How to calculate temperature of sample injection on column

[ChEmfan]

A sample consist: 4-ethylanisole, acetanisole and 1-(4-methoxyphenyl)ethan-1-ol. Temperatures of boiling point at atmospheric pressure are 199 °C , 258 °C, 180 °C, respectively. Solvent is citric acid buffer solution.

Witch temperature set at the inlet of the column?
Is temperature dependent of pressure of carrier gas (He)? (lower at the lower pressure?)
Whole sample has to be vaporized before column?

[jdezeeuw]

You cannot calculate injection port temperature. The temperature you need is to flash-evaporate the small amount (0.5-1um) you inject in the liner. So the whole sample MUST evaporate for a good injection.
Usually a temperature of 250C is chosen. For split injections, often a deactivated plug of glasswool is used, positioned in the middle, like the Precision liner. You need heat capacity,so temperature drop is minimized at injection and split-point when injection and evaporation is done.

If you use a water matrix, the glasswool may hydrolize, causing discrimination. In that case, you may consider double gooseneck or cyclosplitter liners (no glasswool);

Note that buffer salts will also accumulate and you will replace liners systematically.


temperature is not related to pressure.

If you need to see <1 ppm components you may use splitless injections, which will be quite challenging with water matrix

jaap de zeeuw, Restek corporation

[DR]

... Unless you want to go w/ cold trapping.

[ChEmfan]

Citric acid buffer solution is a solvent for reaction. In GC analysis is used methanol as a solvent.

Thanks for a complex answer, I'm just about to start my very first analysis with GC. Generally lover oven temperature is simpler with managing. Substances above should be easy to evaporate because they have strong smell, but I can't be sure due to lack of data in the literature.

[ChEmfan]

Another problem occur.
There's no signal for compound as 4-ethylanisole, acetanisole and 1-(4-methoxyphenyl)ethan-1-ol. Detektor is strictly blind for this substances.
First I've analysed my post-reaction sample. No response - peak area were 0. so I compose standard sample with ratio 1:2 4-ethylanisole and 2-hydroxypropane. There was one peak just at the beginning (0,68 min) witch I assume was 2-hydroxypropane. Finally, last analyze with pure 4-ethylanisole give no signal.

Used Analysis condition:
Injector (PSS): 70°C 2 min then rate 10 deg/min to 250°C hold 15min.
Oven: 70°C 2 min then rate 10 deg/min to 250°C hold 15min. (same as injector)
Detector (FID): 260°C
capillary column, max temp 400°C, i have to check stationary phase, d=320 um, 15m long
carrier gas He, flow 2 ml/min
split gas control, rate 9

boiling point of 4-ethylanisole 199°C

[chromatographer1]

Bring your injector up to 150 C initially.

When you cold trap you dont use a cool injector as you have done. And using methanol will broaden out the peaks if you do splitless. Run test samples 0.1% in methanol in split mode initially to get your retention times and keep diluting until your peaks disappear. Confirm your salt aqueous solution can hold measurable amounts of your analytes.

best wishes,

Rod

[jdezeeuw]

I agree: and start at a higher injection port temperature. Use at least 150C using the split.

If your peaks elutes < 1 minute, you also need a column with more retention (thicker film)

[ChEmfan]

I've done what you told me and still nothing..

[chromatographer1]

And how did this turn out?

" Run test samples 0.1% in methanol in split mode initially to get your retention times and keep diluting until your peaks disappear."

best wishes,

Rod

[Hornet]

Hi, it looks like you have some instrumental issues. Did you check column installation (leaks, column distances at injector and FID), injector split settings, flows? I don't even see the solvent peak in that chromatogram.

[chromatographer1]

I suspect salt deposits blocking the column.

But that is why we do diagnostic injections.

Rod

[ChEmfan]

" Run test samples 0.1% in methanol in split mode initially to get your retention times and keep diluting until your peaks disappear."

I,ve done today sample 0.1% in methanol, 2 rans with the same sample and method give same retention times but peak high was different (670mV, and 850mV), still one peak - I think methanol, this 0,1% of ethylanisole - missing.
I've triad also isothermal program with low and high temperature - there wasn't any peak.
also I've done some literature reaserch and found at least couple of instrumental abnormityies. for example 320um diameter column is installed in injector PSS with wide bore 2mm, when user guide says you have to use pre-column or column witch diameter 530um, there can be some leakage

[Hornet]

Your problem is in the injector or at the injector end of the column.
Either a large leak or deposits of salt like Rod said, especially if you injected water/citric acid solutions like you said in the first post.
If it is so i would start immediately with cleaning procedures, try without any pre-column to eliminate possible causes and just install column correctly. As long as you inject only volatile solvents it won't be a problem.
If it will work then you can start thinking about a pre-column installation. Good luck.

[ChEmfan]

I've finely localized the problem and fixed. the column was to far pushed into detector, over 1,5cm. So the end of the column was in the flame, witch gives negative peaks. The end of the column was damaged, stationary phase burn out and capillary partially melted.

Anyway, i've found at this forum that only reason for negative peaks is CO, CO2 (or other but impossible in my case) so i start think that problem is in detector region.
thanks very much, it was looming large in my mind over one month

[AICMM]

I would add that you probably have the wrong jet installed. Capillary column (0,32) with a capillary jet is not able to get past the jet (on Agilent equipment at least.) Capillary column with packed jet you can over shoot. Does not make too huge a difference though.

Best regards,

AICMM