How a Simple UV-visible Spectrophotometer Works

Professor Davis explains the main components of a UV-visible spectrophotometer schematic, including the source lamp, monochromator, beam splitter, sample compartment, and detectors. Each component's function in separating and measuring light wavelengths is described.

The source lamp is turned on to produce multiple wavelengths of light. The light passes through the monochromator's first slit and prism, which refracts the light into a spectrum so that only a single wavelength passes through the second slit to the beam splitter.

The beam splitter divides the selected wavelength into two equal-intensity beams that pass through reference and sample cells. With no absorbing sample present, both detectors receive identical light intensity, resulting in 100% transmittance.

An absorbing sample is added to the sample cell, causing the light intensity exiting that cell to decrease. The detector current also decreases, and the transmittance drops from 100% to 50% at concentration X.

Additional equivalents of sample are added incrementally to the sample cell. Each addition reduces light intensity by exactly half, demonstrating an exponential relationship between concentration and transmittance rather than a linear one.

Professor Davis explains that the exponential transmittance-concentration relationship is converted to a linear relationship using the Beer-Lambert Law. Taking the negative logarithm of transmittance yields absorbance, which plots linearly against concentration.