Perchlorate Separation

[vockhead]

Hello Everyone,

I just had a question about Ion conductivity detection of perchlorate. I'm using the Hamilton PRP-X110 column, using a 4 mM parahydroxybenzoic acid, 2.5% methanol at a pH of 8.5 via NaOH addition to elute potassium perchlorate samples. USing a Dionex CD25A conductivity detector with temperature compensation on the conductivity cell.

What i'm noticing is that I am getting a strong output signal when trying to detect 15 ppm-ClO4, around 150 mV from our detector, but when trying to get 5 ppm-ClO4, I get nothing, no peak at all, when I would expect a peak around 50 mV. I'm pretty sure I've seen other peaks of heights lower than 50 mV, but I'm going to test other samples like Bromide and Nitrate to see if I can get signals around 50 mV for sure today, but I was just wondering if anyone had any experience where you could see higher concentration peaks by a substantial amount, and then smaller peaks just went missing when they should be easily detectable.

Thanks for any help you can give.

[Markus Laeubli, Metrohm]

I assume that you are not using a suppressor.

Perchlorate is a late eluting ion and the eluent you are using is not very strong. I would expect e pretty high retention time (> 20 min.).
On the other hand it is well known that non-suppressed IC usually shows a system peak (not the injection peak). The system peak is depending on the overal ion concentration in the eluent as well as the eluent itself.
So what you have seen might be that system peak and not perchlorate. An easy test would be injection of tap water. This usually gives a pretty high system peak but does not contain perchlorate in the levels you tested.

[GCMSNoob]

Why are you not using EPA 314.0? I ran that method down to 2ppb.

[vockhead]

We're not using EPA 314 because Dionex AS-16 column is about $2100, and this column was $500 and the budget was pretty tight.

We are getting separation, so I don't think that the column would change all that much.

As for distinguishing the system peak from a perchlorate peak: I've tested with an injection of the eluent and DI and the peak that I'm noticing with perchlorate samples disappears in those blanks, so I'm pretty sure of the peak being perchlorate. I just don't understand why the signal vanishes completely at levels that should be detectable.

[Markus Laeubli, Metrohm]

Another indication that you see the system peak and not perchlorate.