10’11
Anal. Chem. 1982, 5 4 , 1011-1015 d
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a
e
L_-
Time ( m i n . )
_ . 10
15
20
Dilution of the sample by a factor of 10 coupled with a relatively slow eluent flow rate provides, as shown in Figure 1, sufficient peak resolution to allow for the accurate determination of F- and Pod3-concentration. Resolution of the carbonatephosphate peaks is virtually 1. Although significant overlap exists between the fluoride and chloride peaks, the relative concentration ratios combine with minimum resolution to allow for adequate base line identification. As i k s t r a t e d in Table I, analysis of standard reference materials confirms the accuracy of the fusion/instrumental analytical sequence. Data summarized in this table represent at least nine replicate Na2C03 fusions of both NBS 1206 Florida Phosphate rock and the Western Phosphate Organization’s WPO 42. The results show agreement within experimental precision is obtained by the different analytical techniques employed. The comparatively large precision, which is expressed as the standard deviation of the concentration data for the major components (Ca, Si, P) reflects sample inhomogeneity resulting from the small masses of material which can be digested by the carbonate fusion. In Table 11, which summarizes the composition of field phosphorite ore samples analyzed by the various instrumental methods and solubilized by both fusions, the poor reproducibility of the carbonate fusion for major species is again noted while the peroxide fusion, with its larger sample/flux weight ratio, allows for a much more precise analysis. Precision using this digestion is on the order of * l % RSD. We note that once again the agreement between data obtained by the two fusions and the different methods of analysis is sufficiently good that at the 95% confidence level, no statistical difference can be established between techniques.
LITERATURE CITED Figure 1. Chromatogram for WPO 42 analysis. Peak identifications include: (a) sample injection, (b) F-, (c) CI-, (d) C0;-/H2CO3, (e) P O P .
to the presence of an unidentified interferant; therefore, the use of standard additions was required. Direct analysis of the fusion digestant by ion chromatography was prevented due to column saturation caused by the large amounts of chloride and carbonate in the sample matrix.
(1) Hendel, Y.; Ehrenthal, A.; Bernas, B. A t . Absorpt. News/. 197, 12, 130. (2) Chatriand, G., Chief Chemlst, Stauffer Chemical Co., Sllver Bow Plant, Butte, MT, personal communication, 1981. (3) Watson, A. E.;Russell, g. ICP I n f . News/. 1979, 4, 441.
RECEIVED for review September 8,1981. Accepted January 4, 1982.
Microcomputer-Rased Data Acquisition System for Electroanalytical Instrumentation John F. Price, Samuel L. Cooke, Jr., and Richard P. Baldwin” Department of Chemistty, 8Unlversivof Louisville, Loulsville, Kentuce 40292
Applications of microcomputer systems have been reported for virtually every major category of chemical instrumentation, leaving no question concerning the considerable advantages which these types of systems provide to the analyst. In this regard, electroanalytical instrumentation has certainly been no exception; several recently described systems have clearly demonstrated the advantages which can be derived from microprocessor control of electrochemical experiments and of the associated data acquisition, processing, and display functions (I-IO), The systems thusfar reported run the spectrum from relatively expensive commercially available units dedicated to operation with the specific attached electroanalyzer (1)to completely home-built and home-programmed units based on commercially available microprocessor kits (8). However, practically none of the approaches directly addresses the needs of the “ordinary” electrochemist who desires to introduce the advantages of computerized instrumentation into his already outfitted laboratory but who lacks the time, expertisle, or energy to design, construct, program, 0003-2700/82/0354-1011$01.25/0
and debug his own microprocessor system. A compromise approach that would seem to offer several advantages can be made by using one of the new personal microcomputers which have recently gained wide popularity. The attractiveness of this approach derives from the fact that such computer systems can be purchased in immediately operable form. Additionally numerous useful, software-supported peripherals are available which can easily be included with no particular electronics experience required on the part of the user. Thus, the user is freed from concerns regarding the construction and start up of the microcomputer itself and can concentrate directly on his particular application and on implementing the appropriate interface for it. Furthermore, as the operation of these microcomputers usually requires only a knowledge of high level programming languages such ELS BASIC, the user is potentially freed from the need to mastor interfacing and programming at the assembly language level as well. In the electroanalytical realm, only one example of the use of a personal microcomputer has been reported. In 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
