Analytical chemistry

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The Analytical Chemistry deals as a branch of chemistry with the qualitative and quantitative analysis of chemical and biochemical substances (in this context as analytes designated). It plays an important role in almost all chemical sub-disciplines, for example in food and environmental analysis , in forensic analysis (e.g. in the court-proof determination of alcohol, drugs or poisons in blood and urine), in pregnancy tests (through evidence a steroid hormone in urine), the determination of glucose in the blood, in the large field of clinical-chemical analysis (e.g. of metabolic parameters or tumor markers), in the quality control of industrial products such as B. of metals and alloys, of pharmaceuticals and chemical products, in pollutant analyzes directly at workplaces (e.g. solvents, acrylic esters or chlorine), of oxygen (with the help of the lambda probe), sulfur dioxide or nitrogen oxides in car exhaust fumes, or in the Analysis of surface and marine waters.

Methods of analytical chemistry

Probably the most important distinction is that between qualitative analysis , quantitative analysis and structural analysis :

  • The qualitative analysis asks what in the sense of “which substance is it?” If there is not only a chemical compound but a mixture , the question is “Which (bio) chemical substances are present in the sample ?”. The basic task of qualitative analysis is therefore the identification of substances, possibly after prior enrichment, removal of interfering substances, or after separation.
  • The quantitative analysis , on the other hand, asks how much , i. H. according to what amount of a substance (the analyte ) is present in a mixture (the sample).
    By the way, what exactly “how much” should mean is not so trivial. Mostly the substance concentration is meant here, i.e. the number of molecules of a substance in the sample. Where no individual molecules are to be determined, such as B. When determining the total protein or fat content, a mass concentration is given.
  • The structural analysis asks about the molecular structure of a substance (the chemical structural formula or the crystal structure )

The substance to be determined should ideally be known for the analysis, otherwise it may not be searched for at all. For example, melamine was never looked for in milk (which was added to milk in China and India around 2008 in order to increase the nitrogen content and thus simulate a higher protein content in the Kjeldahlian nitrogen determination ; see Chinese milk scandal ) and therefore not in routine examinations found. Reliable analysis was only possible through a combination of HPLC and mass spectrometry. Plasticizers in pond water (see Foil Pond # Polyvinyl Chloride (PVC) ) are not found if they are not searched for in water analyzes by default.

Qualitative and quantitative analytics are often carried out building on each other. A prerequisite for a qualitative analysis is a sufficiently large amount of analyte in the sample, depending on the detection limit of the method used. The structure determination occupies a special position. With the advent of modern coupling methods (see below), structure-determining analysis methods are also becoming increasingly important in qualitative and quantitative analysis.

In addition to the determination of individual substances in a mixture, sum parameters are often determined - especially when it comes to quick basic information about a sample. Examples are the TOC (Total Organic Carbon, a measure of the total content of organic compounds), the COD (chemical oxygen demand as a measure of the total amount of oxidizable substances), the TEAC assay ( antioxidant capacity of a sample), the total content of protein, dietary fiber or sugar in food, or the total content of aromatic hydrocarbons in fuels.

In polymer analysis, the molecular weight distribution of the polymers is of particular interest, since polymers never consist of molecules of the same molecular mass, but are distributed around a statistical mean value; this mean molecular size or the molecular weight distribution are specific properties of the polymer here.

Finally, there are the various surface analysis methods. These mostly instrumental analytical methods are particularly sensitive and at the same time selective. Examples of these methods are electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS), low-energy ion scattering spectroscopy (ISS = LEIS), Rutherford backscattering Spectrometry (RBS), (Surface) Extended X-Ray absorption Fine Structure [(S) EXAFS], X-ray near-edge absorption spectroscopy (XANES = NEXAFS), X-ray small angle scattering (SAXS) or diffraction of low-energy electrons (LEED).

Wet chemical analysis methods

Wet chemical analysis uses mainly chemical methods for the identification and quantification with the help of simple physical phenomena (weight, colored appearance). With the exception of so-called on-site tests, these methods are no longer of great importance. Examples of qualitative methods are:

But quantitative determinations can also be carried out purely chemically:

  • Photometry
    The strength of the coloration of the solution with the analyte is compared with the coloration of solutions of known concentration. In the case of analytes without their own, characteristic color, a colored compound can be generated by a chemical reaction.
  • Titration (volumetry)
    The solution of a reaction partner of known concentration is slowly added to a solution of the analyte. When the analyte has completely reacted, the added reaction partner or an indicator causesa color change, the formation of a precipitate or some other clearly visible event. The concentration of the analyte can be calculated from the volume of the solution of the reaction partner used.
  • Gravimetry
    The analyte reacts with a reaction partner and forms an insoluble precipitate of known composition; the amount of analyte is determined from its weight (hence the name: gravis is Latin and means “heavy”).

