ICP-MS is an analytical technique that enables multi-element analysis of trace elements. This method is optimal for sub-part-per-million analysis, with ideal elemental concentrations ranging in the hundreds part-per-billion (ppb) to parts-per-trillion range. The major components of an ICP-MS are: the sample introduction system and ionization source, an interface for transferring analytes from a high-temperature and atmospheric pressure system to a low-temperature vacuum, a mass filter (called a mass analyzer), and ultimately a detector. The basic principle of ICP-MS involves the introduction of a sample into an inductively coupled plasma—where temperatures range from 6,000 to 10,000 K (that’s as hot as the surface of our sun!). When the sample is introduced into the plasma, molecules are broken down into gaseous atoms that are then ionized into, predominately, cations (ions with a positive charge). The ions are then transferred into an interface region designed put the ions under vacuum. The interface region in an ICP-MS consists of two (or more) metal cone-shaped disks with a single hole in the center of each. The small diameter of the hole allows for sampling of ions concentrated at the center of the plasma—herein lies one of the limitations of ICP-MS as the small diameter of the cones inherently requires samples to be free of particulates, which may not be completely atomized and further ionized, that can clog the cones. Once the ions have traversed the interface, electrostatic lenses then focus the ions from the plasma source into a small beam. Because most ions formed in the plasma have a positive charge, and therefore will be repelled by an electrostatic field of the same polarity (i.e. positive charge), the electrostatic lenses (small, positively charged tubes through which the ions traverse) collimate the ions into a narrow, focused beam. The ion beam is focused into the entrance of the mass spectrometer where the ions are then separated by their mass-to-charge ratio (m/z). The majority of ions produced in the plasma have a single-charge (i.e. +1), so the m/z is effectively equivalent to the mass of each ion. In other words, a copper-63 ( 63Cu+) atom will be ionized to 63Cu + ; the mass (63) to charge (1) ratio is then 63. Ions with a m/z of 63 will be separated from other ions with a differing mass in the mass analyzer. Once separated, the ions with a defined mass are directed to a detector where they are counted.
Quadrupole Mass Analyzer
There are several different mass analyzer techniques, each with its own method for separating ions. One of the most popular techniques in ICP-MS technology is the quadrupole mass spectrometer. The quadrupole mass analyzer works by filtering out everything except ions with a specific m/z. This is achieved by employing electrostatic forces of attraction and repulsion as well as inertia. The electrostatic forces at play in the quadrupole are explained by Coulomb’s law, which tells us that particles with a different charge attract and particles with the same charge repel. The tendencies of the particles to react to the attractive or repulsive forces, however, are also governed by inertia. The laws of inertia dictate that the more massive and object, the greater its tendency to resist changes in its state of motion. Arranging 4 rods in a diamond-like orientation with a space in the middle creates the electrostatic field of the quadrupole. Each rod is paired with a rod to its diametric opposite. One rod-pair has positive direct current (DC) and an alternating current (AC) potential applied to the rods, while the other rod-pair has a negative DC as well as an AC potential applied. The magnitude of the AC and DC voltages controls the trajectory of ions as they travel through the quadrupoles, allowing only ions with a specific mass/z through to the detector at any given time.
To better understand the theory behind the quadrupole, envision the rods aligned along a Cartesian coordinate, where one rod-pair is aligned along the x-axis and the other is aligned along the y-axis. Imagine the rod-pair in the x-direction have a positive DC and AC voltage applied, while the rod-pair in the y-direction have a negative DC and an AC voltage applied. Because lighter ions are more readily affected by electrostatic variations, these ions will readily respond to changes in alternating current. When the alternating current on a rod-pair is positive, the rods will repel these smaller cations. When the AC is negative, the rods will attract the smaller cations. If the cations make contact with the rods, they will be neutralized and disposed from the system. At a given AC voltage, then, all cations with a mass less than the mass of interest will have an unstable trajectory and will not be detected.
