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Method for determining concentration in a solution using attenuated total reflectance spectrometry

  • US 5,416,579 A
  • Filed: 07/21/1994
  • Issued: 05/16/1995
  • Est. Priority Date: 07/23/1993
  • Status: Expired due to Fees
First Claim
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1. A method of using an attenuated total reflectance (ATR) spectrophotometer having an optical wave guide with a curved optical path to determine concentration of solute in a test fluid of unknown concentration, comprising the steps of:

  • a) providing a plurality of optical wave guides for said ATR spectrophotometer having differing values of D, where D is the ratio of the bend radius of said optical wave guide to the radius of said optical wave guide, each of said plurality of wave guides having an index of refraction greater than that of said fluid of unknown concentration, and each of said wave guides having a characteristic effective path (Peff) and a characteristic number of internal reflections (z) for light of a given wavelength;

    b) providing a plurality of calibration fluids comprising the materials of said solute and test fluid at differing known concentrations of solute (C);

    c) determining values of transmittance (T) in said ATR spectrophotometer when combining each of said known concentrations with each of said optical wave guides having differing values of D;

    d) calculating absorbance units (AU) for each of said combinations of C and D according to the equation;

    
    
    space="preserve" listing-type="equation">AU=-log(T.sup.1/z);

    e) selecting a wavelength of light (λ

    ) for said determination;

    f) measuring the index of refraction of said test fluid at said selected wavelength of light;

    g) selecting an operating range of said concentration values to be determined by said ATR spectrophotometer;

    h) selecting an optical wave guide for said ATR spectrophotometer having a D value (Dtest) which provides AU values in the working range of sensitivities of said ATR spectrophotometer over said operating range of said concentration values;

    i) immersing said selected optical wave guide having said Dtest value in said fluid of unknown concentration;

    j) determining the AU value for said fluid; and

    k) comparing said AU value for said fluid to said values of AU determined for said calibration fluids with said element having said Dtest value to determine the value of said solute concentration in said fluid of unknown concentration.

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