Analytical chemistry

Analytical chemistry is concerned as a branch of chemistry with the identification and quantification of chemical substances (in this context called analytes). She plays in almost all chemical sub-disciplines a significant role and is often the subject of current public debate, such as in environmental analysis.

Basic types of analytical chemistry

The most important distinction is between qualitative analysis, quantitative analysis and structural analysis:

• The qualitative analysis asks for the What the meaning of " What a substance that is? " If there is not just one chemical compound but a mixture, the question " What substances are present in the sample? ". Basic task of the qualitative analysis is therefore the identification of substances ( performing a detection reaction, optionally after previous suppression or separation ).
• In contrast, the quantitative analysis asks about the how much, that is, after what amount of a substance ( the analyte ) in a mixture ( the sample) is available. What "how much" is intended to mean exactly the way, is not so trivial. Most here is meant the amount of substance concentration, so as to specify how much moles in caffeine per liter of coffee.
• Structural analysis asks for the molecular structure of a substance ( chemical structural formula or of the crystal structure )

Qualitative and quantitative analysis are often based successive performed: For the quantitative analysis of the substance to be determined should ideally be known. Requirement for a qualitative analysis is a sufficiently large amount of analyte in the sample, depending on the detection limit of the method used. A special position is occupied by the structure determination. With the advent of modern coupling methods (see below) but are structure - determining methods of analysis and in the qualitative and quantitative analysis more important.

In addition to the determination of individual substances in a mixture often sum parameters are determined - especially when it comes to fast basic statements about a sample. Examples are the TOC (total organic carbon, a measurement of the total content of organic compounds ), the COD (chemical oxygen demand, as a measure for the total amount of oxidizable material), or TEAC assay ( anti-oxidant capacity of a sample).

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

Finally, there are the various methods of surface analysis. The special feature of these analytical methods is that they can represent a particularly sensitive way and at the same time selectively surface properties. 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 = TAL ), Rutherford Backscattering spectrometry (RBS ), ( Surface) Extended X -ray absorption fine structure ((S) EXAFS ), X-ray near edge absorption spectroscopy ( XANES = NEXAFS ) or low energy electron diffraction (LEED ).

Wet chemical analysis

The wet - chemical analysis makes use of for the identification and quantification of chemical methods only; any instruments to help take the physical methods, can not be used, but this device for the automation of the analysis (for example, continuous flow analysis ) is not excluded. Examples of qualitative methods are:

• Detection reactions colored complex formation reactions or precipitation by precipitation reactions
• Flame color For example, many metal ions stain a Bunsen burner flame in a characteristic manner

However, quantitative determinations can be carried out purely chemical:

• Photometry The analyte reacts with a reagent to form a colored complex. The intensity of the color is then compared with the color of solutions of known concentration.
• Titration ( volumetric ) To a solution of the analyte solution of known concentration of a reactant is added slowly. If the analyte is completely reacted, the added reactant and an indicator produces a color change, precipitate formation, or some other highly visible event. Of the volume of the spent solution of the reactant can calculate the concentration of the analyte.
• Gravimetry The analyte reacts with a reactant and forms an insoluble precipitate known composition; from the weight of which is the amount of analyte determined (hence the name: gravis is Latin and means " heavy" ).

Instrumental Analysis

→ Main article: Instrumental Analysis

More important than the purely chemical evidence are the methods of instrumental chemical analysis, the number has almost become unmanageable nowadays. The methods are essentially based on physical principles of measurement. Many of these methods are useful for both qualitative and quantitative determinations. Again, just a few examples:

