Atomic emission spectrometry

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The atomic emission spectrometry ( AES ), often optical emission spectrometry ( OES ) or flame spectroscopy ( flame photometry ), is a method of atomic spectroscopy . It is used for the quantitative and qualitative analysis of solid, liquid and gaseous samples. The method is based on the fact that excited atoms emit electromagnetic radiation characteristic of the chemical element and thus provide information about the composition of the sample. The atoms are excited by an external supply of energy, e.g. B. via a flame, an arc , a spark or an inductively coupled plasma (ICP), and the transition to the plasma state .

Flame Atomic Emission Spectrometry (F-AES)

The structure of an F-AES corresponds to that of an F-AAS without a hollow cathode lamp

In flame atomic emission spectrophotometry (F-AES), a material sample (analysis substance ) is placed in a flame, e.g. B. evaporated as a solution and the vapor fed to the flame. The external valence electrons are excited by the thermal energy of the flame and raised to a higher energy level. When returning to the basic state, the previously supplied energy is released as light energy; the atoms emit their element-specific spectrum, which is dispersed and analyzed in the spectrometer. The flame emission spectrum is measured with a flame photometer or, as it is now more common on the market, with a flame atomic absorption spectrometer in emissions mode. The most common application of emission measurement with the flame is the determination of alkali metals in the field of pharmaceutical analysis. The method is very sensitive and easy to carry out.

A laboratory flame photometer

In favor of cheaper analysis (no lamp required), the essential advantages of a measurement in atomic absorption (better linearity, wider working range, less dependence on the flame temperature) are dispensed with. In the pharmaceutical sector, there are sometimes 30-year-old regulations that prescribe measurement in emissions, although atomic absorption has now become established in all other areas.

Optical emission spectrometry with inductively coupled plasma (ICP-OES)

ICP-OES stands for “ inductively coupled plasma optical emission spectrometry ”, ie “optical emission spectrometry using inductively coupled plasmas”. The "A" in the older name ICP-AES stands for atomic , which is somewhat misleading, since ion lines play a dominant role in OES and not atomic lines.

The inductively coupled plasma method is based on the use of a very hot (approx. 10,000 K) argon plasma to excite the optical emission of the elements to be analyzed. Greenfield and Fassel worked out the basics independently of one another in 1964/65. The first commercial device was introduced in 1975 and has been in routine use in industry since around 1985. The ICP-OES technology is now very widespread in environmental analysis, materials research, and the metal and pharmaceutical industries.

principle

A plasma is an ionized gas that contains electrons and ions in addition to atoms. Because of its high ionization energy (15.76 eV) compared to the elements to be determined , its chemical inertness , its comparatively low price and the lack of band spectra, argon (monatomic gas) is usually used as the gas. After ignition, the energy is transferred by a Tesla spark through the high-frequency field in the coils. Free electrons are now accelerated by the applied field and heat the plasma by colliding with the atomic cores. Due to the high particle density in the plasma, the plasma and sample aerosol heat up to 6,000–12,000 K (depending on the RF power of the high frequency generator ). The prevailing temperatures are locally different, a distinction is made between ionization, electron and excitation temperatures. The excitation temperature of approx. 6,000 K is particularly important. The sample aerosol is directed through the middle of the plasma flow without affecting its stability / equilibrium.

construction

The most important parts of an ICP spectrometer are the high frequency generator (27 MHz or 40 MHz), plasma torch, sample nebulizer and the actual spectrometer. The monochromator of modern OES is mainly set up in the echelle arrangement, as this technology requires a much better resolution than in the AAS due to the continuous emission of the spectrum. A polychromator is used most frequently , as it enables the simultaneous measurement of many elements in a short time and in a very stable manner. Usually echelle polychromators are used in conjunction with a CCD area detector . The properties of argon plasma can best be used in this combination:

  • Large dynamic measuring range
  • Multi-element technology
  • Good long-term stability for large series of measurements

The electromagnetic waves can be picked up by the plasma torch at two different locations.

  1. axial, d. H. from the end of the torch (extended axis) and
  2. radial, d. H. of the page.

