Looking at new method. Anyone any advice or tips.

There are a variety of columns which can be applied to this application but things to consider are column capacity and stability of the stationary phase in the presence of an oxidizing analyte. Generally, this stability is achieved with phases that are of rather hydrophilic. A relatively recent column which is frequently applied to this application is the IonPac AS16 column from Dionex. It provides good chemical stability, good capacity and offers the option to do gradient separations.

Another point to keep in mind is that dichromate does not instantly revert to chromate upon dilution. The kinetics of chromium species are rather slow and since dichromate is much more damaging than chromate, you can get better column life by diluting and waiting 30-60 minutes prior to injection to allow the conversion to proceed. Alternatively, briefly heating the sample in a microwave oven for 60 seconds will accomplish the same thing (apparently microwaves substantially accelerate the process).

We already have a very old method for doing this analysis but with newer technologies in both columns and the chromium solutions themselves we are looking to update. We are aware of the problems concerning capacity but I was unaware of the connection to dichromate so thanks for the tip!!
Our biggest problem is the separation of the sulphonic acids that are in modern chromium VI solutions. These seem to coelute with both the fluoride and the chromate peaks. Information on the way to aproach these separations seems very hard to come by.

I have developed simple method which can ressolve fluoride peak from acid 1 and chromate peak from acid 2.
I cannot post details here but you can contact me directly. Vojtech

here is a link you can look at.
http://www.metrohm.com/docs/app/notes/pdf/s51.pdf

Link is very useful. Thanks. Only problem with this is that our current system is non-solvent compatable but are hoping to convince the "powers that be" to upgrade - hence looking for new method.

Also have tried non-Dionex columns before and found very low capacity, have things changed? We currently expect to put 15,000 injections of a 1000x dilution through each column. (1000 through each guard column). But using AG5/AS5 only get sulphate figure that's usable due to coelution.

What means "non-solvent compatible"? We do IC of chromium plating bath routinely and we change the guard column after about every 100 samples. Why do you expect to run 1000 samples through 1 guard column?

Not all of the columns from Metrohm (or other non-dionex columns) required the use of acetone in the eluent. Other anion exchange (some with very large capacity) columns from other manus will do the job too!!. There are plenty on the market which will do this separation, without the requirement of acetone. So no pump modification necessary. You can add the Metrohm suppressor module 753 (10 year warranty on this suppressor) and the 732 conductivity detector to your existing equipment via an anlogue output. (Some other methods use a PCR with UV detection)
15,000 injections is a lot of samples on to a column, though if you are only looking at just Chromium at high levels and you do very large dilutions (X1000) then I guess you are not putting to much on to the column.

I was I little too secretive but as I have seen the sulfonic acids have been generally known as additives for hard chromium plating, so my method is:
Hamilton PRP-X110 (4,1x150mm) with guard column.
Mobile phase: 2 mM sodium 4-hydroxybenzoate; 1 mM sodium tetraborate and 2,5% methanol.
Flow rate: 1,5 ml/min. and the analysis is completed in 20 min.
Detection: direct conductivity with additional pulse damper between pump and sampling valve. The damper really helps to reduce baseline noise.
The column is very rugged and can be cleaned with 100% organic solvent (very useful).
The column is in operation for over two years now with no decrease in performance.

If the sulfonic acids you are referring to are methanesulfonic acid, methanedisulfonic acid and ethanedisulfonic acid, we recently developed a method on the a new column based on covalently immobilized cryptand (the Cryptand A1 column) which can perform the necessary separation of these components in a chromate plating (along with chromate, of course). We used a fairly simple eluent system, utilizing just lithium hydroxide and sodium hydroxide to accomplish this separation. If you're interested in more details, please let me know and I can forward a PowerPoint file with relevant information.

Dear Joanna Gail,
to make the system solvent compatible, I assume that you just need to change the piston seals ??
what do you want to achieve with the analysis ??
just chromate ..anything else?

Of all the separations in the method the one we need most is the separation of MSA from Fluoride. This tends to be very near the front of the trace and is the first one to be lost from what we have seen so far. We analyse many different chromium samples from various sources and need to be able to quantify all the sulphonic acids listed plus sulphate and fluoride and occasionally nitrate and chloride. We find that the chromate figure is not of use in the industry as customers prefer a figure for total chromium VI as chromic acid.

Dear Joanna,
when you do upgrade, try the Metrohm Metrosep A Supp 5-250 column. The chromatographyis like the dual 2 colum(in the link above) but has better front end separation. Retension for Fluoride 4.6 min, Chloride 8.0 min , Sulphate 19 mins. All Performed isocratiacally
No solvent in eluent 1.0mM NaHCO3/3.2mM NaCO3
You should get the MSA resolved too.

Joanna,

The Cryptand A1 column and the IonPac AS16 column will both readily deal with the separation of F- and MSA.

I can not see the cryptand A1 column listed on Dionex web site ?
what are the RT for the components listed above?
Is that isocratic or gradinet ?

Information on the Cryptand A1 column is available at the link below: http://www.dionex.com/app/tree.taf?asset_id=278325 although this link does not include information about the components listed above. Under the conditions of the separation, fluoride elutes at two minutes, methanesulfonate elutes at 2.5 min., methanedisulfonate elutes at five minutes and ethanedisulfonate elute at seven minutes. The separation involves a step from 25 mM sodium hydroxide to 25 mM lithium hydroxide. I can forward a PowerPoint presentation with the information if you e-mail me your contact information.