This technical article details the following HPLC topics:
Columns for HPLC method developers:
Simply screen these four columns in your desired mobile phase, using your preferred column.
To simplify the transferring of your HPLC method, see our HPLC Method Transfer Calculator.
Typically, Ascentis C18 is the first choice for starting a new method. However, when a C18 doesn’t give the desired separation or your sample contains compounds that are known to be difficult to retain or resolve on a C18, consider changing the stationary phase. The range of selectivity provided by Ascentis and Discovery phases makes this easy. The flow chart below helps guide the selection of Ascentis or Discovery phase based on the particular compound type or separation challenge.
The column selection guide below gives recommendations for improving retention or resolution, based on compound class and separation challenge on C18.
A partial list of common buffers and their corresponding useful pH range is supplied. Perhaps the most common buffer in HPLC is the phosphate ion. Although, with the growth of LC-MS, volatile buffers such as TFA, acetate, formate, and ammonia are becoming more frequently used. Remember, the purpose of a buffer in the mobile phase is to inhibit a pH change in the mobile phase after the introduction of a sample. When developing a method, it is important to select a mobile phase with a final pH at least one pH unit away from any analytes pK value. As a rule of thumb, one should work within a ±1 pH unit of the buffer pKa. Typical buffer concentrations for HPLC tend to be 10-100 millimolar level.
Slight variations in pH and buffer concentration could have a dramatic affect on the chromatographic process; consistent and specific techniques should be a regular practice in the preparation of mobile phases. A common practice is to place a sufficient amount of pure water into a volumetric flask and add an accurate amount of buffer. The pH of the solution should be adjusted, if necessary, and then dilute to final volume of water prior to adding or blending of organic solvents. Then, add a volumetrically measured amount of organic solvent to obtain the final mobile phase. Thorough blending, degassing, and filtering prior to use is also recommended.
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