1. Luminescence spectroscopy
Luminescence is radiation emitted from relatively cool bodies. There are several classes of luminescence spectrochemical methods.
chemiluminescence and bioluminescence
Electroluminescence
Triboluminescence
Thermoluminescence
Photoluminescence

2. Chemiluminescence and bioluminescence
excited analyte species are produced by chemical reactions, and the resulting emission is measured.

3. Electroluminescence
results from the movement of electrons in a sample and may be caused by an
electrical discharge,
recombination of ions and electrons at an electrode
interactions of materials with accelerated electrons as in a cathode ray tube.

4. Triboluminescence
Results from the mechanical separation of charges followed by a discharge (e.g., broken crystals of sugar).
Triboluminescence is when the breaking of chemical bonds in a material generates visible light. This is achieved by scratching, crushing, rubbing, ripping and pulling apart materials. In particular, this event can be seen in fractured crystals such as sugar 

5. Thermoluminescence
Thermoluminescence is the enhancement of other types of luminescence by the addition of heat.
Thermoluminescence (TL) dating is a technique that is based on the analysis of light release when heating crystalline material.
seminar

6. Photoluminescence methods
utilize an external radiation source for excitation (as in absorption methods), but the sought-for information is the radiation emitted by the sample as shown in Figure 2-7.

7. For low absorbances (abc < 0
For low absorbances (abc < 0.01), the luminescence radiant power is directly proportional to the analyte concentration and to the radiant power incident on the sample ɸ0
Luminescence methods then involve measurements of ɸL, to obtain the analyte concentration c.
Why?

8. Scattering methods
In addition to being absorbed by the sample, radiation from an external source can be scattered; the intensity, frequency, and angular distribution of scattered radiation can be used in spectrochemical methods.
scattering
Elastic
Inelastic
Rayleigh scattering
Debye scattering
Mie scattering
Brillouin and Raman scattering
Particles< λ
Particles> λ

9. Rayleigh scattering , Particles< λ
Particles smaller than the wavelength of the incident radiation can scatter that radiation elastically without a change in its energy.
Small-particle scattering is called Rayleigh scattering; it typically occurs with atoms or molecules.
Rayleigh-scattered radiation occurs in all directions from the scattering particle.

10. Debye scattering
Here the scattered radiation is of the same frequency as the incident radiation, but the angular distribution of the scattered radiation, unlike Rayleigh scattering, is not uniform.
Mie scattering
Debye or Mie scattering Can be used to determine particle sizes and is important in turbidimetry and nephelometry where suspended particles are the scatterers.
seminar

11. Brillouin and Raman scattering
Brillouin and Raman scattering are forms of inelastic
scattering which involve a change in the frequency
of the incident radiation.
Brillouin scattering results from the reflection of radiant energy waves by thermal sound waves, whereas Raman scattering involves the gain or loss of a vibrational quantum of energy by molecules.
seminar

12. SELECTION OF OPTICAL INFORMATION
Sorting out all the optical information that might be produced in the encoding step is a major step in a spectrochemical measurement.
It is a rarity that all the information produced is useful or desirable.
In analytical procedures the selection step allows us to separate the analyte optical signal from a majority of the potential interfering optical signals.
The selection process is not perfect, however, and we must be aware of the limitations of the instruments and components used.

13. Wavelength Selection
Wavelength selection in spectrochemical instruments can be based on absorption or interference filters, spatial dispersion of wavelengths, or interferometry.
Wavelength selectors which disperse the spectral components of the optical signal spatially are the most common, and some of the major configurations are shown in Figure 2-8.

14. Figure 2-8
spectrograph
Monochromator_photodetector
polychromator_photodetector
Instrumentation for spatial dispersion and detection of optical signals.
Some of the radiation from the spectrochemical encoder enters the entrance slit and strikes the dispersion element.
The dispersion element and image transfer system cause each wavelength to strike a different position in the focal plane where different photodetector configurations can be used. .

15. If a photographic plate or array detector is placed in the focal plane as in (a), the device is called a spectrograph.
If one exit slit is used in the focal plane to define the range of wavelengths to be passed to the photodetector as in (b), the dispersion device is called a monochromator.
If multiple exit slits with a photodetector detector for each slit are employed as in (c), the dispersion device is called a polychromator

16. the name of the dispersive wavelength selector depends on the arrangement of apertures or slits in the focal plane where the spectrum is dispersed as well as the type of detection used.
In a spectrograph, a large aperture in the focal plane allows a wide range of wavelengths to strike a spatially sensitive detector such as a photographic plate.
In recent years, solid-state video-type detectors have become
available and are often employed in spectrographs in place of film. These detectors are actually an array of large number of closely spaced miniature photoelectric detectors.
They have the advantage that the spectrum can be obtained immediately without the time required for film development, for obtaining the density of the lines recorded, and so on.

17. A spectroscope is a device that allows a visual observation of the spectrum. is a spectrograph that uses a viewing screen for observing the spectrum in the focal plane.
In a monochromator, an exit slit about the same size as the entrance slit is used to isolate a small band of wavelengths from all the wavelengths that strike the focal plane.
One wavelength band at a time is isolated\ and different wavelength bands can be selected sqeuentially by rotating the dispersion element to bring the new band into the proper orientation so that it will pass through the exit slit.
If the focal plane contains multiple exit slits so that several wavelength bands can be isolated simultaneously, the wavelength selector is called a polychromator

18. A spectrometer is a spectrochemical instrument which employs a monochromator or a polychromator in conjunction with photoelectric detection of the isolated wavelength band(s).
The photodetector is placed just outside the exit slit. If a polychromator is employed with a separate photodetector for each exit slit, the instrument is often called a direct-reading spectrometer.
Some spectrometers use optical components to sweep the spectrum quite rapidly across a single exit slit. These rapid-scanning spectrometers can obtain a spectrum in a few milliseconds.

