Anal. Chem. 1986, 58, 509-510
the high logic level is transmitted to the D - flip-flop 74LS74 (device C) via two Schmitt-triggered NAND gates (1/4 74LS132). The use of Schmitt triggering avoids changes in logic levels due to short-term hysteresis or noise; capacitors at the input and output stages of the comparators are also designed to avoid spurious oscillations by limiting dV/dt. As the input to device C becomes high, the Q output (pin 9) goes high and turns on the counting status LED L through a driver (1/2 75452). The Q output (high) of device C is also inverted through a NAND gate F (1/4 74LA14) to produce a low output. This output is used to enable or disable the counter; when this output is low, the counter is enabled. On the trailing edge, as the signal falls below the preset voltage selected by Rg, device B output goes high and a low logic status is transmitted to D - flip-flop E via inversion by a Schmitt triggered NAND gate (1/4 74LS132). A low input to device E produces a high Q (pin 6 ) and a low Q (pin 5) output. The Q output signals an optional external data acquisition device such as a microcomputer that data are ready (low logic level). The Q output is also sent to the second input of NAND gate F. At this point both inputs to device F are high, so the output of F goes high and the counter is disabled. The high Q output from device E is also used to turn off the count status LED L through its driver chip and turn on a piezoelectric alarm P through a monostable multivibrator (1/2 74LS123, device M1A) and a driver (1/2 75452). The alarm is disabled, if desired, by turning off switch S1. The high Q output from E also latches the counter and displays the elapsed time by sending a latch output to the counter by a second monostable multivibrator (1/2 74LS123, device MlB). The output of device M1B in turn is directed to monostable multivibrator M2 (1/2 74LS123) which provides a reset pulse to the “AUTO” terminal of the SPDT switch 52. When this,switch is in the “AUTO” position, the flip-flops C and E are automatically reset and the counter is cleared for the next task at the completion of counting via device B. When S2 is in “MAN” mode, flip-flop reset and clearing of the counter is accomplished by manually activating monentary switch 53; this can also be done when counting is in progress. With switch S2 in the “EXT” mode, flip-flop reset and counter clearance is controlled by an external device (e.g., a microcomputer), which must send a low logic status signal to the counter for this purpose. Figure ICshows the heart of the device, the counter itself. A 1MHz crystal oscillator output is divided through a divider chain of four decade counters (74LS90) resulting in a clock period of 10 ms. Obviously, by omitting one or more of the decade counters or by adding one or more, the resolution and the maximum counting capacity of the instrument can be
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tailored for other specific needs. The clock output is sent to a series of five integrated TIL306 devices; each of these combine the functions of a counter, necessary latches, decoders, display drivers, and a seven-segment LED display and also provides parallel BCD output. The most significant digit (MSD) and the least significant digit (LSD) are indicated on the respective devices in Figure IC. Counting begins as the leading edge of the signal goes above the preselected “start” voltage and count enable status is activated by NAND gate F. Counting stops and the elapsed time is latched on the display as the trailing edge of the signal falls below the preselected “stop” voltage and the display latches via M1B; the counter itself is simultaneously reset by the clear counter output from device E. If overflow occurs in the MSD counter (elapsed time >lo00 s), the decimal points in all the display digits light via a 74LSOO inverter and an overflow status signal is also sent to defice E, which turns on the alarm and resets the start and stop flip-flops. Figure l a shows the necessary power supply. The assembled instrument is available from Berne TechLite Corp., Lubbock, TX.
RESULTS The performance of the counter was tested by sending rectangular wave pulses of variable duration (0.5-600 s) generated by a Shawnee digital electronic timer switch and a D cell. For any pulse duration at least 10 measurements were made and the counter was always found to be within one least significant digit (10 ms) of the period selected by the timer. For measurements with a flow injection system, the precision of repeated measurements was found to be 0.5%, which was the flow precision of the pump used, suggesting that the overall precision is controlled by the pumping system. Successive measurements could be made with the autoreset function as fast as the normal throughput rate of the system allowed. If throughput rate is too fast such that sample carryover occurs between successive injections, the counter is subject to the same errbrs as experienced by a conventional system. The counter is susceptible to false triggering if signal is accompanied by spurious noise (21mV), especially if the “start” and “stop” voltages are set to be the same. Such noise may be minimized by active filtering.
LITERATURE CITED (1) Rhee, J.-S.; Dasgupta, P. K., submitted fot publication
RECEIVED for review April 18, 1985, Accepted September 9, 1985.
Continuous-Flow Injector for Flow Injection Analysis Amando F. Kapauan* a n d Marcelita C. Magno Department of Chemistry, Ateneo de Manila University, P.O. Box 154, Manila, Philippines Sample injection into the flowing carrier stream in flow injection analysis (FIA) is a weak link in this analytical technique. Manual syringe injection through an elastomer septum and rotary or sliding valve loop injection are the two most common methods currently used. The former is simple and inexpensive, but the reproducibity is dependent on operator skill, while the latter is expensive and has problems of its own (1). We have designed and fabricated a simple, inexpensive sample injector that controls fluid flow at the point where sample is pumped into the carrier stream, which is split be0003-2700/86/0358-0509$0 l .50/0
tween flow to the detector and flow to waste. A t no point during the injection cycle are any flow rates altered.
