= m
= eV
= Hz


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UV Light
Beer's Law
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Quadrupole Mass Spectrometer
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Optical RotationCircular Dichroism
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The Quadrupole Mass Spectrometer

The Quadrupole Mass Spectrometer, either as a single quadrupole or as a triple quadrupole can also provide MS/MS spectra (at technique that will be discussed later. A diagram of a Quadrupole Mass Spectrometer is shown in figure 68.


Figure 68. The Quadrupole Mass Spectrometer.

The operation of the quadrupole mass spectrometer is completely different from that of the sector instrument. The quadrupole spectrometer consists of four rods that must be precisely straight and parallel and so arranged that the beam of ions is directed axially between them. Ideally the rods should be hyperbolic in cross section but in practice less expensive cylindrical rods are nearly as satisfactory. A voltage comprising a DC component (U) and a radio frequency component (Vocosωt) is applied between adjacent rods, opposite rods being electrically connected, as shown in figure 68. Ions are accelerated into the center, between the rods, by a relatively small potential ranging from 10 to 20 volts. Once inside the quadrupole, the ions oscillate in the (x) and (y) dimensions as a result of the high-frequency electric field.

The stability of the oscillating ions are determined by the magnitude of two parameters (a) and (q) which are defined by the following equations:

a = 8eU/(mr02w2) and q = 4eVo/(mr02w2)

where, (r0) is half the distance between opposite rods of the quadrupole system and the other symbols have the meaning previously ascribed to them.

Theory predicts that oscillations of the ions will only remain stable for certain combined values of (a) and (q). Outside these values the oscillations become infinite and the ions will strike the rods and become dissipated. The relationship between (a) and (q) is shown in figure 69

Figure 69. Conditions for Stable Oscillation in a Quadrupole Mass Spectrometer

It is seen that there is a very restricted range of values of (a) and (q) that can permit the mass spectrometer to operate in a stable mode. The mass range is scanned by changing both (U) and (V0) while keeping the ratio (U/V0) constant.Quadrupole Mass Spectrometer is compact, rugged and easy to operate and consequently is a popular instrument for general use in mass spectrometry.

Unfortunately, the mass range of the quadrupole spectrometer does not extend to such high values as the sector instrument but as already has been discussed, under certain circumstances multiple charged ions can be generated and identified by the mass spectrometer. This, in effect, significantly increases the effective mass range of the device.

The Quadrupole Mass Spectrometer can also be constructed to provide MS/MS performance. This is achieved by combining three quadrupole units in series. A diagram of a triple Quadrupole Mass Spectrometer is shown in figure 70.

Figure 70. The Triple Quadrupole Mass Spectrometer.

The sample enters the ion source and is usually fragmented by either an electron impact or chemical ionization process. In the first analyzer the various charged fragments are separated in the usual way and then pass into the second quadrupole section sometimes called the collision cell. The first quadrupole behaves as a straightforward mass spectrometer.

Instead of the ions then passing to a sensor, they pass into a second mass spectrometer. In this way a specific ion can be selected by the first quadrupole for further study. In the center quadrupole section, the selected ion is further fragmented by collision ionization and the new fragments then pass into the third quadrupole which functions as a second analyzer. The second analyzer segregates the new fragments into their individual masses, which are detected by the sensor, producing the mass Spectrum (originally from ions of one mass only). In this way, the exclusive mass Spectrum of a particular molecular or fragment ion can be obtained from the myriad of ions that may be produced from the sample in the first analyzer. It is seen that this can be an extremely powerful analytical system that can handle exceedingly complex mixtures and very involved molecular structures. The system has more than adequate resolving power and is valuable for structure elucidation.

Most modern quadrupole mass spectrometers are employed in conjunction with a separation unit such as a chromatograph. Due to the compact nature of the mass discrimination unit (compared with a Sector Mass Spectrometer) the combination of a chromatograph and a Quadrupole Mass Spectrometer can be manufactured as a bench top instrument.

A photograph of the triple quadrupole mass spectrometer tandem system consisting of a gas chromatograph combined with a mass spectrometer which was designed for drug analysis and manufactured by Varian Inc. is shown in figure 71.

