Monochromator Bandpass

As we have already mentioned, atomic absorption lines are very narrow (about 0.002 nm). They are so narrow that if we were to use a continuous source of radiation, such as a hydrogen or deuterium lamp, it would be very difficult to detect any absorption of the incident radiation at all. Absorption of a narrow band from a continuum is illustrated in Fig. 6.4, which shows the absorption of energy from a deuterium lamp by zinc atoms absorbing at 213.9 nm. The width of the zinc absorption line is exaggerated for illustration purposes. The wavelength scale for the deuterium lamp in Fig. 6.4 is 50 nm wide, and is controlled by the monochromator bandpass. If the absorption line of Zn were 0.002 nm wide, its width would be 0.002 x 1/50= 1/25,000 of the scale shown. Such a narrow line would be detectable only under extremely high resolution (i.e., very narrow bandpass), which is not encountered in commercial AAS equipment. 

Fig. 32a-d. Resonance Raman spectra of Rps. sphaeroides reaction centers as a function of potential of the silver electrode a) —0.7 V vs SCE b) 0.0 V vs SCE c) Blank (buffer, electrolyte, and LDAO) d) BChl (10 M) dissolved in CH2CI2 with sufficient pyridine to form BChl pyridine laser excitation wavelength 457.9 nm laser power 20 mW monochromator bandpass 5cm (Cotton and Van Duyne, Ref. 

Hgure 8 A demonstration of the overlap of excitation and emission monochromator bandpass when and Xem are close to one another thus explaining the observation of Rayleigh scattering of Xex-... 

Effect of the monochromator s slit width on noise and resolution for the ultraviolet absorption spectrum of benzene. The slit width increases from spectrum (a) to spectrum (d) with effective bandpasses of 0.25 nm, 1.0 nm, 2.0 nm, and 4.0 nm. 

A detachable monochromator (19) developed by Spex Industries, was another approach in minimizing stray light. It is a modified Czerny-Turner spectrograph which can be coupled to the exit slit of a double monochromator and function as a variable bandpass, variable frequency filter. This accessory, while providing the versatility of a triple monochromator, does not add much mechanical and optical complexity and can be removed when not wanted. 

Quin-2 and PHPA are the only combination we have foimd that allows us to look at Ca and oxidant production simultaneously. For this experiment, Quin-2 is detected at 490 nm through a bandpass filter and PHPA is monitored at 400 nm through the monochromator. Under... 

SI(220) double crystal monochromator was used with entrance slit (1 mm high 20 m from the source point) chosen to give a bandpass of 2 eV at the Pt edge, 11,563.7 eV.( ) The operation of the catalyst... 

Profiles of an absorption line, an emission line from a sharp line source and the bandpass of a monochromator. 

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. ... 

In practice, the steady-state fluorescence intensity If(7f) measured at wavelength AE (selected by a monochromator with a certain wavelength bandpass AAF) is proportional to F (/.f) and to the number of photons absorbed at the excitation wavelength AE (selected by a monochromator). It is convenient to replace this number of photons by the absorbed intensity 1a( e), defined as the difference between the intensity of the incident light 10( e) and the intensity of the transmitted light Jt( e) ... 

When the emission monochromator of the spectrofluorometer is set at a certain wavelength AF with a bandpass AAF, the reading is proportional to the number of photons emitted in the wavelength range from AF to AF + AAF, or in the corresponding wavenumber range from to 1/AF to 1/(AF + AAF). The number of detected photons satisfies the relationship ... 

If grating monochromators are used, the bandpass AA is constant and the scale is linear in wavelength. The expression in wavelength must then be used for the integral calculation. 

The addition of a single monochromating crystal gives the device great flexibil ity, since one has independent control over the collimation, by the number of reflections, and the bandpass, controlled by the width of the reflection of the monochromating crystal. Figure 2.16 shows one variant, a three-reflection... 

CCC to give very good angular resolution, plus a Ge 220 monochromating crystal which has a high bandpass. 

Figure 2. Experimental set-up for Raman spectroscopy. The desired laser line is isolated from other plasma lines by a narrow bandpass filter or broadband prism monochromator, then focused onto a sample in a capillary tube. A collecting lens placed at a 90° angle to the incident beam focuses the scattered light onto the entrance slit of a monochromator with output to a photomultiplier tube (in the case of a scanning instrument) or a diode array detector.

These are the only type of interference that do not require the presence of analyte. For AAS the problem of spectral interference is not very severe, and line overlap interferences are negligible. This is because the resolution is provided by the lock and key effect. To give spectral interference the lines must not merely be within the bandpass of the monochromator, but actually overlap each other s spectral profile (i.e. be within 0.01 nm). West [Analyst 99, 886, (1974)] has reviewed all the reported (and a number of other) spectral interferences in AAS. Most of them concern lines which would never be used for a real analysis, and his conclusion is that the only real problem is in the analysis of copper heavily contaminated with europium The most commonly used copper resonance line is 324.754 nm (characteristic concentration 0.1 pg cm- ) and this is overlapped by the europium 324.753 nm line (characteristic concentration 75 pg cm- ). 

Phosphorescence spectra (uncorrected, front face) were recorded on a Perkin-Elmer LS-5 fluorescence spectrometer using a pulsed excitation source ( 10 ps) and gated detection. The instrument was controlled by a P-E 3600 data station. The samples were typically excited at 313 nm using the instrument s monochromator and an additional interference filter. Excitation and emission bandpasses were 2 nm. Typically the emission spectra were recorded using a 50 ps delay following excitation and a 20 ps gate. The samples were contained in cells made of 3x7 mm2 Suprasil tubing, under a continuous stream of N2, 02 or 02/N2 mixtures of known composition. 

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