Ultraviolet/visible spectroscopy sensitivity

Ultraviolet/visible spectroscopy is one of the most common methods for online monitoring of analytes from a chromatographic column. This type of detector is fast (many data points can be taken every second) and sensitive (parts per million concentration levels can register a significant response). When two or more compounds elute through the flow cell at the same time, the UV signal is typically the superposition of the signals from each... 

Despite these apparent limitations, fluorescence methods are employed for the determination of a wide variety of compounds. The selectivity of these analyses arises from the choice of both excitation and emission wavelengths, whereas the sensitivity of the analyses arises from the fact that absolute as opposed to relative measurements of light emission are made. This can be compared to ultraviolet-visible spectroscopy, where the ratio of incident to transmitted light is determined. Fluorescence measurements also have the advantage of a wide linear range of analysis. 

The use of ultraviolet (UV) spectroscopy for on-line analysis is a relatively recent development. Previously, on-line analysis in the UV-visible (UV-vis) region of the electromagnetic spectrum was limited to visible light applications such as color measurement, or chemical concentration measurements made with filter photometers. Three advances of the past two decades have propelled UV spectroscopy into the realm of on-line measurement and opened up a variety of new applications for both on-line UV and visible spectroscopy. These advances are high-quality UV-grade optical fiber, sensitive and affordable array detectors, and chemometrics. 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

Ultraviolet-visible (UV-Vis) spectrophotometric detectors are used to monitor chromatographic separations. However, this type of detection offers very little specificity. Element specific detectors are much more useful and important. Atomic absorption spectrometry (AAS), inductively coupled plasma-atomic emission spectroscopy (ICPAES) and inductively coupled plasma-mass spectrometry (ICP-MS) are often used in current studies. The highest sensitivity is achieved by graphite furnace-AAS and ICP-MS. The former is used off-line while the latter is coupled to the chromatographic column and is used on-line . 

The application of ultraviolet and visible spectroscopy to the identification and measurement of carbenium ions derived from aromatic and dienic monomer has already been discussed (see Sect. II-G-2). The use of this technique to monitor stable carbenium salts is also well known. We have finally stressed in a preceding section that the fate of certain anions could be followed spectrophotometrically during a cationic polymerisation. The limits of detection allowed by the values of the extinction coefficients of all these species and by the sensitivity of present-day instruments is 10 to 10 M. 

Atomic spectroscopy in the ultraviolet-visible region involves transitions of valence shell electrons and the spectra are thus sensitive to the chemical and physical form of the element of interest. For sensitive quantitative work the sample is normally converted to free atoms in the gas phase. This can be achieved by vaporization from a furnace, by... 

Analysis is an integral part of research, clinical, and industrial laboratory methodology. The determination of the components of a substance or the sample in question can be qualitative, quantitative, or both. Techniques that are available to the analyst for such determinations are abundant. In absorption spectroscopy, the molecular absorption properties of the analyte are measured with laboratory instruments that function as detectors. Those that provide absorbance readings over the ultraviolet-visible (UV-vis) light spectrum are commonly used in high-performance liquid chromatography (HPLC). The above method is sufficiently sensitive for quantitative analysis and it has a broader application than other modes of detection. 

Once the transient species has been formed, it has to be monitored by some form of kinetic spectroscopy, typically with ultraviolet-visible absorption or emission, infrared (time-resolved infrared or TRIR) (74), or resonance Raman (time-resolved resonance Raman or TR3) (80) methods of detection. The transient is usually tracked by a probe beam at a single characteristic frequency, thereby giving direct access to the kinetic dimension. Spectra can then be built up point by point, if necessary, with an appropriate change of probe frequency for each point, although improvements in the sensitivity of multichannel detectors may be expected to lead increasingly to the replacement of the laborious point-by-point method by full two-dimensional methods of spectroscopic assay (that is, with both spectral and kinetic dimensions). 

Finally, it has been shown that sulfuranyl radicals, for example, 52 (cf. Scheme 6), exhibit absorption spectra, which can be detected with ultraviolet-visible (UV-Vis) spectroscopy absorption maxima were found to be sensitive to the a- versus 7t-electronic configurations of these radicals and thereby to be a sensitive guide in the assignment of the electronic stmcture of sulfuranyl radicals <1985JOC2516>. 

Organic stractures can be determined accurately and quickly by spectroscopic methods. Mass spectrometry determines mass of a molecule and its atomic composition. NMR spectroscopy reveals the carbon skeleton of the molecule, whereas IR spectroscopy determines functional groups in the molecules. UV-visible spectroscopy tells us about the conjugation present in a molecule. Spectroscopic methods have also provided valuable evidence for the intermediacy of transient species. Most of the common spectroscopic techniques are not appropriate for examining reactive intermediates. The exceptions are visible and ultraviolet spectroscopy, whose inherent sensitivity allows them to be used to detect very low concentrations for example, particularly where combined with flash photolysis when high concentrations of the intermediate can be built up for UV detection, or by using matrix isolation techniques when species such as ortho-benzyne can be detected and their IR spectra obtained. Unfortunately, UV and visible spectroscopy do not provide the rich structural detail afforded by IR and especially H and NMR spectroscopy. Current mechanistic studies use mostly stable isotopes such as H, and 0. Their presence and position in a molecule can... 

The simplest method for the determination of amino acids is reaction with ninhydrin. Ninhydrin reacts with both primary and secondary amino acids to produce Ruhemann s purple, which can be detected by ultraviolet (UV)-visible spectroscopy. The reaction requires heat, and a reducing agent is generally added to stabilize the color formation. Primary amines are detected with the greatest sensitivity at 570 nm, while the absorption maximum for secondary amines is 440 nm. If both primary and secondary amines are to be determined, a common absorption wavelength of 500 nm is employed however, this leads to decreased sensitivity. Under optimal... 

The derivatization of analytes is very important in several branches of analytical chemistry. It expands the fields of application of various spectroscopic techniques (ultraviolet-visible (UV-vis), fluorimetry, nuclear magnetic resonance (NMR), and mass spectroscopies), and in several cases increases also the selectivity and sensitivity of these techniques. Derivatization is also an inevitable tool in all chromatographic and electrophoretic techniques. In gas chromatography (GC), the main importance of derivatization is the improvement of the volatility/thermal stability of the analytes, and in all of the discussed separation techniques it has the potential of increasing the selectivity of the separation (including enantiomeric separations) and the sensitivity of the detection. 

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