Spectral bandpass

Lasers can be coupled efficiently to fiber optic devices to deHver intense monochromatic light precisely to the desired region of the body, including internal organs (see Fiber optics). As in other cases of laser-induced photochemistry, biphotonic effects may be important (87). Lasers also offer the advantage of being able to concentrate the incident energy in a spectral bandpass matched to the absorption band of the sensitizer. 

Figure 2. The fluorescence intensity of PuF Cg) excited at 1064 nm is shown as a function of the energy of the emitted photons. The spectral bandpass and intensity uncertainty are indicated. To within experimental error, the same emission spectrum is found when 532 nm excitation is used.

Important performance characteristics of UV/Vis detectors are sensitivity, linearity, and band dispersion. These are controlled by design of the optics and the flow cell—more specifically by spectral bandpass, stray light characteristics, and the volume and path length of the flow cell. 

Analytikjena Atomic Absorption Spectrometer equipped with an aluminum hollow cathode lamp was used to measure aluminum concentrations. The wavelength and spectral bandpass were set at 309.3 and 1.2 run, respectively. Gas/Oxidant ratio was 0.553. A digital pH meter model Metrohm was used for pH measurements. 

FIGURE 9.7 Schematic of an in-line interferometer. The anti-Stokes local oscillator field is collinearly overlapped with the pnmp and Stokes beams on a dichroic mirror (DM). All fields are focused by a microscope objective (MO) into the sample (S), and the total signal at the anti-Stokes frequency is detected throngh a spectral bandpass filter (F) at the photodetector. 

Figure 5. Emission spectra of anthracene, ptjrene, and diphenylstilbene. (a) 1 eg/ml solution of the three compounds. Experimental conditions excitation wavelength, 340 nm 400 scans spectral bandpass, 3 nm (b) individual emission spectra of same three compounds (c) Computer-deconvoluted emission spectra of...

Figure 2-7 Raman spectra of CCI4 (488.0-nm excitation) obtained under different conditions using a Spex Model 1403 double monochromator equipped with 1,800 grooves/mm gratings and a Hamamatsu R928 photomultiplier, (a) The effect of spectral bandpass (0.2-cm-1 increments per data point), (b) The effect of size of increments between the data points (1-cnT1 slit width accumulation time for all spectra was 1 second per data point.)...

In AFS there is no need to isolate a single wavelength in the fluorescence emission spectrum from nearby, less-intense emission wavelengths, since all lines contribute to the fluorescence signal. Therefore quite large spectral bandpasses are often employed in flame AFS, especially when a low-background flame is being used. Indeed, as seen in Chapter 2, section 14, non-dispersive, filter-based systems may sometimes be employed.7,8... 

The function of the monochromator in AES is to isolate the determinant spectral wavelength of interest from the emission from all concomitant matrix emitting elemental or molecular species. This frequently means that a narrow spectral bandpass must be selected. It is however generally slightly easier to make sure in AES than in AAS that the optimal wavelength is being employed since emission spectra often may be scanned directly. 

Silicate, nickel, and cobalt tend to interfere in the air-acetylene flame, although nickel and cobalt are rarely present in sufficient excess to cause a problem. Silicate interference may be eliminated at modest excesses by the use of lanthanum as a releasing agent or by using a nitrous oxide-acetylene flame. Very careful optimization is sometimes necessary, for example in the analysis of freshwaters, when concentrations are very low. It is important to use a narrow spectral bandpass and to make sure that the correct line is being used, because the hollow cathode lamp emission spectrum of iron is extremely complex. If you have any doubts about monochromator calibration, check the sensitivity at adjacent lines ... 

Manganese may be determined with good sensitivity by flame AAS or AFS, the former technique being very widely used in environmental analysis. The detection limit at 279.5 nm by AAS in a lean air-acetylene flame is about 10 ng ml-1, which is quite adequate for most environmental analyses. A narrow spectral bandpass should be used, and care taken to make sure that the 279.5 nm line is being used rather than one of the adjacent lines at 279.8 or 280.1 nm. The detection limit at 403.1 nm by flame AES using a nitrous oxide-acetylene flame is usually slightly better, at around 5 ng ml-1, than that obtained by flame AAS. 