• Spectroscopy Here, the wavelength- dependent absorption or emission of electromagnetic radiation is used, which is characteristic for the respective analyte. Electromagnetic radiation can in this case the visible or UV light to be (UV / VIS - spectroscopy), infrared light (IR - spectroscopy, Raman spectroscopy ), X-ray ( X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF) analysis ) or gamma radiation ( Mössbauer effect). For the quantitative elemental analysis by atomic absorption spectroscopy, atomic emission spectroscopy and inductively coupled plasmas are mainly coupled with optical emission spectroscopy (ICP -OES) or coupled with mass spectrometry ( ICP -MS ) are used.
• Mass spectrometry ( MS) The analyte molecules are ionized in any manner and brought into the gas phase in vacuo. The masses of the intact molecular ions and the so-called fragment ions ( molecular ions can shatter and may form fragments) can be determined. There are a plethora of different ionization methods and types of detector ( electron impact ionization (EI ), chemical ionization ( CI), electrospray ( ESI), atmospheric pressure chemical ionization (APCI ), matrix assisted laser desorption ionization ( MALDI), fast atom bombardment (FAB ), secondary ion mass spectrometry (SIMS ), field desorption (FD), field ionization (FI ), thermal ionization ( TIMS ), ICP mass spectrometry (ICP -MS), sector-field mass spectrometer, quadrupole mass spectrometer, time of flight mass spectrometers, ion trap mass spectrometer, etc. ).
• Nuclear magnetic resonance spectroscopy (NMR) In this particular type of spectroscopy magnetic interactions between nuclei and electrons are exploited in the analyte molecules. There are a vast number of specific detection methods ( for example COESY, NOESY ), so-called 1D, 2D and 3D NMR etc. A particular variety of NMR is the so-called MRI (magnetic resonance imaging ), which as an imaging method has gained significant importance in medicine.
• Chromatography The objective here is the separation of different substances. For this, the analyte mixture is dissolved in a solvent (mobile phase), then the solid support (stationary phase ) by flow ( liquid chromatography ). Alternatively, the analyte mixture may be evaporated out of the stationary phase by ( gas chromatography). By varying degrees of interaction with the stationary phase, some analytes are fast, others slowly transported in the forward direction. The rate of migration is characteristic of the particular analyte.
• Electroanalytical methods of measurement Here electrochemical parameters ( redox potential, electric current, conductivity, etc. ) may be used to perform qualitative and quantitative analyzes. Tags are voltammetry / polarography, coulometry, amperometry, potentiometry, Conductometry, Electrogravimetry etc.
• Chemosensors This substance to be absorbed on a sensor layer and the change of physical variables, such as capacitance, electrical resistance or piezoelectricity detected. Widely finds gas sensors in use.

Have spectroscopic methods. On its application in the classical analysis, 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 determining crystal structures.

In practice, there are often overlaps of wet - chemical and instrumental analysis: Often, a sample is first wet - chemically processed to be suitable for an instrumental method. As hard as evaporable substances must be chemically modified ( derivatization ), so they can be analyzed by gas chromatography, or it must be particularly complex mixtures initially be separated by wet - chemical methods before instrumental analysis can come in to.

Applications

The many analytical methods allow a large number of applications, for example:

• The structure elucidation is used to identify new chemical compounds in chemical synthesis or in the exploration of new natural products and the understanding of their properties.
• Especially in the environmental and food analysis enormous progress in the performance of analytical measurement methods and their detection limits were made in recent years. Here, as well as in forensic chemical substances must be identified and quantified.
• In the production of chemical, pharmaceutical and cosmetic products and of food in the context of quality control chemical analyzes are essential.

To monitor production, a distinction between discontinuous and continuous analysis. For batch process, samples are taken and analyzed in the laboratory. For continuous process, the sample is taken from the production flow, and fed directly to an analyzer. The determined value serves for regulating, monitoring or quality assurance. Analyzers of continuous analysis including infrared NDIR photometer, gas sensors, Wärmeleitgeräte, devices based on electrochemical methods such as potentiometry and amperometry, gas chromatograph or rarely used as automatic titrator.

Under automated analysis refers to the coupling of analytical instrumentation and data processing, carried out by maximum automation of sampling or entry and the analytical determination of the first analog data acquisition and data processing after digitization with the assistance of computers. Here are many methods of instrumental analysis especially in routine determinations or fully automatic molding machines are used.