Microwave Plasma Torch Atomic Emission Spectrometry (MPT-AES)

Microwave plasma torch atomic emission spectrometry (MPT-AES) is a trace analysis method and is used for sensitive elemental analysis . The advantage of this method is the relatively simple structure and the use of inexpensive components. In particular, the gas consumption is significantly lower compared to the ICP-OES.

Here, too, a distinction is made between spectral interference and non-spectral interference. Chemical disturbances are of little concern, as most chemical compounds are dissociated by the high temperatures in the induction zone (10,000–12,000 K) of the plasma. The spectral disturbances arise from emission lines of the foreign elements (interferences) and molecules in the sample matrix. Which includes:

  • Direct superposition of lines
  • Continuum radiation from the matrix
  • Emission of molecular bands such as: -OH, C 2 , CN, NO, N 2

These disorders can be caused by:

  • suitable underground adaptation
  • the consideration of several lines per element
  • Spectral unfolding of the line by measuring blank solution / analyte / interferer
  • Inter-element correction
  • Standard addition method
  • changed direction of observation

remove.

The non-spectral interferences include the physical properties of the sample solutions, such as:

  • density
  • Surface tension
  • viscosity

These can have a lasting effect on the nebulization properties, nebulizer chamber aerodynamics and sample transport. Furthermore, non-spectral interferences include changes in the excitation conditions in the plasma due to:

  • Temperature changes
  • Changes in the number of electrons in the plasma (impedance)

They can be eliminated through suitable matrix adjustments and standard addition methods .

Comparison with other procedures

Detection limits (NWG) (3 times background noise)
method Detection limits in ppt *
ICP / OES pneumat. Atomizer > 30
ICP / OES ultrasound approx.> 10
Graphite furnace / AAS > 0.1
ICP / MS > 0.02
* These NWG can only be reached under optimal conditions.

The advantage of ICP-OES compared to flame atomic absorption spectrometry (F-AAS) is a considerably higher temperature of the plasma compared to the flame (10,000 K compared to 2800 K). This not only increases the degree of atomization ( Boltzmann distribution ), but also ionizes the excited atoms of the elements to be determined. This in turn has a decisive advantage over AAS, since ion lines, in contrast to atom lines, are quite insensitive to excitation disturbances at high temperatures. In addition, a longer dwell time and better temperature homogeneity, precision / reproducibility and detection limits are achieved. A simultaneous multi-element analysis of up to 70 elements is state of the art today.

application

The ICP-OES is mainly used today in trace and water analysis . In the environmental sector, soil, plants, waste and fertilizers are measured for their elemental composition using ICP-OES. ICP-OES is ideally suited for the analysis of highly radioactive samples, as no disruptive radioactivity is introduced into the analyzer part (optics, semiconductor chip, amplifier), only the emitted light is analyzed (unlike, for example, with mass spectrometry ) and a Almost 100 percent suction of the plasma gases is state of the art today (from around 2000).

See also

literature

  • DA Skoog, JJ Leary: Instrumental Analytics. Springer, Berlin 1996, ISBN 3-540-60450-2 .
  • DC Harris: Quantitative Chemical Analysis 7th Edition, WH Freeman and Company, New York 2003, ISBN 0-7167-7694-4 .
  • K. Cammann: Instrumental Analytical Chemistry. Spektrum Akademischer Verlag, 2000, ISBN 3-8274-0057-0 .
  • J. Nölte: ICP emission spectrometry for practitioners. Basics, method development, application examples. Verlag Wiley-VCH, Weinheim 2002, ISBN 3-527-30351-0 .
  • G. Wünsch: Optical analytical methods for the determination of inorganic substances. In: Göschen Collection. Vol. 2606, Verlag de Gruyter, Berlin, ISBN 3-11-003908-7 .
  • J.-M. Mermet, E. Poussel: ICP Emission Spectrometers: Analytical Figures of Merit. In: Applied Spectroscopy. 49, 1995, pp. 12A-18A ( doi : 10.1366 / 0003702953965588 ).

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