this instance, von Wandruszka and Maraschin (9) described the interfacing of an Apple I1 microcomputer to a commerical potentiostat. Unfortunately, their system was employed only for linear sweep and cyclic voltammetry and not for the more analytically useful pulsed voltammetric techniques. Furthermore, data acquisition which was performed under interrupt control and thus required assembly language interface servicing routines was hardware limited to a maximum sampling rate of only 4 Hz. Recently, Woodard, Woodward, and Reilley (11)reviewed the principal characteristics of microcomputer systems that affect their usefulness in the analytical laboratory and suggested the use of a modular microcomputer-interface approach for the interested user/developer. In this approach, the computer and interface are divided into a number of individual hardware modules, each of which is designed separately to carry out a specific function and can be used, bypassed, or modified as required by the demands of the particular laboratory situation. In this paper, we describe the design and construction of a modular interface which allows data from commonly available analog electrochemical analyzers to be acquired and processed by a TRS-80 microcomputer. No knowledge of assembly language programming is required, as the BASIC programming language is utilized throughouteven for data acquisition. Even so, 12-bit data acquisition rates as high as 60 Hz are attainable, thereby making the system suitable for most routine electroanalytical applications. Moreover, because of its modular nature, the interface could easily be adapted for use both with nonelectrochemical instrumentation and with personal microcomputer models other than the TRS-80.
EXPERIMENTAL SECTION Overview of System. Our system design was based on the use of a commercially available personal microcomputer, the TRS-80 Model I or Model I11 (Radio Shack, Ft. Worth, TX) and a modular interface designed in-house to provide a functional data acquisition unit. This approach allows the user to concentrate on the design of the interface rather than on the construction of the microcomputer. An additional priority was the use of BASIC for all system sohare, including that required for data acquisition. Most microcomputer data systems require assembly language for this purpose; and, as a result, the implementation of real-time data acquisition on these systems represents an arduous and time-consuming learning experience for the beginner. BASICcontrolled data acquisition is certainly considerably slower than that achievable by using the conventional assembly language approach. However, the attainable sampling rate is still fast enough for most routine electroanalytical methods. Additionally, assembly language subroutines could be added at a later date wtih no major hardware modifications if faster data acquisition should prove advantageous. An overall block diagram of the electrochemical experiment, microcomputer, and interface is given in Figure 1. In all cases, conventional analog voltammetric instruments were employed to carry out the usual potentiostat functions including the generation of the appropriate potential waveform, the application of the waveform to the electrochemical cell, and the initial conditioning (i.e., current-to-voltage conversion and amplification) of the resulting redox current signal. Linear sweep, cyclic, and various pulsed forms of voltammetry were utilizied. Microcomputer. All the work described here was performed by using TRS-80 Model I or Model I11 microcomputer systems, both of which use the 8-bit 2-80 microprocessor chip and are capable of directly accessing a maximum of 64K bytes of readjwrite memory. Level I1 BASIC was utilized for all software. This form of BASIC contains two general purpose I/O device commands, “OUT” and “INP”,whose usage is essential in carrying out the datapcquisition approach suggested (12). Our own microcomputer system included 48K of random access readlwrite memory as well as the following TRS-E@peripherals: Floppy Disk Drive Kits 1and 2 (175K bytes each), a Model VI11 line printer, and a Graphics Enhancement package. It should be emphmized,
..:pg..m POTENTIOSTAT
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INTERFACE
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Figure 1. Overall block diagram of the electrochemical instrument, interface, and microcomputer.
