Back-flush mode

Figure 14.17 Schematic diagram of the on-line coupled LC-GC system VI, valve foi switcliing the LC column outlet to the GC injector V2, valve for switching the LC column to back-flush mode V3, LC injection valve RI, refractive index monitor detector UV, ulti avio-let monitor detector FID, flame-ionization detector.

Shim et al. [59] developed an HPLC method, with column switching, for the determination of omeprazole in plasma. The plasma samples were injected onto a Bondapak Phenyl/Corasil (37-50 ym) precolumn and polar plasma components were washed with 0.06 M borate buffer. After valve switching, the concentrated drug were eluted in the back-flush mode and separated on a /i-Bondapack Ci8 column with acetonitrile-phosphate buffer as the mobile phase. The method showed excellent precision, accuracy, and speed with detection limit of 0.01 /ig/ml. Total analysis time per sample was less than 20 min and the coefficients of variation for intra- and interassay were less than 5.63%. The method has been applied to plasma samples from rats after oral administration of omeprazole. 

This reactor can be operated in two ways. When the substrate is fed to the shell side of the hollow fiber module ("back flush mode"), the substrate comes in contact with the enzyme in the fiber wall and product passes into the lumen of the fiber from which it exits the module. When the substrate is fed to the lumen of the fibers (with all permeate ports closed), it will pass from the lumen to the shell side where it contacts the enzyme and products will recycle back to the lumen ("recycle mode") (see Figure 3.72). The "recycle mode" has the advantage over the "back flush mode" in that the substrate does not have to be free of suspended matter. In the "back flush mode", particles in the substrate would plug the sponge wall. 

Rock samples were extracted using methylene chloride and a Soxhlet apparatus and the resulting extract was further fractionated by a semi-quantitative SARA separation. After asphaltenes were removed from the concentrated extract by precipitation with excess pentane, the pentane soluble portion of the sample was separated by medium-pressure liquid chromatography (MPLC) using a deactivated silica gel precolumn and an activated silica gel main column by eluting the saturate and aromatic hydrocarbon fractions from the activated silica column with hexane in the forward and back-flush modes respectively. Polar nonhydrocarbons were backflushed from the precolumn with methylene chloride-methanol. Carbon isotopes were done on a subset of hydrocarbon fractions at Coastal Sciences Labs, Austin, TX (Table 3). 

Ding et al. described an automated on-line SPE-LC-MS/MS method for the determination of macrolide antibiotics, including erythromycin, roxithromycin, tylosin, and tilmicosin in environmental water samples. A Capcell Pak ME Ph-1 packed-column RAM was used as SPE column for the concentration of the analytes and clean-up of the sample. One millilitre of a water sample was injected into the conditioned SPE column, and the matrix was washed out with 3 ml high-purity water. By rotation of the switching valve (see Fig. 4.2), macrolides were eluted in the back-flush mode and transferred to the analytical column. The limits of detection and quantification obtained were 2-6 and 7-20 ng/1, respectively, which is suitable for trace analysis of macrolides. The intra- and inter-day precisions ranged within 2.9-12% and 3.3-8.9%, respectively. At the three fortification concentrations tested (20, 200, and 2000 ng/1), recoveries of macrolides ranged from 86.5% to 98.3%. 

SPE—HPLC mode with conventional analytical columns. To lessen this shortcoming, the elution of analytes from the loaded pre-column must be conducted in the back-flush mode. 

Desorption of analytes from the precolumn (C-1) and subsequent loading on the analytical column (C-2) can be carried out either in the back-flush or forward-flush modes. The back-flush mode provides better peak shape, but it may lead to precolumn packing disturbances, as well as closing of the analytical column when real samples are analyzed. Forward desorption allows the precolumns to also act as guard columns however, depending on the dimension and particle size, this mode can provide additional band broadening of analyte. Both desorption modes are used in practice. 

