Gas chromatography

The gas chromatography (CPG) is, like all the techniques of Chromatographie, a technique which makes it possible to separate from the Molécule S of a mixture possibly very complex of very diverse nature and volatility. It applies mainly to the compounds gas or likely to be vaporized by heating without decomposition. It is used more and more in the principal fields of the Chimie. The mixture to be analyzed is vaporized at the entry of a column , which contains a substance activates solid or liquid called stationary phase , then it is transported through this one using a carrying gas . The various molecules of the mixture will separate and leave the column the ones after the others after a certain amount of time which is function of the affinity of the stationary phase with these molecules.

History

In 1952, A.J.P Martin and A.T. James announced the birth of gas chromatography. This technique lived its golden age between 1955 and 1960, with the invention of the capillary tubes by M.J.E. Golay (1957), of the detector with Ionization with Argon (1958), followed detector to ionization of flame (1958) and detector with capture of electron S (1960). As of the years 1960, progress was directed towards the instrumentation and made it possible to make viable all these inventions. End of the year 1970 at the end of the years 1980, enormous research was undertaken to allow the analysis of all the chemical families of compounds, grace in particular to the development of new injectors and the capillary tubes.

Taking into account its many applications in all the fields of sciences, the chromatography of great effectiveness is regarded as a major change of the XXe century in the field of the analytical Chimie.

Apparatuses

The apparatuses of gas chromatography are called chromatographs . They are mainly made up:

of a furnace (standard revolving heat) which allows adjustable programming of temperature of 20°C (- 100°C for certain systems) 450°C and which is also equipped with a fast cooling system;
of a system of injection , which will make it possible to introduce and make volatile the sample to be analyzed. The injection can be done in a manual or automatic way using a sampler;
of a column ( capillary or filled ), on which the various molecules of the sample injected will separate according to their affinities with the stationary phase;
of a system of detection , which will make it possible to measure the signal emitted by the various molecules and of being able to identify them. For the recording of the signal emitted by the detector, of the software on PC replace advantageously the analogical recorders on paper;
of a system of pressure reducer-regulator for gases used (Helium, Hydrogen, Nitrogen and Compressed air). On the modern chromatographs, one finds electronic systems for the regulation of the gases which are also purified by filter cartridges.

There exist chromatographs various sizes. That goes from the portable (approx. 10 kg) designed for the analyzes on the ground , with those used in the purification of rare gases. Thus Liquid air designed and manufactured a chromatograph of big size for the purification of the Krypton and the Xénon.

To supplement the analyzes, the chromatographs are often coupled with other analytical instruments, in particular for the Spectrometry mass and the infra-red spectroscopy. In perfumery, one uses also the nose, detector extremely sensitive to certain odorous molecules;

Principle of operation

The sample (a volatile liquid) is initially introduced at the head column via a microseringue which will cross a rubber pastille, called septum , to find in a small room upstream of the column called injecting . The injector is crossed by the gas carrying and carried at a temperature appropriate to volatility of the sample. The quantities injected can vary from 0.2 to 5.0 μl.

Then, once made volatile, the various compounds of the sample will be carried by the carrying gas (or carrier gas) through the column and will be detached from/to each other according to their affinity with the stationary phase . The stationary phase can be a liquid not (or little) volatile (chromatography gas-liquid) or an adsorbing solid (gas-solid chromatography). In both cases, the stationary phase will cause a chromatographic phenomenon of retention with the various compounds (called aqueous solutions ). The more the compound has affinity with the stationary phase, the more it will spend time to leave the column. The rough experimental size is called time of retention . In fact the time passes between the injection of the sample and the appearance of the maximum signal of the aqueous solution to the detector. To support the transport of all the compounds through the column ( elution ), it is necessary to determine the good temperature of the furnace. In general, the temperature must be higher than the boiling point of the compounds. One can work at a constant temperature, i.e. with a fixed temperature during all the analysis or with a program of temperature which varies.

At the exit of the column, the compounds meet an essential body which is called detecting . This apparatus evaluates uninterrupted the quantity of each component separated within gas carrying thanks to measurement various properties physique the gas mixture. The detector sends an electronic signal towards a recorder (kind of printer) which will draw the curves of each peak according to their intensity (curve of the Gaussienne type). The whole of the peaks is called chromatogram . Currently and more and more, the software replaces advantageously the recorders papers for the interpretation of the signals sent by the detectors.

