Electrospray interface nebulization

Nilsson, S. L., Bylund, D., Joernten-Karlsson, M., Petersson, P., and Markides, K. E. (2004). A chemometric study of active parameters and their interaction effects in a nebulized sheath-liquid electrospray interface for capillary electrophoresis-mass spectrometry. Electrophoresis 25, 2100-2107. 

Ionization is accomplished in the electrospray interface by passing the HPLC effluent down a heated metal capillary tube along which an electric charge differential is applied. The evaporating liquid sprays out of the tube end as charge droplets rapidly decreasing in size. A gas nebulizer often... 

Detector UV 254 MS, Fisons Quattro II quadrupole, column effluent passed throu a splitter and 40 p.Li/min passed throu a 150 cm X 75 fjim fiised-silica capillary to the electrospray probe, nebulizing gas nitrogen at 12 L/h, drying gas nitrogen at 280 Uh, interface 60", capillary voltage 4 kV, con (orifice) voltage 30-80 V, photomultiplier 550 V,... 

Capillary IC features a flow rate in the range of 10-30 pF/min, thus requiring modifications and reoptimization of existing electrospray interfaces that are usually optimized for either analytical flow (100 pF/min to several milliliters/minute) or nanoflow (<1 pF/min) rates. The optimization of interface parameters such as probe temperature, nebulizer gas, needle voltage, type of desolvation solvent, and the flow rate plays a critical role in establishing instrument sensitivity. When capillary IC is operated at a flow rate of 10-20 pF/min, a probe temperature of 300 °C, a needle voltage of 3 kV, a nebulizer gas pressure of 65 psi, and the use of... 

Detector MS, Micromass Quatro Micro LC triple quadrupole, electrospray interface, positive ion mode, capillary voltage 3.5 kV, source 100°, desolvation 150°, nebulizing gas nitrogen at 365 L/h, collision gas argon at 3.5 jrbar, cone 40 V, collision energy 38.0 eV, m/z 612.40-100.10 (The make-up liquid was EtOHilO mM ammonium acetate 95 5 pumped at 0.25 mL/min, which mixed with the column effluent. A splitter was used so that 0.2 mL/min entered the detector.)... 

Detector MS, PE Sciex API-Ill Plus quadrupole, electrospray, negative ion mode, electrospray interface - 3700 V, orifice - 62 V, nebulizer gas nitrogen at 50 psi, curtain gas nitrogen at 1.8 L/min, collision gas argon, collision offset energy 25 eV, m/z 313-118... 

The design of a pneumatically assisted ESI interface differs from the sinple electrospray interface in that it provides a pneumatic assistance for the spray process. This is achieved by supplying a concentric flow of an inert gas such as nitrogen around the electrospray plume [58-60]. Assistance by a nebulizer gas stream of about 1-21 min allows for higher liquid flow and for a reduced influence of the surface tension of the solvent [61], Pneumatically assisted ESI can accommodate flows of 10-200 pi min . In fact, all modem ESI interfaces equipped with a nebulizer gas or sheath gas line enclosing the spray capillary. Thus, most routine ESI measurements are done using pneumatically assisted ESI. 

The liquid-based ionization technique in API-MS is applicable in electrospray interfacing as well as with a heated nebulizer introduction under certain conditions, i.e. without switching the corona discharge electrode on, and for certain compounds. Here, the discussion is restricted to electrospray interfacing. 

The nebulization and evaporation processes used for the particle-beam interface have closely similar parallels with atmospheric-pressure ionization (API), thermospray (TS), plasmaspray (PS), and electrospray (ES) combined inlet/ionization systems (see Chapters 8, 9, and 11). In all of these systems, a stream of liquid, usually but not necessarily from an HPLC column, is first nebulized... 

Aerosols can be produced as a spray of droplets by various means. A good example of a nebulizer is the common household hair spray, which produces fine droplets of a solution of hair lacquer by using a gas to blow the lacquer solution through a fine nozzle so that it emerges as a spray of small droplets. In use, the droplets strike the hair and settle, and the solvent evaporates to leave behind the nonvolatile lacquer. For mass spectrometry, a spray of a solution of analyte can be produced similarly or by a wide variety of other methods, many of which are discussed here. Chapters 8 ( Electrospray Ionization ) and 11 ( Thermospray and Plasmaspray Interfaces ) also contain details of droplet evaporation and formation of ions that are relevant to the discussion in this chapter. Aerosols are also produced by laser ablation for more information on this topic, see Chapters 17 and 18. 

Another big advance in the appHcation of ms in biotechnology was the development of atmospheric pressure ionization (API) techniques. There are three variants of API sources, a heated nebulizer plus a corona discharge for ionization (APCl) (51), electrospray (ESI) (52), and ion spray (53). In the APCl interface, the Ic eluent is converted into droplets by pneumatic nebulization, and then a sheath gas sweeps the droplets through a heated tube that vaporizes the solvent and analyte. The corona discharge ionizes solvent molecules, which protonate the analyte. Ions transfer into the mass spectrometer through a transfer line which is cryopumped, to keep a reasonable source pressure. 

Fig. 1.15 Desorption electrospray ionization interface. The sample, in this case a pharmaceutical pill, is placed in front of the orifice and is hit by nebulized droplets. Desorbed ions are then sampled into the mass spectrometer.

In IC-MS systems, the core of the equipment is the interface. In fact, inside the interface evaporation of the liquid, ionization of neutral species to charged species and removal of a huge amount of mobile phase to keep the vacuum conditions required from the mass analyzer take place. Two main interfaces are used coupled to IC, namely electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). In the ESI mode, ions are produced by evaporation of charged droplets obtained through spraying and an electrical field, whilst in the APCI mode the spray created by a pneumatic nebulizer is directed towards a heated region (400°C-550°C) in which desolvation and vaporization take place. The eluent vapors are ionized by the corona effect (the partial discharge... 

Mass spectrometry (MS) is now an integrated detector for liquid chromatography. This is due to the advent of atmospheric pressure ionization (API) interfaces. In an API interface, the column effluent is nebulized into an atmospheric pressure ion region. Nebulization is performed pneumatically in atmospheric pressure chemical ionization (APCI) by a strong electrical field in electrospray or by a combination of both in ion spray. Ions are produced from the evaporating droplets... 

Fig. 8.9. Schematic of a CEC—APCI-MS interface with electrospray nebulization, heated...

S. Zhou and M. Hamburger, Application of liquid chromatography-atmospheric pressure ionization mass spectrometry in natural product analysis. Evaluation and optimization of electrospray and heated nebulizer interfaces, J. Chromatogr., A, 755, 189-204 (1996). 

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