The fluorimetric assay of quinine is based on the principle that quinine exhibits natural fluorescence when exposed to ultraviolet (UV) light. The intensity of this fluorescence is directly proportional to the concentration of quinine in a sample, making it suitable for quantitative analysis. Here's an overview of the process:


Principle:

Quinine, when dissolved in an acidic medium (typically sulfuric acid), emits fluorescence when exposed to UV light (typically around 350-360 nm). The emitted fluorescence is measured at a specific wavelength (often around 450-470 nm). The intensity of the fluorescence is used to quantify the concentration of quinine.

Procedure:

  1. Sample Preparation:

    • A known quantity of quinine is dissolved in a dilute sulfuric acid solution. This acidic environment enhances the fluorescence of quinine.
    • Prepare a series of standard quinine solutions with known concentrations to create a calibration curve.

  2. Fluorimetric Measurement:

    • The samples are excited with UV light at a specific excitation wavelength (350-360 nm).
    • Measure the fluorescence intensity at the emission wavelength (450-470 nm) using a fluorimeter.

  3. Calibration Curve:

    • A calibration curve is plotted by measuring the fluorescence intensity of the standard quinine solutions and plotting their intensities against their known concentrations.

  4. Sample Analysis:

    • The fluorescence intensity of the unknown sample is measured, and its concentration is determined by comparing the fluorescence with the calibration curve.

Factors Affecting Fluorescence:

  • pH of the solution: The fluorescence of quinine is strongly influenced by pH, with acidic conditions (sulfuric acid) enhancing its fluorescence.
  • Presence of other substances: Certain substances can quench or enhance fluorescence, which may lead to inaccurate results.
  • Temperature: Changes in temperature can affect the fluorescence intensity, so measurements should be done under controlled conditions.

Applications:

This method is commonly used for the determination of quinine in pharmaceuticals, beverages like tonic water, and biological samples. It's a sensitive method and can detect quinine even at low concentrations.