Sample preconcentration 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. 

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). 

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-... 

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... 

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. 

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. 

Fig.6.3 FI Donnan dialysis preconcentration system for flame AAS. X ion-exchanger membrane tubing S, sample solution B, magnetic stir-bar, V, injector valve (crossed circles arc blocked channels) C, carrier, and AAS, flame AA detector [13].

However, as shown in Sec. 6.5.3, Dorman dialysis may be used to preconcentrate trace analytes and achieve EF values exceeding 100, but at low sample throughputs of only about 6-8 h The low sampling frequency is rather objectionable for a detector which has to maintain a burning flame during the on-line dialysis preconcentration period. Therefore the use of multiple dialysis cells to achieve higher sample throughputs of 20-30 h has been proposed. 

The coupling of an FI manifold to capillary electrophoresis equipment enables sample preparation prior to introduction into the capillary. FI preconcentration by liquid-solid steps, dialysis, and liquid-liquid separations have been developed in a precapillary position. 

Pereiro Carcla et al. [53] described an online preconcentration of aluminum from dialysis concentrates using cation and anion exchangers, Chelex 100, and a synthetic resin, followed by FAAS determination. There is no doubt that there are many more application possibilities when the wealth of chemical literature on ion exchange, chelate formation, and solvent extraction is reexplored for online FI sample handling. Online preconcentration could become even more attractive if it is combined with online sample digestion [54,55]. It was demonstrated that... 

Different sample pretreatment operations include dilution, membrane-extraction (gas diffusion, dialysis), liquid-phase extraction techniques (liquid/liquid extraction, liquid-phase microextraction, single-drop microextraction) and solid reactors and packed columns aiming to facilitate online chemical derivatization, chromatographic separation of target species, removal of interfering matrix compounds, enzymatic assays, or determination of trace levels of analyte via sorptive preconcentration procedures (Marshall et al., 2003 Economou, 2005 Miro and Hansen, 2006 Theodoridis et al., 2007 McKelvie, 2008 Ruzicka, 2014). In this context, BIA and the LOV configurations are particularly useful. Acid-base titrations can also be automated using simple SIA manifolds and potentiometric (van Staden et al., 2002) or photometric (Kozak et al., 2011) detection. Typically, a zone of the sample to be titrated is sandwiched between two zones of titrant by aspiration. In the case of photometric detection, an additional zone of a suitable pH-sensitive colored indicator is aspirated. The stacked zones are delivered to the detector and the width of the peaks is monitored and related to the pH of the solution. 

For determination of nitrites in clean aqueous solutions such as water, there is no need for extraction it is generally enough to filter the sample before it is injected into the FIA system. In some cases preconcentration of the sample may be necessary for determination of nitrite by FIA or sequential injection analysis (SIA). One such example would be direct nitrite determination in nonpolluted water samples where colorimetry is not sensitive enough to determine NO2 (Miro et al., 2000,2001). Preconcentration processes have been based on anionic exchange resins (Dowex 1-X8) (Wada and Hattori, 1971) or on ion pairs coprecipitated with microcrystalline biphenyl (Puri et al., 1998), or with naphthalene (Wang et al., 1998). Dialysis-FIA systems have been proposed for determination of nitrite in residual waters, and even a solid phase (monofunctional Cjg) which is held inside a glass column for an automatic SIA has been tried (Miro et ah, 2000). 

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