Background -- Analysis by Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES), is designated ICP. It is a multi-element analysis technique that will dissociate a sample into its constituent atoms and ions and cause them to emit light at a characteristic wavelength by exciting them to a higher energy level. This is accomplished by the use of an inductively coupled plasma source, usually argon gas. Various atomic spectroscopy methods are useful including for flame, graphite furnace AAS, hydride generation, ICP, as well as ICP/MS. Each technique has its own benefits and limits of detection. The Optima 2100 is a sequential optical emission spectrograph with an ICP excitation source. This technology reflects historical developments. Also a stepwise procedure is described.
A monochromator can separate specific wavelengths of interest, and a detector is used to measures the intensity of the emitted light at specific wavelength. This information can be used to calculate the concentration of that particular element in the sample.
The plasma is a sample cell which excites atoms. When these excited atoms return to the ground state, they emit energy as light at a characteristic wavelength. The monochromator directs these wavelengths to a detector. Initially, argon gas will pass through the quartz tube and exit from the tip. The tip of the quartz tube is surrounded by induction coils that create a magnetic field. The AC current that flows through the coils is at a frequency of about 30 MHz and a power level around 2 kW. The stream of argon gas that passes the coil has been previously seeded with free electrons from a Tesla discharge coil. The magnetic field excites these electrons, and they then have sufficient energy to ionize the argon atoms by colliding with them. The cations and anions present from the initial Tesla spark accelerate due to the magnetic field in a circular pattern that is perpendicular to the stream exiting from the top of the quartz tube. By reversing the direction of the current in the induction coils, the magnetic field is also reversed. This changes the direction of the excited cations and anions, which causes more collisions with argon atoms. This results in further ionization of the argon atoms and intense thermal energy. As a result, a flame shaped plasma forms on top of the torch.
A second stream of gas is usually needed to cool down the inside of the quartz tube. This is provided by a stream of argon that provides a vortex flow. The flow also provides a way of centering and stabilizing the plasma.
When the sample flows into the plasma (in an aerosol form), the atoms present are excited by the extreme temperatures from 6,000 to 10,000 K. These excited atoms will emit energy at a characteristic wavelength. Due to the sample being introduced as an aerosol through the innermost concentric tube of the plasma source at a frequency of about 27 MHz, the skin effect occurs. This effect causes the plasma to have a toroidal shape. This shape increases the time that the sample is in the high-temperature zone of the plasma for about 2 msec. The extended resident time increase the detection limits for several elements.
Shear gas -- A compressed nitrogen or air shear gas (18-25 L/min) is used to remove the plasma tail from the optical path, minimizing interferences and extending the dynamic range. The shear gas design offers a maintenance-free and lower-cost (no use ofArgon) approach to removing the cooler plasma zone. This reduces interferences caused by self-absorption in the cool plasma tail. The term self-absorption refers to the process whereby analyte emission is absorbed by ground state atoms in the plasma. With a radial plasma, this normally occurs at high analyte concentrations and results in a nonlinear calibration curve, limiting the useful analytical range for an analyte. Ground state atoms occur where the plasma is cooler, whether localized in the normal analytical zone or in the tail plume of the plasma. With the radially viewed plasma, this is not a problem since the tail plume is not in the optical path. To eliminate the adverse effects of this cooler tail plume, the axially viewed Optima configurations use an air shear gas to displace the tail plume out of the optical path. See: Spectrochimica Acta Part B: Atomic Spectroscopy 59, 41-58 (2004).
In an ICP analysis, the plasma will reach a temperature in the range of 6,000 - 10,000 K, which will efficiently atomize most elements. The resulting detection limits are very low, and they usually range from 1-10 ppb (part per billion. A well defined tail is present on the tip of the torch, which contains all the analyte atoms that have been excited by the intense heat of the plasma. The optimum region for analysis is just above the apex of the primary plasma cone and under the base of the flame-like glow. By using this region for analysis, the high background from the current-carrying part of the plasma is effectively excluded. A simple background spectra and a high signal to noise ratio, which gives better detection limits.
The sample is introduced as an aerosol by use of a nebulizer or atomizer. Pneumatic nebulization is the most often used method. A Meinhard nebulizer is shown at the right.
Detectors -- The two most common types of detectors for ICP-AES are photographic emulsion and photoelectric transducers. Photographic emulsion detectors have several advantages. These include being able to integrate impinging radiation during the entire exposure time, and they also record all spectral features simultaneously over a large range of wavelengths. Due to these characteristics, weak spectral lines can easily be detected by using extensive exposure times. The processing and analyzing of the spectra obtained from photographic emulsion detectors can be very tedious. They also display a non-linear relationship to radiation intensity. When making quantitative determinations, they provide low precision and lack sensitivity. The photoelectric transducers, like photomultiplier tubes and junction photodiodes, Charge-Coupled Device CCD and CID have been used.
The Optima 2100 DV is described and has the improved design techniques and modified technologies. ICP system is ideal for routine use but also able to perform a variety of special analysis. The Optima 2100 DV ICP-OES (optical emission system) uses various technologies to allow flexibility and good analytical performance for varied and moderate sample loads. A stepwise procedure is described.
