Liquids introduction

Direct liquid introduction interface. An interface that continuously passes all, or a part of, the effluent from a liquid chromatograph to the mass spectrometer the solvent usually functions as a chemical ionization agent for ionization of the solute. 

Seven different LC-MS interfaces are described in Chapter 4, with particular emphasis being placed on their advantages and disadvantages and the ways in which the interface overcomes (or fails to overcome) the incompatibilities of the two techniques. The earlier interfaces are included for historical reasons only as, for example, the moving-belt and direct-liquid-introduction interfaces, are not currently in routine use. The final chapter (Chapter 5) is devoted to a number of illustrative examples of the way in which LC-MS has been used to solve various analytical problems. 

El may be used with the moving-belt and particle-beam interfaces. Cl with the moving-belt, particle-beam and direct-liquid-introduction interfaces, and FAB with the continuous-flow FAB interface. A brief description of these ionization methods will be provided here but for further details the book by Ashcroft [8] is recommended. 

The direct-liquid-introduction (DLI) interface was made available commercially just after the moving-belt interface to which, as no company produced both types, it was an alternative. At this time, therefore, the commercial LC-MS interface used within a laboratory was dictated by the manufacturer of the mass spectrometer already in use unless a new instrument was being purchased solely for LC-MS applications. The development of LC-MS in the early 1980s was such that this was very rare and it was therefore unusual that a scientific evaluation was carried out to assess the ability of a type of interface to solve problems within a particular laboratory. 

The direct-liquid-introduction interface is shown schematically in Figure 4.2. This system is effectively a probe, at the end of which is a pinhole of approximately 5 p.m diameter, which abuts a desolvation chamber attached to the ion source of the mass spectrometer. The eluate from an HPLC column is circulated... 

Figure 4.2 Schematic of a direct-liquid-introduction LC-MS interface. From applications literature published by Agilent Technologies UK Limited, Stockport, UK, and reproduced with permission.

Moving wire/belt Direct liquid introduction Thermospray Particle beam... 

Niessen, W.M.A., Advances in instrumentation in liquid chromatography-mass spectrometry and related liquid-introduction techniques, ]. Chromatogr. A, 794, 407, 1998. 

LC-MS inlet probes support all conventional HPLC column diameters from mobile phase must be eliminated, either before entering or from inside the mass spectrometer, so that the production of ions is not adversely affected. The problem of removing the solvent is usually overcome by direct-liquid-introduction (DLI), mechanical transport devices, or particle beam (PB) interfaces. The main disadvantages of transport devices are that column... 

In reduced-flow LC-MS systems, the solvent flow into the spectrometer is reduced to a level where the pumping system can cope. Essentially, three such systems have been developed direct-liquid-introduction (DLI), flowing FAB [531] and electrospray [532]. An alternative approach to belt transport interfacing is to deliver the column eluate directly into the MS source and use Cl techniques. Methods based on this principle are called direct-liquid-injection systems, which are comprised of capillary flow restrictors, diaphragms,... 

DLI (1) Direct laser ionisation (2) Direct liquid introduction (3) Direct liquid interface... 

J.D. Henion, A comparison of direct liquid introduction LC/MS techniques employing microbore and conventional packed columns, J. Chromatogr. Sci., 18 (1980) 101-115. 

Direct liquid introduction (DLI) is the simplest and most straightforward approach. It was first attempted and reported by Tal Rose et al. 

The thermospray interface overcame many of the problems encountered with the moving-belt and direct-liquid-introduction interfaces and with the advent of this, LC-MS became a routine analytical tool in a large number of laboratories. This was reflected in the fact that this was the first type of interface made available commercially by the majority of the manufacturers of mass spectrometers. 

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