ICP-MS Introduction

Inductively coupled plasma mass spectroscopy (ICP-MS) was developed in the late 1980's to combine the easy sample introduction and quick analysis of ICP technology with the accurate and low detection limits of a mass spectrometer. The resulting instrument is capable of trace multielement analysis, often at the part per trillion level. ICP-MS has been used widely over the years, finding applications in a number of different fields including drinking water, wastewater, natural water systems/hydrogeology, geology and soil science, mining/metallurgy, food sciences, and medicine.

Instrument Description and Theory

Schematic of ICP-MS main processes.

ICP technology was built upon the same principles used in atomic emission spectrometry. Samples are decomposed to neutral elements in a high temperature argon plasma and analyzed based on their mass to charge ratios. An ICP-MS can be thought of as four main processes, including sample introduction and aerosol generation, ionization by an argon plasma source, mass discrimination, and the detection system. The schematic below illustrates this sequence of processes.

Sample Introduction

Generation of aerosol by nebulizer.

Unlike the atomic emission spectrometer, ICP-MS spectrometers can accept solid as well as liquid samples. Solid samples are introduced into the ICP by way of a laser abalation system which can usually be purchased as an accessory. Aqueous samples are introduced by way of a nebulizer which aspirates the sample with high velocity argon, forming a fine mist. The aerosol then passes into a spray chamber where larger droplets are removed via a drain (Jarvis et al., 1992). Typically, only 2% of the orginial mist passes through the spray chamber (Olesik, 1996). This process is necessary to produce droplets small enough to be vaporized in the plasma torch.

Argon Plasma/Sample Ionization

Photographs of argon plasma in operation & ICP torch body.

Once the sample passes through the nebulizer and is partially desolvated, the aerosol moves into the torch body and is mixed with more argon gas. A coupling coil is used to transmit radio frequency to the heated argon gas, producing an argon plasma "flame" located at the torch (Jarvis et al., 1992). The hot plasma removes any remaining solvent and causes sample atomization followed by ionization.

In addition to being ionized, sample atoms are excited in the hot plasma, a phenomenon which is used in ICP-atomic emission spectroscopy. Shown to the right is an ICP torch. The aerosol moves into the bottom of the torch body. The green ports on the right side of the body are where more argon is introduced to the flow. At the top are two high quality quartz tubes and an inner alumina injector tube.

ICP-MS Interface

Quadrupole mass filter separating ions.

Because atomization/ionization occurs at atmospheric pressure, the interface between the ICP and MS components becomes crucial in creating a vacuum environment for the MS system. Ions flow through a small orifice, approximately 1 millimeter in diameter, into a pumped vacuum system. Here a supersonic jet forms and the sample ions are passed into the MS system at high speeds, expanding in the vacuum system (Jarvis et al., 1992).

The entire mass spectrometer must be kept in a vacuum so that the ions are free to move without collisions with air molecules. Since the ICP is maintained at atmospheric pressure, a pumping system is needed to continuously pull a vacuum inside the spectrometer. In order to most efficiently reduce the pressure several pumps are typically used to gradually reduce pressure to 10-5 mbar before the ion stream reaches the quadrupole. If only one pump were used, its size would be excessive to reduce the pressure immediately upon entering the mass spectrometer.

Mass Spectrometer (MS)

Photograph of quadrupole rods from mass spectrometer.

In the first stage of the mass spectrometer ions are removed from the plasma by a pumped extraction system. An ion beam is produced and focused further into the actual unit. There are several different types of mass analyzers which can be employed to separate isotopes based on their mass to charge ratio. Quadrupole analyzers are compact and easy to use but offer lower resolution when dealing with ions of the same mass to charge (m/z) ratio. Double focussing sector analyzers offer better resolution but are larger and have higher capital cost.

The quadrupole mass filter is made up of four metal rods aligned in a parallel diamond pattern. A combined DC and AC electrical potential is applied to the rods with opposite rods having a net negative or positive potential. Ions enter into the path between all of the rods. When the DC and AC voltages are set to certain values only one particular ion is able to continue on a path between the rods and the others are forced out of this path. This ion will have a specific m/z ratio. Many combinations of voltages are chosen which allows an array of different m/z ratio ions to be detected. Shown below is animation of this process. Three mass fragments enter into the quadrupole vacuum chamber. The voltage of the rods is set so that only the pink mass fragment passes completely through the quadrupole rod array and into the detector. The green and blue fragments are unstable at this voltage combination and their path eventually brings them into contact with the rods so that they never reach the detector.

Quadrupole rods require periodic maintenance and cleaning due to the buildup of ions which are removed during the mass discrimination process. These ions form a film which eventually builds up and dulls the metallic surface. To remove this film the vacuum chamber must be repressurized and disassembled. This process can be time consuming and very delicate but is essential to keep a mass spectrometer performing well.

An actual quadrupole, removed from a mass spectrometer for cleaning, is shown below. Note the scale of the rods compared to a wristwatch. The inscription on the rod ends is used at the factory to precisely match the manufactured rods so that they have exactly equal dimensions.
Quadrupole rods require periodic maintenance and cleaning due to the buildup of ions which are removed during the mass discrimination process. These ions form a film which eventually builds up and dulls the metallic surface. To remove this film the vacuum chamber must be repressurized and disassembled. This process can be time consuming and very delicate but is essential to keep a mass spectrometer performing well.

Detector

Electron Multiplier Schematic and Electron Multiplier Removed for Cleaning.

The most common type of ion detector found in an ICP-MS system is the channeltron electron multiplier. This cone or horn shaped tube has a high voltage applied to it opposite in charge to that of the ions being detected. Ions leaving the quadrupole are attracted to the interior cone surface. When they strike the surface additional secondary electrons are emitted which move farther into the tube emitting additional secondary electrons. As the process continues even more electrons are formed, resulting in as many as 108 electrons at the other end of the tube after one ion strikes at the entrance of the cone (Jarvis et al. 1992). The drawing below is an illustration of electron multiplying and the photograph is an actual electron multiplier removed from a mass spectrometer for cleaning. Importance of cleaning is similar to that of the quadrupole rods.

Detection Limits
One of the great advantages to ICP-MS is extremely low detection limits for a wide variety of elements. Some elements can be measured down to part per quadrillion range while most can be detected at part per trillion levels. The table below shows some common detection limits by element:

Element Detection Limit (ppt)
U, Cs, Bi less than 10
Ag, Be, Cd, Rb, Sn, Sb, Au 10-50
Ba, Pb, Se, Sr, Co, W, Mo, Mg 50-100
Cr, Cu, Mn 100-200
Zn, As, Ti 400-500
Li, P 1-3 ppb
Ca less than 20 ppb

References

  • B'Hymer, Clayton, Judith A. Brisbin, Karen L. Sutton, and Joseph A. Caruso. 2000. "New approaches for elemental speciation using plasma mass spectrometry." American Laboratory. 32(3):17-32.
  • Jarvis, K. E.; A. L. Gray; and R. S. Houk. 1992. Handbook of Inductively Coupled Plasma Mass Spectrometry. Chapman and Hall: New York.
  • Newman, Alan. 1996. "Elements of ICPMS." Analytical Chemistry. 68(Jan 1): 46A-51A.
  • Olesik, John W. 1996. "Fundamental Research in ICP-OES and ICPMS." Analytical Chemistry. 68(Aug 1):469A-474A.
  • Worthy, Ward. 1988. "Scope of ICP/MS expands to many fields." Chemical and Engineering News. 66(June 27):33-4.
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