Chromatography Forum

I am running a modified version of ASTM D6729 on a refurbished HP 5890 II Plus GC/FID with a HP 7673 autosampler using the following conditions:

Injection: Split, 150:1
Volume: 0.5 uL
Oven ramp:
Initial: 30C
Ramp 1: 1.00 C/min to 50C
Ramp 2: 0.5 C/min to 75C
Ramp 3: 30C/min to 270C
Column: Restek Rtx-DHA fused silica column - crossbond 100% dimethyl polysiloxane. L: 100m (has been cut to ~90m), ID: 0.25mm, df: 0.5um
Head pressure: 37.8psi, constant flow mode
Carrier: He

I am analyzing samples from a liquid hydrogenation reactor loaded with Pd catalyst running a synthetic feed of 5% 1-octene and 1% styrene in hexane solvent. The hydrogenation products are n-octane, ethylbenzene, ethylcyclohexane, and 5-6 octene isomers.

I have two issues (which may or may not be related) with this instrument:
1) the % RSD will be 4-6% for the analytes of interest. This is from repeated injections from separate vials of the same standard. I have seen better (1% or less %RSD) in the past so I know the instrument and the method are capable of better.
2) Excessive noise/spikes on the chromatogram as seen on the picture below. These two samples were injected within 25 hours a part as part of a continuous run of 30+ samples. The noise will come on and dissipate at random times. I have also observed the noise while opening the oven door while the FID was OFF. I have also observed it right after an injection while the autosampler turret was moving, however, this doesn't happen on every injection. I am almost certain the problem is electronic in nature. I do not know how to track down the source.

I have changed the EPC, selenoid, the entire inlet assembly (including split vent line), main board, the entire FID assembly including the heater, PRT, and FID board. I am running out of things to swap out! I do not have a spare EPC board or cable and am finding it difficult to convince myself that would make a difference.

Any wisdom in this area would be appreciated. I feel like i could track down the source if I was better with a multimeter ?



imgur link in case the above image does not display: http://imgur.com/MUAjAw9

I am sure one of my technicians at GenTech would be able to assist you! If you would like to try and call in when you have some time to trouble shoot, you can reach us at 585-492-1068.

If you have any questions you can contact me directly at atrinkle@gentechscientific.com

You first need to split the problem in two, is it electronic noise or is it something else. Turn the flame off. If the noise is still there then you have an electronic problem. If the noise disappears then you have something like a leak, column problem or dirty gases going to the FID.

Disconnect the column and block off the inlet to the detector. With the flame on, do you still have noise. If yes, this points to dirty gases or a dirty detector.

Gasman