Ethyl acetate recovery for residual solvent method

[srdales]

Past deadline.

If anyone can assist we are having ethyl acetate recovery issues for residual solvent method on PE 40XL.

The recovery is 70%. The specification is 80%.

We have tried bumping up the thermostat time, temp and flow.

All other residual solvents recovery well within the specifications.

Do we need to put a second IS in just for ethyl acetate?

The API is cetirizine HCL.

Details of the method follows:
Specification
Cyclohexane: ≤ 200 ppm
Acetone: ≤ 200 ppm
Ethyl acetate: ≤ 200 ppm
Methyl ethyl ketone: ≤ 200 ppm
Isopropyl alcohol: ≤ 200 ppm
Dichloromethane ≤ 100 ppm

Carrier Gas: Helium
Flow Rate: 4.4 psi (approximately 30 kPa)
Injector: Split, packed with deactivated glass wool
Split Ratio: 10:1
Injector Temperature: 180°C
Run Time: 40 minutes
Column: Rtx-1, 30 m x 0.32 mm ID, 5.0 µm phase thickness
Column Temperature: 40ºC; hold for 4 minutes; ramp at 5ºC/minute to 110ºC; hold for 4 minutes; ramp at 20ºC/minute to 250ºC; hold for 11 minutes.
Equilibration Time: 2.0 minutes
Detector: FID
Detector Temperature: 270°C
Detector Range: 1
Attenuation: -6
Detector Gases: Air 450 mL/min; H2 45 mL/min
Instrument: Perkin-Elmer AutoSystem XL with HS40 XL

Headspace Conditions
Vial Size: 22-mL
Flow Rate: 10 psi, Helium
Sample Oven: 85°C
Needle: 100°C
Transfer Tube: 110°C
Thermostat Time: 10.00 minutes
Shaker: On
Injections per Vial: 1
GC Cycle Time: 55.0 minutes
Pressurization Time: 3.0 minutes
Injection Time: 0.20 minutes
Withdrawal Time: 0.5 minutes
Vent: On


4. Internal Standard
Pipet 0.5-mL of tetrahydrofuran (THF) into a 100-mL volumetric flask containing approximately 50-mL of purified water. Dilute to volume with purified water and mix well.
Pipet 1.0 mL of the stock solution into a 100-mL volumetric flask, and dilute to volume with purified water. Mix well.


5. Standard

5.1 Stock Solution
Accurately weigh approximately 200 mg each of cyclohexane, acetone, ethyl acetate (EtOAc), methyl ethyl ketone (MEK), and isopropyl alcohol (IPA), and 100 mg of dichloromethane (MeCl2) into a 100-mL volumetric flask containing approximately 20 mL of DMA. Dilute to volume with DMA and mix well. This solution contains approximately 2000 g/mL each cyclohexane, acetone, EtOAc, MEK and IPA, and 1000 g/mL MeCl2.

5.2 Intermediate Stock Solution
Pipet 5.0 mL of the stock solution into a 50-mL volumetric flask. Dilute to volume with DMA, and mix well. This solution contains approximately 200 g/mL each cyclohexane, acetone, EtOAc, MEK and IPA, and 100 g/mL MeCl2.
5.3 Working Standard Solution
Pipet 5.0 mL of stock solution into a 50-mL volumetric flask. Dilute to volume with DMA and mix well. This solution contains approximately 20 g/mL each of cyclohexane, acetone, EtOAc, MEK, and IPA, and 10 g/mL MeCl2.

Pipet 5.0 mL aliquots of working standard and 2.0 mL of internal standard solution into headspace vials, and immediately seal the vials.

6.Sample
Accurately weigh approximately 500 mg of sample into a headspace vial. Pipet 2.0 mL of internal standard solution into the vial and briefly swirl to dissolve the sample. Pipet 5.0 mL of DMA into the vial, and immediately seal the vial.

7.Blank
Pipet 5.0 mL of DMA and 2.0 mL of internal standard solution into a headspace vial. Immediately seal the vial.

Can someone help me?

[chromatographer1]

Your problem is your internal standard solution.

