Acetic acid by GCMS

[Golie Mengistu]

I was using a FFAP column (30 m x .25x.25) to determine acetic acid in water by GCMS. There are two more compounds beside acetic acid in my sample. I used a 4mm liner w/o wool since injecting 0.2µL to avoid back flash. Good separation of all three compounds but there is acetic acid carryover in my blank (water). I have done cleaning the system, changed liners, multiple syringe wash and so on. But still acetic acid carry over in the blank. Any help would be much appreciated!! (oven prg. 40C for 2', 8C/min to 150C, 10C/min to 210 fro 5' with inlt temp of 250)

Mgolyad

[Peter Apps]

Welcome to the forum.

Are you doing split or splitless injection ?, if split what is the split ratio, if splitless what is the splitless time ?

What make and model of instrument are you using ?

Peter

[Golie Mengistu]

Splitless, 50/0.50

[GOM]

Hi

Is the carry over reducing with multiple injections of your blank water?

Are your septa PTFE faced?

What pH is your sample - around 3?

Regards

Ralph

[Peter Apps]

Splitless, 50/0.50

OK, you are asking for help from people on the forum, and you reply to an enquiry for information that will help me help you with a 17 character text string, on which only the first word makes sense.

Try again, this is not Twitter or text messaging

Peter

[MSCHemist]

Is this headspace or direct inject. I'll assume direct inject. The small acids tend to stick to active surfaces. Are you using the autosampler syringes with the detachable needles. I am coming to hate them because I find material seems to get into the joint and hide there and come off a little with each injection. Also make sure you use a gas tight syringe with teflon as it can probably also hide between the plunger and wall.

[mckrause]

0.2 uL splitless? I would definitely start there. Move to a larger volume injection (no, you don't get "backflash") using a split injection; say 1-2 uL with a split ratio of somewhere between 5 and 10.

As MSChemist points out, the small organic acids stick to everything. Using a split injection many times helps that.

[crong]

0.2 uL splitless? I would definitely start there. Move to a larger volume injection (no, you don't get "backflash") using a split injection; say 1-2 uL with a split ratio of somewhere between 5 and 10.

As MSChemist points out, the small organic acids stick to everything. Using a split injection many times helps that.

Hi Mark, I am having a very similar problem as the one described in this post. Using a 0.5 uL syringe I'm doing 0.2 uL splitless injections. Samples are volatile fatty acids in 50% methanol 50% water, and same type of column but detection is by FID. My liner also does have wool.

All of my acids are having carry over, however acetic is the worst. For example, a sample with 46 mg/L acetic acid, followed by 50% methanol blank, the blank shows 16 mg/L. Successive blank runs have smaller peaks but the rate at which it decreases is very slow. It takes about 5 runs for the acetic acid peak to be completely gone. I have progressively increased the number of rinses to 15x with water post injection, and 15x with water then 15x with 50% methanol and finally 15x with sample, pre injection. I have replaced the syringe, inlet septum, liner, always use fresh rinse solvent, replaced rinse and waste cap septa (I'm using pre-slit septa as recommended by the syringe manufacturer), still the carry over is there.

When I ran a no injection blank, there were no peaks.

Do you think it would help in my case to switch to split injection using the larger volume? I don't have a teflon coated syringe.

Thank you in advance

[anahita.b]

I have the same issue but with GC-FID. The carry over is worse for more concentrated compounds. I currently run 2 blanks between each sample to handle the issue but it would really help to get a more permanent solution.

[mckrause]

1. Splitless injections are wonderful for analytes that have significantly higher boiling points than the solvent and that give you nice, pretty peaks on relatively non-polar columns at trace levels. That is most definitely NOT acetic acid! Use split injections, and if possible get rid of the liner wool.

2. In general direct injection VFAs are problematic. You get carryover, you get tailing, you get shifting calibrations and poor detection limits. As long as you recognize that you are not dealing with a +/- 5% accuracy type of analysis then you can live with some of this. The alternative, of course, is to derivatize, but that brings its own issues. IMHO you are better off living with the vagaries of direct injection.

3. I have, in general, moved to headspace for VFAs. It has it's own set of weirdness but you avoid all of the matrix effects in the injector. I use a straight liner, no wool and a valve system for injection under split conditions. High, high salt concentrations in the HS vials and I get them HOT. You give up a bit of sensitivity but I can analyze digester sludge directly, which is pretty cool (and rather smelly!)

[MSCHemist]

What I like to do for small acids is FET headspace esterfication. You put 2.5ul sample, 2.5ul strong H2SO4 and 5ul of whatever alcohol you want to esterfy with can be methanol, ethanol, propanol, or whatever in a 10ml headspace vial. Use an internal standard I use 4-methyl pentanoic and incubate the sample for 60min at 110-120 deg C and do headspace. Nice, clean, and esters give nicer chromatography.

[crong]

I have the same issue but with GC-FID. The carry over is worse for more concentrated compounds. I currently run 2 blanks between each sample to handle the issue but it would really help to get a more permanent solution.

I think someone earlier posted that over time more active surfaces accumulate on the liner and septa which the VFAs to stick to and causes carry over.

I myself have given up on the aqueous injection. With splitless injection the carryover is getting worse and worse. Tried a split injection and response is just too low even with 1 uL injection and low split ratio (5). I have now changed over to a liquid liquid extraction with MTBE, which has much higher response (on FID) and much lower carryover. I think with VFAs in general maybe the inlet consumables need to be changed more frequently and especially so if aqueous injections are done. This is what I concluded from reading forum posts and my limited experience.