ESI Technique

Electrospray ionization was first reported by Fenn in 1984, and further developed in 1988.11 This technique relies on the generation of a spray from a liquid upon application of a high voltage. Typically, the sample to be analyzed is introduced in a liquid phase in a capillary. A strong electric field is created (high voltage of several kilovolts) between the liquid and a counter electrode (i.e. inlet of the mass spectrometer) placed some centimeters in front of the capillary. Upon application of the electric field, the liquid breaks into a gas of 

There are now two commonly defined regimes for electrospray analysis. These regimes are distinguished by the inner diameter of the capillary source, the liquid flow rate and the applied ionization voltage, and as already mentioned above these three parameters dictate the size of the generated droplets. These two regimes are 

The miniaturization of the technique started in 1994 with the description of microESI by the group of Caprioli.12,13 A further step towards miniaturization was reported by Wilm and Mann in the 1990s with the development of nanoESI.14 The miniaturization of the technique is driven by the improvement 

Additionally, miniaturization of the sources also leads to a decrease of the sample flow rate and the use of a lower ionization voltage. Ionization conditions are smoother, and the ionization source can be placed closer to the mass spectrometer inlet. Consequently, not only more ions are formed, but also more ions enter the mass spectrometer for their analysis. 

The enhancement in performance brought about by nanoESI compared to classical ESI is crucial for some fields of applications, such as bioanalysis, and especially proteomics. Miniaturizing ESI provides increased sensitivity of the analysis and enables its application for the analysis of complex samples with a wide range of analyte concentrations. When working with nanoESI the probability of ionizing molecules is higher, and this is of great importance for the analysis of complex real-world samples where some compounds are present as traces. 

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In contrast to direct mass spectrometry used in the El mode, ESI often requires specific pretreatments of the samples to purify the components of interest, to increase their yield of ionisation and consequently to improve selectivity and sensitivity. It is thus not a preliminary step of analysis but a method that forms part of an analytical strategy that allows the presence of well preserved high molecular long chain compounds to be shown before their fine characterisation by ESI techniques (Regert et al., 2003a Mirabaud, 2007 Mirabaud et al., 2007). 

The electroscopy ionization (ESI) technique is widely used in environmental analysis [75,83,90,538- 541,543]. In most ESI techniques, the source consists of... 

NSI Nanospray ionization (NSI) is a low-flow (10-500-nL/min) ESI technique with many advantages over conventional-flow ESI ( 200 xL/min) for the analysis of drugs, metabolites, peptides, proteins, and other macromolecules. Advantages of NSI over ESI include decreased sample consumption and increased sensitivity (Wilm and Mann, 1996 Corkery et al., 2005). NSI can be used for LC-MS or direct-infusion MS analysis of molecules (Wickremsinhe et al., 2006 Ramanathan et al., 2007c). 

Several modifications of the ESI technique have been introduced, principally micro-electrospray14 and nano-electrospray15 that have the advantage of using much lower flow rates, reducing the amount of analyte needed for a mass-spectrometric analysis this is performed using adapted probed tips. When performing nanospray, the sample is loaded into a fine hollow needle with a... 

The use of high performance liquid chromatography (HPLC) on-line or off-line is an essential feature for peptide mapping to integrate the removal of buffers and salts (purification) and the separation of analytes (preconcentration) with mass spectrometry. With on-line LC/MS approaches, low flow rates (<100pL/min) have been demonstrated to provide maximum sensitivity with ESI techniques for the analysis of proteins. In the work performed by Arnott and... 

The first approach to interfacing CE with MS was reported by Smith et al.48 when they incorporated the electrospray ionization (ESI) technique introduced by Dole et al.66 This development was based on the recognition that it is not necessary for the detection end of the CE capillary to be immersed in the buffer reservoir as conventionally practiced, as long as it is biased negative of the cathode potential (assuming a cathodic detector end). The ESI was created directly at the terminus of the CE capillary, avoiding any postcolumn region that would contribute to extracolumn band spread or analyte adsorption. A quadrupole mass filter was combined with ESI to produce the first on-line MS detection with capillary electrophoresis. 

Strege summarized the technique of high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI-MS) in dereplication of natural products. In contrast to earlier electron impact ionization (El), ESI technique is applicable to virtually any ion in solution with a soft ionization process. A comparison of ESI with fast atom bombardment (FAB), matrix assisted laser desorption ionization (MALDI), atmospheric pressure chemical ionization (APCI) and other techniques demonstrates its superior sensitivity, compatibility and reliability when coupled with HPLC [51]. 