FIGURE 20.7 Position 1 = loading step position 2 = elution step. In position 1, the sample is injected onto the trapping (enrichment) column via pump A. Over the course of a few minutes, the analytes of interest are stacked at the top of the column, while salts and other polar contaminants are guided to waste. When the valve switches to position 2, the analytes of interest are eluted in back-flush mode of the trapping column via pump B onto the analytical (separation) column. Source Reproduced from Bioanalysis, Jurw 2011, Vol. 3, No. 11, Pages 1271—1285 [195] zvith permission of Future Science, Ltd. 

The evaporative light-scattering detector has been shown applicable to the analysis of many surfactants. This detector has only limited tolerance for the salts usually added to the mobile phase during analysis of anionic surfactants and is not applicable in many situations. In back-flush mode, the ELS detector can be used to determine total inorganic salts, sulfonated or sulfated surfactant, and unsulfonated or unsulfated material (30). 

Simple and comprehensive 2D HPLC was reported in a reversed-phase mode using monolithic silica columns for the 2nd-D separation (Tanaka et al., 2004). Every fraction from the lst-D column, 15cm long (4.6 mm i.d.), packed with fluoroalkylsilyl-bonded (FR) silica particles (5 pm), was subjected to the separation in the 2nd-D using one or two octadecylsilylated (Cig) monolithic silica columns (4.6 mm i.d., 3 cm). Monolithic silica columns in the 2nd-D were eluted at a flow rate of up to lOmL/min with separation time of 30 s that provides fractionation every 15-30s for the lst-D, which is operated near the optimum flow rate of 0.4-0.8 mL/min. The 2D-HPLC systems were assembled, as shown in Fig. 7.6, so that the sample loops of the 2nd-D injectors were back flushed to minimize band broadening. 

An active flux maintenance procedure was initiated at this point (about 330 h TOS), beginning with a 2 s back-flush of clean permeate through the filter membrane. This active flux maintenance cycle was continued every 30 min for just over 24 h. The flux initially recovered to 0.90 lpm/m2 (32.0 GPD/ft2), but declined again within 24 h to a baseline value of 0.76 lpm/m2 (26.7 GPD/ft2) without clean permeate back-flush. The flux maintenance method was then returned to passive (no back-flush with clean permeate) mode, only increasing the flux off-time to 60 s every 30 min. Thereafter, the flux steadily declined over the next 120 h TOS from 0.77 to 0.58 lpm/m2 (27.3 to 20.4 GPD/ft2). At 480 h TOS, a 1 h flux off-cycle was attempted, resulting in an increase of the flux back to 0.82 lpm/m2 (29.1 GPD/ ft2), a 42.6% increase. When the flux off-cycle was returned to the 60 s off-cycle for the next 48 h, it was found that the permeate flux decreased to 0.62 lpm/m2 (21.9 GPD/ft2). Applying another 1 h flux off-cycle returned the flux to 0.721pm/... 

An injector valve operates in two modes— the fixed-loop mode or the partial-loop mode. In the fixed-loop mode, a sample is overfilled into the loop at 2-4 times the loop volume and the entire loop content is injected. In the partial-loop fill mode, a variable sample aliquot, measured precisely by a syringe at <50% of the loop volume, is injected. Note that the sample slug is introduced into the end of the sample loop and is back flushed onto the column to minimize band dispersion by the sample loop (Figure 9). Due to the emphasis on productivity, manual injectors are seldom used in the pharmaceutical laboratory except for preparative applications. 

A gas chromatograph is used for the primary separation of the components in gas oil. An automatic unit feeds the chromatograph with samples and a back-flushing unit has heen added in order to remove heavy hydrocarbons, which might otherwise choke the column. TTie carrier gas (nitrogen) is controlled by three electropneumatic valves, as shown in Fig. 4.4. In the normal mode with valve A open and valves B and C closed, carrier gas flows through columns 1 and 2 to the detector. 

Sample, vacuum gas oil solvent, n-heptane (1 mL/min) sample volume, 100-/iL loop (dilutions used to reduce sample size) column, 1-ft i-Porasil mode, back-flush operation for aromatics. 

