Seven residual solvents, Peaks missing!!!!

[dpogre]

Hi all,

I am having some issues trying to write an analytical method to combine seven different residual solvents together in a diluent to establish seven peaks to be visible by headspace GC. The column i'm using is Durabond DB-WAX 30 m x 0.530 mm, 1 ufilm.

The seven residual solvents and their concentrations are: 250 ppm Acetone, 100 ppm Methanol, 30 ppm Dichloromethane, 20 ppm Toluene, 18 ppm Chlorobenzene, 16 ppm Triethylamine, and 250 ppm Acetic Acid.

I prepared a standard in purified water with them all however, only five peaks showed up (acetone, methanol, dichloromethane, toluene, and chlorobenzene). Triethylamine and Acetic Acid did not have peaks present, i think this is due to TEA and Acetic Acid reacting in water.

Next, i prepared TEA and Acetic Acid individually in purified water to see if the peaks would show up. TEA concentration was 14.935 ppm and Acetic Acid concentration was 249.25 ppm. Separately, i got a shift in retention time for TEA at 3.098 (before when i injected TEA in 2 mL purified water i got a retention time of 2.339) and a peak area of 8.5 pA*s (which to me does not sound right), Acetic acid also had a small peak at 13.816 retention time and a peak area of 8.7 pA*s. When combining the two with purified water, neither peak showed up.

I have concluded that these two solvents cannot be quantified in purified water, probably because TEA-acetate is produced??

Then, I prepared individual vials of 1 drop TEA + 2 mL DMF, 1 drop Acetic Acid + 2 mL DMF, and i prepared one vial containing 1 drop of TEA and 1 drop Acetic Acid + 2 mL DMF. Individually, TEA had a retention time at 2.399 and Acetic Acid at 13.714. Combined, both peaks showed up too at roughly the same retention time (2.401, 13.749 respectively).

Thinking I'm all good, i prepared a standard solution with all residual solvents in DMF. I prepared the standard with the same concentrations as above...yet again when i came in the morning, i had missing peaks, this time I was missing three/four peaks ( i say that cause when i zoomed in at the retention time where toluene should be, i discovered a tiny tiny peak and Toluene showed up for the 120% linearity with a peak area of 14 pA*s at a concentration of 24 ppm). The peaks present were Acetone, Methanol, and chlorobenzene (DMF was detected at retention time 12.913). The areas of these peaks were definitely not as high as they were in purified water but i'm not too sure how relevant that is. TEA, Dichloromethane, and Acetic Acid were missing.

Next i tried the same preparation except in DMSO. Individually, 1 drop Acetic Acid + 2 mL DMSO, 1 drop TEA + 2 mL DMSO, 1 drop chlorobenzene + 2 mL DMSO, 1 drop TEA + 1 drop Acetic Acid + 2 mL DMSO, and 1 drop TEA + 1 drop Acetic Acid + 1 drop chlorobenzene + 2 mL DMSO. All peaks showed up, even for the two vials that had them combined. HOWEVER!! when all seven solvents were combined together in DMSO with the concentrations above, there was an overlap of what i think would be TEA and Acetone (2.738, 2.876 respectively). Dichloromethane, and Acetic Acid did not show up!

I do not know how to write a method for all seven of these residual solvents if peaks are missing and its constantly the same peaks that are missing. Please help! Can i just not combine all seven of these solvents. Should i separate two different methods and have some residual solvents in one method and others in another method. I know i can validate Acetone, Methanol, Dichloromethane, and Toluene together (maybe even Cholorbenzene), but what do I do with TEA and Acetic Acid.

Please let me know if you have any questions or have any answers for me.

Thanks!

[rb6banjo]

Triethyl amine (tertiary amine) and acetic acid can't make an amide. You can only make amides from primary and secondary amines (or ammonia) and carboxylic acids. If your compounds react, it's an acid-base reaction to make salts. Salts are not volatile and thus you would not see them by headspace analysis.

