Dispersive Raman typically employs visible lasers, a grating, and CCD detectors (silicon array charged coupled detectors) as a means to collect data. The laser acts as a light source irradiating the sample. Most of the radiation will elastically scatter or "reflect off" the sample at the same energy as the incoming laser radiation, a process known as Rayleigh scattering. However, a small amount, approximately one photon out of a million (0.0001%), will scatter from that sample at a wavelength slightly shifted from the original wavelengths.
FT-IR spectrometers record the interaction of IR radiation with experimental samples, measuring the frequencies at which the sample absorbs the radiation and the intensities of the absorptions. This is a single beam instrument.
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Wavelength range refers to the dispersion of the grating across the linear array; also expressed as the "size" of the spectra on the array.
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Resolution is the width of an analytical peak at half its height expressed in nanometers. It's a measure of the instruments ability to separate two overlapping peaks. At the point of resolution, two peaks of equal height are said to be resolved if you can see a dip between the tops of the two peaks.
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Accuracy is the agreement between a mean measured value and a true or accepted value as quantified by error. Related to determinate errors and quantified by bias.
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A monochromator is a spectrometer optical system capable of measuring a single wavelength, which can be scanned through a wide wavelength range. A common form of monochromator is the Czerny-Turner design, consisting of fixed entrance and exit slits, fixed focusing mirrors and a rotatable diffraction grating. As the grating rotates, a different wavelength is focused onto the exit slit. The wavelength range of a monochromator varies with the choice of grating, but commonly they can scan from 160 nm to 500 nm or ever wider ranges. The spectral resolution depends on the widths of the slits, the choice of grating and focal length, but commonly can be less than 10 pm for high resolution OES.
Filter systems may also be referred to as a polarizer. The filter system uses Near Infrared filters mounted on a temperature controlled filter wheel and controlled by a microprocessor.
Acousto-optical tubable scanning (AOTF) devices, which are usually designed for visible and near IR applications, also overcome the mechanical problems associated with multi-wavelength filter instruments, and likewise, they are based on the use of a single- element detector.
A source of infrared energy illuminates a device called an interferometer. In the interferometer, the infrared energy is split in to two beams, and then optically recombined. In this way the two beams interfere with each other through a constructive / destructive process to produce a complex waveform. This waveform is directed towards the sensor surface of the instrument where it interacts with the molecules of the sample and where the wavelengths are selectively absorbed.
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MCT: Mercury Cadmium Tellurium. HgCdTe is a ternary semiconductor compound which exhibits a wavelength cutoff proportional to the alloy composition. The actual detector is composed of a thin layer (10 to 20 µm) of HgCdTe with metalized contact pads defining the active area. Photons with energy greater than the semiconductor band-gap energy excite electrons into the conduction band, thereby increasing the conductivity of the material.
DTGS: deuterated triglycine sulfate. A very sensitive detector for mid infrared range measurement. Most routine scanning instruments feature the more expensive and more sensitive DTGS detector, which is one or two orders of magnitude more sensitive than lithium tantalate, depending upon implementation. One drawback of the DTGS detector in a process environment is that it stops functioning as a detector at temperatures higher than 41 degrees C; the detector depolarizes above this temperature.
Lithium tantalate detectors are used as low-cost thermal sensing devices (including fire alarms) for which the detector element can cost as little as $10. They are also used in filter-based instruments and in low- cost scanning instruments.
In the long wave spectral region (14.6 to about 7 microns), only photoconductive HgCdTe detectors (PC-CMT) are able to meet the specifications on low noise contribution and electronics bandwidth.
At the shorter wavelengths (7 to 4 microns), photovoltaic HgCdTe detectors (PV-CMT) are the best choice. The detector elements are cooled to about 70 K to reduce their internal noise contribution.
The photoacoustic technique is based on energy transfer from a modulated infrared beam into a mechanical vibration. The incident beam is absorbed by the sample which results in a thermal transfer from the sample to a surrounding inert gas envelope. The gas expands to give small pressure fluctuations detected by a sensitive microphone.
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The panel uses digital keypads or menus for programming.
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A port, or interface, that can be used for serial communication, in which only one bit is transferred at a time. Examples include RS232, RS422, RS485, USB, etc.
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No integral display. Output is read and displayed remotely.
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4-20mA or any other analog current output. Often called a transmitter. A current is imposed on the output circuit proportional to the measurement. Feedback is used to provide the appropriate current regardless of line noise, impedance, etc. Useful when sending signals long distances.
The ''output'' is a change in state of a switch or an alarm.
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Portable spectrometers are modular devices that are designed for ease of transport and assembly. Rugged enclosures, PC interfaces, and customizable software packages are common features for the portable device.
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Is it necessary for the device to withstand and function within hostile or extreme environments, including high or low temperature, humidity, shock, vibration, etc.?
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This is the full required range of ambient operating temperature.
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This is the full required range of ambient operating humidity.
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