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Anal. Chem. 1904, 56,2995-2997
Diode-Array Centroid Monitor for Rapid Plasma Stabillty Measurement James A. Stewart’ and Alexander Scheeline* School of Chemical Sciences, University of Illinois, 1209 West California Avenue, Urbana, Illinois 61801 Positional stability has been shown to be an important parameter in determining analytical performance of the high-voltage spark discharge (1-7). Real-time monitoring of spark positional stability is thus a useful tool in quality control of spectrochemical analyses. A recent report (8)included a description of a diode-array observation approach for measuring the position of a spark on an electrode. This consisted of a diode array which was read out to a digital oscilloscope between successive discharges. The raw data were then processed by the oscilloscope to determine the position of the center of the discharge. Only four successive sparks could be monitored due to limited storage in the oscilloscope memory. The goal of the work reported here was to build a device which could monitor the position of every spark in a train. As typical determination procedures may involve thousands of sparks, it became quickly evident that some data reduction would be necessary before the storage of individual spark positional data. Without some compression, the data from the 256 element diode array employed would quickly fill all 64 kbytes of memory available on our LSI-11 computer. The hardware reported requires digitization of only two numbers per spark and has been successfully used to observe a train of 2000 sparks. The duty cycle was sufficiently low (no more than 720 digitizations per second) that the computer could have performed other tasks simultaneously.
EXPERIMENTAL SECTION The principle upon which the diode-array centroid monitor works is expressed in eq 1. This is derivable from statistical 266
C IiXi
x- = in0 -266
C Ii
i-0
procedures for computing expectation values of a variable (here x ) weighted by a distribution function (here I). The sum of the product of intensity with the position at each diode is divided by the total intensity to give the mean position. I refers to intensity, x refers to position, and the subscript i refers to a particular diode (numbered 0 to 255 for the array employed). Since the position can only be found in discrete increments, it is sufficient to record the position as an integer representing the diode number along the array. Because the diodes are on 25-pm centers, absolute position can be recovered by scaling the centroid position given as a diode number, multiplying by the separation of the individual diodes, and dividing by the magnification of the optical system imaging the spark being observed onto the array. Here, a glass lens with f = 176 mm, f / k 2.8 was employed with a magnification ranging from 1.68 to 3.28 (the array viewed a region with a width of 1.95-3.81 mm, selected as a function of the amount of wander). The cathode space charge region was focused onto the 26-pm aperture height of the array. Olesik and Walters (8) demonstrated that this arrangement results in monitoring of the cathode spot position. A block diagram of the electronics appears in Figure 1, and a timing diagram appears in Figure 2. Wiring details are contained in Figures 3 and 4. In Figure 1,the digital delay generator (1) (California Avionics Inc., Palo Alto, CA) triggers both the spark source (2) (9) and centroid monitor at fixed intervals. A threephase dc power supply (not shown) was employed to charge the source’s capacitor (IO). Photodetection was with a Reticon RL256G diode array and RC301 controller (3) (EG&G-Reticon, Current address: Crime Laboratory, Texas Department of Public Safety, P.O. Box 4143, Austin, TX 78765.
