Chromatography Forum

Does anyone know if the use of ethanol as a diluent in GC onto a polyethylene glycol column phase will cause any sort of phase deterioration? Does the polarity of the ethanol overwhelm/interact too much with the polar wax phase? I didn't expect any issues with ethanol as a diluent in this method developmenet, but I am seeing weird baselines...

Are you using absolute ethanol? If not then you are injecting about 5% water when you inject ethanol.

Liquid water CAN damage the PEG phase and its bonding linkages to the FSOT surface, causing polarity and activity changes and later, column phase bleed.

Not much more can I say with the limited information supplied.

best wishes,

Rodney George

We've been injecting/assaying aqueous dilute ethanol samples (dilutions of hand sanitizer products) on the same Supelco Nukol 15m x 0.53mm column for 14 years now, same column (the backup column just sits in the drawer). Yes, we have oxygen trap on the helium carrier gas, as the operating temperature is 40C for this. We do take to 150C for a few minutes prior to running a sequence. Occasionally we assay other samples on this column as well. We leave a tiny amount of He carrier flowing through when installed and not in use. When we remove from the GC we plug the ends, but not sure how fast oxygen will attack PEG at room temperatures, understand that high temperatures and oxygen is very bad for them.

CPG,

You are quite correct that oxygen can damage a PEG type phase. Water also (in liquid phase) is quite harmful as well, Thus your moderate temperature conditioning (140°C) is a very good practice.

I am certain that Supelco would be happy to sell you another column however, whenever you felt the need to replace it.

Given clean samples and care of the purity of helium and other carrier gases the modern capillary column can last for a very long time as long as temperatures are kept below the reaction temperatures of the column materials.

Rodney George
consultant

We also use USP <611> on a 624 type column for ethanol assays; our QA still made us validate it though, thought "it would be easier to defend" if audited if based on a USP assay. That uses ACN as internal standard.

Good ol' USP, just about everyone else in the world uses n-propyl alcohol for that (which does resolve well from ethanol using the USP <611> column and conditions, but USP somehow chose ACN. Go figure.

The ethanol is 200 proof, so I would say that's as pure as you can get
So it sounds like no one sees any issues with using ethanol as a diluent, correct? B/c I'm extracting glycerol into ethanol and then injecting 1uL of the ethanol extract into the PEG column (50-1 split). I am wondering if something else is going on to give me the wierd baseline. Thanks for everyone's tips regarding PEG column deterioration!

The ethanol is 200 proof, so I would say that's as pure as you can get :)
So it sounds like no one sees any issues with using ethanol as a diluent, correct? B/c I'm extracting glycerol into ethanol and then injecting 1uL of the ethanol extract into the PEG column (50-1 split). I am wondering if something else is going on to give me the wierd baseline. Thanks for everyone's tips regarding PEG column deterioration!

Doesn't it take temperatures close to the limit of the PEG column to elute glycerol? And how's that glycerol peak shape? We've never been that happy with the glycerol peak shape underivatized on PEG, but better on 624 columns like in the USP glycerin monograph than on PEG for us.

Typically, we extract into DMF and use trimethylsilyl derivatization for glycerin assays and for similar glycols and glycol ethers.

It doesn't take much, the inlet is set at 220, and the flow rate is aggressive (5mL/min) and the ramp (if I remember correctly) is 130C - 220C @ 15C/min...then we hold after that. The peak comes out around 4-5 minutes!

You are giving that column a lot of thermal stress.

Also understand that PEG column temperature limits you have on the box from suppliers are NOT isothermal limits. Anytime you exceed 200°C for a PEG column you are initiating reactions with the phase and the tubing which cause damage, especially if there anything on the column from your extractions that are reactive. More time = more damage = less lifetime

Continue as you have been doing and you will be buying new columns regularly. If it is required for your analysis then that is the cost of doing business. But if you can find another phase that will perform the analysis as well, you might consider it.

Temperature limits are for programmed ovens, not isothermal.

best wishes,

Rodney George
consultant

The max. operating temperature on the box is 250oC so I think I should be ok

We use ZB 624 column for IPA and ethanol analysis regularly. When testing ethanol we ust IPA as internal, when testing IPA we use ethanol and internal. We use a ratio of one to the other. Sample are diluted in water. Products are 70% alcohol usually.

t_chem_1

Your naiveté is common. Believe what you wish. The laws of physics do not care what marketing people put on boxes. And just pretend the wavy baseline doesn't mean anything. I worked for a manufacturer of capillary columns and understand how these limits are determined and the business decisions are made (they all do it). Customers can make presumptions but that doesn't make them true. Remember what you wrote?

"but I am seeing weird baselines..."

Make sure your budget next year has a line for new columns.

best wishes,

Rodney George