Direct interfacing

The El ion source pressme is typically less than 10 r. The main reasons for maintaining these pressures are to, 

Since the source pressure has to be maintained within the pumping capacity or conductance of the vacuum system, low carrier gas flow rates are necessary or various types of interface are used to reduce the carrier gas component in the GC effluent. Interfaces and transfer lines have to be maintained at or above the maximum column temperature used. 

Direct interfacing is used in bench-top GC-MS instruments which are equipped with narrow bore WCOT capillary columns and the carrier gas 

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MACCS-II enables direct interface with other database management systems, such as the Relational Database Management System (RDBMS) and Oracle, so that databases which contain text and numeric data for which special interfaces are normally needed can be constmcted. Eor example, an Oracle MACCS-II linked system is currendy being used by the National Institute on Dmg Abuse (113) to develop a database that will allow scientists to determine the molecular stmctures of cocaine and other controlled substances as well as designer dmgs. 

This method requires about 40 g of tobacco which are extracted with ethyl acetate in the presence of ascorbic acid. A trace amount of C-NDELA is added as an internal standard for quantitative analytical work. The filtered extract is concentrated and NDELA is enriched by column chromatography of the concentrate on silica gel. The residues of fractions with p-activity are pooled and redissolved in acetonitrile. Initially, we attempted to separate NDELA on a 3% OV-225 Chromosorb W HP column at 210 C using a GC-TEA system with direct interface similar to the technique developed by Edwards a. for the analysis of NDELA in urine (18). We found this method satisfactory for reference compounds however, it was not useful for an optimal separation of NDELA from the crude concentrate of the tobacco extract (Figure 4). Therefore, we silylated the crude concentrate with BSTFA and an aliquot was analyzed by GC-TEA with direct interface. The chromatographic conditions were 6 ft glass column filled with 3% OV-... 

A biosensor is an analytical device comprising a biological recognition element directly interfaced to a signal transducer, which together relates the concentration of... 

Ab-initio CAChe features all of the above plus ab-initio and density functional methods. This program requires a workstation (Windows NT minimum or SGI and IBM unix-based machines) and can be used to build and visualize results from ab-initio programs (e.g., Gaussian, see description under Gaussian, Inc.). Also, CAChe directly interfaces to Dgauss , a computational chemistry package that uses density functional theory to predict molecular structures, properties, and energetics. 

Identification of the component peaks of a chromatogram, which may be numerous, can be achieved in two ways comparison of retention times (discussed below) trapping the eluted components for further analysis by other analytical techniques such as infrared and mass spectrometry or by direct interfacing of these techniques with a gas chromatograph. This latter approach is discussed on p. 114. 

The identification of GC peaks other than through retention data, which are sometimes ambiguous or inconclusive, can be facilitated by the direct interfacing of GC with infrared spectrometry (p. 378 et seq.) or mass spectrometry (p. 426 etseq.), so-called coupled or hyphenated techniques. The general instrumental arrangement is shown in Figure 4.29(a). 

When nano LC is combined with mass spectrometer detection, attamole detection can be achieved for low abundance components in biological fluids, drug metabolites, and natural products such as Chinese herb medicines. Nano LC-MS-MS has become an essential tool for complex biological and drug metabolite studies. Nano LC-MS presents two significant differences from conventional analytical HPLC (1) large enhancement factor for sample detection and (2) direct interface to MS without flow splitting. The enhancement in MS ion counts relative to a conventional 4.6 mm ID column is proportional to the ratio of the square of the column diameter ... 

For a 75 /.mi ID nano LC column as an example, the MS detection enhancement factor (ion count) in comparison to a 4.6 mm column is much higher than (4.6/0.075)2 = 3761 because of the reduction in sample molecular zone dilution and because a nano LC solvent flow rate at 0.02 to 2 /iL/min can be 100% directly sprayed into the MS ion source. No post-column flow splitting is required for nano-LC-MS as that required when 1 mL/min is used in a 4.6 mm ID column. This large enhancement of MS detection and the ability to directly interface with MS presents nano LC-MS as the best tool for life science research. 

Atmospheric pressure ionization (API) was the first technique to directly interface solution phase with a mass analyzer. [26] In API, a solution of the analyte is injected into a stream of hot nitrogen to rapidly evaporate the solvent. The vapor passes through a Ni source where electrons emitted from the radioactive Ni isotope initiate a complex series of ionizing processes. Beginning with the ioniza-... 

With the advent of capillary GC, [50-54] the need for separators and the concomitant risk of suppression of certain components vanished. Capillary columns are operated at flow rates in the order of 1 ml min and therefore can be directly interfaced to EI/CI ion sources. [48,49] Thus, a modem GC-MS interface basically consists of a heated (glass) line bridging the distance between GC oven and ion source. On the ion source block, an entrance port often opposite to the direct probe is reserved for that purpose (Chap. 5.2.1). The interface should be operated at the highest temperature employed in the actual GC separation or at the highest temperature the column can tolerate (200-300 °C). Keeping the transfer line at lower temperature causes condensation of eluting components to the end of the column. 

Bi-directional interfacing to existing business systems (LIMS, SAP, etc.). 

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-... 

Wherever possible, analytical systems are automated and directly interfaced to a laboratory data system. This mainly receives information from an Optical Mark Registration (OMR) system, or from down loaded computer files, and then produces a work schedule which is printed for the analyst and transferred electronically to the instrument. 

Two principal GC-MS interfaces are available for open-tubular GC columns. The so-called direct interface provides the highest possible detector sensitivity, whereas the open-split interface offers the least possible interference with chromatographic separation. With the direct interface, the column exit is routed from the GC oven through a heated transfer line directly into the ionization chamber. As long as the vacuum-pumping system can remove the carrier gas and maintain a sufficiently low pressure, the MS detector will function. Also, little chance exists for adsorptive loses of solute because the analytes contact only the GC column. 

However, a direct interface subjects the exit of the column to vacuum conditions. Tire vacuum may lower the inlet pressure required to obtain the desired mass-flow rate of the carrier gas and also changes its linear-velocity profile across the column. These conditions can cause poor retention-time and peak-area precision and can even make the inlet system stop delivering carrier gas to the column. Thus, analysts should use direct interfaces only with long, narrow-bore columns... 

Application of F.t.-i.r. spectroscopy to biological systems and carbohydrate mixtures or dilute solutions is of particular interest, because of the ease of analysis of data by use of such techniques as absorption subtraction or factor analysis. This is possible owing to the direct interfacing of the computer to the spectrometer, which allows arithmetic manipulation of the spectra in an imaginative way, as will be seen in the following Section. 

MACCS-II enables direct interface with other database management systems, such as Ihe Relational Database Management System RDBMS) and Oracle, so that databases that contain text and numeric data, for which special interfaces arc normally needed, can be constructed. 

Some analyzers do not use a sampling system the analyzer probe is put directly into the process line. Many spectroscopic analyzers can be operated with a probe directly interfaced with the process (called in-line analysis). Some concerns with taking this approach are calibration, and how to introduce a real standard to the system for instrument verification. These are questions one must be asking. 

Li and coworkers [193] reported a hybrid technique for rapid speciation analysis of Hg(I) and MeHg(II) by directly interfacing an NCE to atomic fluorescence spectrometry. Both mercury species were separated as their cysteine complexes within 64 seconds. The precision (RSD, n = 5) of migration time,... 

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