Chemical Analysis: Modern Instrumentation Methods And Techniques - Hardcover

Rouessac, Francis; Rouessac, Annick

Hardcover

ISBN 10: 0470859024 ISBN 13: 9780470859025

Verlag: John Wiley & Sons Inc, 2007

Inhaltsangabe

Completely revised and updated, Chemical Analysis: Second Edition is an essential introduction to a wide range of analytical techniques and instruments. Assuming little in the way of prior knowledge, this text carefully guides the reader through the more widely used and important techniques, whilst avoiding excessive technical detail.

• Provides a thorough introduction to a wide range of the most important and widely used instrumental techniques
• Maintains a careful balance between depth and breadth of coverage
• Includes examples, problems and their solutions
• Includes coverage of latest developments including supercritical fluid chromatography and capillary electrophoresis

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Über die Autorin bzw. den Autor

Francis Rouessac and Annick Rouessac are the authors of Chemical Analysis: Modern Instrumentation Methods and Techniques, 2nd Edition, published by Wiley.

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Chemical Analysis

Modern Instrumentation Methods and TechniquesBy Francis Rouessac

John Wiley & Sons

Copyright © 2007 Francis Rouessac
All right reserved.
ISBN: 9780470859025

Chapter One

General aspects of chromatography

Chromatography, the process by which the components of a mixture can be separated, has become one of the primary analytical methods for the identification and quantification of compounds in the gaseous or liquid state. The basic principle is based on the concentration equilibrium of the components of interest, between two immiscible phases. One is called the stationary phase, because it is immobilized within a column or fixed upon a support, while the second, called the mobile phase, is forced through the first. The phases are chosen such that components of the sample have differing solubilities in each phase. The differential migration of compounds lead to their separation. Of all the instrumental analytical techniques this hydrodynamic procedure is the one with the broadest application. Chromatography occupies a dominant position that all laboratories involved in molecular analysis can confirm.

1.1 General concepts of analytical chromatography

Chromatography is a physico-chemical method of separation of components within mixtures, liquid or gaseous, in the same vein as distillation, crystallization, or the fractionated extraction. The applications of this procedure are therefore numerous since many of heterogeneous mixtures, or those in solid form, can be dissolved by a suitable solvent (which becomes, of course, a supplementary component of the mixture).

A basic chromatographic process may be described as follows (Figure 1.1):

1. A vertical hollow glass tube (the column) is filled with a suitable finely powdered solid, the stationary phase.

2. At the top of this column is placed a small volume of the sample mixture to be separated into individual components.

3. The sample is then taken up by continuous addition of the mobile phase, which goes through the column by gravity, carrying the various constituents of the mixture along with it. This process is called elution. If the components migrate at different velocities, they will become separated from each other and can be recovered, mixed with the mobile phase.

This basic procedure, carried out in a column, has been used since its discovery on a large scale for the separation or purification of numerous compounds (preparative column chromatography), but it has also progressed into a stand-alone analytical technique, particularly once the idea of measuring the migration times of the different compounds as a mean to identify them had been conceived, without the need for their collection. To do that, an optical device was placed at the column exit, which indicated the variation of the composition of the eluting phase with time. This form of chromatography, whose goal is not simply to recover the components but to control their migration, first appeared around 1940 though its development since has been relatively slow.

The identification of a compound by chromatography is achieved by comparison: To identify a compound which may be A or B, a solution of this unknown is run on a column. Next, its retention time is compared with those for the two reference compounds A and B previously recorded using the same apparatus and the same experimental conditions. The choice between A and B for the unknown is done by comparison of the retention times.

In this experiment a true separation had not been effected (A and B were pure products) but only a comparison of their times of migration was performed. In such an experiment there are, however, three unfavourable points to note: the procedure is fairly slow; absolute identification is unattainable; and the physical contact between the sample and the stationary phase could modify its properties, therefore its retention times and finally the conclusion.

This method of separation, using two immiscible phases in contact with each other, was first undertaken at the beginning of the 20th century and is credited to botanist Michael Tswett to whom is equally attributed the invention of the terms chromatography and chromatogram.

The technique has improved considerably since its beginnings. Nowadays chromatographic techniques are piloted by computer software, which operate highly efficient miniature columns able to separate nano-quantities of sample. These instruments comprise a complete range of accessories designed to assure reproducibility of successive experiments by the perfect control of the different parameters of separation. Thus it is possible to obtain, during successive analyses of the same sample conducted within a few hours, recordings that are reproducible to within a second (Figure 1.2).

The essential recording that is obtained for each separation is called a chromatogram. It corresponds to a two-dimensional diagram traced on a chart paper or a screen that reveals the variations of composition of the eluting mobile phase as it exits the column. To obtain this document, a sensor, of which there exists a great variety, needs to be placed at the outlet of the column. The detector signal appears as the ordinate of the chromatogram while time or alternatively elution volume appears on the abscissa.

* The identification of a molecular compound only by its retention time is somewhat arbitrary. A better method consists of associating two different complementary methods, for example, a chromatograph and a second instrument on-line, such as a mass spectrometer or an infrared spectrometer. These hyphenated techniques enable the independent collating of two different types of information that are independent (time of migration and `the spectrum'). Therefore, it is possible to determine without ambiguity the composition and concentration of complex mixtures in which the concentration of compounds can be of the order of nanograms.

1.2 The chromatogram

The chromatogram is the representation of the variation, with time (rarely volume), of the amount of the analyte in the mobile phase exiting the chromatographic column. It is a curve that has a baseline which corresponds to the trace obtained in the absence of a compound being eluted. The separation is complete when the chromatogram shows as many chromatographic peaks as there are components in the mixture to be analysed (Figure 1.3).

A constituent is characterized by its retention time [t.sub.R], which represents the time elapsed from the sample introduction to the detection of the peak maximum on the chromatogram. In an ideal case, [t.sub.R] is independent of the quantity injected.

A constituent which is not retained will elute out of the column at time [t.sub.M], called the hold-up time or dead time (formerly designated [t.sub.0]). It is the time required for the mobile phase to pass through the column.

The difference between the retention time and the hold-up time is designated by the adjusted retention time of the compound, [t'.sub.R].

If the signal sent by the sensor varies linearly with the concentration of a compound, then the same variation will occur for the area under the corresponding peak on the chromatogram. This is a basic condition to perform quantitative analysis from a chromatogram.

1.3 Gaussian-shaped elution peaks

On a...

Details zur Ausgabe

• Verlag: John Wiley & Sons Inc
• Erscheinungsdatum: 2007
• Sprache: Englisch
• ISBN 10: 0470859024
• ISBN 13: 9780470859025
• Einband: Gebundene Ausgabe
• Auflage: 2
• Anzahl der Seiten: 574
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: Nicht verfügbar