IPA and Ethanol separation

[bhaskarr]

Hi I am using this method for many times . But the resolution between Ethanol and IPA will not be acheived every time. I have to struggle for 2-3 days, 3rd or 4th day without any changes the resolution will be acheived. I am not able to understand , wht really happening here,

My GC conditions are

Column: CP Sil 5 Cb 60m*0.32mm 1.0u
Oven 100°C 8min 20°C/min 200°C
Inj:120°C
Det: 250°C
Flow: 20 Psi
Split flow:50ml/min
Auxillary flow:30ml/min

My resolution requirement is not less than 1.5.
Because of the peak shape the resolution is going down. If peak shape is sharp, then the resolution will be acheived. Can anyone help me.

[krickos]

Hi

Do not use 60m columns very often but most likely the peak shape issue is related to that your start temp in oven (Oven 100°C ) is way over the boiling points of ethanol and 2-propanol (78 & 82°C). Even though the 5% part of phenyl phase gives some increased selectivity, the column mainly separate homologs of alcohols based on boiling point. Due to the oven temperature these two will start moving directly and will not be focused in a narrow start "band" at the injection.
The reason for achiving 1,5 resolution from time to time most likely is due to that the column is new or/and recently cut.
Lowering the oven start temperature under boiling points of those solvents will likely solve this issue more permanently.

[WK]

bhaskarr
Try using a 624 type column.
WK

[krickos]

bhaskarr
Try using a 624 type column.
WK

Same issue will arise at least with a 30m 0,53mm id 3um 624 type column as there is only a ~0,5min RT difference between those solvents at 40°C on the 624 column

I still vote for fixing the oven temp

[i.horsting]

I think you can seperate them more effectively when you start your oven at 40°C and than ramp it to 100°C with 3°C/min

And please use a smaller ID column. 0.25 mm. this will help your peak shapes and thereby the resolution.

And what is the Auxillary flow for?

[Consumer Products Guy]

Actually, it's not a slam dunk to separate ethanol and IPA well and get good peak shapes by capillary GC. As strange as it seems, we'd likely go back to packed-column for this, with a 40 inch x 1/8 inch Tenax-GC column.

[gcguy]

You could use a 0.18 624 and enter the world of fast gc.

[WK]

Hi,
I use 624 1.8um 0.32mm x 30m with helium at 14psi and FID.
I get about 30secs difference between apex at 50degC isothermal.
Split 100:1.
WK

[bhaskarr]

Thankyou all for your replys.

Bhaskar

[Victor]

I agree with Consumer products guy. If you are only interested in the analysis of ethanol and IPA (and not in some complex mixture of other compounds too) using a packed column like he described (or Chromosorb 101 or Porapak Q-these are porous polymer columns) is simple. You could drive a truck between the peaks of ethanol and IPA on this column-you can inject water directly if you want to, and the column will last for a long time. And if you contaminate it you just throw out the first few cm of packing, refill it and start again....No problems with injections either-you can just put the needle directly into the glass wool at the top of the column.

[rbardsley]

You did not list what else is in the mixture or the ratio of ethanol to isopropanol. I just completed a blood alcohol HS project and the BAC-1 column worked very well. I used a 30 m 0.53 2um column at 40C isocratic.

[Ron]

A Q-Plot column also works very well for this separation, especially at higher concentrations. Plot columns will allow more mass on column before overloading.

[chrom]

use phenyl substituted column,with less flow (0.7-1.0mL flow). Initial oven temperature 45 deg and hold upto 10mts.

thanks