Atomic emission spectroscopy

Atomic emission spectrometry (AES ), often also optical emission spectrometry ( OES) called, is a method of atomic spectroscopy. It is used for quantitative and qualitative analysis of solid, liquid and gaseous samples. The method is based on that excited atoms emit characteristic electromagnetic radiation for the chemical element, and thus provide information about the composition of the sample. The excitation of the atoms via an external power supply, such as a flame, an electric arc, a spark or an inductively coupled plasma ( ICP), and the transfer into the plasma state.

Flame atomic emission spectrometry (F -AES)

In flame - atomic emission spectrometry (English: flame atomic emission spectrophotometry, F AES ) a sample of material ( analyte ) is placed in a flame, e.g. evaporated as a solution and supplied to the vapor of the flame. Due to the thermal energy of the flame, the outer valence electrons are excited and raised to a higher energy state. When relapse in the ground state, the previously supplied energy is emitted as light energy; while the atoms emit their element-specific spectrum is dispersed and analyzed in the spectrometer. The flame emission spectrum with a flame photometer or, as more frequently represented in the market, with a flame atomic absorption spectrometer in emission mode measured. The most common application of the 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 perform.

In favor of a cheaper Analytics ( no lamp required) is dispensed with significant advantages of a measurement by atomic absorption ( better linearity, additional workspace, lower dependence on the flame temperature ). In the pharmaceutical industry, there are some 30 year old provisions requiring a measurement in emission, although in all other areas since the atomic absorption has prevailed.

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

ICP -OES stands for English " optical inductively coupled plasma emission spectrometry ," meaning " by means of inductively coupled plasma optical emission spectrometry ". The 'A ' in the earlier designation ICP-AES stands for atomic, but this is somewhat misleading because in the OES ion lines play a dominant role and not atomic lines.

The method of inductively coupled plasma, based on the use of a very hot (about 10,000 K) argon plasma for the excitation of the emission of the optical elements to be analyzed. The Basics developed independently Greenfield and Fassel 1964 /65. The first commercial instrument was introduced in 1975, since about 1985, is routinely used in the industry. Thus, the ICP -OES technique is now very widely used in environmental analysis, materials research, metallurgical and pharmaceutical industries.

Principle

Plasma is an ionized gas which contains electron and ion addition atoms. As gas is due to its relation to the elements to be determined large ionization energy ( 15.76 eV), its chemical inertness, its relatively low price, and the lack of band spectra, usually argon ( monatomic gas ) used. The energy transfer takes place after ignition by a spark Tesla by the applied RF field in the coils. Free electrons are now accelerated by the applied field and heat by collision with the atomic cores on the plasma. Due to the high particle density in the plasma is plasma and aerosol samples heated to 6,000-12,000 K (depending on the RF power of the RF generator ). The prevailing temperatures are locally different, a distinction ionization, electron and excitation temperatures. Particularly important excitation temperature of about 6,000 K. The sample aerosol is thereby passed through the center of the plasma stream without affecting its stability / balance.

Construction

The most important parts of an ICP spectrometer is high-frequency generator (27 MHz or 40 MHz ), plasma torch, and the actual Probenzerstäuber spectrometer. Monochromator modern OES is mainly built up in the echelle arrangement, since in this technique, a significantly better resolution than in the AAS is necessary because of the continuous emission of the spectrum. Most commonly a polychromator is used because the simultaneous measurement of many elements in a short time and very stable is possible with him. Echelle polychromators usually come into contact with a CCD area detector is used. The properties of the argon plasma can be used best in this combination:

• Large dynamic measurement range
• Multi-element technique
• Good long-term stability for large time series measurements

The electromagnetic waves can be recorded at two different positions of the plasma torch.

Microwave plasma torch atomic emission spectrometry (MPT -AES)

Microwave plasma torch atomic emission spectrometry ( engl. microwave plasma torch atomic emission spectrometry, MPT -AES) is a method of trace analysis and enables the sensitive element analysis. The advantage of this method lies in the relatively simple construction and in the use of inexpensive components. In particular, the gas consumption is significantly lower compared to the ICP- OES.

Again, a distinction spectral interference and non- spectral interferences. Chemical interference is of little importance, since most of the chemical compounds by the high temperatures in the induction zone ( 10,000-12,000 K) of the plasma is dissociated. The spectral interference caused by the emission lines of the external elements ( interferent ), and molecules in the sample matrix. These include:

• Direct superposition of lines
• Continuum radiation from the matrix
• Emission bands of the molecule, such as: -OH, C2, CN, NO, N2

These disorders can be obtained by:

• Taking appropriate adaptation
• The study of several lines per element
• Spectral deconvolution of the line by measuring blank solution / analyte / interferer
• Inter- element correction
• Standard addition method
• Changed the direction of observation

Eliminate.

To the non- spectral interferences are also here the physical properties of the sample solutions, such as:

• Density
• Surface tension
• Viscosity

This can have a lasting influence on the atomization characteristics, Zerstäuberkammeraerodynamik and sample transport. Furthermore, among the non- spectral interferences changes in the excitation conditions in the plasma by:

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

They can be eliminated by suitable matrix adjustments and standard addition method.

Comparison with other methods

ICP-OES advantageous with respect to the flame atomic absorption spectrometry ( AAS - F ) is a much higher temperature of the plasma with respect to the flame ( 10,000 K to over 2800 K). This not only increases the Atomisierungsgrad ( Boltzmann distribution ), but the excited atoms of the elements to be determined are additionally ionized. This in turn has a decisive advantage over AAS, since ion lines in contrast to atomic lines at high temperatures are fairly insensitive to excitation disorders. Also it takes a longer residence time and better temperature uniformity, precision / reproducibility and limits of detection. A simultaneous multi-element analysis of up to 70 elements is state of the art.

Application

Application finds the ICP -OES is mainly in trace and water analysis. In the environment by means of ICP -OES soils, plants, waste and fertilizers are measured in the elemental composition. ICP -OES is ideal for the analysis also highly radioactive samples, since no interfering radioactivity in the analyzer section (optics, semiconductor chip amplifier ) is introduced, it will only analyze the emitted light ( unlike, for example, in mass spectrometry ) and a almost 100 percent extraction of the plasma gases is now (from about 2000) the prior art.