Two process modes, namely, dead-end and cross-flow modes, are widely used for microfiltration (14). For the dead-end mode, the entire solution is forced through the membrane. The substances to be separated are deposited on the membrane, which increases the hydraulic resistance of the deposit. The membrane needs to be renewed as soon as the filtrate flux no longer reaches the required minimum values at the maximum operation pressure. This mode is mostly used for slightly contaminated solutions, e.g., production of ultra-pure water. For the cross-flow mode, the solution flows across the membrane surface at a rate between 0.5 and 5.0 m/s, which prevents the formation of a cover layer on the membrane surface. A circulation pump produces the cross-flow velocity or the shear force needed to control the thickness of the cover layer. The system is most widely used for periodic back flushing, where part of the filtrate is forced in the opposite direction at certain intervals, and breaks up the cover layer. The normal operating pressure for this mode is 1-2 bars. 

Researchers should therefore study the performance of the derivatization reactions used with their samples to avoid the possibility that biases are introduced due to the derivatization reactions (QC samples of the type employed in LC-MS can provide much useful data in this respect) (44). Another issue is the need for extensive column cleanup. In most cases, injection occurs in splitless mode. Introduction of excess derivatization reagents may damage the injection liner and the analytical column. Frequent replacement of the liner is thus necessary. Cleanup of the column can be done by back-flush (when this feature is available) or by application of high temperatures (bake-out). 

Procedure Determination of Unsulfonated Alkylbenzene and Sulfone Content by HPLC (13) A methanol solution of the surfactant, about 10% concentration, is injected on an HPLC system consisting of a Ci8 reversed-phase column, 4.6 x 150 mm, a mobile phase of 94 6 MeOH/H20 at 1 mL/min, and both refractive index and UV detectors. The RI detector is thermostatted at 35 C, while the UV detector is used at 220 nm. In the normal mode of operation, LAS is eluted immediately, followed by unsulfonated material in order of chain length, followed by sulfones over the range 10-35 min. For rapid quantitative analysis, the system is back-flushed after 4.5 min, after elution of the LAS. The total nonpolar material is then eluted through the RI detector, at a retention time of 9 min after injection. 

It is my contention that the optical and physical properties and the optical structure produced during the destructive distillation or thermal decomposition of vitrinite is closely related to mode of carbonization and, in the case of pitch, is intimately related to the method of pitch preparation. For instance, a pitch may be produced from a high or low temperature tar, from a primary cooler tar, or from a flushing liquor tar. In addition, it may be air blown, thermally or chemically treated, straight distilled, or cut back, just to mention a few. Under similar carbonization conditions almost any one of these pitches will produce a coke which has certain characteristics that are related to the parent pitch. Even pitches similarly processed from the tar can differ in the content of quinoline- and benzene-insoluble material and P-resin, and can contain more than one distinct liquid phase. None of these points of difference has been discussed by Dr. Taylor or even recognized in the preparation. To interpret the structure of pitch coke divorced from a knowledge of the pitch source and/or carbonization conditions can lead to erroneous conclusions. These are pertinent data omitted by the authors. 

System flushes are typically used when an RO system goes off-line, comes back on-line, and during stand-by mode. The purpose of the off-line and stand-by flushes is to rid the feed/concentrate side of the membrane of either high concentrations of feed water species or to stir up materials that may have settled on the membrane during down time. The on-line flush (when the membranes come back on line) is to reduce the conductivity in the RO permeate before sending the permeate on to further processing or to the ultimate use. Flush water is typically sent to drain. 

Laboratory pyrolysis of HDPE was first carried out using the batch mode reactor (Eigure 13.2). After flushing the system with an inert gas, the reactor was lowered into the floor furnace. The furnace was heated from room temperature to the pyrolysis temperature in 15-20 min and held at that temperature for 1 h before cooling back to room temperature. There was complete conversion of the HDPE in all runs and the reactor was clean at the end of the run. Several runs were carried out to optimize the temperature and pressure conditions. 

In the event of a lire, wear full protective clothing and NIOSH-approvcd self-contained breathing apparatus with full facepiece operated in tlie pressure demand or other positive pressure mode. Use water spray to blanket (ire, cool fire exposed containers, and to flush non-ignitod spills or vapors away from fire. Vapors can flow along surfaces to distunt ignition source and flash back. 

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