Hyphenated Mass Spectrometry Methods

In theory, the MS system used in these techniques is no different from any other detection system (e.g. a UV detector or flame ionization detection), except that it is a good deal more expensive. However, its main advantage over other detectors is the additional information about the analyte that it provides and the greater sensitivity it offers. 

The power of GC-MSand LC-MS techniques lies in their potential ability to take a mixture of many different compounds and provide structural information on each component. There is some overlap in the types of compounds that may be analysed using these two techniques, but, in the main, the role of both techniques may be considered to be complementary. In which circumstances then do we use GC-MS and in which do we use LC-MS As a general rule, if the analyte is non-polar and therefore volatile, we would probably use GC-MS. On the other hand, if the analyte is polar and/or non-volatile, the most appropriate technique will probably be LC-MS. 

as a chromatographic technique, is naturally suited to MS because it produces analyte molecules that are already in the gas phase (a requirement for any MS analysis). Interfacing a GC system to an MS instrument is, therefore, relatively straightforward as the compounds eluting from a GC column have already been volatilized and merely require separation from the carrier gas and ionization (usually by El or CI Section 5.2.1) before mass analysis. 

This technique has found great application in (orenaic science, e.g. in the detection of drugs in athletes or racehorses, 

as a technique, is very much dependent upon ionization (and ion vaporization) techniques that are suited to LC conditions, i.e. techniques where a relatively large solvent flow can be accommodated, which restricts us to just two ionization methods electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCl). Both techniques are very similar in their modes of operation (see Section 5.2.1), relying on the formation of a spray from a solvent flow at atmospheric pressure, and hence they are ideally suited to use in LC-MS applications. 

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Derive data on mass accuracy from high-resolution mass spectra Describe selected hyphenated mass spectrometry methods for... 

Whenever the goals of curve resolution are achieved, the understanding of a chemical system is dramatically increased and facilitated, avoiding the use of enhanced and much more costly experimental techniques. Through multivariate-resolution methods, the ubiquitous mixture analysis problem in chemistry (and other scientific fields) is solved directly by mathematical and software tools instead of using costly analytical chemistry and instrumental tools, for example, as in sophisticated hyphenated mass spectrometry-chromatographic methods. 

Benzene in gasoline can also be measured by infrared spectroscopy (ASTM D-4053). But additional benefits are derived from hyphenated analytical methods such as gas chromatography-mass spectrometry (ASTM D-5769) and gas chromatography-Fourier transform infrared spectroscopy) ASTM D-5986), which also accurately measure benzene in gasoline. The gas chromatography-mass spectrometry method (ASTM D-5769) is based on the Environmental Protection Agency s gas chromatography/mass spectrometry (EPA GC/MS) procedure for aromatics. 

See also Atomic Absorption, Methods and Instrumentation Atomic Absorption, Theory Atomic Emission, Methods and Instrumentation Biomedical Applications of Atomic Spectroscopy Forensic Science, Applications of Atomic Spectroscopy Hyphenated Techniques, Applications of in Mass Spectrometry Inductively Coupled Plasma Mass Spectrometry, Methods Inorganic Chemistry, Applications of Mass Spectrometry. 

The possibiHties for multidimensional iastmmental techniques are endless, and many other candidate components for iaclusion as hyphenated methods are expected to surface as the technology of interfacing is resolved. In addition, ternary systems, such as gas chromatography-mass spectrometry-iafrared spectrometry (gc/ms/ir), are also commercially available. 

In the case of the low abundance of some compounds, there are difficulties with signal overlap. To overcome these difficulties, there have been developments involving NMR hyphenation with techniques such as HPLC and mass spectrometry. In LC/NMR methods of analysis, NMR is used as the detector following LC separation and this technique is capable of detecting low concentrations in the nanogram range. This technique has been reported for the detection and identification of flavanoids in fruit juices and the characterization of sugars in wine [17]. 

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

Gas and liquid chromatography directly coupled with atomic spectrometry have been reviewed [178,179], as well as the determination of trace elements by chromatographic methods employing atomic plasma emission spectrometric detection [180]. Sutton et al. [181] have reviewed the use and applications of ICP-MS as a chromatographic and capillary electrophoretic detector, whereas Niessen [182] has briefly reviewed the applications of mass spectrometry to hyphenated techniques. 

Further developments of this topic are in progress, both in developing theoretical models and in applying the procedures to real experimental cases. In particular, the methods can be extended to hyphenated techniques to investigate the complex signals obtained from mass spectrometry detection (Pietrogrande et al., 2006b). 

Hyphenated techniques like combination of optical detection methods based on reflectometry or refractometry and separation techniques are of future interest. The same is valid for the intention to couple SPR or RIfS with mass spectrometry like MALDI33. 

Figure 2.9. Schematic of a matrix-assisted laser desorption/ionization (MALDI) event. The SEM micrograph depicts sinapinic acid-equine myoglobin crystal from a sample prepared according to the dried drop sample preparation method. In the desorption event neutral matrix molecules (M), positive matrix ions (M+), negative matrix ions (M-), neutral analyte molecules (N), positive analyte ions (+), and negative analyte ions (-) are created and/or transferred to the gas phase. Reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

Chromatographic methods are also often used as part of systems that are called hyphenated methods, (see Chapter 15) where the output of the chromatographic section is used as the input for an identification method such as mass spectrometry. These hyphenated methods are also most often referred to by their acronyms, for example, GC-MS—gas chromatography-mass spectrometry and HPLC-MS—high-performance liquid chromatography-mass spectrometry. Note that although ultraviolet-visible (UV-Vis) is hyphenated, it is not a hyphenated method in that it does not consist of two different methods of analysis. Hyphenated methods will be discussed fully in Chapter 15. 

Fascinating possibilities are opened by combining ACE with different mass spectrometry (MS) methods in order to overcome the main disadvantage of classical CE and IJV detection the lack of sensitivity. The hyphenated methods are very sensitive and allow us to characterize interactions of very small quantities of molecular entities. 

The combination of ACE with mass spectrometry (hyphenation methods) will increase significantly its applicability in pharmaceutics and biopharmaceutics. 

The elution character of the FFF techniques allows for it to be used in combination with other methods for further on-line or off-line characterization of the analytes (see Figure 12.1). FFF can be hyphenated with selective detection systems like mass spectrometry, multiangle laser scattering and can be combined with different separation techniques in multidimensional modes. In Figure 12.3, the trend in the number of published papers is reported. 

Because ICP-MS with different instrumentations and sample introduction systems (besides solution nebulization, also laser ablation or hyphenated methods, such as HPLC, CE, SPME) is today the most frequently used analytical technique for precise and accurate isotope ratio measurements, the following section will mainly focus on this form of mass spectrometry with an inductively coupled plasma source. 

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