Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS is an elemental analysis technique first introduced commercially in 1983, which can detect and quantify the elemental composition of an introduced sample,. ICP-MS has many advantages over other elemental analysis techniques including:
Detection limits for most elements equal to or better than those obtained by Graphite Furnace Atomic Absorption Spectroscopy (GFAAS) Higher throughput than GFAAS The ability to handle both simple and complex matrices with a minimum of matrix interferences due to the high-temperature of the ICP source Superior detection capability to ICP-AES with the same sample throughput The ability to obtain isotopic information.
ICP-MS essentially consists of two parts combined together ICP and MS. Liquid samples are pumped into the machine to the inductively coupled plasma (ICP), this is a high temperature ion source which breaks down the sample from a molecular species through to a collection of elemental ions. These ions are then passed through a series of ion lenses to a second part of the technique a mass spectrometer (MS). An MS is a detection device with a mass range of 2 to 260 amu. This follows full coverage of all elements and their isotopes (Li to U). The Agilent 7500 series is a quadrupole scanning spectrometer that allows all the elements to be mass separated during a single analysis of a sample. Once the elements are separated they are detected using a dual mode detector that allows linear detection limits from ppm to sub ppt ( 9 orders of magnitude). Sample Introduction Liquid samples are pumped with a peristaltic pump through the nebuliser into a spray chamber where it enters as a fine mist. Only the finest droplets exit the spray chamber and enter the plasma. The heavier droplets collide with the wall of the spray chamber and are drained away. The carrier gas flow rate determines the degree of nebulisation. Plasma The plasma is an electrical discharge caused through the inductively coupling of free electrons by a rapidly oscillating magnetic field. The energy is then transferred to Argon plasma gas through collisions , heating up the gas to about 10000 K causing a plasma. The sample which has been converted to an aerosol in the spray chamber is passed into the plasma where it is atomized and ionized. Interface After ions have been produced in the plasma at atmospheric pressure they are then extracted from the atmospheric pressure plasma into the high vacuum region of the mass analyzer via the interface. The interface consists of two water cooled orifices called cones (sampler and skimmer). The sampler cone samples the atmospheric plasma. The cone has a hole in the centre with a diameter of 1 mm. This is the optimum size to allow transmission of the ions formed in the plasma into the interface without allowing the atmosphere outside the plasma to enter. As the plasma travels through the sampler cone into the lower pressure region, the sampled analytes and argon are subjected to an increase in speed to the point where
they are supersonic. The second cone, the skimmer cone has a much smaller orifice of 0.4 mm which allows the transmission of the supersonic ions into the high vacuum region of the ion optics and mass analyzer. Ion Optics System There are a series of 4 ion lenses in the high vacuum region. The extraction lenses extract ions and accelerate them towards the Einzel lenses which focus the ion beam. The next lenses called the Omega lenses offset the beam to remove neutral ions and photons. The ions are then focussed in the QP focus lens before entering the quadrupole mass analyzer where the ions are detected. Quadrupole Mass Analyzer The quadrupole consists of two pairs of rods which are arranged perpendicular to each other. The centre space between the rods is aligned with the ion beam. An RF and DC voltage is applied to diagonally opposed rods while a similar negative voltage is applied to the other rods. Altering the voltages creates a electromagnetic field which affects the ion beam allowing only ions of individual m/z ratios to pass through the quadrupole and hit the detector at certain voltage settings. All the m/z ratios covering the mass range 5-260 can be sequentially acquired by scanning the voltages applied to both pairs of rods. Detector The detector is an electron multiplier detector, as ions strike the surface of the detector it creates a shower of electrons which are pulled into the detector. These hit the next dynode with each electron giving a rise to a further shower. This process continues and the signal intensity is multiplied by about a million fold from the surface of the detector to the amplifier at the base.