mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

, the quadrupole mass analyzer (or quadrupole mass filter) is a type of

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

analyzer originally conceived by Nobel laureate

Wolfgang Paul Wolfgang Paul (; 10 August 1913 – 7 December 1993) was a German physicist, who co-developed the non-magnetic quadrupole mass filter which laid the foundation for what is now called an ion trap. He shared one-half of the Nobel Prize in Ph ...

and his student Helmut Steinwedel. As the name implies, it consists of four cylindrical rods, set parallel to each other. In a quadrupole mass spectrometer (QMS) the

quadrupole A quadrupole or quadrapole is one of a sequence of configurations of things like electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure re ...

is the ''mass analyzer'' – the component of the instrument responsible for selecting sample

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s based on their

mass-to-charge ratio The mass-to-charge ratio (''m''/''Q'') is a physical quantity Ratio, relating the ''mass'' (quantity of matter) and the ''electric charge'' of a given particle, expressed in Physical unit, units of kilograms per coulomb (kg/C). It is most widely ...

(''m/z''). Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

s that are applied to the rods.

Principle of operation

The quadrupole consists of four parallel metal rods. Each opposing rod pair is connected together electrically, and a

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...

(RF) voltage with a DC offset voltage is applied between one pair of rods and the other. Ions travel down the quadrupole between the rods. Only ions of a certain mass-to-charge ratio will reach the detector for a given ratio of voltages: other ions have unstable trajectories and will collide with the rods. This permits selection of an ion with a particular ''m/z'' or allows the operator to scan for a range of ''m/z''-values by continuously varying the applied voltage. Mathematically this can be modeled with the help of the Mathieu differential equation. Quadrupole mass analyzer

Ideally, the rods are

hyperbolic Hyperbolic may refer to: * of or pertaining to a hyperbola, a type of smooth curve lying in a plane in mathematics ** Hyperbolic geometry, a non-Euclidean geometry ** Hyperbolic functions, analogues of ordinary trigonometric functions, defined u ...

, however cylindrical rods with a specific ratio of rod diameter-to-spacing provide an easier-to-manufacture adequate approximation to hyperbolas. Small variations in the ratio have large effects on resolution and peak shape. Different manufacturers choose slightly different ratios to fine-tune operating characteristics in context of anticipated application requirements. Since the 1980s, the

MAT A mat is a hard or soft floor covering that generally is placed on a floor or other flat surface. Mats serve a range of purposes including: * serving to clean items passed over it, such as a doormat, which removes dirt from the soles of shoe ...

company and subsequently Finnigan Instrument Corporation used hyperbolic rods produced with a mechanical tolerance of 0.001 mm, whose exact production process was a well-kept secret within the company.

Multiple quadrupoles, hybrids and variations

A linear series of three quadrupoles is known as a

triple quadrupole mass spectrometer A triple quadrupole mass spectrometer (TQMS), is a tandem mass spectrometry, tandem mass spectrometer consisting of two quadrupole mass analyzers in series, with a (non-mass-resolving) Radio-frequency quadrupole, radio frequency (RF)–only quadr ...

. The first (Q₁) and third (Q₃) quadrupoles act as mass filters, and the middle (q₂) quadrupole is employed as a collision cell. This collision cell is an RF-only quadrupole (non-mass filtering) using Ar, He, or N₂ gas (~10⁻³ Torr, ~30 eV) for collision induced dissociation of selected parent ion(s) from Q₁. Subsequent fragments are passed through to Q₃ where they may be filtered or fully scanned. This process allows for the study of fragments that are useful in structural elucidation by

tandem mass spectrometry Tandem mass spectrometry, also known as MS/MS or MS2, is a technique in instrumental analysis where two or more stages of analysis using one or more mass analyzer are performed with an additional reaction step in between these analyses to increa ...

. For example, the Q₁ may be set to 'filter' for a drug ion of known mass, which is fragmented in q₂. The third quadrupole (Q₃) can then be set to scan the entire ''m/z'' range, giving information on the intensities of the fragments. Thus, the structure of the original ion can be deduced. The arrangement of three quadrupoles was first developed by

Jim Morrison James Douglas Morrison (December 8, 1943 – July 3, 1971) was an American singer, songwriter, and poet who was the lead vocalist and primary lyricist of the rock band the Doors. Due to his charismatic persona, poetic lyrics, distinctive vo ...

La Trobe University La Trobe University is a public university, public research university based in Melbourne, Victoria (Australia), Victoria, Australia. Its main campus is located in the suburb of Bundoora, Victoria, Bundoora. The university was established in 1 ...

in Australia for the purpose of studying the

photodissociation Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...

of gas-phase ions. The first triple-quadrupole mass spectrometer was developed at Michigan State University by Christie Enke and graduate student Richard Yost in the late 1970s. Quadrupoles can be used in

hybrid mass spectrometer A hybrid mass spectrometer is a device for tandem mass spectrometry that consists of a combination of two or more '' m/z'' separation devices of different types. Notation The different ''m/z'' separation elements of a hybrid mass spectrometer ca ...

s. For example, a sector instrument can be combined with a collision quadrupole and quadrupole mass analyzer to form a hybrid instrument. A mass-selecting quadrupole and collision quadrupole with time-of-flight device as the second mass selection stage is a hybrid known as a quadrupole time-of-flight mass spectrometer (QTOF MS). Quadrupole-quadrupole-time-of-flight (QqTOF) configurations are also possible and used especially the mass spectrometry of peptides and other large biological polymers. A variant of the quadrupole mass analyzer called the monopole was invented by von Zahn which operates with two electrodes and generates one quarter of the quadrupole field. It has one circular electrode and one V-shaped electrode. The performance is, however, lower than that of a quadrupole mass analyzer. An enhancement to the performance of the quadrupole mass analyzer has been demonstrated to occur when a magnetic field is applied to the instrument. Manifold improvements in resolution and sensitivity have been reported for a magnetic field applied in various orientations to a QMS.

Applications

These mass spectrometers excel at applications where particular ions of interest are being studied because they can stay tuned on a single ion for extended periods of time. One place where this is useful is in liquid chromatography-mass spectrometry or gas chromatography-mass spectrometry where they serve as exceptionally high specificity detectors. Quadrupole instruments are often reasonably priced and make good multi-purpose instruments. A single quadrupole mass spectrometer with an electron impact ionizer is used as a standalone analyzer in residual gas analyzers, real-time gas analyzers,

plasma diagnostics Plasma diagnostics are a pool of methods, instruments, and experimental techniques used to measure properties of a plasma, such as plasma components' density, distribution function over energy (temperature), their spatial profiles and dynamics, whi ...

and SIMS surface analysis systems.

References

External links

{{Mass spectrometry Mass spectrometry

Principle of operation

Multiple quadrupoles, hybrids and variations

Applications

See also

References

External links