Table 2. Calculated Relative lntensftles for The Cluster Ion HgBr2+ Mass Number
Relative %'
354 356
0.167 11.799 19.281 49.100 52.878 96.949 48.099 100.000 14.500 48.037 7.513
357 358 359 360 361 362 363 364 366
'Isotwic abvndancestaken horn ref. (39).
isotopic cluster in question, e.g., AzB3, then extracting from that equation the abundance expected for each mass number possible for the cluster. Recognition that the form of the equation involved will differ with the number of atoms present in the cluster led us t o write subprograms for 2-atom, 3-atom, 4-atom, . . . etc. clusters rather than attemptine to write self-modifvine code to facilitate such calculation. i m a x i m u m 6-atom capibility has proved to be adeauate for our needs althouah modification to accommudrrte larger numhers of atoms is-easily possible rivcn thereneralitv of the approach. This was true in part, at feast, because envelopes are &ten identified by tbeir-profile as much as by their actual mass numbers. It is therefore possible to omit monoisotopic elements such as fluorine or iodine from the cluster formula without changing the envelope profile that reduces the total number of atoms that must beincluded in the cluster formula. Once the choice of a particular subprogram has been made, the program requests as user input the numher of isotopes that exist for each atom of the cluster and the mass numher and percent ahundance of each isotope. These values are stored in annronriate .. . arrav variables that are then used to calculate mass number sums and ahundance products for all possible cluster isotope combinations. Finally, all contributions to each allowed cluster mass number are summed and the totals normalized to 100%. Output in the form of mass numherabundance values is sent to the console screen and, optionally, to the printer. An ahundance histogram is constructed in a vertical format on a single page with a maximum of 20 or 33 peaks depending.on whether an 80 or 132 column printer is &ailable. Figure 10a represents the parent ion envelope observed in the mass spectrum of BCb (33)('OB = 19.78%,llB = 80.22%. W 1 = 75.53%, 37CI = 24.47%) compared to the envelope produced by the ABCD.BAS program (Fig. lob). The histogram-plotting routine in the program scales the vertical peak height to 50 lines for the 100% RA ion in order to keep the plot on a single page. For that reason abundances less than 2% are not disnlaved in the histoeram as can he seen for the small peak at'm/k 121, calculated"ahundance ON%, visible in Figure 10a hut absent in lob. Otherwise a close correspondence can be seen between the experimental and calculated spectra. Often metal halide com~oundsprovide examples of cluster ions detected in their mass specira. One such instance involving polyisotopic elements is HgBr%+,the spectrum of which (Fig. l l a ) was reported by Glocking (38). The calculated profile for the ion (Fie. l l b ) clearly resembles the experimental profile in spite i f small differences in relative intensity for mle 361,363,365, and 366. Glocking points out that mass spectra of even very pure metal halide; often give the impression that impurities are present in the sample with extra peaks and unexpected relative intensities of peaks in the spectra owing to facile chemical interchange between the 530
Journal of Chemical Education
sample and the ion source contaminants. We believe this phenomenon is responsible for the differences noted above since %RAvalues seem lareer for the experimental spectrum in every case where there is a difference. There are numerous factors that can and often do cause differences hetween the spectral envelopes obtained experimentallv and tw calculation. Some of these orieinate from the instrum&t, h i t probably more common areufactors arising from the sample, such as impurities or other sample components which overlap or interfere with the envelope of the component of interest. Our experience with the prozrams so far suggests that obtaining c~eHnmass spectral ehveiopes for comparison is not as easy as one might wish. Nevertheless, calculated profiles have helped us identify solution species on several occasions even when interfering ions were present. The five programs described are written in MBASIC (Microsoft Basic), hut an effort was made to avoid dialect-specific constructs so that translation to other versions of BASIC should not require major rewriting. Each program requires about 4 K of disk storage. Memory requirement during execution is somewhat greater owing to storage of the input and calculated parameters. Execution time varies from almost no delay in a case such as CO to a few minutes for 6-atom clusters with numerous isotooes. Printout of the relative ahundances and histogram is opiional. Copies of the programs on 5.25-in. diskettes along with listings and program notes can be obtained for $20 to cover the costs of media and shipping. We can provide the programs on diskettes with North Star format (10 hard sectors) single or double density and Morrow Micro Decision MD2 format (soft sector). We can also write the programs onto disks previously formated by the Osborne 1(single density) or Xerox 820 hut cannot format those here. Checks or money orders should be made out to the University of Houston-Department of Chemistry a t the above address. Acknowledgment
The author gratefully acknowledges support of this work by the Robert A. Welch Foundation under grant E-439.