Advancing methods in mass spectrometry (MS) have made it easier to determine the amino acid sequence in these peptides, and on ever smaller amounts of the compound. Direct tissue and single neuron analyses by matrix-assisted laser desorption/ionization-MS are particularly successful (Chapter 9.10). By these methods about 440 neuropeptides have been identified, and some 450 by electrospray ionization (ESI) techniques. Further techniques hold promise for more peptides.10 The neuropeptides have great importance for the insect physiologist,11 but hold less interest for the structural chemist. Their three-dimensional folding is largely undetermined. 

While the ESI technique has revolutionized the analysis of biomolecules such as peptides, proteins, polysaccharides, and nucleotides, the constraints imposed by its dependence on solution chemistry for ionization limit its applicability in the realm of small- to medium-sized nonpolar molecules (i.e., weakly basic or neutral compounds).120 Atmospheric pressure chemical ionization (APCI), on the other hand, takes advantage of gas-phase processes... 

The ESI technique is an online ionization technique because the liquid flowing from a syringe or from a liquid chromatography column is coupled directly to the ESI source. ESI sources have been coupled to many different types of mass analyzers and these will be discussed after the next section on MALDI. 

There is no doubt that for the analysis of all these compounds, mass spectrometry is the first choice to obtain information about the molecular masses of the investigated compounds. Mass spectrometry offers high information content with extremely low sample consumption. In principle, any mass spectrometric method can be employed. From the diverse ionization techniques which are available for the analysis of combinatorially sythesized compounds, especially the ESI-technique but also the MALDI-technique have gained most common acceptance. The reasons for this development are various and are discussed in the following sections. 

In the ESI technique [9, 10] a solution of the sample in a polar, volatile solvent (H2O, MeOH, CH3CN, etc.) is sprayed at atmospheric pressure into the ion source. During evaporation of the droplets, ions are liberated and transferred into mass spectrometer. The main advantages of ESI are simplicity of sample preparation and measurement of the spectrum, relatively low cost, and ability to transfer even weakly bonded complexes from the liquid to the gas phase. Importantly, the ESI ion source can be coupled directly to the outlet of an HPLC column making possi-... 

SEC/ESl also has been developed (see Chapter 4), although the ESI technique is mass limited for synthetic polymers, due to the effect of molecular entanglement processes in high MM polymers during electrospray experiments. 

While the stopped-flow ESI technique represents a straightforward adaptation of the canonical spectroscopic methodology for MS measurements, many studies cited in this and other chapters of the present book report on continuous flow interface systems. Indeed, the possibility to feed the dynamic samples continuously to the ESI source is the forte of this ionization technique. In the case of real-time ESI-MS monitoring, it is important to... 

The ESI technique was applied to the analyses of synthetic isomers of triacylglycerol hydroperoxides of eicosapentaenoic acid, and other oxidation products, including hydroxides, epoxides and triglyceride core aldehydes... 

Four tetracyclines (tetracycline, chlortetracycline, oxytetracycline, and doxycy-cline) and three fluoroquinolones (enrofloxacin, ciprofloxacin, and marbofloxacin) were analyzed by HPTLC-ESI technique. HPTLC silica gel 60 glass plates were functionalized to obtain ethylenediaminetetraacetic acid-modified silica gel plates. 

Electrospray ionization of polymers was introduced in 1968 (7) in an effort to study polystyrene by ms. In 1984, the technique was further developed for biopolymers (8). Spectra of poly(ethylene glycols) (PEG) up to molecular masses of 17,500 were also obtained using esi (9). In 1992, esi techniques were used to obtain ms on a poly(ethylene oxide) (PEO) of about 5,000,000 (2). Since esi techniques multiply charge macromolecules, both these studies were able to get spectra on a quadrupole ms with an miz of less than 2000, where z is the number of charges. However, with many different ra-mers in a normal narrow MMD of a synthetic pol5mier, the effect of z often as high as 40 means that one needs to obtain resolution in mIz of a small fraction of a mass unit to see the entire MMD. 

Preservation of the Chaise in the Transit of Ions from Solution to the Gas Phase Using the ESI Technique... 

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