Headspace Residual Solvents Acetic Acid Problem

[sdegrace]

We are having problem developing a residual solvents headspace method to detect acetic acid. We are running out of ideas to determine the reason and to try and fix it. As we have comparatively limited experience with headspace GC, we would appeciate any advice anyone can offer. We can give a fairly complete description of the problem. The method is given at the end of this post.

We are using an Agilent G1888 headspace sampler and an Agilent 6890 GC. The column is an Rtx 5 from Restek (35 m, 530 μm ID, 5.00 μm film), equivalent to a USP G27. We can produce a good peak for acetone in our standards, but with acetic acid we are getting what I can only describe as "garbage" ahead of the peak and overlapping with it in such a way as to make it useless for quantitation (Figure 1 is a normal standard concentration, Figure 2 is 10X this concentration). HS sample prep is 1 mL of solution into a 20 mL headspace vial.

Figure 1: 500 μg/mL acetone and acetic acid, using headspace, in water, G27 column


Figure 2: 5000 μg/mL acetone and acetic acid, using headspace, in water, G27 column


Using the exact same GC method but directly injecting a 2 μL sample instead of using the headspace, we get the chromatogram in Figure 3, which does not exhibit the problem:

Figure 3: 500 μg/mL acetone and acetic acid, direct inject 2 μL, in water, G27 column


To see whether the use of water as a diluent could be the problem, we tried making the sample in DMF instead. As seen in Figure 4, the peak shape issue may possibly be resolved, but we see a dramatic drop in sensitivity:

Figure 4: 500 μg/mL acetone and acetic acid, using headspace, in DMF, G27 column


We tried using a different column, a ZB-624 from Phenomenex (same ID and film thickness, 30 m nominal length), equivalent to a USP G43. The resulting chromatogram of a 500 μg/mL solution in water shows some differences with the same sample run using the Rtx 5, but is still not really usable from our perspective (Figure 5).

Figure 5: 500 μg/mL acetone and acetic acid, using headspace, in water, G43 column


We also tried contacting the vendor of the headspace sampler, Agilent, for some advice. They suggested trying increasing the vial pressure to 15 psi and the vial pressurization time to 0.6 min. The chromatogram in Figure 6 resulted; this is still not an acceptable result, but there are significant changes, suggesting to us that our problem might just be a matter of finding the right headspace parameters.

Figure 6: 500 μg/mL acetone and acetic acid, using headspace, in water, G27 column, Vial Pressure = 15 psi, Vial Pressurization Time = 0.6 min


The method parameters we have been using follow. Thanks to anyone who can a) suggest improvements that might solve our problem or b) explain what is happening!!

Thanks,

Stephen

Headspace Method:

Device: Agilent G1888 Headspace Sampler
Vial Size: 20 mL
Oven Stabilization Time: 1 min
Multi HS Extr: Off
Extractions Per Vial: 2
GC Cycle Time: 26 min
Inject Time: 1.00 min
Loop Equilibration Time: 0.10 min
Loop Fill Time: 0.20 min
Loop Temperature: 95 °C
Oven Temperature: 80 °C
Shake: LOW
Transfer Line Temperature: 105 °C
Vial Equilibration Time: 40.0 min
Vial Pressurization Time: 0.40 min
Carrier Pressure: 4 psi
Vial Pressure: 10 psi

Agilent 6890 GC Method:

Oven Program:
Initial Temp 40 °C, hold 0 min. Ramp at 15 °C/min to 100 C. Ramp at 20 °C/min to 200 °C. Run time 13 min.

Inlet:
Mode: Split
Temp: 140 °C
Pressure: 4.26 psi
Split Ratio: 9.99:1
Split Flow: 48.4 mL/min
Total Flow: 55.7 mL/min
Gas Saver : On
Saver Flow: 20.0 mL/min
Saver Time: 2.00 min
Carrier Gas: Helium

Column:
Mode: Constant pressure
Pressure: 4.26 psi
Nominal Initial Flow: 4.9 mL/min
Average Velocity: 35 cm/sec

Detector:
Temperature: 250 °C
Hydrogen Flow: 30.0 mL/min
Air Flow: 400.0 mL/min
Mode: Constant makeup flow
Makeup Flow: 25.0 mL/min
Makeup Gas: Nitrogen

[GasMan]

How are you connecting the HSS to the capillary inlet.

Gasman

[sdegrace]

I'm not sure what the options might be... we are using the Agilent-supplied fitting to connect the HSS transfer line to the GC inlet to which the capillary column is connected again using the Agilent fillings.

[modonovan]

You might want to try increasing the temperatures in the flow path. Acetic acids bp is 118°C so a loop/tr. line/inlet temp of 125°C?

[GasMan]

The above is a good tip. You should also increase the temp of the capillary inlet and put some insulation around the tubing and connector to the capillary inlet, as it would also be possible for the acetic acid to condense in this area.

Gasman

[chromatographer1]

This is typical of acetic acid and headspace.

Presence of water causes the acid to disassociate.

The pH and the temperatures have to be adjusted and the flow path deactivated. Even then it can be difficult.

Most labs do not try to use HS but ion chromatography or NMR to measure trace amounts of HoAc.

best wishes,

Rod

ps I am on vacation in AZ and am enjoying the sunshine.

[sdegrace]

I envy your southern vacation, Rod - it's still snowy and cold here in Atlantic Canada and not warming up nearly fast enough!

Do you think it might be beneficial to use DMF or DMSO instead of water and much higher temperatures to attempt to boost acetic acid recovery and minimize somewhat the effects affecting peak shape? If we want to make HS work for acetic acid that's the approach I was thinking of going in, especially after hearing the advice I've received thusfar, although I'm also intrigued by the possibility of doing something completely different to quantitate residual HOAc and will look into that as well.

There's also direct injection, although we've looked at that in the past for something else with this same product and the GC can't take too many injections before you have to be changing liners.

Does anyone happen to have a link to any good info on quantifying residual HOAc?

Stephen

[chromatographer1]

You will need to add H2SO4 or some non volatile acid. DMSO would be a good solvent choice. Temperature of less than 100C should be Ok.

It is so much easier to do HPLC, methods are common, fast, and easy. Check column web sites for examples.

Good luck. Validate thoroughly changing water, salt, and impurities variation before trusting in your analysis method.

Rod

[sdegrace]

Thanks for all your advice, everyone who replied! Insofar as we have some time left for method development I think we'll try to fiddle with the headspace and see if we can get it to work for acetic acid, or perhaps we'll bite the bullet and do direct injection, changing the liner frequently. I also plan to try HILIC LC because I have a feeling that might be a good way to go, and I've been itching to find a good excuse to try HILIC for something . I'll also look into ready-made HOAc methods available out there.

If we manage to get something to work decently well on HSS, as unlikely as that seems right now, we'll post what we did.

[Bruce Hamilton]

The FID response for acetic acid is very poor and, as Rod has said, the presence of water causes even more issues. What works one day may not work the next, as the acetic seems to activate any junk in the splitter. Even under the best conditions the peak shape often varies with other components present.

Direct injection isn't that great either, and I would recommend switching to another technique, probably ion chromatography.

Please keep having fun,

Bruce Hamilton

[sdegrace]

I'm really excited about HILIC for residual acetic acid - I plan to try it when I get the opportunity. The reversed selectivity to reverse phase is really enticing as it promises a method which might have very broad general applicability, i.e., shoot off your product which is well-retained in reverse phase right at the outset and hopefully resolve the acetic acid present. I think I'll wander over to the LC section when I get the chance and maybe post about it and see what comments I get.

Stephen