What are some mobile phase considerations when using UV detection?

With any separation mode it’s important to use low UV absorbing modifiers, especially when analytes have low UV absorbance.

With RP separations, phosphoric acid is commonly used to acidify mobile phase as it has a very low UV absorbance, especially in comparison to organic acids such as TFA or formic acid. Dibasic phosphate buffers are also used to maintain pH near neutral. 

Note that buffers some buffers with basic pH and low UV absorbance, such as triphosphate or triethylamine, are outside of the workable pH range of some HPLC materials.

With HILIC methods, dilute sulfuric acid (0.005 N) has been used as it is also a low UV absorber and offers unique selectivity. In addition, steep gradients where the organic component of the mobile phase is changed rapidly can result in an unstable baseline and should be avoided.