Instrumental bandwidth measuring

The resolution of a monochromator is the smallest frequency interval the instrument can separate. The limiting resolution is the bandwidth measured at half height when scanning across an infinitely narrow intense source 22). As already mentioned, the broader excitation line width of Ar+ lasers (0.15 to 0.25 cm-1) compared to that of the He-Ne lasers (0.05 cm-1) means a lower resolution limit when the Ar+ laser is used as a Raman source. 

Double beam spectrophotometers allow differential measurements to be made between the sample and the analytical blank. They are preferable to single beam instruments for measurements in problematic solutions. For high performance instruments, the bandwidth can be as low as 0.01 nm. 

Until quite recently, direct measurements of o(>d2)(X) were limited by the very real experimental difficulties associated with the highly efficient deactivation of O ( D2) by O3, as well as the need to provide a sensitive probe for atomic oxygen atoms in the ground Pj state as well as in the electronically excited D2 state. The development of resonance spectroscopic techniques for time-resolved detection of O ( Pi) has permitted monitoring of this state at densities of ca. 10 cm with an instrumental bandwidth in excess of 10 MHz. When combined with the use of high intensity photolysis sources such as the excimer lasers and frequency quadrupled Nd/YAG, it has proved possible to measure directly the yield of 0( D2) and O( Pj) at several discrete wavelengths in the middle ultraviolet. 

This instrumental bandwidth can be physically measured as follows ... 

There are commercial instruments which measure excitation and emission spectra directly in pwatts/A bandwidth, thereby obviating the necessity of mecisuring absorption spectra. The quantity we wish to obtain is the total energy emitted (so as to be able to compare separate phosphors), namely ... 

The Fellgett or multiplex advantage deals with the fact that a Fourier transform spectrometer records data from the entire spectral region throughout the experiment. This is quite different to the case with a dispersive spectrometer, as the grating or prism instrument only measures a narrow bandwidth at any time. The measurement bandwidth of the dispersive spectrometer is regulated by the instrument s exit slit. This difference has important effects on the acquisition of data. 

If the central plane of the near-confocal FPI is imaged by a lens onto a circular aperture with sufficiently small radius b < (Ar ) / only the central interference order is transmitted to the detector while all other orders are stopped. Because of the large radial dispersion for small p one obtains a high spectral resolving power. With this arrangement not only spectral line profiles but also the instrumental bandwidth can be measured, when an incident monochromatic wave (from a stabilized single-mode laser) is used. The mirror separation d = r - is varied by the small amount e and the power... 

Electronic instrumentation is available for the measurement of D.C. and A.C. voltage, current and power as well as impedance. Such instruments usually have higher sensitivities, operating frequencies and input impedance than is normally found in the electromechanical instrumentation described above. However, they may need to incorporate amplifiers and they invariably need power to operate the final display. Hence, an independent power source is needed. Both mains and battery units are available. The accuracy of measurement is very dependent on the amplifier, and bandwidth and adequate gain are important qualities. 

Accuracy of data The microprocessor should be capable of automatically acquiring accurate, repeatable data from equipment included in the program. The elimination of user input on filter settings, bandwidths and other measurement parameters would greatly improve the accuracy of acquired data. The specific requirements that determine data accuracy will vary depending on the type of data. For example, a vibration instrument should be able to average... 

If we consider FTIR instrumentation then the situation is trickier, since the equivalent resolution in nm varies across the spectrum. But even keeping the spectrum in its natural wavenumber units, we again find that, except for rotational fine structure of gases, the natural bandwidth of many (most) absorbance bands is greater than 10 wavenumbers. So again, using that figure shows the typical user how he can expect his own measured spectra to behave. 

Insufficient resolution leads to a decrease in the extinction coefficient across the wavelength axis, and therefore inaccurate quantitation results. The sensitivity of the measurement is also compromised. From a qualitative point of view, the fine features in the spectrum may be lost. The resolution of a UV-Vis spectrophotometer is related to its spectral bandwidth (SBW). The smaller the spectral bandwidth, the finer the resolution. The SBW depends on the slit width and the dispersive power of the monochrometer. Typically, only spectrophotometers designed for high-resolution work have a variable slit width. Spectrophotometers for routine analysis usually have a fixed slit width. For diode array instruments, the resolution also depends on the number of diodes in the array. 

The CD spectrometer is usually required to work near the limits of sensitivity—e.g., reading AA values of <10 4 at a total absorbance of 1. Thus, particular care needs to be taken with cleanliness and orientation of cells and with settings of scan rate, time constant, and bandwidth. It is also important, especially when recording far-UV spectra, that the lamp is not old and that the mirrors are not clouded from radiation and traces of ozone. Because the spectrometer is a single-beam instrument, it is essential always to watch carefully for evidence of instrumental drift during measurements of sample and baseline. 

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