Direct Liquid Introduction DLI

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. 

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,... 

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

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. 

The first approaches to the coupling of liquid-phase separation techniques with mass spectrometry were designed for HPLC needs, starting in the 1970s with since-forgotten techniques such as direct liquid introduction (DLI) and moving belt. In the 1980s, techniques such as thermospray, continuous-flow-fast atom bombardment (CF-FAB), and particle beam arose. 

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... 

The scientific curiosity to explore the utility of mass spectrometry to compounds that could not be analyzed by conventional GC/MS was supported by the need to extend the technique into the expanding field of biochemistry. While the development of LC/MS is still undergoing rapid evolution as evidenced by the number of reviews published at regular intervals, three main technological approaches have been constructed which continue to gain popular acceptance for practical use. These three introduction interfaces that are available commercially are the moving belt or transport interface (MB1), direct liquid introduction (DLI), and thermospray (TSP). This review will concentrate on these three interface types that are currently in widespread use. 

In the work described here the utility of solvent adduct ions in TSP LC-MS which consist in the use of novel additives in the chromatographic eluent, such as ammonium formate or chloroacetonitrile, will be demonstrated for confirmation of structure of a variety of herbicides including triazines, phenylurea and chlorinated phenoxyacids. Complementary adduct ion information to the conventional TSP LC-MS mode of operation will be obtained. Because TSP LC-MS involves mainly a chemical ionization process where the vaporized eluent acts as chemical ionization gas, it will be of interest to compare the different adduct ions obtained here with those using other interfacing systems such as direct liquid introduction (DLI) (13-18). 

The use of filament-on thermospray LC-MS in environmental pesticide analysis is a valuable technique with points of similarity in the ionization process with other hyphenated systems such as direct liquid introduction (DLI) LC-MS and chemical ionization GC-MS. The relative merits of ammonium formate as ionizing additive in PI and NI modes TSP LC-MS for three different... 

Direct liquid introduction (DLI) methodology has been described elsewhere (9). A thermospray probe and source (TSP, Model TS 360Q) were acquired from Vestec Corp. (Houston, TX) and later modified to include a probe tip heater incorporated in a copper block. Because of thermal contact, however, there was considerable interaction between the source temperature, the probe tip temperature and the temperature of the capillary. For this reason, attempts to optimize the ion currents by adjusting temperatures had limited success. Typical temperatures of the tip of the interface were 199- 205°C with source temperatures of230-240°C. The two mobile phases used were 0.1M ammonium acetate and 4 1 (v/v) 0.1 M ammonium acetate acetonitrile. The flow rate was 1.0 ml/min 1. Samples were admitted to the TSP probe via a Rheodyne (Model 7125) valve. 

Significant advances with the ionization source have been made. The pioneering work of many outstanding scientists with moving belt [34-36], direct liquid introduction (DLI) [37-39], thermospray ionization (TSI) [40,41], and ESI [42-44] set the stage for the development of modem ionization sources for high-performance LC-MS analysis. 

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