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7.1: General Instrument Designs

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  • The two most important spectroscopic methods in analytical chemistry are based on the absorption of emission of light.

    Instruments for absorption spectroscopy are of two types: single beam and double beam.  A shown in Figure 1(a), in a single beam instrument light passes through the sample from the source to the photoelectric transducer (detector).  Absorbance values are based on first measuring the transmittance of light through a sample containing an absorbing species, then switching the sample for a reference sample (a blank), and then measuring the transmittance through the reference sample.  In a double beam instrument and as shown in Figure 1 (b), the light path alternates between passing through the sample with absorbing species and a reference sample.


    Figure \(\PageIndex{1}\):Block diagrams for (a) a single beam instrument and (b) a double beam instrument for absorption spectroscopy

    Instruments for fluorescence spectroscopy are based on measurements of the intensity of emitted light following excitation of the sample with light of a shorter wavelength.  As shown in Figure 2, the emitted light is collected at an angle perpendicular to the path of the excitation light.


    Figure \(\PageIndex{2}\):Block diagram for an instrument for fluorescence spectroscopy.

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