Chromatography Forum

Hello everyone,

I took over a GC analysis for phthalate testing from a co-worker, and I am completely lost over what I'm doing right or wrong. I am unfamiliar with the capillary column, as I work mostly with water solvents and extracts thus packed column is my forte.

Per the recommendation of a test method, these are the parameters of the Agilent 6890N I'm using right now to run 0.05 ug/mL, 0.5 ug/mL and 5.0 ug/mL of DINP standards:

Instrument: Agilent 6890N
Column: Chrompack fused silica, CP-Sil-5CB, 50 m x 0.32 mm, 0.12 um id
Temperature Program: Start at 150°C, ramp 5°C/min to 280°C, hold at 280°C for 5 minutes
Carrier gas: Helium at 44 mL/min, column head pressure: 19.5 psi
Injector: Split, 300°C, ratio 20:1
Detector: Flame Ionization, 300°C
Make-up Gas: Nitrogen at 30 mL/min

I am using a brand new column which has already been conditioned for 2 days, and am currently running more blank solvents and standards to determine the DINP peak from the chromatograms. I am on a very tight deadline, any input at this point will definitely help. I'm also using the Agilent guide on a method that was performed per the Chinese standards on phthalate testing on children's toys, if anyone can recommend any other tweaks to the methods.

Your efforts are truly appreciated!

I guess you're having trouble identifying the DINP in your chromatograms. Your GC conditions look reasonable to me. If I were you, I'd dip my needle in some DINP and inject that. You should definitely be able to determine the RT for the material with that much yet not completely saturate your system.

Isn't DINP a mixture of isomers? I'm not sure that you'll see one discrete peak. I think you'll see a distribution of things with similar retention times.

Carrier gas: Helium at 44 mL/min, column head pressure: 19.5 psi
Injector: Split, 300°C, ratio 20:1


If the column flow is actually 44 ml/min, that is very high for that column. If it is total flow, a 20:1 split means column flow 2.2 ml/min - that is not bad, though a bit low.

I think the FID will have some trouble detecting anything but the high standard at 20:1 split (though the true split is often different from the nominal setting), particularly for a multicomponent analyte. Rb6banjo has the right idea, inject something at a higher concentration, see what comes out.

The comment by Yama001 about the flow is correct. If the head pressure of helium is 19.5 psig on a 50 m x 0.32 mm column at 150 °C, the flow at the flame is 1.9 mL/min. Methane (unretained) should come out at about 2.6 minutes.

Again, these are reasonable conditions for this analysis. The linear velocity is in the 25-35 cm/s range (good for helium). Inject the pure material (only a needle full at most) and see what happens.

Thank you rb6banjo and Yama001 for the responses.

If the column flow is actually 44 ml/min, that is very high for that column (This is the total flow). If it is total flow, a 20:1 split means column flow 2.2 ml/min - that is not bad, though a bit low. My protocol states the Helium flow should be at 55 mL/min, does this mean my split ratio is correct but my total flow should be higher?

Isn't DINP a mixture of isomers? I'm not sure that you'll see one discrete peak. I think you'll see a distribution of things with similar retention times.Yes, I do see a distribution of things with similar retention times. I'm just having trouble getting them to appear in the chromatogram as much as they should.

I see that yours is a 6890 so all of the calculations are done for you. If you set the head pressure and the split ratio in your method, you should not concern yourself with the column flow UNLESS you suspect that it's not what you think. At 44 mL/min through that column, the DINP should come flying through.

Column flow will be a combination of the head pressure on the column and the flow out the split and septum purge vents. This utility has been very helpful to me in calculating pressures and split-vent flows on manual (non-EPC) systems:

http://www.chem.agilent.com/en-US/Techn ... ators.aspx

Agilent also has an app (iTunes store, not sure about Android) you can get for your smart phone if you have one. The app does the same things.