Dionex Thermo or Metohm?

[Victor]

We are an environmental laboratory that has need for the determination of common anions, and also sometimes cations such as ammonia in a variety of samples. We have atomic absorption equipment so determination of metals is not a big issue for us.

For about the same price, we have been offered a Thermo Dionex 1100 or a Metrohm 930 IC. There does not seem to us a huge difference between these options, although we do not have a lot of experience of ion chromatography, but have some in HPLC. Apparently, a suppressor is not available for cation determinations using the Metrohm-would this reduce the sensitivity for ammonia determination on this instrument? However, as mentioned, anion determination would be the major use of the equipment.

Could anyone give us advice on this choice? We have already considered service and repair arrangements, but any suggestions on this topic or the relative capabilities of the instruments would be appreciated.

[Markus Laeubli, Metrohm]

Dear Victor

About ammonium with or without suppression, you will get not large differences in response factors, actually almost identical. The signal in suppressed cation IC is proportional to the conductivity of NH4+ plus OH- (sum of the equivalent conductivities is 271). In non-suppressed cation IC the signal is proportional to H+ minus NH4+ (difference -277, negativ peak).

In the case of suppressed mode the signal is NH4OH in pure water. Here dissocation might not be 100%. This yields in slightly non-linear calibration curve.
In non-suppressed IC the ionic strength is not changed. Therefore NH4+ is fully dissociated and an abolutely linear calibration curve is obtained.

But both modes will deliver good results for ammonium.

Which instrument I would recommend is obvious (see below).

[James_Ball]

We currently have one of the older Metrohm 761 compact IC units and two of the Metrohm 881 units doing suppressed Anion analysis. The older unit we have had 10 years now and except for normal maintenance such as pump seals, check valves and piston replacements the only repair we have had was having a service tech adjust the pulse that steps the suppressor so it would rotate completely and not give higher back pressure. It was a minor adjustment and was working even when it wasn't perfectly aligned. That suppressor is still working great even after 10 years. Our old Dionex unit would need a suppressor replacement in the 6 to 12 month range as it is considered a consumable by them.

We run anywhere from 50-75 samples per day per instrument. They are environmental samples ranging from drinking waters to waste effluents to sludge/soil samples. If sulfate begins to drift low on calibration checks ran ever 10 injections we find that if we replace the filter membrane on the autosampler it usually comes back to normal and we keep running. Calibrations will last us from a week to over 3 months depending on what samples we get.

Pretty much we load them on Monday and keep them running until sometime on Saturday without much problems at all. They would be the only ICs I would recommend currently.

[Victor]

These replies are very helpful. It is also nice to see a carefully measured reply from a manufacturer too-thank you.

If anyone else has a view on this choice, please post.

Thanks a lot, Victor

[malbertson]

Hi Victor

If ammonium response is of concern I would go with the salt convertor suppressor on the ICS-1100 this re-injects a small amount of Methane Sulphonic Acid MSA (the mobile phase) post suppressor on the ICS-1100. This will increase the ionic mobility for ammonium and give a similar almost equivalent response as the fully disassociated cations at the same concentration. I would also suggest you look strongly at Column capacity for cations. When dealing with some wastewaters, particularly if the matrix has significant amounts of Sodium present, column capacity will be the key consideration to getting a good separation and qauntitation of ammonium.

Mark Albertson
Thermo Fisher Scientific
Sales Mgr ANZ.
HPLC, IC, Sample Prep