Chemical Analysis. Francis Rouessac

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Название Chemical Analysis
Автор произведения Francis Rouessac
Жанр Химия
Серия
Издательство Химия
Год выпуска 0
isbn 9781119701347



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      2D GC. Optimization sometimes involves dual chromatography on two different stationary phases (for example, polar and nonpolar) with a single set‐up including a carrier gas inlet shared by the two columns. The first chromatograph is used to isolate a peak, possibly corresponding to an unresolved mixture, which then goes through a second column for another separation.

      2.9.1 ‘Fast’ and ‘Ultra‐Fast’ Chromatography

GC type Ramp (°C/min) Analysis time (min) Peak width (s) Column length (m) Internal diameter (μm)
Conventional Conventional oven (1–20) ~30 5–10 15–100 250–320
Fast chromatography Conventional oven (20–100) 5–10 0.5–5 5–15 100–250
Ultra‐fast chromatography Resistive heating (60–1,200) ~1 0.05–0.2 2–5 50–100
Schematic illustration of ‘Ultra-fast’ chromatogram.

      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.

      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

Schematic illustration of kovats straight line graph.

      On a graphical representation, the carbon number n versus log t’R(n) usually yields a series of well‐aligned points:

      (2.3)log left-parenthesis t Subscript upper R left-parenthesis n right-parenthesis Baseline minus t Subscript upper M Baseline right-parenthesis equals log t prime Subscript upper R left-parenthesis n right-parenthesis Baseline equals a dot n plus b

      The adjusted retention time t’R(n)