apparatus for the automation of solvent...

Warren et al Automation of a UV visible monochromator

greater than the monochromator’s specification, it should benoted that the monochromator used in this study is over tenyears old. Because this error is a rather smooth and stablefunction of wavelength, it is possible to calculate the wave-length of an unknown emission line in a complex sample toan accuracy better than the monochromator specification.The measured wavelength is slightly dependent on temper-ature, as shown in Table 4. Thus, for the best accuracy, themonochromator should be calibrated at the t.emperature atwhich it is to be used. Also, data can be collected for a fewpoints in the region of interest, and the wavelength shift canbe calculated for the operating temperature by comparingthese data to the calibration values at a standard temperature.The microprocessor controller enables the overall performanceof the monochromator to be improved beyond its basicspecifications, as well as providing the automation features.

Software for the control programs, artwork for the PC boardsand complete documentation are available from the authors.

REFERENCES[1] GCA Corporation, "Scanning Monochromator EU-700 and

EUE-700 Series", Acton, MA, USA, 1968.[2] Fractional and Subfractional Horsepower Electric Motors.

C.G. Veinott, McGraw-Hill Book Company, New York, 1970.[3] Cordos, E., and Malmstadt, H.V., (1975) Anal. Chem. 45,

425(2), "Programmable Monochromator for Accurate HighSpeed Wavelength Isolation".

Table 4. Wavelength accuracy reproducibility and temperaturedata for the. EU-700 monochromator

Temperature(C)24.0

25.1

27.1

Averagevalue

253.54365.06404.73435.90546.10253.55365.09404.77435.91546.12253.57365.10404.77435.92546.13

Literaturevalue (6)

253.65365.01404.66435.84546.08253.65365.01404.66435.84546.08253.65365.01404.66435.84546.08

Errornm

-0.11+0.05+0.07+0.06+0.02-0.10+0.08+0.11+0.07+0.04-0.08+0.09+0.11+0.08+0.05

[4] Avery, J., Ph D. Thesis, University of Illinois, School of ChemicalSciences, Urbana, IL, 1978.

[5 Lovse, D., Ph D. Thesis, University of Illinois, School of ChemicalSciences, Urbana, IL, 1977.

[6] The Chemical Rubber Company, Handbook of Chemistry andPhysics, Cleveland, OH, 1970, p E-220.

Novel apparatus for the automation ofsolvent extraction

John G. Williams, Peter B.. Stockwell*, Michael Holmes, Derrick G. Porter.Laboratory of the Government Chemist, Stamford St., London SE1 9NQ, UK.

IntroductionChemical analyses usually require pretreatment of the sampleprior to measurement. When solutions are employed as trans-port media for the analytes, convenient minimisation ofphysical and chemical interferences may be brought aboutby solvent extraction. Previously, automatic solvent extrac-tion systems have been based on manual techniques, wherebythe eye of the human operator is replaced by some form ofphase boundary detector and the tap of the separating funnelis replaced by an electromechanical valve. Most phase bound-ary sensors have broadly similar characteristics. Apart fromoperating problems they all rely on the differential trans-mission of electromagnetic radiation on either side of theboundary. Trowell has described phase boundary detectorsbased on differential conductivity [1] and differential capa-citance [2]. Associated with the latter, relying on a dielectricchange, are methods using a change of refractive index [3] tocontrol a bistable valve as an interface flows past a fixedpoint in the system. Recent, unpublished work performedat this Laboratory has shown that ultrasonic transducers arealso capable of phase boundary detection.

Other well tried forms of solvent extraction (other thanchromatographic) involve the migration of the species ofinterest across a semi-permeable membrane under the influ-

*Present address: Plasma Therm Ltd., 6 Station Road, Penge, UK.

Volume 3 No. 2 April 1981

ence of either a concentration gradient or a potential gradi-ent, or a combination of the two. Methods relying on thegravity separation of two completely immiscible phases aresometimes employed in continuous-flow air-segmentedanalytical systems; when well designed they are relativelytrouble free. Vallis [4] designed a somewhat differentapproach to automated solvent extraction based on a rotat-able cup-shaped-vessel with a porous lid attached to the lip.The cup is placed inside a collecting vessel; if the porouslid is made from hydrophilic material such as sintered glass,water will pass into the collecting vessel at low rotationspeeds leaving the organic phase in the cup. An increasedrotation speed then ejects the organic phase. The use of ahydrophobic material such as sintered PTFE enables thepreferential rejection of the organic phase.A prototype separator which .has been designed and

built at this Laboratory using a completely new approach, iscurrently the subject of a patent application [5]. In principle,separation is effected by absorption of both phases into aporous nickel-chrome alloy disc mounted on a motor-drivenshaft. Controlled angular acceleration and centripetal forceon the droplets within the pores enables one phase to beseparated from the other. The speed of rotation of theporous disc is coupled microelectronically to the verticalcomponent of its motion so that separated droplets leavingthe disc tangentially are trapped by hitting the walls of

