Quinine fluorescence

The dynamic range of the fluorescence experiment is related to a number of factors but it can be orders of magnitude. It is possible, for example, to determine quinine in water from nanomolar to millimolar concentration by direct measurement. Quinine fluorescence is familiar to most people that have noticed the blue glow of quinine tonic water in sunlight. 

To verify this point, a much more sensitive analytical method, fluo-rescencet was used to search for traces of quinine in the permeate solution. The magnitude of the absorbance in Fig. 18C indicates that the pyridine concentration in the permeate is 7 X 10 M. With fluorescence analysis it is possible to detect 5 X 10 M quinine in the presence of 7 X 10 M pyridine. However, no quinine fluorescence could be detected from the... 

Quinidine, C20I isomeric with quinine, fluorescent, probably the most powerful... 

Although the quantum yield of fluorescence is readily determined by reference to quinine fluorescence as described by Calvert and Pitts (1966), those of the other processes can only be obtained by difference. Phosphorescence is usually too weak to be observed in solution at room temperature, but can be measured if the drug is held in a glassy matrix at low temperature. The usual procedure is to dissolve the drug in ethanol and immerse in liquid nitrogen. The phosphorescence accessory of the fluorimeter incorporates a mechanical chopper enabling the phosphorescence to be observed free of interference from any fluorescence. Because of the difference in temperature and matrix, it is not possible to compare the phosphorescence yield with that of fluorescence. Nevertheless, phosphorescence is worth measuring because it is an important indicator of the capacity of a molecule to populate its triplet state. 

Figure 5.46 Absorption and fluorescence spectra of anthracene and quinine Curve A, anthracene absorption Curve B, quinine absorption Curve C, anthracene fluorescence Curve D, quinine fluorescence. (From Guilbault, used with permission.)...

FIGURE 15.6 Schematic catalyst library screening by fluorescence, (a) Ethanol electro-oxidation reaction, (b) pH-dependent Quinine fluorescence, (c) Screening of catalyst library film during ethanol oxidation in ethanol/ quinine solution monitored by a UV Vis lamp. 

Description of Method. Quinine is an alkaloid used in treating malaria (it also is found in tonic water). It is a strongly fluorescent compound in dilute solutions of H2SO4 (f = 0.55). The excitation spectrum of quinine shows two absorption bands at 250 nm and 350 nm, and the emission spectrum shows a single emission band at 450 nm. Quinine is rapidly excreted from the body in urine and is easily determined by fluorescence following its extraction from the urine sample. 

Chloride ion is known to quench the intensity of quinine s fluorescent emission. For example, the presence of 100 ppm NaCI (61 ppm Ch) gives an emission intensity that is only 83% of that without chloride, whereas the presence of 1000 ppm NaCI (610 ppm Ch) gives a fluorescent emission that is only 29% as intense. The concentration of chloride in urine typically ranges from 4600 to 6700 ppm Ch. Flow is an interference from chloride avoided in this procedure ... 

Samples of urine that do not contain quinine still contain a small amount of fluorescent material after the extraction steps. Flow can the quantitative procedure described earlier be modified to take this into account ... 

One approach is to prepare a sample blank using urine known to be free of quinine. The fluorescent signal for the sample blank is subtracted from the urine sample s measured fluorescence. 

The fluorescent emission for quinine at 450 nm can be induced using an excitation frequency of either 250 nm or 350 nm. The fluorescent quantum efficiency is known to be the same for either excitation wavelength, and the UV absorption spectrum shows that 250 is greater than 350- Nevertheless, fluorescent emission intensity is greater when using 350 nm as the excitation wavelength. Speculate on why this is the case. 

Cmchonine, C19H22ON2. This alkaloid is usually present in cinchona and cuprea barks. One of the best sources is Cinchona micrantha bark. It occurs in the crude quinine sulphate mother liquors. The mixed alkaloids recovered from these may be extracted with ether to remove quinidine and cinchonidine and the insoluble residue boiled with successive small quantities of alcohol, from which cinchonine crystallises on cooling. The crude alkaloid is neutralised with dilute sulphuric acid and the sulphate recrystallised from boiling water. Cinchonine so prepared contains quinidine, from which it may be freed by crystallisation from boiling alcohol until it ceases to exhibit fluorescence in dilute sulphuric acid. It will then still contain 10 to 15 per cent, of dihydrocinchonine, which may be removed by reprecipitation as the cuprichloride, B. 2HC1. CuClj, or by the simpler mercuric acetate process of Thron and Dirscherl. ... 

