tailieunhanh - Introduction to Modern Liquid Chromatography, Third Edition part 16

Introduction to Modern Liquid Chromatography, Third Edition part 16. High-performance liquid chromatography (HPLC) is today the leading technique for chemical analysis and related applications, with an ability to separate, analyze, and/or purify virtually any sample. Snyder and Kirkland's Introduction to Modern Liquid Chromatography has long represented the premier reference to HPLC. This Third Edition, with John Dolan as added coauthor, addresses important improvements in columns and equipment, as well as major advances in our understanding of HPLC separation, our ability to solve problems that were troublesome in the past, and the application of HPLC for new kinds of samples. . | 106 EQUIPMENT Figure 3-12 Check-valve designs a Simple ball and seat b c active check valves. pump head. Ceramic check valves may be used in some pumps and sometimes a small spring is used to assist check-valve closure. Operation of rbal-typc check valve hpqrçte simple when ehipressute oil the inlet side of the valve below the ball in Fig. is higher than that on the outlet side the ball will be displaced from the seat and solvent will flow through the valve. whenthe pressureishiaher an the hutletsidh above thebhllin Fig. the ball will bepesheh agoinet lheseat whseh ceeuies a seal so that no liquid passes through sphve and iSlustrutenin tkemlet and outleta heslivuio allaw thepumpheaatdalteniatelyfill with mobiiephasa from the eeaetaoisanadeSiver mehiSe phase tothe totha ball-type chethvalatss the actiua chuck halve shownin Figuit de sc rikeil in Section . PUMPING SYSTEMS 107 c Figure HPLC pump designs with corresponding flow and pressure profiles. a Single-piston reciprocating pump b effect of a shaped driving cam c dual-piston pump d accumulator or tandem-piston design. The single-pistonpumpwithia ionsre S-toeesVlinear dtivg will spend half of rhe time flhng end half of rhe timedehvermgsolveirt. Theresnlnng flowfarrd pressure profile extreme pulses in flow and pressure are undesirable for HPLC applications. An alternative single-piston design uses a specially shaped . elliptical driving cam on the motor ora veeisble-speed iSepphv-drlvenmotot tg movn the piston. Width tOis modificatith tht pisloeseesO gonOa vnrisdivithmoachpump cycle so thalmoee yhsn halfedshe timni-spent in thedeliverscyeleanh a small fraction ot Tietime tsspesit inPii thlc hle h g- . Some pumps refine this 108 EQUIPMENT process further so that the pump controller remembers the pressure on the prior stroke and moves the piston forward rapidly at the beginning of the .