I need an IC method for the analysis od Phosphonoformate, Dichoromethylphosphonate and Monochloromethylenebiphosphonate using an old Waters conductivity detector. Anyone got any ideas ??
thanks

Yes go and buy a brand new Dionex DX120.

We are an Educational establishment, with little to no money for new kit.
I am told that the Dionex suppressor frequently fails. It is not covered by a warranty and is very expensive to replace.
I would prefer a non-suppressed method.
Thanks in advance for any constructive comments.

Anon above:

Don't believe everything that you been told. I was just visiting with the customer who finally had to their suppressor after six continuous years of operation.

The fact is that the vast majority of all anion analysis involving non-chromaphoric anions are performed using a suppressor. The difficulty of accomplishing separations and detection without such a device increases as the valency of the anion increases. Thus, your application involving a potentially tetravalent anion will be rather difficult to accomplish without the use of a suppressor. Nonetheless, if you're funding situation prevents you from operating with a suppressor, I would suggest that you pursue an ion pair separation at neutral pH on a reversed phase column. If you use an eluent containing two millimolar tetrabutylammonium hydroxide and adjust the pH of your mobile phase to 7.5 with boric acid and you add 15-20% acetonitrile, you should be able to retain and separate your analytes. Your sensitivity won't be very good but maybe you can get away with using such as system depending on your analytical requirements. For better sensitivity, you can still use suppressors and stay within your budget if you pack your own suppressor columns using 200-400 mesh strong acid cation-exchange resin in 6 mm by 50 mm column hardware.

Dear Chris,
Thanks for the information. I thought Ion pair chromatography was very temperamental with long stability times for the column.
I note that the Dionex method for doing phosphonates is to use an AS 7 anion column with HNO3 20 to 60 mmol/L eluent at 0.4 ml/min acid. A PCR of Fe(III)NO3 and perchloric acid was used with UV detector.
Do you think the AS 7 column with a modified eluent of 2mmol/L HNO3 and 8mmol/L Sodium Nitrate will work to get the phosphonates off the column? The conductivity of the eluent will be within range of the detector about 1200uS/cm. The nitrate in the eluent I assume is the component which is competing for cation sites on the column. Levels of phosphonates are about 150ppm levels. I would assume we would get a reasonable conductivity signal for the phosphonates in this modified eluent.
What do you think?
regards
Ted

Ted

It's a bit difficult to predict how well your proposed method will work without trying it. As a bit of introduction to the method to which you refer, the IonPac AS7 column is designed for the separation of chelating agents under acidic conditions. Because some of the more common chelating agents are amphoteric, the column has both cation and anion exchange sites which are independently measured as part of the QC for the column. The optimum choice of eluent pH is dictated by the nature of your analytes (in your case, the analytes are not amphoteric so the cation-exchange mechanism is not relevant), whether or not transition metals may be present in your sample and the valency of analyte anions. When transition metals may be present in samples, it's generally advisable to work at pH values low enough to prevent formation of a stable complex. Otherwise, numerous sample dependent artifact peaks will be observed in the chromatogram. So the answer to the question regarding whether or not you can switch to a significantly higher pH depends upon your sample and the properties of your analyte (in terms of the maximum pH at which you can operate without formation of stable complexes). Also, assuming metals in your sample are not an issue, I still think it is likely that you'll have difficulty eluting monochloromethylenebiphosphonate with the eluent you describe in a reasonable time on the AS7 column. This analyte should have significant divalent character in 2 mM acid which may make its elution difficult under the conditions you propose.

Finally, using conductivity detection in the nonsuppressed mode, you need to consider not only the total conductivity of your eluent but the relative conductance of your analyte and your eluent species. Although I couldn't find any reference information on the limiting equivalent conductance values of your analytes, my guess would be that they will be too similar to that of nitrate to provide acceptable conductivity sensitivity in nonsuppressed mode. I think you would be better off using an aromatic analyte such as toluene sulfonate or sulfosalicylic acid, since these analytes will have significantly lower limiting equivalent conductance values as well as greater elution potency thus reducing eluant conductance. Finally, nonsuppressed applications are generally done on stationary phases with significantly lower capacity then the IonPac AS7 column in order to minimize background conductance. You might be better off with a lower capacity column like the IonPac AS4A for such an application.

Chris