The time spent in the mobile phase is the same for all sample components, but the retention is compound specific.
A linear isotherm delivers a symmetric solute band peaks and the peak maximum is independent of the solute amount. A nonlinear isotherm results in unsymmetrical solute bands and the location of peak maximum depends on the solute amount. A nonlinear isotherm can either be formed convex or concave. In case of a concave isotherm, K increases with increasing concentrations resulting in a shallow frontal edge and a sharp rear edge of the peak. This is called fronting. As a consequence, the peak maximum moves to higher retention times see Chap. In the opposite case, the convex isotherm, K, decreases with increasing concentrations resulting in a sharp frontal edge and a shallow rear edge of the peak.
This is called tailing. The peak maximum moves to lower retention times. In practice, linear distribution isotherms are only found if the solute and stationary phase are structurally similar. However, as Fig. One should keep in mind to work at low concentrations in the quasi- linear range if retention values are used for identification see Chap.
Depending on the shape of the distribution isotherm, we distinguish between linear and nonlinear chromatography for the description of chromatographic processes. We further divide into ideal and nonideal chromatography. Ideal chromatography implies a reversible exchange between the two phases with the equilibrium being established rapidly due to a fast mass transfer.
Diffusion pro- cesses that result in band broadening are assumed to be small and are ignored.
In ideal chromatography the concentration profiles of the separated solute should have a rectangle profile. The Gaussian profile obtained in practice demonstrates that these assumptions are not valid. In case of nonideal chromatography these assump- tion are not made. In GC, the mostly used partition chromatography can be classified as linear nonideal chromatography. In that case, almost symmetric peaks are obtained and band broadening is explained by the kinetic theory according to van Deemter . The nomenclature and symbols used in the literature to describe retention para- meters are rather inconsistent, which can be confusing especially while reading older papers.
Abundance B. A summary of the IUPAC nomenclature together with additional and outdated terms is given in the appendix.
Practical gas chromatography : a comprehensive reference
As already mentioned, the chromatographic separation of mixture is based on the different distribution of the components between the stationary phase and the mobile phase. A higher concentration in the stationary phase results in a longer retention of the respective solute in the stationary phase Fig. A separation requires different values of the distribution constants of the solutes in the mixture. The time spent in the chromatographic column is called retention time tR based on the Latin word retenare retain.
Figure 2. The detector signal is proportional to the concentration or mass of the solute in the eluate leaving the column. With older recorders the signal was measured in mV while modern computer-based systems deliver counts or arbitrary units of abun- dance.
If no solute is leaving the column an ideally straight line the so-called baseline is recorded, which is characterized by slight fluctuations called the base- line noise see also Chap. If a solute is leaving the column, the baseline rises up to a maximum and drops then back down again. This ideally symmetric shape is called a chromatographic peak.
Practical gas chromatography a comprehensive reference - Technische Informationsbibliothek (TIB)
Please note that signals in mass spectrometry likewise are called peaks, but they are representation of abundance in mass to charge ratio. The hold-up time tM is the time needed to transport the solute through the column, which is the same for all solutes in a mixture. The hold-up time can be determined by injecting a compound, an inert or marker substance, that is not retained by the stationary phase, but that can be detected with the given detection system, e. In reality, the compounds listed above are not ideal inert substances.
Depending on the chromatographic column, the conditions must be chosen in a way that the retention by the stationary phase is negligible, e. The hold-up time can also be determined based on the retention time of three consecutive n-alkanes or other members of a homologous series:. Furthermore, tM can be calculated based on the column dimensions and carrier gas pressure:. Software tools are available from major instrument manufacturers, such as the flow calculator from Agilent Technologies , which can be used to calculate tM. Furthermore, retention times are influenced by the column dimensions and the operation condi- tions column head pressure, gas flow, temperature.
The reproducibility of these parameters was quite limited in the early days of gas chromatography, but has improved tremendously with the modern instruments in use these days. Multiplying the retention time with the gas flow Fc of the mobile phase results in the respective volumes: retention volume, adjusted retention volume, and hold-up volume:. A more reproducible way to characterize retention is the use of relative retention values instead of absolute values.
The retention factor is dimensionless and expresses how long a solute is retained in the stationary phase compared to the time needed to transport the solute through the column. Assuming the distribution constant K is independent of the solute concentration linear range of the distribution isotherm , k equals the ratio of the mass of the solute i in the stationary Wi S and in the mobile phase Wi M at equilibrium:.
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The higher the value of k, the higher is the amount of the solute i in the stationary phase, which means the solute i is retained longer in the column. Consequently, k is a measure of retention. Using Eqs. The retention time is directly proportional to the column length and indirectly proportional to the average linear velocity of the mobile phase according to this equation.
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If two analytes have the same retention time or retention volume on a column, they are not separated and we call this coelution. A separation requires different reten- tion values. The bigger these differences, the better is the separation efficiency or selectivity of the stationary phase for the respective pair of analytes. Often, one analyte is used as a reference and the retention of the other analyte is related to this retention standard see also Chap.
The selectivity of liquid stationary phases is mostly determined by two para- meters: the vapor pressure of the solutes at column temperature and their activity coefficients in the stationary phase. The liquid stationary phase can be considered as a high boiling solvent with a negligible vapor pressure and the analytes are dissolved in this solvent. The partial vapor pressure of the solutes is equal to their equilibrium concentration in the gas phase above the solvent.
The correlation between the concentration of a solute in solution liquid stationary phase and in the gas phase is described by Henrys law:. If we assume that adsorption processes at the interfaces are negligible and taking further simplifying assumptions into account e. R gas constant T absolute column temperature [Kelvin] VS molar volume of the liquid stationary phase MS molecular weight of the liquid stationary phase dS density of the liquid stationary phase.
If we examine the separation of two analytes, we obtain the following equation for the separation factor or the relative retention by combining Eqs.
Practical Gas Chromatography a Comprehensive Reference 2014
The log-transformed version of Eq. The equation contains two terms: the vapor pressure term, sometimes not quite correctly called boiling point term, and the activity term, which is also called solubility or interaction term. According to Eq. The vapor pressure term depends on the structure of the two analytes and is independent of the chosen stationary phase. However, it is influenced by the column temperature. This term does not contribute to the separation if the two analytes possess the same vapor pressure.
A separation is only possible in this case, if the activity coefficients are different. As we will see later in this chapter, the two terms can act concordantly or contrarily.
The activity term expresses the strength of the intermolecular forces between the solvent stationary phase and the dissolved solutes. The stronger the force of attraction, the higher is the portion of the solute in the stationary and consequently the retention time. The intermolecular forces depend on the structure of the interacting partners, solvent and solute, such as type and number of functional groups, and spatial alignment sterical hindrance. Examples are ethers, aldehydes, ketones, alcohols, and nitro- and cyano- compounds.
This requires polarizable structures in the molecule such as double bonds or aromatic structures. Nonpolar molecules are molecules without a dipole moment that are not prone to the induction of a dipole moment. Typical examples are the saturated hydrocarbons. Intermolecular forces are forces of attraction or repulsion between valence- saturated, electrical neutral molecules that are in close proximity.
The interaction energy decreases with increasing distance between the interacting partners; more precisely, it is inversely proportional to the sixth power of the distance. While we are mostly interested in chromatographic retention caused by the intermolecular forces, they are also responsible for the so-called cohesion properties such as melting and boiling point, solubility, miscibility, surface tension, and interface phenomena.
In gas chromatography the following intermolecular forces van der Waals force are important:.