Phosphorescence, analytical method

P. E. Nelson and L. M. Boyd, Phosphorescence detection in high-performance liquid chromatography of drugs. In Analytical Methods in Forensic Chemistry (M. H. Ho, ed.), Ellis Horwood, New York, 1990, pp.225-235. 

Phenanthridine, like other monoazaaromatics which fluoresce only weakly in nonpolar solvents, is subject to marked fluorescence activation by hydroxylic solvents. Recent studies have shown this to result from the effects of solvent on vibronic interactions between re,77 and 77,77 electronic states, and the effect of solvent changes on the phosphorescence half-life has been similarly explained.237 Measurements of fluorescence and, more especially, phosphorescence characteristics have been proposed as analytical methods for mixtures containing phenanthridines238, 239 and detailed studies of the emission spectra of phenanthridine,240, 241 its cation,241 9-methylphenanthri-dine,242 and phenanthridine-iV-oxide243 have been reported. A... 

Phosphorescence of metal ion complexes with organic ligands is used in inorganic analysis much less frequently than fluorescence even though phosphorimetric-analytical methods are promising because the use of phosphoroscopes results in significantly lower detection limits, due to the reduction of blank magnitude caused by fluorescence of the... 

The energy captured during the course of photon absorption can be released through a variety of processes that occur concurrently with emission of a photon (Figure 9.1c). Phosphorescence and fluorescence are transformations of this kind, which are exploited in other analytical methods (cf. Chapter 11). 

The experiments described above produce accurate decay constants only when sir is negligible. The apparent kinetic parameters that are obtained when this is not the case involve all of the k and W values simultaneously. It is not possible under these circumstances to obtain the actual decay constants from these experiments. Experimental methods have been developed, however, that allow extraction of the individual k and W values in the presence of slr. One method applies to the regime where sir is dominant, that is, W > k, whereas the other is applicable when W < k. The latter method has been applied to biopolymer ODMR and involves deconvolution of phosphorescence decays measured during continuous microwave saturation of pairs of triplet sublevels. Microwave saturation creates a pseudo-two-level system whose decays are easily deconvoluted and are amenable to analysis. The analytical development of the microwave-saturated phosphorescence decay method is rather lengthy, so it is not discussed in this chapter. Detailed descriptions of the method may be found elsewhere. 

In recent years, there has been a rapid growth in the number of publications that report the use of surfactant monomers or micelles to improve the analytical perfommice of various spectroscopic (UV-visible spectrophotometry, fluorimetry, phosphorimetry, chemiluminescence and atomic spectroscopy), and electrochemical (especially amperometry) methods [1]. The unique properties of surfactants have been recognized as being very helpful to overcome many problems associated with the use of organic solvents in these methods. Surfactant-modified procedures yield sensitivity and/or selectivity improvements in determinations commonly performed in homogeneous solution, whereas certain analytic methods (such as room-temperature phosphorescence in solution) can be exclusively conducted in organized media. 

SS-RTP exhibits in all cases poorer detection limits than low-temperature phosphorescence. However, it is potentially useful as a routine analytical method because (1) no cryogenic equipment, expensive and rather cumbersome to use, is needed (2) no time-consuming degassing of the solvent is mandatory and (3) chromatographic separations can be performed on the substrate before the analysis. These features make SS-RTP particularly suitable to exploit new detection schemes. So, SS-RTP is a convenient means of observing delayed fluorescence in those... 

Despite of the need for less expensive optics and light sources, PL wiU certainly constantly remain the most significant analytical luminescence method. PL can be divided into two main subclasses fluorescence and phosphorescence, and these analytical methods, including CL, are often reviewed in various articles [4, 5] and in several other sources [6-8]. 

CL-based analytical methods have several advantages such as rapidity, specificity and cost-effectiveness one minor disadvantage is that they require semi-skilled personnel. There is no requirement for an excitation source as in fluorescence and phosphorescence, a monochromator (often not even a filter), or the use of radioactive or hazardous chemicals. Thus, CL is advantageous for routine analysis. 

