ANALYTICAL CHEMISTRY, VOL. 50, NO 9, AUGUST 1978 (3) C. W. Spicer, "The Fate of Nitrogen Oxides in the Atmosphere", in Adv. Environ. Sci. Techno/., 7 , 163-261 (1977). (4) C. W. Spicer, G. F. Ward, and B. J. Gay, Jr., "A Further Evaluation of Microcoulometry for Atmospheric Nitric Acid Monitoring", Anal. Len., I 1 (1) (1978). (5) C. W. Spicer, "Photod-emical Atmospheric Pollutants Derived from Nbcgen Oxides", Atm. Environ., 11, 1089 (1977). (6) C. W. Spicer, P. M. Schumacher, and D. F. Miller, "A Integrated Method for Sampling Atmospheric Nltric Acid", manuscript in preparation, January 1978. (7) T. Okita, S. Morimoto, S. Izowa, and S.Kouno, "The Measurement of Gaseous and Particulate Nitrates in the Atmosphere, Arm. Environ., 10, 1085 (1976).
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(8) A. L. Lazarus and B. W. Gandrud, "Progress Report on Distribution of Stratospheric Nitric Acid", Proc. Third CIAP Conf., 161 (1974). (9) Mast Development Co. product brochure, Mast Development Co., Davenport, Iowa, 1968. (10) B. Appel and J. Tokiwa, AIHL, personal communication, January 1978. (1 1) C. W. Spicer, P. M. Schumacher, J. A. Kouyoumjian, and D. W. Joseph, "The Collection and Analysis of Atmospheric Particulate Nitrate", Battelle-Columbus final report to EPA (Contract No. 68-02-2213), January 1978.
RECEIVED for review February 13, 1978. Accepted April 12, 1978.
Switchboard for Experimental and Computer Networking Steven B. Schram and David H. Freeman* Department of Chemistry, University of Maryland, College Park, Maryland 20742
The use of computer compatible equipment in analytical laboratories is becoming very prevalent. Laboratory experiments and computer components are usually coupled together using teletype-compatible communications links. This technique is limited if more than two devices are to be coupled in parallel. In that case, it is advantageous to connect them in a network that is conducive for full automation, so that data input, analysis, control, and output can be performed automatically. This process would tend to be viewed as data bus architecture in terms of computer engineering, while the logic of component interconnection is of principal concern to the flow of analytical data. When the latter becomes complicated, the use of switchboard networks become advantageous. Such networks have been described in the analytical and engineering literature (1-6). For example, a recent article by Dessy (7) discusses many of the options that are available. In this article, we will present one possible solution to the problem of connecting serial communications devices together in a versatile fashion. This is a real problem, and although this solution is not the only one possible (and indeed it may not be useful in some circumstances), this represents a simple, strictly mechanical approach that can be easily implemented by any interested party. The problem can be approached quite simply in the following way. Teletype compatible devices send on one wire, and receive on another. The signals are encoded in current (20-mA current loop) or voltage (RS-232 in USA; CCITT in Europe) changes. Current devices appear less frequently in newer equipment, and these can be modified to the voltage format using simple transistor circuiting (8,9). Voltage signals are easily networked provided that the transmission characteristics are standardized between all of the connected devices. Standardization between devices must include the transmission rate (Baud rate) and the transmission language. The usual languages are ASCII (American Standard Code for Information Interchange), BAUDOT (similar to ASCII), or EBCDIC (Extended Binary Coded Decimal Interchange Code). After the devices are rate and code compatible, network interfacing can be achieved in a convenient way. The most frequently encountered serial connector is the standardized 25-pin EIA-RS-232 interface (Cannon DP-25, M or F) as shown in Figure 1. Figure 2 diagrams the closed loop that is formed if this connector is used to link two devices. Note that the two devices can communicate only with each other. Although 25-pin connections are involved, it should be noted that only three of the connections are actually used. 0003-2700/78/0350-1403$01 . O O / O
Figure 1. Picture o f EIA-RS-232 interface plug SEN2
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3
RECEIVE
UIWICOMPUTER GROUND
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Figure 2. The standard connector allows t w o devices t o communicate with e a c h other
EXPERIMENT
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MICROPROCESSOR
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MINICOMPUTER
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PRINTER
1
Figure 3. The modified connector allows a device t o receive from one device, and transmit to another. The arrow (+) refers to the direction of signal transmission. The ground connection is not s h o w n
I t may be desirable to link several devices serially, as shown in Figure 3. However, this requires that the connector be modified to carry out this initial use of a network arrangement. We have modified our connectors by attaching a mating plug that has been altered by shorting pins 5, 6, 8, and 20 together. This process provides pins 5 , 6, and 8 (clear to send; data set ready; carrier detect) with a positive voltage from pin 20 (data terminal "ready"); this is normally provided by the conventional connection. Information flow depends on two other data pins plus a ground connection pin: pin 2 is used for received data; pin 3 for transmitted data, and pin 7 is the ground. Broad flexibility can now be achieved if wires from these three pins are brought from each mating plug, one for 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
