Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS is a very powerful tool for detecting and analyzing trace and ultra-trace elements. ICP-MS is the technique of choice in many analytical laboratories for providing accurate and precise measurements needed for today’s demanding applications and for providing required lower limits of detection. In keeping with a tradition of providing high-quality trace element analysis and technological innovation, Elemental Analysis, Inc. (EAI) offers customers a choice of two ICP analysis methods: |
 ICP-MS Limits of Detection (LODs) |
The PQ3 Quadrupole system offered by EAI is capable of handling high volume samples and is suitable for a majority of applications where limit of detection requirements are in the part-per-million (ppm) and low part-per-billion (ppb) range.
The High-Resolution ICP-MS offered by EAI is the VG AXIOM high-resolution system that is capable of providing higher sensitivities along with higher mass resolutions. Limits of detection are in the parts-per-trillion and parts-per-quadrillion (ppq) ranges.
ICP-MS Operating Principles
How it works: In ICP-MS, a plasma or gas consisting of ions, electrons and neutral particles, is formed from Argon gas, which is then utilized to atomize and ionize the elements in the sample matrix. These resulting ions are then passed through a series of aperatures (cones) into a high vacuum mass analyzer where the isotopes of the elements are identified by their mass-to-charge ratio. The intensity of a specific peak in the mass spectrum is proportional to the amount of the elemental isotope from the original sample.
The heart of the ICP-MS is the inductively-coupled plasma ion source. Since the source operates at temperatures of 7000° K, virtually all molecules in a sample will be broken into their component atoms. A radio frequency signal (RF) is fed into a tightly wound, water-cooled, coil where it generates an intense magnetic field. Within the center of this coil is a quartz plasma torch where the plasma if formed, which is generated by “seeding” the argon gas with a spark from a Telsa unit (a device similar to a spark plug). When the spark passes through the argon gas, some of the argon atoms are ionized and the resultant cations and electrons are accelerated toward the magnetic field of the RF coil. Stable, high temperature plasma, is then generated as the result of the inelastic collisions created between the charged particles and the neutral argon atoms.
These concentrations of electrons in the plasma reach equilibrium very quickly, after which the plasma will remain “lit” as long as the RF field in maintained and a constant supply of argon gas is injected into the plasma.
The plasma torch is designed in such a manner that the sample is then injected directly into the heart of the plasma. The injected sample consists of a fine aerosol, which can be derived from, but not limited to, nebulized liquids and ablated solids. As this aerosol sample passes through the plasma, it collides with free electrons, argon cations, and neutral argon atoms, causing any molecules initially present in the aerosol to be quickly and completely broken down into charged atoms. Some of these charged atoms will recombine with other species in the plasma to create both stable and meta-stable molecular species, which will then be transmitted into the mass analyzer along with the charged atoms. At this point, a special set of metal cones and ion-focusing elements are used to extract the charged atoms from the plasma into the mass analyzer. The tongue of the plasma is directed across the tip of the sampler cone, which has a small hole opening into the mass spectrometer. The ignited jet of gasses are directed through the sampler at a rate in excess of the speed of sound which then terminates at a “Mach disk” as the gasses begin to slow. |
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The expansion chamber terminates with another cone known as the “skimmer” cone, which is situated with its orifice located in the shock zone of the Mach disk. The ions that successfully pass through the skimmer cone orifice are first accelerated by a high voltage potential gradient, which are then passed through a series of focusing lenses into the mass analyzer. This series of cones and expansion chamber are commonly referred to as the interface to the mass spectrometer, which is where we begin to see the difference between the Quadrupole ICP and the High-Resolution ICP instruments.
Mass Analyzers: Differences between Quadrupole and High-Resolution
The Quadrupole mass analyzer consists of four cylindrical rods onto which are applied both RF and DC electrical fields. These four rods are arranged in such a manner that they form one pair in the X plane, and one in the Y plane.
As
ions enter the Quadrupole, they begin to oscillate in both the
X and Y planes, thus causing the lower m/e ions to be destabilized
in the Quadrupole whenever the alternating (RF) component of the
electric field exceeds the direct (DC) component. In this condition,
the lower m/e ions will be thrown out of the Quadrupole
and not reach the detector, thereby creating an effective low
mass filter.
If the direct component exceeds the alternating component then the high m/e ions become unstable, while the lower m/e will be stabilized by the presence of the alternating component making for an effective high mass filter. In the Quadrupole system, the mass analyzer is created by connecting the two pairs of rods in such a manner that the X plane acts as a low mass filter and the Y plane acts as a high mass filter. By carefully matching the two fields, only ions of a particular mass are able to resonate at the correct frequency and pass through the Quadrupole at any time. In this regard, the Quadrupole mass analyzer is a very fast and efficient system.