The MINC Computer in the Physical Chemistry Laboratory Geoffrey S. Waldo, Carol A. Schulre, and Rubln BaHlno Wright State University Dayton. OH 45435
We purchased a MINC computer system (Digital Equipment Corp.) including a printer, X-Y recorder, and dual 8-in. disk drives several years ago on an NSF cost-sharing grant. The intent was to modernize our physical chemistry laboratory using this system. First, all students taking the laboratory courseare required to learn BASIC and to work on the machine. A set of detailed eet on the machine and includine notes to heln the students to " sample exercises is given out at the beginning of the course. The computer is used a t several levels: for teachine Drogramming, for doing selective homework problems f r o g the lecture part of the course, and for routine data processing for several of the experiments. In particular, we have written programs to process the data for a bomb calorimeter experiment where students manually record time-temperature data for subsequent keying into the computer. Temperatures are determined using a DVM output from a transducer that linearizes the signal from a thermistor to give 0-1 V in the range 20-30°C. The output from our Parr solution calorimeter is recorded directly into a file in the computer. This stored data is then processe-d with programs that-will calculate the heat capacity uf the system (using THIS, a calorimeter standnrd)
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or the enthalpy change for an unknown. The computer procram oromots mixine of the reagents. we-founi one experiment t h 2 could he readily controlled from start to finish by the computer-the vapor pressure of a volatile liquid as a function of temperature. For this we used a Yellow Springs Instrument Co. thermilinear thermistor whose signal was transformed t o give 0-1 V in the range O10O0C(i.e.. 10 mVPC) bv a YSI Thermivolt Thermistor Simal ~unditionrr.We used-a Data Instruments, Inc. pressure transducer Model AB-25 PSIA/AI)ISS which gives 1 V fur full scale of 25 PSlA when run through a 5-V power supplv built here. The accuracy of the pressure transducer is 1%,and the students develop calibration curve for its output as a separate experiment. For this calibration students read the pressure on-a manometer. correct for barometric oressure. and kev this value into the cokputer which automa&ally reeords thk sienal from the oressure transducer. Our oromam . " "VAPCAL" does this. The controlling main computer program permits students to use an existing calibration or the one they generate via VAPCAL. The liquid initially fills two-thirds of an ampoule that has a 9-mm 0.d. stem to fit a 3/s-in. Swagelok fitting and a body that is 10 cm long and 30 mm 0.d. The liquid is thorouehlv degassed while in the amooule and then the t&nsd;cer (in contact with the vapor phase) and the liquid are isolated from the rest of the system. The ampoule and pressure transducer and thermistor are loaded into a 20-L water bath. The computer program is loaded and the operator enters the starting and final temperatures. The program automatically selects temperature intervals to yield a total of 15 data points. The romp&er raises the temperature of the bath to the starting temperature, say 2 5 T . It maintains cunstant LO within +0.02°C for 3 min and then takes 100 temnerarure ~ - - - ~ pressure and temperature readings at 10-msec intervals which i t then averaees and stores. Next it increases the bath temperature by the appropriate increment and repeats the process until the end of thr rxneriment. Wedevised a two-tier heatine system: all heaters &on full-blast to within 1°C of the targe't temperature a t which point a subroutine involving proportional control takes over. We are currently developinga system to automate a kinetics experiment and a coefficient of expansion experiment. Listings of our programs are available upon request.
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Acknowledament T o G. B. Skinner for much of the initial work, to J. Pollock for the interfaces and general trouhle-shootinn. to Yellow Springs Instrument ~ o m ~ and a nJ. ~Campbell for the gift of the temperature sensor and signal conditioner, and to the National Science Foundation for the cost-sharing grant.