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however, that the minimum package required for a working data acquisition system is only the Model I or I11 microcomputer with 16K memory and the interface modules described below. The total cost for this minimum electrochemical data sampling and processing package is approximately $1200. Interface. The interface consisted of a series of hardware modules-an approach which allows the prospective user to select only the hardware required for his particular electrochemical application. Each of the modules was constructed on either 4.5 X 10 in. or 4.5 X 6 in. printed circuit cards, which could be interconnected and powered by insertion into a standard 44conductor interface bus. Our particular interface was built to accommodateup to five modules and is easily expandable, should future applications require additional hardware elements. The present application required three modules: a buffer board; a 12-bit A/D board; and a 10-bit D/A board. Buffer Board. This module (see Figure 2) serves as the direct linkage between the TRS-80 microcomputer and the interface bus. As such, its functions include (1)the isolation of the interface from the microcomputer, thereby protecting the microcomputer system bus from encountering potentially destructive signals, and
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
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(2) the decoding of the data and address information transmitted between the microcomputer and the interface. As the output structures differ for different microcomputer models, the configuration of this board could be altered somewhat so that a microcomputer system other than the TRS-80 could be empolyed. Buffering of the TRS-80’s eight least significant address bits (&-A7) and two 1-bit control signals and OUT) is accompplished by means of two SN74365 three-state bus driver chips which are always enabled. The 8-bit data bus (Do-D7) utilizes two DM8216 bidirectional buffer chips which are normally maintained in the output mode. When an input operation is required, the pulse generated by the BASIC “INP” instruction momentarily (- 1.3 bs) reverses the direction of the bus, allowing information to be passed from the interface to the microcomputer. The availability of 8 address bits permits a maximum of 256 different 1/0 ports to be specified (although some are reserved for system use). The address decoding circuitry employed here, however, limits the selection to a set of only 16 consecutive decoded 1 / 0 port addresses for interface use. This is accomplished by initially comprxring the four upper address lines, A4-A7, to binary value set by four external switches. When these address bits match the switch-selected value, a pulse sent to a 4- to 16-line decoder (SN74154) enables the decoding of the lower four address bits, &-A3. The 16 unique address ports that result are available on the interface bus for subsequent interface use. The circuitry described here requires only 10 port addresses, thereby leaving six others immediately available for system expansion. Possible additions could include more A/D or D/A converters and provisions for the discrete control of experimental parameters such as light pulses, reagent addition, gas flow, etc. Further expansion beyond 16 port addresses would be possible by replication of this circuit and selection of a different set of 16 port addresses. It should be nobd that the TRS-80 Model I11 has been provided by the manufacturer with an internal fully buffered 1/0port which can be software enabled and allows access only to the eight least significant address bits (&-A7). As in the Model I, an input pulse is used to reverse momentarily the direction of the data bus which is normally maintained in the output mode. Thus, the buffer board module appropriate for use with the Model I11 needs to provide only the &bit address decoding circuitry described above. For a detailed discussion of TRS-80 1/0 architecture and recommended interfacing approaches, the interested reader is advised to consult ref 13. AID Converter Board. This module (see Figure 3) utilizes the Analog Devices (Norwood, MA) AD574KD chip, a 12-bit successive-approximation A/D converter. The AD574 was selected
74100
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Flgure 4. D/A conversion module.
since it provides tristate output buffers and a self-contained reference voltage and is capable of both unipolar (0 V to +10 V or 0 V to +20 V) and bipolar (-5 V to +5 V and -10 V to +10 V) operation. The interface module contains two A/D converters which, working in unison, allow the simultaneous digitization of two separate signals (e.g., current and potential for an electrochemical experiment). Both converters have been wired so that sample-and-hold amplifiers are activated and the conversion sequence is initiated when an ”OUT” instruction is encountered and executed in the BASIC main program. Upon completion of the conversion, the sample-and-hold amplifiers are automatically reset to the sample mode to await the next conversion. Two “INF”’ statements then instruct each A/D to transfer to the data bus first an 8-bit nibble and then a 4-bit nibble which together correspond to the 12-bit converted values. D / A Converter Board. This module (shown in Figure 4) consists of a pair of double buffered 10-bit Analog Devices AD561KD D/A converters. Two D/A converters were utilized
