Anion analysis with Dionex 600 (unknown peak)

[Josep Tom]

Hello,

I am using Dionex 600 to measure anions. The eluent is Na2CO3 and NaHCO3 mixture. I always found an unknown peak comes out at the retention time of around 11.7 min when i inject DI (18.3MOhm-cm) water. Although I cleaned the both guard and analytical columns and change the DI water and eluents, i still see the peak.
That peak interfere with the sulphate peak which usually come out at 11.3 min. How can i eliminat that unknown peak?

Thanks in advanced,
Tom.

[Chris Pohl]

Tom,

I couldn't say for sure without knowing a bit more information about the column you are using but most likely the extra peak is due to an organic acid, probably oxalic acid. Since you say you're seeing a peak when you inject deionized water, this suggests the problem is caused by contamination of your deionized water. First off, I would try obtaining some deionized water from a different source to verify this hypothesis. If this isn't an option, you may need to inspect your deionized water system to see whether or not it's in need of maintenance. A common problem with deionized water systems is contamination of the line leading from the final filter with either bacteria or algae. This is usually apparent as the supply tubing is discolored by this contamination (green and brown discoloration being common). Often ionic byproducts arising from this contamination create analytical problems of the sort you describe. If such contamination is evident, you'll need to decontaminate your deionized water system (the supplier of the deionized water system should have information on how to accomplish this).

[Josep Tom]

Hello Chris,
Thank you for your guidance.
My system details are as follows:

DX600
CD25Detector
GP50 Gradient Pump
ASRS 4mm Supressor
AS12A 4mm Column

One more- my supressor is leaking. My water resistivity is 18.3 M Ohm. With this resistivity it is possible contamination is from water source?

Regards,
Tom.

[PeteMartin]

Hey Tom,

Sorry to jump in, but just on your side note on the leaking supressor. I've experienced a leaking ASRS as well. It seems that the ASRS ultra II's are not as tolerant to backpressures as their pedecessors.

I assume you have it in recycle mode; change the tubing a little after the cell to the regen-in port of the ASRS to the clear 1/8" tubing (you'll have to step it back down if you go through the column switching valve) and make sure there are no blockages in your waste line (it should already be the 1/8"). That, and a new supressor, fixed it for me.

HTH

edit: Sorry Tom, Kannan is correct in me assuming you are using a column switching valve in his post below. I should have asked first.

[kannan_srinivasan]

Hi Tom

I am responding to your comment on the leaking suppressor. Pete is assuming that you have a column switching valve interface.

The leakage issue is possibly related to

a) High backpressure from one of your lines. Measure the backpressure to the cell and ensure that this is less than 40 psi by trimming the backpressure coil.

Remember that all tubings post suppressor would contribute to the backpressure. For 4 mm applications we recommend that you use 0.01" black PEEK tubing for all connections upto the backpressure coil on the cell. From the cell backpressure coil to the suppressor and waste use 1/8" clear tubing.

b) Measure the system pressure A and then disconnect the line out of the eluent out port and measure the system pressure B.
The difference A-B should be < 100 psi. If this is > 100 psi and you have the correct backpressure then the high pressure may be from your cell.

c) If you are running external regenerant ensure that the regenerant flow is 3-5 ml/min. Use 1/8" tubing for making the connection to and from the suppressor. Reduce the applied pressure to the regenerant bottle if needed to ensure that the regenerant flow rate is within the recommended values.

d) Ensure that the suppressor is powered only with the eluent flow and regenerant flow on. If the suppressor is powered without flow you can damage the suppressor.

If you have any further questions feel free to contact me at: kannan.srinivasan@dionex.com.

Cheers,

Kannan

[Chris Pohl]

Tom,

I agree if you're deionized water system is giving you 18.3 megaohms it's probably not your water (although you can sometimes have deionized water contamination arising from the line leading from the final filtration unit to the DI water sytem outlet). The next most likely culprit are your reagents used to produce your eluent. At this point, I would suggest producing your eluent from a fresh set of analytical reagents. Such a problem can sometimes arise from contamination of the carbonate or the bicarbonate. It's remotely possible this problem is related to column contamination, so it is sometimes helpful to swap with a different column said just to see if the problem follows the eluent or the column. You might want to try this if you have another column handy (it doesn't have to be a selectivity identical to the column set you're currently using).

[Mat]

With the new suppressors (Diones SRS Ultra II) it can be difficult to set up the system correctly. We used to have a problem of gas bubbles in the conductivity cell, which got solved by adding a short backpressure coil. This is not possible anymore with the new suppressors in our system. Also, using red tubing (0.005") between the column and the cell is not possible, but the difference in resolution is not really noticeable.

Fortunately, we changed other things as well, including adding an EG50 eluent generator for KOH eluent, and the bubbling issue has solved itself (I guess due to the added degas assembly in the EG)...

Nonetheless, our SRS Ultra II started leaking internally. We got it replaced and now it seems to work fine. But backpressure wise we are right at the limit (but not over it).

Our system is set up to work for cations as well as for anions, and the bubbling issues only disappeared after we used two eluent generators (We had it on the line without the EG). Without an eluent generator, the bubbling was a problem, which would mean that we would have had to add or replace a coil each time we switched columns...

And Kannan, just to complete the information, is the value of 3-5 ml/min for external regenerant flow also valid for 2 mm systems?

Back to the original poster, did you measure the conductivity of your water separately, or did you just check the reading on the water system? If the latter is the case, you might want to replace the outlet filter, it is a nice breeding ground for all sorts of organisms, and it is downstreams from the measurement...

[kannan_srinivasan]

Hi Mat

The recommended regenerant flow rate for 2 mm applications is in the 1-2 ml regime.

Cheers,

Kannan

Kannan.Srinivasan@dionex.com