Dear Members of Chromatography Forum:

We want to use our HP-6890 equipped with a capillary column HP-Innowax (PEG Column, 0.25mm*30m*0.25um) for analysis of
reaction intermediates such as organic acids(oxalic, formic, maleic, fumaric and
4-hydroybenzoic acids) and quinones (p-benzoquinone, o-benzoquinone, catechol, and hydroquinone).
We follow a method recommended for this column for acids {oven:100ºC(5 min),100-250ºC(12ºC/min), Carrier: Helium at constant pressure of 148.2 kPa, injection: Split (50:1), 1 ul, 180ºC, detector: FID, 275ºC} and we obtain poor results for all the acids and benzoquinones, we tried various temperature
ramps, splits, fluxes, pressures, isothermal temperature programs and so on.
What do you recommend? Our samples are a prepared pattern diluted in water and with concentrations of
about 10000 ppm except for 4-hydroxybenzoic acid and fumaric acid(1000 ppm).
Can anybody help us? Now we are thinking in the esterification of our samples (a complicated work with real samples) or buy another column like DB-FFAP.
If someone can suggest something to us, we appreciate very much

Can you expand upon what you mean by "poor results". Are you referring to peak shape problems, lack of sensitivity, poor RSD, etc.?

You might try injecting some standards containing neutral analytes (ie. hydrocarbons) to see if the problem also occurs for these compounds. If so, then I would look at sample introduction as the potential source of the problem. If not, then it implies that the problem is associated with analyte activity.

If you would like, I would be glad to discuss this further with you off-line. You are welcome to fax me chromatograms and more details and we can try to come up with a solution.

Best regards,
Jason Ellis
GC Column Technical Support
Agilent Technologies

Hmmm...I think you need to think a bit more about what you are doing.

Formic acid has little or no response on an FID detector. If I remember rightly, oxalic acid decomposes to formic acid in the injection port. Indeed, oxalic acid is sometimes added to samples to produce formic acid so that the column may be deactivated for other acids....

Some of the acids you are using are really quite polar and will stick like glue to any active sites you have in your column or instrument (injector etc) Have you thought about doing these by HPLC?

Regards,
David

This a answer for Jason Ellis:
Hello and thanks for your coments, By "Poor results" I mean that we don`t obtain any signal for these acids in fact we have good peak resolution and sensibility with phenols, catechol, aniline, and so on.
In fact we contacted the agilent support team at Barcelona, Spain and they suggest some injection improvements and other things, but without any, results, we will try to use other column like FFAP.
Tell me if you want to see our chromatograms...
Thanks

This is an Answer to David...
Hello and thanks for your coments, other people in our faculty explain to us, how the FID, detect the compounds and is true what you say, but we don`t have any signal for other acids with more than 1 Carbon bounds, except by acetic acid and other compounds like phenols, aniline, catechol, hydroquinone and so on...
Previously we used HPLC for this compounds but now our HPLC are broken and we have to wait for a year to buy another. So, our option is to try to doing in GC, Probably esterification of samples would be a good solution. What dou you think about FFAP column?
With best regards
Marie and Magda

I think David brings up some excellent points. After looking at your analyte list again I think this is going to be very difficult to do by GC. You may be able to do it if you derivatize the acids, however my guess is that you will still have trouble with the formic as it may be lost due to volatility (after deriv).

If you deriv, then you would have to quench the excess deriv reagent prior to injection onto your wax column (otherwise you'll deriv the wax phase, and you don't want that!). Rather than using the wax I would try a nonpolar column such as a "1" or "5" phase after the deriv. With the nonpolar column you don't have to worry about damaging the phase with excess reagent.

Regarding the FFAP column: once the acids are derivatized it's not necessary to use the FFAP phase because they no longer have acid functionality.

I agree with Jason. It will be very difficult to do without derivatization. I would suggest that Propyl esters be the approach rather than methyl esters (FID sensitivity and extraction yields) and that hexane be the extraction solvent rather than toluene or methylene chloride, which carry over too much water. If possible do not dilute the samples in water and try to dry the samples as much as possible before the derivatization reaction is performed. You may be able to determine the approximate % yield with TLC and a pH sensitive reagent. Be sure to freeze your reaction vial before extraction and to minimize the time the vessel is open to the air. Oxalic and Fumaric acids requires about 15-30 min to derivatize in high yield at 110°C.

You may get enough information to monitor your process effectively. Do not expect high yields from a GC esterification derivatization procedure with water present.

Good luck.

Rodney George
Supelco

Marie Magda,

If you want to see what is possible with underivatised acids on FFAP columns, you can look at the results I got in Journal of High Resolution Chromatography 1989 12 465. You should be able to get nice peaks for acetic, propionic, butyric and the other straight chain acids which can be injected in water. However, you can see that already there is a problem with lactic acid, which tails; dicarboxylic acids may be irreversibly adsorbed.

The problem if you want to derivatise these acids is your sample matrix. If they are in water, then you have a big problem because the partition coefficients of acids between organic solvents and water are extremely unfavorable. This is a separate problem from the water carry over mentioned by Rodney. I would expect the recovery of dicarboxylic acids extracted from aqueous solution with hexane to be extremely low. I hope your sample matrix is not water. There are some lengthy and cumbersome column chromatgraphy procedures which are an alternative to solvent extraction, if you have an aqueous matrix.

I would try to borrow an HPLC!

Regards, David McCalley

Marie-Magda

David is quite right. Water interfers with the estification reaction. The extraction from an aqueous solution must be saturated with salt. Even then oxalic acid esters are difficult to extract but if you use propyl ester it is better.

I have used 50/50 chloroform-hexane organic to extract these esters from a solution of sat NaCl(aq).

But since the samples were diluted in water (that was my understanding) they might be derivatized before dilution. In any case there might be enough accuracy to monitor and control your process, but I like David's idea to beg, rent, borrow or steal (from within the company of course) a HPLC.

It might cost a little money but labor, time, and equipment wasted is also expensive.

Rodney George
Supelco

Marie-

I have the solution!!! For the last 2 weeks, I have been tackling a similar problem but with a tougher acid--a perfluorinated acid which is not only more acidic (pka 1.0) but more volatile. Using an FID, I can detect down to 2 ppm on the Innowax.

Here is what you do. Order a gas tight 2ml syringe ($70) and some headspace vials with tops with a crimper. You will be getting into the fascinating world of HS-GC. Don't be afraid, it is easier than you can imagine and this is all you need in addition to your current set up.

You will be making methyl esters using 80/20 MeOH/sulfuric acid. At first, this seems preposterous but trust me, this is the derivatizing agent used for trifluoroacetic acid. If your acids are in water, you will need to make a Na salt, dry it down, and then deriv. In short, place 100 microliters of x ppm standard of oxalic (which is made up in MeOH) into a 10 ml HS vial, add 1 ml deriv agent, heat for 15 min at 60C, inject 1000 microlites of the gas space (head space) and viola.....run the GC in 10:1 split mode...I have details on this in addition to a paper so email me if you need further clarification.

Unfortunately, this is not simply an analytical question. You need to throw in some organic chem logic here.
Vem

Hello
I want to ask some question relating to GC. I would like to know that....Is there any possibility of dimers or trimers when the polymer is esterified. eg. Polyhydroxybutyrate. Does the esters of dimer and monomer give different retention time in GC. If u kindly hepl me i would really be grateful to u.
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