Instrument-based analytics

The number of methods of instrumental chemical analysis has become almost unmanageable. The methods are essentially based on physical measuring principles. Many of these methods can be used for both qualitative and quantitative determinations. Here are just a few examples:

  • Spectroscopy
    Here the wavelength-dependent absorption or emission of electromagnetic radiation is used, which is characteristic of the respective analyte. Electromagnetic radiation can be visible or UV light ( UV / VIS spectroscopy ), infrared light ( IR spectroscopy , Raman spectroscopy ), X-rays (X-ray photoelectron spectroscopy ( XPS ), X-ray fluorescence analysis ( XRF )) or gamma radiation ( Mössbauer effect ). For quantitative element analysis,     atomic absorption spectroscopy , atomic emission spectroscopy and inductively coupled plasmas coupled with optical emission spectroscopy ( ICP-OES ) or coupled with mass spectrometry ( ICP-MS )are mainly used.
  • Mass spectrometry ( MS )
    First, molecules are ionized in a high vacuum or at atmospheric pressure in the gas phase. Electron impact ionization (EI) is most commonly used in a high vacuum . The analyte molecules are ionized by electrons with an energy of 10 to 15 eV. A voltage of 70 volts is often applied to the ion sources in order to be able to compare the mass spectra of different devices with similar source geometries. The methods most commonly used at atmospheric pressure are electrospray ionization and atmospheric pressure chemical ionization . There are other ionization methods : Atmospheric Pressure Photoionization (APPI), Atmospheric Pressure Laser Ionization (APLI), Chemical Ionization (CI), Direct Analysis at Real Time (DART) , Desorption ElectroSpray Ionization (DESI), Fast Atom Bombardment (FAB), Field Desorption (FD) , Field ionization (FI), matrix assisted laser desorption ionization (MALDI), secondary ion mass spectrometry (SIMS); Thermal ionization (TIMS). After ionization, the ions are transported into the analyzer as an ion stream via acceleration electrodes (individual lenses). The masses of the intact molecular ions and the so-called fragment ions (molecular ions can break and form fragments) are determined. The mass-selective separation can be carried out with various analyzers: sector field mass spectrometer, quadrupole mass spectrometer , time-of-flight mass spectrometer , ion trap mass spectrometer , ICP mass spectrometry (ICP-MS).
  • Nuclear magnetic resonance spectroscopy (NMR)
    This special type of spectroscopy makes use of magnetic interactions between atomic nuclei and electrons in the analyte molecules. There is a vast number of special detection methods (for example COESY, NOESY), so-called 1D, 2D and 3D NMR, etc. A special variant of NMR is so-called MRT (magnetic resonance tomography), which is used as imaging Procedure in medicine has gained considerable importance.
  • Chromatography
    The aim here is to separate different substances. For this purpose, the analyte mixture is dissolved in a solvent ( mobile phase ), which thenflows througha solid carrier substance ( stationary phase ) ( liquid chromatography ). Alternatively, the analyte mixture can also be evaporated past the stationary phase ( gas chromatography ). Due to the varying degrees of interaction with the stationary phase, some analytes are transported quickly, others slowly in the direction of flow. The migration speed is characteristic of the respective analyte.
  • Electroanalytical measurement methods
    Here, electrochemical parameters (redox potential, electrical current, conductivity, etc.) are used to carry out qualitative and quantitative analyzes. Keywords are voltammetry / polarography , coulometry , amperometry , potentiometry , conductometry , electrogravimetry, etc.
  • Chemical sensors and biosensors
    Here, substances are absorbed on a specifically developed sensor layer and transmitted through changes in physical parameters, such as B. current flow, voltage, electrical resistance, absorbance or fluorescence detected. The sensor layer must ensure that the sensor is as specific as possible for the analyte. Research in the field of sensor materials is an important branch of materials science. Gas sensors are widely used. The oxygen lambda probe is the most widely produced chemical sensor in the world.

In addition to their application in classical analysis, spectroscopic methods are of considerable importance for the structure elucidation of chemical compounds. In particular, the combination of several spectroscopic methods is a very effective tool, especially in organic chemistry. In addition, the X-ray structure analysis plays an important role in the elucidation of crystal structures .

In practice, there is very often an overlap between wet-chemical and instrumental analysis: Often a sample is first prepared wet-chemically so that it can be used for an instrumental method. Prior concentration is often required in trace analysis. Many analytes have to be chemically modified ( derivatization or labeling) so that they can be analyzed instrumentally.

