The Direct-Liquid-Introduction Interface

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 most important part of this type of interface, from a number of points of view, is the pinhole which, in conjunction with the pumping capacity of the mass spectrometer, controls the flow of eluate into the mass spectrometer. This flow, and therefore the properties of the spray being introduced into the mass spectrometer, is affected by a change in the viscosity of the mobile phase. The use of gradient elution has therefore to be approached with some caution as the sensitivity of the mass spectrometer can change significantly during the course of an analysis. 

The maximum flow rate that can be accommodated while still allowing the mass spectrometer to operate is in the range of 10-20 pdmin-1. Typical flow rates used in conventional HPLC separations are between 500 and 1000 ttlmin-1 and therefore only between 1 and 4% of the column eluate, and therefore ana-lyte(s), enter the mass spectrometer source. The sensitivity, or more accurately the lack of sensitivity, of the DLI interface is one of its major limitations. 

This can potentially be overcome by the use of microbore HPLC columns with flow rates which are directly compatible with mass spectrometer operation, although the necessary decrease in injection volume results in little overall gain in the concentration of sample reaching the mass spectrometer. In addition, at the time that the DLI was available, the use of microbore HPLC, which introduces another set of potential problems related to chromatographic performance, was probably as widespread as the use of LC-MS It has been assessed [2] that in around 25% of the reported applications of DLI, microbore HPLC has been utilized. 

The liquid jet from the DLI probe has to be initiated at atmospheric pressure, i.e. before insertion of the interface into the mass spectrometer, and, for best performance, the spray direction has to be coaxial to the probe. Any deviation from this, however slight, tends to produce changes in the mass spectrum obtained. 

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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... 

In order to avoid precipitation of non-volatile analytes at the iimer wall of the tube, tapering of the outlet was investigated [29-31]. The difficulty with this approach is that tapering is art rather than science. The latter problem was solved by the introduction in 1980 of a small diaphragm as solvent restriction [32], leading to the direct liquid introduction interface (Ch. 4.5). 

Atmospheric-pressure chemical ionisation was favoured by Horning et al. [3], whereas (2) Scott et al. [4] applied a moving-wire system which became transformed and finally resulted in the moving-belt interface. (3) The research of Arpino and his co-workers [5] led further in the direction initiated by Talroze [2], which after all had brought about the direct liquid introduction interface. 

The first two commercially available LC-MS interfaces were the moving-belt interface and the direct liquid introduction interface. These hyphenated techniques promoted pharmacological research at several stages of drug development. The polar pharmaceutical compounds that were under research in pharmacological experiments, their polar by-products from chemical synthesis or even the very polar... 

First paper on LC-MS by the group of Tal roze 1974 A variety of LC-MS interfaces presented at the 9th International Symposium on Advances in Chromatography, Houston, Texas 1976 Commercial introduction of the moving-belt interface 1980 Commercial introduction of the direct liquid introduction interface... 

In the past 10 years, the manner in which LC-MS analysis is performed has significantly changed. While in the past it was necessary to choose the most appropriate LC-MS interface for a particular application from a list of five possibilities, e.g., the moving-belt interface, the direct-liquid introduction interface, the thermospray interface, the particle-beam interface, and the continuous-flow fast-atom bombardment interface, today all LC-MS technologies are based on API. The two most important... 

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 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. 

Direct liquid introduction interface. An interface that continuously passes all, or a part of, the... 

For LC-MS to become a reality an interface had to be designed which was capable of providing a vapour sample feed consistent with the vacuum requirements of the mass spectrometer ion source and of volatilising the sample without decomposition. Various enrichment interfaces have been developed such as the molecular jet, vacuum nebulising, the direct liquid introduction inlet and thermospray systems. 

