I advice you to see this guide

https://www.agilent.com/cs/library/catalogs/Public/5990-9867EN_GC_CSG.pdf

These compounds should work fine on a typical volatiles column, such as a DB-624 or DB-VRX (or equivalent phase from other manufacturers). I would also think they wouldn't need the cryo cooling during the loading process since they are no so volatile as to prevent their focusing (an thus achieving a narrow peak) on teh head of the column. I'm not an expert on using the TDU as you are for this application, but I wander if this could be the reason for your splitting peak. Also, what column stationary phase and dimensions are you using?

Thanks!

I've been using a 30m 25mm DB-5.

The compounds do show up, but the peaks are often hard to work with.

The TDU may be a reason for the splitting peak, but there's not much I can do about it.

What is your film thickness? This stationary phase and diameter is typically applicable to semi-volatile type compounds. If you are using a film thickness of 0.25, or similar, I suspect the issue causing your splitting peaks is the column and may not be the TDU.

Omer

You should use a trap with sorbent -even with crio option it will provide more narrow band. The cplumn should have a thick film. DB624 is very good for 0,25mm ID the thickness shouls be 1.4 um. What is the flowrate during desorption? Probably spli mode with higher flowrate through the trap will give better effects.

Regards

G

Thanks! I'm actually not sure about the film thickness (away from the lab in the next weeks). Would you recommend thicker or thinner film for very volatile compounds?

And generally - my problems are always at the beginning of the chromatogram. So choosing a polar column on which these compounds elute later should already help, shouldn't it?

Omar,

Use thinner film and switch to a 624 type column. Then you should be fine.

Thank you all!

Hi Omer,

I strongly recommend a thicker film for your VOC analysis.

It is well known, that highly volatile components are lost by using thin film capillaries like 0,25µ .

At least you will observe broad peaks for the low boilers.

I do NOT agree to Justin Fischedicks opinion to use a thinner film.

We are analysing actually about 360 target VOC compounds an even more in a non-target search routinely by using an 60 m DB-625 column / 1,4 and a 50 m DB-5 column 0,5µm, respectively for nearly 2 decades with success by GC/MS with Agilents 5890/6890 and 5971 to 5973 MS machines together with Cryotrapping TDS.

Hope, this hints are useful,

best regards, Peter

Hi Peter,

Thanks a lot. Very informative.

In this project I'd like to optimize the system for highly and high volatile aliphatic esters and alcohols.

I'll probably have to purchase a column for this analysis, which will have thicker film that what we normally use. Would you also recommend it to be long (>50m)? And what about polarity?

Hi Omer,

the DB-624 column as a medium polar "all purpose" type is a fine selection for VOC analysis - especial for esters and alcohols.

I strongly suggest you to select a 1,4 µm film.

50 m for VOC analysis seems to be a minimum; however, I always prefer 60 long types. Very often you are confronted with a large amount of additional signals to be separated from the VOCs of interest.

Using the lower to medium polar DB-624 you would have fine results for volatile aliphatic esters as well as alcohols without loss of low to nonpolar VOCs most often present in VOC analyses.

Keeping the Kovàts data in mind, you will notice a nonpolar column like DB-1 or DB-5 to be less meaningful.

Hope this will make your selection a little bit easier.

Best regards,

Peter

Dear Mr. Omer,

Your samples are C4 to C6 esters. Please note that esters can be better analyzed on a polar or semi-polar column. DB-5 column, which you are using now, is almost a non-polar column with 5% phenyl modification. This is not the right column for polar samples. For your samples, a better column will be Rtx-624 (or DB-624) which should be 30 mt in length, 0.32 mm I.D. and 1.8 micron film thickness. I do not think these compound will require Thermal Desorption. You can get a good separation with good peak shape on this column. With the respective standards, you can do the quantification also.

No one working with volatile compounds would consider your compounds "highly volatile". I have used thin film DB-5 columns very successfully for compounds significantly more volatile than mid-range aliphatic esters. People routinely use -5 phases for BTEX analysis, where benzene and toluene are significantly more volatile than the compounds you are dealing with.

Given that you are using the Gerstel unit coupled to a cryotrap I would suspect the injector. Are you trying to use splitless? You almost have to do this with a low level split flow. If you don't you get exactly what you have observed, broadened or split front end peaks. Your starting temperature needs to be at least 20C lower than the lowest boiling point of your compounds, preferably lower than that. Given that butyl acetate boils at 126 C that shouldn't be an issue for you. You can start your oven at 60 and be just fine.

You don't mention the heating rate of the glass wool trap. I assume it is fast (several seconds); for the compounds you are dealing with it needs to be. You also probably need to hold your initial temperature for at least 45 seconds to give you a chance to recondense the compounds on the head of your column.

Hi Mark,

Thanks a lot. It's very useful.

I guess that "highly volatile" is just relative to what you're used to working with.

yes - 'highly volatile' is indeed relative .....

However, Acetaldehyde a an official compound according to the VOC list from UBA or the German AGÖF organisation.

Some doubts are there, how to separate this volatile compound on thin film DB-5 columns...

Regards, pb

Peter, he specifically stated that he is looking for C4-C6 aliphatic esters. These are clearly not "volatile" in terms of the conventional use of the word. Acetaldehyde is NOT a C4-C6 aliphatic ester; my comment does not apply to acetaldehyde (although you can do it with -5 phases).

Hi Mark,

according to the German UBA-list, the AGÖF list, the Wikipedia etc., Volatile Organic Compounds (VOCs) are defined to have a boiling range from 50°C up to 240°C.

C4-C6 aliphatic esters (butyformiate @ 107° bp. up to Hexylacetate @ 171° bp. are well within the rannge of the so called "volatile compounds".

Regards, Peter

for MS detector, better use DB WAX ultra Inert.0.25mm. would give u better resolution and peak sharpness