Chromatography Forum

When we were doing wipes testing for methamphetamine we ignored the weight of the wipe completely.

Take the wipe(which had an unknown amount of analyte) and put into a 4 ounce jar and add 50ml solvent. Shake for a few minutes to extract the analyte. Inject extract and compare to calibration curve constructed using the same analyte dissolved in the same solvent calibrated in units such as ug/ml or mg/ml.

Result from instrument is mg/ml of analyte in 50ml solvent so:

mg/ml x 50ml = mg analyte per wipe.

Should be able to do the same thing with these wipes using either Acetonitrile or Water as the solvent, which ever is preferable for the GC and separating column. If you want to use organic solvent and Acetonitrile elutes too near to Ethanol then use a higher boiling solvent, maybe Toluene or Ethyl Acetate.

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).

When we were doing wipes testing for methamphetamine we ignored the weight of the wipe completely.

Take the wipe(which had an unknown amount of analyte) and put into a 4 ounce jar and add 50ml solvent. Shake for a few minutes to extract the analyte. Inject extract and compare to calibration curve constructed using the same analyte dissolved in the same solvent calibrated in units such as ug/ml or mg/ml.

Result from instrument is mg/ml of analyte in 50ml solvent so:

mg/ml x 50ml = mg analyte per wipe.

Should be able to do the same thing with these wipes using either Acetonitrile or Water as the solvent, which ever is preferable for the GC and separating column. If you want to use organic solvent and Acetonitrile elutes too near to Ethanol then use a higher boiling solvent, maybe Toluene or Ethyl Acetate.

Rather than expressing the result in mg analyte per wipe, how would I calculate to get the result in v/v % or w/w% (the concentration of ethanol solution that was on the wipe)?

When we were doing wipes testing for methamphetamine we ignored the weight of the wipe completely.

Take the wipe(which had an unknown amount of analyte) and put into a 4 ounce jar and add 50ml solvent. Shake for a few minutes to extract the analyte. Inject extract and compare to calibration curve constructed using the same analyte dissolved in the same solvent calibrated in units such as ug/ml or mg/ml.

Result from instrument is mg/ml of analyte in 50ml solvent so:

mg/ml x 50ml = mg analyte per wipe.

Should be able to do the same thing with these wipes using either Acetonitrile or Water as the solvent, which ever is preferable for the GC and separating column. If you want to use organic solvent and Acetonitrile elutes too near to Ethanol then use a higher boiling solvent, maybe Toluene or Ethyl Acetate.

Rather than expressing the result in mg analyte per wipe, how would I calculate to get the result in v/v % or w/w% (the concentration of ethanol solution that was on the wipe)?

Are you looking at % ethanol in the original solution or % ethanol compared to the weight of the wipe? If compared to the weight of the wipe, do you need it as the weight of a dry wipe or saturated wipe?

If you determine the mg ethanol in a wipe as I posted above, then you can calculate the mg ethanol in a weight of the solution by taking a wipe, weighing it wet, then drying it and subtracting the weight of the wipe to get the weight of wetting solution then divide mg ethanol/mg solution to get w/w% in solution. The same calculation can be done using the weight of a wet wipe or dry wipe to get the w/w% for either of those.