DEVICE
*
MICROPROCESSOR
PLOTTER
I I
I 1
P I N NUMBERS’
’
5
‘
8
7
2
7 2
3
7
1
2
3
communication
Figure 4. Plug lines. Pin 7 is the common ground EXPT/
MICRO-
RINI-
PLOTTER
PROCESSOR
COMPUTER
KEYBOARD/ PRINTER
EXPT/ PLOTTER
MODEM
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Figure 5. The lines signify jumper cables. The direction of data is indicated by the arrow. Data comes from the experiment, to the microprocessor, from the microprocessor to the minicomputer, and from the minicomputer to the printer
each device, to a n interface switchboard. In the switchboard, all grounds from pins 7 are tied together. Paired input-output sets of banana jack receptacles, one pair for each device, are located on the switchboard panel. Lines from pins 2 connect to one side of the receptacles while lines from pins 3 connect t o the other side (Figure 4). A short jumper cable can now be used to carry an input signal to one or more of the desired output terminals. To illustrate this, the configuration in Figure 3 is obtained using the switchboard as shown in Figure 5 . Other configurations are easily visualized. Figure 6 shows how a Texas Instruments printer can sequentially monitor the output of two different devices. Note that no buffering or handshaking capability is included, which means that if there is a lack of suitable control or priority, then there could be a mixture of data and subsequent jumble of output. In this case, our Wang 2200 minicomputer output is sent simultaneously t o our Technic0 9900 microprocessor and printer. Since the microprocessor does not respond until the entire instruction is received (and hence completely printed), the response to the printer is not mixed, and is printed separate from the original instruction. Thus, provided that only one signal, a t one time, is sent to the printer, any number of output devices can be monitored. Since this operation implies a logical “or”, all devices must have a reasonably high output impedance (most do) for this to work as described. Any
RICROPROCESSOR
MINICOPPUTER
KEYBOARD/ PRINTER
MODEM
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Figure 6. The printer sequentially monitors instructions sent by the minicomputer to the microprocessor, and the microprocessor’s response to the instruction EXPT/ PLOTTER
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MICROPROCESSOR
MINICOMPOTLR
KEYBOARD/
PRINTER
MODEM
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Figure 7. Experiment data are sent to the microprocessor and minicomputer. The microprocessor sends averaged data to a plotter. The minicomputer sends reduced data to a modem and a printer. The modem sends it to another computer. The returned data is received by the modem and also sent to the printer
other RS-232 serially interfaced output device, (plotter, oscilloscope) could also be used instead of the printer. Various combinations of signal transfers can be achieved. We have used the switchboard to provide the configuration shown in Figure 7 . This configuration is not possible with a n unmodified connector. Use of the modification with switchboard provides both reliability and flexibility. Since all signals are present a t one location, design changes are easy to adapt. New configurations are easy to test and implement. This represents a great savings in time, and offers the user expanded versatility with regard to signal processing.
ANALYTICAL CHEMISTRY, VOL. 50, NO 9, AUGUST 1978
LITERATURE CITED (1) (2) (3) (4) (5) (6)
R. E. Dessy and J. A. Titus, Anal. Chem., 45, 124A (1973). T. J. Williams, Chimia, 27, 669 (1973). R. E. Dessy and J. A. Titus, Anal. Chem., 46, 291A (1974). P. Lykos, Ed., ACS Symposium Series, Voi. 19 (1975). J. M. McQuiiian and D. C. Waiden, Comput. Networks, 1 (5) 1 (1977). W. Greene and U. W. Pooch, Computer, 10 (11) 12 (1977).
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(7) R. E. Dessy and J. A. Titus, Anal. Chem., 49, llOOA (1977). (8) H. V. Maimstadt, C. G. Enke. and S. R. Crouch, "Electronic Measurements for Scientists", W. A. Benjamin Inc., Menio Park, Calif., 1974, pp 226-262. (9) D. G. Larson, P. R. Rony, and J. A. Titus, Am. Lab., 7 (6), 76 (1975).
for review February
Accepted
279
24~
1978.
Variable-Temperature Cryogenic Trap for the Separation of Gas Mixtures David J. Des Marais Extraterrestrial Biology Division, Ames Research Center, NASA, Moffett Field, California 94035
Cryogenic traps offer a clean, simple, and relatively rapid means for resolving simple gas mixtures. This report describes a continuously variable-temperature cold trap which can both purify vacuum-line combustion products for subsequent stable isotopic analysis and isolate t h e methane and ethane constituents of natural gases. Murakami and Okamoto ( I ) , Crosmer e t al. ( Z ) , and Stump and Frazer ( 3 )have previously described variable-temperature cold trap designs for specific applications. I feel the trap described here embodies all the important capabilities of these former designs, yet is generally easier and less expensive t o build and operate.
