Why OPC or OPA is used to adjust pH of buffer solutions in High performance Liquid Chromatography

 In High-Performance Liquid Chromatography (HPLC), the pH of the mobile phase is a critical parameter that can significantly impact the separation and detection of analytes. pH adjustments are often required to optimize the chromatographic conditions for specific analytes, as their ionization state and hydrophobicity can vary with pH.

One of the common methods to adjust the pH in HPLC is by using an Organic pH Modifier or an Organic Phase Modifier, commonly known as OPC (Organic Phase Control) or OPA (Organic Phase Adjuster). OPC is a small organic molecule that is added to the mobile phase to achieve the desired pH.

Here's why OPC is used to adjust pH in HPLC:

1. Compatibility with Organic Solvents: HPLC often uses organic solvents as the mobile phase, which is essential for the separation of non-polar and semi-polar compounds. Traditional pH adjustment using inorganic acids or bases may not be compatible with organic solvents, leading to precipitation or phase separation. OPCs are carefully selected organic molecules that can effectively change the pH of the mobile phase without causing any undesirable phase separation.

2. Wide pH Range: OPCs are available in a wide range of molecular structures, allowing for pH adjustments over a broad range, typically from acidic to basic conditions. This versatility is crucial for accommodating the separation of a diverse range of analytes with varying pH sensitivities.

3. Ionization Control: The ionization state of analytes can significantly influence their retention time and chromatographic behavior. By adjusting the pH with OPC, one can control the ionization of acidic or basic analytes, which can enhance the chromatographic separation and improve peak shapes.

4. Improved Separation: OPC can help in fine-tuning the selectivity of the HPLC separation. In certain cases, minor pH adjustments with OPC can lead to better resolution between closely eluting peaks.

5. Reproducibility: OPCs are usually stable and well-defined, ensuring consistent pH adjustments, which is crucial for achieving reproducible chromatographic results.

It's important to select the appropriate OPC for the specific HPLC method, taking into account the nature of the analytes, the composition of the mobile phase, and the desired separation conditions. The amount of OPC added should be carefully optimized to achieve the desired pH without excessive interference with the chromatographic process.