anion IC question

Record: 102

Category: Column Selection
Product: IC
Sample: nitrate, nitrite, and sulfate

Question
I am in Mexico City this week visiting a couple of customers and one is an atmospheric sciences professor who just bought an IC system. They are analyzing for nitrate, nitrite, and sulfate.

Their current system is very old and has a Perkin Elmer pump, manual injector, and a conductivity detector from a place called LDC Analytical. I’ve never heard of it and it is at least 20 years old, probably more. They also have an Alltech suppressor but they said they have not been using it for this analysis. The column is a Hamilton PRP-X100 and the mobile phase is 4 mM hydroxybenzoic acid with 2.5% methanol, pH 8.5.

They are doing low level analysis and their calibration curve uses 0.1, 0.5, 1, 3, and 7ppm of each of the 3 ions. From all of your literature, the recommendation is to use a suppressed system for these low levels, and for these anions. Our conductivity detector manual also indicates that the hydroxybenzoic acid mobile phase is for a non-suppressed application.

With our system, at the highest gain setting of the CDD, we could see the 7ppm standards fine. At 0.1ppm the nitrate and nitrite were very small peaks, but we could not see the sulfate (it may have been in baseline interference because we didn’t let the cell stabilize long enough). With the high concentration of standards the sulfate peak eluted around 14 minutes at a flow rate of 2 mL/minute and 40C oven temperature. The customer can see all the ions at all of the concentrations in their current system.

They did buy a suppressor with this system but at the time I didn’t know what their mobile phase was. My main question for you is will this suppressor (information is attached) work with the hydroxybenzoic acid mobile phase? I think it will only work with sodium carbonate mobile phases, and that the HBA is only for non-suppressed.

Also, is it typical that a non-suppressed system will be effective for these levels of ions? Since they can see everything with their current system, I was hoping that ours would be the same or better because our conductivity detector is very good.

I showed the customer the application notes from your website with the SI-90 4E column (the separation has sulfate elute around 10 min at 1.5 mL/min) and the customer was very interested in trying that. I think we have that column in our lab in MD that I will send him to try.

Would you please let me know if the suppressor will work with the HBA mobile phase, and if you think a non-suppressed system will be good for these concentration levels? Please also reply to include Patty, our distributor, so she can let me and another colleague know your opinion because we will be at the customer site all day and I will not be able to get email during the day.

Thank you so much, once again, for all of your help and advice. I think the best recommendation for this customer is to try your column with the carbonate mobile phase and the suppressor. They will be able to reduce the run time and I’m sure will get much better peak shapes than with their current system. Thanks again and I look forward to your suggestions,

Answer
One information I found was that uses a strong cation exchange type suppressor, with this baseline of HBA can be suppressed.
http://www.flowinjection.com/SIA_Addons/SIA_IC.aspx

However, as you mentioned, for those three anions, I think our IC columns would have good chances of providing good separations in short run time.

One option is as you said, use SI-90 4E with carbonate buffer and suppressed conductivity.
http://shodex.net/index.php?seitenid=1&applic=690

The other option is to use I-524A, phtalic acid + Tris buffer (pH4.2) with non-suppressed conductivity detector.
With this method, those three anions can be determined within 9 min.
http://shodex.net/index.php?seitenid=1&applic=935
So if the customer does not want to use suppressor, this would be a good choice.
For the lower (0.1 ppm) level measurement, SI-90 4E with suppressor will provide much better detection limit than others.
http://shodex.net/index.php?seitenid=1&applic=694

With I-524, it could be bit difficult for measuring <0.1ppm. but it’s high-performance type, NI-424 might be possible.. If the customer does require very low conc. Measurement, my best recommendation would be SI-90 4E.     Additional Comments

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