Chromatography Forum

We just ran a set of MDL's spiked @ 0.4 ppb and the %RSD's for the compound areas are not what we expect. They range from 10-25%! I did some inlet maint. and re ran them and the numbers got a little better ( 6-15%) but I still don't like them. I am now cleanning the source with hopes that the RSD's will alll be below 10%.
Am I expecting to much? What do others doing this method get.

Info:
Autotrace extractor using Restek C 18 6 ml cartrige 1 G bed weight. 1000 mls of sample.10 min dry with N2 at about 7 psi. Elute by rinsing sample bottle with 5ml ethyl acetate to elute, then 6 ml MeCl2 to elute then, the Autotrace elutes with more ea followed by MeCl2 for a final volume of about 18ml. Blow down to ~1ml with Turbovap @~ 56C with N2.
Agilent 6890/5973 using a modified autotune ( drop the ion foccus so the 198/442 passes).

Another related question ( sorry).
Are there better test probes for this method other than the endrin DDT breakdown? I can easily meet this criteria but some more reactive compounds can suffer ( 2 isomers of dinitrotoluene being the worst).

Which compounds are you having issues with?

Your RSD% values seem reasonable, you should not expect too low as you may fail the MDL check criteria:
10 x MDL > spiked conc., you can find more info on page 15 of the document: http://dnr.wi.gov/regulations/labcert/d ... dguide.pdf

Update. I rebuilt my injector ( iinstalled clean one as I choose not to "gun brush"), clip column and cleaned my source. The dinitro toluenes look better from a non extracted standard. I'm going to build a test probe with these cpds to monitor the condition of my inlet. Before inlet maintenance the method test probes ( DDT and Endrin) were giving mr about 7% total breakdown. Meets method criteria, but we suffer for it!

Update. I rebuilt my injector ( iinstalled clean one as I choose not to "gun brush"), clip column and cleaned my source. The dinitro toluenes look better from a non extracted standard. I'm going to build a test probe with these cpds to monitor the condition of my inlet. Before inlet maintenance the method test probes ( DDT and Endrin) were giving mr about 7% total breakdown. Meets method criteria, but we suffer for it!

I would first inject a series of non extracted standards to get an idea of the reproducibility of the instrument, then follow with the extracted MDL study which will have a higher RSD because of the extraction process.

I have begun develop on 525.3 so that I can use SIM mode to drive my limits down enough to report the pesticides at the same levels we do on ECD and NPD along with reaching the level for Benzo-a-pyrene from method 550 so that we can run all four tests (525, 550, 507 and 508) compounds on a single injection. The states we work with require the lowest limits possible, so they search through any promulgated methods and set limits to the lowest they find.

Please keep us posted on the progress of 525.3. I have read it but am not a fan. I don't know if I can still write programs for our old Autotrace (floppy driven).