81

Williams et al Automation of solvent extraction

concentrically arranged glass vessels. Valves are providedat the base of each system so that separated droplets may beremoved for further processing. By applying a potentialbetween the rotating disc and a rigid electrode situatedabout 5 mm from the edge of the rotor, a current can besensed as soon as the speed of rotation has increased suf-ficiently to effect spin-off of liquid droplets. This signalmay be used to instruct the motor to continue to run at aconstant speed. Alternative provisions are included to enablethe conditions required for droplet throw-off to be electroni-cally memorised. Continuous extraction is effected by therepetitive up and down motion of the spinning disc. It is atpresent designed for the separation of two liquid phasesalthough provision for a third phase is easily added.

ApparatusThe apparatus is shown in Figure 1. The body of the extrac-tion vessel is made of Pyrex glass. Separation is effected byabsorption of a batch containing both phases into a porous2 cm diameter nickel-chrome alloy disc (A) the upper surfaceof which is domed. The disc is mounted on the end of a stain-less steel shaft (B) turned by a geared high torque electricmotor. The disc-shaft-motor assembly can be transportedalong its axis of rotation to any of three stations. The assemblyis shown at its bottom station, with the porous disc within theinner vessel (C), around which is a collar (D) forming the firstannular pocket (E). The collar itself forms the inner wall ofthe second annular pocket (F), the outer wall of whichextends upwards to support a Perspex lid (G). The innervessel and both annular pockets are fitted with drain valves.A stiff piece of platinum wire passes through the lid into theglassware as far as the level of the first annular pocket.

In operation, the mixed liquid to be separated is pumpedinto the vessel, covering the disc at its bottom station. Thedisc is set to spin at high speed, thoroughly mixing theliquid. The spinning of the disc is stopped and the discraised electromechanically to a position just above the topof the upstanding collar. At the same time, the motor startsto spin the disc, the speed of which is smoothly increaseduntil droplets of the first phase come off and a significantcurrent flow is observed between the rotating disc and theplatinum wire. The rotor continues to spin at a constantspeed for fifteen seconds, sufficient time for the first phaseto be thrown off the disc. The disc is then raised to its topstation and accelerated, throwing off the aqueous phase.The rotating disc remains in this position for a further fifteenseconds after which it returns to the lower position. The pro-cess is then repeated. The linear electromechanical actuatorand the motor used for spinning the disc were both obtainedfrom Portescap (U.K.) Ltd. of Reading. Either phase may beselected for one hundred percent purity by adjusting the sen-sitivity.of the droplet detector. In general, the second phaseis 70%-75% pure. The apparatus has been used with severalsolvent combinations including chloroform/water.

The nickel chrome alloy is available from Dunlop AviationGroup and has the trade name of "Retimet". Retimet is suit-able although its affinity for the organic phase is greater thanfor the aqueous phase; this can be used to advantage. However,in order to produce approximately equal affinity for eachphase the discs were gold plated for ten minutes using a cur-rent of 300 mA. It is preferable that the alloy is shaped sothat the pore structure is maintained on the surface aftermachining. Spark erosion is found to be successful and isavailable from Pantograph Precision Ltd, Slough. An altern-ative material for the disc is vitreous carbon. Suitable poresizes depend on the solvents to be separated but that corres-ponding to 60 pores per linear inch is appropriate for mostof the common laboratory solvents.

The solvent separation presumably depends on the com-bined effects of density and relative affinities of the porousdisc for each solvent. Table illustrates the relative affinityfor various solvents for the case of a one phase system. The

-" ()

Secondposition

(c)

Figure 1. Schematic layout of centrifugal separationsystem.

figures given represent the amount of each solvent withdrawnby a porous disc from a reservoir after being submerged eachtime to a constant depth. The disc used for these simpleexperiments was as supplied by the manufacturers prior toplating.

Electronic controlThe electronic control provides accurate repeatable controlof the angular acceleration of the porous disc and controlsthe upward and downwa.rd movement of the disc after timedintervals in the cycle. A microprocessor-baed system waschosen as this provides the necessary accuracy by the employ-ment of digital techniques whereby all the timed periods arederived from the quartz crystal controlled clock of the micro-processor. The microprocessor approach also allows versa-tility as any modifications can be made simply by alteringthe program rather than. by redesigning the circuit board.This aspect was found particularly useful during the develop-ment stage.

Table 1. Results obtained at room temperature.Mass of porous disc used 3.33 g.