I think your problem centers more on the fact that acetic acid and triethyl amine are extremely water soluble and tend to not partition into the headspace above the water. To get TEA by headspace, you might have to raise the pH of the solution. At pH = 7, the majority of it will be protonated and thus rendered nonvolatile. For acetic acid, you'll need to be at a pretty low pH. At pH = 7, a substantial percentage of it will be in the acetate form (again, nonvolatile).

That's a tough combination to get all in one shot. To push more of them into the headspace (once you get the pH think worked out), you might try to increase the ionic strength (add salt like sodium chloride) to drive them into the headspace.

Good luck.

[chromatographer1]

While it may be possible to do TEA salts by total evaporative headspace (I have done this) to do TEA from liquid water, especially with HoAc present, well, it is a fool's errand.

Develop two different methods, separate the solvents to be measured with each one, and proceed accordingly.

Adding salts won't buy you much, and it may actually reduce the recoveries and linearities. Try for yourself, you might find a work around.

Good luck, but ion chromatography analysis would be so much better. Using a Wax column with both acidic and basic sites in the phase (nature of the beast) is another issue.

best wishes,

Rod

[dpogre]

thanks for the replies.

I think i'm going to have to validate TEA and Acetic Acid separately.

I'm working with Acetone, Methanol, Dichloromethane, Toluene, and Chlorobenzene to validate together. I decided to try and use 1N NaOH as the diluent. Yet now i ran into a new problem.....when Im prepared the stock solutions for Toluene and Chlorobenzene, i can see that they are not soluble in 1N NaOH and I'm not getting the recovery (85% - 115%) the next day when I made the second 100% reference standard. Also the linearity is coming out to be around the same number for the 100% and 120%. Should I use a different diluent? I'm using 1N NaOH because that way my API sample dissolves fully and gives me more accurate residual solvent peak area responses.

Thanks
Dina

[Meerkat]

please try using either 0.5 or 0.25 molar hydroxide, this ought clear up the issue while still enabling 100% dissolution of your sample

[Peter Apps]

please try using either 0.5 or 0.25 molar hydroxide, this ought clear up the issue while still enabling 100% dissolution of your sample

And zero acetic acid in the headspace

Peter

[chromatographer1]

Yes, a different diluent will be necessary for the acetic acid determination.

But acetic acid by HS is not a trivial task. And using a Wax column for it is a waste of time. A Wax column is NOT pH neutral. To do acids it must be converted to a FFAP phase, which is why they were invented in the first place.

Acetic acid should be done by ion chromatography. If not, then HPLC can be tried. To do HS an acidic solvent like 1N H2SO4 must be used. Even then getting accurate results requires a bit of luck with the interacting chemical reactions, not to mention the removal of all basic sites in the flow path of the HS sample.

Better to do six solvents well and miss one, than to do all seven poorly.

I spiked my aqueous samples by adding the solvents dissolved into DMAc and using a minimum of it, like 10-25 microliters in a 250 microliter volume of water or 0.25M NaOH. Don't expect standards dissolved into aqueous solutions to be stable.

Don't be silly.

best wishes,

Rod

[maliheh.1367]

While it may be possible to do TEA salts by total evaporative headspace (I have done this) to do TEA from liquid water, especially with HoAc present, well, it is a fool's errand.

Develop two different methods, separate the solvents to be measured with each one, and proceed accordingly.

Adding salts won't buy you much, and it may actually reduce the recoveries and linearities. Try for yourself, you might find a work around.

Good luck, but ion chromatography analysis would be so much better. Using a Wax column with both acidic and basic sites in the phase (nature of the beast) is another issue.

best wishes,

Rod

Dear chromatographer1

I would be thankful if you suggest any method (containing column type, Mobile phase, Diluent and cinematographic condition) for determination of acetic acid in API using HPLC.

Thanks & Best regards
Maliheh