Chromatography Forum

Anyone using a CTC GC-PAL autosampler (either with a GC-FID or GC-MS)?

We're having problems with reproducibility (the entire system is brand new and we've changed the liner, septum, checked column depth, changed the GC method to different parameters to see if we'd get better results) but our RSD's typically vary between 5-10 %, which is really not that great.

Any optimal numbers for filling strokes, speeds of syringes, fill volumes, etc.? We're washing the syringe twice in each wash vial (9 uL on 10 uL syringe) then aspirating the sample (4 filling strokes of 8 uL) at 1 uL and injecting (inlet T is 250, oven set at 100, split flow 100:1, 1.3 mL/min at constant flow mode) - Agilent 6890 GC and 5975 MS in EI mode.

I've tried several things and I am starting to be at a loss...

Thanks!

What is your injection solvent? Your inlet temp might be too hot for that much volume, or you might be injecting too much for your liner.

On my AS200 autosampler I usually prewash the syringe with 5-6 µL four or five times with both cleaning solvent and sample prior to injection. Do you also pull up 1 µL of air after loading the syringe?

What injection port do you have, is it a PTV. or the classic Split/Splitless.

As the PTV we have discovered has a terrible %RSD, as the liners have
only 200ul volume. so we switched to rear injection port with the classic Split/Splitless and have gotten great %RSD.

We also use the double goose neck liners (no glass wool)

regards

Alex

Also make sure your injection penetration depth is set at the maximimum (45 mm).

Hi there,

My first injection liner was an Agilent 5062-3587 (splitless, single taper with glass wool). Hadn't realized that's what was in there and we were doing split injections so we switch to a split/splitless liner from Phenomenex (AG0-7515), which is a tapered FocusLiner deactivated with glass wool (4 x 78.5 IDxL mm), but unfortunately we did not get better reproducibility.

Also tried switching the syringe to no avail. My injection solvent was isopropanol.

Tried adding sample wash but did not improve results - neither did taking up an air volume after the sample - btw, is this a common procedure in GC? We've never taken up an air volume after the sample before... Isn't the injection of air unto the column not that great for it?

Our inlet is classic split/splitless. Tried modifying the injection depth to 45.0 mm too but again, better results were not obtained...

May try a different liner or may try contacting Agilent - we just got this system and I find this terribly frustrating!

If you have any suggestions, please do share them.

Thanks.

Wrong injection speed? I use between 50-80 ul/s.

Roxanne42,

What do your peaks look like? Would you say that your reproducability is poor because of integration or because of material on column changes so much. I would suggest you either start 20 C warmer or 40 C colder since you are fairly close to the boiling point of the IPA.

Also, what is your column heigth?

Best regards.

Tried up to 80 uL/s but no major improvement...

Peak looks great - also tried raising oven temp to 120, but no better results. The syringe drive does make a funny sound when it pulls up the plunger but I watched the whole process carefully, and it looks good...

Column length is 29.5 m.

I have spent like 3 days trying various things because I dreaded calling Agilent but I think I'm at that point.

Thanks for all the suggestions though!

Oh, what RSD's do you guys typically get with your CTC autosampler (or other) and on how many injections?

Some of my numbers are like;

59214479
60631200
58648698
57605642

which gives 2.13 % RSD - that's one of my better sequences; usually get between 3-7 %!




Thanks,

Roxanne.