What kind of solvent used for GC FID autosampler

[orsfph]

Hello,

I plan to use GC FID to test VFAs. For the autosampler, the syrindge will be washed by a Solvent A or B.

What kind of solvents are normally used? Can I use aceton in here? But if aceton is used, will there be a peak for aceton in the result? Then can I just use DI water?

Thank you!

[Don_Hilton]

You may use one or two solvents - the selection of solvent includes considerations: Will it clean the syringe and Will it interfere with the analysis.

YOu should include at least on wash with a solvent before drawing the sample - not to clean the syringe, but to be sure there is a liquid seal along the syringe plunger - to keep air from leaking in and forming a bubble.

What you put in the syringe will tend to be in the syringe when you draw up the sample. However, if you need a solvent that you do not want to see in the injection, I would suggest that you use that as the A solvent and then a different solvent as the B solvent and use multiple rinses with both solvents after injection. Be sure that solvents A and B will mix and that B does not interfere with the analysis. And expect a small quantity to solvent A to show up in the injection.

Be sure that the solvent you rinse with just before injection is compatible with your sample. Do not use solvents that are corrosive to the syringe or will damage the GC inlet and column.

Before you consider water, I would suggest you give careful thought to the many volatile organic solvents availble, including THF, MTBE, and such. (Water may not show on the FID, but it can cause other problems.)

[Peter Apps]

The final solvent rinse before injection needs to be with the same solvent as you use to dissolve the samples. If your samples are dissolved in water you can expect a range of problems and you should seriously consider getting them dissolved into something else.

What sample B is (presuming that your autosampler can handle two rinse solvents) depends on what your sample is - it has to dissolve everything in the sample in order to effectively clean the syringe, and this applies to sample components other than your target analytes.

Peter

[orsfph]

Thank you very much for Peter and Don's helpful reply.

My samples are normal aqueous solution. I will pretreat the samples by centrifuging and filtering first, and then acitify them by mixing H3PO4 and use crotonic acid as internal standard.

In this case, aceton seems good as a solvent. But since I will test VFAs like acetic acid, will the aceton residue affect the signal of acetic acid, since both them are 2 carbon?

[Peter Apps]

If I understand correctly your samples will end up dissolved in phosphoric acid ?

This will destroy the syringe, inlet and column. Do not do it. Where did you get this method from ?

What column are you proposing to use ?, with a robust packed column you could make aqueous injections (but not with a non-volatile inorganic acid dissolved in them), but on a capillary these will cause all sorts of problems.

The "carbon number" of acetic acid and acertone are irrelavent, a wax type phase will easilt separate acetic acid from acetone.

Peter

[orsfph]

Hi, Peter,

Yes, the samples will dissolved in phosporic acid. You means the acid will cause damage to syringe etc? But for FID test, is it required to acidify samples to a certain pH, say pH 2, before injection?

I got this method from a journal paper, Bioresource Technology, 2009, 100, 4133-4138.

Anyway, my plan is to test the VFAs of effluent from a digestor using FID with a capillary column. Could you please refer me a detail procedure?

Thank you very much for your kind help!

If I understand correctly your samples will end up dissolved in phosphoric acid ?

This will destroy the syringe, inlet and column. Do not do it. Where did you get this method from ?

What column are you proposing to use ?, with a robust packed column you could make aqueous injections (but not with a non-volatile inorganic acid dissolved in them), but on a capillary these will cause all sorts of problems.

The "carbon number" of acetic acid and acertone are irrelavent, a wax type phase will easilt separate acetic acid from acetone.

Peter

[bisnettrj2]

For those without access to the paper, here's the OP's referenced analytical method:

2.5. Analytical procedures
The samples from each reactor were centrifuged at 5000 rpm for 15 min (Centrifuge 4226 Pacisa) and the supernatants were filtered through 0.45 μm glass-microfibre filters (Albet). pH values were continuously monitored by using a pH-checker (Hanna). The filtered samples were used to establish alkalinity, soluble COD (CODs), soluble total Kjeldahl nitrogen (TKN) and soluble ammonia nitrogen. Total and volatile solids (TS and VS) were also determined in the raw effluents. These parameters were determined according to standard methods (APHA-AWWA-WPCF, 1998 APHA-AWWA-WPCF, 1998. Standard Methods for the Examination of Water and Wastewater, twentieth ed. Washington, DC.APHA, 1998). Specifically, ammonia nitrogen was established according to the 4500-E standard method. Total COD (CODt) was analyzed in the raw effluents according to the method proposed by Raposo et al. (2008).

The VFAs from C2 to C7 including iso-C4, iso-C5 and iso-C6 were analyzed by using a Gas Chromatograph (Shimadzu GC 2010) equipped with a flame ionization detector (FID) and a capillary column filled with Nukol (polyethylene glycol modified by nitroterephthalic acid). Prior to injection, 900 μL of the sample was mixed with 150 μL of H3PO4 (1:2 V:V) to adjust pH below 2.0 and 150 μL of a solution of crotonic acid (2000 mg L−1) as an internal standard. This mixture was centrifuged to remove any solids and transferred to a 1500 μL gas chromatography (GC) vial; the sample injection volume was 1 μL. The temperatures of the injector and detector were maintained at 200 °C and 250 °C, respectively, while the column temperature was increased from 120 to 160 °C with an increasing rate of 10 °C min−1.

[orsfph]

Thank you Bisnetterj2 's help for posting the paper.

[Don_Hilton]

While that procedure may work, I wonder about the robustness of it... And it brings to mind advice given me by a chemist who did analytical work in industry, many years ago: "Most of the work you find in the literature is by graduate students trying to get out." While the statement may not be entirely fair, there are some papers of marginal value that do make it into the literature, so be careful.

With the solution acidified, it may be worth considering a SPME technique. While the SPME fiber will take only so much acid exposure, it is less expensive to discard than a column and should not transfer phosphoric acid to the GC inlet. The literature may give you some ideas of other techniques that would not requrie the injection of phosphoric acid into a GC.

[Peter Apps]

Thanks bisnettjr2 for posting the method - in this case seeing is not believing !

With the aqueous phase acidified to pH less than 2 headspace might be a better option, and headspace SPME would hardly require any more hardware.

Peter

[orsfph]

Thank you very much, Peter, Don and all other's kind reply.

As for the solvent, I will try aceton or water to see the different. But I got confused about pretreatment of samples.

Do I need to acidify samples? What's the pH values of samples for normal measurment? And normally do we need adjust pH?

Thank you and best regards!