8) Explain the principle of operation of mass spectrometric detector, which is used for analysis of gases.

Mass spectrometry (mass-spectroscopy, mass spectroscopy, mass spectral analysis, mass spectrometric analysis) is a method for studying a substance by determining the mass-to-charge ratio (quality) and the number of charged particles produced by a particular process of action on a substance. The history of mass spectrometry is conducted from the fundamental experiments of John Thomson at the beginning of the 20th century. The term "-metry" was termed after the ubiquitous transition from the detection of charged particles by photographic plates to electrical measurements of ion currents. The essential difference between mass spectrometry and other analytical physico-chemical methods is that optical, X-ray and some other methods detect radiation or energy absorption by molecules or atoms, and mass spectrometry directly detects the particles of substances themselves. Mass spectrometry and chromatography: it is possible to know the mass of a molecule, its gross formula, chromatography, while helping to analyze the mixture by separating the components at the time of entry into a detecting mass spectrometer. In the presence of databases, it is easy to identify the components of the mixture and the compounds themselves. High sensitivity (in some cases - nanograms sample). Easy to understand by a chemist. Mass spectrometers are vacuum devices that determine the masses of atoms (molecules). Apparatus uses the physical laws of motion of charged particles in electric and magnetic fields to produce a mass spectrum.

One of the element of the simplified mass spectrometer described by us is a charged particle detector. The first mass spectrometers used a photographic plate as a detector. Now we use dynode secondary electron multipliers in which an ion, falling on the first dynode, knocks out a beam of electrons from it, which in turn, reaching the next dynode, knock out even more electrons from it, etc. Another option is photomultipliers, Recording glow that occurs when bombarded with phosphor ions. In addition, microchannel multipliers, diode array type systems, and collectors are used to collect all the ions that hit a given point in space (Faraday's collectors). The most important technical characteristics of mass spectrometers are sensitivity, dynamic range, resolution, scanning speed. The most important characteristic in the analysis of organic compounds is sensitivity. In order to achieve as much sensitivity as possible with an improvement in the signal-to-noise ratio, one resorts to detection by selected selected ions. The gain in sensitivity and selectivity is enormous, but when using low-resolution devices one has to sacrifice another important parameter - reliability. Tasks to be performed by the MS: Identification of substances; Chemical analysis of mixtures; Elemental analysis; Isotope analysis; Isotope separation.

Principle of the method: 1. The sample is introduced into the ionization source, where the molecules are ionized. 2. The resulting positive ions are derived from the ionization zone, accelerated by an electric field and simultaneously focused into a beam. Neutral molecules are removed by a vacuum pump. 3. The stream of accelerated ions enters the mass analyzer, where the ions are separated by Mass. 4. Separated ion beams enter the detector, where the ion current is converted into Electrical signal, which is amplified and recorded.

Detectors of ions: First, a photographic plate was used as a detector; Currently, dynode secondary-electronic; Multipliers in which an ion, falling on the first dynode (ie, an electrode in Photoelectric multiplier, which serves to enhance the flow of electrons due to their secondary Emission (emission of electrons by a surface of Me)), knock out a beam from it Electrons, which in turn, falling on the next dynode, Knock out even more electrons from it, and so on. 2. microchannel multipliers, diode array type systems and Collectors that collect all the ions that hit this point Space (Faraday's collectors).

In gas chromatography, a wide cool theater is used in which one can be divided into integral (а) and differential (б). Integral registers changes in the time of the total number of all components. The differential measures the instantaneous concentrate. Differential dictators are in turn subdivided into concentration and streaming in the concentration detectors. The signal is determined by the current concentration in the cell and is repeatedly recorded depending on the detector's flow rate. A detector of this type. The stream detector detects the signal once, the signal is determined by the instantaneous value of the concentration, it does not depend on the flow velocity. An example of such a detector is a flame ionization detector.

The general requirements for detectors are: sufficient sensitivity to solve a particular problem; Low inertia; A small dependence of the readings on the parameters of the experiment (i.e., temperature, pressure, flow rate, and so on); Linear relationship between readings and concentration in a wide range of its variation; Zero line stability; Ease of recording the signal and transferring it to a distance; Simplicity, cheapness.

Universal is the katharometer detector for thermal conductivity. The principle of operation is based on changing the temperature of heated children of sensitive elements depending on the thermal conductivity of the surrounding gas which is determined by its composition. The detector measures the difference in the thermal conductivity of a pure carrier gas and carrier gas mixture.