ethanol and isopropyl alcohol separation

[tellest81]

Hi!!!

I have since the last few days a co-elution problem of ethanol and isopropyl on my new column Agilent HP-INNOWax
19091N-133 30 m x 0,25 mm, 0,25 μm, recommended for better separation of alcohols. to analyse my sample i use the following program
Helium 1 ml/min, 35°C, constant flow
oven : 35 °C during 6 min
35 à 150 °C at 5 °C/min
150 à 250 °C at 15 °C/min
250 °C during 2 min
Splitless Injection, 220 °C (in waiting of a split liner)
vol injected: 1 μl

the others molecules in my sample are very well separated due to their polarity, but the ethanol and isopropanol (isopropanol was the internal standard) are not separated. which is not theoretically logical. so I equilibrated the column, Check for leaks. I find always with the same problem. therefore, I removed isopropanol by acetonitrile : another co-elution . I am in search of appropriate maintenance to perform elution . I work with an Agilent GC 7890A Series MSD. does someone can help me?

[WK]

Hi ,
I don't know about the separation on wax type phase - but I use a 624 type phase and get very good separation of methanol ethanol isopropanol n-propanol etc at 50degC.
Regards
WK

[Peter Apps]

Unless you are diluting your sample, a splitless injection is probably just giving peaks that are so big that they overload the column, and with the low starting temperature for the column you could easily have condensation on the column and all sorts of messy solvent effects. You can do split injections with a splitless liner, so try a split injection with a split ratio of 50:1 and see if that helps.

Unless you need to elute heavy compounds do not abuse your column by heating it to its maximum temperature.

Peter

[chromatographer1]

A wax phase column will give poor if any separations, as you have discovered.

As WK wrote, the 624 or 1301 phase column will give excellent separation of these analytes.

good luck,

Rod

[tellest81]

Thanks!

Rod, its not what is said in the data sheet from Agilent concerning alcohol also, i have a good separation between methanol and ethanol! and I get a better separation of all my other compounds present in my sample!. Because I have a wax column, I may need to change the internal standard! But, what standard can i choose? the HP-Innowax is a column of PEG with a high polarity.

[chromatographer1]

normal propanol is a good and common choice. Just don't pick t-butanol, because guess what !

When I wanted to separate acetone, acetonitrile, methanol, ethanol, IPA, nPrOH, and many other solvents at the same time I put a 3 meter piece of wax capillary in front of a 30m 624 capillary.

Journal of Analytical Chemistry June 1997

best wishes,

Rod

[tellest81]

thanks rod!
im very happy to know that! can you give me your issue number or the complet reference of your article please ?
but now what i should do to improve my separation? the problem is it on my GCMS system and no on my column?

[chromatographer1]

Analytical Chemistry Vol.69, No. 11, p2221-3

Your flow of helium at 1mL/min is higher than optimum. 0.6mL/min would be better. Having vacuum at the end of the column also increases the flow rate through the column. Try different (lower) flow rates.

Minimize the sample size to improve separation. Increase the split or the sample volume injected.

Good luck.

Rod

ps CHANGE YOUR COLUMN TO A 624

[Consumer Products Guy]

I also would EXPECT ethanol and IPA to co-elute on a PEG column. And almost EVERYONE (except the USP, of course, <611>) uses n-propyl alcohol as internal standard anyway.

What ^&%#$%^&$%$##^&# had the great idea to use IPA as internal standard anyway, and on a PEG column ????

By the way, we use both PEG and 624 capillaries for our alcohol products. And have done our own internal test method validations (required by our QA).

[chromatographer1]

Oops ! I forgot a word previously.

"Increase the split or the sample volume injected."

Increase the split or DECREASE the sample volume injected.

Rod

[tellest81]

please, Consumer Products Guy!!!

can you explain your :

What ^&%#$%^&$%$##^&# had the great idea to use IPA as internal standard anyway, and on a PEG column ????

because i don't think that is a ^&%#$%^&$%$##^&# idea to use the IPA as interne standard in this polar column!!!!

[tellest81]

Oops ! I forgot a word previously.

"Increase the split or the sample volume injected."

Increase the split or DECREASE the sample volume injected.

Rod

Thanks, i will try this!

[chromatographer1]

tellest81 wrote:

"please, Consumer Products Guy!!! can you explain your :

What ^&%#$%^&$%$##^&# had the great idea to use IPA as internal standard anyway, and on a PEG column ????

because i don't think that is a ^&%#$%^&$%$##^&# idea to use the IPA as interne standard in this polar column!!!! "

Well I do. I agree with Consumer Products Guy !

Having an Internal Std which is almost impossible to separate from your analyte of interest is a not-brainer, instead of a no-brainer choice. It sounds like a desk chemist never had to do the work in the lab wrote the method. Or he meant to write n-propanol or isobutanol in place of IPA and was too embarrassed to change it after submission.

BWDIK

Rod

[tellest81]

tellest81 wrote:

"please, Consumer Products Guy!!! can you explain your :

What ^&%#$%^&$%$##^&# had the great idea to use IPA as internal standard anyway, and on a PEG column ????

because i don't think that is a ^&%#$%^&$%$##^&# idea to use the IPA as interne standard in this polar column!!!! "

Well I do. I agree with Consumer Products Guy !

Having an Internal Std which is almost impossible to separate from your analyte of interest is a not-brainer, instead of a no-brainer choice. It sounds like a desk chemist never had to do the work in the lab wrote the method. Or he meant to write n-propanol or isobutanol in place of IPA and was too embarrassed to change it after submission.

BWDIK

Rod

wow! Gently!

I am only at the beginning of method development. I chose this column because theoretically, it represented a good compromise for the separation of analytes! this column is ok for me in regarding of the separation of ethanol and the others molecules contained in my sample (this results had been difficult to obtain with a semi-polar column DB 17ms)! theoretically propanol and ethanol should be separate on this PEG column. it's for that i have choosen IPA as intern standard. i have any problem concerning change of this internal standard or modification of my method! that's why I come seeking help on this forum!!

[chromatographer1]

Hey, I wasn't talking about YOU, but about the guy who wrote the USP method.

You are excused as it was a learning experience. But the USP guy should have known better.

best wishes to you.......

Rod