A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials.
1. Sources of Radiation:
The source in an IR spectrophotometer provides the necessary IR radiation which will pass through the sample.
The source should be stable, have a long lifetime, and cover the desired wavelength range.
A. Globar:
It's a silicon carbide rod which, when electrically heated, emits radiation in the mid-IR region. Its spectral radiance is nearly constant over this range.
B. Nernst Glower:
Made of rare earth oxides, this source has a spectral radiance that increases with wavelength.
It's a ceramic rod which, when electrically heated, emits radiation in the mid-IR range.
C. Tungsten Filament:
This is generally used for the near-IR region. It emits over a broad range, covering both the visible and NIR region.
D. Mercury Arc Lamp:
This is useful for the far-IR region.
2. Wavelength Selectors:
These devices separate the broad spectrum emitted by the source into individual frequencies, allowing for the sequential or specific analysis of each frequency.
A. Monochromators:
These use prisms or gratings to disperse radiation.
As the radiation passes through, different wavelengths are bent by different amounts, effectively separating them.
B. Interferometers:
Used in Fourier Transform Infrared (FTIR) Spectrometers.
They split the radiation beam into two, reflecting them off mirrors, and then recombine them.
The resulting interference pattern (interferogram) contains information about all wavelengths at once, which is then Fourier transformed to give the spectrum.
3. Detectors:
Detectors are devices that convert the IR radiation, which is transmitted or reflected by the sample, into an electrical signal.
A) Golay Cell (Pneumatic Detector):
It's a thermal detector that operates on the principle of constant volume gas thermometry.
It consists of a closed-ended, mirrored cylinder with a diaphragm at the end. The chamber is filled with a gas, and IR radiation heats the gas, causing a pressure change that moves the diaphragm. This movement is detected and converted into an electrical signal.
Application: Widely used in FTIR due to its wide frequency range and fast response.
B) Bolometer:
It employs a blackened platinum or gold strip maintained at constant temperature.
When it absorbs IR radiation, the temperature rises and resistance changes.
Application: Used for detecting far-IR radiation because of its high sensitivity in this region.
C) Thermocouple:
It consists of two different metals joined together. When one junction is heated by IR radiation, a small voltage is produced.
Application: Suitable for the mid-IR range due to its moderate sensitivity and wide frequency response.
D) Thermistor (Thermal Resistor):
Made from semiconducting materials whose resistance changes significantly with temperature. When IR radiation heats it, this resistance change is measured.
Application: Used in the mid-IR range, it provides higher sensitivity than thermocouples.
E) Pyroelectric Detector (e.g., DTGS - Deuterated Triglycine Sulfate):
These detectors generate a voltage when they are heated or cooled. When IR radiation falls on the detector, it heats up and produces a voltage proportional to the change in temperature.
Application: Widely used in FTIR instruments due to their broad spectral response and compatibility with room temperature operation.
