an XPT but have a customer who has one so I will check with them. Having said that, I am a bit further confused by the high desorb flow with the low split flow. I am assuming the carrier for the GC is spliced into the transfer line to the injector?

You can take your trap a little hotter but not a lot. The other possibility is that you are sinking the naphthalene deep enough into the trap (with high dry purge for example) that it is having a tough time coming off the trap. You might try no dry purge for a couple of runs just to see what it does to the naphthalene.

Some things to look at. Sorry I don't have more at the moment.

Best regards

AICMM, the "desorb flow" that co408 indicates is not really the flow used to transfer analytes from trap to gc.
Solatek indicates this as DESORB flow but is the flow used to drain the vessel .
The desorb flow is only inlet condition dependant.

CO408: we have a system like your, but with #9 trap (new patent pending by tekmar). No problem with naphtalene or other heavier components.
In the past , on another instr we have had problem with heavier solved with clean source (also if appear good on tune)

Good luck

Sounds like the problem I have been chasing for ~2 yrs after swapping a 5970 for a 5973. My problem seems to be all the water transfered to the MS oxidizes the source.
A simple test is to clean your source and re-try your calibration.

If that works I would suggest you use the gas saver option to up the flow rate to 100-150 cc/min @ 0.75 min ( 0.5X desorb time). This will get rid of most of the water in the transfer line. If you have the flexibility to shorten the desorb time to 0.75-1.25 min would also help. If you can add time to the max temp of the oven program would help the MS to purge water between runs. I hold mine for just under 10 min without affecting run to run time.
Drop your dry purge flow to 40 and DP for 6 min.
The highest split rate you can use without impacting sensitivity is recommended. I purge 25ml and use a 75:1 split.

My problem is on going but I have made great strides by using the suggested changes.
Please post your GC parameters, column flow rate temp program etc.