Inmass spectrometry, the quadrupole mass analyzer (orquadrupole mass filter) is a type of mass analyzer originally conceived[1] by Nobel laureate Wolfgang Paul and his student Helmut Steinwedel. As the name implies, it consists of four cylindrical rods, set parallel to each other.[2] In a quadrupole mass spectrometer (QMS) the quadrupole is the mass analyzer – the component of the instrument responsible for selecting sample ions based on their mass-to-charge ratio (m/z). Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating electric fields that are applied to the rods.[2]
Image from US Patent "Apparatus for separating charged particles of different specific charges"[1]
The quadrupole consists of four parallel metal rods. Each opposing rod pair is connected together electrically, and a radio frequency (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.[2] Mathematically this can be modeled with the help of the Mathieu differential equation.[3]
Ion path through a quadrupole
Ideally, the rods are hyperbolic, 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 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.[4]
Multiple quadrupoles, hybrids and variations[edit]
Hybrid quadrupole time-of-flight mass spectrometer
A linear series of three quadrupoles is known as a triple quadrupole mass spectrometer. 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 elucidationbytandem mass spectrometry. 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 MorrisonofLa Trobe University in Australia for the purpose of studying the photodissociation of gas-phase ions.[5] The first triple-quadrupole mass spectrometer was developed at Michigan State University by Christie Enke and graduate student Richard Yost in the late 1970s.[6]
Quadrupoles can be used in hybrid mass spectrometers. For example, a sector instrument can be combined with a collision quadrupole and quadrupole mass analyzer to form a hybrid instrument.[7]
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).[8][9] Quadrupole-quadrupole-time-of-flight (QqTOF) configurations are also possible and used especially the mass spectrometry of peptides and other large biological polymers.[10][11]
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.[12] 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.[13][14]
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 spectrometryorgas 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 and SIMS surface analysis systems.[citation needed]
^ abUS 2939952, Paul, Wolfgang & Steinwedel, Helmut, "Apparatus for separating charged particles of different specific charges", published 1960-06-07, assigned to Wolfgang Paul
^ abcde Hoffmann, Edmond; Vincent Stroobant (2003). Mass Spectrometry: Principles and Applications (Second ed.). Toronto: John Wiley & Sons, Ltd. p. 65. ISBN978-0-471-48566-7.
^Morrison, J. D. (1991). "Personal reminiscences of forty years of mass spectrometry in Australia". Organic Mass Spectrometry. 26 (4): 183–194. doi:10.1002/oms.1210260404.
^Shevchenko A; Loboda A; Shevchenko A; Ens W; Standing KG (May 2000). "MALDI quadrupole time-of-flight mass spectrometry: a powerful tool for proteomic research". Anal. Chem. 72 (9): 2132–41. doi:10.1021/ac9913659. PMID10815976.
^Steen H; Küster B; Mann M (July 2001). "Quadrupole time-of-flight versus triple-quadrupole mass spectrometry for the determination of phosphopeptides by precursor ion scanning". J Mass Spectrom. 36 (7): 782–90. Bibcode:2001JMSp...36..782S. doi:10.1002/jms.174. PMID11473401.
^Maher S; Syed S; Hughes D; Gibson J; Taylor S (2013). "Mapping the stability diagram of a quadrupole mass spectrometer with a static transverse magnetic field applied". Journal of the American Society for Mass Spectrometry. 24 (8): 1307–1314. Bibcode:2013JASMS..24.1307M. doi:10.1007/s13361-013-0654-5. PMID23720050. S2CID45734248.