Graphics with a Dot-Matrix Printer Benson Floss Sundhelrn New York University Washington Square New York, NY 10003 High quality graphs, mathematical, and chemical symbols for eauations and enlareed t m e faces for oreoarine transparencies for overhead projectors generally require special orinters. I t is oossible. however. to obtain these from avarietv bf low-cost, dit-matrix printers'(such as the Epson MX series? driven hv microcomouters. The orint heads of these computers carry a set (us;ally 9) of vekically aligned wires which can be "fired" in various oatterns to svnthesize the desired characters. The typeface ?font) is usukly determined by a seauence of such firines stored in the small on-board memorv ol'ihe printer itsell's" that the computrr merely signals thk ASCII rode for t h ~rrquired . symhul and the printer dori the rest. On almost all of thrsp printers there isat least theoption of a graphics mode where these firings may be controlled by
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the microcomputer directly. This capability has several apnlications to the teachine of chemistrv. One application is the preparation ofhigh quality characters in various fonts and sizes with s ~ e c i achemical l and mathematical symbols for use in charts and manuscripts. For examole. we mav want arrows and double arrows t o reoresent irr;ve;sible o; reversible chemical reactions; subsc;ipts in chemical formulae; different fonts to distinguish between, e.g, the Faraday and the free energy or the EMF and the energy; special svmhols to desimate oartial derivatives. summations. divergeice operators, &c. Fortunately, it is unnecessary for each of us to program a computer to draw all of the required symbols in all of the useful sizes since there is available2 (public domain) the Hershey database which lists the basic dot structure of about 1000 symbols. Furthermore, there are commercially availahle software packages that utilize these data to devise a set of symbols, select and scale the symbol and font, code the appropriate signal to the printer, and execute the printing of copy prepared with any kind of editor or word-processor.Since the orinters are usuallv.caoable . of small motions of the order of % of a dot sparing hori7ontally and ' 2 of a dot spacing verticallv. it is rwss~bletoobtain a resolution not onlv far hetter thau f i r thenormal m ~ d of r the dot-matrix but a'lso considerahlv bptter than an ordinarv tvoewritrr. The characters look as though they had been printed on a press. In one such package3 print sizes are supplied from 8/72 to 2 in. in roman, boldface, italic, sans serif, script, and gothic fonts, together with a wide array of special symbols. T o further simplify the procedure, a "driver" package4 can he used which provides simple commands to manage rather sophisticated formatting along the lines used in typesetting systems. It is only necessary to prepare the desired copv, from a page to a manuscript, with an kdiior (we use a word &ocessor & tGe "non-document mode") and invoke the driver program in order to produce the final printed material. Since-the-printer must make many more strikes to synthesize the typefaces than it does for the s&lard printerrh&arters, the process is considerably slower, requiring ahuut 10 midpage. As an example, Figure 12 shows a page containing several typefaces and font sizes that could he used to compose sheets for coovine .. onto a transoarencv bv xeroeraohv for use in an overhead projector. hes scale ii importak%e symbols are readilv seen. The oaee need not be crowded as would be the case with ordinaryiy$ng, and the lettering is crisp and solidly black. In another example, Figure 13displays a few lines of characters that might occur in a technical manuscript or material prepared for physical chemistry classes. The ready availability of the symbols and fonts produces professional looking results. The preparation of graphs and similar figures using a microcomputer is constrained principally by the size of the availahle memory. An 8 X 10-in. plotting region can contain about 2 million individual dots using the graphics mode of most dot-matrix printers. This reduces to 250 khytes, far too much for the averaee micro to handle. If we reduce the resolution somewhat w; find that a matrix of 512 X 480 dots can be stored in 32 khvtes which is in ranee of most microcomputers and which corresponds to a dut spacing of about 'hof an inch. This is acceotahle hut not verv- hieh resolution in hard copy. For comparison, if it were reduced to a 3-in. column, it
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Created by Alan V. Hershey for the National Bureau of Standards: Wolcott. NBS Special Report #424. e.g., "Fancy Font" distributed by Soft Craft, 8726 S. Sepulveda Blvd., Suite 164, Los Angeles. CA 90045. e.g., "tex" by Mike Meyer, P.O. Box 1749, Norman OK 73070. Mr. Meyer has indicated that he may permit free general dislriblrtion of his material provided that it is properly acknowledged by users.
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Volume 61
Number 6 June 1984
531
