Offline interfacing

Similar chip design was also used for the offline interface with matrix-assisted laser desorp-tion/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). In contrast to inline ESI, MALDI provides for time flexibility. Separated compounds are collected on the plate and archived, thus enabling reanalyzing the sample later if so desired. Also, MS or MS/MS performance can be optimized independently in this implementation. An important part of the system is the noncontact electric field-driven deposition system mounted on an x,y,z stage, enabling computer-controlled deposition of sample spots across the plate. 

Offline-surface mediated approach Collect effluent from multiple LCs on surface place surface in MS interface use fast, surface-mediated ionization methods such as MALDI, DESI, etc. to sample effluents... 

A winding machine is typically equipped with computer control, and its programming is done offline. Winding pattern programs are often provided that interface with CAD programs and finite element analysis code. Such tools greatly enhance the process of concurrent engineering. Moreover, computer control allows the precise placement of the fiber on the part such that gap/overlap is minimized and the fiber orientation is controlled precisely. The... 

Shortly afterwards, this work was extended by the incorporation of a mass spectrometer into the system, thus enabling on-line NMR and MS data to be obtained with on-line collection of the eluent for off-line FT-IR spectroscopy [22]. The incorporation of the mass spectrometer required the addition of a small proportion of ammonium acetate, dissolved in methanol, to the deuterated chloroform used as the eluent in order to promote the ionisation of the analytes. The inclusion of methanol and ammonium acetate to the solvent obviously introduced new signals into the NMR spectra, and in addition resulted in the loss of exchangeable protons from the analytes which had been observable when chloroform alone was used as the solvent. This work demonstrated the feasibility of multiple hyphenation ( hypernation ) but the off-line nature of the FT-IR data acquisition, with the inevitable delay inherent in offline analysis, represents a slight disadvantage. In addition, volatile components may well be lost as the solvent is evaporated. This can be a problem that, together with analyte instability, is exacerbated with such interfaces when reversed-phase eluents are used since these require heating in order to ensure removal of the solvent. 

Of all tandem methods, SFE-HPLC is the most difficult coupling system to use. The main reason lies in linking a separation step at high pressure and generating a gas in the interface with a chromatographic technique, which commonly works under lower pressure and uses a liquid as a mobile phase. For this reason, in most applications, the analytes are collected offline and later analyzed by HPLC. SFE-HPLC has been performed for only a few specific applications. " ... 

MS is becoming the detection system of choice for LC by virtue of its flexibility and high selectivity for individual solutesHowever, LC-MS is always less sensitive than GC-MS as a result of the need to transfer the analytes from the liquid phase into a high-vacuum gas phase. Other limitations of LC-MS combination include the inability to use nonvolatile buffers, the narrow optimum range for eluent flow rate influence of the proportion of organic modifier on the sensitivity, and the narrow choice of ionization methods.Nevertheless, LC-MS has been widely accepted as an advantageous choice for the determination of carbamate pesticides in water matrices, which is more robust and flexible in the absence of derivatization. Thermospray and particle-beam interfaces are probably most commonly used for offline and online determination of carbamates in Atmospheric pressure sources such as... 

Cylinders of the flashed gas were analysed for hydrocarbon gas composition on a Carle gas chromatograph (GC) system equipped with both a thermal conductivity detector (TCD) and a flame ionization detector (FID). An offline preparation system and dual inlet mass spectrometer (MS) were used to analyse the carbon and hydrogen isotopic values of hydrocarbon components. A customized Gow Mac GC was interfaced with a vacuum/combustion system to separate hydrocarbons from other components and combust to CO2 and water that were purified and sealed into Pyrex tubes for isotopic analysis. The CO2 was analysed directly on one of three dual inlet mass spectrometers Finnigan Delta S, Finnigan Delta + XL or VG SIRA II. The water was reacted with zinc turnings and converted to hydrogen gas, which was analysed on either the Delta S or Delta + XL MS. 

There is one report of an offline SPME (or perhaps more properly SPE) extraction system for CE analysis in which the offline extraction is automated. The extraction medium is a monolith placed in the loop position of a six-port loop injector. An injector loop upstream contains a large loop with sample. The sample from the loop in the upstream injector is passed through the extraction medium for a controlled time at a controlled flow rate. Following a rinse, the back extraction solvent is then pumped through the monolith into a collection vial. This system has the advantage of the reproducibility of an automated system, and simultaneously avoids the complexity of the interfacing process. 

While still in early stages of development, microfluidic devices with MS detection are capable of successfully performing a number of bioanalytical tasks and have been demonstrated in various application areas. In particular, areas requiring capabilities for high-throughput processing of small sample amounts, such as proteomics and drug development, have been pursued. Online and offline sample infusion, preparation, and separation tasks have been implemented on chips interfaced to both ESI and MALDI-MS detectors. 

While MALDI is a typical off-line ionization technique/interface, one can also implement classical interfaces to carry out off-line (or at-line) analyses of reaction mixtures. Aliquots can be obtained from a reaction mixture (a dynamic sample/matrix) at specific time points, and injected to the ion source of a mass spectrometer for analysis (e.g., [49-54]). In some cases, quenching is conducted [49, 50]. If the transient intermediates (e.g., radicals) are to be detected, it is important to assure that the reaction has not finished at the time of ionization [55]. Short-lived intermediates can be reacted with auxiliary compounds in order to enable subsequent measurements in the methodology referred to as spin-trapping (see, e.g., [56, 57]). The off-line analyses based on aliquoting of dynamic matrices, and subsequent separation, provide temporal resolutions in the order of a few minutes (see, e.g., [58]). Nonetheless, they are uncomplicated, and - in some cases - they may enable offline sample treatment prior to detection by MS. For instance, inorganic ions, present in the reaction mixture, can readily be removed on an exchange resin to render the collected samples compatible with MS [59]. 

Extraction of zones on TLC plates into vials using the TLC-MS interface was reported for offline HPTLC-attenuated total reflectance (ATR)-1R spectrometry and... 

Combination of TLC with NMR spectrometry was reported by Gaugler et al. [19]. Three natural substances occurring frequently in plant extracts, that is, caffeic acid, chlorogenic acid, and rutin, were transferred offline from an HPTLC plate to an NMR spectrometer at concentrations above 10 pg/zone. A silica gel 6OF254 plate prewashed with methanol and dried under vacuum was developed with formic acid-ethyl acetate-water-methyl ethyl ketone (5 30 6 18) in a CAM AG twin trough chamber (TTC). After 5 min drying, elution of zones into respective vials was performed using the TLC-MS interface with the round elution head and methanol eluent at a flow rate of 0.3 mL/min for 6 min. The NMR instrument used was a 400-MHz Bruker Advance I with 5 mm BBO sample head under TOPSPIN 2.1. 

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