Monochromator wavelength calibration

Table I. Spectral Lines from External Sources Used for Monochromator Wavelength Calibration...

Care should be taken to make sure that the wavelength calibration of the monochromator is correct. For elements with complex emission spectra, such as iron, a small wavelength calibration error may result in the wrong wavelength being employed accidentally. This may seriously adversely effect sensitivity and precision. 

The calibration of excitation and emission monochromator wavelengths should be checked regularly by the use of sharp lines from the instrument s own radiation source (e.g. xenon lines at 450.1, 462.4,... 

Accuracy of wavelength calibration is maintained by thermostatic control of the monochromator. Water vapor may be harmful to optical components and in all instruments the internal atmosphere must be controlled by drying, gas filling, or evacuation. Carbon dioxide and water vapor absorb in the infrared and in single-beam instruments separate recordings of blank and sample spectra must be made. This is inconvenient, and double beam instruments, with automatic blank compensation and improved stability, are more commonly used. 

The first and/or second dye lasers were tuned to the specific wavelength(s) to populate the desired level(s). The final laser in the excitation sequence (either the second or third laser) was then continuously scanned to obtain the Rydberg or autoionization spectrum. The spectrum and wavelength calibrations were recorded simultaneously on a two pen recorder. Wavelength calibration was obtained by directing a portion of the scan laser radiation to a monochromator that was preset at known U or Th emission lines from an electrodless lamp. 

The dispersive spectrometers suffer from greater wavenumber errors, of a less predictable form, owing to their general mechanical and thermal instability and can also be affected by non-uniform illumination across the monochromator entrance slit [26]. FT-spectrometers typically use a He-Ne laser as a reference beam to monitor the displacement of the moving optical element, so providing an active internal absolute wavelength calibration... 

Figure 7.2a shows the intensity (as measured by a calibrated silicon photodiode) from a monochromator set to a 20 nm bandpass attached to a 1 kW Xe lamp. The wavelength calibration is set to the center of the bandpass. 

Continuous xenon (Xe) arc lamps are very high intensity visible light emitters with good yield of UV which decreases in intensity from about 400 nm downwards emission also extends out into the near IR (Fig. 14.3c). Emission is a continuum with some high intensity emission lines from xenon, particularly around 420 80 nm and in the near IR, superimposed. Most fluorimeters use Xe-arc lamps as excitation source and these emission lines can be used for internal wavelength calibration e.g., the line at 467 nm can be used to calibrate the position of the excitation monochromator in a fluorimeter. Xe-arc lamps also make excellent high intensity irradiation sources for photochemical reactors. When fitted with a suitable Air Mass filter they give a reasonable approximation to the solar spectrum and are widely used in solar simulators. 

Both spectrometers are equipped with a self-controlling wavelength calibration routine using the possibility to avoid the order pre-selection in fl-ont of the echelle monochromator. This takes up the well-known method from classical VUV spectrometry, where VUV lines diffracted from echellettes or concave gratings in higher orders were calibrated... 

The original motivation for the preparation of regular metallic multilayers of carefully controlled periodicity was the need for X-ray reflectors, both to calibrate unknown X-ray wavelengths and to function as large and efficient monochromators, especially for soft X-rays of wavelengths of several A. This was first done by... 

As mentioned previously (11), the wavelength position and stability of spectral lines from xenon or mercury excitation sources of spectrofluorometers may be variable with time-, and such sources are difficult to use with certainty for the calibration of monochromators. ... 

Effect of bandpass and choice of wavelength on a Beer s law plot. Curve A represents a calibration curve using a narrow bandpass monochromator at k. Curve B represents a calibration curve using a wide bandpass filter at X or a narrow bandpass monochromator at. ... 

Calibrations which are generated on the top-of-the-line instrument can be transfered to a less expensive instrument for routine use by less skilled personnel. Computer programs handle all the details, such as a filter transform (to make data collected via monochromator look like it was collected via interference filter), a data reduction (to limit a large data base of as many as 700 wavelengths to only those 19 wavelengths represented by the filters in a smaller instrument), and a calibration (the creation of an equation by examining known standards and maximizing the correlation as described above). 

When a luminescence spectrum is obtained on an instrument such as that used to produce the spectra in Figure 7.23, it will depend on the characteristics of the emission monochromator and the detector. The transmission of the monochromator and the quantum efficiency of the detector are both wavelength dependent and these would yield only an instrumental spectrum. Correction is made by reference to some absolute spectra. Comparison of the absolute and instrumental spectra then yields the correction function which is stored in a computer memory and can be used to multiply automatically new instrumental spectra to obtain the corrected spectra. The calibration must of course be repeated if the monochromator or the detector is changed. 

The tail of the plasma formed at the tip of the torch is the spectroscopic source, where the analyte atoms and their ions are thermally ionized and produce emission spectra. The spectra of various elements are detected either sequentially or simultaneously. The optical system of a sequential instrument consists of a single grating spectrometer with a scanning monochromator that provides the sequential detection of the emission spectra lines. Simultaneous optical systems use multichannel detectors and diode arrays that allow the monitoring of multiple emission lines. Sequential instruments have a greater wavelength selection, while simultaneous ones have a better sample throughput. The intensities of each element s characteristic spectral lines, which are proportional to the number of element s atoms, are recorded, and the concentrations are calculated with reference to a calibration standard. 

One of the major advantages of the dispersive instrument is the possibility of computer adjusting the electronics at any spectral point such that the signal can be processed under optimum conditions. This is possible because the signal varies slowly with time, and because the data are not multiplexed. Furthermore, synchronization of the PEM retardation with the wavelength transmitted by the monochromator can be achieved, which results in maximum circular polarization at each wavelength, and therefore, an automatically calibrated signal at optimum amplitude. 

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