GC Troubleshooting for Solvent Peak separation

[Miguel Angel Perez]

Greetings everybody

My HP 5890 has been experiencing some troubles for separating organic Peaks: Ethyl Acetate and N-Propyl Acetate have 77°C and 101.6 °C boiling points respectively, but my chromatogram gives me only one peak. Same is happening to Acetone (56.53°C) and Isopropyl Alcohol (80.3°C) mixtures. For years we have been working with 12 Organic solvents, but market needs pushed us to use a 16 solvent base. Timing for run is 10 min
we have tried Temperature Ramps, Isotherms, Flow variations, But nothing seems to work. This is strange mainly because it is the first time we calibrate with all 16 solvents. also because single solvent retention times are as follows:
N-Propyl acetate = 4.415 @ Flow=6.2
Ethyl acetate = 4.717 @ Flow=6.2
Those two when mixed gives me a single peak with retention time of 4.724
The temperature Ramp used was: 40°C up to 4 min, 40-70°C for 1 min, 70 - 105°C for 1 min ending at 140°C last minute.

My column Characteristics are: 007 series Bonded phase Fused silica Capillary Column (Methyl Silicone), 25 meters, 0.53 mm ID, 8.0u Film Thickness
The whole list of solvents to be analyzed are: Methyl alcohol, ethyl alcohol, Acetone, Isopropyl alcohol, N-propanol, MEK, ethyl Acetate, n-Propyl acetate, Hexane, Isipropyl acetate, Crisanol, Methyl Isobutyl Ketone, etoxypropanol, Toluene, butyl acetate, Propasol

Can anyone give me a hint on what to look for? or if there is a Trouble shooting Guide where i can view possible failures

thanks

[Bruce Hamilton]

Well, if the column still passes it's certificate performance tests, and you have plenty of sensitivity, I'd suggest first reducing the amount of sample going on the column so you can assess whether the column actually resolves all the peaks you are interested in above the detction limits you need.

Also make sure that your solvent, oven, and injector settings aren't adversely affecting resolution. ( usually less injected = improved resolution, lower oven temperature improves resolution of volatile solvents, injector temperature should quickly volatilise all sample and solvents ).

If you need for greater resolution, you could consider a different column phase ( with more retention, rather than just boiling point separation ) or increase the number of plates of the same column phase by increasing column length.

If you haven't tested the column to ensure it still has good separation ability and peak shape, that should be the first step. Then inject small amounts of dilute solutions to see is maximum resolution will provide all the desired separations, if not consider another column.

Please keep having fun,

Bruce Hamilton

[Peter Apps]

Hi Miguel

It would be really helpful if you could post a chromatogram. From the retention time of the combined peak being longer than the retention of ethyl acetate alone you are probably overloading your column. If this is the case it should help to increase the split ratio.

Your temperature programme seems to be a series of very steep ramps separated by short temperature holds. This seldom works very well. Try running one ramp from 40 to 140 at 10 degrees per minute.

Check the column suppliers' applications databases for the separation that you are looking for and replicate their conditions.

Peter

[chromatographer1]

I suspect one of your solvents is mis labeled. An 8micron methyl silicone not separating ethyl and propyl acetate? NO WAY !