Читать книгу Chemical Analysis - Francis Rouessac - Страница 60

1 Chromatography is a separation method and also an analytical method. It is a very reliable method, with continued innovation in order to separate, purify, identify, and quantify chemical species.

2 It is based on the adsorption or partition, depending on various parameters and interactions, of chemical species between two immiscible phases, one fixed or stationary (SP) and the other mobile (MP).

3 There are many chromatographic techniques that differ in the process used, the nature of phases, physico‐chemical equilibria, the physical state, etc., and just as many classifications are possible depending on the criterion retained!

4 To each compound, we assign a distribution factor K = CS /CM as a function of its concentration in the two phases. For each type of chromatography, thermodynamic relationships apply. From K (which is not a constant), we can calculate ΔH, ΔS, and ΔG for the separation in question.

5 A document inseparable from any chromatographic analysis, the chromatogram, or elution curve, represents the graphic recording of concentration over time. It provides information on the analysed sample and on the quality of the separation in progress.

6 One peak = one compound is the ideal situation. At column outlet, we treat the ideal peak as a Gaussian peak (hence the parameters σ, δ, ω), but the reality may be different. An ideal chromatogram has peaks that do not overlap and which are at least somewhat spaced out. A peak alone does not enable the absolute identification of a compound.

7 A number of parameters can be used to better characterize each separation: efficiency, retention, separation, and resolution. The flow rate of the mobile phase has an influence on efficiency. Many empirical equations, not reported in this chapter, are offered to mathematically define the behaviour of analytes during elutions with a gradient.

8 The speed of progression of the mobile phase with respect to the stationary phase has an essential influence on separation quality. A hyperbolic function, formed from the addition of three independent terms, known as the Van Deemter equation, demonstrates the existence of an optimal linear speed, i.e. leading to a minimum value of HETP.

9 First established for GC, and later adapted to HPLC, the three terms of the previous equation refer to preferential paths (turbulent diffusion), to longitudinal diffusion, and to resistance to mass transfer.

10 The quantification of analytes using chromatography is founded on the relationship between the mass and the area of the corresponding peak on the chromatogram. There are three methods, called the external standard method, internal standard method, and internal normalization method. The first is by far the most used because it is the quickest when it comes to processing a large number of analyses. Chromatograms equipped with a sample holder carousel and an automatic injection device make this method very reliable.