Resolving power monochromator

The spectral resolution can be measured directly from a spectrum. It depends on the emission peak width AA, at half height, measured in spectral units. Its value depends on the monochromator and its tuning. R defines the resolving power of a spectrometer that is, its ability to separate two lines of very close wavelength ... 

The besl isolation of radiant energy can he achieved with flame spectrometers that incorporate either a prism sir grating monochromator, those with prisms having variable gauged entrance and exii slits. Both these spectrometers provide a continuous selection of wavelengths with resolving power sufficient lo separate completely most of the easily excited emission lines, and afford freedom from scattered radiation sufficient lo minimize interferences. Fused silica or quartz optical components are necessary to permit measurements in Ihe ultraviolet portion of the spectrum below 350 nanometers Sec also Analysis (Chemical) Atomic Spectroscopy Photometers and Spectra Instruments. 

Prism monochromators made of quartz have the great disadvantage of not being very useful below about 2500 A. As one goes to shorter wavelengths the transparency decreases and with it the resolving power. Much quartz also fluoresces in its own right. 

One of the main advantages of CARS and also of other nonlinear Raman spectroscopies is the high resolution that can be achieved in spectra of gases at low pressures. The reason for this is that the instrumental resolving power in these techniques depends only on the convoluted linewidths of the lasers used for excitation, whereas in linear Raman spectroscopy the resolution is determined by the monochromators used to disperse the observed scattered Raman light. 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

The Problem If a conventional continuum source is used to provide the incident radiation to the analyte, the intensity of the required radiation is too weak and worse there is no moderately priced monochromator available with sufficient resolving power to isolate the required radiation from the rest of the output from the source. One of the fundamental requirements for quantitative absorption is that the width of the radiation which is incident on the analyte must be smaller than the width of the absorption line. 

Resolving Power of Mnnochrumalurs. The resuhirig poner R ol a monochromator describes Ihe limit of its ability to separate adjacent im.ages that hate a slight difference in wavelength. Here, l)v deliriitlon... 

The resolving power of a monochromator is equal to A/ SX, where A is the average of the wavelengths of the two lines to be resolved and SX is the difference in wavelength between these lines. In the present example the required resolution is... 

Figure 117 Wavelength scan which demonstrates the high resolving power of an echelle monochromator (ARL)...

Most monochromators for atomic absorption are grating devices, although a few prism monochromators are in use. The monochromator must isolate the desired spectral line from the source and no other spectral lines should occur within the spectral band pass of the monochromator. Commonly used atomic absorption sources emit very narrow lines, usually of one element only. The spectrum therefore usually is not highly complex thus very high resolving power usually is not required. Some exceptions to these generalizations occur with some elements. 

Spectral interferences occur whenever any radiation overlaps that of the analyte element. The interfering radiation may be an emission line of another element, radical, or molecule, unresolved band spectra, or general background radiation from the flame, solvent, or analytical sample. If the spectral interference does not coincide or overlap the analyte element, spectral interference may still occur if the resolving power and spectral band pass of the monochromator permit the undesired radiation to reach the photoreceptor. 

The monochromator used in AAS does not require the high resolving power... 

The theoretical resolving power of a grating monochromator is identical to that derived for a prism. Hence the smaller the intergroove distance, the greater the resolution. 

Resolution of a monochromator The ability to disperse radiation is called resolving power. Alternative designations include dispersive power and resolution. For example, in order to observe... 

In practice, the ability to resolve between atoms exhibiting slightly different chemical shifts is limited by the peak widths which are governed by a combination of factors especially the intrinsic width of the initial level and the lifetime of the final state, the line-width of the incident radiation (which for traditional X-ray sources can only be improved by using X-ray monochromators) and the resolving power of the electron-energy analyzer. In most cases, the second factor is the major contributor to the overall line width. 

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