Direct liquid interface

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

While the first coupling of gas chromatography and mass spectrometry had been reported in the late fifties [4] one had to wait for almost another 20 years before the direct interfacing of liquid chromatography with mass spectrometry (LC-MS) was described by Arpino et al. [5]. With the direct liquid interface (DLI) the effluent of the chromatographic column was directly introduced in the electron impact source. Contrarily to gas chromatography coupled to mass spectrometry (GC-MS), LC-MS did do not catch on as rapidly. One of the reasons was that the MS interface could only handle LC fiow rates of a few microliters per minute. Another limitation was that electron impact or chemical ionization was not suit-... 

For LC/MS the main problem is the large amount of mobile liquid phase that must be removed to get the effluent reduced to the high vacuum of the MS. Microbore columns are desirable for this reason.22 The three most popular devices have been summarized by Majors23 direct liquid interface (DLI), moving belt transport, and thermospray.24 The thermospray device consists of a small bore capillary tube that is heated to produce a stable, high-velocity jet consisting mostly of vapor with a small amount of mist. It not only provides an interface to the MS, it also causes the ionization of analytes necessary for the MS. Some think it may find more widespread use as a transport device. 

Figure 2. Schematic of a direct liquid interface (DLI). From reference 24 with permission]...

A direct liquid interface, where the mobile phase is nebulized into the source, vaporized, and condensed onto a cold surface... 

This section represents a continuation of Section VII-5, which dealt primarily with the direct estimation of surface quantities at a solid-gas interface. Although in principle some of the methods described there could be applied at a solid-liquid interface, very little has been done apart from the study of the following Kelvin effect and nucleation studies, discussed in Chapter IX. 

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. 

In contrast to many other surface analytical techniques, like e. g. scanning electron microscopy, AFM does not require vacuum. Therefore, it can be operated under ambient conditions which enables direct observation of processes at solid-gas and solid-liquid interfaces. The latter can be accomplished by means of a liquid cell which is schematically shown in Fig. 5.6. The cell is formed by the sample at the bottom, a glass cover - holding the cantilever - at the top, and a silicone o-ring seal between. Studies with such a liquid cell can also be performed under potential control which opens up valuable opportunities for electrochemistry [5.11, 5.12]. Moreover, imaging under liquids opens up the possibility to protect sensitive surfaces by in-situ preparation and imaging under an inert fluid [5.13]. 

Although physical studies of the electronic structure of surfaces have to be performed under UHV conditions to guarantee clean uncontaminated samples, the technique does not require vacuum for its operation. Thus, in-situ observation of processes at solid-gas and solid-liquid interfaces is possible as well. This has been utilized, for instance, to directly observe corrosion and electrode processes with atomic resolution [5.2, 5.37]. 

The liquid alone pattern showed no entrained bubbles or gas-liquid interface in the field of view. The capillary bubbly flow, in the upper part of Fig. 5.14a, is characterized by the appearance of distinct non-spherical bubbles, generally smaller in the streamwise direction than at the base of the triangular channel. This flow pattern was also observed by Triplett et al. (1999a) in the 1.097 mm diameter circular tube, and by Zhao and Bi (2001a) in the triangular channel of hydraulic diameter of 0.866 mm. This flow, referred to by Zhao and Bi (2001a) as capillary bubbly... 

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.

The thermospray interface overcame many of the problems enconntered with the moving-belt and direct-liquid-introdnction 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 mannfacturers of mass spectrometers. 

Surface forces measurement is a unique tool for surface characterization. It can directly monitor the distance (D) dependence of surface properties, which is difficult to obtain by other techniques. One of the simplest examples is the case of the electric double-layer force. The repulsion observed between charged surfaces describes the counterion distribution in the vicinity of surfaces and is known as the electric double-layer force (repulsion). In a similar manner, we should be able to study various, more complex surface phenomena and obtain new insight into them. Indeed, based on observation by surface forces measurement and Fourier transform infrared (FTIR) spectroscopy, we have found the formation of a novel molecular architecture, an alcohol macrocluster, at the solid-liquid interface. 

Food typically is a complicated system with diverse interfaces. Stable air-water or oil-water interfaces are essential for the production of food foams and emulsions. Interface phenomena, therefore, attract great interest in the food industry. AFM provides enough resolution to visualize the interface structures, but it cannot be directly applied on air-liquid or liquid-liquid interfaces. Fortunately, the interface structure can be captured and transferred onto a freshly cleaved mica substrate using Langmuir-Blodgett techniques for AFM scan. Images are normally captured under butanol to reduce adhesion between the probe and the sample. Then, sample distortion or damage can be avoided (Morris et al, 1999). 

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