Chemistry

Chromatography

Chromatography



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Virtual gas chromatograph

Learning objective: The structure and mode of operation of chemical devices, for example a gas chromatograph, are very complex. A virtual device, reduced to a few important aspects, should therefore first convey the basic functionality and operation of the real device and thus facilitate entry (theoretical basic knowledge of gas chromatography is assumed). Based on the basic structure of knowledge obtained in this way, the many other functions of a real device can be more easily understood and integrated into the existing knowledge. We expect this step-by-step approach to make the internship more efficient for you.

Please follow the instructions for the virtual GC.

Please note the occupational health and safety for the GC laboratory.


Chromatography

Chromatography, physico-chem. Process for the analytical or preparative separation of a mixture of substances between two immiscible phases. C. is used as adsorption chromatography, partition chromatography, ion chromatography, permeation chromatography or affinity chromatography. The separation is based on the different migration speeds of different types of particles in one mobile phase along the separation distance due to different dwell times at one stationary phase. The material separation takes place through the continuous exchange of substances and heat between the two phases and the connection of a primary separation effect with kinetic phenomena. Depending on the state of aggregation, the following separation phase combinations are possible: Liquid-solid chromatography (engl. liquid-solid-chromatography, abbr. LSC), Liquid-liquid chromatography (engl. liquid-liquid-chromatography, abbr. LLC), Gas-solid chromatography (engl. Gas-solid-chromatography, abbr. GSC) and Gas-liquid chromatography [engl. Gas-liquid-chromatography, abbr. GLC). A distinction is made according to the geometric design of the isolating distance Column chromatography and Layer- respectively. Flat bed chromatography. The chromatogram development takes place according to the elution technique, displacement technique or frontal technique. The area of ​​application of C. is in the molecular weight range from 10 1 to 10 15.

theory. The C. consists of a multiple repetition of one Separation process as the distribution of a substance between the stationary and mobile phase, obeying a separating function. as Separation function adsorption isotherms, distribution or exchange equilibria are primarily possible. For the mathematical treatment of the separation process, models are used that represent a certain simplification through abstraction.

the Theory of B & # 246den describes the separation distance as consisting of strung together sections, which are also called theoretical b & # 246den and their length or height HETP (engl. Height eequivalent to one ttheoretical plate) is called. The theoretical plate number results from the height of the separation stage and the length of a separation distance n, which is directly proportional to the retention time and inversely proportional to the width of the analog signal (peak).



Chromatography: Fig .: Van Deemter curve.

The dynamic theory is the Van Deemter equation based on: HETP = A. + B / u + C u, in which the scattered diffusion caused by the nature of the stationary phase A., diffusion in the mobile phase B. as well as diffusion and mass transfer between the mobile and stationary phase C. depending on the linear flow rate u the mobile phase are discussed (Fig.).


Separation of substances and phase equilibria

Every substance consists of tiny particles (atoms, ions), inside & # 8221 of this substance (according to the simple particle model) every particle is surrounded on all sides by a & # 8220binding partner & # 8221. In this case there is an equilibrium of forces (of attraction and repulsion), it & # 8220 penetrates & # 8221 no interaction & # 8221 to the outside.

But if we now consider the interface on a substance (at the particle level), not all particles are surrounded by partners. Therefore, a force (e.g. Van der Waals forces) acts from the interface to the outside. These interaction forces at the interfaces cause many physical and chemical "phenomena". One of these phenomena is the so-called & # 8220 adsorption & # 8221. Adsorption is understood to mean the accumulation of gases or liquids on the interface / surface of a solid or liquid due to the interaction forces (also referred to as free surface forces in physics).

With adsorption you have the following & # 8220 phase equilibrium & # 8221


Working group of Prof. Björn Braunschweig

Work areas

The main research areas of the working group are in the field of fluid interfaces and interface-controlled materials. The focus is on disperse systems such as foams or colloids, thin films and the electrochemistry of electrode surfaces. With the help of defined molecular building blocks and their self-organization, tailor-made interfaces and materials are developed from the nano to the macro scale via structure-property relationships. The elucidation of the molecular structure at the respective interface as well as its physicochemical properties under equilibrium and non-equilibrium conditions is an essential part of the research and is based on the application and further development of modern non-linear optical spectroscopy with ultra-short laser pulses.

