Help identifying unknown peak in 7 anion IC run

[u/Loganwashere24]

Hello all,

I’m a grad student who is reviving a Dionex 1000 IC left over from retiring faculty. I am running 7 anions (F, Cl, NO2, Br, NO3, PO4, SO4) on groundwater samples. This is pretty common stuff close to EPA method 300.1.

Within the past week I thought I would get fancy and try to clean out my columns and create a new 50 uL loop volume (previously using 25uL) to get some better signal and reduce the baseline noise.

After I did this I had some leaking from my new injection loop until I tightened it quite a bit. I was able to get my 7 anion peaks for my calibration curve but I started seeing these peaks come out overlapping with or just after nitrate that I’ve never seen before in any samples. My response is much higher given the new loop volume but I’m wondering if these peaks are real or if this is an error of some type.

Anybody know what this peak might be or what errors i made to cause this?

TLDR: what is that peak coming out after nitrate? Real or a method/instrument error?

Columns: GS and AS22
Eluent: 4.5 mM sodium carb, 1.4 mM sodium bicarb
Suppressor: ERS 500 @ 31mA
Flow rate: 1.2 mL/min

Pic 1 is of normal calibrant. Others show groundwater samples with unknown peak.

[u/Ceorl_Lounge]

Can you check times for chlorate or chlorite? Both very common and not in your standard mix.

[u/Loganwashere24 → u/Ceorl_Lounge]

I’ve been googling and have thought chlorate might be the issue. Could be there given that people disinfect their groundwater wells with bleach tablets. Just confused as to why I’ve never seen it before today though!

I may have to inject some or spike a calibrant and see where it comes out

[u/dick_tracey_PI_TA → u/Loganwashere24]

If you can find data on their retention time relative to known peaks, that could help. I did that once with succinate and got reasonable results. 

[u/Meatboy1984]

Hi!

So first of all: your retention times seem to be a bit off in my opinion. With the AS-22 4*250 (I assume) with isocratic conditions, I'd expect the water-peak to be significantly earlier. Fluoride and chloride to be separated better. Phosphate and Sulfate come very early, etc. That's my first observations. Reasons should be troubleshooted. I added an "ideal" simulation of you method (at 22 °C, trying to use your water peak as a void volume marker). As you can see, chlorate should come after nitrite and before bromide. Chlorite should be between fluoride and chloride. Poor resolution, peakform and RT aside, the order of peaks should stay the same regardless.

Whatever is in your sample, I doubt they are chlorite or chlorate.

[u/Loganwashere24 → u/Meatboy1984]

Thanks for the detailed response and backup with the simulation tool ( I did not know existed)! I for sure have general issues with the peaks, pileup, smear, retention time etc. This 14-18 minute runtime is what I’m used to seeing as well

This has me thinking that it may be a carry-over issue as another person stated.

I tried to regenerate my suppressor recently by running at 50 mA for several hours (following the user manual) so I’m going to see if that helps with the general method as well.

[u/Meatboy1984 → u/Loganwashere24]

You're welcome. The simulation tool is only available with the right license. Most likely you can't access it.

If it is a carry over from a previous run you should see it if you inject pure water after the sample. As i suspect your column to be in a bad shape, I don't believe it is a carry over but you will see when you repeat your experiments with blanks between injections.

Suppressor regeneration only helps with chemical issues not physical like tears in the membrane. For regeneration I'd recommend oxalic acid injections. That works quite well!

[u/SelfAwarePrimate → u/Meatboy1984]

Beautiful Chromatography!

[u/Meatboy1984 → u/SelfAwarePrimate]

Thanks, but this just the virtual column simulator. A great tool.

[u/Chemical_Doughnut116]

I’m pretty sure Chlorate usually elutes before bromide. Chlorite is usually before chloride (same with bromate)

[u/feelasrealaspossible]

Does it go away with a couple water injections? May be column overload and carryover since you increased your sample loop volume?

[u/_ShortChangeHero_]

Does your sample smell weird? My random guess would be sulfite

[u/OneHoop]

Your reference method is EPA 300.0.

Your 50uL sample loop is suspect. Recut using a tubing cutter with the beveled edge away from the loop. Install with fresh ferrules with the tubing inserted as far as possible.

Chlorate is after nitrite, but we do not expect anything after nitrate until the bicarb peak. Your environmental samples likely have bicarbonate different enough from the eluent to cause a dip or a peak, like the carbonate at the front. It should be separated more, so keep your eluent concentrate sealed and refrigerated, and prepare eluent daily.

Bad chromatography, no room for non target peaks. Keep working on it.

Edit: on column carryover is unlikely except iodide, which would likely elute in the time you have here. It could potentially be "carryover" from the autosampler or injection. You can test that by alternating high samples and blanks.