Residuals ethanol and caprylic acid by GC

[mael]

Hello,

We are trying to develop a GC method for the analysis of both ethanol and caprylic acid in a water solution at limit of 10 ppm.
I was told that since caprylic acid is not volatile enough it should first go through esterification. well, we tried the two residuals as is anyway but it seem that caprylic has a teribble carry- over.
Do you have any recommendations?
Do you think it is even possible?

[Consumer Products Guy]

I think I'd plan to have separate test methods for these.

[rb6banjo]

I don't have any trouble with these analytes when I use SPME to sample the headspace. My advice is that you should use a wax phase because as a general rule, the carboxylic acids behave badly on non-polar phases. Alcohols chromatograph nicely on this phase as well.

Is the water medium acidic or do you have the ability to add acid to your sample prior to headspace analysis? You don't want to ever add something if it's going to have an adverse effect on your analysis in some other way. If caprylic acid is what controls the pH of the solution, at 10 ppm (6.93 x 10^-5 M) about 34% of it will be ionized to form the conjugate base which will raise your detection limit for this molecule. Adding acid will ensure that you can get the maximum amount of acid into the headspace. If the pH is higher than 5.2 or 5.3, your probability of success is going to go down fairly dramatically.

I was curious about the ethanol part so I did a quick experiment. I added 1.4 µL of 190 proof ethanol (density is 0.789 g/mL) to 100 mL of water and ran it with my "starting point" SPME method. I was able to see EtOH with a flame detector at 10.5 ppm.



The S/N ratio for that response is 294. If you take the LOQ as the concentration where the S/N ratio is 10, then I can quantitate EtOH at 0.4 ppm. Likewise if the LOD is the concentration that gives a response where the S/N ratio is 3, then the LOD is 0.1 ppm. Not bad for something so water soluble. The split ratio used here is 5:1. I could improve the detection limit if I didn't split the injection as dramatically.

The peak at 5.6 minutes is ethanol. The blue trace is the water blank used to make the standard.

SPME Conditions: Carboxen/PDMS/DVB-2cm, 60 °C preheat for 15 min., followed by 30 minutes headspace sampling at 60 °C. I used a dry-bath heater for this work.

Column: Zebron-Wax (30 m x 0.53 mm x 1.0 µm) from Phenomenex.
Inlet: 230 °C, head pressure 2.5 psig, 5:1 split ratio, helium carrier gas
Oven: 40 °C for 2 min., 40-230 °C at 10 °C/min., 230 °C for 14 min.
Detector: FID, 230 °C, 30:300:25 H2:Air:He makeup

I was able to detect 10.5 ppm EtOH using headspace sampling at room temperature as well - although the detection limit is not as good as when I heated it.

I believe you can do what you desire in this case if you sample with SPME.

[MSCHemist]

I am doing residual ethanol now with SPME. What I ended up doing for ethanol is using an Innowax column and doing standard addition spiking 1, 10, and 100ppm ethanol and 1ppm isobutanol ITSD ( I tried isopropanol but it was not well resolved and the ions are the same for mass spec). I'd have prefered to have a thicker phase wax column for ethanol but went with what I had which was 0.25um 30m .25mm ID.

Acids can also be done well with SPME and a wax column. I see it routinely all the way up to stearic.
http://pubs.acs.org/doi/abs/10.1021/jf0 ... ode=jafcau

My typical SPME
2cm carb/dvb/pdms
Restek 20973 liner

Incubate sample in 50deg water bath with vigorous stiriing 15 min
1g NaCL 1g solid sample 3ml aqueous

incubate with fiber for 30 min

desorb 1 min at 260 pulsed splitless 30psi, splitless, or 2:1 split
then flush inlet with 65ml/min
40 deg 5 min
+3 175
+20 260 5 min

[mael]

Thank you for your seriousness. these are really usufull ideas.
Hope it will work.