Internal Standard in the Diluent

[wchriste]

Dear Experts,
Have any of you ever worked with a method that used a diluent that contained an internal standard? (vs. a method that would require diluting the sample and adding the internal standard in two seperate steps)

Thanks!
W.C.

[GaryR]

This is not that unusual. We have several methods (HPLC and GC) which use an internal standard which is prepared as the diluent.
It probably depends on the preference of the method development and validation people.

The other extreme would be to make microlitre/millilitre additions of a concentrated internal standard prior to making the final dilution to volume.

The error associated with including the internal standard in the diluent is much less than when making internal standard additions.

[tom jupille]

The error associated with including the internal standard in the diluent is much less than when making internal standard additions.

On the other hand, you lose one of the collateral benefits of using an internal standard, which is to correct for errors in dilution.

[Consumer Products Guy]

I do this routinely with one of my assays, combines two steps which must be done quickly due to volatile analyte.

[Peter Apps]

Depending what you want the internal standard to correct for, you can add it at any stage.

Peter

[gbalock]

I have typically used an internal standard where there were difficult extractions and the internal standard had a similar extraction profile as the analyte. I would not add the internal standard to the extraction solvent in that case as I want to measure the efficiency of my extraction. If the IS is used to overcome injector irreproducibiity, then putting it into the diluent is okay.