Off-line filtration

For waters with much less than 0.5 mg/L SPM, the amount of material obtainable from the typical 0.5-2 L sample volume is insufficient for subsequent precise and accurate TE analyses. Off-line processing of the much larger volumes of water then necessary becomes very laborious and should be replaced by on-line filtration from appropriate samplers (Fig. 12-30). To this end, the filter holder is attached to the drain spigot, and the necessary additional pressure is applied via the sampler s air-vent. The suspended particulate matter is collected by filtering all the water held by the sampler. To remove the seasalt from the loaded filters, the on-line filter holder is disconnected from the sampler and attached to a plastic syringe. Manually, two 20 mL portions of high-purity distilled water are drained through the filters. The further treatment of the loaded, salt-free filters is identical to the procedure applied to filters obtained by off-line filtration. 

Low pressure hydraulic filters are typically designed for operating pressures up to 28 bar, with flows up to 1200 1/min. Generally they have a wide application range, particularly for industrial and mobile applications, including high flow tank top and tank top return line filters. Many versions are of the spin-on type and can be provided as dual flow assembly off-line filtration. Core-less low pressure filler elements contain no metal and are designed to minimize waste disposal. 

The preferred method of extracting hydraulic fluid from containers is by the use of a portable pump and filter unit, or by means of an off-line filtration unit fitted to the system. The filter unit of the portable system should be checked to ensure that it has the correct rating. 

A convenient position for installing off-line filtration is in the reservoir. Care is required in positioning the tank connections to prevent any settled contaminant or water being re-entrained. This system may be combined with the filling system it may be run as a clean-up system when the main pump is not in use, and should be running continuously when the main system is operating. 

As a result, there is a growing trend to off-line clean reverse-pulse filters by using bags with multiple compartments. These sections allow the outlet-gas plenum serving a particular section to be closed off from the clean-gas exhaust, thereby stopping the flow of inlet gas. On the dirty-side of the tube sheet, the isolated section is separated by partitions from the neighboring sections where filtration continues. Sections of the filter are cleaned in rotation as with shaker and reverse-flow filters. 

Arc furnaces are operated in conjunction with large capacitor banks and harmonic filters to improve the power factor and also to filter the harmonic frequency currents so they do not unduly affect other power users sharing the same power fines. It is not uncommon to see arc furnaces supplied from dedicated utility power fines to minimize their impact on other power users. The presence of large capacitance in an electrical system can result in voltage rise due to the leading reactive power demands of the capacitors, unless they are adequately canceled by the lagging reactive power required by the loads. This is why capacitor banks, whether for power factor correction or harmonic current filtration, are switched on when the furnace is brought on line and switched off when the arc furnace is off line. 

Arce et al. [39] developed a flow injection analysis (F1A) system (Fig. 5.3) for online filtration of water samples prior to CE analysis. They also constructed a pump-driven unit for extraction and filtration of soil samples combined with CE in an online mode (automated sample transfer between pre-CE sample preparation step and the CE) [40]. The method was precise and four times faster than conventional methods of sample preparation with an off-line unit. Blood samples are centrifuged immediately to remove red blood cells and the serum is stored as discussed above. Sometimes, urine samples also contain precipitates which are removed by centrifuge. 

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. 

Sample preparation in CE is similar to sample preparation in HPLC (Section 3.8). It is usually performed off-line in CE, because commercial instrumentation does not allow for on-line sample preparation. The exception is sample preconcentration, which is readily performed on-line. Sample preparation includes any manipulation of the sample, such as weighing, filtration, dilution, or derivatization, prior to introduction into the CE system. 

Off-line determination of biomass concentration by classical gravimetric methods requires cell separation, washing steps and drying to constant weight. The separation of cells can be made either by centrifugation or by filtration. 

Dead end filtration is a batch process. That means that the filter will accumulate and eventually blind off with particulates such that water can no longer pass through. The filtration system will need to be taken off line and the filter will need to be either cleaned or replaced. 

Fig. 6 shows the variation in and residual pressure drop during six hours of operation. K is calculated from Eq. 4. Calculated mean specific cake resistance equals 6.5-10 [s ], with a standard deviation of approx. 15%. is defined as the velocity across the exposed filter surface (vy 2 U,). Observed fluctuations in pressure build-up did not result in any increase in the residual pressure drop. The residual pressure drop could be maintained at a constant level. The average filtration efficiency was 0.9983. Filter regeneration was conducted with off-line pulsing (P,a = 2barg and total sand spill of 10 kg). 

Residues of oxytetracycline, tetracycline, and chlortetracyline in bovine milk were determined and confirmed after centrifugation of the milk, filtration over a 25 kDa cut-off filter, and SPE on a C,8 cartridge. Methanol-oxalic acid-acetonitrile was used as mobile phase. Four ions for each tetracycline from the negative-ion mass spectra were used for confirmation at 100 ng/ml level [97]. Carson et al. [98] reported the determination of tetracycline residues in milk and oxytetracycline residues in shrimp. Off-line metal-chelate affinity chromatography on Cu " -loaded chelating Sepharose in combination with SPE on polymeric ENVI-ChromP material was used for sample pretieatment. LC is performed using a PLRP-s polymeric material and 5 mmol/1 oxahc acid in the mobile phase. The method is vahdated with samples spiked at 30 ng/ml in milk and 100 ng/g in shrimps. 

There are many sample preparation techniques listed in texts, from a simple filtration or centrifugation to many other kinds of extraction procedures, including both liquid-liquid and solid-phase extraction. When any type of sample preparation is used, it often is done manually if only a few samples are involved. If a large number of samples are to be analyzed, the entire procedure should lend itself to automation. Regardless of the number of samples, most sample preparation is done off-line that is, the samples are prepared first with one of the methods listed, then placed into an automated sample injection system for sequential analysis of all samples. 

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