Spectral line sources modulation

The line-shape of a true (undistorted) EPR spectrum should be independent of the acquisition parameters, and therefore to assess spectral distortion one can compare spectra acquired with different parameters. Figure 15.6 illustrates the effect of modulation amplitude on EPR line-shape. The central line-width (peak-to-peak width AHpp = 1.6 G) remains unchanged when the modulation amplitude is increased from 0.5 to 1 G while at a modulation amplitude of 10 G, distortion and line-broadening (AHpp = 6.4 G) can be clearly observed. The main sources of spectral distortions are modulation amplitude, microwave power, and scanning rate (speed). These are discussed in the following sections. 

In the TDLAS detector, a diode laser is scanned across a chosen absorption line. The selection and application of the 02 spectral line is proprietary to most vendors, but it is usually one of the 02 lines around 760 nm (Figure 3.37). The laser source wavelength is modulated as the absorption line is scanned, which makes it possible to use the spectroscopic oxygen technique, which previously was not considered possible. 

The function of the spectrometer is to accept as much light from the source as possible and to isolate the required spectral lines. This may be impossible where there is a continuous spectrum in the same region as the analytical line for example, the magnesium line of 286.2 nm coincides with a hydroxyl band. In direct reading instruments, electronic devices may be used to supplement the resolution of the spectrometer by modulating the intensity of the analytical signal. In absorption and fluorescence the light source is modulated in emission the spectral line is scanned (816) or the sample flow modulated (M23). 

The lamps may be operated as continuous wave sources or be modulated or pulsed. EDLs have a poor stability or poor shelf life in some cases, but they have high intensities, produce narrow spectral lines, and are relatively inexpensive. 

Vaks et alf have designed a novel spectrometric approach for gas analysis based on a BWO and low-Q absorption cell. The BWO spectral output was split between a reference cell and aim waveguide measurement cell. The gas in the 0.2 m pathlength waveguide reference cell was used to lock the BWO frequency to the spectral line at the 304 GHz OCS spectral line and the frequency modulated spectral source directed to the analytical cell. Detection was with a Schottky barrier diode mixer. 

The apparent oxygen absorption linewidth pressure dependence in air derived from the fitting to this simple model showed an effect of greater FM deviation as it became comparable with the narrower spectral line at low pressures. This phenomenon is because of the interaction between the electric field of the modulated wave and that of the relaxing molecule during its transition (ref. 15, p. 139, and ref 20) see Section 1.2. The effect is to increase the measured spectral absorption linewidth because the molecules are oriented in LTE with the instantaneous electric field due to the source, as they relax after a collision. As the FM deviation increases, so therefore does the bandwidth of the radiation field they experience, and this causes an increase to the linewidth. 

To avoid spectral interferences when non-dispersive AFS instruments are used, line-like radiation sources are required. Spectral interferences using narrow line sources are uncommon, while when using continuum sources they are quite common. Corrections can be made by wavelength or amplitude modulation, which can be performed with filters. Different wavelengths are separated from each other by filters whose advantage is their low price and good spectral transmission. 

Spectral line interference is less critical in atomic absorption than it is in flame emission. This is due to the fact that absorption is usually concerned with one spectral line only for each element and that, by proper modulation of the source signal, extraneous spectral lines that do not actually overlap the desired line are not detected. It is wise, however, to use as narrow a slit width as possible to keep the spectral band pass of the monochromator to a minimum. Actual spectral line overlap cannot be corrected by this means. If spectral line overlap occurs, as might happen, e.g., with palladium at 3404.6 A and cobalt at 3405.1 A, the only solutions are (1) to use another spectral line of the element or (2) remove the offending element from the analytical sample. 

A second type of spectral interference is due to thermal emission from the flame of the sample. For interference to occur in this case the thermally excited spectral lines also must lie within the spectral band pass of the monochromator slit width, and this also results in an erroneously enhanced signal. In this case, however, the interference can be eliminated by use of a modulated source and an amplifier tuned to the frequency of modulation since the undesired signal will be a steady (dc) signal. 

The procedure to obtain the fluorescence intensity for the desired element after the preliminary adjustments have been made depends in part on the type of equipment in use. If an ac system with a chopper or electrically modulated source is in use and is properly adjusted, any dc signal component of the flame cell can be ignored since the amplifier will not respond to the dc signal. This includes any continuum and any thermally excited spectral lines within the band pass of the monochromator. 

One of the greatest advantages of AAS, namely, its specifity, is based on the use of element-specific radiation sources that emit the spectrum of the analyte element in the form of very narrow spectral lines. While the quality of an instrument in other spectrometric techniques frequently depends on the resolution of the monochromator or on its spectral bandpass (the range of radiation that passes through the exit slit), these factors are not of primary importance in AAS. If the element-specific radiation is modulated and the amplifier tuned to the same frequency, AAS is selective and free of spectral interferences caused by overlapping of atomic lines of different elements (see Sec. 1.6). 

A pulsed dye laser is the most practical excitation source for LEI spectrometry. Amplitude-modulated CW dye lasers have displayed a useful immunity to electrical interferences at any excitation position 38), but because of the inefficiency of the frequency doubling process, CW dye lasers yield low power emission in the ultraviolet spectral region. Most metals have their strongest resonance lines in the ultraviolet. In addition, ultraviolet transitions terminate nearer the ionization limit, producing the highest LEI sensitivity. 

The spectrum near 600 cm of the V2 band of the CH3 radical generated by a glow discharge in di-tert-butylperoxide (Yamada et. al. 1981) provides a second example of this selectivity. With Zeeman modulation all the observed lines arise from CH3, but with source frequency modulation many additional lines from diamagnetic species are detected in the same spectral region. 

Fourier domain OCT measures interference fringes in the spectral separated domain either by a spectrometer with a high-speed line-scan camera (Fig. 2) or a swept laser source-based system that uses a single detector [3, 6, 8]. Modulation of the interference fringe intensity in the spectral domain is used to determine the location... 

The most important components are a radiation source, which emits the spectrum of the analyte element, an atomizer in which the atoms of the analyte element are formed, a monochromator for the spectral dispersion of the radiation and separation of the analytical line from other radiation, a detector permitting measurement of radiation intensity, followed by an amplifier and a signal-processing unit with a readout device. The primary radiation is modulated either mechanically or electrically at a fixed frequency, and the amplifier electronics are turned to the same frequency. In such a system only the element-specific radiation having the modulation frequency is amplified while any other radiation emitted by the atomizer, which is not modulated, is neglected. 

Spectral interferences are due to incomplete isolation of the radiation absorbed by the analyte from other radiation or radiation absorption detected by the instrument. In AAS spectral interferences by thermal emission of concomitants transmitted by the monochromator or received by the detector as stray light are eliminated by modulating the primary radiation and tuning the amplifier to the same modulation frequency (see Sec. 1.1). Spectral interferences can therefore only be caused by absorption of radiation by overlapping atomic or molecular lines, or by scattering of source radiation by nonvolatilized particles formed by the concomitants. Spectral interferences are best corrected for by an efficient background corrector (see Sec. 1.4). It must be stressed that the method of additions (see Sec. l.S) by definition cannot be used to correct for any spectral interferences. 

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