EXPERIMENTAL SECTION Sample Injector and FIA System, Figure 1shows the important details of the sample injector, while Figure 2 shows how it is incorporated into the FIA system. The sample injector body is acrylic (1/4-in.sheet stock) with one straight-through channel and two opposing side channels about 6 mm from each other. Short pieces of 18-gaugetubing from stainless-steel hypodermic syringe needles are epoxied on so that PVC tubing can be attached. The tubing labeled 0 in Figure 1 has an inner tubing, I (20-gauge 0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
IS
0
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I CM.
Flgure 1. Continuous-flow sample Injector. All the lettered components except P are stainless-steel tubing epoxied to channels bored In acryllc plastic (shaded portions). Tubing I slide fits Into tubing 0. P is a plunger connected to a dc solenoid. PUMP
SA
output of a monostable multivibrator based on a type 555 integrated circuit (2). Switch-selected resistors vary the pulse width of the monostable,allowing injection of different sample volumes. The necessary trigger can be a manually operated pushbutton or a computer-generated signal pulse. Precision Testing. Three systems using two different detectors were used to evaluate the reproducibilityof the FIA system as configured. The first detector used was an 80-pLflow-through cell mounted in a Perkin-Elmer Model 55E single-beam spectrophotometer, and the second was a three-electrode electrochemical cell in a PAR 174A polarographic analyzer system operated in a speeded up DPV mode (3). By use of the spectrophotometricdetector, reproducibility was evaluated for (a) 6 mg/L methyl orange solution injected into a 0.01 M HCl carrier and monitored at 510 nm and (b) 0.80 mM sodium chromate solution injected into a 0.01 M NaOH carrier and monitored at 375 nm. With the electrochemical detector, a 4 mM zinc sulfate solution was injected into a deaerated 0.5 M ammonium chloride/0.5 M ammonium hydroxide buffer stream. An amalgamated copper working electrode, a platinum counter electrode, and a silver-silver chloride reference were used in the cell, while a 0.2-5 pulse rate, 100-mVmodulation amplitude, and a steady -1.25-V (vs. SCE) cell potential were set on the PAR 174A operated in the DPV mode. Flow rates in all of the tests reported here were as follows: carrier solution FC, 4.50 mL/min; sample FS, 0.64 mL/min; and waste FW, 1.72 mL/min. The flow rate through the detector FD was therefore 3.42 mL/min. The solenoid was activated for 3.3 s in all trials, resulting in the injection of 35-fiLof sample at the flow rates specified above. Ten trials were run on each test, and results were recorded on a Varian Model A-6 strip-chart recorder. All 10 readings were used in calculating the relative standard deviation. In the DPV mode, readings were also recorded digitally on a microcomputer-controlleddata acquisition system equipped with a fast 12-bit A/D converter.
RESULTS AND DISCUSSION
Figure 2. Flow injection analysis system. SS is a surge suppressor, DE is the detector, and WA is waste. See text for other detalls.
stainless steel) slide fitted into it whose tip inside the device can be located accurately either at the junction of the side channel to W or at about 2 mm below the junction of the channel to C. This positioning of the inner tube, I, is done by means of a small dc solenoid, a return spring (not shown), and adjustable stops. In this setup tube I is up when the solenoid is off and down when it is on. Three of the four channels of a Gilson Minipuls 2 variable-speed peristaltic pump are used, with the flow rate relationships as follows: FC > FW > FS and FD = FC + FS - FW, where the rate at which carrier solution is pumped from reservoir CA is FC, from the sample vial SA is FS, and to waste is FW. FD is the resulting flow rate through the detector. In operation, when the tube I is up, sample input through S is completely diverted to waste through W, and when the tube is down, the sample is completely taken up into the carrier stream, which goes to the detector through D. Precise control of the time that the tube I is down controls the amount of sample injected. This is accomplished by activating the solenoid with the amplified
Typical overall relative standard deviations for the two spectrophotometric detection systems were 0.72 and 0.59%, while that for the electrochemical detection system was 50.56%. Aside from the variability of the injection system, factors contributing to the observed reproducibility include (1)variations in fluid flow rates and (2) variable response of the detectors. At the worst, this injector system is reproducible to 0.5-0.7%, which is equal to or better than the reproducibility obtained in manual injection by a skilled operator. The system described has been in continuous trouble-free use for 6 months.
ACKNOWLEDGMENT We thank Peter W. Alexander of the University of New South Wales, Australia, for providing the electrochemical cells and for his valuable help in putting together and starting up our FIA system.
LITERATURE CITED (1) Harrow, Jeffrey J.; Janata, Jlfi Anal. Chern. 1883, 55, 2461-2464. (2) Lancaster, Don “TTL Cookbook”; Howard W. Sarns & Co., Inc.: Indlanapolis, IN, 1974; pp 180-183. (3) Alexander, Peter W.; Akapongkul, Umaporn Anel. Chirn. Acta 1984, 166. 119-127.
RECEIVED for review July 16, 1985. Accepted September 9, 1985. Partial support for this project was provided by the Philippine National Science and Technology Authority and the Australian International Development Program.