The instrument is fitted with dual injectors and can provide both electron impact and chemical ionization facilities and MS/MS features if required.

Courtesy of Varian Inc.

Figure 71. The Varian Saturn 2200 Quadrupole Mass Spectrometer Combined with a Gas Chromatograph for Drug Analysis

Another example of the many tandem instruments that are available is that shown in figure 72. This also is designed round the Saturn 2200 quadruple mass spectrometer including a gas chromatograph.

Courtesy of Varian Inc.

Figure 72. The Varian Saturn 2200 Quadrupole Mass Spectrometer Combined with a Gas Chromatograph for Pesticide Residue Analysis

This instrument is fitted with an auto sampler in addition to the gas chromatograph, dual injectors and a second GC detector such as the electron capture detector.

A photograph of the layout of the triple mass discrimination unit is shown in figure 73, In the top left hand corner is the combined electron impact and Chemical Ionization sources with a hexapole ion guide interface to the mass analyzer unit that significantly improves the performance of the ionization process (improves the ionization efficiency by ensuring more ions that are formed enter the mass analyzer). Slightly left of center is a single turbo pump providing differential pumping for the source and the analyzer compartment. After the ions are produced they are conducted by the hexapole ion guide into the first quadrupole where the initial mass discrimination takes place. After selecting the ion(s) of interest the selected ions pass into the second quadrupole or collision cell. The collision cell is curved which ensures efficient dissociation. The selected ion(s) then pass into the third mass analyzer. Pre- and post-ion guides improve the transmission efficiency of each quardrupole. No lenses are employed which simplifies the tuning and further improve the magnitude of the signal. The ions after discrimination are accelerated by a 5kV gradient, which provides the same ion conversion efficiency as a ± 15kV dynode without suffering the noise associated with dynodes.

Courtesy of Varian Inc.

Figure 73. The Layout of the Triple Quadrupole Mass Discrimination System of the Varian 2200 Quadrupole Mass Spectrometer

The operating conditions of the mass analyzers and their associated equipment are continually available to the analyst in the form of a chart as shown in figure 74.

Courtesy of Varian Inc.

Figure 74. The Status of the Varian 2200 Quadrupole Mass Spectrometer

The action of the triple quadrupole system is depicted by the mass spectra shown in figure 74A, 74B, and 74C provided by Varian Inc. The many ions from the ionization source (and these will be parent ions of the substance(s)) are fed into the sample system together with any fragment ions and these will be separated in the first mass analyzer (i.e. the first quadrupole). The number of charged fragments produced will depend on the ion source. If the ions are produced by electron impact then many charged fragments are likely to be produced. If the ions are formed by Chemical Ionization then any ions formed are all likely to be parent ions or simple ‘charged addition’ ions. Thus, the mass discrimination afforded by the first quadrupole analyzer is shown in figure 74A.

Courtesy of Varian Inc.

Figure 74A. The Mass Analysis Taking Place in the First Quadrupole

This allows a specific ion (or group of ions having the same m/z values) to be selected and passed into the second analyzer. In figure 74 A the ions of m/z 263 pass into the second analyzer. This is depicted in the spectra shown in figure 74 B

Courtesy of Varian Inc.

Figure 74B. The Selected M/Z Ions that are Allowed to Enter the Third Analyzer.

In the second quadrupole collision induced fragmentation takes place and the products pass into the third quadrupole where the fragments are mass analyzed providing the Spectrum shown in figure 74C

Courtesy of Varian Inc.

Figure 74C. The Mass Analysis of the Collision Induced Fragments Produced in the Second Quadropole.

The unique collision induced fragments are presented against a virtually noise free background and yield better quantitative date and more informative data for structure elucidation. The triple quadrupole system, as well as providing more confident compound identification also provides high sensitivity. An example of the application of the system to drug analysis is shown in figure 75

Courtesy of Varian Inc.

Figure 75. The Results from the Analysis of 6-Monoacetylmorphine in Urine

The Combination of the triple quadrupole mass spectrometer with a separation technique such as a gas chromatograph or a liquid chromatograph is probably one of the most powerful analytical tools available to the contemporary chemist.



About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.

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