Gas chromatography method with FET

[alemaggot]

Hi guys!

I must create a method for determination of residual solvent in my pharmaceutical product. This isn't soluble in organic solvent, such as DMF, DMSO, and in water. Pratically in nothing, only in diluted acid and alkaline hydroxide.

For this reason I wan't create a FET (full evaportation technique) method.
But it's the first time that I apply this method. Someone of you have been used this technique? In internet I've see that I must use an internal standard. Do you confirm it?

Can you tell me the important things that I must consider?

Thank yo very much

[chromatographer1]

It is not necessary to have an internal standard. You should ALWAYS use standard addition to quantify your solvent, not external standards, but many disagree. The burden of proof is then always on the use of external std that partition is exactly the same as from the sample (which it never is, but it may be close.) Std addition avoids that controversy and has a solid scientific basis. It does require the sample to be homogeneous and three samples to be prepared. (worth every minute of preparation time and the cost of material as it is self validating)

You have to show in your method that the solvent can be recovered consistently, reproducibly, and accurately over a wide range of levels.

Dilute non-volatile acids are best suited for headspace. Hopefully your solvent is not affected by acids or pH.

Good luck in your research.

Rod

[krickos]

Hi

As an intial comment have you confirmed that the sample do not dissolve durig heating and if possible shaking in the headspace oven (or normal oven) at various temperatures and times? It is not necessary that sample dissolves at room temperature, you can weight directly into headspace vial and add solvent.

Also, why would you rule out static headspace injection provided the diluted acid don not cause any troubles?

[Axar]

Hi!!!
I guess, best way to determine residual solvents is use of HeadSpase.
I would use water as solvent (in a case you don't need to determine solvents, insoluble in water, such as hexane, coz you will have problems with standart solution).
It is not necessary to dissolve your substance complitely. A dosage of UV will drain organics from your substance. Adding solts, such as Na2SO4 is option. In a case of water, it will boost evaporation for polar organics, and decrease it for nonpolar.
I do not think you need an internal standart in this case. I have reprodused a lot of residual solvents methods, and I have met internal standart rarely in them.
Best wishes!!!

P.S. sorry for my bad English here.

[alemaggot]

I must determiante this three solvents: Methanol, isopropanol and cyclohexane.

This first and second are water soluble, the third no. It's soluble in DMF but I've two problem with this solvent: In my results I've found many interference peaks with my analyte and my analyte dosn't dissolve in it. And in this solvent I can't use salt addition.

I've try create a mixture of water/DMF but cyclohexane dosen't be soluble in it.

Then I've got most problem with standard solution. I can't prepare it! !

This bring me to FET method.

[chromatographer1]

Axar wrote:

I guess, best way to determine residual solvents is use of HeadSpase.

Yes, it is highly effective.

I would use water as solvent (in a case you don't need to determine solvents, insoluble in water, such as hexane, coz you will have problems with standart solution).

There is no problem in doing std addition. Put all solvents in a carrier solvent such as DMF or DMAc. I have done solvents such as hexane, methylene chloride, methanol, dioxane, ethanol at the same time, same sample.

If you water sample solution is 200 microliters, then add 50 microliters of DMF containing the required amounts of solvents for which you are testing. Add the same amount of DMF to each sample but change the solvent content for the std addition requirements

It is not necessary to dissolve your substance complitely. A dosage of UV will drain organics from your substance. Adding solts, such as Na2SO4 is option. In a case of water, it will boost evaporation for polar organics, and decrease it for nonpolar.

IT IS REQUIRED to dissolve the sample completely. Your analysis of solvent content will be inaccurate otherwise. Adding salts can be useful but almost always in NOT required, unless you use too large a sample size WHICH IS NOT RECOMMENDED. UV light? Why would one use such a variability technique which could produce decomposition products?

I do not think you need an internal standart in this case. I have reprodused a lot of residual solvents methods, and I have met internal standart rarely in them.

Rodney George
Best wishes!!!

[chromatographer1]

test your sample to see if it dissolves into DMF/acidic water in any ratio, but 25/225 or 50/200 or even 10/490 could be used.

Your std addition of solvents can be prepared by weighing methanol, IPA, and cyclohexane into a volumetric flask of DMF which is not full, then filling to the mark AFTER the solvents are added.

This will give you wt/vol and you can accurately dispense the DMF (mixed well) with solvents into your HS vial, after:

weighing a known weight of sample, followed by adding the acidic water before the DMF.

As long as the sample dissolves into the mix after hearing in the HS Analyzer, you will have no problems.