Methods

  • Sum frequency vibration spectroscopy (SFG)
  • Optical frequency doubling (SHG)
  • Surface Enhanced Infrared Spectroscopy (SEIRAS)
  • Tensiometry and interfacial rheology
  • Voltammetry

Possible topics for work

  • In situ investigations of catalytically active surfaces under potential control
  • Molecular structure and self-organization of polyelectrolytes and proteins at electrolyte / gas interfaces
  • Molecular self-organization at solid / liquid interfaces

Mobile and stationary phase

Your mobile phase is just that Mixture of substanceswhich moves and is to be separated. Your mobile phase needs a solvent that carries the substance mixture through the stationary phase. The solvent can depending on the one liquid (Hexane, ethyl acetate, ...) or an unreactive one gas (Helium, nitrogen, ...).

The stationary phase, on the other hand, does not move. You can do this from one gel, one Solid or one liquid exist. Your mixture of substances in the mobile phase becomes intermolecular because of it Interactions separated with the stationary phase.


The gel filtration & # 8211 basics

As mentioned at the beginning, the gel filtration is based on different molecule sizes or diameters. The gels in the chromatography separation columns are comparable to a sieve that separates larger molecules with the mobile phase & # 8220 by & # 8221 and small molecules. Since you can choose the pore size of the gel, you can often use gel filtration to separate impurities (small molecules). Since there is a separation between small and large molecules, gel filtration is often also referred to as exclusion chromatography or gel chromatography. Even if the comparison with & # 8220 is simplistic & # 8221, you can compare the gel filtration with the effect of a sieve, but in a reciprocal manner (the sieve separates large components and lets small components through).

Since there is no separation by chemical (e.g. hydrolysis) or physical (e.g. electric field) methods during gel filtration, gel filtration is a very gentle method for separating mixtures. This is why gel filtration is often used to separate proteins and carbohydrates.

As already mentioned, the principle of gel filtration is based on a porous gel with which a chromatography column is equipped. This porous gel serves as a stationary phase (the gel & # 8220 moves & # 8221 does not move with the mobile phase, the so-called eluant). If molecules of different molecular sizes now move in the mobile phase over the gel, the stationary phase, molecules with a small diameter penetrate into the pores, while molecules with a larger diameter move on in the mobile phase without & # 8220 obstruction & # 8221. Therefore, larger molecules with the mobile phase also migrate faster through the acid than smaller molecules (diameter smaller than the pore diameter).


Chromatography separation principles

All chromatographic separation processes are based on the different interactions (affinity) of substances with a stationary and a mobile phase.

The mobile phase transports the individual components contained in a sample through the chromatographic system with a transport force that is the same for all particles. During this transport, the sample components change very many times from the mobile to the stationary phase and back. As a result, they are slowed down in comparison to the speed of the mobile phase - and the more so, the longer they stay in or on the stationary phase. Since this dwell time is a specific material property in a given chromatographic system, the individual sample components can be spatially and temporally separated from one another. At the molecular level, two basic principles are used to interpret the substance-specific affinities: the equilibrium of adsorption and distribution. The following images show as a model different phases of a chromatographic separation:

Substance 1 (square symbol) is preferably on the surface of the solid stationary phase due to stronger adsorption or due to better solubility in the liquid stationary phase. Substance 2 (round symbol) has a greater affinity for the mobile phase.

After some time, the flowing fluid has transported the dissolved constituents of the mixture to unoccupied “free” areas of the solid or liquid stationary phase. At the same time, pure flow agent reaches the places of the stationary phase that are covered with adsorbed or dissolved particles. At both points, the substances redistribute themselves according to their affinities between the two phases

After the equilibrium adjustments have been repeated many times, the substances are completely separated from one another.