1014
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
1
Table I. Sample Data Acquisition Program 0.32
BASIC statement 5 for I = 1 to 1024 1 0 OUT 236,16 20 OUT 96, 1
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30 A(1) = INP(96)*16 + INP( 97)/16 40 E(I) = INP( 98)*16 + INP( 99)/16 50 NEXT I
function specify that 1024 pairs of data are to be taken enable 1 / 0 port (for Model I11 only) instruct A/D converters to begin a conversion input and store potential value (in two nibbles) input and store current value (in two nibbles) continue loop
necarboxaldehyde and, in all four cases, were virtually identical with those obtained by direct recording of the analog data. I Peak and plateau currents were found to be reproducible to 0 0 01 0 2 0: 04 0 5 0 6 07 08 09 +10 within &0.5%on successive runs. E IV vs SCEl The data acquisition rate for the system was software limFlgure 5. Microcomputer-acquired differential pulse voltammogram ited to approximately 60 points/s. The limiting factor was M ferrocene and 8.4 X M for solution contalning 5.5 X the execution time of the BASIC instructions required to input ferrocenecarboxaldehyde. The voltammogram was obtained at a Pt and store data from the dual A/D converters and perform the worklng electrode uslng a Prlnceton Applied Research Model 174A necessary program bookkeeping. The BASIC program repolarographic analyzer. Initial potential was 0.0 V vs. SCE, pulse quired to carry out the data acquisition is listed in Table I. amplitude was 50 mV, and scan rate was 5 mV/s. Note that only six instructions were required. The practical in order to permit direct interaction with usual electrochemical effect of this sampling limitation was illustrated by acquiring display devices, such as X-Y recorders, which require two indecyclic voltammograms a t various potential scan rates and pendent voltage inputs. Ten-bit converters were selected so as comparing the microcomputer calculated peak currents and to match as closely as possible the resolution of most commercially peak potentials to those measured conventionally. No sigavailable X-Y recorders. In operation (i.e., when an appropriate nificant differences were noted for scan rates slower than 600 “OUT” instruction is executed), the interface data bus first loads mV/s. In fact, the available sampling rate using the system the lower 8 bits into the 8-bit data latch and then the two most described here generally permits the use of higher scan rates significant bits into a separate latch. Subsequently, these sepathan is possible with analog electrochemical systems limited rately formed portions of the 10-bit word are simultaneously transferred to the D/A for conversion and output. The douby the response speed of an associated X-Y recorder. The ble-buffering approach is employed in order to prevent the ocmaximum usable scan rate for this system is determined currence of “glitches” resulting from nonsimultaneous transfer primarily by the minimum potential axis resolution required of data to the D/A converters. Fast settling operational amplifiers by the specific application under investigation. (Analog Devices Model 509) permit the adjustment of the D/A Numerous types of data manipulations of direct benefit to gain. the electrochemist are easily made possible by this specific An optional pen drop circuit for use with an analog X-Y plotter microcomputer-interface combination. In our laboratory, the is also located on the D/A board used in this work. system is being employed primarily to perform functions Electrochemical Instrumentation. Princeton Applied Reinvolving data logging, background correction, peak integrasearch (Princeton, NJ) Models 174A and 364 polarographic analyzers and a Bioanalytical Systems (West Lafayette, IN) Model tion, and display. Undoubtedly, its most useful characteristic CV-1B cyclic voltammetry controller were used to provide elecis its data storage capaility which makes possible a variety trochemical data in this work. In each case, the potentiostat of postexperiment BASIC-language data manipulations. The output could be input directly to the interface system without system is relatively simple to construct and yet is extremely any instrument modification. However, since the input range of versatile. With virtually no modifications, the interface should the A/D converter was 0 to +10 V and the potential-axis output be suitable for use with most commercially available and many of the Princeton Applied Research devices is only 0 to +1.5 V, home-built electroanalytical instruments. At the same time, approximately 21/2 bits of A/D resolution would be unused. use of the interface with many nonelectrochemical transducers Consequently, a simple modification