Element Analytical wavelength (nm) AAS Spectral bandpass (nm) Flame conditions8 ETA Graphite furnace conditions ... 

The two lines at 249.68 nm and 249.77nm separated by 0.09nm may be difficult to resolve a narrow 0.1 nm or less spectral bandpass is required... 

With the 309.27/309.28 doublet, sensitivity depends on spectral bandpass a narrow 0.2 nm bandpass is recommended to minimise intense emission of the flame. Absorbance depends critically on flame stoichiometry and observation height. S/N can be improved by increasing lamp current and optimizing fuel flow. 

The doublet at 318.34/318.40 nm can be resolved from the adjacent 318.54 nm line with a 0.03 nm spectral bandpass. As all three lines have similar absorption sensitivity, transmission of the three lines using a 0.2nm spectral bandpass is recommended for increased detectivity and precision. Optimum sensitivity is obtained with a slightly fuel-rich nitrous oxide-acetylene flame ionization should be suppressed with 1000 jug K ml-1. Variable enhancement of absorption by Al, Fe, Cr and other elements is removed by addition of 2000 jug Alml-1. 

A narrow spectral bandpass of ca. 0.2 nm is required with the 357.87 chromium line to eliminate nearby 357.66 nm and 358.23 nm argon lines when an argon-filled light source is used. In an air—acetylene flame, sensitivity and chemical interferences are critically dependent on flame stoichiometry and observation height. 

Even the narrowest spectral bandpass obtainable with most spectrometers will not isolate the most sensitive 248.33 nm iron line from the nonabsorbing iron line at 248.42 nm and other nearby lines. Iron calibration curves, therefore, will usually be non-linear. The 302.05/302.06 nm iron line has been reported to have a better signal/noise ratio [194]. 

The rich nickel spectrum in the vicinity of the most sensitive atomic absorption Ni line, 232.00 nm, leads to pronounced curvature of the calibration graph. For highest sensitivity and somewhat reduced calibration curvature, a 0.2 nm or less spectral bandpass must be employed to attempt to isolate the 232.00 nm line from adjacent non-absorbing Ni lines at 231.72nm and 232.14nm. 

With the sensitive 213.86 nm absorption line, spectral bandpass is not... 

Absorbance is highly dependent on lamp current and flame stoichiometry. Hollow-cathode lamps must be operated at fairly low currents to prevent self-absorption good lamp stability, however, permits use of high instrumental scale expansion. The spectral bandpass used is not critical. For aqueous solutions very few chemical interferences have been reported in the air—acetylene flame depression of Cd absorbance is caused by large amounts of silicon. 

The fuel-rich nitrous oxide—acetylene flame provides for interference-free operation. Somewhat greater (ca. two-fold) sensitivity is observed at the 235.48nm line over absorption at the 286.33 nm line a narrow 0.2nm (or less) spectral bandpass must be used to avoid spectral interferences at the latter line. Measurements should be conducted at 286.33 nm when non-atomic absorption is encountered at 235.48nm. Ionization in the nitrous oxide—acetylene flame should be suppressed by the addition of 1000 jug ml-1 of potassium or cesium. 

Slits help focus light onto the monochromators and the detector they regulate the wavelength range that excites and is emitted by the sample. Smaller slit widths are more selective, producing a narrower range of spectral bandpass (bandwidth at half the peak transmittance), but a decrease in transmitted light is noted and, therefore, there is a decrease in sensitivity. There are two types of slits fixed and variable. 

In the case of atomic absorption and atomic fluorescence the selectivity is thus already partly realized by the radiation source delivering the primary radiation, which in most cases is a line source (hollow cathode lamp, laser, etc.). Therefore, the spectral bandpass of the monochromator is not as critical as it is in atomic emission work. This is especially true for laser based methods, where in some cases of atomic fluorescence a filter is sufficient, or for laser induced ionization spectrometry where no spectral isolation is required at all. 

Filter monochromators are now used almost only for flame photometry. They make use of interference filters, which may have a fairly low spectral bandpass (less than a few nm). However, it is also possible to use such filters for dynamic measurements of line and background intensities, and for transient signals, as occur in gas chromatography. The use of oscillating filters has been described, where the wavelength bandpass is slightly shifted by inclining them towards the radiation beam [65]. 

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