Chromatography Forum

Hi,

I am a PhD student and I am very grateful for your help in the forum when I posted a question regarding using CAD to detect Oxytetracycline and you did help me and provide me with a method using UV and I have tried it but instead of using methanol ,I am using 15 % acetonitril and 85 % of 0.1 phosphoric acid at 355 nm and I got weird results ; at 0.1 ppm the area looks small but = 6.9 on the other hand for 0.5 ppm the area looks bigger but = 3.9 and for the concentrations 1 , 5 , 10 ppm it is logic that is why I think 0.1 ppm is the saturation point for the instrument ( I have no idea)
so would you please help me with that and I would like also to know what is the detection limit of UV/VIS for Oxytetracycline and Sulfachloropyridizine.

Thank you very much in advance

Emi

Detection limit depends on signal to noise ratio, this is system dependend.
So if I give you a detection limit of 0.001 ppm, then this would tell.... nothing.

Can't you provide us with chroms, so that we can see what is going on?

Ace

We need to know your injection volume as well as model of UV/vis detector. I used to have a detection limit for OTC near 0.001 ppm with an older detector at 355 nm, but I was injecting 600 uL on a gradient system. So detection limit was near 0.6 ng OTC on-column. Depending on column, you may not get good peak shape for OTC using a phosphate buffer-based mobile phase. I am also not sure that 15% ACN will be enough to pull it off.

I didn't think 355nm was an optimal wavelength for SCP. I thought you need to go lower, like 265nm.

If your peaks look small,
1) are you sure you are looking at the right peak;
2) change your scale.