Table I. Pulse Widths of Monostables monostable 1 2
3
pulse width and range 102.0 f 0.3 ps 1.203 f 0.003 ws 640 i= 8 ns
monostable 4 5
6
pulse width and range 442 f 3 ns 158.5 f 1.6 ns 5.67 f 0.16 ps
Sunnyvale, CA). Timing electronics (4) consist of monostable multivibrators to generate necessary timing pulses, assorted TTL gates to sequence those pulses, and a binary counter to generate the integer values corresponding to x+ Additional details are given below. As the array output is pulsed, a sample and hold circuit ( 5 ) is employed for pixel acquisition. A multiplying digital-toanalogue converter (6) with a 1-MHz analogue bandwidth is used to compute the product of I,and x,. After the diode-array readout commences, the first clock pulse does not result in video output containing any optical information, so the logic starts the multiplication and accumulation process on the second clock pulse after readout starts. The summations given in eq 1are conducted with gated integrators (7 and 8). For each readout cycle, only two numbers must be digitized, corresponding to the numerator and denominator of eq 1,respectively. A multiplexer (9) selects between the numerator sum (multiplexer input A) and denominator sum (multiplexer input B) of eq 1 and transmits the appropriate data to a 12-bit analogue-to-digital converter (10) (Data Translation Inc., Marlboro, MA). During a train of sparks, the LSI-11 computer (11)(Digital Equipment Corp., Maynard, MA, and Scientific Microsystems Inc., Mt. View, CA) records pairs of summation data for each spark. As data were collected for a train of sparks running at 360 Hz, the data digitization rate was only 720 Hz, which could be handled by many less sophisticated microprocessors. Due to memory limitations,dak were collected for bursts of 2048 sparks. Data collection for each spark was initiated by a pulse from the digital delay generator. This triggered the spark and cleared the gated integrators (reset pulse in Figure 2). Following completion of the spark (typicallyafter 100 ps), readout of the array w a initiated. ~ The RC301 controller was modified to allow external triggering per the manufacturer’s instructions. Also, a 10-kQtrim pot was added in parallel to the zero adjust pot provided on the RC301 to improve the resolution of zero trimming. Digitization began 4 ps after array readout was completed. Data collection was sufficiently rapid to have allowed the stability of 2-kHz trains to be monitored, although this was not actually attempted. Timing and control pulse generation hardware is shown in Figure 3, with pulse widths of the various monostables listed in Table I. A NOR buffer driver (74LS128), not shown in Figure 3, was placed adjacent to the array and controller board to transmit clock pulses from the 1-MHz clock provided on the array controller to the electronics shown in the figure. This prevented loading of the clock circuit. Trigger 1was connected to the digital delay generator output; trigger 2 could be strobed by the computer. Figure 4 shows analogue electrical circuitry, with specific values of components listed in Table 11. Operational amplifiers were balanced so that when a darkened array was read out, both sum I and sum Zx outputs were nulled.
RESULTS AND DISCUSSION Initial operation revealed substantial interference with the raw video signal due to 60-Hz fluctuations in amplifier gain on the RC301 card. With shielding of the power supply and transmission cables, capacitive decoupling of the supplies, and careful routing of wires, 60-Hz fluctuations were eventually reduced to 1-mV p-p. This is sufficiently low to produce negligible positional errors. Radio frequency noise produced by the spark was not a problem for analogue signals since
0003-2700/84/0356-2995$01.50/00 1984 American Chemlcai Society
2096
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 VIDEO
Overflow
w GATED INTEGRATOR
Flgure 1. Block dmgram of video centroid monitor. Start pulse initiates the readout of the array data. Output of timing controls to gated integrators consists of gate and reset signals. Further details in the text.
-Timing Diagram
DDG Reset Spark Current
A
~
Figure 4. Analogue signal processing circuit diagram. For details, see Table 11.
~
Strobe 3-IOms
SPS
Flgure 2. Timing diagram for main sequencing pulses. 0
RST/
-3
3
0 POSITION
(mm)
Flgure 5. Histogram of spark posltlon data for a train of 2048 sparks. The Inset is an iterate map of successive positions, demonstrating stochastic wander behavior of the discharges ( 11). Argon flow rate is 59 mL s-‘.
m 5
Flgure 3. Digital timing logic. All components are 74LS series gates.
Time delays for monostables are listed in Table I. A 74LS128 is interposed between the RC301 clock and the clock Input NAND gate as a line driver.
