Optical spectrometers, atomic

Interaction between the magnetic field of the electrons and the nuclear spin is the basis for various techniques that are broadly applied in chemistry, atomic physics, nuclear physics and solid-state physics. The magnetic field of the electrons is due to their spin and orbital angular momentum and much larger than the magnetic field of the nucleus. Consequently, the nuclear spin is oriented in relation to the field produced by the electron shell. This leads to hyperfine spectra which can be resolved by means of optical spectrometers of very high resolution. 

Atomic spectrometric methods of analysis essentially make use of equipment for spectral dispersion so as to isolate the signals of the elements to be determined and to make the full selectivity of the methodology available. In optical atomic spectrometry, this involves the use of dispersive as well as of non-dispersive spectrometers. The radiation from the spectrochemical radiation sources or the radiation which has passed through the atom reservoir is then imaged into an optical spectrometer. In the case of atomic spectrometry, when using a plasma as an ion source, mass spectrometric equipment is required so as to separate the ions of the different analytes according to their mass to charge ratio. In both cases suitable data acquisition and data treatment systems need to be provided with the instruments as well. 

Inductively coupled plasma has become the ionization method of choice for elemental mass spectrometry. It was initially developed as the excitation source for multi-element optical spectrometers, because at typical plasma temperatures of 5000-10,000°C virtually all elements on the periodic chart emit detectable light. Most molecules are also atomized at these temperatures, which makes inductively coupled plasma ideal for mass spectrometry monitoring of elemental composition as well. Fassell and co-workers introduced the first inductively coupled plasma interfaced to a mass spectrometer in 1980 (Houk et al., 1980). Elemental mass spectrometry normally requires only low-resolution analysers because unit mass resolution is typically required (i.e. the mass difference between elements, which is always equal to or greater than 1 Da). 

Material from a specimen is removed and excited by cathodic evaporation via glow discharge. The light, emitted by the atoms, is detected wavelength dispersively by use of an optical spectrometer. The determined intensities are proportional to the mass of the particular chemical element and its concentration in the specimen. All elements from the periodic table can be analyzed. As this method removes material from the surface, GDOS can be used for the detection of gradients of the chemical composition in the subsurface. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

The main features of f.a.b.-m.s. are shown schematically in Fig. 1. The hardware consists of (i) an atom gun (or ion gun, see later) which is either mounted on the source housing of the mass spectrometer or, if small enough, inside the housing on the source itself, (it) a sample probe to the end of which is attached a small metal target onto which the sample is loaded, and (Hi) suitable source-optics for the efficient extraction of ions into the analyzer of the mass spectrometer. 

Plasma sources were developed for emission spectrometric analysis in the late-1960s. Commercial inductively coupled and d.c. plasma spectrometers were introduced in the mid-1970s. By comparison with AAS, atomic plasma emission spectroscopy (APES) can achieve simultaneous multi-element measurement, while maintaining a wide dynamic measurement range and high sensitivities and selectivities over background elements. As a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multi-element trace determinations. With several techniques, absolute detection limits are below the ng level. 

Mass spectrometry is the only universal multielement method which allows the determination of all elements and their isotopes in both solids and liquids. Detection limits for virtually all elements are low. Mass spectrometry can be more easily applied than other spectroscopic techniques as an absolute method, because the analyte atoms produce the analytical signal themselves, and their amount is not deduced from emitted or absorbed radiation the spectra are simple compared to the line-rich spectra often found in optical emission spectrometry. The resolving power of conventional mass spectrometers is sufficient to separate all isotope signals, although expensive instruments and skill are required to eliminate interferences from molecules and polyatomic cluster ions. 

Accelerator mass spectrometry Acousto-optical tuneable filter Acousto-optical tuneable spectrometer/scanning Atom probe... 

The instrument which uses this plasma torch is called an inductively coupled plasma atomic emission spectrometer (ICP-AES) or an inductively coupled plasma optical emission spectrometer (ICP-OES). It is similar to an... 

Figure 1.2 shows the basic instrumentation for atomic mass spectrometry. The component where the ions are produced and sampled from is the ion source. Unlike optical spectroscopy, the ion sampling interface is in intimate contact with the ion source because the ions must be extracted into the vacuum conditions of the mass spectrometer. The ions are separated with respect to mass by the mass analyser, usually a quadrupole, and literally counted by means of an electron multiplier detector. The ion signal for each... 

Atomic emission from the plasma is focused on to the entrance slit of the monochromator using a combination of convex or plano-convex lenses or a concave mirror. The combination of focusing optics, monochromator and detector is generally referred to as a spectrometer, although the heart of the device is the monochromator. A monochromator is an instrument that... 

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