Emission control spectral analysis

Fluorescence microspectrophotometry typically provides chemical information in three modes spectral characterization, constituent mapping in specimens, and kinetic measurements of enzyme systems or photobleaching. All three approaches assist in defining chemical composition and properties in situ and one or all may be incorporated into modem instruments. Software control of monochrometers allows precise analysis of absoiption and/or fluorescence emission characteristics in foods, and routine detailed spectral analysis of large numbers of food elements (e.g., cells, fibers, fat droplets, protein bodies, crystals, etc.) is accomplished easily. The limit to the number of applications is really only that which is imposed by the imagination - there are quite incredible numbers of reagents which are capable of selective fluorescence tagging of food components, and their application is as diverse as the variety of problems in the research laboratory. 

Lorentzen, C.J., Carlhoff, C., Hahn, U., Jogwich, M. (1992) Applications of laser induced emission spectral analysis for industrial process and quality control. Journal of Analytical Atomic Spectroscopy,... 

Figure 1 shows a rotatable Polaroid sheet, p, for determination of the emission polarization with respect to v Bandpass and coloured glass cut off filters replace this Polaroid sheet for coarse spectral analysis. Output from a bialkali cathode photomultiplier tube, pmt, is monitored by a photon counter signal averager at the hexapole modulation frequency and stored on a microcomputer that also controls the various field voltages and polaroid sheet positions. 

The composition of steels or other metals is commonly analyzed by emission or X-ray spectrometry during and after the production process. Both methods have to be calibrated by solid samples. These are either exactly analyzed samples taken from the same process or synthetic melted mixtures of the matrix with added accompanying elements (RMs). Available CRMs are then used to control the slope of the calibration function. Today, available RMs and CRMs are increasingly and exclusively used in spectral laboratories as the chemical analysis became much restricted and typical control laboratories were totally closed (Slickers 1993). 

Spark sources are especially important for metal analysis. To date, medium-voltage sparks (0.5-1 kV) often at high frequencies (1 kHz and more), are used under an argon atmosphere. Spark analyses can be performed in less than 30 s. For accurate analyses, extensive sets of calibration samples must be used, and mathematical procedures may be helpful so as to perform corrections for matrix interferences. In arc and spark emission spectrometry, the spectral lines used are situated in the UV (180-380nm), VIS (380-550nm) and VUV (<180 nm) regions. Atomic emission spectrometry with spark excitation is a standard method for production and product control in the metal industry. 

Relatively recently, AIS Sommer GmbH of Germany delivered a laser-induced fluorescence (LIP) analyzer for quality control in minerals and mineral processing (Broicher 2000). The LIP analyzer includes two light detector systems with three photomultipliers each, which evaluate three spectral bands in two time windows each. It was done in the Kiruna phosphorous iron ore mine, Sweden. The limitation of LIP analysis is that its accuracy depends on the complexity of the composition of the ore and the concentration and fluorescence properties of the critical minerals in relation to all the other minerals present. The phosphorous iron ore in Kiruna is ideal for LIP analyzes, because its iron minerals are practically non-luminescent, while magmatic apatite is strongly fluorescent with intensive emissions of Ce and Eu ". ... 

At a high enough temperature, any element can be characterised and quantified because it will begin to emit. Elemental analysis from atomic emission spectra is thus a versatile analytical method when high temperatures can be obtained by sparks, electrical arcs or inert-gas plasmas. The optical emission obtained from samples (solute plus matrix) is very complex. It contains spectral lines often accompanied by a continuum spectrum. Optical emission spectrophotometers contain three principal components the device responsible for bringing the sample to a sufficient temperature the optics including a mono- or polychromator that constitute the heart of these instruments and a microcomputer that controls the instrument. The most striking feature of these instruments is their optical bench, which differentiates them from flame emission spectrophotometers which are more limited in performance. Because of their price, these instruments constitute a major investment for any analytical laboratory. 

Emission spectroscopy is widely used for both qualitative and quantitative analysis. The high sensitivity and the possible simultaneous excitation of as many as 72 elements, notably metals and metalloids, makes emission spectroscopy especially suited for rapid survey analysis of the elemental content in small samples at the level of 10 /ug/g or less. With control over excitation conditions to maintain constant and reliable atomization and excitation, the spectral line intensities can be used for quantitatively determining concentrations. An analytical curve must be constructed with known standards, and often the ratio of analyte intensity to the intensity of a second element contained in, or added to, the sample (the internal-standard method) is used to improve the precision of quantitative analyses. Preparation of standards for arc and spark techniques requires considerable care to match chemical and physical forms to the sample this is not commonly required for ICP discharge. 

In the period 1860-1900 a number of practical advances in analytical emission spectroscopy occurred. In 1874, Lockyer stated that the length, brightness, thickness, and number of spectral lines were related to the quantity of the element present in the sample. Hartley s studied the spectra of metals at varying concentrations and proposed a method of analysis based on the last line principle. The most persistent lines were those visible at the lowest concentrations of the element and thus served to measure the lowest concentration of the element that produced spectral lines under controlled excitation conditions. 

This is not to say that the measurement of emissivity is unimportant. For example, the spectral emissivity of materials is an important property for calculations of radiant heat transfer. Radiant heat transfer analysis is critical to designing the temperature-control systems of satellites and spacecraft, since convection and conduction are negligible in outer space. Radiant heat transfer analysis is also important... 

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