Chromatography Forum

I would like to test cyanide amount in pharmaceutical substance and EurPh provides for liquid chromatography with amperometric detection. My laboratory could not buy an ion chromatography so I would like to know if it's acceptable to perform ion chromato with a basic HPLC system and appropriate ion exchange column and detector. Should I change HPLC pipe for peek pipe?
What are the problems I may encounter?

thanks a lot!

Dear tp51

Just use your basic IC and add an electrochemical detector.
You might replace stainless steel capillaries by PEEK. But this is not absolutely crucial.
Anyway if you have stainless steel in your flow path, it must be passivated before use e.g. with 0.1 molar nitric acid.

The mobile phase for this application is pH 13. You should make sure that all wetted materials are rated for that. Especially the rotor seal on your injection valve; many models use Vespel which is not stable above pH 10. Basic mobile phases are hard on pump seals, so if you can, use the seal wash option. Cyanide binds to metals, so you should minimize metal contact, and clean and passivate all the remaining metal components.

Markus,
is that 0.1 M nitric for passivation a "feeling", or whatever, or is it based on some hard evidence? I took the following out from some notes which could be from refs out of an earlier discussion chain on this:

LC-GC - June 2000

Stainless Steel Surfaces in LC Systems, Part I - Corrosion and Erosion

Guest Authors
Kenneth E. Collins, Carol H. Collins, and Celso A. Bertran

.................Several simple laboratory tests showed us that nitric acid at concentrations of 10-30%, which
corresponds to 1.5-5 mol/L, corroded small samples of stainless steel but that more concentrated
nitric acid - 50-100% or 7.5-15 mol/L - did not. We thought we had discovered something new - until
we found that this strange concentration dependence has been known for centuries (1) and was
studied extensively by Michael Faraday in the 1830s (2). Thus, chromatographers who use nitric
acid for cleaning or passivating the inner surfaces of their LC systems should have at least some
awareness of the vulnerability of their stainless steel surfaces to corrosion processes in general and
to nitric acid in particular. ......

Dear HW Mueller

It is longterm experience. We are working with amperometric detection on stainless steel systems since over 20 years in HPLC and IC. The mentioned passivation (actually we use 0.2 M nitric acid) worked good all the times.

Markus, are you shure that your apparatus needed passivation? I have been very uncomfortable doing these high conc passivations for more than 20 years. At less than 1/10000 of the HNO3 concentration one could even drink the solution (no need for precautions).

Dear HW Mueller

First of all sorry for my typing error. The passivation solution we used was 0.2 M (corrected in the post of July 03).

Of course passivation of steel systems is by far not a pay to day procedure. We usually performed it when we faced problems which we could not allocate to column performance. In such cases passivation help quite often and never harmed.

Also sorry. I thought that was 0.2 mM. But still, this 0.2 M is low enough to warrant a very thorough investigation in view of the refs I gave above.

I used to do a lot of passivations on LC equipment (dsDNA has a nasty habit to attach itself to any "active" stainless steel surfaces through the phosphate groups...). I used 8mol/L (~35%) nitric acid with great success but once in a while I also used concentrated nitric acid (~70%) without any problems.
The important thing is to very carefully rinse the system afterwards . A water rinse followed by a phosphate buffer rinse (pH=7) and again a water rinse usually was enough and all was done in about 1 or 2 hours.
Autosamplers were passivated in a similar fashion.
Disclaimer:
Always check with the LC manufacturer first before pumping 8mol/L nitric acid through a LC-system!

Thank you very much for your advice. I am very happy to learn it is possible to perfom ionic chomatography with classical HPLC system. I work with Waters Alliance system and I have asked to Waters how I can passivate the system. Technical support have answered to passivate the system with 1 N nitric acid during 1 or 2 hours and rince with water until solution will be neutral. Problem may occur on injector with an higher amount of nitric acid (6N). The mobile phase I should use (PhEur) is a blend of LiOH and sodium ededate. I have heard that Li+ may cause problem with stainless steel, what do you think about it?

Thanks and best regards