You can use additives such as trifluoroacetic acid or formic acid to improve the resolution. A complete range of high purity LC-MS additives is available in bottles or ampoules.

Ion pair reagents rely upon the addition of ionic compounds to the mobile phase to promote the formation of ion pairs with charged analytes. These reagents are comprised of an alkyl chain with an ionisable terminus. When used with common hydrophobic HPLC phases in the reversed phase mode, ion pair reagents can be used to selectively increase the retention of charged analytes.

• Transition of the sample from an autosampler to an instrument, especially when impurities are absorbed to the inside and outside surfaces of the needle dipped into the vial
• If an impurity in the sample has the same absorption wavelength as the compound of interest
• The water used in the mobile phase is not pure or fresh enough
• Old mobile phase
• Poor quality buffers or additives added in the mobile phase
• Phthalates or plastic additives contained in the plastic containers used during mobile phase preparation

• Try to use pre-mixed solvents
• Use fresh eluants with salts
• Filter your HPLC solvents
• Use high quality water (18,2 mOhm, TOC<5 ppb) or gradient grade water
• Use good quality buffers and salts
• Control glassware cleaning

When samples contain ionizable compounds, mobile phase pH can be one of the most variable in the control of retention in reversed phase RP HPLC separation. However, if it is not controlled properly, pH can be a source of many problems. Since most compounds analyzed by RP HPLC contain one or more acidic or basic functional groups, most mobile phases require pH control. For this reason, buffers are widely used.