Use as little acid as possible. Use another carrier solvent other than DMF if necessary. Short heating times <10 min should not produce any artifacts from the DMF.

best wishes,

ROd

[alemaggot]

Acetic acid is absorbed by sthationary fase of most column.

If I use it as solvent, also if it is present in small concentration, I'll found a residue of it in my gas phase and it will be injected in the gc.

I think that will damage my column. What you think about it Rod?

[chromatographer1]

Acetic acid won't damage most phases: methyl or phenyl silicone, wax, etc. I would try to stay away from poly cyano phases.

Why do you bring up acetic acid? This is new to the discussion.

Potassium and sodium hydroxide is also not a problem usually. I would not use them with DMF or DMAc, however.

Rod

[Axar]

Axar wrote:

IT IS REQUIRED to dissolve the sample completely. Your analysis of solvent content will be inaccurate otherwise. Adding salts can be useful but almost always in NOT required, unless you use too large a sample size WHICH IS NOT RECOMMENDED. UV light? Why would one use such a variability technique which could produce decomposition products?

Rodney George
Best wishes!!!

Well, sorry, I meant Ultrasound.
And if you use method of standart addition it is not nesessary to dissolve substance. You might add standart solution in different concentrations to your substance in HS vial. (example: 1g + 1ml standart + 4 ml solvent, 1g + 2ml standart + 3 ml solvent, ..., 1g + 5ml of standart) Then you make a calibration curve and calculate concentration in substance. With this approach you will minimize inaccuracy for insoluble substance.

[Ashita]

Hi!!!
I guess, best way to determine residual solvents is use of HeadSpase.
I would use water as solvent (in a case you don't need to determine solvents, insoluble in water, such as hexane, coz you will have problems with standart solution).
It is not necessary to dissolve your substance complitely. A dosage of UV will drain organics from your substance. Adding solts, such as Na2SO4 is option. In a case of water, it will boost evaporation for polar organics, and decrease it for nonpolar.
I do not think you need an internal standart in this case. I have reprodused a lot of residual solvents methods, and I have met internal standart rarely in them.
Best wishes!!!

P.S. sorry for my bad English here.

I also think the best way is headspace. there is no need of internal standard. i agree and sure about the method.

[Peter Apps]

[quote="Axar

And if you use method of standart addition it is not nesessary to dissolve substance. You might add standart solution in different concentrations to your substance in HS vial. (example: 1g + 1ml standart + 4 ml solvent, 1g + 2ml standart + 3 ml solvent, ..., 1g + 5ml of standart) Then you make a calibration curve and calculate concentration in substance. With this approach you will minimize inaccuracy for insoluble substance.[/quote]

There are likely to be major problems with equilibration of the added standard with the sample, and of the residual solvents already in the sample with the headspace, because if the sample is a solid then diffusion within the sample matrix is extremely slow. There are ways around this, but they are not as simple as just weighing sample into vials and adding standard. And if the sample does not disolve, what is the solvent doing in this scheme ?

Peter

[Axar]

There are likely to be major problems with equilibration of the added standard with the sample, and of the residual solvents already in the sample with the headspace, because if the sample is a solid then diffusion within the sample matrix is extremely slow. There are ways around this, but they are not as simple as just weighing sample into vials and adding standard. And if the sample does not disolve, what is the solvent doing in this scheme ?

Peter

Of course you are right. Sample must be prepeared to injection with mixing heating and ultrasounding. Solvent in this scheme makes volume. If solvent is not added (in a case of 20ml vial for example) for first vial headspase would be 19ml, second - 18ml ... fifth - 15ml.

[alemaggot]

I've try to use pure DMF as solvent with high purity grade. But There's really most interferent peaks. If I try to add acetic acid, the problem dosn't go out!

I must try the FET method. But I've problem with standards preparation. I don't understand how can I made standard solution.
I can't weight pure sample because it will evaporate instantly if i weigh it directly in the flask ( I can't add nothing in the flask). If I use a automatic pipette I can't adjust exactly volume to withdraw.

Have you got some idea?

[alemaggot]

Maybe I've resolve my weigh problem.

I weigh first methanol and isopropanol ( the component with high value of K) and for last cyclohexane. I weigh there component in a 5 ml vial (In this mode I've got a small volume where component can evaporate). Immediately after the weigh I close vial with septum and I write the sample weight. I remove the septum and I weight the succesive component and I repet all for the other compound. At the last I preleve 10 ul of this solution and I put it in the HS vial ready for the analisis.