Desalting techniques

Desalting techniques include ultrafiltration, which simultaneously desalts and concentrates the sample membrane dialysis (Cooksy, 1992) and simple dilution in low ionic strength buffer or water. The latter method is especially useful when glycerol, Triton X-100, or formamide has been added to the PCR mixture. It should be noted that loss of DNA from adsorption onto the filters in ultrafiltration has been reported (Butler et al., 1994). Thus, as a rule, sample desalting by this method should be used as a last resort. 

Removal of salt from a protein solution may be necessary prior to chromatography. For example, injection of high concentrations of salt would result in irreproducibility in retention of early eluting peaks on an ion-exchange column. Concentrated salts may also be incompatible with some organic mobile phases in reversed-phase chromatography. Common desalting techniques are listed in Table 8. 

Process Flow The schematic in Fig. 22-56 may imply that the feed rates to the concentrate and diluate compartments are equal. If they are, and the diluate is essentially desalted, the concentrate would leave the process with twice the salt concentration of the feed. A higher ratio is usually desired, so the flow rates of feed for concentrate and feed for diluate can be independently controlled. Since sharply differing flow rates lead to pressure imbalances within the stack, the usual procedure is to recirculate the brine stream using a feed-and-bleed technique This is usually true for ED reversal plants. Some nonreversal plants use slow flow on the brine side avoiding the recirculating pumps.. Diluate production rates are often 10X brine-production rates. 

The immunoglobulin fraction from each bleed was purified by use of a recirculating isoelectric focusing (RIEF) technique (Bier et al. 1979). The whole serum was diluted 1 3 with urea, to yield a final urea concentration of 3 M, and then desalted by electrodialysis. The urea was added to prevent precipitation under hypotonic conditions. Ampholine (1 percent w/v, pH 3.5 to 10, LKB... 

Blood should be deproteinized by some technique which leaves no extra salt, acid, or alkali in the supernatant Some suitable techniques are with tungstic acid, with ethanol (BIO), or with zinc sulphate and barium hydroxide (S21). The supernatant is desalted in the same way as urine and, if necessary, concentrated before applying to the paper. Subsequent technique is as for urine. 

The application of the SMB-technique to the downstream processing of biotechnological products requires some specific changes to meet the special demands of bioproduct isolation. Some exemplary applications are given including separations of sugars, proteins, monoclonal antibodies, ionic molecules and optical isomers and for desalting. 

Desalting is the name given to the technique of removing unwanted small molecules from preparations of macro-molecules. 

A two-dimensional technique involving initial separation by high voltage electrophoresis at pH 2.0 followed by chromatography is a useful means of separating similar amino acids and short peptides and does not require desalting or excessive purification of the sample (Figure 10.17). 

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