Detection. Cinchonidine is distinguished from quinine and quinidine by not being fluorescent in dilute sulphuric acid, and by not giving the thalleioquin reaction and from cinchonine in being laevorotatory and more soluble in ether, and in the sparing solubility of its tartrate. 

Example 9. Quinine may be determined by measuring the fluorescence intensity in 1M H2S04 solution (Section 18.4). Standard solutions of quinine gave the following fluorescence values. Calculate the correlation coefficient r. 

If the fluorescence intensity of the test solution containing quinine was found to be 16.1, then an estimate of the concentration of quinine (x fig mL 1) in this unknown could be... 

Important organic applications are to the determination of quinine and the vitamins riboflavin (vitamin B2) and thiamine (vitamin Bj). Riboflavin fluoresces in aqueous solution thiamine must first be oxidised with alkaline hexacyanoferrate(III) solution to thiochrome, which gives a blue fluorescence in butanol solution. Under standard conditions, the net fluorescence of the thiochrome produced by oxidation of the vitamin Bj is directly proportional to its concentration over a given range. The fluorescence can be measured either by reference to a standard quinine solution in a null-point instrument or directly in a spectrofluorimeter.27... 

Procedure. Measure the fluorescence of each of the above solutions at 445 nm, using that containing 62.0 mL of the dilute quinine solution as standard for the fluorimeter. Use LF2 or an equivalent primary filter (/cx = 350 nm) and gelatin as the secondary filter if using a simple fluorimeter. 

Now prepare test solutions containing, say, 0.00025 and 0.00045 mg quinine per mL. Determine their concentrations by measuring the fluorescence on the instrument and using the calibration curve (see Note). 

Quercitrin 149, 279, 280, 323 Quinaldic acid 171 Quinine alkaloids 88 Quinine, pH-dependent change of fluorescence color 91 Quinoline alkaloids 66... 

Figure 7. Dependence of the fluorescence quamum yield of BMPC on solvent viscosity ( ) in linear alcohols, from methanol to dccanol, at 25°C, (o) in absolute ethanol between 200 and 298 K. The quantum yields were measured on optically thin samples (Am <0.2). The value in ethanol, 5.7x10, was determined relative to quinine sulfate in 0.5 mol 1" HjSO ((j)p=0.55 [62]) and 9,10-diphenylanthracene in cyclohexane (4ip=0.90 [63]). It was then used as a reference for the determinations in the other alcohols.

If the refractive indices of the solvents used for the sample and the fluorescence standard are not the same, a further correction must be made. For example, quinine sulfate in 0.1 N H2S04 (Or = 0.5) is commonly used as a fluorescence standard. If the fluorescence of the sample whose relative quantum yield is desired is determined in benzene, a correction factor of 27% must be applied in determining the relative areas under the fluorescence bands. If ethanol is used, this correction is only 5.5%. 

The first photoelectric fhiorimeter was described by Jette and West in 1928. The instrument, which used two photoemissive cells, was employed for studying the quantitative effects of electrolytes upon the fluorescence of a series of substances, including quinine sulfate [5], In 1935, Cohen provides a review of the first photoelectric fluorimeters developed until then and describes his own apparatus using a very simple scheme. With the latter he obtained a typical analytical calibration curve, thus confirming the findings of Desha [33], The sensitivity of these photoelectric instruments was limited, and as a result utilization of the photomultiplier tube, invented by Zworykin and Rajchman in 1939 [34], was an important step forward in the development of suitable and more sensitive fluorometers. The pulse fhiorimeter, which can be used for direct measurements of fluorescence decay times and polarization, was developed around 1950, and was initiated by the commercialization of an adequate photomultiplier [35]. 

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