was performed in order to or with microcomputers other than the TRS-80 should easily utilize fully the available dynamic range of the A/D. For the be carried out following only minor modifications. Finally, Model 174A, the modification consisted of the replacement of resistor R118 (60 kQ, 0.1%) on the Programmer/Potentiostat when not in use for on-line electrochemical analysis, the board with a 9-kQresistor (also 0.1%) or an appropriately trimmed TRS-80 and its peripherals are readily available for unrelated 15-turn 10-kQpotentiometer. This change transforms the pooff-line computer applications. Detailed system documentential-axis output of the potentiostat to a 0 to +10 V range. An tation is available on request. In a subsequent paper, we will analogous modification replacing resistor R17 can be performed describe a BASIC software package which is capable of perfor the Model 364 as well. forming a variety of commonly used transformations on labRESULTS AND DISCUSSION oratory data acquired via this interface system and stored in appropriate data files. Linear sweep, cyclic, normal pulse, and differential pulse voltammetry were carried out with the use of several popular LITERATURE CITED electrochemical analyzers; and the resulting current-potential Bond, A. M.; Grabaric, A. S.Anal. Chim. Acta 1877, 88,227-236. information was sampled by a TRS-80 microcomputer via the Edmonds, T. E. Anal. Chim. Acta 1878, 108,155-160. Barrett, P.; Davidowski, L. J.: Copeland, T. R. Anal. Chim. Acta 1080, modular interface described in the Experimental Section. 122,67-73. Figure 5 illustrates a typical differential pulse voltammogram Skov, H. J.; Kryger, L. Anal. Chim. Acta 1880, 122,179-191. obtained by using this approach and subsequently output for Bond, A. M.; Norris, A. Anal. Chem. 1080, 52,367-371. Dayton, M. A,; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. display on an X-Y recorder. The voltammograms were taken Chem. 1080, 5.2, 946-950. under standard experimental conditions for solutions conCheng, H. Y.; White, W.; Adams, R. N. Anal. Chem. 1880, 52, 2445-2440. taining the model redox compounds ferrocene and ferroce-
Anal. Chern. 1982, 5 4 , 1015-1017
(8) Anderson, J. E : Bagchi, R. N.; Bond, A. M.; Breenhiii, H. B.; Hender-
son, T. L. E.; Walter, F. L. Am. Lab. (Fairfield, Conn.) 1981 (Feb),
21-32. (9) von Wandruszka, R. M. A.; Maraschln, M. Anal. Lett. 1981, 74, 463-478. (lo) Anderson, J. E:.: Bond, A. M. Anal. Chem. 1981, 5 3 , 1394-1398. (11) Woodard, F. E ; Woodward, W. S.; Rellley, C. N. Anal. Chem. 1981, 53, 1251 A-1266 A. (12) "TRS-BO Model I l l Operation and BASIC Language Reference Manual"; Radio Shack: Ft. Worth, TX, 1980.
10'15
(13) Titus, J. A. "TRS-80 Interfacing"; Howard W. Sams & Co., Inc.: Indianapolis, IN, 1979;Book 1, Chapters 1-3.
RECEIVED for review October
16, 1981. Accepted February 8, 1982. This work was supported by the National Science Foundation Grant 77-06911 and by the School of the University of Louisville.
Chemililminescence Measurement of Atmospheric Ozone with an Oil-Coated Paper Filter Fumltake Chisalka" and Shigeru Yanagihara Mechanical Engineering Laboratory, 1-2 Namiki, Sakura-rnura, Niiharl-gun, Ibaraki, 305, Japan
Sensitive chemiluminescence techniques have recently been developed to monitor ambient ozone concentrations. The Nederbragt et al. technique (1) for ozone monitoring is based upon the chemiluminescence light produced in the reaction between ozone and ethylene. Ethylene is combustible and toxic. In addition, it polymerizes in the gas flow system, thus causing difficultiles with flow meters, regulators, and needle valves. As air filters resolve ozone molecules, a useful UV absorption ozone monitor is disturbed by fine paticulates in air. An ozone detector using chemiluminescence from the reaction of ozone with polymers such as poly(tetraflor0ethylene) and polyethylene has also been developed by Neti et al. (2),but this device has not been tested as an urban ozone monitor. As a very useful and sensitive tecnique, the chemiluminescent reaction between peroxyacetyl nitrate (PAN) and ozone with trieth!ylamine vapor, developed by Pitts et al. (3), can be used to monitor these atmospheric concentrations. Our supplementary examination of the method has been carried out by using a common cellulosic paper fiiter impregnated with liquid triethylamine. Our reexamination of the method with particular attention being paid to the stability of the vaporization of triethylamine led to our discovery of chemiluminescence in the reaction of ozone with oil-coated paper filters. This reactive paper filter is prepared by impregnating common cellulosic paper filter with a common lubrication oil. As an application of this phenomenon, a prototype atmospheric ozone monitor has been constructed and tested. Only cellulosic paper filter shows the effect, and the light which occurs on the surface of the reactive paper filter has not yet been phytiically accounted for. But, we have found that the results of some measurements on the characteristics of the chemilumiinescent light are as follows.
