I need to use my LC system for both cations and anions. When I am looking for cations, do I have to bypass the DS-Plus Auto Suppressor?

The DS-Plus is intended to suppress the mobile phase for anion analysis it will not suppress for cation analysis. I would advise that you bypass the DS-Plus for cations. This will help eliminate extra sources of dead volume and will ensure that detection of cations is not compromised. Feel free to contact Alltech and ask for technical support if you need more help. 1-800-Alltech (255-8324) or email alltech@alltechemail.com

You definitely need to bypass your DS-Plus suppressor for cation analysis. This product is designed for anion analysis and its function is for removing cations from your eluent so your cationic analytes will never make it through this device. Furthermore, you should make sure that you flush your system well when switching back and forth between anions and cations. Otherwise, you will have unstable baselines and lots of noise. My advice would be to rinse your system with deionized water until the conductivity is below one microsiemen before switching from one mode to the other. This usually requires multiple rinse outs of eluent bottles. If your instrument has a low-pressure proportioning valve, it's best to wash out all eluent lines, even the ones you're not using since low level bleed from these valves can cause drifting baselines. You may also need to throw the injection valve a few times clear any residual contamination from the valve.

By the way, switching back and forth between anions and cations on a stainless-steel system isn't a very good long-term strategy. The eluent systems typically used with nonsuppressed cation separations utilize chelating agents which can be corrosive to stainless-steel. Furthermore, switching back and forth between alkaline and acidic eluent systems will also damage the passivation layer on stainless-steel.

We have a method for analysis of cations and anions with out ion chromatography. The combination of Primesep columns and ELSD is a new alternative to ion chromatography. You can analize ions (cations and anions) and organic molecules in a single HPLC run. Visit www.primesep.com or request additional information. We can send you a brochure :Primesep with ELSD: A new alternative to ion chromatography. No suppressors, no ion chromatograph just regular HPLC system equipped with ELSD

AllsepTech,

I think you would do well to mention the limitations of your method instead of broacasting this message with such bravado. Your method may be a replacement for conductivity detection for some ions when sensitivity isn't important but this is hardly the same as "a new replacement to ion chromatography". You could use a similar arguement for ion detection with RI but in fact RI is rarely used for ion detection because while sometimes sensitivity isn't important, you still end up needing "ion chromatography sensitivity" sometimes. So, you still typically need an ion chromatograph anyway.

Thank you for your guidance! I will take into consideration all of your comments.

Dear Chris,
I am sorry for not mentioning limitations of our approach. I guess I am too excited about this. My statement was "a new alternative" (not "new replacement"), and of cause your are right we have limitations of our technology. The LOD for our methods is about 10 ppm. In many instances especially in pharma applications 10 ppm is more then enough . We are not trying to replace ion chromatography, we just showing a simple alternative for certain applications. The world still needs ion chromatography. As for RI, our method is not effected by temperature fluctuations and we can do gradient elution, which is a necessity in reverse phase chromatography. Our column can be used for reverse phase LC in addition to ion-exchange. But thank you for mentioning this; I did not want to mislead people.

An addition to Chris Pohl,October 23.

Switching back and forth between very differing eluents is always a not very good strategy. We did this on stainless steel and PEEK systems and on both you will see slight alterations with time. That means on stainless steel and PEEK you will see higher LOD's with time. The only difference is that stainless steel systems can be re-passivated to the original performance.
But as these systems are not well accepted in IC the best long term strategy is to run dedicates anion and cation systems. Especially if low concentrations have to be determined. For ppm levels switching is absolutely no problem.