Excitation Source

Any excitation source for analytical atomic emission spectroscopy must accomplish the following processes (1) the analytical sample must be vaporized (2) it must be dissociated into atoms (3) the electrons in the atoms must be excited to energy levels above the ground state. The three steps 

The energy required to produce spectral emission can be provided in several ways, including discharge tubes, flames, electric arcs, electric sparks, plasmas, and lasers. The first two, discharge tubes and flames, are not discussed here. Flames are treated in Chapter 9 (Flame Emission Spectroscopy) and discharge tubes are discussed in Chapter 10 (Atomic Absorption Spectroscopy). 

The typical excitation sources for TCSPC experiments are listed below. 

Light Source Wavelength Range nm Pulse Width Rep. Rate (typ) ps MHz Power (CW) mW Cost Maintenance and Alignment Effort 

Another benefit of TCSPC applications is that the diodes can be operated at almost any pulse repetition rate up to more than 100 MHz. Moreover, pulsed diode 

To obtain best results, some peculiarities of diode lasers should, however, be taken into regard. 

Diode lasers have an extremely small cavity. Most lasers in the power range below 200 mW (CW) are single-mode lasers, i.e. the height and width are so small (a few pm) that only one transversal mode is excited. This implies that the radiation ean, in prineiple, be focused into a diffraction-limited spot. However, beeause the eavity is only a few pm long, the light is emitted over a wide angle. The general beam profile of a laser diode is shown in Fig. 7.1. 

S5mchrotron radiation can also be used to provide excitation ptilses for performing time-resolved fluorescence measurements. We will limit our comments, however, as the necessary support facilities required for a synchrotron radiation source restrict them to being located at major universities and national laboratories. Such facilities include the Aladdin Biofluorescence Center (ABC) at 

Coimnercially available time-resolved fluorescence spectrometers,typically use photomultiplier tubes (PMTs) to convert fight into an electronic signal for further processing. As was pointed out in Footnote 3, however, PMTs distort the fluorescence decay profile by broadening it this places a limit on how short a fluorescence decay time a photomultiplier-based time-resolved system can measure. PMTs are described in Section 4.6 of Chapter 2 the reader can also find additional information in (36). 

faster detectors that can be used in place of a conventional PMT include microchannel plate photomultiplier tubes (MCP-PMTs) (31) and streak cameras (37). Because of their expense, the use of these devices is usually confined to home built fluorimeters foimd in dedicated fluorescence laboratories, and is therefore not discussed here. 

In Table 3.2 a non-exhaustive summary of laser Hght sources for single molecule spectroscopy is presented with a brief indication of their typical 

Continuous wave 488 Rhodamlne Green, Alexa Flour 488, GTPFITC Gas phase argon ion [9,21,62] 

Pulsed (single photon) 635 DID,Cy5 Diode laser, direct emission [58,68,69] 

Pulsed (multi- 770 Yellow/green dyes Titanium Sapphire [70,71] 

The various essential components of a reasonably good emission spectrograph are as follows, namely (/ ) Excitation sources, 

The excitation sources may be sub divided into the following two heads, namely  

Note (1) The temperature of the flame and the composition of the flame afford a direct influence on interferences which may give rise to erroneous results, 

It is worthwhile noting that the arc-gap temperature in this case is considerably lower than the direct-current arc, due to the stop-and start nature of the source, which ultimately offers a much lower sensitivity. 

The electrodes normally employed in emission spectroscopy are of two types, namely  

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Using Equ. (3.1), we can now compute the optimum frequency for cracks in various depths (see Fig. 3.2). For comparison, the optimum excitation frequency for a planar wave or a sheet inducer (300 x 160 mm) is also displayed. One finds that for a planar excitation source, a much lower excitation frequency is required, which causes a reducfion in the response signal of the crack of up to an order of magnitude in case of a small circular coil. 

We can then observe ( figure 2 ) the excitation coil length influence towards the range of the signal when the probe is moved in the tube, depending of the excitation source mode ( current I, tension U or power P constant) ... 

The eombination in a compact system of an infrared sensor and a laser as excitation source is called a photothermal camera. The surface heating is aehieved by the absorption of the focused beam of a laser. This localisation of the heating permits a three-dimensional heat diffusion in the sample to be examined. The infrared (IR) emission of the surface in the neighbourhood of the heating spot is measured by an infrared detector. A full surface inspection is possible through a video scanning of the excitation and detection spots on the piece to test (figure 1). 

