Chromatography Forum

I am trying to figure out the best way to perform alcohol determination on alcohol pre-moistened wipes with respect to USP <611>. What would be the best way to extract the alcohol from the pads for quantification?

Anyone have experience with this?

Is headspace an option for you?

I am trying to figure out the best way to perform alcohol determination on alcohol pre-moistened wipes with respect to USP <611>. What would be the best way to extract the alcohol from the pads for quantification?

Anyone have experience with this?

If you aren't set up for headspace, I ask whether your "sample" is to be the liquid squeezed from a wipe, or the moist wipe itself. Because most likely you'd be extracting into water for analysis.

Be sure to use 0.5 microliter injection as the maximum, water expands a ton in the inlet.

Is room temperature headspace injection an option. Using a gas tight syringe on your autosampler should allow you to do room temperature analysis of the ethanol. You should be able to program the autosampler to sample higher up in the vial than normal.
You will need a suitable internal standard of course.
This is a low tech solution but might do the job
Kevin

Is room temperature headspace injection an option. Using a gas tight syringe on your autosampler should allow you to do room temperature analysis of the ethanol. You should be able to program the autosampler to sample higher up in the vial than normal.
You will need a suitable internal standard of course.
This is a low tech solution but might do the job
Kevin

Headspace is an option, but I wasn't sure if I could use water as a diluent because of it's expansion. Would it be better just to vial the wipe by itself or place it in some sort of diluent?

Is headspace an option for you?

yes I have headspace as an option, but not sure how to go about.

I am trying to figure out the best way to perform alcohol determination on alcohol pre-moistened wipes with respect to USP <611>. What would be the best way to extract the alcohol from the pads for quantification?

Anyone have experience with this?

If you aren't set up for headspace, I ask whether your "sample" is to be the liquid squeezed from a wipe, or the moist wipe itself. Because most likely you'd be extracting into water for analysis.

Be sure to use 0.5 microliter injection as the maximum, water expands a ton in the inlet.

I have headspace as an option. In theory, would it be better to first start off with headspace of direct injection by extracting into water for analysis?

Interesting problem. Water in your sample - like if you extract the wipes - is tough on a GC system. The posters above are trying to indicate that with all of their comments about injection volume when the sample is substantially aqueous.

If you have a static headspace sampler, you might be able to just do it directly.
Known mass of wipe in the vial, seal, and analyze directly. This gets you out of the whole water mess. Another take on it would be to add a known mass of wipe and a known volume of water (keep the phase ratio in the vial known and constant). Since you're not injecting anything more than water vapor, you don't have to worry about the expansion of water in the hot inlet. If I were going that direction, I'd probably calibrate using the method of standard addition.

Interesting problem. Water in your sample - like if you extract the wipes - is tough on a GC system. The posters above are trying to indicate that with all of their comments about injection volume when the sample is substantially aqueous.

If you have a static headspace sampler, you might be able to just do it directly.
Known mass of wipe in the vial, seal, and analyze directly. This gets you out of the whole water mess. Another take on it would be to add a known mass of wipe and a known volume of water (keep the phase ratio in the vial known and constant). Since you're not injecting anything more than water vapor, you don't have to worry about the expansion of water in the hot inlet. If I were going that direction, I'd probably calibrate using the method of standard addition.

Thanks for your reply.

If I am not interested in the mass of the wipe, rather just the solvent contained in the wipe would headspace still be suitable and how would you calculate?

You have to be concerned about the mass of the wipe because that's the only way you can control your sample size.

How is the content of the ethanol in the wipes expressed in the specification? wt%, ppm? g/ft^2?

You have to be concerned about the mass of the wipe because that's the only way you can control your sample size.

How is the content of the ethanol in the wipes expressed in the specification? wt%, ppm? g/ft^2?

Typically the alcohol is expressed in v/v% or w/w% in solution which wetted the wipe.

First pass - this is rough but it's a start.

- Weigh a vial, then add one drop of ethanol, reweigh it to get ethanol weight. Seal it. (It would be more correct if you could put a wipe in here that does NOT contain any ethanol)

- Run it (set HS oven to something like 75C). Find ethanol peak.

- Calculate standard concentration (g ethanol/ 1 g) and calibrate. We're assuming a 1 gram sample weight basis.

- Seal a wipe in a vial, get the wipe weight and run it. Enter the wipe weight to correct the concentration to the 1 g assumed weight.

Can you obtain the un-moistened wipes? If you can then you can do in effect a standard additions calibration. This shouldn't be too difficult, just use different concentrations of ethanol in water to wet the wipe and use this for the headspace analysis.

You have to be concerned about the mass of the wipe because that's the only way you can control your sample size.

How is the content of the ethanol in the wipes expressed in the specification? wt%, ppm? g/ft^2?

Typically the alcohol is expressed in v/v% or w/w% in solution which wetted the wipe.

What if you simply extracted the wipe with acetonitrile? I'll bet the extraction would be pretty quantitative in 1 shot and it gets you out of the water problem for direct injection GC. I've seen where people use acetonitrile as an internal standard in EtOH analysis so there must be some chromatographic conditions to separate the two.

This is an interesting sampling approach for me so I pursued it a little. It might help me on another project.

One of my lab paper towels can take on about 15 mL of liquid - it's pretty saturated by that point but for "proof of concept" it's pretty good. I took 9.5 mL of my 95% EtOH and diluted it to 15 mL with DI water. That's about 60% EtOH in the solution:

9.5 x 0.95 x 100/15 = 60% (ABV)

I added it to a 500 mL beaker and added 1 of my paper towels to the beaker. Allowed the liquid to wick up and saturate the paper towel. I pulled it from the beaker and allowed it to drip until the drips pretty much stopped. It didn't take long. I took 1.0 gram of that saturated paper towel, added it to a 22 mL headspace vial, added 10 mL of acetonitrile and sealed the vial. Shook it for 2-3 minutes. Collected the extract and repeated (2 extracts total).

GC conditions are essentially the same as USP:

https://www.usp.org/sites/default/files ... -m1238.pdf

I've been using a Rtx-1701 (30 m x 0.53 mm x 1.0 µm) because that's what I had. It gives good resolution for their critical analytes.

Analysis of the 2 extracts and using the theory of serial extractions (see any textbook that describes how multiple-headspace-extraction (MHE) works, I see that 90% of the ethanol is removed from the paper in 1 extraction. The sensitivity is such that you could do 2 extractions, pool them, and still detect the ethanol easily.

This is the 40 °C isothermal part of the GC analysis. EtOH is easily resolved from the acetonitrile. EtOH is the peak at 2.6 minutes.

Once you know the concentration of EtOH in the acetonitrile extract you know the mass of EtOH in the extract. Once you know the mass in the extract, it is the mass that's in the sample size of wipe you have.

Good luck! This approach will help me sample some aqueous ethanol samples that have too much ethanol in them to get it by headspace SPME (my go-to for volatiles).