It is much more difficult to couple HPLC on-hne with El and Cl sources (Ardrey 2003) since the voliune of vaporized mobile phase is up to 10 times larger than that of the liquid, far beyond the pumping capabilities of any realistic vacuum system unless the flow rate is restricted by a post-column split to a few xL.min, as in the so-called direct liquid introduction interfaces (Abian 1999) for Cl and (less commonly) El these interfaces... 

High performance liquid chromatography is well established as a tool for the separation of mixtures of labile biological substances, but its combination with mass spectrometry (LC/MS) remains fraught with difficulties as a valid analytical procedure. Several new types of direct liquid introduction interfaces (as opposed to transport interfaces) have been developed recently and appear to offer promise for the future. The thermospray interface 44) has been used successfully for amino acids, small peptides, nucleosides, antibiotics and glucuronides. The spectra are the relatively simple ones commonly obtained from soft ionization methods and resemble those obtained by FABMS. A gas nebulizer interface has enabled representative spectra (resembling those generated from DCI) to be... 

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,... 

Many interfaces have been developed to meet these demanding challenges. Some of these coupling methods, such as the moving belt or the particle beam interface, are based on the concomitant elimination of the solvent before it enters the mass spectrometer. Other methods such as direct liquid introduction (DLI) or continuous flow FAB rely on splitting the flow of the liquid that is introduced into the interface in order to obtain a flow that can be directly infused into the ionization source. However, these types of interfaces can only handle a fraction of the liquid flow from the LC. 

An alternative to direct liquid introduction is the moving belt, or moving-wire, transport interface. Because all l.c. solvents are evaporated before the sample is transported into the ion source, fewer restrictions are placed on solvent type, flow rates, or buffer composition. This system has been used for analysis of mixtures of pentoses, hexoses, and disaccha-rides. ... 

Most of the direct and indirect (transport) interfaces described here use chemical ionization (c.i.) ion-sources, which are not well suited to such polar, non-volatile compounds as tri- and higher oligosaccharides. The thermospray interface, which can operate on an ion-evaporative mode, is capable of producing intact molecular ions from such nonvolatile, polar molecules and should be useful in oligosaccharide analysis. Molecules of this type, however, can also be easily analyzed by fast-atom-bombardment ionization, and use of this technique, coupled to direct liquid introduction and moving-belt interfaces, has been reported. The latter system has been applied to complex oligosaccharide analysis. ... 

Even a technique as complicated as direct liquid-introduction mass spectrometry has been coupled with reactor systems to provide real-time compositional analysis, as described in a series of articles by Dell Orco and colleagues.32-34 In their work, these authors used a dynamic dilution interface to provide samples in real time to un-modified commercial ionization sources (electrospray (ESI) and atmospheric pressure chemical ionization (APCI)). Complete speciation was demonstrated due to the unambiguous assignment of molecular weights to reactants, intermediates, and products. 

Arpino, P. J. Guiochon, G. Krien, R Devant, G. 1979. Optimization of the instrumental parameters of a combined liquid chromatograph-mass spectrometer, coupled by an interface for direct liquid introduction. . /. Chromatogr., 185,529-547. 

Different methods are used to tackle these problems [10-13], Some of these coupling methods, such as moving-belt coupling or the particle beam (PB) interface, are based on the selective vaporization of the elution solvent before it enters the spectrometer source. Other methods such as direct liquid introduction (DLI) [14] or continuous flow FAB (CF-FAB) rely on reducing the flow of the liquid that is introduced into the interface in order to obtain a flow that can be directly pumped into the source. In order to achieve this it must be reduced to one-twentieth of the value calculated above, that is 5 pi min. These flows are obtained from HPLC capillary columns or from a flow split at the outlet of classical HPLC columns. Finally, a series of HPLC/MS coupling methods such as thermospray (TSP), electrospray (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) can tolerate flow rates of about 1 ml min 1 without requiring a flow split. Introducing the eluent entirely into the interface increases the detection sensitivity of these methods. ESI can accept flow rates from 10 nl min-1 levels to... 

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