Calibration Curve Method (Determination of Concentration by Calibration Curve - Introduction)

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• Calibration curve method (intensity method) introduction
  • Concentration measurement - Calibration curve -

    　There are not many handy devices that allow you to feed an environmental sample into an analyzer and automatically finish the measurement and output a concentration value. Or, such devices are not very reliable*. So, how do you convert measurement results into concentration? The most common method is the intensity method using calibration curves.

    * If the performance of the instrument changes little over time, a calibration curve can be created in advance and stored in the instrument. Then, the concentration value can be automatically output from the signal intensity.

    
    

    Intensity method

    　Prepare several standard samples of known concentration (see figure below), measure them, and obtain the signal intensity. A calibration curve is created by plotting the concentration of the standard samples on the vertical axis and the signal intensity on the horizontal axis. The signal intensity obtained from the measurement of the unknown sample is then converted to concentration by substituting it into the regression equation of the calibration curve.
    
    

    
    

    (Calculation methods and exercises will be covered in the next course.)

    Reference book：Introduction to Environmental Analytical Chemistry for the Future (Edited by OGUMA Koichi et al., Kodansha Scientific)
    

  • 　As shown in the picture above, several types of standard samples with known concentrations are prepared. These are analyzed to obtain the signal strength (analysis result). The analysis result is not obtained as a concentration value. They are obtained as "signal intensities" detected by the analyzer. In the example below, the signal intensity of nitrate ion (NO3-) was obtained by ion chromatography as the "peak height" of the chromatogram, and plotted as signal intensity on the horizontal axis and concentration on the vertical axis. The results of the standard sample measurement were plotted and the regression curve (straight line) is represented by a light pink thick line. This is the calibration curve.
    

    

    
    

    　Next, the unknown sample is analyzed and the obtained signal intensity is fitted to a calibration curve graph. The value on the vertical axis is the concentration of the unknown sample (calculated value).
    
    　This method of measuring a standard sample, drawing a calibration curve, and calculating the concentration by fitting the signal intensity of the unknown sample to the calibration curve is called the "calibration curve method" or "intensity method".