Solvent Amount withdrawn (mmoles)

Watern-Hexanetrichloromethane2-propanonediethyl etherglycerolethanol

66+/- 320+24+ 235+ 325+ 232_+ 1046+

82 Journal of Automatic Chemistry

Williams et al Automation of solvent extractionII IIII

CRYSTAL

DRIVEr- 822418.432MHz

Figure 2. Microprocessor board.

AO-10

A15

C.P.U.

8080ARSTRDYSYNC0102 AO A1

EPROM

2708

CS

SYSTEMCONTRO

8228DBO-7

WR

PARALLELINTERFACE

8255A

PORT A

PORTBPORT C

+12V

+5V

15K 100n

1/24529 ,0 .-I 1/24528

4.7n

120

BFR81

PORT C

GND

DACZN425E

CURRENT SENSOR

10K

10n

1K BC183

MOTOR

+30V

INPUT-

Figure 3. Motor speed control.

L.P.FI LTER PORT A

GND

Volume 3 No. 2 April 1981 83

Williams et al Automation of solvent extraction

The electronic system of the apparatus can be divided intotwo main parts: (i) The microprocessor board consisting ofthe central processing unit (CPU), erasable programmableread only memory (EPROM) and the associated peripheraldevices. (ii) The Control board consisting of the digital toanalogue converter (DAC), motor controller, droplet detectorcurrent sensor and the relays.

The microprocessor board, shown in Figure 2, is based onthe Intel 8080A CPU and its associated peripheral devices.The program is stored in the 2708 EPROM and uses approxi-mately 300 of the 1024 bytes available. No random accessmemory is used as the limited amount of data handlingrequired can be accomplished using the six internal registersof the CPU. Communication between the microprocessorboard and the rest of the system is via the 8255 programmableparallel interface which is programmed to provide sixteenoutputs and eight inputs. Seven of the outputs from port Cof the 8255 are used to provide data inputs for the DAC onthe control board. The other output is used to activate therelay which raises the actuator. Port B provides seven outputsto the displays which give an indication of the speed, theother output being used to lower the disc at the end of thecycle. Of the eight inputs to port A, one is used for the out-put from the circuit which senses the current produced whendrops spin off, the other seven inputs are used to select apreset speed when the droplet sensor is not used. By acti-vating these inputs one of seven preset speeds stored in thememory can be selected.

The control board is shown in Figure 3. The speed controldata from the microprocessor board is converted by the DACinto a reference voltage for the motor control circuit. AFerranti ZN425E DAC is used as this device provides a voltageoutput as opposed to the current output of most otherdevices and is substantially chealSer. The bit seven input ofthe DAC is not used, bit eight activates the full speed stirringwhich mixes the two phases together at the bottom of theapparatus. The least significant six bits are used gradually toincrease the speed in sixty increments at the rate of one persecond. This produces reproducibly uniform acceleration ofthe disc.

The motor speed control circuit operates by pulsing themotor on and off. This approach allows feedback of themotor speed to be obtained by sensing the back e.m.f.during the period when the motor is turned off. Duringthis off period, which is fixed at 1.3ms, the motor is con-nected to capacitor C by the analogue switch (A2). Theback e.m.f, is retained on the capacitor during the on periodof the motor and compared with the voltage output fromthe DAC. The output of the comparator controls the analogueswitch (A1). This switch operates in a quasi-linear mode andvaries the length of the on period to keep the back e.m.f.(and hence the speed) close to the reference voltage suppliedby the DAC. The method used in this Circuit of only varyingthe on period to provide speed control is preferable toother methods as it allows smooth control at slow speeds.This is because the total on/off period is shortest at slowspe6ds i.e. when the on period is small.

The current detector circuit is necessary to detect thecurrent of a few nanoamps which flows in the presence ofdrops when a potential of 30V is applied between the discand the detector wire. The circuit uses a CA3140 MOSinput operational amplifier with a typical input current of10pA at 25 degrees centigrade. The sensitivity of this circuitcan be as low as lnA and is adjustable. To prevent falsetriggering by electrical interference, two more operationalamplifiers are used to produce a fourth order low passfilter with a cut-off frequency of 10Hz. In addition, a smallamount of software filtering has been incorporated.

The program was written in assembly language and con-sists mainly of timing loops with various breakpoints whereoperations are performed such as activating the actuator orincrementing the motor speed and displays.

REFERENCESTrowell, F. (1969) Laboratory Practice, 18, 44.

[2] Trowell, F. (1967) U.K. Pat. Application, 17329/67.[3] Porter, D.G., Jackson, C.J., Bunting, W., (1974) Laboratory

Practice, 23, 111.[4] Vallis, D.G., (1967) U.K. Pat. Application, 14964/67.[5] Williams, J.G., Stockwell, P.B. (1980) U.K. Pat. Application,

8023547.

84 Journal of Automatic Chemistry

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