Icp ms how does it work

These are then attracted to the positively charged second dynode, where more electrons are emitted. The electron emission process continues in this manner throughout the entire cascade of individual dynodes usually , resulting in a geometrically amplified signal.

Integrated circuitry for this detector, involving simultaneous measurement of signal intensity at mid- and final-dynode positions, can obtain a wide linear dynamic detection range spanning eight orders of magnitude, while also protecting the detector by excluding extreme high intensity signals. High intensity signals are measured from the final dynode as a voltages analog mode , which are subsequently normalized to cps equivalents from pulse-to-analog conversion factors previously optimized during tuning of the instrument.

The dual-stage functionality of this detector achieves a linear detection range extending from ppt to s of ppm within a single multi-element analysis, commonly obviating the need for additional dilution. Energy achieved by the Ar ICP is sufficient to cause the majority of sample atoms passing through it to exceed their first, but not second, ionization potentials Fig 3.

It is important to realize that although most elements are substantially ionized in the high temperature Ar plasma Fig 4 , Houk ; Douglas and Tanner, , some elements having first ionization potentials approaching or exceeding that of Ar Arsenic 9. By comparison, sulfur Figure 4. Within the quadupole electrostatic field, ion paths become spiral. Unstable flight paths result in ion collisions with one of the quadrupole rods, whereupon they are removed by neutralization.

Ion paths either adjust sufficiently maintain sufficiently narrow spirals to pass through the length of the quadrupole, or obtain terminal collision trajectories spiral paths too wide. The quadrupole mass filtering process is extremely fast. Employing signal averaging techniques, the entire amu range amu can be scanned times in about 3 minutes to obtain a robust full spectral scan.

A quadrupole consists of four parallel poles, or rods, two that obtain a net negative charge and two that obtain a net positive charge Figs The polarity and strength of the electromagnetic field achieved by each set of opposing rods results from the simultaneous application of DC and RF AC voltages, which can be increased or decreased, but are always maintained in a fixed ratio typically on the order of The applied DC voltages are constant but of opposite polarity — one rod set positive, the other negative.

Meanwhile, the applied RF current alternates between positive and negative polarity in the range of MHz, with the polarity alternations maintained exactly out-of-phase between the opposing rod sets Fig 6. In other words, while one set of opposing rods has an applied positive RF potential, the other set has an equally applied but negative RF potential. It is when the magnitude of the RF polarity field strength exceeds that of the oppositely applied DC polarity field strength that a given set of opposing rods obtains its effective net polarity, which then affects ion paths as a function of their inertial masses.

Ion trajectories are ultimately governed by both sets of rods, but are more easily understood in terms of the local electrostatic field between a given rod set. In this simplification, one set acts as a high-pass filter high masses stable through the quad.

The other acts as a low-pass filter low masses stable through the quad. Figure 5 Left. Black wires on the external housing supply opposing rod sets with applied RF and DC potentials. Figure 6 Right. Lower diagram shows how the voltage potentials applied to opposing rod sets vary through one RF cycle. High Pass Condition : When opposing rods obtain an overriding positive polarity field Fig 6, rods 2 and 4 above , a massive ion passes through the entire length of quadrupole, whereas a less massive ion is sufficiently attracted by the negative pulses of the applied RF such that its trajectory ultimately collides with a rod.

Simultaneously, smaller ions are sufficiently attracted by the alternating positive and negative RF polarity pulses to maintain a stable path through the length of the quadrupole.

Because the two sets of rods operate simultaneously and the RF component on each set is out-of-phase, the resulting magnetic field changes dynamically with each successive RF cycle such that ion trajectories actually spiral as they move through the quadrupole. The number of RF cycles affecting ion trajectories depends on the length of the quadrupole rods, the frequency of AC, and the mass velocity of the ion.

For example, a massive ion e. CRC technology is an especially significant advancement whereby polyatomic interferences are reduced or eliminated by passing the ion beam, prior to reaching the quadrupole mass filter, through a cell that can be pressurized with either an inert collision gas e.

He or a reactive gas e. Multiple modes, using collision, reaction, or neither no gas , can be used during an analytical session to achieve virtually interference free results on analytes throughout most of the periodic table. The main purpose of Q-ICP-MS is to determine high quality precise and accurate elemental concentrations of various materials.

Owing to its high sample throughput capabilities and wide range of detection and elemental coverage, ICP-Q-MS is routinely used for the elemental analysis of geological, environmental, chemical, biological, industrial, semi-conductor, metallurgical, petrochemical, medical, forensic, nuclear, and archaeological samples.

No Particles: Samples for solution mode ICP-Q-MS should be particulate free in order to prevent clogging of sample tubing and the nebulizer capillaries particularly for concentric and micro-concentric types. Samples containing solids even nanoparticles will not ionize the same way as liquid samples, and thus can negatively affect quantitative analysis.

