Standard addition was used to accurately quantify for quinine in an unknown urine sample containing approximately 100 ìg cm‑³ of quinine.

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Instrumental Methods of Analysis

Determination Of Quinine In Urine By Fluorescence Spectroscopy

Name:                 Andrew Holmes, Tana Epsom and Rachel Nicholls

Course:         BSc Forensic Science

Year:                 2

Unit:                 Instrumental Methods of Analysis

Date:                 09.10.02

Summary

Standard addition was used to accurately quantify for quinine in an unknown urine sample containing approximately 100 μg cm³ of quinine. The fluorescence intensity of each standard addition solution was measured using a fluorescence spectrophotometer. The result was found to be that the unknown urine sample contained a concentration of 121 μg cm³ of quinine. This was found by extrapolating the data on the standard addition graph of the variation in fluorescence with added quinine sulphate concentration.

Aims:

  • To determine the variation in fluorescence with quinine sulphate concentration of standard addition solutions.
  • To determine the quinine sulphate concentration of an unknown urine sample.
  • To assess the data and judge whether other components of the urine interfere.
  • To adhere to all safety regulations required when working in a laboratory.
  • To carry out the experiment to a high level of accuracy.
  • To employ standard addition techniques.

Introduction

Fig.1

Structure Of Quinine

Quinine is a white solid, C20 H24 N2 O2.3H2O. It is a poisonous alkaloid found in the bark of the South American cinchona tree, although it is now usually produced synthetically. It forms salts and is toxic to the malarial parasite, and so quinine and its salts are used to treat malaria. In small doses it may be prescribed for colds and influenza. In dilute solutions it has an astringent taste and is added to some types of tonic water.

The analysis of quinine in urine is important in forensic science as quinine is frequently used as an adulterant in illicit heroin samples. Its presence can therefore be tested for in order to determine the presence of heroin in the body.

Fluorescence and Phosphorescence

Fluorescence and phosphorescence are phenomena associated with transitions between more than one excited state for a species. After excitation to a higher level, an electron drops by a non-radiative process to an intermediate level and then to the ground state giving rise to emission at a longer wavelength than that of the exciting radiation. In fluorescence the process occurs very rapidly. Fluorescence and phosphorescence come under the general heading photoluminescence. In fluorescence the energy transitions do not involve a change in electron spin whereas, phosphorescence does involve a change in electron spin and therefore, occurs more slowly than fluorescence. Because the vibrational levels of both ground and excited states are similar, the fluorescence spectrum is often a sort of mirror image of the exciting absorption spectrum. The lifetime of an excited singlet state is usually 10-9-10-6 seconds and fluorescence lifetimes fall in this range.  The lifetime is defined as the time required for the population of the excited state to decrease to 1/e of its original value after the excitation source is turned off.

Fluorescence is expected in molecules that are aromatic or contain multiple – conjugated double bonds with a high degree of resonance stability. Both classes of substances have delocalised π-electrons that can be placed in low-lying excited singlet states. In polycyclic aromatic systems where the number of π-electrons available is greater than in benzene, these compounds and their derivatives are usually much more fluorescent than benzene and its derivatives.

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As can be seen from the structure of quinine (fig.1), quinine is a polycyclic aromatic compound and as such, quinine can be estimated by fluorescence spectroscopy at levels as low as 0.1μg/cm3

Changes in the system pH, if it has an effect on the charge status of the chromaphore, may influence pH. A change in the pH of the solution may alter the shape of the excitation spectrum of the fluorescent compound. The presence of anions, such as chloride, bromide, iodine and nitrate may affect fluorescence. Quinine sulphate is highly fluorescent in 0.1M H2SO4, but becomes non-fluorescent in 0.1M ...

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