Chromatography Forum

In sample prep, I use nitrogen to evaporate solvent until there's about half mL, and then I have to make it 1.0 mL. What I am currently doing is to use the sample vials which have the marks (0.5; 1; 1.5mL) but it's really not accurate. Anyone have a solution?

Thanks

If you add a known mass of a surrogate standard before processing you will have an internal control.

I think your statement begs a question: Why do you evaporate to half a mL? To half a mL from what volume? Why not take it to dryness? Why evaporate at all?

In any case, you can begin your evaporation in a 1-mL volumetric; once you've evaporated to the level you currently evaporate to, bring up to volume using the mark on the volumetric, then transfer to an injection vial. That way, you know your final volume is 1 mL.

*edit* -You could also evaporate from whatever volume you want to less than 1 mL, then transfer the remaining sample (plus a small amount of extra solvent used to wash down the sides of the original container, also transferred) to the 1-mL volumetric, then bring it up to the 1-mL mark.

Thank you for replying.

In my case, evaporating to dryness may lead to loss of analytes.
I start from 15ml (more or less)
I know that using istd can solve the problem, but it's more expesive and tedious. For my analysis, it may be the final solution.

One variation of a theme:

If you have a very small amount of analyte in your solvent and your solvent does not contain any other contents but has a defined density at a known temperature you can weigh the vial before adding the sample and then add your partially evaporated solution to it and add by weight enough solvent to make a volume of 1 mL.

But measuring the weight three time, dry vial, partially filled vial, and then filled to 1mL vial, is probably more troublesome than adding an internal standard. If you are working with volatile analytes that are easily lost you have a lot of work validating both the method and the technician for any results to be considered valid.

Perhaps if you just weigh the vial before and after filling it ONCE and calculate the volume from the weight you can determine concentration.

These comments presume that your analyte amount is quite small (less than 1mg?) and your solvent weight is at least 600 mg.



Otherwise a really finely graduated and accurate vial would be required.



and a really big magnifying glass. ( you realize this part is a joke, right?)

good luck,


Rodney George

Dear chromatographer1,

I think your idea is rather practical because in my case, the analytes are very little (ppb range) and the solvent used is known and not a mix. I will try it and hope the matrix doesn't interfere much.

haiedc, you are doing analysis on hope?

I have been doing separation on hopes, dreams and reality for the past 10 years, if you need help HW.

Is this a typical result?

Hope<LOD
Dreams>Upper LOQ
Reality = Internal Standard?

haiedc, you are doing analysis on hope?

Sometimes, Yes.
There are times your analysis go wrong day by day and you can only hope for the best (find out what is the reason).

Well. after reducing from ~15 mL and not knowing what weight of matrix is there, I wouldn´t even dream to hope. bisnettrj2 gave you a method with which you don´t need to hope.

haiedc,

If you do not want something that is tedious, I would suggest that you aviod tring to bring samples to a fixed wieght with the addition of solvent. The addition of internal standards does not have to be particularly tedious or expensive. You only need a compund that elutes at a retention time different from your analytes and should be of a similar chemistry in the solvent and GC.

Bring your samples to volume, cap the GC vials, and add an internal standard mixture with a microliter syringe, through the cap. If you do not mix the vials, the syringe will not be in contact with anything that is contaminated by the sample and you can go down a row of samples adding internal standard. Then mix. Careful work with a microliter syringe can give variability well below that expected for a GC anaysis and would have insignificant contribution to method variability.

On further thought - the solution I posted will allow you to correct for dilution of the sample, but does not account for losses in extraction and concentration. Addition of a standard as early on in the procedure would be good also, particularly as you are evaoprating a sample with analytes that could be lost. The pair of standards becomes a check of sample handling. Excessive loss of the internal standard added earlier on becomes an inidcation that analytes have probably been lost - and because, typically, the analytes will not be lost at the same rate, the results from the sample are not used.

Hi Don,

Yes, I think your way is good and straightforward. At first I thought istd is used for the whole sample prep procedure and it's not easy to find an istd that is similar to the analytes. Anyway the method does not require an istd. So your method can only adjust for the dilution of the sample (to 1ml), and that's much easier.

Thanks

Good answer Don. I concur.

Rodney George