19. A spectrophotometer is an instrument similar to a spectrometer except that it allows the ratio of the radiant powers of two beams to be obtained, a requirement for absorption spectroscopy.
A photometer is a spectrochemical instrument which uses a filter for wavelength selection in conjunction with photoelectric detection.
Interferometers are nondispersive devices in which the constructive and destructive interference of light waves can be used to obtain spectral information. Several important interferometer types are discussed in Section 3-7.

20. Other selection criteria
Since many of the optical phenomena employed for spectrochemical analyses are time dependent, or can be made so, it is no surprise that time discrimination techniques are often employed for improving selectivity.
The time dependence of the optical signal from the analyte can be used to distinguish the analyte signal from time-independent or steady-state background signals.
If background signals are also time dependent, measurements can be made at a time interval which maximizes the S/B if the background signals have a different time dependence than the analytical signal.
Time discrimination is usually used along with the wavelength
selection techniques discussed previously.

21. There are many additional ways to distinguish the desired optical signal from interfering signals.
With many atomic emission sources, the intensity of the analyte
and background emission vary with the spatial region of the source viewed.
The S/B can be significantly improved, if the analyte emission can be monitored froma region that is relatively free from background emission, by adjustment of viewing position.
In some spectrochemical methods, the selectivity for the analyte can be improved by using chemical reactions of the analyte with selective reagents.

22. The change in the optical signal as a result of the reaction is measured and related to the analyte concentration.
The degree of polarization of the optical signal is yet another criterion that can aid in selecting the analyte signal.
The incorporation of computers into spectrochemical instruments has made possible the collection and display of data as a function of more than one variable at a time.
In many cases the information from multidimensional instruments can be used for optimization purposes, including improvement of the selectivity for the analyte.
A great deal of research is under way that will help us treat the vast quantities of data that such multidimensional instruments can produce. The development of computer software to handle and reduce this
information will remain a challenging task for many years to come.

23. Measurement of optical signal
All spectrochemical techniques that operate in the UVvisible
and IR regions of the spectrum employ similar instrumental components

24. The major instrumental differences between emission, photoluminescence, and absorption techniques occur in the arrangement and type of sample introduction system, encoding system and information selection system. All techniques depend upon the measurement of radiant power.
The radiant power monitor or optical transducer-signal processing- readout system is shown in block diagram form
in Figure 2-9.
The specific transducers and signal processing devices used in various regions of the spectrum in specific spectrochemical techniques are described in Chapter 4.
In this section we explore how the analytical signal is extracted from the readout data in spectrochemical methods.

26. Analytical Signal
The analytical signal for emission and chemiluminescence
techniques is defined as the signal to be displayed by the readout device due only to analyte emission.
It is given the symbol EE, and we presume that EE, is directly related to the radiant power of emission ΦE.
Similarly, the analytical signal in photoluminescence
techniques, EL, is the measured signal due only to radiationally produced emission of the analyte.
In the case of absorption methods, the analytical signal is the absorbance A due only to absorption of radiation by the analyte species.

27. Because of the presence of extraneous signals, such as signals from concomitants, the sample cell, and room light, at least two measurements are required to obtain the analytical signal.
The background or extraneous signal that registers on the readout device is due to two primary sources.
The first source is the dark signal Ed of the radiant power monitor, which is the signal present when no radiation is impingent on the transducer.
The second source is the background signal, EB due to background radiation that strikes the transducer.
The background radiation is composed of radiation from all sources other than the desired optical phenomenon from the analyte.

28. The data domains of the different signals in a spectrochemical
instrument prior to the signal processing system are dependent on the observation point.
For example, in Figure 2-9, prior to the transducer the data
are present as an optical radiant power in watts.
The transducer can convert this optical signal to an electrical
current, voltage, or charge. Normally, the output of the signal processing system to be displayed on the readout device is an electrical voltage.
Hence, in general in this book, analyte and background signals will be written as voltages E.

29. However, we will often need to look back at the magnitude and form of the signal prior to this point in the instrument in order to relate it to phenomena being measured, and in some cases the signals may be expressed in the frequency domain.

30. Emission and Chemiluminescence Spectrometry
The basic instrumental configuration for wavelength resolved
emission spectrochemical methods is shown in Figure 2-10.
FIGURE 2-10
Instrumentation for emission spectrochemical methods.
The excitation source provides the external energy necessary to excite the analyte species.
For example, the excitation source could be a flame, a plasma, a high-voltage spark discharge, or a chemical reaction. The sample container holds the sample.
The wavelength selector passes a selected wavelength band emitted by the sample to the radiant power monitor

31. The emission that results from excitation of the analyte species by a flame, a plasma, or a chemical reaction encodes the concentration of the analyte as the radiant power of emission ɸE
In some spectrochemical methods the excitation source and sample container are an integral unit, as in the nebulizer-burner used in flame emission and the reaction cell used in chemiluminescence.

32. When the analytical sample is present in the same
cell, a total or composite signal EtE, is obtained.
This total signal is the sum of :
To extract the analytical signal, a second measurement is
required to obtain the sum of the dark signal and the
background emission signal.
EE:
Analytical signal
Ed:
Dark signal
EbE:
The background emission signal
This second measurement usually made by replacing the analytical sample with blank that is ideally identical to the analytical sample except that the analyte is missing.

33. If desired, the dark signal can be obtained separately by locking all radiation from reaching the radiant power monitor.
The background emission signal could then be obtained from Ebk- Ed.
In many instruments the blank solution is used to adjust the readout device to read zero by suppression of the blank signal.
This esblishment of the zero position is still, however, a measurement of the blank signal.