Gas

The carrying gas (or carrier gas), is the mobile, dynamic phase of gas chromatography. It is in its flow that one injects the mixture to be analyzed, and it is him which conveys it to the detector through all the column.

In the majority of the cases, it must be inert with respect to the aqueous solutions and of the stationary phase . There are thus four type of gases used: Helium, Hydrogen, Nitrogen and Argon. It can be provided either by cylinders of gas or products by generators (case of Hydrogen and Nitrogen). These carrier gases must be pure, of water, oxygen and light hydrocarbons to avoid all reactions with the aqueous solutions and the stationary phase. This is why specific filters are affixed at the entry of the chromatograph.

The principal property of the carrier gases is their insolubility in the liquids. Their electrical signal will not appear on the chromatogram.

Injectors

Linjector is placed in a metal block whose temperature is controlled in order to ensure a good thermal homogeneity of the system. The sample will be introduced, through a car-sealing pastille called septum , via a microseringue. The sample will be vaporized and the aqueous solutions will cross the injector through a tube out of glass (sometimes metal) called liner (or insert ), thanks to carrying gas, to the column heading. The interest of the liner is to retain the nonvolatile components of the sample, unsuitable by nature with the Chromatographie.

There exist 2 types of injectors. Those for the columns filled (not very many currently) and those for the capillary tubes (most frequent…). The principle remains the same one, it is right a question of design of the room of vaporization as well as connection with the column which changes.

In the case of the capillary tubes, 3 modes of injections can be presented:

injection with division ( Split ): the sample is vaporized and mixed in carrying gas, then the mixture is divided into two parts. More the small portion arrives on the column whereas most important is evacuated. It is called the escape . The ratio of division is regulated on the machine. This mode of injection makes it possible to inject minor amounts of samples concentrated without having to dilute them as a preliminary. Dilution which is sometimes impossible for certain products (essential oils, oil products…) because the Solvant would mask the detection of made up most volatile. On the other hand, it is often necessary to use temperatures of high injections which could lead to the degradation of certain aqueous solutions.

injection without division ( splitless ): the sample is vaporized and mixed in carrying gas, but the mixture is not divided into two parts. There remain a few seconds in the liner before being transferred on the column (approx. 95% of the product). The 5% remainder are evacuated by the opening of the valve of escape. This method is used when the sample to be analyzed is very diluted and possibly very dirty (containing not-volatile residues). It also makes it possible to analyze the very volatile compounds (more volatile than solvent of dilution) by concentrating them on the column heading which will be colder than the injector.

injection in the column ( one-column ): there is no stage of Vaporisation. The sample is directly mixed with the carrier gas and is cold injected on the column. This method requires a syringe and a specific injector (without septum and heating). The advantages are to be able to inject the sample in liquid form without causing selective vaporization in the needle (more precision on the volume of injection) and at the lowest possible temperature (that of the column) to avoid the degradation of the thermolabile compounds . The undesirable effects of the septum (traces of Polymeric S carried by carrying gas) are also eliminated. On the other hand, of the disadvantages as the accumulation of made up nonvolatile in the column can present itself and the reliability of this technique is not always with go.

Column

The column is placed in a furnace to maintain a temperature sufficient in order to keep the aqueous solutions in gas phase during the analysis.

The column consists of a more or less long tube (which can be in Silice, Stainless steel…) furnished with an inert solid support (such as for example a Zeolite) and in rather fine particles. This support is chemically impregnated of a product called stationary Phase whose affinity with the components of the product to be analyzed is largest. By injecting a sample at the entry of the column, the product is vaporized by heating and the difference in affinity of the components towards the stationary phase makes it possible to retain more or less a long time certain components with respect to the others. The carrying gas will convey these components towards the exit.

There exist two types of column: filled 2 meters and capillary 20 to 100 meters. The difference between those is due to the stationary type of phase which is contained there: for the columns filled, they are silica grains on which rests a liquid film whereas for the capillary tubes the film is directly deposited on the walls of the column. The film can be simply deposited or grafted. If it is necessary to reach high temperatures, one will choose the Clerc's Office, question of stability.

