Medium-pressure LC

An aqueous extract of P. hysterophorus (collected in Puerto Rico) was partitioned into methylene chloride at pH 7, pH 10 and pH 2. Bioassays of the methylene chloride soluble fractions, using the bean second internode bioassay (13), showed that the highest activity was concentrated in the methylene chloride extract at pH 7. Extensive chromatographic purification (flash chromatography, medium pressure LC, preparative TLC) monitored by bioassay led to the isolation of the four sesquiter-... 

Determination of ppm concentrations of nitriles in light naphtha was carried out by capillary GC, by either of the following procedures. Method 1 Clean-up and preconcentration by medium pressure LC removed the hydrocarbons and increased the nitrile concentration ten fold the samples were analysed using an FI detector. Method 2 Direct injection into a capillary GC using a chemiluminescence nitrogen detector ... 

Recently, several online separation and detection systems are available for the qualitative and quantitative analysis of elute/solutes from the columns to speed up the isolation process. Using this principle, a lot of separation techniques were evolved including (1) flash chromatography (FC), (2) vacuum liquid chromatography (VLC), (3) preparative planar chromatography, (4) low-pressure LC (LPLC), (5) medium-pressure LC (MPLC), and (6) high-pressure LC (HPLC). 

With the successful implementation of differentially-pumped external ion sources, FTMS is rapidly becoming a routine mass spectrometric technique. Medium-pressure interfaces for the coupling of GC, LC, FAB, and liquid SIMS into the external ionizer are currently under development, and should become available in the near future. 

Unfortunately, temperature dependent UV-Vis spectra of polymers, 31, were not reported. Films of the polymers were irradiated (medium-pressure Hg lamp) and the reactions monitored by UV-Vis and FTIR spectroscopy. Homopolymer, 31 (jc = 6), was irradiated at 120°C in the N phase. A gradual decrease in the main absorption band of the chromophore at 352 nm was observed and the film eventually became completely insoluble. Interestingly, no initial increase in absorbance was observed, as had been seen for a small molecule LC model and for MCLC polymers [64] 30 (vide supra) and previously attributed to disruption of... 

Medium-pressure Cl at ion-source pressures between 1 and 2000 Pa is widely used. In LC-MS, it is important in particle-beam and thermospray interfacing. Either an externally-added reagent gas like methane, isobutane, or ammonia is... 

The newer LC techniques of flash column chromatography and short path or medium pressure column chromatography though offering resolution approaching that of thin layer are still comparatively slow and expensive in terms of packing and solvents. The principal advantage of these techniques is in their preparative capability. 

At present, the most powerful and promising interfaces for drug residue analysis are die particle-beam (PB) interface that provides online EI mass spectra, the thermospray (TSP) interface diat works well with substances of medium polarity, and more recently the atmospheric pressure ionization (API) interfaces that have opened up important application areas of LC to LC-MS for ionizable compounds. Among die API interfaces, ESP and ISP appear to be the most versatile since diey are suitable for substances ranging from polar to ionic and from low to high molecular mass. ISP, in particular, is compatible with the flow rates used with conventional LC columns (70). In addition, both ESP and ISP appear to be valuable in terms of analyte detectability. These interfaces can further be supplemented by preanalyzer collision-induced dissociation (CID) or tandem MS as realized with the use of triple quadrupole systems. Complementary to ESP and ISP interfaces with respect to the analyte polarity is APCI with a heated nebulizer interface. This is a powerful interface for both structural confirmation and quantitative analysis. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

Since its introduction in the 1960s, SFC has experienced several ups and downs in its development. Either a gas or a Uquid above its critical temperature and pressure is used as the mobile phase for SFC. In most cases, COj is used because of its favorable critical parameters (i.e., a critical temperature of 31 °C and a critical pressure of 7.3 MPa). Moreover, CO2 is cheap, nontoxic, and nonflammable. A high-pressure pump delivers the mobile phase through either a packed (pSFC) or capillary column (cSFC) to the detector. The mobile phase is maintained under supercritical or subcritical conditions via an electronic controlled variable restrictor that is positioned after detection (pSFC) or via a fixed restrictor positioned before a gas-phase detector (cSFC). The retention characteristics of the analytes are influenced by the properties of the stationary phase and by the polarity, selectivity, and density of the CO2 mobile phase. The density is controlled by variation of the temperature and pressure of the supercritical medium. Furthermore, the elution of very polar compounds under high densities can be achieved with a precolumn addition of polar modifiers such as methanol. Nowadays, pSFC formats use the same injector and column configurations as LC methods. Consequently, pSFC formats are considered to be more useful for routine operation than cSFC. The most remarkable... 

Determination. GC two different SFE systems are compared high temperature method in some cases yields a more exhaustive extraction, but also less clean extracts whereas medium temperature may sometimes cause problems with quantitative recoveries, but it yields very clean extracts The use, advantages and disadvantages of silica sorbents, polymeric, functionalized, carbon-based and mixed available sorbents are discussed Determination. LC—MS best conditions, extraction solvent methanol—acetone (1 1, v/v) temperature 50°C pressure 1500 psi two static cycles recoveries >10%. LOD between 1 to 5 /ag/kg loss of volatile molecules is produce at elevated temperatures Determination GC-MS Best conditions extractant methanol temperature lOffC, pressure 100 atm combined with 15 min static and then 10 min dynamic recovery 111% (RSD 4%) and 106% (RSD 5%) extraction efficiency of the PLE was compared with conventional Soxhlet and bath ultrasonication GC-MS an extraction time of 1 h was employed. 

The specific fluidic system involved, while not the main topic of this entry, is closely linked to the detection system and warrants a brief explanation. Separation techniques such as CE and HPLC utilize channels or columns that pass a fluidic medium carrying the analytes of interest. Bulk flow is actuated using either gravity or pressure, in the case of LC, or electroosmotic forces, in the case of CE. Separation of analytes occurs due to physical or chemical interactions with what is incorporated into the column (a packing material that completely fills the column or is... 

Among the currently available interfaces for drug residue analysis, more interesting appear the particle-beam (PB) interface, the thermospray (TSP) interface that works well with substances of medium polarity, and the atmospheric pressure ionization (API) interfaces that have opened up powerful application areas to LC-MS for ionizable compounds. Among API interfaces, most versatile appear to be the electrospray (ESP) interface and its variants including the ion spray... 

Solid-phase microextraction (SPME) [63] is normally used for sample collection, pre-concentration and desalting before analysis. It is usually used off-line. However, SPME can be implemented in temporal monitoring of biomolecules with low to medium temporal resolution (minutes, hours) [64]. In one method, SPME fibers were used to sample metabolites from live animals followed by analysis using LC-MS [65]. The method enabled extraction of metabolites directly in the tissue of moving animals. It was not necessary to withdraw a representative biological sample for analysis. In this case, the amount of analyte extracted into the SPME fiber was independent of the sample volume [65]. Recently, SPME was also coupled on-line with a MS ion source operated at atmospheric pressure [66]. 

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