548.1 Endothall, erratic Internal Standard recoveries

[catdoc27]

Has anyone experienced erratic ISTD recoveries when analyzing for Endothall? Our calibration will run anywhere from 15-30% RSD, and extracted QC/samples will be anywhere from 80-160% from the calibration average.....We've tried different standard lots, different vendors & even different reagents......we're at a loss since the ISTD is added after extraction/derivatization. Any information/advice would be greatly appreciated-I'm ready to pull out my hair!

[Steve Reimer]

If you add the internal standard solution to dcm at the expected concentration and analyze do you see the variability? If not, is it that the final composition of the extract isn't consistent?
It has been 12 years since I ran that method but I don't remember any problems with the internal standard. Hard water gave lots of problems with the extraction step and we were looking at an ATP for it, instead we stopped running it.

[catdoc27]

No-we're using the same ISTD for 525.2 & have consistent recoveries. We've questioned the extracts, since the calibration (while variable) typically falls within a certain response and then the SPE QC/samples will have a response quite different from the cal (80-150% of average).....There's not much we can do with the extracts, since method 548 uses the non-extracted calibration-which led us to investigate reagents & standards.

[lovemymoe]

This test is the worst one we do, in fact it's why I'm here (our recovery is suffering and we are getting emulsions in our sepatory funnels.) The only thing I can think of is how far down are you concentrating them? We stop at the 0.7 mL mark because much lower than that and they will get dark and then the internal standard areas do start to vary. Also, are you making sure that the gas isn't blasting the extract or that if you are using the water bath of a concentrator, that the temp isn't cranked up super high? During concentration, we try to make sure that they are "wafting and not denting" for lack of a better term to avoid any volatilty from a hard stream of nitrogen. Also, I assume you are using a micro syringe for the internal standard delivery(?). I only ask because I have found that especially with solvent based reagents, mechanical pipettors sometimes struggle with repeated dispensing. Lastly, did you change anything about your consumables or how you prepare your internal standard? It's an obvious place to start but God only knows that with this test, that the devil is in the details and things that are "identical" are often anything but...Let us know what you find, especially because that will probably be the next problem we encounter as soon as we get this one ironed out. Good luck!

[James_Ball]

This test is the worst one we do, in fact it's why I'm here (our recovery is suffering and we are getting emulsions in our sepatory funnels.) The only thing I can think of is how far down are you concentrating them? We stop at the 0.7 mL mark because much lower than that and they will get dark and then the internal standard areas do start to vary. Also, are you making sure that the gas isn't blasting the extract or that if you are using the water bath of a concentrator, that the temp isn't cranked up super high? During concentration, we try to make sure that they are "wafting and not denting" for lack of a better term to avoid any volatilty from a hard stream of nitrogen. Also, I assume you are using a micro syringe for the internal standard delivery(?). I only ask because I have found that especially with solvent based reagents, mechanical pipettors sometimes struggle with repeated dispensing. Lastly, did you change anything about your consumables or how you prepare your internal standard? It's an obvious place to start but God only knows that with this test, that the devil is in the details and things that are "identical" are often anything but...Let us know what you find, especially because that will probably be the next problem we encounter as soon as we get this one ironed out. Good luck!

Have you tried using solid phase extraction? We us that and get fairly good recoveries with no emulsion problem. I would need to check with our extraction guy to see exactly which cartridge we use, it is either Restek or UCT, I just can't remember which.

[mdevay]

What conditions are you using to concentrate the extract and what is your internal standard? I'm assuming DCM is your solvent. We use acenapthene as internal standard. Samples are concentrated on an N-Evap concentrator, and if the nitrogen flow is too high or the temperature of the water bath too high, then we get some volatilization of the internal standard.