Table 11. Components for Centroid Monitor
item
description
PMI OP-27 CJ operational amplifier Intersil SHM-BE sample and hold module PMI OP-16 GJ operational amplifier OA3-6 OA7 TI uA741CP operational amplifier MDAC PMI DAC-08CQ multiplying digital-to-analogue convertor; input and reference resistors 4.99 kQ, 1% analog switches intersil IH5041CPE dual SPST FET switches R1 10 kQ RZ 5 kQ R3 4.1 kQ R4 512 Q , 1% RS 2.2 kQ R6 18 kil C1 0.10 p F OAl
S/H
CZ
0.047pF
readout occurred during interspark intervals. False triggering of digital signals was a problem but was overcome by both capacitive decoupling of signal lines and judicious use of software to recognize and ignore rf-generated trigger pulses. Such pulses could be detected either by their effect on various computer status bits or by comparison of digitized data values to the range producible by the array and properly functioning electronics. Any values leading to negative computed centroid positions were clearly erroneous. Calibration was performed by illuminating the array with a helium-neon laser. By iterative adjustment of the position of the array using an oscilloscope to monitor the video signal, the laser beam was centered on diode 128. Upon digitization, it waa found that the ratio in eq 1was 0.3406 f 0.0022. Thus, calibration was approximately f1 diode. Similarly, readout was linear with a zero-intercept f l diode. Application to measurement of spark positional instability is demonstrated in Figure 5. For the combination of current wave form and electrodes employed (40peak A, unidirectional, brass planar cathode), optimal stability occurs for an argon flow rate of 9.5 mL s-l. For the data reported in the figure, the flow rate was 59 mL s-l. The plot is a diode-by-diode histogram (rescaled to the projected size of the array at the spark discharge plane) of the probability of the spark anchoring at a given position. While most of the sparks anchor within a region 1mm wide slightly to the left of the alignment axis, some sparks anchored over a range of 3 mm. Of all the sparks 91% had centroid locations to the left of 0.5 mm, with the remaining 9% to the right. The value 0.5 mm waachosen as a demarcation as being halfway between the two main peaks
2997
Anal. Chem. 1984, 56,2997-2999
in the distribution. The inset in Figure 5 is an oscilloscope picture of an iterated map (11) of the position of spark N (horizontal axis) vs. the position of spark N + 1(vertical axis) in the train, where N runs from 1to 2047. The diffuse group of points in the upper right of the inset corresponds to those instances in which two successive sparks were centered in the right lobe of the distribution. Eighteen such points can be counted. For purely stochastic behavior, in which each spark attacks the electrode independently, the probability of having two successive sparks in the right-hand lobe should be the square of the probability of any spark being so located. One would thus predict that 17 pairs of sparks would sample the right lobe sequentially. Thus, it is demonstrated in this instance that wander is random within the f l limit inherent in asynchronous counting measurements. The data are consistent with independence of each spark in attacking the electrode. While similar measuremenb have been made for different gas flow rates, they have not yet been attempted for different spark repetition rates. Were two centroid-monitor assemblies available, they could be arranged to view the discharge at a 90° separation, allowing for the position of the cathode spot of each spark to be located in a Cartesian plane. Such an arrangment would be useful for surface mapping of electrode inhomogeneity (8) in situations where the desired discharge positional stability could not be achieved.
positional stability of a spark or arc during routine analyses. By condensing raw video data before digitizing for computer storage, positional data for entire trains of sparks can be realistically stored.
CONCLUSION A simple device has been demonstrated which can compute the centroid position of a non-point-light source or shadow. An example application has been given where the positional stability of a train of spark discharges has been monitored. The device could be employed for on-line monitoring of the
RECEIVED for review June 11,1984. Accepted August 27,1984. This work was supported by the National Science Foundation (Grant CHE-81-21809). Portions of this work were presented at the 10th Annual Federation for Analytical Chemistry and Spectroscopy Societies Meeting, Philadelphia, PA, September, 1983.
Note Added in Proof. Similar hardware has recently been reported (12). ACKNOWLEDGMENT Engineering advice by C. Hawley and software assistance by M. A. Lovik are appreciated. LITERATURE CITED Sacks, R. D.; Walters, J. P. Anal. Chem. 1970, 42, 61-84. Waiters, J. P.; Goldstein, S. A. ASTM Spec. Tech. Publ. 1973, 540, 45-7 1.
Walters, J. P.; Eaton, W.
