Chromatography Forum

Could anyone provide guidance on what phosphate salts (type, manufacturer, specification) provide the cleanest baselines please?

We often use potassium phosphate for method development at low and mid pH but very often see system peaks at the higher organic (typically acetonitrile) end of our gradients which can interfere with quantification of sample. We never go above 80% v/v so I don't believe it is precipitating out. We also use ultra pure water and have checked that this isn't the source of the contamination.

Any comments would be very welcome, particularly if anyone has found 'cleaner' mobile phases can be produced by titrating phosphoric acid (not tried this yet).

What wavelenghts and buffer concentrations are you using?

Typically solid buffer concentration is 10-25 mM (method dependent) prior to pH adjustment. Typical wavelengths used are in the range 210-230 nm (obviously compound dependent).

We use VWR or Fisher HPLC grade phosphate and don't have any significant problems. We do still see solvent peaks though. You are right that at 10 to 25 mM it is unlikely that it will be precipitating. I suspect you are seeing 'crud' washing off the column or from the HPLC system and that flushing the system at 90% acetonitrile until you have a stable baseline before starting your sequence will minimise the problem.

Here is a trick we have used in the past to get cleaner buffers...make a small amount of concentrated buffer and pass it over a reversed phase SPE syringe cartridge, then dilute it with water to make up your mobile phase.

Are you sure about your water quality?
You can buy the cleanest phosphate you want, but if your water quality isn't ok....

We also used SPE for cleanup in the past, but we had to rinse first with methanol and afterwards with water, to clean the cartridge first, otherwise we got more peaks than we started with.


Ace