POLY CHROMATE

Compared to flame excitation, random fluctuations in the intensity of emitted radiation from samples excited by arc and spark discharges are considerable. For this reason instantaneous measurements are not sufficiently reliable for analytical purposes and it is necessary to measure integrated intensities over periods of up to several minutes. Modern instruments will be computer controlled and fitted with VDUs. Computer-based data handling will enable qualitative analysis by sequential examination of the spectrum for elemental lines. Peak integration may be used for quantitative analysis and peak overlay routines for comparisons with standard spectra, detection of interferences and their correction (Figure 8.4). Alternatively an instrument fitted with a poly-chromator and which has a number of fixed channels (ca. 30) enables simultaneous measurements to be made. Such instruments are called direct reading spectrometers. 

We employ method B to study effects of this type. In this mode, our apparatus yields relative high-resolution fluorescence spectra at different time windows after excitation of the sample by the 355 nm pulse. The spectra are acquired by the upconversion method. The upconverted fluorescence spectrum is recorded simultaneously at all monitored wavelengths by an optical multichannel analyzer. It is constructed from a poly-chromator (HR320 Instruments SA) and an intensified silicon photodiode array detector (Princeton Applied Research Model 1412). The detector is interfaced to our Cromemco computer. 

A stopped-flow setup can relatively easily be equipped with multispectral detection. The fluorescence light is emitted from a small spot in the flow channel. Therefore the flow cell can be placed directly in the input slit plane of a poly-chromator. The spectram is detected by a multianode PMT. The photons detected in the spectral channels are recorded simultaneously by a TCSPC device and a router. However, although the implementation is relatively simple, no spectrally resolved TCSPC-based stopped-flow system has yet been described. 

In practice, the only feasible solution is often to transfer the light to the poly-chromator slit plane by an optical fibre. The slit is removed, and the numerical aperture at the input of the fibre is reduced to match the numerical aperture of the polyehromator. Because only moderate wavelength resolution is required, a relatively thick fibre (up to 1 mm) can be used. Therefore a reasonably high coupling efficiency with a single fibre can be obtained, even for nondescanned detection systems. The fibre should be not longer than 50 cm to avoid broadening of the IRF by pulse dispersion. 

Fig. 4.67. Wavemeter for pulsed and cw lasers, based on a combination of a small poly-chromator and three FPI with widely differing free spectral ranges [4.77]...

Fig.4.69a-e. Output signals at the poly-chromator and the three diode arrays of the FPI wavemeter, which had been illuminated by a cw HeNe laser oscillating on two axial modes (a-d). The lowest figure shows the ring intensity pattern of an excimer-pumped single-mode dye laser measured behind a FPI with 3.3 GHz free spectral range [4.70]... 

Simultaneous multichannel instruments are of two general tfp s polychromators and spectrographs. Poly-chromators contain a series of photomultiplier tubes for detection, but spectrographs use two-dimensional charge-injection devices (CIDs) or charge-coupled devices (CCDs) as transducers. Older instruments used photographic emulsions as transducers. 

Because of the high excitation temperatures attained in ICP, the most serious disadvantage of this plasma is the relatively large number of spectral interferences. Spectral overlap becomes very likely in complex samples containing many elements over a wide range of concentrations. The potential spectral overlaps are often remedied with the use of high-resolution monochromators (poly chromators). 

Atomic emission spectroscopy (AES) and atomic absorption spectroscopy (AAS) use the emission and absorption of light for elemental composition measurement, respectively. In an AES analysis, all atoms in a sample are excited simultaneously, and can be detected at the same time using a poly-chromator with multiple detectors. This is the major advantage of AES compared to AAS, which uses a monochromator and therefore only one single element can be analyzed at a time. 

---