Chromatography Forum

I'm trying to develop a new method to separate phyrethrins : Prallethrin and and D-allethrin using an old GC14 with DB1 (G2) column and FID detector 280 C.
I'm using He as carrier with constant flow. The initial temp is 175o C, Rate 30o C/min, hold time : 2 min, final temp 250o C, But the retention time between these two compound still similar and the resolution still under 2.0
Even I'm trying to lower the initial temp. until 60o C but the resolution didn’t change.
Is the injection temp. effect the separation? Because I never change the injection temp (210o C)
What should I do to separated those two compound and get the best resolution?

Regards

Vienna

When you 'blast' those two analytes off the column at 30 degrees per min you are expecting good resolution?

How about trying 4 degrees per min and see if the separation improves?

best wishes,

Rod

When you 'blast' those two analytes off the column at 30 degrees per min you are expecting good resolution?

How about trying 4 degrees per min and see if the separation improves?

best wishes,

Rod

Yes, I've tried.
before I 'blast' them, I've tried rate from 3 until 10 degrees/min. the RT difference between them still 0.4 min.
but I have not tried lower initial. temp with 4 or 5 degrees per min.

Thanks

Vienna

Traditional techniques, film thickness, change to narrower ID column, change selectivity of column 5% Phenyl, 20% Phenyl, the 1301 phase and 1701 phase offer multiple mechanisms of separation.

If at first you don't succeed, try all the columns you have.

Rod

A separation of 0.4 min (=24 s) should give you baseline separation unless the peaks are wider than they should be from a capillary column.

Dirty inlets, degraded columns and poor choice of solvent spring to mind as possible causes of wide peaks. There are many others.

Peter

Traditional techniques, film thickness, change to narrower ID column, change selectivity of column 5% Phenyl, 20% Phenyl, the 1301 phase and 1701 phase offer multiple mechanisms of separation.

If at first you don't succeed, try all the columns you have.

Rod

OK thanks

Vienna

A separation of 0.4 min (=24 s) should give you baseline separation unless the peaks are wider than they should be from a capillary column.

Dirty inlets, degraded columns and poor choice of solvent spring to mind as possible causes of wide peaks. There are many others.

Peter

the tailing factor is 1.2 when I use acetone as solvent, but the resolution is 1.5.
but when I use methanol as solvent the tailing factor is 2.2 but the resolution is higher than 2.0

vienna

Obviously you have some solvent effects going on. We need to know the details of your analytical conditions and the nature of the samples.

Peter

Obviously you have some solvent effects going on. We need to know the details of your analytical conditions and the nature of the samples.

Peter

the sample is pesticides pyrethroids class (Pralletrhin and d-alethrin) with concentrate 0.079 % w/v, they are non polar compound. solvent is acetone

GC condition is : The initial temp is 175o C, Rate 5.0o C/min, hold time : 2 min, final temp 250o C. He as carrier with constant flow. I'm using capillary column 100% dimethylpolysiloxane.
do u have any suggestion what kind of column should I use?

Regards

Vienna

If you feed liitle bits of your method at a time this is going to go back and forward multiple times before we have enough information to help you.

What is the length, diameter and film thickness of the column ?. What is the carrier flow rate (NB "constant" does not answer this question) ? What volume do you inject, split or splitless ?, split ratio or splitless time as appropriate. What inlet liner do you have, when did you last change it or clean it ?.

Peter

If you feed liitle bits of your method at a time this is going to go back and forward multiple times before we have enough information to help you.

What is the length, diameter and film thickness of the column ?. What is the carrier flow rate (NB "constant" does not answer this question) ? What volume do you inject, split or splitless ?, split ratio or splitless time as appropriate. What inlet liner do you have, when did you last change it or clean it ?.

Peter

Length 15 m, diameter 0.53 mm and thickness 1.5 um. I use splitless injection with glass insert, 2.0 ul. last time we clean it three week ago.
Carrier flow not more than 30 ml/min

OK, now I can make sensible suggestions.

A 15 m 0.53 mm column does not give much resolution (in capillary GC terms). A 30 m 0.32 or 0.25 mm column with a 0.5 um film will work better.

You are probably overloading the column - you are putting about 2 ug of each analyte onto it with a splitless injection. Try diluting your sample, or use a split injection.

"Not more than 30 ml/min" does not provide useful information; if you are working at 30 ml/min the flow is much too fast, if at 1 ml/min it is much too slow. Set the flow rate at 5 ml/min for your existing column, 2 ml/min for a 0.32 mm column and 1 ml/min for a 0.25 mm column.

Nearly all inlet liners are made of glass - what shape is yours ?

You ignored my question about splitless time.

Peter

OK, now I can make sensible suggestions.

A 15 m 0.53 mm column does not give much resolution (in capillary GC terms). A 30 m 0.32 or 0.25 mm column with a 0.5 um film will work better.

You are probably overloading the column - you are putting about 2 ug of each analyte onto it with a splitless injection. Try diluting your sample, or use a split injection.

"Not more than 30 ml/min" does not provide useful information; if you are working at 30 ml/min the flow is much too fast, if at 1 ml/min it is much too slow. Set the flow rate at 5 ml/min for your existing column, 2 ml/min for a 0.32 mm column and 1 ml/min for a 0.25 mm column.

Nearly all inlet liners are made of glass - what shape is yours ?

You ignored my question about splitless time.

Peter

OK, I understand now. My inlet liners is deactivated fused silica (deactivated wool). I'm sorry, I forgot to inform you that I'm using very old GC (Shimadzu GC-14 with chromatopac). Not connected to PC. And the injection is direct to column.
Thanks, I try to set my flow as you suggested

regards

Vienna

Hi Vienna

If you have a liner packed with glass wool you cannot be injecting directly to the column - this requires the needle of the syringe to enter the top of the column. I suspect that you are doing a splitless injection. Since the GC is quite old it might have a packed column inlet that has been converted to work with a wide bore capillary column. This will mean that you cannot do split injections so if you do have this type if inlet you need to dilute your sample by about 20 times to avoid overloading the column.

Please confirm what type of inlet you have - I am not familiar with Shimadzus so just telling me the model number will not help.

Peter