Online sample conditioning

Figure 10.15. Online sample conditioning using turbulent-flow mixing column (Hendricks, 2005).

The quality of data obtained by each instrument is optimized through proper online sample conditioning. 

Initial efforts were focused on the optimization of the online sample conditioning and data interpretation. In order to make absolute measurements on a heterogeneous system containing oil, water, monomer, polymer, initiator, and surfactant, the monomeric and polymeric contents of the inverse emulsion or latex must be spilled out from the discrete phase droplet in a period of seconds and the resulting detectors signals must be interpreted to allow differentiating among the complex mixture of components. 

Figure A2.1 Waters ProMonix On-Line HPLC analyzer. The upper compartment door contains a keypad for programming and operation of the analyzer. The upper window allows viewing of indicator lights and a liquid crystal display that provides the operator with analyzer interface, programmed parameters, and instrument status results. The lower chamber contains the pumps, valves, injector, and detector(s) required for the chromatographic separation. The sample conditioning plate for online process monitoring is to the right of the analyzer. This is a typical process HPLC. (From Cotter, R.L. and Li, J.B., Lab Rob Autom., 1, 251,1989. With permission of VCH Publishers.)...

As mentioned before, when pressure-based sample injection is employed, the maximum injection volume must be less than the effective length of the capillary. At least 10% of the effective capillary length must remain available for separation. To inject a larger sample volume, electrokinetic injection must be employed. In most online sample preconcentration techniques, the maximum amount that can be injected without loss of separation efficiency is certainly less than the capillary volume. However, with a large volume sample injection under cathodic EOF conditions in SDS MEKC, a sample devoid of the micelle can be continuously electrokinetically injected for a volume equivalent to seven times the capillary volume without significant loss of separation efficiency under favorable... 

Kritsunankul et al. [76] proposed flow injection online dialysis for sample pretreatment prior to the simultaneous determination of some food additives by HPLC and UV detection (FID-HPLC). For this, a liquid sample or mixed standard solution (900 pL) was injected into a donor stream (5%, w/v, sucrose) of a FID system and was pushed further through a dialysis cell, while an acceptor solution (0.025 mol/L phosphate buffer, pH 3.75) was held on the opposite side of the dialysis membrane. The dialysate was then flowed to an injection loop of the HPLC valve, where it was further injected into the HPLC system and analyzed under isocratic reversed-phase HPLC conditions and UV detection (230 nm) (Figure 24.6). The order of elution of five food additives was acesulfame-K, saccharin, caffeine, benzoic acid, and sorbic acid, with an analysis time of 14 min. This system has advantages of high degrees of automation for sample pretreatment, that is, online sample separation and dilution and low consumption of chemicals and materials. 

Modifiers can be used very effectively in on-line SFE-GC to determine the concentration levels of the respective analytes. This presents an advantage in terms of the use of modifiers in SFE, since they appear as solvent peaks in GC separations and do not interfere with the target analyte determination. Although online SFE-GC is a simple technique, its applicability to real-life samples is limited compared to off-line SFE-GC. As a result, on-line SFE-GC requires suitable sample selection and appropriate setting of extraction conditions. If the goal is to determine the profile or matrix composition of a sample, it is required to use the fluid at the maximum solubility. For trace analysis it is best to choose a condition that separates the analytes from the matrix without interference. However, present SFE-GC techniques are not useful for samples... 

Online SPE after conditioning of the SPE cartridge, the sample solution is flushed into the cartridge by the washing solvent after a positional switch of the injection valve. A large volume of washing solvent is used to clean up the sample. 

Isobaric interferences (especially those arising from the plasma itself, e.g., ArO+ on Fe) can be eliminated using cool-plasma conditions, sometimes in combination with a shield torch. This option is not suitable for seawater samples because a cool plasma, in the presence of a heavy matrix, cannot fully ionize elements with high first ionization potentials, notably Zn, Cd, and Hg. Protocols have thus been established for analysis of 10-fold diluted seawater on instalments with sufficiently high resolution to separate most of the affected isotopes from their isobaric interferences [1], To circumvent the issue entirely, others have used online chemical extraction to separate analytes of interest... 

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