Infrared Spectrometers

An infrared spectrometer (IR spectrometer) is an instrument that measures the frequencies of infrared light absorbed by a compound. It then generates an infrared spectrum (IR spectrum). The IR spectrometer detects how a molecule's absorption of energy varies with wave number.

The wave number (cm^–1) is the number of wavelengths of the wave in a centimeter or the reciprocal of the wavelength (in centimeters). The wave number is proportional to the energy/frequency of the vibration of the bonds. Infrared light causes covalent bonds to vibrate based on the stiffness of the bond and atomic size. The wave number is proportional to the energy required to make covalent bonds vibrate. A downward stretch on the spectrum represents absorption at a specific wave number; because the absorption of energy corresponds to specific values, it is possible to identify various functional groups within the molecule.

All IR spectrometers measure how much light a sample absorbs at each wavelength. Accordingly, IR spectrometers use the same general setup: a radiation source (infrared radiation), a sample, a reference, and a detector to analyze the signal.

The spectrometers can be divided into two categories: monochromatic radiation absorption and Fourier transform. The first category uses as the source either a broadband radiator emitter followed by a monochromator (a device that will transmit only a narrow frequency of light from a broader source of light) or a generator that can be tuned to a specific, single frequency. The light source is split so that one beam of light passes through a reference cell and the other beam of light passes through the sample cell. IR software will subtract any stretches that appear in the reference cell (such as nitrogen from the air) from the final spectra so that it only shows stretches attributable to the sample. Next, a chopper modulates the intensity of the light to increase the signal noise ratio. Finally, a monochromator will only allow a specific, narrow signal to pass into a detector that has been designed to sense that specific frequency range. The monochromator contains a diffraction grating that separates light by wavelength. If multiple frequency ranges need to be tested, then the sample must be scanned at each individual frequency range. Fourier transform spectrometers also use a broadband source of radiation, but this type of spectrometer passes the radiation through an interferometer. The interferometer is a configuration of mirrors—one of which moves—inside the instrument that periodically blocks and transmits the beam of light from the radiation source. As the mirror moves, it blocks different wavelengths so that each time the beam is transmitted, it is a different spectrum of light. The light reflects off the mirrors and back to the beam splitter where it is recombined to form a single beam composed of light with the same wavelengths but shifted peaks. This light then passes through the sample to the detector where the differences in the parts of the beam produce interference patterns. A computer processes the raw data using the Fourier transform algorithm. Each spectrum of light is considered a data point, and the computer will analyze the data to determine how much light was absorbed by the sample at each wavelength. While the two categories of IR spectrometer seem similar, there is one large difference between the two techniques. In the monochromatic absorption technique, each frequency range must be scanned individually and a spectrum generated for that individual range. In the Fourier transform, all the frequencies can be scanned, and the computer can generate a spectrum based on all the ranges much faster.