The High-Resolution mass analyzer consists of several components acting in unison. First, the ion beam passes through a narrow slit, which only allows those ions traveling along eh correct axial plane of the mass spectrometer to pass through, resulting in a narrow beam of ions all traveling parallel to each other.
In
the next stage, the electro static analyzer (ESA), which
consists of two curved plates applied with DC voltage, causes
the inner plate (negative polarity) attracts the positively charged
ions, while the outer plate (positive polarity) repels the ions.
The ion beam passes through the two plates and is both
focused and curved through an angle of approximately 40°.
Since only ions with a narrow range of kinetic energy are able
to pass through the ESA, it thus forms an effective energy filter.
After the ion beam passes through the ESA, all ions will then pass into the HR magnet, which creates a uniform magnetic field causing the passing ions to have a similar kinetic energy. The ion beam then passes through the final part of the system which is a narrow slit situated at the focal point of the magnet known as the collector slit. High resolutions are thus achieved by making both of the slits very narrow so that the beam reaching the detector has only a very narrow bandwidth of mass at any given time. Therefore, the high-resolution mass analyzer in known as a double focusing system because it is able to focus both energy and mass/charge.
Instrument Calibration
Any sample entered into the mass spectrometer under similar conditions will return a count rate that can be converted directly to the concentration for each element from an established calibration curve. Since the response of the mass spectrometer in counts per second is directly proportional to the concentration of a given element in a sample, this allows the system to be calibrated by incorporating a series of external standards of differing concentrations. The challenge, therefore, is to ensure that conditions are identical for each sample, and that potential variables that can affect the analysis are recognized and compensated. Possible factors that can affect the sampling conditions of the ICP-MS are:
- Variations in plasma ionization efficiency
- Possible clogging or erosion of cone apertures
- Differing matrix concentrations in samples that could result in matrix suppression
- Temperature and humidity fluctuations in the laboratory environment
- Formation of molecular species within the sample that may interfere with another element in an unexpected manner
Any one of these variations or conditions can render the accurate analysis of the respective sample difficult or impossible unless certain methods or procedures are employed to minimize the potential for such difficulties. Among the steps that can be taken, and that are utilized by EAI chemists are the following procedures:
- Incorporation of an external calibration series encompassing the elements to be analyzed. This is designed to cover a range of concentrations that will completely bracket the concentration of analyte in the sample. In the event the sample is found to fall significantly outside the bracketed range, it can then be diluted and run again so that it falls within the desired range.
- Internal standards can be incorporated for each sample at known concentrations for the desired element(s) to compensate for any variation in the intensity of the element signal, which can them be corrected to the known concentration. By applying this same correction to other elements in the matrix solution, the correct element concentration can then readily be calculated.
- For potentially difficult matrices, the chemists can incorporate the use of spiked samples. This procedure involves the preparation of duplicate sample(s) spiked with each element of interest, which can then be utilized to measure the recovery efficiency of each element so that obvious discrepancies can be determined and investigate in more detail.
Advantages and Capabilities Comparison
Quadrupole ICP-MS Speed: The Quadrupole mass analyzer is Mass Stability: Since there are no magnetic fields with the Quadrupole system, it is able to move from mass to mass with excellent precision. This allows the operator to incorporate the technique of “peak hopping” to acquire a single point of data at the very top of the peak at each element during the analysis. Sensitivity: Modern Quadrupole systems such as those utilized by EAI are able to detect trace elements consistently at levels in the low-ppb range, and oftentimes ppt range. Reliability: Quadrupole ICP systems have become the workhorse of the industry due to their ability to accurately and consistently turn out data on a day in and day out basis, and can even be left to operate overnight unattended. |
High-Resolution ICP-MS Variable Resolution: The High-Resolution ICP is capable of varying its resolving power by means of the two swinging gate slits. By varying the resolution power, this allows the analysis to be tailored in such a manner that each element is analyzed at a resolution enabling it to be fully resolved from any interference without over-resolving an element of interest. High Sensitivity: High-Resolution systems are known for their extremely high ion transport efficiency which gives rise to very high levels of sensitivity. The VG Axiom used by EAI typically returns a count rate of greater than 1,000,000 counts per second for a solution of 1 ppb of 115Indium. Low Noise: Due to the design of the High-Resolution mass analyzer there is little opportunity for stray photons to traverse the entire instrument, resulting in an extremely low background noise. This low noise level, combined with high sensitivity capabilities, allow the High-Resolution system to return unparalleled limits of detection. For some applications, the instrument is capable of achieving LOD’s below 1 ppq. |
