with channelling and uneven distrib

with channelling and uneven distribution of reagents through the resin bed. In recent years, these supports have been superseded by materials based on polyethylene glycol (PEG). The two most frequently utilized are Tentagel 1 and PEO-PS 2, which are prepared by grafting of PEG to low cross-linked polystyrene. The beads of these materials are spherical, pack well into reaction columns, are able to withstand high back-pressures, and can be used in batchwise as well as continuous-flow synthesis. More recently, supports based on cross-linked PEG have also been developed (PEGA 3 and CLEAR 4)—although not widely used, indications are that their performance compares favourably with the previously described composite PEG-PS supports. The properties of the resins discussed above are summarized in Table 2. The requirement for small amounts of large numbers of peptides, particularly for antigen-mapping, has led to the development of various novel non-beaded supports designed especially for this purpose. The two most notable examples are PINS and SPOTS, the uses of which are described in Chapter 14.