Poor Naphathalene Recovery on VPH Massachusetts Method

[ELIkjw]

Hi Everyone,

I’m running the VPH Massachusetts method on a PID/FID. My VPH CCV’s are passing very nicely (~95%-105%) except for Naphthalene (last eluting compound) which is recovering at ~%70 on both the PID and FID. This is occurring on my 3400 Varian with a 3000 Tekmar P&T. Here is a short recap of what I have tried to fix this issue.

Replaced the MCS Loop (multiple times)
Replaced the He converter tube, I have also tried using a different heater box wondering if it wasn’t getting hot enough
Monthly maintenance on the air compressor (replacing desiccant and draining water from the tank)
Baked out the Trap and detectors
Rinse sample lines with MeOH
Ran 25% MeOH blanks to try to clean the entire system
Replaced the Silco steel sample lines leading from the sample syringe to the sparger
Verified the transfer line/column connection was ok
Verified the Purge flow of 40ml/min (mine was actually a little high at 48ml/min)
Replaced the Trap
Replaced the Trap heater
Cleaned the Sparger
Replaced the Glass Sparger
Increased the desorb temp to 250
Bypassed the MCS Loop all together (this actually dropped Naphth down to a ND….interesting)
Decreased and increased dry purge time

I think this is most of what I have tried. I’m kind of at a loss. I was convinced this was a moisture issue. Usually when my naphthalene starts to decrease I just replace the MCS loop and that fixes it. This time however nothing I have done has improved the naphthalene recovery. Also, I haven’t had any RT problems through this whole trouble shooting process. It is also worth noting that I have substantial Naphthalene carry over on the following instrument blanks. I have been thinking about replacing/cleaning the sample lines in the P&T, but I would like to exhaust every other avenue until I embark on that job.

Here are my P&T paramaters.

Purge Flow Rate: 40 mL/min
Purge Time (Min.):11
Desorb Preheat: 225
Desorb Temp.: 225
Desorb Time:4
Trap Bake Temp.: 225
Trap Bake Time (Min.): 10
Line Temp.: 100
MCS Bake Temp.: 310
Dry Purge Time (Min.): 3
Trap: Supelco Cat#: 24916

[rb6banjo]

Has this worked in the past? Naphthalene is not particularly volatile (solid at RT). Compare its boiling point (218 °C at atm pressure) to decane (174 °C at atm pressure). I'm sort of not surprised that it will be your problem child. I don't do a lot of purge and trap but my first thought to get naphthalene would not have been that.

Do you heat the sample while you purge it?

[ELIkjw]

Yes, Naphthalene has worked wonderfully in the past! I noticed Its recovery started to slowly decrease over time. Like I said the usual maintanence is replacing the MCS loop on the purge and trap. After this maintanence Naphthalene usually jumps right back up to normal levels. If that doesnt work replacing the trap is the next course of action and that always has solved the problem.

I am not using a sample heater.

I should also note that I have verified the purge and trap sample flow, trap pressure control, and sample pressure. All are within the manufacturers recommendations and are the same values I have been running at for years.

Thanks!

[rb6banjo]

Sounds like you have tried everything. Sorry, but I'm not much more help.

[Steve Reimer]

My first thought when faced with a loss of heavy, late eluting compounds is that somewhere in the system is an accumulation of "goo."
If you have replaced or cleaned all the easy places then the more difficult ones are the next step. I have had to go as far as replacing the rotor on a six port valve. I hope you don't have to resort to that. (Mine was on a Vacuum Distiller running waste solvent samples)

[Yama001]

I would wonder if the fitting where the MCS links to the trap is fouled or corroded. It may be worth removing that and cleaning it up. The round tee and the mount are good suspects as well.

I think James has mentioned that it can help to lower the MCS bake temp to avoid charring.

[ELIkjw]

Steve,
I was thinking something similar. Naphthalene being a heavy compound is probably getting stuck somewhere in the system due to "goo" build up like you said. Especially since I'm having bad carry over of Naphthalene on instrument blanks following a CCV. Naphthalene is definately getting caught up somewhere and gradually entering the GC when it shouldn't be. I have had some people tell me to try replacing the silco lines in the purge and trap but I haven't considered the 6-port value! Thanks for the insite!

James,
I haven't thoroghly inspected that fitting. I have replaced the ferrule and leak checked everything around it, but it would definately be worth looking into that a little further. Thanks for your input.

[rb6banjo]

That transfer-line temperature is kind of low for a heavy material like naphthalene. Have you tried elevating that from 100 °C? I do aroma work and the olfactory port transfer line is more like 165 °C and it's not nearly as long as the the transfer line from our purge-and-trap sampler.

[Bigbear]

Never ran a 3000, jumped from an LSC-2 to a Velocity. I had isssues with the Velocity mount ( part that the sparge vessel attached to). It needed changing every 12-18 months.
What do your other "late eluters" look like? If they are all low, you may have a cold spot in the system. Is your transfer line warm to the touch?

[ELIkjw]

rb6banjo the line temp is a misprint. my appoligies, it is actually 150.

Good news though! I replaced the 6-port valve and cleaned the 3 port value on the top of the purge and trap along with all the sample lines. Now naphthalene is back up to normal levels and the carryover on subsequent blanks is much more managable. Thanks for all the input!!

[Yama001]

Good to hear it. I hate that pair of valves.