Chromatography Forum

Hello folks,

I'm Chris a researcher/procestechnologist at a papermaking company in the Netherlands. Recently we wanted to start measuring Short Chain Fatty Acids in our proces- & wastewater by GC/FID.

I am having trouble measuring samples from our proces- & wastewater
(aprox ph7). The only sample preparation I am doing is filtration of the water and then direct injection into the GC/FID. But I am not getting any peaks or what so ever (although i'm sure the proces water contains atleast 1000mg of FA). When injecting the calibration standard (C2-C5) I am getting good peaks.

What could be wrong? Do the samples require more preparation? Acidification or converting to methyl esthers?

Would like to hear from you & thanks in advance!

Hi Chris

Welcome to the forum.

Some more chromatographic details would be useful.

Having said that, you usefully stated that your sample pH is 7. Try reducing that to 3 to keep the fatty acids in the free acid form

See many previous posts on the conditions best suited to aqueous injections if your sample is water based.

Then let us know how you get on. Derivatisation should not be necessary for C2-C5 fatty acids

Regards

Ralph

Hi Chris

Welcome to the forum.

Some more chromatographic details would be useful.

Having said that, you usefully stated that your sample pH is 7. Try reducing that to 3 to keep the fatty acids in the free acid form

See many previous posts on the conditions best suited to aqueous injections if your sample is water based.

Then let us know how you get on. Derivatisation should not be necessary for C2-C5 fatty acids

Regards

Ralph

Hello Ralph,

Thank you for your reply and advice. Reducing the pH to 3 did the trick.
You are awsome!

King regards,

Chris

Hello Ralph,

Thank you for your reply and advice. Reducing the pH to 3 did the trick.
You are awsome!
Chris

Chris - you just proved once again that short chain soaps are not volatile !!!

Hi Chris

Thanks for the update - glad that we could help

It always nice when a poster takes the time to reply with an update to the forum - thank you.

I just happened to be first to jump in - many other regular contributors would have offered the same advice

Regards

Ralph

Measuring our proces & wastewater for short chain fatty acids is going great.
The next step we are trying to do is measuring the SCFA concentration in our produced tissue paper product. The challenge here is extracting the SCFA from the paper into the aqueus form.

I'm now trying to extract the fatty acids out of the paper with diluted acid and shaking the sample during an certain amount of time. The thing is that in the paper product is alot of calcium carbonate wich i am dissolving with the acid, hopefully adding more acid will do the trick.

If i run into some trouble ill give you guys an update

Oooooh - investigations!

I might try filtering out the calcium carbonate, maybe even making higher pH first before filtration, keeping the volatile fatty acids as the soaps. Then reduce the pH for the analysis.

Oooooh - investigations!

I might try filtering out the calcium carbonate, maybe even making higher pH first before filtration, keeping the volatile fatty acids as the soaps. Then reduce the pH for the analysis.

Thanks for the tip, I will definatly try this.


By the way people, I notice that when im running blanks i still get small peaks of C2-C5 in the chromatogram. How is this possible? I guess there are alot of reasons this can happen. But what are the most commen ones?



................ C2 C3 iC4 C4 iC5 C5 Total (mg/l)
Calibratie....1000 1000 1000 1000 1000 1000 6000
Blanco 1.....39,2 16,3 3,1 13,9 5 24,1 101,6
Blanco 2.....13,1 5,7 0,9 4,5 1,4 7,3 32,9
Controle.....1051 1070 1079 1095 1078 1184 6560
Blanco 3.....45,8 20 4,1 17,8 7 27,5 122,2
Blanco 4.....19,3 8,2 1,3 6,6 2,3 10,5 48,2

This would mean i would be running more blanks than samples? This cant be right?

you are getting carryover from somewhere or your water source has some FFAs. Most likely the water or the injection syringe. I find with the detachable needles stuff tends to hide in the connection crevices or if it is not a gas tight the glass in between the plunger and side of syringe.

I personally dislike injecting water. I'd at least use a retention gap and replace that frequently. Otherwise I'd esterify. There is a headspace esterfication technique but it might no yield the sensitivity you need and you really need an ITSD for it.

Thanks for all the advice.

I have another question, they want to measure fatty acids in a starch slurry.
How smart would it be to direct inject the slurry in the GC (around 5% solids)?

That starch is going to make a really big mess out of your inlet and probably column. It is going to char in the instrument and your whole inlet and liner will turn black in short order.

I'd either consider the headspace esterfication technique or I have one that uses isobutyl chloroformate for free fatty acids or for total fatty acids I just use the fast methanolysis with isobutanol/2.4% HCl made from 35% HCl.

http://chromforum.org/viewtopic.php?f=2 ... 7&start=15

http://chromforum.org/viewtopic.php?f=3 ... te#p117980