Detection limit for antibiotics by using UV-Vis

[Emi]

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

[aceto_81]

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

[MaryCarson]

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.