An Inexpensive Digital Gradient Controller for HPLC James E. Brady and Peter W. Carr' Kolthoff & Smith Hall, University of Minnesota, Minneapolis, MN 55455
The use of gradient elution techniques in high performance liquid chrom&graphy (HPLC) is of& essential for the direct separation of complex mixtures. Gradient elution HPLC generally requires the use of two solvent pumps and a gradient controller. Since gradient elution is a very powerful chromatographic technique, it is unfortunate that the high cost of commercially availahle controller units (which often exceed the cost of a pump) inhibits the use of this technique in typical teaching situations. Recognizing that most commercial gradient controllers have features that are of only marginal value for pedagogical purposes, we devised a low cost controller capable of illustrating the essential features of gradient elution. Since the maior of eradient elution work utilizes " oortion . linear ramps, the controller design can he simplified greatly from that which is availahle on most commercial equipment. The flow rate in virtually all simple HPLC pumps is &&olled bv a DC voltage, is reduced to the - . thus the design . problem . generation of two complementary ramps of adjustahle dnration (typically 1-30 min) and magnitude. In this manner, the sum of the two signals (and hence the total flow rate) is maintained constant while the relative proportion of the two eluents delivered by the system is a liiearfunction of time. Once the ramp has been completed, it is advisable to maintain the strong solvent conditions for an additional period of time to elute completely any stronaly retained species. Thus, the waveform ultima&ly desired is a capped ramp. The simplest and most accurate method for generating a linear ramp that will he stable over the time frame (30 min) typically encountered in gradient elution work is through the use of a pulse train inpnt to a counting circuit followed by conversion of the digital count to a proportional analog voltage. Although the final result is a staircase ramp rather than a continuous linear ramp, use of a digital to analog converter (DACI with good resolution (10-12 hits) will result in a virtually continuous solvent ramp. Moreover, since the most significant hit (msh) inpnt to the DAC can he used to disable the square wave, geneiation of the desired capped ramp is achieved in a straightforward fashion. The specifics of the approach are as follows and are illustrated in the figure below. The leads between the potentiometer controll& pump flow rate and the main circuitry of the . pump are disconnected on each pump. The lead from the flow rate potentiometer of one of the pumps (assume it to be the weak solvent pump) is connected to the controller circuit at voltaee follower 0A4. The o u t ~ uof t thisvotentiometer, V;,, and hence the flow rate dial of the weak soivent pump, will he used to set the total flow rate of both Dumps. . . The input signal Vi, is inverted with unity gain at OA5, yielding -Vim A fraction of this voltage, -aVin, is tapped off potentiometer (10 turn) P2 via voltage follower OA6. This voltage is added to the output of the ramp circuit, -r(t), in inverting fashion at OA7 to yield aVi, r(t). This signal is used to control the flow rate of the strong solvent pump. In addition, this signal is added to the output of 0A5, -Vim, in inverting fashion to yield (1 or)V:, - r(t)\at OA8. This signal is used to control the weak solvent pump. Generation of the ramp signal ( r ( t ) )proceeds
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L Schematic diagram of the controller.
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mines whether these chips are in a cleared state (output of 0) or a counting state (enabled).The output of the counter is the input to a digital to analog converter (DAC). The most significant hit (msh) innut to the DAC is used to disable the clock , a t IClB t h r o u g h ~ ~Thus, i ~ . the ramp is terminated when a count of 2"-' in = number of hits input to DAC) is reached. This count wiil be maintained until switch S1 is reset to CLEAR. The output of the DAC is amplified at OA2. The adjustment of potentiometer P 1 depends on the voltage reauirements of the Dumps used. In general P 1 would he set so chat at a count of 2"l, the voltage butput of OA2 corresponds to a flow rate of 5 mllmin. Potentiometer P 3 (10 turn) is then used to determine the magnitude of the ramp in mllmin (i.e., final % strong - initial % strong1100 X total flow rate). I t should he noted that in this configuration the relative resolution of the staircase ramp is independent of the ramp magnitude and on the order of 0.5% of the total ramp for a 10 hit DAC. A minor disadvantage of this design is that a ramp hack to the initial conditions is not provided, the change to initial conditions being a step function. This renders the present design inapplicable for use with resin type support materials or with eluent systems where buffer precipitation may occur during the switch to the initial conditions. For the purposes of a teachine lab, this constraint is minor. A controlyer based on this design has been used to operate Dumps a air of HPLC . - (Altex ll0A) for gradient elution exp&rnents in a separations laboratory course offered at this university. The performance of these cont,rollers has been remarkable in view of the simplicity of the design and overall per unit cost (-$200.00). A parts list and negative for a printed circuit board on which this design is based are availahle from the authors on request. ~~
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&or*%as s.pportco oy 0 grant lrom lhc i.noergraoualc St Aenl nstr~ctionalPraqram ol the NSk .%nofendspro$ (led oy the EducX ona Development Program of the University of Minnesota. Author to whom correspondence should be addressed. Tn s
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Volume 60
Number 1 January 1983
83
