Off-line preparation

For the extraction of sulfates and total sulfur a suitable acid and reducing agent, such as tin(II)-phosphoric acid (the Kiba solution of Sasaki et al. 1979) is needed. The direct thermal reduction of sulfate to SO2 has been described by Holt and Engelkemeier (1970) and Coleman and Moore (1978). Ueda and Sakai (1984) described a method in which sulfate and sulfide disseminated in rocks are converted to SO2 and H2S simultaneously, but analyzed separately. With the introduction of on-line combustion methods (Giesemann et al. 1994), multistep off-line preparations can be reduced to one single preparation step, namely the combustion in an elemental analyzer. Sample preparations have become less dependent on possibly fractionating wet-chemical extraction steps and less time-consuming. 

Due to the nature of the process the reinforcement must be continuous, either in the form of roving packages or rolls of fabrics or mats. The reinforcement is stored in a creel stand that is of the simplest possible rack construction because no significant loads, except the weight of the reinforcement, normally need to be supported. The entire creel stand is often placed on wheels to allow off-line preparation of reinforcement supply and rapid reconfiguration of a pultrusion line. 

Samples rarely come in a form that can be injected directly into the instrument some form of sample preparation usually is required. Sample preparation includes any manipulation of the sample prior to analysis, including techniques such as weighing, dilution, concentration, filtration, centrifugation, and liquid- or solid-phase extraction. Sample preparation can be performed either on-line or off-line, but it is usually performed offline. Off-line preparation can be time-consuming and tedious, and the more steps that are required, the more susceptible the analytical method is to operator error and irreproducibility. 

Fig. 3.33 Catalyst cells (a) high temperature (> 600 K), flow-through, Swagelok sealed stainless steel cell and (b) high temperature gold wire sealed zirconium cell. O-ring sealed aluminium cells (c) low volume, (d) high volume and (e) flow-through, (f) Aluminium cell for off-line prepared samples (g) low temperature (<350 K) indium wire sealed aluminium cell, (h) High temperature, flow-through, Conflat sealed stainless steel cell for use on MARI, all the others are for TOSCA. The scale in the foreground is 0.3 m.

Microsystems for MALDI-MS analysis can be divided into three groups microfabricated MALDI target plates, microsystems for off-line preparation of samples and microsystems that integrate both the sample preparation steps and the MALDI targets, with an on-line or off-line analysis of the samples. 

In the direct-determination procedure, carbonate carbon is first removed from the sediment subsample by treatment with dilute (3N) hydrochloric or phosphoric acid, washing and drying the carbonate-free residue, and then measuring the carbon content of the residue with an elemental analyzer. An alternative direct-determination procedure employs acid vapors to remove inorganic carbon (Yamamuro Kayanne, 1995). A related procedure employs direct reaction with HCl that is added to the sediment sample while it is in the tin boat that is used in the elemental analyzer. After reaction for 24 hours, the acid is evaporated prior to carbon analysis. Yet another alternative direct-determination method involves measuring the amount of CO2 released from carbonate-free sediment subsamples during off-line oxidation of the residual carbon in preparation for carbon isotope analysis. Off-line preparation lines for isotopic analyses can be manometrically calibrated to yield this measurement. Regardless of the procedure, TOC concentrations of lake sediments are usually expressed on a whole-sediment basis. 

CE-IRMS consists of an IRMS coupled on-line to a device that produces the CO2 to be analyzed. In the case of sediment organic matter, the on-line connection is to an elemental analyzer. The coupled elemental analyzer-IRMS has greatly simplified isotopic measurements key improvements over conventional off-line preparations include significant reduction of sample size and analysis time. Some loss in precision occurs, although the reproducibility of (5 C values is commonly 0. l%c. 

Determining the N-isotopic composition of sedimentary OM requires that N2 gas be prepared from the sample. The gas is then fed into a mass spectrometer for analysis. An excellent, comprehensive review of mass spectrometry, and the various sample preparation techniques that may be used for N-isotope analysis, is given in Owens (1987). Attention here will be confined to some of the more pertinent practical aspects of isotopic analysis with particular reference to the system used at UB. As indicated in the Introduction, the adoption of nitrogen isotopes as a standard technique in the earth and biological sciences was hindered by the time-consuming, off-line preparation techniques that most studies required. A few labs have stuck to the original, wet-chemistry sample preparation, feeling... 

Off-line preparation A method that prepares a sample for isotopic analysis that uses a vacuum system in which the gaseous forms of the elements of interest are produced, purified, and put into vials for subsequent analysis in an isotope ratio mass spectrometer. 

Application of rotating coiled columns has become attractive for preparative-scale separations of various substances from different samples (natural products, food and environmental samples) due to advantages over traditional liquid-liquid extraction methods and other chromatographic techniques. The studies mainly made during the last fifteen years have shown that using rotating coiled columns is also promising for analytical chemistry, particularly for the extraction, separation and pre-concentration of substances to be determined (analytes) before their on-line or off-line analysis by different determination techniques. 