Electronic fluorescence techniques may be used to detect inorganic compounds, which exist as ions in solution, using one of several methods. Photoluminescence (or simply luminescence) refers to the photoexcitation of an analyte in solution, caused by absorption of visible or ultraviolet radiation, followed by emission at a longer wavelength. Luminescence is classified as either fluorescence or phosphorescence. Fluorescence is an allowed radiative transition from the first excited singlet state, while phosphorescence is a spin-forbidden transition from the first excited triplet state. Typical fluorescence lifetimes are nanoseconds to sub-microseconds, while typical phosphorescence lifetimes are microseconds to seconds. For many inorganic substances, fluorimetric methods are the best analytical methods. 

In Reference 50, a CE method was developed, for the separation of bupropion, with detection based on phosphorescence in both the direct and indirect modes. In case of direct phosphorescence, the phosphorescent analyte itself is excited by the light of an appropriate wavelength. The indirect mode may increase the intensity of the signal when... 

The active state of luminescence spectrometry today may be judged ly an examination of the 1988 issue of Fundamental Reviews of Analytical Chemistry (78), which divides its report titled Molecular Fluorescence, Phosphorescence, and Chemiluminescence Spectrometry into about 27 specialized topical areas, depending on how you choose to count all the subdivisions. This profusion of luminescence topics in Fundamental Reviews is just the tip of the iceberg, because it omits all publications not primarily concerned with analytical applications. Fundamental Reviews does, however, represent a good cross-section of the available techniques because nearly every method for using luminescence in scientific studies eventually finds a use in some form of chemical analysis. Since it would be impossible to mention here all of the current important applications and developments in the entire universe of luminescence, this report continues with a look at progress in a few current areas that seem significant to the author for their potential impact on future work. 

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

However, since the second half of the eighties, practically no more phosphorescence appears, at least in the analytical literature for quantitative estimations, being nearly completely substituted by sophisticated fluorescence and laser-induced fluorescence methods, mostly applied as detection tools in diverse flowing streams. 

The uses of micelles in chemical analysis are rapidly increasing (Hinze, 1979). Analytical reactions are carried out typically on a small scale and are based on spectrophotometry. At the same time, undesired side reactions can cause major problems, especially when the analytical procedure depends on reactions which are relatively slow and require high temperatures, exotic solvents or high reagent concentrations for completion. Micelles can suppress undesired reactions as well as speed desired ones and they also solubilize reagents which are sparingly soluble in water. In addition it is often possible to make phosphorescence measurements at room temperature in the presence of surfactants which enormously increases the utility of this very sensitive method of detection. 

Luminescence spectroscopy involves three related optical methods fluorescence, phosphorescence, and chemiluminescene. These methods utilize excited molecules of an analyte to give a species whose emission spectrum can provide information about the molecule. In fluorescence, atoms can be excited to a higher energy level by the absorption of photons of radiation. Some features of luminescence methods are increased sensitivity (in the order of three magnitudes smaller than absorption spectroscopy), larger linear range of concentration, and method selectivity (Parsons 1982). 

A review of chemiluminescent and bioluminescent methods in analytical chemistry has been given by Kricka and Thorpe. A two-phase flow cell for chemiluminescence and bioluminescencc has been designed by Mullin and Seitz. The chemiluminescence mechanisms of cyclic hydrazides, such as luminol, have been extensively analysed. " Fluorescence quantum yields of some phenyl and phenylethynyl aromatic compounds in peroxylate systems have been determined in benzene. Excited triplet states from dismutation of geminate alkoxyl radical pairs are involved in chemiluminescence from hyponitrite esters. Ruorophor-labelled compounds can be determined by a method based on peroxyoxalate-induced chemiluminescence. Fluorescence and phosphorescence spectra of firefly have been used to identify the multiplicity of the emitting species. " The chemiluminescence and e.s.r. of plasma-irradiated saccharides and the relationship between lyoluminescence and radical reaction rate constants have also been investigated. Electroluminescence from poly(vinylcarbazole) films has been reported in a series of four... 

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