Mechanical slit width

This means that if a fixed slit width is used, the radiation transmitted will have different bandwidths depending upon the region of the spectrum. The mechanical efficiency of the mobile slit is therefore extremely important in the ability to select specific wavelengths. 

A slit adjustable for both length and width is desirable, but improvisation with black tape can compensate for the absence of proper adjusting mechanisms. A slit width of about 1 mm is needed to provide enough light for the classic barrier-layer photocell, but as narrow a slit as possible should be used to minimize the degree of uneveness in the area covered. Toward the edges of the paper strip the protein (and therefore the dye) concentration of the zones decreases (variably because of different diffusion rates) to zero. The length of the slit should be such that only the central, protein-filled part of the paper strip is scanned (Fig. 

BP = Dj1 x SW, where Djl is the inverse linear dispersion of the grating and SW is the mechanical slit width. 

Slit. The intensity of light emitted through any filter or monochrometer may be too intense or too weak for a light-sensing device to record. It is therefore necessary to be able to adjust the intensity of the incident light If) by placing a pair of baffles in the light path to form a slit. Simple colorimeters often have a fixed slit, but more sophisticated spectrophotometers usually have a variable-slit-width adjustment mechanism. 

Variable slits are used more commonly than fixed slits. There are two types available unilateral or bilateral. Both involve the use of beveled blades attached to micrometers that allow the blades to shift a specified distance, although unilateral slits have only one movable blade the other is fixed. Bilateral slits are preferred over unilateral slits because a constant center line is maintained with the former whereas that using the latter continuously changes as the slit width changes. Variable slits are more expensive but are more convenient for routine analysis. With constant use, the micrometer mechanism and blade edges become worn, resulting in a decrease in sensitivity and reproducibility. This is not considered a major problem in analytical work unless a precise knowledge of the slit widths is imperative. 

The entrance and exit slits of a monochromator isolate narrow portions of the light beam and direct them on to the grating. The slit width may be varied automatically by instruments to increase or decrease radiant power reaching the sample and detector. The grating is automatically driven by what is called a sine-bar mechanism, in which the bar that holds the grating is attached to a stepper motor-driven micrometer screw. Thus, the instrument covers a range of approximately 200-900 nm, with wavelength variation of 0.2 nm. 

The spectral range of interest for AAS spans from the near infrared (852.1 nm for Cs) to the vacuum ultra violet (193.6 nm for As). A monochromator is a device which separates, isolates, and controls the intensity of a narrow region of the radiant energy which is transmitted to the detector (characterised by its spectral slit width). It consists of an entrance slit, a dispersing unit (a diffraction grating) and an exit slit. The greater the intensity of the radiation transmitted to the detector the lower the signal amplification required (which contributes to electronic noise). However, the entrance and exit slits must be of similar mechanical widths (aperture size) so that, in practice, a compromise must be chosen (slit width versus narrow... 

The mechanical slit is placed before the detector to limit further the number of frequencies that impinge simultaneously upon it. Transfer optics may differ from one type of monochromator to another and yield different spatial spreads of the frequencies that arrive at the mechanical slit. An adjustment of the slit width effects... 

The physical distance between the jaws of the slit is called the mechanical slit width. Instruments normally have a micrometer scale attached so that one can read off the... 

If the mechanical slit width were made wider, the spectral bandpass would simultaneously increase and vice versa. The spectral bandpass is one of the components of the spectrometer that affects resolution. For example, with the mechanical slit settings described, it would not be possible to resolve an emission line at 229.0 nm from the... 

What is the effect of mechanical slit width on resolution ... 

Commercial AAS instrumentation may be purchased with fixed shts or with variable slits. Fixed mechanical slit widths are available so that the resolution and sensitivity are acceptable for most analytical purposes at lower cost than instruments with variable sht widths. Variable slit widths are desirable for maximum flexibility, especially if samples are varied and complex. Instruments that have both flame and graphite furnace atomizers often have separate sets of slits of different heights for each atomizer. The furnace slits are usually shorter to avoid having emission from the small diameter incandescent furnace reach the detector. In general, the analyst should use the widest slit widths that minimize the stray light that reaches the detector while spectrally isolating a single resonance line for the analyte from the HCL. 

The geometric slit width is associated with the effective mechanical widths (in mm or )um) at the entrance and exit slits for a given spectral bandpass. The entrance and exit slits, thus, control the portion of the radiation from the source that enters the monochromator and falls on the detector. By use of a wide entrance slit, large amounts of radiation energy reach the detector. In this case, the noise is small compared to the signal, and lower amplification can be employed. When the noise is low, the signal is stable and precise and low detection limits can be measured. The entrance and exit slits should have very similar mechanical dimensions. 

The physical distance between the jaws of the slit is called the mechanical slit width. Instruments used to have a micrometer scale attached so that one read off the mechanical slit width directly modern computer-controlled instruments set and read the slit width through the software that controls a stepper motor operating the slit mechanism. In UV absorption spectroscopy, mechanical slit widths are of the order of 0.3-4 pm. In IR spectroscopy, slit widths between 0.1 and 2.0 mm are common for dispersive instruments. There are no slits in FTIR spectrometers. 

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