S. Anal. Chem. 1983, 55, 57-64.
Barnhart, S. G. Ph.D. Thesis, University of Wisconsin-Madison, 1983. Washburn, D. N.; Walters, J. P. Appl. Spectrosc. 1982, 36, 510-519. Ekimoff, D.; Walters, J. P. Anal. Chem. 1981, 53, 1644-1655. Walters, J. P.; Goldstein, S. A. Spechochlm. Acta, Part 8 1984, 398, 693-728.
Oiesik, J.
W.; Walters, J. P. Appl. Spectrosc. 1983, 37, 105-119. Tran, T. V.; Scheeline, A. Appl. Spectrosc. 1981, 35, 536-540. Coleman, D. M.; Walters, J. P.; Watters, R. W. Spectrochim. Acta, Part B 1977, 328, 287-304. Hardas, B. R.; Scheeiine, A. Anal. Chem. 1984, 56, 169-175. Bertani, D.; Cetlca, M.; Ciliberto, S.; Franclni, F. Rev. Scl. Instrum.
1884, 55, 1270-1272.
Interface for the Direct Coupling of a Second Gas Chromatograph to a Gas Chromatograph/Mass Spectrometer for Use with a Fused Silica Capillary Column James F. Pankow* and Lome M. Isabelle Department of Chemical, Biological, and Environmental Sciences, Oregon Graduate Center, 19600 N .W. Walker Road, Beauerton, Oregon 97006 For a laboratory which employs a gas chromatograph/mass spectrometer/data system (GC/MS/DS), many days of expensive instrument time can be lost each month while reconfiguring the GC for the differenttypes of analyses performed. Such reconfigurations can take the form of changing columns, mounting other equipment on the GC (e.g., a purge and trap apparatus), etc. The time lost during these equipment changeovers can be costly to research and service analytical laboratories alike. If two GCs were interfaced to the same MS in a given GC/MS/DS system, then such reconfigurations could take place on one GC while analyses were proceeding and being completed on the other. Thereafter, the next type of analysis could begin with minimal delay. Such thoughts have led Kalman (1) to interface a second GC to a Finnigan 4000 GC/MS/DS. The interface was heated resistively by using an electrical current. This interfacing was facilitated by the presence of a normally unused flange on the right-hand side of the Finnigan 4000 MS source manifold. In its standard configuration, the flange’s only purposes are to (1) allow the introduction of MS calibration gas and (2) provide a measurement port for the pirani gauge which senses the MS source pressure. This paper describes a GC/MS interface that is similar in function but fundamentally different in design. It was constructed primarily of copper. This metal was selected due to its high thermal conductivity. External to the MS, heating 0003-2700/84/0356-2997$01.50/0
of the interface is provided with cartridge heaters. Heat is transmitted to the portion of the interface which is internal to the MS by conduction down the axis of the interface. Conductive heating has been used rather than direct electrical resistive heating (1) or circulating hot oil heating (2) due to its greater simplicity. In this manner, the difficult-to-access portion of the interface inside of the MS is heated from an external source. The interface has been designed for use primarily with fused silica capillary columns in a directly coupled mode. It has been built into the right-hand side manifold flange of a Finnigan 4000 GC/MS/DS, and it allows the continued introduction of MS calibration gas from that side of the MS source. In this paper, the term “GC-1” will refer to the original GC fitted to the GC/MS/DS; the term “GC-2”will refer to the retrofit GC connected to the auxiliary GC/MS interface to be described.
EXPERIMENTAL SECTION The interface is presented in Figures 1 and 2. Its positional relationship to the MS source pirani gauge and the calibration gas inlet is shown in block diagram form in Figure 2. It was built around a 19.1 cm long copper tube with an 0.d. of 0.295 cm and an i.d. of 0.0572 cm. The tube was constructed from a 19.1 cm long piece of 0.159 cm (0.0625 in.) o.d., 0.0572 cm (0.0225 in.) i.d. copper tubing and a 19.1 cm long piece of standard 0.318 cm (0.125 in.) o.d., 0.173 cm (0.068 in.) i.d. copper tubing. The smaller 0.d. tube was inserted into the larger 0.d. tube, and the ends were 0 1984 American Chemical Society
