Fluorescence spectrophotometry detectors

Since chlorophylls have distinct spectroscopic properties, absorption or fluorescence spectrophotometry can be employed for their identification and quantification (118). Usually wavelengths between 430 and 440 nm and 645 and 660 nm, respectively (93,115), are used for spec-trophotometric detection. With spectrophotometric detectors, detection limits of approximately 80 ng chlorophyll (119) and 1 ng chlorophyll (120), respectively, can be achieved. Fluorescence... 

The main advantage of fluorescence techniques is their sensitivity and measurements of nanogram (10—9 g) quantities are often possible. The reason for the increased sensitivity of fluorimetry over that of molecular absorption spectrophotometry lies in the fact that fluorescence measurements use a non-fluorescent blank solution, which gives a zero or minimal signal from the detector. Absorbance measurements, on the other hand, demand a blank solution which transmits most of the incident radiation and results in a large response from the detector. The sensitivity of fluorimetric measurements can be increased by using a detector that will accurately measure very small amounts of radiation. 

Photodiode detectors have already been cited in this chapter in relation to near-IR fluorescence measurements on singlet oxygen,(8 16 18) in decay-time temperature sensing,(50) in liquid chromatography,(62) the study of proteins labelled with Nile Red,(64) and diode laser spectrometry,(67) Photodiodes are also conveniently packaged for many applications in an array form enabling rapid data acquisition e.g., in spectrophotometry, (35)... 

A phosphorus-specific thermionic detector was also adapted from GLC (See Section III.3.b) for use with small-bore HPLC columns208,307,330,334. Based on an electrically heated rubidium salt bead, it permits detection limits of 0.2-0.5 ng of phosphorus and its response is linear with the amount of phosphorus over several orders of magnitude. This detector yields good results with phosphates which cannot be detected by UV spectrophotometry or by fluorescence measurements. 

A number of analytical techniques have been used for measuring aluminum concentrations in environmental samples. These include GFAAS, FAAS, NAA, ICP-AES, ICP-MS, spectrophotometry using absorbance and fluorescence detection, phosphorimetry, chromatography and gas chromatography equipped with an electron capture detector (GC/ECD) (Andersen 1987, 1988 Benson et al. 1990 ... 

HPLC units have been interfaced with a wide range of detection techniques (e.g. spectrophotometry, fluorimetry, refractive index measurement, voltammetry and conductance) but most of them only provide elution rate information. As with other forms of chromatography, for component identification, the retention parameters have to be compared with the behaviour of known chemical species. For organo-metallic species element-specific detectors (such as spectrometers which measure atomic absorption, atomic emission and atomic fluorescence) have proved quite useful. The state-of-the-art HPLC detection system is an inductively coupled plasma/MS unit. HPLC applications (in speciation studies) include determination of metal alkyls and aryls in oils, separation of soluble species of higher molecular weight, and separation of As111, Asv, mono-, di- and trimethyl arsonic acids. There are also procedures for separating mixtures of oxyanions of N, S or P. 

The most commonly-used detectors are those based on spectrophotometry in the region 184-400nm, visible ultraviolet spectroscopy in the region 185-900nm, post-column derivativisation with fluorescence detection (see below), conductivity and those based on the relatively new technique of multiple wavelength ultraviolet detectors using a diode array system detector (described below). Other types of detectors available are those based on electrochemical principles, refractive index, differential viscosity and mass detection. 

Emphasis was therefore put on analytical procedures able to determine many elements in parallel and/or requiring almost no previous separation. procedures preferred were X-ray fluorescence using a Am source and Si(Li)-detector, atomic absorption spectrophotometry, gamma spectrometry using tracer isotopes and Ge(Li)-detector and acid-base titrations with recording of the pH-volume derivative. Table 2 summarises the use of these methods for the different elements, and it also gives a rough indication of interferences, sensitivity and accuracy obtained. 

