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8.4.4: UV - Vis (and Near IR) Instruments with Double Dispersion

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    The most expensive instruments for absorption spectroscopy in the UV and visible regions of the spectrum contain two monochromators to better reduce stray light.  These instruments which cost approximately $60k are coveted by researchers studying highly absorbing materials such as Heme containing proteins.  Also, because of the cost of these instruments, manufacturers often extend their usefulness further into the near IR (3200 nm) by incorporating a lead sulfide detector along with the photomultiplier detector (PMT) . 

    A general layout of a UV-vis spectrophotometer is shown below in Figure \(\PageIndex{1}\).

    double monochromator.jpg

    Figure \(\PageIndex{1}\). The general optical layout of an instrument with double dispersion.


    As shown in Figure \(\PageIndex{1}\), light captured from the source is filtered to select a narrow band of wavelengths by the first 0.140 m monochromator.  The light in the narrow band of wavelengths is dispersed again in the second, 0.278 m, monochromator.  This double dispersion helps to halve the contributions from stray light to the detector signal and extend the Beer's Law limit of linearity to greater than 6 absorbance units.

    8.4.4: UV - Vis (and Near IR) Instruments with Double Dispersion is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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