LDC/Milton Roy HPLC Metering Pumps Priced from $975
pares favorably with current gradient elution analysis methodology; however, resolution by the optimized isocratic system is superior. There are many other advantages of an isocratic separation relative to gradient elution, such as simpler equipment, improved quantitation, and the lack of need for reequilibrating the column with the initial mobile phase. pH and Ionic Effects
L DC/Milton Roy offers a complete family of precision metering pumps for high performance liquid chromatography (HPLC). •
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miniPump* series metering pumps • The first metering pump manufactured and used for HPLC. Priced from $975.00. Single piston, 0-6000 psi, solvent delivery from 16-920 mL/hr, flow rate stability + 0.3% per hour. miniMetric™series metering pumps - The newest addition to the LOC/Milton Roy family of metering pumps. Priced from $1752.* Fast-refill single sapphire piston, 0-6000 psi, pulseless solvent delivery from 50 ^ L - 5 mL/min, flow rate stability ± 0.3% per hour, gradient compatible. constaMetric* series metering pumps - The ultimate in pulseless flow performance. Priced from $2292.* Dual reciprocating sapphire pistons, 0-6000 psi, pulseless solvent delivery from 0.1 to 9.99 mL/min, flow rate stability ± 0.3% per hour, gradient compatible.
• When purchased with a gradient system. For more information or to order call: Toll Free: 800-327-6182 In Florida: (305) 844-5241 LDC/Milton Roy Analytical Instruments · a tradition of quality and performance.
LDC / Milt on Roy 3661 Interstate Park Rd. N. P.O. Box 10235 Riviera Beach, FL 33404
The use of pH and other ionic effects can be quite helpful in modifying separation selectivity, particularly in reversed-phase systems. Optimized separation of a nine-component mixture of weak acids, weak bases, and zwitterionic compounds has been demonstrated by this approach (21 ). In this case, the pH and concentration of octylamine hydrochloride ion-interaction agent were used as discrete variables, and a four-level two-factor (42) experimental design was employed to specify 16 mobile-phase compositions. These consisted of all combinations of four pH values (3.6, 4.4, 5.2, and 6.0) and four concentrations of octylamine hydrochloride (0, 1.5, 3.0, and 5.0 mM). From separations with these mobile phases, retention time maps were obtained for all nine species as a function of both pH and concentration of octylamine hydrochloride. Two of these maps are shown in Figure 8. A window diagram was used to determine the optimum set of experimental conditions. The resulting optimum mobile phase separated all nine components in 45 min at pH 3.7 and 0.75 mM octylamine hydrochloride. A similar experimental design has been used to examine the effects of mobile-phase strength, pH, concentration of a phosphate buffer, and concentration of camphorsulfonic acid ion-pairing agent for the separation of four alkaloids (15). Other Effects
Although not as generally applicable, changes in temperature also have been used to achieve selectivity optimization (73). Changes in relative retention values are often observed for ionizable compounds. Temperature optimization also can be combined with other effects, such as mobilephase and stationary-phase characteristics as well as pH effects, for more comprehensive optimization possibilities. Use of secondary chemical equilibrium (16) to affect selectivity could also be utilized in optimization schemes. Conclusion
Optimization in LC can be very powerful if care is taken to choose the proper parameters for change, and if a valid experimental design is selected. (continued on p. 336 A )
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332 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 2. FEBRUARY 1983
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