Electrospray ionization small molecules

Smyth WE, Brooks P. 2004. A critical evaluation of high performance liquid chromatography ionization-mass spectrometry and capillary electrophoresis-electrospray-mass spectrometry for the detection and determination of small molecules of significance in clinical and forensic science. Electrophoresis 25 1413. 

Electrospray ionization mass spectrometry (ESI-MS) is a powerful technique for analyzing non-covalent complexes formed between small molecules and proteins. 

Much data on the structure of flavonoids in crude or semipurified plant extracts have been obtained by HPLC coupled with MS, in order to obtain information on sugar and acyl moieties not revealed by ultraviolet spectrum, without the need to isolate and hydrolyze the compounds. In the last decade, soft ionization MS techniques have been used in this respect, e.g., thermospray (TSP) and atmospheric pressure ionization (API). However, the most used methods for the determination of phenols in crude plant extracts were the coupling of liquid chromatography (LC) and MS with API techniques such as electrospray ionization (ESI) MS and atmospheric pressure chemical ionization (APCI) MS. ESI and APCI are soft ionization techniques that generate mainly protonated molecules for relatively small metabolites such as flavonoids. 

Kondo, T., Ueda, M., Yoshida, K., Titani, K., Isobe, M., and Goto, T. 1994. Direct observation of a small-molecule associated supramolecular pigment, commelinin, by electrospray-ionization mass-spectroscopy. J. Am. Chem. Soc. 116 7457-7458. 

Dunayevskiy, Y. Vouros, P. Carell, T. Wintner, E. A. Rebek, J. Jr. 1995. Characterization of the complexity of small-molecule libraries of electrospray ionization mass spectrometry. Anal. Chem., 67,2906-2915. 

Plastic microdevices for high-throughput screening with MS detection were also prepared for detection of aflatoxins and barbiturates. These devices incorporated concentration techniques interfaced with electrospray ionization MS (ESI-MS) through capillaries [2], The microfluidic device for aflatoxin detection employed an affinity dialysis technique, in which a poly (vinylidene fluoride) (PVDF) membrane was incorporated in the microchip between two channels. Small molecules were dialyzed from the aflatoxin/antibody complexes, which were then analyzed by MS. A similar device was used for concentrating barbiturate/antibody complexes using an affinity ultrafiltration technique. A barbiturate solution was mixed with antibodies and then flowed into the device, where uncomplexed barbiturates were removed by filtration. The antibody complex was then dissociated and electrokinetically mobilized for MS analysis. In each case, the affinity preconcentration improved the sensitivity by at least one to two orders of magnitude over previously reported detection limits. 

Electrospray ionization involves the introduction of a liquid solution directly into the atmospheric pressure source through an emitter. The liquid forms a droplet at the end of the emitter, where it is exposed to a high electrical field (Fig. 1). This results in a buildup of multiple charges on the surface of the droplet. The coulombic forces from these charges ultimately result in the droplet s expulsion from the surface. The ions produced in the ion source are then extracted into the mass analyzer. ESI is now widely used for identifying small molecules, proteins, studying large non-covalent complexes, structural analysis, and as a detector for separation methods such as HPLC and capillary... 

Mass spectrometry (MS) is playing an increasingly visible role in the molecular characterization of combinatorial libraries, natural products, drug metabolism and pharmacokinetics, toxicology and forensic investigations, and proteomics. Toward this end, electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo-ionization (APPI) have proven valuable for both qualitative and quantitative screening of small molecules (e.g., pharmaceutical products) [9-14]. 

Capillary electrokinetic chromatography (CEKC) with ESI-MS requires either the use of additives that do not significantly impact the ESI process or a method for their removal prior to the electrospray. Although this problem has not yet been completely solved, recent reports have suggested that considered choices of surfactant type and reduction of electro-osmotic flow (EOF) and surfactant in the capillary can decrease problems. Because most analytes that benefit from the CEKC mode of operation can be effectively addressed by the interface of other separations methods with MS, more emphasis has until now been placed upon interfacing with other CE modes. For small-molecule CE analysis, in which micellar and inclusion complex systems are commonly used, atmospheric pressure chemical ionization (APCI) may provide a useful alternative to ESI, as it is not as greatly affected by involatile salts and additives. 

Contemporary MS has evolved rapidly since the first reports of matrix-assisted laser-desorption/ionization (MALDl) [4] and electrospray ionization (ESI) [5]. The phamiaceutical industry is in a phase where an engineering ethos is center stage for characterization of small and large molecules alike. As MS tools proliferate, increased efficiency, such as those applications reviewed in this book, will enable an increased number of analyses per day, decreased sample volumes, and higher molecular weight analytes [6,7]. 

Griffey, R.H. Sannes-Lowery, K.A. Drader, J.J. Mohan, V.O Swayze, E.E. Hofstadler, S.A. Characterization of Low-Affinity Complexes Between RNA and Small Molecules Using Electrospray Ionization Mass Spectrometry, J. Am. Chem. Soc. 122, 9933-9938 (2000). 

Jiang, Y Lee, C.S. On-Line Coupling of Hollow Fiber Membranes with Electrospray Ionization Mass Spectrometry for Continuous Affinity Selection, Concentration and Identification of Small-Molecule Libraries, /. Mass Spectrom. 36, 664-669 (2001). 

Nuclear magnetic resonance (NMR) spectrometry and electrospray ionization mass spectrometry are other techniques used to measure the affinity of a small test molecule (the ligand) bound to a receptor. In varying the collision... 

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