Off-Line Interfacing

For many experiments, including the slow rate pyrolyses discussed above, it may be advantageous or necessary to install the pyrolysis device away from the analytical instrument, with a collection or trapping system between. This may be accompUshed in a variety of ways, both manually and automatically. 

An analyst performing many off-line pyrolyses or reactant gas studies will probably select an automatic means of isolating the pyrolysis end of the experiment from the analytical. This is typically done by employing a valve in a heated oven to place the collection device alternately on-line with the pyrolyzer or with the analytical device. Commercial systems are available that incorporate this valve in a programmable controller to automate the process, as well as in manually operated modes. The pyrolysis chamber carrier gas is directed through the valve to the trap. 

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The ability of SPE to be online or off-line, interfaced with major analytical instrumentation, is considered as another dimension and an advantage over the classical liquid extraction. 

A major barrier to development of MALDl as an off-line interface between HPLC and MS for trace quantitation has been the chemical noise (peak-at-every-m/z background, particularly at low mh <500) where matrix-derived peaks can dominate the spectrum. However, the overall problem can be resolved into constituent parts, namely, development of efficient deposition of eluant to provide adequate representations of the chromatographic separation, and efficient MALDl-MS analysis of the deposited analytes on a timescale that should certainly be no longer than the chromatographic time, and also methods to provide acceptable accuracy, precision, repeatability, LOD, LLOQ and dynamic range of the quantitation. 

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

Traditionally, LC and GC are used as separate steps in the sample analysis sequence, with collection in between, and then followed by transfer. A major limitation of off-line LC-GC is that only a small aliquot of the LC fraction is injected into the GC p. (e.g. 1 - 2 p.1 from 1 ml). Therefore, increasing attention is now given to the on-line combination of LC and GC. This involves the transfer of large volumes of eluent into capillary GC. In order to achieve this, the so-called on-column interface (retention gap) or a programmed temperature vaporizor (PTV) in front of the GC column are used. Nearly all on-line LC-GC applications involve normal-phase (NP) LC, because the introduction of relatively large volumes of apolar, relatively volatile mobile phases into the GC unit is easier than for aqueous solvents. On-line LC-GC does not only increase the sensitivity but also saves time and improves precision. 

One of the attractive features of SFE with CO2 as the extracting fluid is the ability to directly couple the extraction method with subsequent analytical methods (both chromatographic and spectroscopic). Various modes of on-line analyses have been reported, and include continuous monitoring of the total SFE effluent by MS [6,7], SFE-GC [8-11], SFE-HPLC [12,13], SFE-SFC [14,15] and SFE-TLC [16]. However, interfacing of SFE with other techniques is not without problems. The required purity of the CO2 for extraction depends entirely on the analytical technique used. In the off-line mode SFE takes place as a separate and isolated process to chromatography extracted solutes are trapped or collected, often in a suitable solvent for later injection on to chromatographic instrumentation. Off-line SFE is inherently simpler to perform, since only the extraction parameters need to be understood, and several analyses can be performed on a single extract. Off-line SFE still dominates over on-line determinations of additives-an... 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

Klink [135] recently discussed sample preparation procedures for LC-MS. SPE can be so well integrated into the concept of LC-MS, that in many automated applications no clear distinction exists between SPE and LC [135]. In on-line LC-MS mode, the possibilities for changing the eluent are rather limited, because of the tolerance of the eluent for the interface. Moreover, the conventional gradient mode may lead to strong fluctuations in the response of the MS detector. Here the off-line mode, using SPE for concentration followed by selective elution, enables very far-reaching preseparation, due to the differences in the polarity of the eluents applied and their mixtures. Although the overall benefits of SPE for LC-MS applications are positive, extracts... 

Consequently, the major experimental options for the analyst are packed or capillary SFC, mobile phase with/without modifier and off-line or on-line mode, namely direct deposition (DD-SFC-FUR) vs. flowcell. Both small-bore packed columns and narrow-bore open-tubular columns have been used for SFC-FTIR analysis using a pressure-stable, thermostated, flow-cell or solvent elimination interfaces. 

For microbore HPLC, with a flow of less than lOOpLmin-1, off-line LC-FT1R has been developed using matrix isolation techniques. The solutes are deposited on a moving IR salt window [504] or on a rotating plated disc [486], and are measured afterwards with the aid of a FITR microscope or a reflectance accessory. FTIR detection was first applied to the analysis of microbore HPLC eluent by Teramae and Tanaka [505]. In microbore HPLC-FTIR the amount of mobile phase required for separation is much less than for conventional scale HPLC. This simplifies both flow-cell and mobile-phase elimination interfaces. Flow-cell... 

Principles and Characteristics Multidimensional gas chromatography (MDGC) is widely used, due to the mobile-phase compatibility between the primary and secondary separating systems, which allows relatively simple coupling with less-complicated interfaces. In its simplest form, 2DGC can be carried out in the off-line mode. The most elementary procedure involves manual collection of effluent from a column, followed by reinjection into another column of a different selectivity (e.g. from an apolar to a polar column). Selecting proper GC-column combinations is critical. In on-line mode, the interface in MDGC must provide for the quantitative transfer of the effluent from one column... 

SEC has sometimes been used with off-line IR spectroscopy for the detection of polymer additives, such as dioctylphthalate, as well as on-line [39]. Dissolutions of PVC/DEHP and of PC/pentaerythritoltetrastearate (release agent) were analysed by SEC-FTIR using the thermospray/moving belt/DRIFT interface [40]. The detection limits of the method were in the 100 ng range, depending on the IR sensitivity and volatility of the solutes. This is not extremely sensitive. 

Chen, H.S., Rejtar, T., Andreev, V., Moskovets, E., Karger, B.L. (2005). High-speed, high-resolution monolithic capillary LC-MALDI MS using an off-line continuous deposition interface for proteomic analysis. Anal. Chem. 77, 2323-2331. 

In off-line coupling of LC and MS for the analysis of surfactants in water samples, the suitability of desorption techniques such as Fast Atom Bombardment (FAB) and Desorption Chemical Ionisation was well established early on. In rapid succession, new interfaces like Atmospheric Pressure Chemical Ionisation (APCI) and Electrospray Ionisation (ESI) were applied successfully to solve a large number of analytical problems with these substance classes. In order to perform structure analysis on the metabolites and to improve sensitivity for the detection of the various surfactants and their metabolites in the environment, the use of various MS-MS techniques has also proven very useful, if not necessary, and in some cases even high-resolution MS is required. 

MS techniques have met this need in the analysis of involatile, polar surfactants after coupling techniques of liquid chromatographic methods with MS became available. Different types of interfaces for off-line and on-line coupling of liquid chromatography (LC) and MS in the analyses of surfactants had been in use [7,16] while the methods applied at present were performed predominately with soft-ionising atmospheric pressure ionisation (API) interfaces [16-19],... 

Of the analytical techniques available for process analytical measmements, IR is one of the most versatile, where all physical forms of a sample may be considered - gases, liquids, solids and even mixed phase materials. A wide range of sample interfaces (sampling accessories) have been developed for infrared spectroscopy over the past 20 to 30 years and many of these can be adapted for either near-lme/at-lme production control or on-line process monitoring applications. For continuous on-line measurements applications may be limited to liquids and gases. However, for applications that have human interaction, such as near-line measurements, then all material types can be considered. For continuous measurements sample condition, as it exists within the process, may be an issue and factors such as temperature, pressure, chemical interfer-ants (such as solvents), and particulate matter may need to be addressed. In off-line applications this may be addressed by the way that the sample is handled, but for continuous on-line process applications this has to be accommodated by a sampling system. 

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