indirect UV for sulphate ions

[aceto_81]

Hi,

I'm trying to detect sulphate ions through indirect UV.
My mobile phase is 4g sodium tetraborate + 50 mg ftalic acid in 1000ml water, brought at a pH of 6.0 with HCl, running at a flow of 1.0ml/min.
Column is a Hypersil SAX column 250x 4.6 5µm.

I used 1mg/ml Na2SO4 as a reference, but couldn't detect any peaks.
(Except at 3 min, but it seems that's water)

I have no experience with ion chrom or indirect UV, only reversed phase, so I guess I need a little help.

Thanks
Ace

[DJ]

What wavelength are you monitoring?

[aceto_81]

Hi,

I'm on a PDA, so I'm scanning from 200 - 400nm.
But I think I found 2 peaks, one at 30 minutes, and a negative one at 55 minutes.
After increasing my borate concentration, the peak retention time decrease.

Both peak areas seem to increase and decrease with the sodium sulphate concentration.
But I think I can't see a "normal" positive peak for sulphate while using indirect UV?

How is this possible?

Ace

[tom jupille]

But I think I can't see a "normal" positive peak for sulphate while using indirect UV

No, but you *can* see a "system peak" (which would be positive in this case) for phthalate that is displaced by the injection process.

[Markus Laeubli, Metrohm]

Dear Aceto_81

You found all you need. Tom is right with the system peak, this is the positive peak which you found. But the second, negative peak is your sulfate.

As you are running indirect UV, your sulfate peak will be negative.
The reason is quite simple. Your eluting anion is phthalate with a high absorbance. In the ion-exchange process on the column some of the phthalate ions are replace by sulfate ions (no absorbance). Therfore the signal will be reduced.

I would recommend to use an eluent with phthalate only and use KOH for pH setting.

[aceto_81]

Dear Aceto_81

I would recommend to use an eluent with phthalate only and use KOH for pH setting.

I'm currently running on an Hypersil SAX column, what eluent would you suggest?
Something along the lines of 50mg/l phtalate @ pH 6.0 with KOH?
No buffer at all?


Thanks for your comments

Ace

[Markus Laeubli, Metrohm]

I would start with e.g. 2 mmol/L at pH = 5.

Phthalic acid itself is a buffering component. Any addition of a further buffer would influence the separation.
The column you use is quite high in capacity. I do not have experience with this type, you might need to adjust the eluent concentration. Also run other ion which mitght be in your sample to make sure that you do not get co-elution.

[aceto_81]

Thanks for your quick reply, I will try this out this day and I will keep you informed!

Thanks

Ace

[aceto_81]

Markus,

thanks for your help.
At pH 5, I had a runtime of about 45 minutes, and coelution between other components of my ointment.
But now at pH 6.4, everything works fine!
Today is the big testing day with lots of repetitions, blancs, spiking, linearity, ... so hopefully it turns out great!

Thanks to all for your input!

Ace

[Markus Laeubli, Metrohm]

Sounds good. At pH 6.4 of the phthalate is in the divalent form. This speed up sulfate more than other monovalent anions.

Good luck!