XPS is also often perfonned employing syncln-otron radiation as the excitation source [59]. This technique is sometimes called soft x-ray photoelectron spectroscopy (SXPS) to distinguish it from laboratory XPS. The use of syncluotron radiation has two major advantages (1) a much higher spectral resolution can be achieved and (2) the photon energy of the excitation can be adjusted which, in turn, allows for a particular electron kinetic energy to be selected. 

Ultraviolet photoelectron spectroscopy (UPS) is a variety of photoelectron spectroscopy that is aimed at measuring the valence band, as described in sectionBl.25.2.3. Valence band spectroscopy is best perfonned with photon energies in the range of 20-50 eV. A He discharge lamp, which can produce 21.2 or 40.8 eV photons, is commonly used as the excitation source m the laboratory, or UPS can be perfonned with synchrotron radiation. Note that UPS is sometimes just referred to as photoelectron spectroscopy (PES), or simply valence band photoemission. 

Even while Raman spectrometers today incorporate modem teclmology, the fiindamental components remain unchanged. Connnercially, one still has an excitation source, sample illuminating optics, a scattered light collection system, a dispersive element and a detechon system. Each is now briefly discussed. 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

The intensity of fluorescence. If, is proportional to the amount of the radiation from the excitation source that is absorbed and the quantum yield for fluorescence... 

The intensity of fluorescence therefore, increases with an increase in quantum efficiency, incident power of the excitation source, and the molar absorptivity and concentration of the fluorescing species. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

Precision When the analyte s concentration is well above the detection limit, the relative standard deviation for fluorescence is usually 0.5-2%. The limiting instrumental factor affecting precision is the stability of the excitation source. The precision for phosphorescence is often limited by reproducibility in preparing samples for analysis, with relative standard deviations of 5-10% being common. 

Sensitivity From equations 10.32 and 10.33 we can see that the sensitivity of a fluorescent or phosphorescent method is influenced by a number of parameters. The importance of quantum yield and the effect of temperature and solution composition on f and p already have been considered. Besides quantum yield, the sensitivity of an analysis can be improved by using an excitation source that has a greater... 

Atomization and Excitation Atomic emission requires a means for converting an analyte in solid, liquid, or solution form to a free gaseous atom. The same source of thermal energy usually serves as the excitation source. The most common methods are flames and plasmas, both of which are useful for liquid or solution samples. Solid samples may be analyzed by dissolving in solution and using a flame or plasma atomizer. 

Choice of Atomization and Excitation Source Except for the alkali metals, detection limits when using an ICP are significantly better than those obtained with flame emission (Table 10.14). Plasmas also are subject to fewer spectral and chemical interferences. For these reasons a plasma emission source is usually the better choice. 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

Better detection limits are obtained using fluorescence, particularly when using a laser as an excitation source. When using fluorescence detection, a small portion of the capillary s protective coating is removed and the laser beam is focused on the inner portion of the capillary tubing. Emission is measured at an angle of 90° to the laser. Because the laser provides an intense source of radiation that can be focused to a narrow spot, detection limits are as low as 10 M. 

The high performance of modem spectrographs means that low power lasers can be used as excitation sources. These are typically 10—100-mW devices which are air-cooled and can be operated from 117-V a-c lines. In the green, the Ar" (514.5-nm) laser remains the most popular but is being challenged by the smaller and more efficient frequency doubled Nd YAG (532-nm). In the nir, diode lasers (784-nm) and diode-pumped alexandrite... 

Fig. 1. Common phosphor excitation sources, energies, and the type of soHd-state excitation caused by these sources. Following the initial excitation, high...

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

The analysis was performed by XRF method with SR. SRXRF is an instrumental, multielemental, non-destructive analytical method using synchrotron radiation as primary excitation source. The fluorescence radiation was measured on the XRF beam-line of VEPP-3 (E=2 GeV, 1=100 mA), Institute of Nuclear Physics, Novosibirsk, Russia. For quality control were used international reference standards. 

The interesting aspect of torsional problems in turbomachiner s terns is that the first indication of a problem is usually a ruptured shah oi coupling in the field. Silent and deadly, a torsional response can lurk a synchronous or non-synchronous frequencies, and be steady or transieri. in nature. Once a torsional problem is found in the field and the excua tions are determined to be inherent in the system, the only solution avail able, to put the system back on line quickly, is to decouple the excitation source or to dampen the system response. 

The displacements of the system at resonance will be a function of the magnitude of the driving or excitation source and damping. 

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