Regardless of which of the two different molecular mechanisms is responsible for the different distribution of the sample components, such distributions between two phases can theoretically be described by a distribution coefficient K. The partition coefficient is defined as the ratio of the equilibrium concentration of a substance in the stationary (sorbing) phase (cs) to that in the mobile phase (cm):

This relationship is valid for a certain temperature and only for very small concentrations of the amount of substance at which saturation of the stationary phase (cinfinite ) is not reached. The graphical representation of the equation for the partition coefficient leads to sorption isotherms. In a particular chromatographic system, two substances A and B can only be separated if they have different sorption isotherms:

Substance B mainly stays in the stationary phase, which means that its chromatographic migration speed is lower than that of substance A:


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Chromatography

Chromatography, physico-chem. Process for the analytical or preparative separation of a mixture of substances between two immiscible phases. C. is used as adsorption chromatography, partition chromatography, ion chromatography, permeation chromatography or affinity chromatography. The separation is based on the different migration speeds of different types of particles in one mobile phase along the separation distance due to different dwell times at one stationary phase. The material separation takes place through the continuous exchange of substances and heat between the two phases and the connection of a primary separation effect with kinetic phenomena. Depending on the state of aggregation, the following separation phase combinations are possible: Liquid-solid chromatography (engl. liquid-solid-chromatography, abbr. LSC), Liquid-liquid chromatography (engl. liquid-liquid-chromatography, abbr. LLC), Gas-solid chromatography (engl. Gas-solid-chromatography, abbr. GSC) and Gas-liquid chromatography [engl. Gas-liquid-chromatography, abbr. GLC). A distinction is made according to the geometric design of the isolating distance Column chromatography and Layer- respectively. Flat bed chromatography. The chromatogram development takes place according to the elution technique, displacement technique or frontal technique. The area of ​​application of C. is in the molecular weight range from 10 1 to 10 15.

theory. The C. consists of a multiple repetition of one Separation process as the distribution of a substance between the stationary and mobile phase, obeying a separating function. as Separation function adsorption isotherms, distribution or exchange equilibria are primarily possible. For the mathematical treatment of the separation process, models are used that represent a certain simplification through abstraction.

the Theory of B & # 246den describes the separation distance as consisting of strung together sections, which are also called theoretical b & # 246den and their length or height HETP (engl. Height eequivalent to one ttheoretical plate) is called. The theoretical plate number results from the height of the separation stage and the length of a separation distance n, which is directly proportional to the retention time and inversely proportional to the width of the analog signal (peak).



Chromatography: Fig .: Van Deemter curve.

The dynamic theory is the Van Deemter equation based on: HETP = A. + B / u + C u, in which the scattered diffusion caused by the nature of the stationary phase A., diffusion in the mobile phase B. as well as diffusion and mass transfer between the mobile and stationary phase C. depending on the linear flow rate u the mobile phase are discussed (Fig.).


Separation of substances and phase equilibria

Every substance consists of tiny particles (atoms, ions), inside & # 8221 of this substance (according to the simple particle model) every particle is surrounded on all sides by a & # 8220binding partner & # 8221. In this case there is an equilibrium of forces (of attraction and repulsion), it & # 8220 penetrates & # 8221 no interaction & # 8221 to the outside.

But if we now consider the interface on a substance (at the particle level), not all particles are surrounded by partners. Therefore, a force (e.g. Van der Waals forces) acts from the interface to the outside. These interaction forces at the interfaces cause many physical and chemical "phenomena". One of these phenomena is the so-called & # 8220 adsorption & # 8221. Adsorption is understood to mean the accumulation of gases or liquids on the interface / surface of a solid or liquid due to the interaction forces (also referred to as free surface forces in physics).

With adsorption you have the following & # 8220 phase equilibrium & # 8221


Chromatogram

That Chromatogram is this Result of chromatography. You differentiate between outer and inner chromatograms.

A external chromatogram you get, for example, from gas chromatography with a detector. The detector registers the individual components of the separated mixture as a function of the Time. So you have the on the x-axis Time and on the y-axis the frequencywith which your components hit the detector at the respective time. That inner chromatogram shows you the distribution of the individual components of the separated mixture depending on the Place. You don't need a detector here, as your separated mixture is simply stored in the stationary phase is distributed. This is the case, for example, with thin layer chromatography.