ICP->  Home | User Info | Detection limits | Schedule | Download Data

ICP-AES, or simply ICP for short, 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. 

A monochrometer 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.  This basic concept is illustrated in the figure above (Figure 1).

The plasma is used here as a sample cell that will excite atoms.  When these excited atoms return to the ground state, they will emit energy as  light at a characteristic wavelength.  The monochrometer can direct these wavelengths to a detector.

A typical plasma source is shown below in Figure 2.  The argon flow is represented by the green arrow, and the sample flow is indicated by the blue arrow.  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. 

Figure 2.  A typical plasma source

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 (6 to 10K Kelvin).  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.

In an ICP analysis, the plasma will reach a temperature in the range of 6,000 - 10,000^o C, which will efficiently atomize most elements.  The resulting detection limits are very low, and they usually range from 1-10 ppb.  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.

There is no electrode contact in the plasma source, which results in spatially separated excitation and emission zones.  This creates 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.  The problem of blockage is usually overcome by using either cross-flow nebulizer or Babington-type nebulizer.  Other methods used include high solid nebulizer and electrothermal vaporizers.

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, are more widely used today.  They display a linear response to radiation intensity over a large range of five to seven orders of magnitude.  They also provide a quick way to obtain data with high precision and sensitivity.  They do cost more, and they can only analyze one spectral line at a time.  The use of multichannel PM arrangements and photodiode arrays have helped address the problem of only being able to analyze one line at a time.  These devices can easily be interfaced with computer systems to provide quick sample collection and processing.

There are two basic types of ICP instruments.  These are the radial and the axial configurations.  In the radial configuration, the plasma is viewed across the narrow emitting central channel of the plasma, which is accomplished by viewing the plasma source from the side.  The axial configuration is when the plasma is viewed horizontally along its length.  This newer configuration increases the path length and has been shown to reduce background noise.  The reduction of background noise can significantly increase the detection limit of the instrument. There are ICP today has both two configuration so the users can select which to use for specific situations.

A simultaneous ICP instrument can easily analyze up to 60 different elements at the same time without compromising the precision of the analysis or the detection limits. Believe or not, 3 ml solution should be enough to get these elements quantified in less than 3 minutes on our ICP.