Meanwhile, the more massive cations, with greater inertia, will resist the forces resulting from the AC potential. Now consider that each rod-pair has a DC potential of opposite sign. Suppose the rod-pair in the x-direction has a positive DC potential and the rod-pair in the y-direction has a negative DC potential. The positive DC potential will focus the cations towards the center of the rods, but the negative DC potential will attract the more massive ions, and any cation with a mass greater than the mass of interest will collide with the negative DC rod pair. With these factors (AC and DC potentials) combined, the quadrupole selectively allows cations with a single m/z through the mass filter. The magnitudes of the AC and DC potentials are adjusted to allow ions to be detected sequentially. At this point, the ions are converted to an electronic signal at the detector and measured.
Because quadrupoles have low resolution and filter ions based on m/z, distinct species with similar m/z will not be distinguishable. This leads to interferences during analysis. There are two major types of interferences in ICP-MS: isobaric and polyatomic.
Isobaric interferences occur when two isotopes have the same mass. For instance, indium (In) and tin (Sn) both have isotopes of mass 115. 115Sn has a mass of 114.9033 and 115In has a mass of 114.9038. Discrimination of these two isotopes requires a mass analyzer with a resolving power greater than 200,000. Since quadrupole ICP-MS generally has a resolving power of 1,000 the quadrupole ICP-MS is incapable of distinguishing these two species. In such a case, both isotopes would be measured at the same time and the concentration of either would not be ascertainable if both were appreciably present in the sample.
The second major type of interference is polyatomic interference. Polyatomic interferences occur when molecules recombine with argon or other matrix components. When these polyatomic (molecular) species have the same mass as an isotope of the element of interest, the two species may be indistinguishable. For example, argon and oxygen recombine in the plasma to form a molecular species with a mass similar to that of the dominant iron isotope. In this example, 40Ar and 16O recombine to form 40Ar 16O + with a m/z of 56, which is similar to 56Fe+.
These mass interferences can be overcome through planning and foresight. Techniques such as the use of cool plasma, reaction or collision cells, or chromatographic separation can be employed to correct or eliminate many of the interferences. Additionally, selecting an alternate isotope, when an option, can help avoid these interferences.
Other Mass Analyzers: The Magnetic Sector and MC-ICP-MS Theory
Unlike the quadrupole, wherein the ions oscillate through the quadrupole and are removed by collision with one of the quadrupoles, in the magnetic sector analyzer ions are accelerated through a flight tube and the path traveled is influenced by a magnetic field. Although the geometry of the flight tube can vary between different types of magnetic sectors, all magnetic sectors deflect the ions in an angular path. The specific angle the ions acquire is dependent on two forces, centripetal and magnetic field, experienced by the ions. These forces are proportional to mass, and as a result, all ions with a specific m/z will have a unique path radius determined by the magnetic field and voltage difference applied. In other words, the deflection of ions through the flight tube is dependent on kinetic energy.
When a constant magnetic field and voltage differential is applied to the flight tube, ions too heavy to be deflected at the corresponding angle of the flight tube will collide with one side of the flight tube. On the other end of the spectrum, light ions experience a greater degree of curvature. When the degree of curvature experienced by lighter ions is greater than the angle of the flight tube, these ions will also collide into the walls of the flight tube. Within the flight tube, ions that have the ideal curvature (as dictated by the magnetic field), relative to the flight tube, will travel unobstructed and experience unique trajectories based on their m/z. Each of these trajectories, or ion beams, can then be focused into a series of detectors. This is the basis of both single and multiple collector -ICP-MS.
In MC-ICP-MS, the ion beams of various isotopes are focused into one of numerous detectors (i.e. multiple collectors)—oftentimes Faraday Cup style detectors. As a result, within certain limitations (e.g. similar mass), various isotopes can be measured simultaneously. This differs from the quadrupole mass analyzer and other single-collector mass spectrometers, which only measure different isotopic species sequentially. The inherent lower precision of sequential analysis limits the applicability of single-collectors. The simultaneous detection and measurement of isotopes, on the other hand, offers an efficient and precise method for measuring and calculating isotopic ratios.