27/03/64 03-27-1984
File MPHH Smpl it (q):
. 0412
Sample: M (Cl04) 2. 6H20 at-ma- l d l g 0 H (J/9>- 22. 96418
Scan Speed(C/m>n) 3 10 Tamp. Rango/K : 310- 3 4 1
mCoI/eec- 32 Run AqaInst: blank
A
TEMP /K
Figure 5. Example DSC scan, output to H-P 7470A plotter. A solid-solid phase transition in Mg(CQ)2-6H20is illustrated.
serted in the computer system board. Through relaysbuilt into the same board, data collection initiation has been automated, by-passing the toggle switch on the DSC; this control also allows the temperature increase to be stopped at the end of a run. Further temperature changes are carried out through the normal use of the toggle switch. Data acquisition, manipulation, peak integration, screen plotting, and the generation of hard copy plots on a Hewlett-Packard 7470A plotter are software-controlled. The output from the DSC is of two types: voltages that indicate the difference in amounts of heat required to keep the rate of temperature rise for the sample and reference pans equal, and flags that occur once per degree of temperature rise. (The latter were formerly used to provide a temperature tick on the two-pen chart recorder.) The interface circuit, which was built for about $200, consists of an input differential amplifier (with a gain of 500), a 3-step binary autorange stage, a 12-bit charge-balancinganalog-to-digitalconverter, and the necessary latch circuit to interface to the IBM PC's buss. Also included was a signal conditioner and an event for the latch marker. The ADC chip takes 24 ms to digitize the output from the DSC. Auto-ranging provides maximum resolution of 1part in 4096 in each of the three input ranges of +2.5,5.0,10.0 mV. The circuit that controls the initiation and termination of a DSC run, which added about $25 to the hardware cost, consists of two relays and drivers, interlocked to operate in parallel with the existing double-pull double-throw center-off switch. The relays are software-controlled. In addition to the interfacelrelay board, an IBM PC with 128K RAM (64K is sufficient to handle a comment-free edition of the software), a color/graphics card, serial and printer ports and a dual disk drive, a composite video monitor, a printer, and a Hewlett-Packard 7470A plotter with an RS-232 interface are required hardware. The software (written in BASICA) controls the hardware, initiates the DSC scan, acquires the voltages and temperatures, plots the data on the screen as it is acquired, terminates the temperature increase, and stores the data onto the disk for future recall and manipulation. To reduce the noise of the DSC trace, every 10 samples of the voltage are averaged to yield one point in the plot. A typical run a t a temperature increase to 10K per minute yields eight data points (80 samplings) per degree. The software can handle up to 2000 points of data, and this value can be increased easily. Soft keys are used to control plotting parameters and the positions of cursors on the screen. The cursors are used to select the start and end points of the segment to be integrated. The area underneath a peak is calculated from the difference between the total integration area and the integration of the baseline from the cursor start and end positions. All integra-
tions are done numerically. The enthalpy of a transition is calculated from the area under the peak by comparison with a transition of known enthalpy, the melting point of indium. (This data is recalled from disk and need only be updated occasionally to check the reproducibility of the DSC.) After integration, a hard-copy plot of variable magnification may be obtained on the HP 7470A plotter. An example of the output is illustrated in Figure 5. Copies of the interfacelrelay circuit and its documentation, a guide to the use of the software and a disk containing the program DSC are available from M. A. White. Please send $10 to cover the cost of the disk, printing, and postage. The authors gratefully acknowledge the financial support of the Dalhousie University Research Development Fund.
The Design of a Computer-Controlled Flow-Injection Analyzer: An Undergraduate Experiment Sam A. McClintock, James R. Weber,3 and William C. Purdy McGill University Montreal, Quebec, Canada H3A 2K6
Much of the work in a modern analytical or clinical laboratory is carried out by highly automated microprocessorcontrolled instrumentation. In the past five years there has been a tremendous increase in the number of such systems. However, probably due to cost factors, many chemistry departments have been slow in developing laboratory experiments that illustrate the operation of these instruments. Even if such instrumentation were available, students would not be encouraged to dismantle it to "see how it works." Yet there is a need for the student to become more aware of the compromises that have been made and the limitations placed on experimental results by instrument design. In many cases automated instrumentation has removed the operator several steps from the measurement process so that he or she is no longer cognizant of how the measurement step is carried out. We have developed an undergraduate experiment that allows the student to become the instrument designer. The student constructs a simple solid-state photometer and uses it as a detector in a flow-injectionor continuous-flowanalyzer. A small single-board computer acquires the data and controls the system. Analysis can be made of any material that can be converted by chemical reaction to a substance that absorbs light of a wavelength emitted by the photometer. In our experiment, acetylsalicylic acid is analyzed by hydrolysis to salicylic acid and reaction with ferric ion, the Trinder reaction (5),to form a reddish-purple-colored complex. Several important concepts are incorporated in this experiment: it demonstrates the principle of continuous-flow or flow-injection analysis; it uses simple analog electronics to run a small single-wavelength photometer; it illustrates microprocessor control of an instrument; and it investigates the limitations that such control places on the measurement process. All of this is accomplished for a very small investment in equipment. Analysis in flowing streams has become a common method of assaying large numbers of samples. Two techniques are presently in use, continuous-flow analysis using an air-segmented stream and flow-injection analysis using a non-segmented stream. Hansen and RGiiEka (6) have developed a student experiment based on the latter technique; that paper can be consulted for technical details. The basic theory (7.8) and the advantages of one system over the other have been covered elsewhere (9,lO).Both systems work with the present detector, and the one chosen for the student experiment will Present address: University of Guelph, Guelph, Ontario, Canada N I G 2W1.
Volume 62
Number 1 January 1985
65
