Gradient elution troubles/loss of acetate and formate

Discussions about IC and related topics

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Hello all,

I am trying to develop an IC method that has a linear gradient elution from 7mM to 40mM to achieve fluoride, acetate, formate, nitrite, nitrate, sulfate, and phosphate. Currently, two separate isocratic methods are being used to achieve all the peaks. The LOQ for the isocratic runs is 10ppb, but when calibrating the gradient, acetate and formate disappear at that level on the gradient. I have tried a method that stays at 7mM for the first 10 minutes to elute those first 3 peaks before ramping and that didn’t work. Does a gradient elution decrease sensitivity? I’m pretty new to the ion chromatography so this may be an easy fix for someone more well-versed.

Any advice on gradient elution methods or on increasing sensitivity on the IC or any advice in general would be greatly appreciated.

System: Dionex ICS-6000
Column: AS18 4 x 250mm
EGC: EGC KOH 500 RFIC
Suppressor: ADRS 600
Flow rate: 0.8 mL/min
Injection Volume: 100µL
I ramp the eluant up from 23 mm to 34mM on an AS-18 column to get the phosphate off faster. I have not noticed any change in sensitivity. I have never run as low as 7 mM though. Could acetate and formate disappear because they are merging with fluoride? I had a guy from Thermo come and test some gradients and he couldn't get one that separated Br and sulfate. Then he said you never see Br so it doesn't matter. Ugh. I don't want that guy coming back.
Hello anionman,

Thanks for your reply. I don't think that acetate and formate are merging with fluoride because the later levels (30ppb, 50ppb, 80ppb...) show good resolution between the three peaks.

I have tried upwards of 20 or so variations of methods and all of them have some sort of "interference" when ramping up the concentration. It is more prevalent on the step gradients, where each step will have a jump in conductivity and level out during the duration of the step and jump up again at the beginning of the next step. Is this "interference" normal? I thought using a linear gradient would eliminate it but it occurs there too.

When you run a water do you experience any jumps in conductivity at all? Does your baseline just follow your gradient with no hiccups?
The Thermo guy couldn't do better with ramps than I have done. The same problem occurs - peaks merge. The conductivity rises when I ramp up the eluant concentration. I only ramp up to 34 mM because if I go any higher it will not come back down before the next injection. I start the increase at 8 minutes and at about 11 minutes the baseline begins to rise. That's fine because nitrate come off at 10 minutes with a new column. That puts phosphate on a slope but I don't quantify or care about phosphate. The rise in baseline is pretty small so it isn't visible when running anything by a blank or MDL. I am not looking any lower than 0.1 ppm for fluoride and nitrite. I run MDLs at 0.01 ppm fluoride and nitrite. I can see a slight rise in the baseline at those levels. There is some acetate and formate contamination and the retention times are fluoride 3.44 minute, acetate 3.60 minute, formate 3.86 minute. I don't have trouble quantifying fluoride because of the contamination usually. I don't care about quantifying acetate of formate so I have never tried to separate them better.

I have looked at as-18 column literature published online and see some ramps from 19-30, but never as low as yours.

https://assets.thermofisher.com/TFS-Ass ... 878-en.pdf
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