In general, the pressure loss is rather large between the entry and the exit of the column, also a certain pressure is applied so that the carrying gas can forward the various components to the exit.

If the sample to be analyzed is a mixture of gas (oxygen, nitrogenizes, methane, ethane…), in order to delay the progression of these components in the column, this one is cooled to the extreme, one puts it in liquid nitrogen which boils with -196°C.

Detector and recorder

At the exit of this column, a very sensitive detector is placed, for example:

a TCD: electric detector, based on the principle of the Wheatstone bridge: the passage of the components will vary the tension, this variation is due to the difference in conductibility of each component;
a FID: detector with ionization of flame: a tension about the hundred volts is maintained between the tube of the flame and an electrode surrounding the latter. When the molecules cross the flame, they are ionized what causes between the electrodes an electric current which is then amplified.
a ECD: detector with electronic absorption: electrons are emitted, in general by a radioactive source (Rayonnement beta), and cross gas; when an electron meets a gas molecule, it can be captured, which varies the intensity of the current of electrons, this intensity being measured uninterrupted.
One ms: Mass spectrometer, using mainly the electronic impact or chemical ionization like modes of ionization.

One records this variation on the recorder according to the time of exit of the peak, known as time of retention. At present, there exist sensitive ultra detectors making it possible to detect some ppm (left per million) of a component.

The nature of the components is given by time to the end of which the peak appears. To connect time and chemical nature, one makes use of a reference sample. To the exit, the chromatogram will provide a series of more or less separate, more or less large and more or less broad peaks. The surface of a peak is, according to the method of detection, proportional to the quantity of product represented by this peak. By measuring the surface of each peak and by reporting it to the entire surface of all the peaks, one determines the percentage of each component contained in the product (method of measurement by standardization). At the beginning, in 1955, this calculation was done with the hand (either using a planimeter, or by cutting of the peak followed by the weighing of cut out paper), but since the beginning of the years 1970 this measurement is done in an automatic way by electronic integrators.

There exist three kinds of chromatographs:

industrial Chromatograph,
Chromatograph with column,
capillary Chromatograph.

First is used for the purification of the products, the second uses a product (stationary phase) impregnated or grafted on an inert solid support with strong capillarity, and in the last, the stationary phase is fixed directly on the internal surface of the hollow capillary tube, whose internal diameter is about the half or quarter of millimetre.

NB: Small information for the smokers. We analyzed the fair cigarette smoke and after counting, this one contains more than 300 different components of which majority in benzene, naphtenic and aromatic form, and produced with very long highly carcinogenic chains.

Since 1962, this technique of analysis is usually used in oil industry. Indeed, to have fast results during drillings, it is essential to have a method of analysis which gives almost instantaneously and automatically reliable results. Since, this technique developed and extends today to all the fields: chemistry, biology, astronomy, pharmacy, industry of platic matters etc This technique can for example make it possible to analyze Polymère S (rubber and plastics). A piece of polymer is pyrolyzed, i.e. subjected to an intense heat which degrades it and transforms it into several gases (hydrocarbons); the polymer can be put on a metal support (ferromagnetic) heated by induction, the abolition of the magnetic properties at a certain temperature (Point of Curie) makes it possible to obtain a reproducible temperature, attack very quickly. The produced gases are separated by gas chromatography, at exit of capillary is placed a detector; for example, a flame which becomes more luminous when a gas molecule leaves, since the gas is flammable (the variation of luminosity is recorded by a diode photoréceptrice); or even the gas is subjected to a bombardment of electrons, the passage of a molecule causes an absorption of the jet of electrons (one records uninterrupted the intensity of the flow of electron crossing gas). If one traces the intensity of the signal (luminous intensity or intensity of the current of electrons) according to time, one obtains a series of peaks, each peak representing the passage of a kind of molecules. Each polymer will give a series of peaks (a reason) which is clean for him and which is to some extent its “signature”.

The chromatography of the type gas-liquid, largely used nowadays compared to that of gas-solid type, is based on the division of the Soluté between a gas mobile phase and a stationary phase liquidates immobilized on an inert support.

External bonds

illustrations
illustrations

See too

ionic Spectrometry of mobility

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