Preconcentration dialysis

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

Different sampling methods often result in collection of different components of the element or compound of interest. Most seawater samples are collected in bottles, filtered to remove particles and analyzed directly or after preconcentration of minor components. Particulate matter from the water column is collected by filtration or with sediment traps. Sediment samples are collected in cores that recover intact chronological sequences and are commonly subsampled, dried and ground to a powder. Pore waters are extracted from sediments by squeezing or suction, dialysis, and centrifugation. 

Online dialysis and subsequent trace enrichment has been further described for isolation/purification of flumequine residues from fish muscle (203), or oxolinic acid and flumequine from chicken liver (193) and salmon muscle (204). This involves online purification by diphasic dialysis membrane and trapping of the analytes onto a liquid chromatographic preconcentration column (reversed-phase Ci8 or polymeric), rinsing of the coextracted materials to waste, and finally flushing of die concentrated analytes onto the analytical column. 

Ion chromatography using spectrophoto-metric detection and on-line preconcentration gives an effective detection limit <1 g/L in aqueous samples. GFAAS is the method of choice for measuring low-ppb levels of aluminum in dialysis fluids (Andersen 1987, 1988 Woolfson and Gracey 1988). 

Kasthurikrishnan, N. and Koropchak, J.A. (1993) Flow injection donnan dialysis preconcentration of trace metal cations for inductively coupled plasma atomic emission spectrometry. Anal. Chem., 65, 857-862. 

Plastic microdevices for high-throughput screening with MS detection were also prepared for detection of aflatoxins and barbiturates. These devices incorporated concentration techniques interfaced with electrospray ionization MS (ESI-MS) through capillaries [2], The microfluidic device for aflatoxin detection employed an affinity dialysis technique, in which a poly (vinylidene fluoride) (PVDF) membrane was incorporated in the microchip between two channels. Small molecules were dialyzed from the aflatoxin/antibody complexes, which were then analyzed by MS. A similar device was used for concentrating barbiturate/antibody complexes using an affinity ultrafiltration technique. A barbiturate solution was mixed with antibodies and then flowed into the device, where uncomplexed barbiturates were removed by filtration. The antibody complex was then dissociated and electrokinetically mobilized for MS analysis. In each case, the affinity preconcentration improved the sensitivity by at least one to two orders of magnitude over previously reported detection limits. 

The use of ion-exchange and chelating resins to preconcentrate the metal ion samples prior to CE application has been reported.An on-line dialysis sample cleanup method for CE analysis has also been presented. Besides, several reports have been published on dialy-... 

Shan H X, Li Z J. Novel ionic liquid as solvent for preconcentration of trace lead in dialysis fluids and its determination by graphite furnace... 

Chrome azurol S was used to determine A1 in tap water, dialysis fluids and alkali metal salts [1] and in mine well water [2]. The preliminary preconcentration of the eomplex on a polyethylene powder was applied [1]. Aluminium species (Al(III) ions and Ali304(0H)24 (H20)i2 ) in aqueous soil extracts and humic waters were determined by FIA method using Chrome azurol S after preliminary retaining of the analyte on a column reactor containing 8-quinolinol immobilized onto Fractogel and selective elution with different elluents [3]. 

Beryllium in tap water, dialysis fluids and alkali-metal salts was determined with Chrome Azurol S after preconcentration of the complex on a column packed with polyethylene powder [1]. Sorption preconcentration of Be on a fibrous sorbent (poly(acrylonitrile)-carboxylated polyethylene-polyamine) prior to the determination of metal with Arsenazo I in sea water was described [2]. 

Preconcentration of T1 in the form of ion associate with rhodamine B on polyethylene powder was applied prior to the determination in tap water, dialysis fluids and alkali-metal salts [1]. 

Depending on the type of membrane used, dialysis may be either passive or active (Donnan type). Passive dialysis employs a neutral jwrous membrane with a fine pore structure. This type of dialysis is used primary to remove sample matrix materials such as proteins and/or fiber or other solids [8,9]. Donnan dialysis can do the same, but can also perform preconcentration of the ions in sample. In Donnan dialysis, an ion exchange membrane is used to exchange sample ions for receiving solution ions [10-13]. Each type of dialysis is described below. 

Sample preparation (Chapter 9) includes simple procedures such as centrifugation or filtration. Other more complex sample preparation procedures include passive or active dialysis, preconcentration, combustion or precipitation of matrix ions. In some cases, choosing the correct eluent/column/detector system will negate the need for sample preparations. A selective detector will be able to detect a minor analyte in the presence of other ions. Ion exclusion allows the passage of strong acid anions prior to separation of weak acid anions. Methods that require the least sample preparation or minimal sample preparation directly prior to injection are the most desirable. 

Donnan dialysis of conjugate bases of weak acids presents an additional factor for control, namely pH. Cox and Cheng (34) investigated preconcentration of a number of anions in this category. Sample pH was adjusted to ensure that a major fraction of the weak acid system was in anionic form. Maximum enrichment factor, defined as ratio of receiver concentration to sample concentration after some given, fixed time (usually 30 minutes), was obtained when pH of the receiver solution was much less than the... 

K. Pyrzynska, Preconcentration and recovery of metal ions by donnan dialysis, Microchim. Acta 153 (2006) 117. 

In order to eliminate interferences, to provide suitable selectivity in the measurement, or to preconcentrate the analyte for more sensitive or accurate measurement, the analyst must often perform one or more separation steps. It is preferable to separate the analyte away from the sample matrix, in order to minimize losses of the analjde. Separation steps may include precipitation, extraction into an immiscible solvent, chromatography, dialysis, and distillation. 

Donnan dialysis Recovery of precious metals, softening of hard water, preconcentration of a trace amount of metal ions for analysis, etc. 

J.E. DiNunzio and M. Jubara, Donnan dialysis preconcentration for ion chromatography, Anal. Chem., 1983, 55, 1013 J.A. Cox and G.R. Litwinski, High sample convection Donnan dialysis, Anal. Chem., 1983, 55, 1640-1642 J.A. Koropchak and L. Allen, Flow infection Donnan dialysis preconcentration of cations for flame atomic absorption spectrophotometry, Anal. Chem., 1989, 61, 1410. 

The presence of a trace element-containing species is usually detected by applying analytical techniques that determine the metal or metalloid constituents. The methods and techniques for such determinations have been adequately dealt with in various chapters of this book. No additional comments are required other than to remark that in cases where the concentration of the analyte is at the limit of detection of the analytical technique a preconcentration step (Poole and Schuette 1984) should be considered. This step could involve sample evaporation, solvent extraction, dialysis, ion-exchange chromatography and/or electrolytic preconcentration. If further analytical procedures are contemplated, for example, characterisation of the species, then it is essential that the species is not destroyed during the preconcentration step. 

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