Chromatography Forum

Does anyone else have difficulty creating linear curves for the HAA5 methods, 552.2 and/or 552.3? I've tried 2 types of extractions, I changed the consumables on my Bruker 450ECD, and now I'm lost on what to do next. My curves start linear, then the middle range concentrations "bubble up and out" of a linear range, and the top end of the curve lines back up with the low end.
Can anyone help explain this or explain another way of extraction?
I'm using Restek CL-Pest1 and CL-Pest2 columns, direct on-column injections, and like I said a Bruker 450ECD.

I run 6 calibration standards from 1ng/mL to 40ng/mL and I run into the same problem. My curve is never linear, but if I use a second order regression (quadratic) it will almost always come out to be 0.999-1.000 which is acceptable by the method. I extract using 552.3 and I believe it requires 0.995 or greater. I believe the problem is the sensitivity the ECD has toward HAA's. At the higher end of the curve the detector is being overloaded somewhat which causes it to "bubble up and out" of the linear range. Hope this helps.

Try reducing the amount of sample on the column by either lowering injection volume or run in split mode.

You may be limited on how you inject by the required sensitivity for monochloroacetic acid. It doesn't respond well on ECDs. If it is a detector overload that one should be least affected.

I have reduced my injection amount to 0.5uL and MCAA is harder to see at the low end of the curve. I can't run in split mode because its direct-on-column injection.
Thanks for the info.
jss37992, it makes me feel better knowing others have the same problem.

Hi to all of you,
I am Alex and im looking forward to apply the EPA Method 552.2 fot ther determination of HAAs in drinking water. I already have a Restek Rtx-5MS capillary column installed on the GC/ECD (Trace GC ultra, Thermo). The method suggests the DB-5.625 column and i would like to ask you if you think that the Rtx-5MS could be used as well a the DB-5.625. From the research i did, the polarity of the columns is almost the same, so i think there will not be any difference.
Thank you

I use RTX-CLPesticides column, I can see all the peaks, but I don't know if you using this column is contributing to my problems or not.

I use RTX-CLPesticide 1 & 2 for most all of my methods. I get really good seperation on my HAA's with a 20 minute run. I could push it much faster if I needed to (12 minutes or less) but I run many samples and like to bake the column out between injections. Hope this helps.

My method is set up to run for 22 minutes but everything comes out in under 12. What are your method parameters? I'm willing to try something else to see if I get better peaks and curves.

Related? question. What should the pH be of the extracts? I just checked my last set and they seemed to have a pH of about 4 ( pH paper). My chromatography has tailing peaks for some of the compounds that I can get a little better with injection port maintenance. Wondering if the extracts are acidic enough to chromatograph poorly.
I do 552 methylate with 1 ml of 10% H2SO4/MeOH, and quench with 4 ml. of sat. sodium bicarbonate.

Wow, the pH of my final extract is around 7. The samples before methylation are acidified to a pH less then or equal to 0.5 with conc. sulfuric acid and 3mLs of 10%sulfuric acid in methanol is added. After methylation 1mL of sodium bicarbonate solution is added which neutralizes my samples. 552.3 doesn't say what the final pH should be. Are you thinking the more acidic they are the better the peaks will look?

When I ran that method I learned the hard way to check the pH of the extracts prior to analysis. If it turned the pH paper red I re-neutralized the extract. Prior to adding that step I destroyed a pair of columns by injecting some of the methanol/acid into the system.

pH is super important i've destroyed brand new columns and a person i was training skipped the pH check and ruined a couple a few weeks later... as for peak tailing, it is the nature of the beast... i say extract a third party certified reference with every other batch and keep a close eye on your surrogate recoveries...
i've been running this for 5 years now and use DB-1701 and DB-5.625 and aside from the pH issue have had no problems, oh and i calibrate with a quadratic curve.

I am planning to apply EPA Method 552.2 and i have a question about the equipment. The method indicates:

"Graduated Conical Centrifuge Tubes with Teflon-Lined Screw Caps"

Do you think that it would make any difference use plastic tubes instead of glass tubes? Could appear any interferences in the matrix from the plastic tubes?

Thank you,
Alex

Plastic often leaches materials into the solvent. Whether your tubes will cause problems with your analysis can only be answered by trying. Also the derivatized acids may be absorbed into the plastic of your tubes. Again, that will depend upon your tubes.
The derivatization is done a elevated temperature in sealed tubes. The tubes must be able to stand the increased pressure without breaking or leaking.