Particles may be removed by filtration or by centrifugation and decanting. Acidification prevents dissolved species from readily sorbing onto tubing and container walls. Nitric acid is preferred because it generates fewer polyatomic interferences compared to other acids. Dilution is also important to ensure that measurement conditions remain consistent throughout the analytical session.

The fine droplets that are created by the nebulizer are passed through a spray chamber before entering the plasma. Different types are again available, but the function remains the same: to allow a high number of the small droplets to enter the plasma while discriminating against the larger droplets which can create analytical issues if permitted into the plasma. The argon plasma generated in the ICP reaches temperatures of between K and is generated by passing argon gas through concentric quartz tubes commonly referred to as the ICP torch that are contained at one end within a radio frequency RF coil.

Energy supplied to the coil by an RF generator couples with the argon gas to produce the plasma. As the liquid droplets enter the high temperature plasma, they are converted to the gaseous state. As they absorb more energy, they will eventually release an electron to form a single, positively charged ion. The interface region where the ions produced by the plasma are introduced to the mass spectrometer presents an engineering challenge.

Secondly, the torch will have been necessarily backfilled with the Ar gas required to generate the plasma, while the mass spectrometer will be under high vacuum conditions. Each manufacturer will have different solutions, but two or more cone structures lenses are used to prevent a wide divergence of the ions as they enter a region of high vacuum, and to focus them into the collision cell or directly into the mass spectrometer.

Ions will not be the only species exiting the plasma — neutral atoms and photons will be present. Photons can give rise to false ion counts so it is important that they are removed from the path of ions. There are variations on a theme as to how the different manufacturers deal with this issue, but a common solution is to place some form of lens element that will selectively bend only the ions into the quadrupole mass spectrometer. This occurs when two ions, an elemental ion e.

Many of these interferences will be difficult to separate based only on the mass resolving power of the mass spectrometer. In collision mode, the cell is backfilled with a partial pressure of inert gas, and both the elemental and molecular ions will lose some of their kinetic energy through collisions with the inert gas atoms as they travel through the cell.

Nonetheless, the detection limit will be adversely affected. In a reaction cell, the inert gas atoms are replaced by a reactive gaseous species. The rationale is that the introduced gas will react with the interfering species to produce a neutral species that can no longer be influenced by the electrostatic fields of the ion optics or the quadrupole.

It will be effectively filtered out. The analyte ion is not affected, and thus compared to the collision cell it is a more powerful method for eliminating mass interferences. Quadrupole mass spectrometers are most commonly found in ICP-MS instruments, although others based on magnetic sectors and time-of-flight are available. Ions of other masses will collide with the rods and be eliminated.

As mass spectrometers can have significant length through which the ion must travel to achieve mass separation, it is absolutely imperative that this region is under high vacuum. Otherwise, analyte ions could collide with gas molecules leading to possible charge exchange reactions and decreasing overall sensitivity and increasing unwanted mass interferences. Data in ICP-MS is generally analyzed either quantitatively or semi-quantitatively, as isotope ratio measurements or in isotope dilution analyses.

With this knowledge, full quantitative analyses can be planned with appropriate standards, and instrument operation protocols adjusted in the event of identified potential mass interference problems. The sampling interface unit consists of two metallic cones, the sampling cone orifice radius about 0. The path of the ions pulled through by the sampling cone and the skimmer cone converge into the mass spectrophotometer through the ion lens. The ion lens and the mass spectrophotometer unit are ventilated to and Pa respectively, by the turbo molecular pump.

The ions sorted by mass with the mass spectrophotometer are detected by the ion detector. One problem with ICP-MS is the spectral interference that occurs when the spectrum of ions or molecular ions with the same mass number as the objective element overlap and interfere. Accordingly, measurement of elements interfered by Ar molecular ions is conducted in a high background condition, making infinitesimal concentration measurements extremely difficult.

Chart 1: Major Ar molecular ions. Chart 1 shows major elements affected by molecular ions of Argon origin. K, Ca and Fe are especially affected, as the Ar molecular ion levels for these elements range from tens to hundreds of ppb when converted to the concentration for each element, and ppt-order analysis under these conditions are nearly impossible.

Cool Plasma Measurement addresses the problem of infinitesimal concentration analysis for elements affected by Ar molecular ions. As its name suggests, Cool Plasma refers to the lower than normal temperature of the plasma.

Ar molecular ions are difficult to be produced in a cool plasma state and the background becomes as low as possible. As a result, the lower detection limit improves. Chart 2 shows detection limit DL and the background equivalent concentration BEC under cool plasma conditions. The background level is reduced to 1ppt or lower, making ppt-order analysis possible. DL: Concentration calculated by multiplying the repeated measurement result of the blank by 3 BEC: The blank value converted to concentration.

Environmental samples such as stream water and lake water contain many matrix components in addition to the measured elements.