On-Column - a possible misunderstanding of the basics

[eljoe]

Hi,

If your column is a 0.53 mm ID try to work without the pre column (procedure use in our lab) since it could be a source of concern. A high flow rate, higher than normally recomended, could also help. I use a 15 ml/min. flow. With those condition we get a 11 min. RT for diethylene glycol. Working @ 70ºC for your 1041 injector with 100ºC as initial temperature should not be a problem. A Internal Standard is also recomended. We used 1,4 butanediol.
Joel

[mbdixon]

Hi

I don't suppose anyone got this method (EN13130-7)work with a split/splitless injector? We have a Zb-Wax Plus column (0.32mm id x 30m x0.25 thickness) which should handle the water carrier. So far I'm just not getting any sensible chromatography in either split or splitless mode.

[tlahren]

I think there was a thread on this a few weeks ago (analysis of glycols etc.). I think you could get near on-column results if you use a direct injection method with a Uniliner. See this article: http://www.restek.com/pdfs/59882B.pdf

[Yama001]

My solution since 2008 has been to sub the work to one of our sister labs - just managed to weasel out of one this morning . This is an annoying test and I hope you have enough work to make it worth the effort.

I last pulled this off with a wax type column - probably DB-Wax, 30m x .25 mm ID, and splitless injection (1-2uL) on an old pre EPC 5890. Mixing the aqueuous samples with Methanol was essential to reduce ghost peaks, also rinsing with methanol in at least one of the wash bottles. I recall a ghost peak that eluted close the ethylene glycol would show up after several injections, though I could demonstrate it was separating by spiking some glycol.

The Restek article looks good - I am guessing a uniliner would be an improvement. Modern GCs seem have more fits with a hot inector and aqueous injections, perhaps a pressure pulse will help as well. I think I had a 25 psi head pressure on the old 5890.

Tim

[mbdixon]

Thanks for the tips chaps.

I'm not going to get funding for purchasing a new injector such as the Restek one so I'm stuck with the current split/splitless one. I'm going to have a go first with a Pulsed pressure injection in splitless mode(Yama001 - did you not get flashback problems injecting 1-2µl?). Then if that improves things I'll try the methanol mix.

I'll let you know how I get on.

[Yama001]

I think flash back was likely a problem; we did not want to reduce the injection volume due to the limited sensitivity of the FID. I believe it worked out for us since the samples were pretty much ground water - dirty samples with real gylcol levels would have given us fits.

[Peter Apps]

Thanks for the tips chaps.

I'm not going to get funding for purchasing a new injector such as the Restek one You don't have money for an inlet liner ?? so I'm stuck with the current split/splitless one. I'm going to have a go first with a Pulsed pressure injection in splitless mode(Yama001 - did you not get flashback problems injecting 1-2µl?). Then if that improves things I'll try the methanol mix.

I'll let you know how I get on.

[mbdixon]

Thanks for the tips chaps.

I'm not going to get funding for purchasing a new injector such as the Restek one You don't have money for an inlet liner ?? so I'm stuck with the current split/splitless one. I'm going to have a go first with a Pulsed pressure injection in splitless mode(Yama001 - did you not get flashback problems injecting 1-2µl?). Then if that improves things I'll try the methanol mix.

I'll let you know how I get on.

My bad, I only glanced at the article and assumed I'd need other adapters for it to work. In practice just how much difference will they make?

[Peter Apps]

I've never tried these exact liners (they don't make them for Varian/Brukers) but I can say that Uniliners, which are similar for "direct" injections give much better transfer to the column than ordinary splitless liners. Their main drawback is that the solvent tail is longer.

For dirty samples direct injections are favoured over on-column. You can swap Uniliners without re-cutting the column - just put them in very gently so that they do not stick to the top of the column - there is no pressure drop across the seal between liner and column so it does not have to be as robust as would be required when e.g. joining 2 columns with a presstight connector.

Peter

[mbdixon]

Hi again,

I have my drilled uniliner in place, and matrix is a 50:50 mix of water and IPA and I'm getting better chromatography but just can't seem to perfect the chromatography which is annoying. My conditions (HP6890GC) are:

Inlet : Pulsed Splitless, 220°C, Pulsed Pressure 70.0 Pulse Time 1.50 Purge Time 3.00Purge Flow 150
Injection 1µl manual (no autosampler)
Column ZB- Wax Plus (30m x 0.32 mm ID x 0.25µm thickness)
Oven 80 to 160°C @ 5°C/min, hold for 2.00min, then 50°C/min to 250 for 2 minutes
FID Detector 250°C (H2 25, Air 350, Make Up off)

The ethylene glycol peak is very broad at 3 minutes!!! (RT8.10min) and the 1.4butane-diol (rt 14.35) and Di-ethylene glycol (rt14.95 are not totally resolved)

Any suggestions for improvements would be greatly recieved.

[Peter Apps]

Are the Uniliners drilled at the top or the bottom ?.

Either way, try without the pressure pulse - what you gain in faster transfer to the column you might lose by band spreading at the very high initial flow rates.

Are the peaks symetrically broadened, tialed, or distorted ?

Peter

[mbdixon]

Thanks Pete
The uniliners are drilled at the top.

Its strange in that the Ethylene glycol is broadened symetrically, whilst the butandiol an DEG peaks are what I consider reasonable(~0.5min at base) and are symetrical too, especially considering the matrix and injection technique.

I'll give it a bash without the pulsed injection and report back.

[Peter Apps]

In this context symmetrical broadening is good - it points to simple band spreading.

Peter

[mbdixon]

In splitless mode the MEG peak is still very broad and symetrical, whilst my DEG/Bitan1,4 Diol peaks are OK.

I have also prepared new standards, though this time with just water as per the method.

Inlet Conditions : Splitless, Temp 250°C, Purge Time 1.00, purge Flow 50.0, saver Flow 20.0, Saver Time 5.00
Injection 1µl manual
Column ZB- Wax Plus (30m x 0.32 mm ID x 0.25µm thickness)
Oven 50 (2mins) to 160°C @ 10°C/min, hold for 2.00min, then 50°C/min to 250 for 2 minutes
FID Detector 250°C (H2 25, Air 350, Make Up 29.0)

Any suggestions??

[Peter Apps]

What is the carrier gas flow rate ?

Peter