You are losing 20 ppm of ethyl acetate when heating it in a solution of API as a HCl salt. I will bet you are hydrolyzing the ester to the acid and alcohol.

Will your API dissolve in DMA (dimethylacetamide I assume, not dimethylamine) after it is heated in the HS vial?

Find another solvent for your API other than water or use a buffer solution instead of water.

How is your recovery without the API? How is it with the API as a free base?

See a difference? I hope this is helpful.

best wishes,

Rod

[chromatographer1]

I forgot to mention that your cetirizine may be part of the equilibrium problem as it is a carboxylic acid.

The buffer will probably solve your problem with ethyl acetate.

AND THANKS for presenting such a detailed and clear post of your analytical conditions. It was a joy to behold ! I wish everyone would be so considerate and helpful...... and thorough !

best wishes,

Rod

[srdales]

Dear Chromatographer 1,

Thank you very much for your assistance.

The recovery of the standard is well within 90% - 110%.
The sample is the issue - recovery than 80%.

The API appears to be so soluble with the IS only (with the additon of the H20).

Thanks for your assistance.
We will give it a try.

[shaun78]

May I also suggest obtaining your weights of solvents from pipeting a fixed volume and using the density. I have seen large amounts of error induced when trying to weigh a volatile solvent into anything.

Also, as pointed out, you might be breaking down the ethel acetate upon heating. Anything state that you must use head space? Can you drop the HS, dissolve something like 250 mg of sample into 10 mL, inject a few microliters and split off however much you need?

Shaun

[GoldLeader]

srdales,
As a colleague, I sincerely hope you are finding a solution to your recovery issue. I know how it is to be past deadline...

Chromatographer1,
I agree that posting as much information as possible is the quickest way to solve a problem here. In fact, this board has been extremely helpful to me in my own career and for that I thank everyone that posts. But...
Remember that in real life several of us may be direct competitors in industry. And while we try to focus on the science, one must always be wary of posting too much. By posting the name of the API, srdales has told everyone that someone out there is working on it. Saying it's past deadline tells us how far along. Identifying the complete residual solvent profile, with specs, tells us which vendor they may be doing business with. And last, srdales has given a method to the competition.

I'm not trying to be negative here. Like I've said, I rely extensively on this board. It's just a reminder to be careful of the details that we post. I just wouldn't want to see anyone get in trouble with the boss for saying too much...

Jeff

[chromatographer1]

Jeff,

I understand your comments and in no way disagree with you.

I often comment that I would prefer to discuss particulars privately. I never share proprietary information. Sometimes more detail like saying an API is a secondary amine with a carboxylic acid functional group can be very helpful in solving a problem without "giving away the farm".

I am here to be helpful to people who believe what they have to state publicly is not confidential. Anyone who makes confidential information public will not keep their livelihood for long, and I do not condone or encourage such behavior. Even a generic question I realize can give away secrets unintentionally.

No one who works for a company with trade secrets should go to a public forum and ask questions or give out information that might compromise the company or themselves.

So EVERYONE, be careful in what you say.

best wishes,

Rod

[chromatographer1]

srdales

One additional comment. You did not state HOW you are determining recovery.

Are you calculating recovery by using a regression line of std additions to the API matrix solution?

If you are comparing the size of ethyl acetate peak from the headspace of an API matrix to a ethyl acetate peak from the headspace of a solution of the same solvent mix but without the API added then your 80% recovery may very well be exactly what it should be.

The addition of an API to a solution can effect a change in partition equilibrium (see Henry's Law).

Linearity is determined by adding different amounts (std additions) of a volatile to IDENTICAL matrices in identical headspace volumes.

Recovery is then calculated by using a different amount of volatile and comparing it to the regression line of the standard additions.

I wish to confirm we were discussing the same thing.

best wishes,

Rod

[GoldLeader]

srdales,

If you are running the method as Rod suggests (Ethyl Acetate Standard in Solvent with no API), you may find it helpful to add a small amount of some salt (such as sodium sulfate) into your standards but not your sample preps. That worked for us with a similar issue.

Jeff

[srdales]

Thank you to all for your assistance and advise.


Shellie