7694 Headspace Maintenance

[thebug799]

I was wondering if someone could offer some tips on maintaining/cleanning a 7694. We have an instrument that is operating pretty poorly (poor sensitivity, lack of reproducibility). Any suggestions other than Agilent's recommended steam cleaning procedure?

Thanks

[aldehyde]

Check the integrity of your vent and vial pressure valves. Hook up a length of tubing to the vent port on the back of the instrument (looks like a 1/4-1/2inch long piece of metal tubing sticking out--you'll need like LC tubing to hook up to it). Stick it in a vial of water and then go into the adv parameters, manual operation. Turn on vial pressure, turn off vent and sample valves. No bubbles should be observed. Turn on vent and a stream of bubbles should come, turn it off and they should (fairly quickly) stop. If the bubbles continue that is your problem.

Repeat the test with vent off, vial pressure on, sample valve on (then vent on, vial P off) and you should get a similar response. If you get bubbles with vent off, or you get a steady stream with vial pressure off you are losing a bunch of sample due to broken solenoid valve.

On some of the older models the off meant open and on meant closed so you may have to do the reverse of what I am describing for some older models.

Next try steam cleaning, if that doesnt work you may need to replace some of the flow path because it may be contaminated. You may also consider the rotor seal in the 6 port valve, both of these fixes would probably require a call to Agilent.

[thebug799]

Thank you for the suggestions. Everything checked out okay so I did the steam cleaning... we'll see if it helped or not.

[chromatographer1]

Make sure that your vial crimping system is working properly. It is a common cause for your problems.

Rodney George

[krickos]

Hi

Beside aldehydes and Rodneys valid comments I would like to add a few things.

-flow path. There are a few things you may change your self with the right training ie sample loop and needle can be changed and cleaned (soak in ethanol (70%) and perhaps low intensity ultrazonic treatment). As stated I would not touch the 6 port valve without proper training/knowhow (not touching it myself).

-sadly the problems you describe is not only related to poor preventive maintenance of 7694A/B or G1888 models. It also common due to poor method settings as seen quite frequently in this forum. In short temperatures in sample loop/needle compartment and transfer line often are set too low versus boiling points of sample solvent/analytes/matrix that evapourates into gasphase. So preventive maintenance may not be the issue alone.

-steam cleaning as per agilent. aye may help sometimes depending on your typical samples. We did that our selves in the past, but later switch to using ethanol and hexane (do not have detailed settings as not in office) as that proved more efficient for our purposes.

-some 10 years ago we had similar issues but by considering the above mention by myself and others our isssues with failing RSD, signal to noise, accuracy and carry overs are considerbly less frequent than in the past.

[larkl]

The PM is going to depend on the samples. With polymer samples, I've seen a lot of issues due to poly/wax build up in the lines and valve. The COLD vent line plugging was the most common. Symptom was higher than expected results, as you're no longer injecting at the same pressure.

[GC_Man83]

-steam cleaning as per agilent. aye may help sometimes depending on your typical samples. We did that our selves in the past, but later switch to using ethanol and hexane (do not have detailed settings as not in office) as that proved more efficient for our purposes.

Hi Krickos, could I possibly get a copy of the operating conditions that you were using to clean your headspace system with ehtnaol and hexane? I have performed the steam cleaning procedure but it hasn't seemed to help with my bonus peaks.

Thank You.

[krickos]

Hi GC_Man

See below, please note the temp settings for valve/needle/transferline. You might also try replacing needle and sample loop (can be cleaned).

Add 3ml hexane and 3ml ethanol to 5 vial each. Also place some ~12 vials that are empty in the sequence after heaxane and ethanol. Sometimes more empty vials are needed to confirm that the last residues of the solvents are gone.

HS settings:
Oven 50°C
Sample valve 150°C (may need increasing if high boilers been used)
Transfer line 150°C (may need increasing if high boilers been used)

GC cycle 5min
Sample equilibration 15min
Vial pressurization 0,10min
Loop fill 1,0min
Loop equilibration 0,05min
Injection time 1,0min

GC settings: We usually have a DB-5 column or similar around to monitor the cleaning.

GC oven isothermal at ~50-60°C for 5 mins.
Split flow 20ml/min this depend a bit on your physical set up, we control the HS flows via EPCs, if you manually controll the HS flow just make sure you have a decent flow through the HS to the GC.

[Karen01]

The COLD vent line plugging was the most common. Symptom was higher than expected results, as you're no longer injecting at the same pressure.

One thing I have encountered is reproducibility issues from solvent condensing in the vent line when running a lot of samples with that instrument ... Yes that vent line SHOULD be heated and/or purged after injection automatically... and I've told that to Agilent engineers and those in the booth at Pittcon several times over the last 10+ years. Don't know what they did with the new one they just introduced.

Anyway if that is the problem, use the procedure mentioned above to blow carrier out the vent for several hours to evaporate solvent...

Then for the long run, if you can, try lowering the HS oven temp 10C (or more if possible)... If you still have enough sensitivity, your assay will put less solvent into headspace so there will be less chance of condensing in the vent. and less issues overall.

That has solved reproducibility problems for me in the past.

- Karen