Preswelled Sephacryl S-1000 was prepared in a K26/100 column (88 X 2.6 cm). Equilibration with 0.005 M NaOH containing 0.002% NaN3 at a flow rate of 0.67 ml/min was achieved after 20 hr. Sample solutions were applied with a 5-ml injection loop. The mass and iodine-complexing potential of separated glucan components was determined off-line for each of the subsequently eluted 5-ml fractions. Based on the determined mass of carbohydrate for each of the fractions, elution profiles such as Fig. 16.1 were constructed. 

FIGURE 16.1 Preparative SEC of short-chain (scb) branched glucans of small" (<3S /u.m) starch granules of potato species Ostara separated on Sephacryl S-1000 (88 X 2.6 cm) eluent 0.005 M NaOH the normalized chromatogram (area = 1.0) was constructed from an off-line determined carbohydrate content of succeeding 5-ml fractions flow rate 0.67 ml/mln V d = 185 ml, V, = 460 ml fraction I high dp fraction fraction 2 low dp fraction. 

Although SFE and SFC share several common features, including the use of a superaitical fluid as the solvent and similar instrumentation, their goals are quite distinct. While SFE is used mainly for the sample preparation step (extraction), SFC is employed to isolate (chr-omatography) individual compounds present in complex samples (11 -15). Both techniques can be used in two different approaches off-line, in which the analytes and the solvent are either vented after analysis (SFC) or collected (SFE), or on-line coupled with a second technique, thus providing a multidimensional approach. Off-line methods are slow and susceptible to solute losses and contamination the on-line coupled system makes possible a deaease in the detection limits, with an improvement in quantification, while the use of valves for automation results in faster and more reproducible analyses (16). The off-line... 

In order to reduce or eliminate off-line sample preparation, multidimensional chromatographic techniques have been employed in these difficult analyses. LC-GC has been employed in numerous applications that involve the analysis of poisonous compounds or metabolites from biological matrices such as fats and tissues, while GC-GC has been employed for complex samples, such as arson propellants and for samples in which special selectivity, such as chiral recognition, is required. Other techniques include on-line sample preparation methods, such as supercritical fluid extraction (SFE)-GC and LC-GC-GC. In many of these applications, the chromatographic method is coupled to mass spectrometry or another spectrometiic detector for final confirmation of the analyte identity, as required by many courts of law. 

In liquid chromatography, in contrast to gas chromatography [see Section 9.2(2)], derivatives are almost invariably prepared to enhance the response of a particular detector to the substance of analytical interest. For example, with compounds lacking an ultraviolet chromophore in the 254 nm region but having a reactive functional group, derivatisation provides a means of introducing into the molecule a chromophore suitable for its detection. Derivative preparation can be carried out either prior to the separation (pre-column derivatisation) or afterwards (post-column derivatisation). The most commonly used techniques are pre-column off-line and post-column on-line derivatisation. 

There are in-line LC/spectroscopic systems available, but in most cases it is easier to carry out a semi-preparative separation, collect the material and carry out the spectroscopic examination off-line. However, for routine quality control analyses, where the sample... 

In the last several years, on-line extraction systems have become a popular way to deal with the analysis of large numbers of water samples. Vacuum manifolds and computerized SPE stations were all considered to be off-line systems, i.e., the tubes had to be placed in the system rack and the sample eluate collected in a test-tube or other appropriate vessel. Then, the eluted sample had to be collected and the extract concentrated and eventually transferred to an autosampler vial for instrumental analyses. Robotics systems were designed to aid in these steps of sample preparation, but some manual sample manipulation was still required. Operation and programming of the robotic system could be cumbersome and time consuming when changing methods. 

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... 

On-line SFE-pSFC-FTD, using formic or acetic acid modified CO2 as an extraction solvent, was used to analyse a dialkyltin mercaptide stabiliser in rigid PVC (Geon 87444) [114]. Hunt et al. [115] reported off-line SFE-pSFC-UV analysis of PVC/(DIOP, chlorinated PE wax, Topanol CA), using methanol as a modifier. Individual additives are unevenly extracted at lower pressures and temperatures, where extraction is incomplete. Topanol CA, the most polar of the three PVC additives studied, could not be fully extracted in the time-scale required (15-20min), even at the highest CO2 temperature and pressure obtainable. However, methanol-modified CO2 enhances extraction of Topanol CA. PVC film additives (DEHP, fatty acids, saturated and aromatic hydrocarbons) were also separated by off-line SFE-preparative SFC, and analysed by PDA and IR [116]. 

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... 

Thermal degradation of Irganox 1076 in air was studied by means of HPLC-UV/VIS and by preparative HPLC-NMR. At 180 °C cinnamate and dimeric oxidation products are formed, and at 250 °C de-alkylation products are observed [660], On-line LC-NMR hardly covers a real need in polymer/additive analysis, as the off-line option is mostly perfectly adequate for that purpose. 

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