Spectrometer wavelength adjustment

As described in Chapter 3, grating spectrometers will respond to harmonically related wavelengths that is, if a grating spectrometer is adjusted to record spectral emission at 6000 A in the first order, spectral emission at 3000 A in the second order also will be recorded at 6000 A and emission at 2000 A will be recorded as third-order emission at the same 6000 A position. Overlapping spectral orders therefore may unnecessarily complicate the observed spectrum and, in some cases, interfere with spectral lines used for analytical purposes. 

With a flow rate adjusted at 10 pL/s, dispense 2000 pL to the Z-flow cell and perform reference and absorbance scan with the spectrometer wavelength fixed at 813 nm. 

The Raman microscope that has been described in Section II can be readily adapted for use with lasers from the near-ultraviolet to near-infrared wavelengths some adjustments are required, however, for optimal operation with two wavelengths that are more than 200 nm apart. A diffraction grating of appropriate groove density must be used and the spectrometer lenses adjusted to allow for the small changes in focal length arising from the imperfect corrections for chromatic aberration. The falloff in the sensitivity of the CCD... 

The precise wavelength adjustment is realized by prism and grating rotation actuated by stepper-motor controlled lever arms. All optical and mechanical spectrometer components are arranged on a rugged base made of cast-iron. A photograph of the DEMON spectrometer module is shown in Figure 3.6. 

Prepare a stock solution of 0.05 M cadmium sulphate or nitrate. By successive dilution prepare a series of solutions in the concentration range of 5 x 10 to 10 M. Set the wavelength of the spectrometer to 228.0 nm (326.1 nm is a secondary wavelength). Adjust the air flow at 8.0 dm min and C2H2 flow at 3.0 dm min. Then measure the absorbance of each solution and plot against concentration. Use the calibration plot to determine the concentration of an unknown solution. 

Adjust the spectrometer variables of spectral band pass, wavelength and lamp current according to the manufacturer s recommended conditions for Ni and V. It will be necessary to employ background correction for the determination with most electrothermal atomisers, especially in the case of Ni. Where available on the spectrometer, set up the deuterium or hydrogen lamp background correction system as recommended by the manufacturer. Otherwise use a nearby non-absorbing line to estimate the background intensity. 

Adjust the spectrometer parameters of spectral band pass, wavelength and lamp current in accordance with the manufacturer s recommended conditions for Pb. It will be necessary to correct for background effects with most instruments. Use automatic background correction facilities where available, otherwise make use of a nearby non-absorbing line. The use of the 283.3 nm Pb wavelength will reduce background effects compared with the 217.0 nm wavelength. 

Apparatus. Perkin-Elmer (PE) 360 and 272 atomic absorption spectrometers, with an air-acetylene or a nitrous oxide-acetylene flame, were employed for the analyses. Standard hollow cathode lamps were used as light sources. In all cases, the instruments were operated as recommended by the manufacturer gas pressures, gas flows, slits, wavelengths, and other controls were adjusted to the prescribed values. The readout was obtained directly using a 5-s integration in the concentration (PE 360) or absorbance (PE 272) mode. 

When Fourier transform infrared (FTIR) spectrometers first appeared on the market in the early 1970s, they were bulky and expensive (more than 100,000) and required frequent mechanical adjustments. For these reasons, their use was limited to special applications in which their unique characteristics (great speed, high resolution, high sensitivity, and excellent wavelength precision and accuracy) were essential. Currently, however, FTIR spectrometers have been reduced to benchtop size and have become very reliable and easy to maintain. Furthermore, the simple models are now priced similarly to simple dispersive spectrometers. Hence, FTIR spectrometers are largely displacing dispersive instruments in most laboratories. 

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