UV-Visible Spectrophotometer Part 1

UV-Visible Spectrophotometer Instrumentation Part 1

• The instrument is used for analyzing substances based on their interaction with ultraviolet and visible light
• Different components work together to produce a spectral output
• Diagram of Cary Double Beam UV-Visible Spectrometer shows the optical layout
• Key components include mirrors, a grating, an entrance slit, a blocking filter, a light source, a sample cell, a reference cell, a half-silvered mirror, and a detector
• A diagram shows how light travels through the instrument, including the paths of the light through the sample and reference cells, and how the two beams are combined and measured

Radiation Sources

• Radiation source requirements include sufficient energy over the entire spectral region and a stable constant power
• Maintaining a stable radiant energy source (P_o) is vital for accurate measurements.
• Double beam designs help maintain a constant power source by comparing the intensities of the two paths
• Regulation of the power source is necessary to ensure a stable and consistent energy output for accurate measurements

Categories of Radiation Sources

• Line sources emit specific wavelengths unique to the element forming the cathode.
• Continuous sources emit a wide range of wavelengths at a constant power.
• Examples of continuous sources include tungsten/halogen lamps and deuterium lamps, which serve specific wavelength ranges.

Tungsten/Halogen Lamp

• Design features include a sealed quartz envelope, evacuated bulb, thin coiled tungsten filament, and small amount of iodine vapor.
• Operation involves applying current to heat the filament to high temperature until it glows.
• Po is proportional to filament temperature, which is proportional to the current flow
• The light from the lamp provides a continuous spectrum of wavelengths in the UV/visible/IR regions.

Tungsten/Halogen Lamp - Filament Reactions

• Reactions that occur within the filament during operation
• Sublimation (W_(s) → W_(g))
• Formation of tungsten oxyhalide complexes
• Redeposition (W_(l) → W_(s))

Tungsten/Halogen Lamp - Advantages

• Longer operational lifetime compare to other similar lamps: 10,000 h vs 1,000 h.
• Operation at higher temperatures; 2000 to 3300 K
• Shifts spectral response to the ultraviolet region

Deuterium Lamp

• UV source 190-350 nm
• Operation involves a current-regulated power source providing 100 W of power, mechanical aperture between anode and cathode to constrain the discharge path to a narrow area.
• Lifetime of approximately 2000 hours
• Reactions include electrical excitation and subsequent emission of photons.

Lenses

• Optical components, typically made of quartz or silica to ensure high transmission rates.
• Designed with two or more refractive surfaces
• Function: collimating light, focusing light to point, collecting radiation and condensing radiation to a point

Front Faced (Concave) Mirrors

• Made with vacuum-coated aluminum or other reflective metals on highly polished substrates (like glass).
• Surfaces are sometimes further coated to prevent oxidation
• Function: collimating radiation to parallel beams, steering/changing direction of radiation, and focusing radiation to a point

Wavelength Selection

• Selects a narrow band of radiation suitable for analyte absorption/emission
• Filters and monochromators are commonly used to select specific wavelengths.
• Filters are simpler and cheaper but may not completely isolate target wavelengths.
• Monochromators provide more effective isolation of wavelengths for more precise and accurate measurements
• Czerny-Turner monochromators use a diffraction grating.

Czerny-Turner Monochromator

• An optical system for dispersing polychromatic radiation into constituent wavelengths.
• Components: entrance slit, collimating mirror, diffraction grating, focusing mirror, and exit slit.

Czerny-Turner Monochromator Components

• Entrance slit manages incoming, polychromatic radiant energy
• Collimating mirror directs incoming light to the grating
• Diffraction grating disperses incoming light
• Focusing mirror focuses light from the grating onto the exit slit.
• Exit slit restricts light passing through, depending on the desired bandwidth, creating a narrow band of light

Slits

• Used to isolate a section of source radiation
• Carefully machined metal with sharp edges
• Slit jaws must be parallel and precisely aligned
• Entrance slit determines the light entering; Exit slit determines light exiting.

Entrance Slit

• Receives continuous polychromatic radiation
• Dictates the light intensity

Diffraction Grating

• Consists of a mirrored surface with precisely spaced parallel grooves.
• Used to separate constituent wavelengths in a light source.
• Grating is typically made from glass or similar material
• UV-visible spectrometers often use 1200-1400 grooves/mm gratings

Diffraction Grating Surface

• Precision manufacturing of the grating surface (groove measurements) is essential for efficient dispersion

Diffraction Grating

• Polychromatic light incident on the grating is dispersed into different wavelengths based on diffraction angle.

Operation of Diffraction Grating

• Constructive interference results in reinforced monochromatic beams
• Polychromatic radiation is dispersed into individual wavelengths

Exit Slit

• Isolates a narrow band of dispersed radiation
• Spectral bandpass depends on grating dispersion, exit slit width and position.
• Individual wavelengths are projected onto the exit slit by the focusing mirror

Effective Bandwidth

• Impossible for monochromators to fully isolate spectrally pure wavelengths.
• This bandwidth depends on the characteristics of the grating, exit slit, and the configuration of the monochromator
• A range of A's (wavelengths) passes through the exit slit.

Slit Width Determination

• Adjusting slit width affects the radiant power and wavelength resolution.
• A trade-off exists between wavelength resolution and light intensity.
• A narrower slit yields greater resolution but less light intensity
• Increasing effective bandwidth improves resolution

Summary of Monochromator Operation

• Video of a tunable monochromator is recommended for further study.

Explore the fundamentals of UV-Visible spectrophotometer instrumentation in this quiz. Learn about its key components, optical layout, and the importance of stable radiation sources for accurate measurements. Dive into the features that enable precise analysis of substances interacting with ultraviolet and visible light.