Tailing in the Dionex ICS 1100 and other issues

[icchlorine]

Hello,
I am working in a lab with a Dionex ICS-1100 using AG & AS 22 columns in order to measure chloride and nitrate in ice samples in the 50ppb range. Our ICS has been giving us quite a bit of trouble, so I plan on posting several questions.
The first problem to come up is tailing. We got the ICS last February, and used it a couple of times and got nice, symmetric peaks with just a hint of a tail. We turned the machine off and didn't use it again until December, and then got quite a bit of tailing in our peaks. We've been running the IC several times since then, and the tailing hasn't gone away, though it has decreased a little bit. Do you have any ideas as to what could be going on? Could not using the machine for 9 months have deteriorated the peaks?

Also, I was wondering, do you consider it best practice to leave an ICS running when you are not using it for more than a few days? When do you usually turn off your ICS?
Thanks for any help you can provide.

[icchlorine]

This might be a simpler question to ask: when we did not use our ICS 1100 for 9 months, we simply left the columns in the machine and had the pump off. Could that have damaged the columns?

Another problem we have had with the ICS is poor peak resolution at concentrations below ~0.5 ppm. Our plates have lately been around 7500-8000. We've tried cleaning the columns with concentrated eluent, nanopure water, and 1 M HCl, but the issue persists. Any ideas?

Thanks again.

[iceyuser]

As far as disuse goes, I've noticed that the suppressor can get funky after a while.

[Chris Pohl]

icchlorine,

It is possible you have a bit of contamination in your suppressor. This could explain both the issues you report. You might try taking the suppressor out and injecting a few milliliters of 5% hypochlorite (i.e. Chlorox bleach) into the suppressor eluent chamber, wait 10-15 and then rinse eluent and regen chambers with 10-15 milliters of DI water and then reinstall the suppressor.

Generally, if you plan on going for weeks without using a column we recommend you store it in 0.1M sodium tetraborate. Then when you are ready to use the column, reconnect it to the system and run the column 10 minutes or so before connecting the column to the suppressor.

[icchlorine]

Thanks for the suggestion. It appears that the issue is after all the suppressor. I am running an ASRS 300 suppressor with a CRD 300 to remove the carbonic acid and reduce the background conductivity to ~1µS (ideally 0.7 µS). The ASRS 300 manual recommends running 0.2N sulfuric acid through the suppressor to regenerate it. Do you think analytical grade is fine for this kind of work or does it require HPLC grade sulfuric? I'm doing ~50 ppb level analysis, and the tailing is kind of annoying. Right now I am at ~15% error, which is workable but I'd like to get within 10% on a regular basis.

[Chris Pohl]

Analytical grade sulfuric acid should be okay for suppressor regenerant. You might want to try disconnecting the CRD, though, to make sure the tailing isn't caused by the CRD.

[icchlorine]

I ran the sulfuric through the ASRS, let it sit for about and hour, then flushed it with water. The peaks are still tailing so I am thinking the issue lies with the CRD. Do you know if I can regenerate the CRD with the sulfuric acid as well?

Thanks for the help.

[Chris Pohl]

Sulfuric acid regeneration (or any other type of regeneration) won't help if the problem is due to the CRD. Most likely, the problem is due to slippage of an internal fluidic connection in the CRD. It's best to make sure that peak shapes are restored to normal if the CRD is bypassed to confirm that the CRD is the root cause. If this proves to be the case, you'll want to check to see if the downstream backpressure is excessive (as this is the most common cause of CRD failure) before replacing the CRD with a new one.

[icchlorine]

Thanks Chris for the tips.
I bypassed the CRD and the tailing was reduced quite a bit and my measurements were more accurate. So yeah, the problem has been the CRD. I am trying to see how much I can get along without it, but will probably have to order a new one eventually (which is a pity as they are not as cheap as I hoped).

I also have a strange peak pooping up between the fluoride peak and the water dip. It went away initially after I bypassed the CRD, but in a subsequent run it reappeared. My lab uses 18.2 MΩ water with Barnstead and Milli-Q systems. Any ideas?

I hope I do not turn this into a never-ending thread, but this machine has presented a number of problems and this is my fist experience with ion chromatography.

[Chris Pohl]

Well, at least you identified the cause of the problem. I can't do anything about the price of the CRD but you should be able to press for a warranty replacement even if it's outside the warranty period if you haven't used it much.

There are relatively few analytes which elute before fluoride. This consists mainly of polyolcarboxylates such as quinate or low molecular weight alkylphosphonates such as ethylphosphonate. I'm pretty sure you don't have any alkylphosphonate in your samples since these are derived almost exclusively from nerve gas but polyolcarboxylates are fairly common in natural samples. It's possible that the ability to detect the mystery peak is dependent upon the background and that the higher background associated with working without the CRD is at least partially masking the artifact. Did you notice any difference in the background between the conditions under which you could see the artifact in the conditions where the artifact was absent?

[mty]

Hi There! I don't want to hijack the thread, but what if I've got tailing in a KOH eluent? I was all excited to find this thread, but I don't have a CRD. I do have a CR-TC...is that similar? And could that be an issue? I'm running fluoroacetate and perchlorate on an AS21 column, starting at 15mM KOH and ramping to 30mM KOH. I tried ramping to 50 mM KOH, to see if that would sharpen the peaks, but it doesn't.
Thanks for any help you might be able to give me!
mty

[M Farooq]

Hi There! I don't want to hijack the thread, but what if I've got tailing in a KOH eluent? I was all excited to find this thread, but I don't have a CRD. I do have a CR-TC...is that similar? And could that be an issue? I'm running fluoroacetate and perchlorate on an AS21 column, starting at 15mM KOH and ramping to 30mM KOH. I tried ramping to 50 mM KOH, to see if that would sharpen the peaks, but it doesn't.
Thanks for any help you might be able to give me!
mty

You can easily track the causes of tailing, due to the hardware, by testing the early eluting peaks such as fluoride and chloride. If these peaks are excessively tailed, focus on fixing the extra-column effects. An old suppressor can also lead to tailing provided it is behaving the same with every column. An aged, not-so-well-treated suppressor shows a wide water dip which comes out later than the normal dead time of the column. As a result, all peaks are tailed. As alluded above, the general cause is excessive backpressure which sort of increases the internal volume of the suppressor. As far as I know, eluent concentration and CR-TC have nothing to do with the peak shape under normal trace conditions in IC (assuming you are not overloading the column).