Chemical Analysis. Francis Rouessac
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Название: Chemical Analysis
Автор: Francis Rouessac
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119701347
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The detector must be able to follow the rapid variations in concentration almost immediately, i.e. at the moment of each analyte’s elution. For detection by mass spectrometry, there is good reason to be attentive to the sweep speed of the m/z ratio; a slow sequential sweep may lead to a situation in which the concentration in the ionization chamber is not the same from one end of the recording to the other. TOF‐MS (Time‐of‐Flight Mass Spectrometry) does not suffer from this drawback.
2.9.2 Micro Gas Chromatography
In parallel with laboratory equipment, portable devices (μ‐GC) have been developed for fast analyses in the field (Figure 2.1). These devices, sometimes referred to as ‘noses,’ must be light and small, despite a carrier gas reserve enabling their stand‐alone use. The detector used most often is the katharometer with its detection and quantification limits, or more specific variants (μ‐katharometer). The flame ionization detector, requiring an additional gas source, or the mass detector would make heavier and more complex devices. These portable devices may include several analytical modules, operating in parallel for multiple, simultaneous analyses. For each module, simply choose a column with a different polarity and different temperature conditions. In fact, each column is covered with a metal envelope supplied with an electrical current, which enables its temperature programming.
2.10 RETENTION INDEXES AND STATIONARY PHASE CONSTANTS
These parameters have been developed to pursue at least three objectives:
To identify a compound by a more general characteristic than its retention time under predefined conditions. As a result, a system of retention indexes has been developed; it is an efficient and cheap means by which to avoid certain identification errors.
To follow the evolution of a column’s performance over time.
To classify all stationary phases in order to simplify the choice of the column best adapted to a particular kind of separation problem. The polarity or chemical nature of a stationary phase does not allow prediction of which column will be optimal for a given separation. For this, the behaviour of stationary phases with respect to several reference compounds should be examined, in order to determine stationary phase constants.
2.10.1 Kovats Straight‐Line Relationship
Retention indexes are determined using the following general principle. When a mixture containing compounds belonging to a homologous series of n‐alkanes is injected onto a column maintained in isothermal mode, the resulting chromatogram is such that the logarithm of the adjusted retention times t′R(n) increases linearly with the number n of carbon atoms present in the corresponding n‐alkane (Figure 2.17).
Figure 2.17 Kovats straight line graph. Left, isothermal chromatogram for a series of five n‐alkanes (C10−C14). Right, Kovats’ relationship for the chosen stationary phase and for the predetermined analytical conditions.
On a graphical representation, the carbon number n versus log t’R(n) usually yields a series of well‐aligned points:
(2.3)
The adjusted retention time t’R(n) corresponds to the retention time tR of an alkane having n atoms of carbon, minus the dead time tM; a and b are numerical coefficients. The slope of the graph obtained depends on the overall performance of the column and the operating conditions of the chromatograph.
This expression follows on from another linear relation seen in thermodynamics linking the variation in free energy and the equilibrium constant K (∆G = −RT ln K), for a homologous family of compounds in which each term differs from the preceding one by an additional CH2 unit. Since K = kβ,
2.10.2 Kovats Retention Index
A compound (X) is now injected onto the column without changing the settings of the instrument. The resulting chromatogram will enable Ix, the Kovats retention index, to be calculated for X on the specific column employed: this is equal to 100 times the equivalent number of carbon atoms nx of the ‘theoretical alkane’ having the same adjusted retention time as X. Two methods can be used to find the number nx of equivalent carbons of X.
The first is based on the Kovats relationship obtained above (Figure 2.17). This leads to a calculation of nx (therefore Ix), using a spreadsheet, for example.
The second can be used to calculate Ix directly from the adjusted retention times of the two n‐alkanes (n and n + 1 C) that bracket compound X on the chromatogram:
In contrast to the Kovats regression line, the retention indexes depend only on the stationary phase and not on the settings of the chromatogram. In particular, they do not depend on retention times.
In practice, to ensure that the experimental conditions for the two injections are uniform, compound X and the n−alkanes mixture are co‐injected (Figures 2.9 and СКАЧАТЬ