EXPERIMENTAL Figure 1 is a schematic drawing of the cold trap. The trap is U-shaped and consists of two 0.635-cm (0.25-in.)diameter by 8-cm long pieces of stainless steel tubing (C) welded to a 30-cm segment of Cajon flexible tubing (G)(part no. 321-4-X-12, Cajon Company. 32550 Old South Miles Road, Cleveland, Ohio 44139). A 20-cm long, welded stainless steel canister (D) surrounds the lower portion of the trap, generating a 3-mm annular air space between the canister and the flexible tubing. Asbestos-insulated 25-gauge chrome1 A resistance wire (E) is wrapped around the flexible tubing and brazed to Fiberglas-insulated copper wires which emerge from the enclosure via a 0.635-cm (0.25-in.) stainless steel tube (F) and a vacuum epoxy feedthrough (B). A 30-gauge chromel-alumel thermocouple (J)is brazed to the bottom of the U-trap, and the wire leads also emerge from the housing via the steel tube (F) and the feedthrough (B). During use, the canister is almost totally immersed in liquid nitrogen. A rubber stopper (A) is inserted in the aperture near the vacuum feedthrough (B) to prevent the condensation of liquid oxygen inside the enclosure. The stopper is preferred to a more permanent closure for reasons of safety. Should a leak permit condensation to occur inside the canister, the stoppered aperture would serve as a pressure release valve for the confined gas. In operation, a gas mixture to be resolved enters the evacuated U-trap at room temperature. The trap canister is then almost totally immersed in a liquid-nitrogen bath, cooling the U-trap as heat flows across the 3-mm annular air space between the flexible tubing and the canister wall. As the trap cools, the gas mixture components condense sequentially according to their relative vapor pressures. As Table I shows, about 12 min is required for the bottom of the U-trap to attain liquid-nitrogen temperature. Measurements made using traps with additional thermocouples located at positions G and H (Figure 1)describe a thermal gradient along the length of the trap. This gradient reflects the steady-state balance between heat flow from the flexible tube t o the canister, and heat flow into the tube from the warmer upper portions of the trap assembly. As discussed by Crosmer et al. ( 2 ) ,such a thermal gradient promotes the fractionation of gas mixture components. After the bottom of the trap (J) has attained liquid-nitrogen temperature, the passage of current through the resistance wire (E) warms the U-trap and permits the distillation of successive gas components at trap temperatures optimal for their resolution. Table I lists the power requirements as a function of temperature for a properly constructed trap. The consumption
Table I. Variable-Temperature Trap Thermal Behavior and Power Consumption time (min)
after temp. ("C) at location ___ LN, G H J immersion
power input
(W 0 0 0
25 a
0 0 0 1
a
2 4 8 12
25 a a a a
a
a
163 - 153 - 137 - 103 - 40 +9
--
190 - 167 - 153 - 125 - 78 -- 38 -
-
25 50
- 108 - 160 - 190
195 - 175 - 165 - 145 --llO - 75 -
0 1 2 4 8
12 b b b b b
Not measured. The trap usually requires about 1 rnin t o increase its temperature 30 'C and attain a new steady-state thermal profile. rate of liquid nitrogen is typically low, even at "warmer" trap temperatures. For example, when trap thermocouple J is at -110 "C, 8 W of heater power is being balanced by a liquid-nitrogen evaporation rate of approximately 4 mI,/min.
RESULTS AND DISCUSSION The ability of the variable-temperature trap to separate gas mixtures is shown in Table 11. The first mixture represents typical vacuum-line combustion products of geochemical samples such as rocks or marine sediments. Separation of the individual gases of this mixture by the cold trap, facilitates the measurement of their abundance and stable isotopic compositions. COz and SO2 aliquots of known isotopic composition were used in this demonstration. T h e gas fractions distilled from the trap were quantified using a mercury manometer, and subsequently analyzed for their purity and isotopic composition using a Nuclide 6-60 RMS mass spectrometer. The gas mixture was condensed in the variable-temperature trap in the manner described earlier. The trap was then warmed to -143 "C and the volatilized C 0 2 was distilled for 5 min from the variable-temperature trap to a liquid-nitrogen trap incorporated into the mercury manometer. Despite the substantially greater abundance of SOz in the original gas mixture, the COZ recovered by this distillation was more than 99% pure, and had sustained little isotopic fractionation. The t r a p was warmed to -85 "C and the SO, was distilled for 10 min into the mercury manometer. The SOz fraction also exhibited a very high purity and had sustained negligible isotopic fractionation. T h e variabletemperature trap was then warmed to room temperature to recover the HzO. The somewhat lower recovery achieved
This article not subject to U.S. Copyright. Published 1978 by the American Chemical Society