EXPERIMENTAL SECTION Apparatus. An experimental counting system for the light emission from the ozone-oiled paper filter reaction has been constructed that allows for convenient change in experimental conditions such as a sample gas flow rate, reaction pressure, and the exchange of suitable light filters. A schematic view of the experimental apparatus is shown in Figure 1. A sample gas to be monitored enter13 a reaction vessel (101 mm i.d., 1.3 L volume, stainless steel) through a sampling pipe (10 mm o.d., 8 mm id., curved stainless steel pipe 200 mm long for blocking room light, and Teflon pipe tubing) and a glass nozzle. Ozone molecules in the sample gas colllide with the reactive oiled paper filter, impregnated with 1 rnL of a common lubrication oil, and a chemiluminescence reaction takes place. The curved head of the nozzle is perpendicularly centered above the paper filter. The sample flow is maintained Iby a vacuum pump and regulated by a valve. The light is emitted through an Pyrex glass window and a light
filter and detected by EL photomultiplier tube (PMT, Hamamatsu TV R464, -1000 V). A thermoelectric cooler at -20 "C was used to stabilize the PMT dark count. The signal is fed into a photon counter device. Procedure. To being an experiment, we secured the reactive paper fiiter to a filter mount and inserted the nozzle into an O-ring fitting connector. The characteristics of used cellulosic paper filter were double acid wash grade No. 2 in JIS standard, 90 mm in diameter, 0.26 mm in thickness, and 115 g/m2 in weight, made by Toyo Roshi Corp. The pressure in the vessel and sample gas flow rate are typically 460 torr and 1.2 L/min, respectively. All experiments were performed at room temperature 21 & 0.5 OC. Blank values for the photon counting were obtained by using synthetic dry air (02/N2= 1/4). As samples of known concentration, the dry air was ozonized (maximum 0.2 ppm (v/v)) by a Bebdix photolytic ozonizer, and concentrations of ozone were measured by a conventional ozone monitor (Model OX-21, Kyoto Denshi Corp., chemiluminescent method using ethylene). For most experiments, photon counts were accumulated for 10 ti.
RESULTS AND DISCUSSION Emission Spectrum. The corrected spectrum of the light from the reactive filter impregrated with spindle oil (Instrument Oil) was obtained in the range from 300 to 600 nm. The relative intensity distribution of the spectrum is shown in Figure 2. The intensity was measured with five light filtem (UV-D2, KL-400, KL-450, KP-500, and KL-550; made b y Toshiba). The corrected relative spectrum Ir, of the light for wavelength =400 nm can be obtained by using three relativle factors Pr,, Frx, and Crf,: Irx = CrfxPr,/Frx (relative value for 400 nm), where Pr, is the relative response of the phototube, Fr, is the relative integrated transmission ratioed to the bandwidth of each light filter, and Crf, is the relative intensity of photon counts obtained for each light filter, respectively. The spectrum thus obtained has two peaks near 400 and 500 nm. The sensitivities to ozone for no filter and three filters UV-D2, Y-44 (Toshiba) and KL-400 are 136.9,25.2, 99.7, and 15.0 s-l-ppm-', respectively. Effect of Sample Flow Rate and Reaction Vessel Pressure. The sample gas supply and total pressure in the reaction vessel are variable quantities and depend in large measure on the shape of the glass nozzle head. The absence of an appreciable change in these quantities is a necessary prerequisite for a uniform light intensity in the vessel and a linear response from the detector. Four glass nozzles were prepared by drawing a heated glass pipe (4 mm i.d., 6 mm 0.d.). For each nozzle, experiments were run in triplicate to demonstrate the feasibility of a nozzle such that the response was not measurably affected by very small changes in the relative position of the nozzle with respect to the filter paper.
0003-2700/82/0354-1015$01.25/00 1982 American Chemical Society
