Mass spectrometry ionization techniques

Electrospray is the softest mass spectrometry ionization technique and electrospray spectra therefore usually consist solely of molecular ions. Electrospray is unique, however, in that if the analyte contains more than one site at which protonation (in the positive-ion mode) or deprotonation (in the negative-ion mode) may occur, a number of molecular ions with a range of charge states is usually observed. For low-molecular-weight materials (< 1000 Da), the number of sites... 

Thus, either the emitted light or the ions formed can be used to examine samples. For example, the mass spectrometric ionization technique of atmospheric-pressure chemical ionization (APCI) utilizes a corona discharge to enhance the number of ions formed. Carbon arc discharges have been used to generate ions of otherwise analytically intractable inorganic substances, with the ions being examined by mass spectrometry. 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

Burrell M, Earnshaw C, Clench M (2007) Imaging matrix assisted laser desorption ionization mass spectrometry a technique to map plant metabolites within tissue at high spatial resolution. J Exp Bot 58(4) 757-763. doi 10.1093/jxb/erll39... 

FAB is most often compared to the soft ionization method known as field desorption (FD) mass spectrometry, a technique in which the sample, deposited on an emitter wire coated with microcrystalline carbon needles, is desorbed under the influence of a high electric field gradient. As usual, bioorganic systems are best represented by both techniques (21, 33). Though FAB is the easier of the two, they are complementary, FAB being particularly suited for the case of extreme thermal lability and FD for the case of chemical lability or matrix interference. Cerny et al. (33) compare the two techniques for the study of coordination complexes and conclude FD is better for molecular-ion determination, while FAB provides better fragmentation information, which is useful in elucidating structures. 

At that time the mass spectrometric ionization techniques of electron ionization (El) [1] and chemical ionization (Cl) [2] required the analyte molecules to be present in the gas phase and were thus suitable only for volatile compounds or for samples subjected to derivatization to make them volatile. Moreover, the field desorption (FD) ionization method [3], which allows the ionization of non-volatile molecules with masses up to 5000 Da, was a delicate technique that required an experienced operator [4], This limited considerably the field of application of mass spectrometry of large non-volatile biological molecules that are often thermolabile. 

The use of mass spectrometry (MS) techniques to monitor SP reactions has recently become possible through the use of matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) spectrometry (137) after in situ cleavage of a small number of resin beads (138-140). Although the compound is cleaved from the resin, the cleavage happens directly onto the center of the MALDI target and the method can be considered on-bead. 

The detection and identification of phenolic compounds, including phenolic acids, have also been simph-fied using mass spectrometry (MS) techniques on-hne, coupled to the HPLC equipment. The electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) interfaces dominate the analysis of phenohcs in herbs, fmits, vegetables, peels, seeds, and other plants. In some cases, HPLC, with different sensitivity detectors (UV, electrochemical, fluorescence), and HPLC-MS are simultaneously used for the identification and determination of phenolic acids in natural plants and related food products.In some papers, other spectroscopic instmmental techniques (IR, H NMR, and C NMR) have also been apphed for the identification of isolated phenolic compounds. 

Mass spectrometry (MS) techniques are based on measurement of the charge/mass (m/z) ratio of molecules ionized by electron impact, laser irradiation, or by interaction with electric fields. Several MS methods have been applied in the study of grape and wine proteins initially as off-line techniques, then coupled to LC and CE techniques. 

S. Hillenkamp, F De Vries, M.S. Comparative Mass Spectrometric Analyses of Photofrin Oligomers by Fast Atom Bombardment Mass Spectrometry, UV and IR Matrix-Assisted Laser Desorption/lonization Mass Spectrometry, Electrospray Ionization Mass Spectrometry and Laser Desorption/Jet-Cooling Photoionization Mass Spectrometry, J. Mass Spectrom. 34, 661-669 (1999). Powell, K.D. Fitzgerald, M.C. Accuracy and Precision of a New H/D Exchange- and Mass Spectrometry-Based Technique for Measuring the Thermodynamic Properties of Protein-Peptide Complexes, Biochemistry 42,4962-4970 (2003). 

One of the most powerful techniques used in Upid analysis today is HPLC coupled with mass spectrometry (HPLC/MS). Several mass spectrometric ionization techniques, such as fast atom bombardment (FAB) [23], electrospray ionization (ESI) [29,30], ionspray ionization (ISI) [31], and atmospheric pressure chemical ionization (APCI) [22,30,32] have been used. By using HPLC/MS, one can get information on the molecular structure of the intact lipids, which helps differentiate molecular species within different lipid classes. By using tandem mass spectrometry (MS/MS), identification of molecular species of different sphingolipids can be achieved in an easier and more sensitive way. There are many other advantages of using MS, such as small sample size, minimal sample preparation, and lack of need for derivatization, speeds, and sensitivity. In the literature, sphingolipids of both animal and plant origin were analyzed by MS. 

Modem mass spectrometry, photoionization techniques, and ion cyclotron resonance spectroscopy revolutionized gas-phase basidty determination. These techniques have permitted extremely accurate absolute gas-phase basidties to be obtained for a few molecules via thermodynamic cycles using electron affinity and ionization energy data. - Absolute gas-phase basicities for even a few molecules provide valuable references for determination of GB values for bases for which direct GB determination is problematic. The mathematical approach is conceptually straightforward. Consider the general GB equations for Bj and B2 ... 

With the development of relatively softer ionization techniques (ESI and MALDI), mass spectrometry has emerged as an option for the determination of conformational changes in proteins [2,16-23], It is believed that the solution-phase structure of a protein is largely preserved during ionization by these two methods. Therefore, the ESI and MALDI mass spectra of a protein reflect the features of its aqueous solution chemistry. The multiple-charging feature of ESI is also a valuable asset because it allows the study of much larger proteins. In this chapter a broad outline of the commonly used mass spectrometry-based techniques is presented. 

See also Derivatization of Analytes. Gas Chromatography Column Technology High-Temperature Techniques High-Speed Techniques Detectors Environ-mentai Appiications Forensic Appiications Petro-chemicai Appiications. Mass Spectrometry Ionization Methods Overview Mass Separation Selected Ion Monitoring. 

See also Atomic Mass Spectrometry Inductively Coupled Plasma Laser Microprobe. Gas Chromatography Mass Spectrometry. Liquid Chromatography Liquid Chromatography-Mass Spectrometry. Mass Spectrometry Ionization Methods Overview Atmospheric Pressure Ionization Techniques Electrospray Liquid Secondary Ion Mass Spectrometry Matrix-Assisted Laser Desorption/ionization. Surface Analysis Secondary Ion Mass Spectrometry of Polymers Laser Ionization. 

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