Electrospray interface advantages

Although on-line HPLC-MS using an electrospray interface is a straightforward technique nowadays, it can be of advantage to analyze specific samples off-line. 

The obvious alternative for the in-line flow-through cell in HPLC-FTIR is mobile-phase elimination ( transport interfacing), first reported in 1977 [495], and now the usual way of carrying out LC-FTIR, in particular for the identification of (minor) constituents of complex mixtures. Various spray-type LC-FTIR interfaces have been developed, namely, thermospray (TSP) [496], particle-beam (PB) [497,498], electrospray (ESP) [499] and pneumatic nebulisers [486], as compared by Som-sen et al. [500]. The main advantage of the TSP-based... 

The ionspray (ISP, or pneumatically assisted electrospray) LC-MS interface offers all the benefits of electrospray ionisation with the additional advantages of accommodating a wide liquid flow range (up to 1 rnl.rnin ) and improved ion current stability [536]. In most LC-MS applications, one aims at introducing the highest possible flow-rate to the interface. While early ESI interfaces show best performance at 5-l() iLrnin, ion-spray interfaces are optimised for flow-rates between 50 and 200 xLmin 1. A gradient capillary HPLC system (320 xm i.d., 3-5 xLmin 1) is ideally suited for direct coupling to an electrospray mass spectrometer [537]. In sample-limited cases, nano-ISP interfaces are applied which can efficiently be operated at sub-p,Lmin 1 flow-rates [538,539]. These flow-rates are directly compatible with micro- and capillary HPLC systems, and with other separation techniques (CE, CEC). 

Issaq, H.J., Janini, G.M., Chan, K.C., Veenstra, T.D. (2004). Sheathless electrospray ionization interfaces for capillary electrophoresis—mass spectrometric detection advantages and limitations. J. Chromatogr. A 1053, 37 42. 

With respect to packing geometry and colunm efficiency, microbore colunms are equivalent to conventional colunms, except with respect to the internal diameter. Since most electrospray (ESI) interfaces are optimized for operation with flow-rates between 50 and 200 pl/min, the use of 1-3-mm-ID microbore columns is advantageous, because no post-column solvent splitting is required. 

The coupling of an MS with CEC or PEC provides several advantages. With the capillary columns of 100 mm inner-diameter (i.d.), flow rates of 1-2 L/min are obtained, which are ideal for electrospray MS [4]. No interface like a liquid sheath flow is required and the sintered silica gel frits allow direct coupling of the packed capillary columns without additional transfer capillaries. The spray is therefore formed directly at the outlet side of the column. Verheij et al. carried out the first coupling of a pseudoelectrochromatography system to a fast-atom bombardment (FAB)-MS in 1991 [6]. However, this required transfer capillaries that caused a loss in efficiency, which was also a problem with other experimentations with this technique. 

GC/MS with capillary columns has been the gold standard for more than 20 years, but LC/MS has become a complementary method due to the success in interface development with atmospheric pressure ionisation (API) for low molecular weight compounds and the appHcation to biopolymers. For many areas of analytical chemistry, LC/MS has become indispensible due to its advantages over GC/MS for polar and thermolabile analytes. A Hmiting factor for LC/MS has been the incompatibility between the hquid eluting from the LC and the mass spectrometer vacuum. This could be overcome in electrospray ionisation with the use of a nebuliser gas ( ion spray ) or additional heated drying gas ( turbo ion spray ) (70, 71]. Due to its high sensitivity and selectivity, APl-MS has become a standard tool for the stracture elucidation of analytes from complex mixtures. 

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