Applications

The many different analysis methods allow a multitude of applications, for example:

  • In recent years, especially in environmental and food analysis, enormous advances have been made in the performance of analytical measurement methods and their detection limits . Here, as in forensic chemistry , substances must be identified and quantified.
  • Chemical analyzes are indispensable for quality control in the manufacture of chemical, pharmaceutical and cosmetic products as well as food .
  • The structure determination is used to identify new chemical compounds in the chemical synthesis or the exploration of new natural products .

To monitor production processes, a distinction is made between discontinuous and continuous analysis. In the case of discontinuous processes, samples are taken and examined in the laboratory. In the case of continuous processes, the sample is taken from the production flow and fed directly to an analysis device. The measured value determined is used for regulation, monitoring or quality assurance. Analysis devices for continuous analysis are, for example, infrared NDIR photometers, chemical sensors , electrochemical methods such. B. potentiometry and amperometry , optical methods such as absorptiometry and fluorescence, separation methods such. B. chromatography or electrophoresis, and - now more rarely - automatic titration .

Under automated analysis refers to the coupling of instrumental analysis and data processing, wherein, after possible automated sampling or entry and execution of the analytical determination of the first analog data acquisition and data processing by digitizing using the computer take place. Fully automatic or semi-automatic machines are used for many methods of instrumental analysis, especially for routine determinations.

literature

  • Ralph L. Shriner, Reynold C. Fuson, David Y. Curtin, Terence C. Morill: The systematic identification of organic compounds - a laboratory manual , Verlag Wiley, New York 1980, 6th edition, ISBN 0-471-78874-0 .
  • Skoog, Leary: Instrumental Analytics. Basics, devices, applications. Springer textbook. Springer Verlag, Berlin 1996, ISBN 978-3-540-60450-1 .
  • Einax, Zwanziger , Geiss: Chemometrics in environmental analysis. VCH Verlag, Weinheim 1997, ISBN 3-527-28772-8 .
  • Kromidas , Stavros: Validation in Analytics , Wiley-VCH, Weinheim 1999, ISBN 3-527-28748-5 .
  • Georg Schwedt, Torsten C. Schmidt and Oliver J. Schmitz: Analytical chemistry. Wiley-VCH, 2016, ISBN 978-3-527-34082-8 .
  • Wächter, Michael: Book of tables of chemistry. Data on analytics, laboratory practice and theory , Wiley-VCH, Weinheim 2012, 1st edition, ISBN 978-3-527-32960-1 (data collection for use in chemical and analytical laboratories )
  • Jander, Blasius, Strähle: Introduction to the inorganic-chemical practical course (including quantitative analysis). Hirzel, Stuttgart, 15th, revised. Edition 2005, ISBN 978-3-7776-1364-2 .
  • Jander , Blasius, Strähle, Schweda: Textbook of analytical and preparative inorganic chemistry. Hirzel, Stuttgart, 16., revised. Edition 2006, ISBN 978-3-7776-1388-8 .
  • Otto: Analytical chemistry. Wiley-VCH, 3rd, completely revised. u. exp. Edition 2006, ISBN 978-3-527-31416-4 .
  • Handbook of Experimental Chemistry; Upper secondary level, volumes 3 + 4, analytical chemistry and environmental analysis I + II. Aulis Verlag Deubner & Co. KG, Cologne.

Individual evidence

  1. ↑ Georg Schwedt, Torsten C. Schmidt and Oliver J. Schmitz, Analytische Chemie , 2016, pp. 320–321, ISBN 978-3-527-34082-8 .
  2. ↑ R. Schiewek, M. Schellträger, R. Mönnikes, M. Lorenz, R. Giese, KJ Brockmann, S. Gäb, Th. Benter, OJ Schmitz: Ultrasensitive Determination of Polycyclic Aromatic Compounds with Atmospheric-Pressure Laser Ionization as an Interface for GC / MS . In: Analytical Chemistry . tape 79 , no. 11 , 2007, p. 4135-4140 , doi : 10.1021 / ac0700631 .
  3. ↑ E. Nicklaus: Continuous Analytics in the Service of Process Management, Chemistry in Our Time, 15th year 1981, No. 1, pp. 27–34, ISSN  0009-2851
  4. ↑ Egon Fahr: Automated Analytics . In: Chemistry in Our Time . tape 7 , no. 2 , 1973, ISSN  0009-2851 , pp. 33-41 , doi : 10.1002 / ciuz.19730070202 .

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