Sensitive Optical Detectors. More sensitive optical techniques that have been used with CE include fluorescence, refractive index, chemiluminescence, Raman spectrophotometry, and circular dichroism. The most sensitive optical detection method used in CE is laser-induced fluorescence (LIE), which is capable of detection limits in the 10 to 10" mol (or better) range. This detection mode is easily accomplished with analytes that are either easily labeled with a fluorescent substrate (e.g., intercalators for double-stranded DNA) or are naturally fluorescent (e.g., proteins or peptides containing tryptophan). CE systems have also been interfaced with mass spectrometers, and electrochemical detection methods have been developed, although such detectors must be isolated electrically from the electrophoretic voltages. 

Both molecular and atomic detectors have been used in combination with SCF extractors for monitoring purposes. Thus, the techniques used in combination with SFE are infrared spectroscopy, spectrophotometry, fluorescence spectrometry, thermal lens spectrometry, atomic absorption and atomic emission spectroscopies, mass spectrometry, nuclear magnetic resonance spectroscopy, voltammetry, and piezoelectric measurements. 

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

Section I covers the more conventional equipment available for analytical scientists. I have used a unified means of illustrating the composition of instruments over the five chapters in this section. This system describes each piece of equipment in terms of five modules - source, sample, discriminator, detector and output device. I believe this system allows for easily comparing and contrasting of instruments across the various categories, as opposed to other texts where different instrument types are represented by different schematic styles. Chapter 2 in this section describes the spectroscopic techniques of visible and ultraviolet spectrophotometry, near infrared, mid-infrared and Raman spectrometry, fluorescence and phosphorescence, nuclear magnetic resonance, mass spectrometry and, finally, a section on atomic spectrometric techniques. I have used the aspirin molecule as an example all the way through this section so that the spectral data obtained from each... 

Different types of detectors have been developed and many are under development. These are based on distinctive physicochemical properties of plastics and employ different techniques such as x-ray, near-infrared spectrophotometry, fluorescence, and optical measurement of transparency and color. Automatic systems consisting of a platform for selection according to plastics topology, a number of identification and detection steps, and adequate checks on the efficiency of separation following detection have been developed. The Poly-Sort system described above is one such example. 

Residues are determined in the purified extracts by chromatographic or immunochemical techniques. In the chromatographic systems, thin-layer chromatography (TEC), liquid chromatography (LC), and GC, the analytes are separated on plates or columns and determined by colorimetry, by spectrophotometry (ultraviolet (UV), infrared (IR, Fourier transform infrared (FTIR)), by fluorescence, by selective detectors (in GC analysis ECD, flame photometric (FPD), nitrogen/phosphorus (NPD, TSD), etc.), or by MS. Separations may also be achieved by... 

Wcssman reviewed a number of instruments used for uranium analyses and ranked their relative measurement sensitivities [32]. The methods include atomic absorption spectrophotometry, colorimetry, neutron bombardment, fission etched track detectors, fluorimetry, laser-induced fluorescence spectrometry, a-spectrometry, isotope dilution mass spectrometry, and spark source mass spectrometry. The majority of urinary bioassay measurements have been performed by fluorimetry, while environmental survey and baseline measurements have been performed by fluorimetry, a-spectrometry, and induced coupled plasma source mass spectrometry. 

The detection methods used include spectrophotometry, chemiluminescence, fluorescence, amperometry, conductometry, thermometry and potentiometry with ion-selective electrodes or gas sensors. We have focused our attention only on the electrochemical detectors. Some examples of applications of reactor biosensors with the specification of enzyme used, reactor type and detection system are summarized in Table 5. 

Note A amperometry BI bead injection CL chemiluminescence F fluorescence FAT flow analysis technique HPLC/DAD high-performance liquid chromatography with diode array detector LOD detection limit MCFIA multicommutated flow injection analysis MPFS multipumping flow systems MSFIA multisyringe FIA P potentiometry RSD relative standard deviation SF sampling frequency SFA stopped-flow analysis SP spectrophotometry. 

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