1899
Anal. Chem. 1982, 5 4 , 1899-1901
e-
nique, for example, agree within k25% of the values obtained by volumetric dilution. No unusual surface effects have been observed with the Au/A1 source. Very sensitive studies of surface catalyzed radical combination reactions under CI conditions showed the Au/A1 surface to be somewhat less reactive than a stainless steel surface ( 4 ) .
LITERATURE CITED a
b
c
d
Figure 1. Cut away view of stacked ring source construction: (a) back plate; (b) Teflon spacer; (c) source ring; (d) iton exit plate.
times calculated from literature values of the ionic mobility (3) are in reasonable agreement with previously reported range of values. Alternatively, one may use the known rate constants and drift times to calculate the concentration of samples in the source without specific calibration factors. Estimates of small concentrations of ethane in methane using this tech-
(1) Price, P.; Swofford, H.; Buttrili, S. Anal. Chem. 1977, 4 9 , 1497-1500. (2) Hatch, F.; Munson, B. J . Phys. Chem. 1978, 82, 2362-2369. (3) Ridge, D. P.; Beauchamp, J. L. J . Chem. Phys. 1976, 6 4 , 2735-2746. (4) Biom, K.; Munson, B. Int. J . Mass Spectrom, Ion Phys., in press.
RECEIVED for review March 26,1982. Accepted June 3,1982. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Purchase of the vacuum deposition equipment was made possible by a grant to HSG from the University of Delaware Research Foundation.
Autozeroing Microcomputerized Boxcar Integrator T. D. L. Pearson, J. N Demas," and Scott Davis Department of Chemistty, University of Virghia, Charlottesville, Virginia 2290 1
Transient recorderia and signal averagers are frequently indispensable data acquisition tools. For slow signals there are many excellent commercial recorders and averagers. For faster signals (
Figure 1. Schematlc diagram of the autozeroing boxcar integrator as used in a nanosecond luminescence decay time instrument: M, monochromator; P1, slgnal photomultiplier (PMT); P2, trigger PMT; TG, trigger’generator;S,sampling oscilloscope; SC, signal conditioner;CI, computer interface. Dashed lines indicate the laser beam. Solid lines indicate electrical connections. corresponds to the instrument base line. (4) This base line point is subtracted from the measured point to give a base line corrected value. Since the base line is collected within a few milliseconds of the transient, slow base line drift does not affect the results. The base line can be collected a number of times and averaged for each datum point collected making base line noise suppression faster than if only one base line point was collected per transient. This scheme only corrects for instrumental base line drift and not for systematic bias as a function of time such as might arise from a luminescent solvent blank or electromagnetic interference (EMI). System control by high-level language programs permits data collection, averaging, reduction, and storage. System components and operation are described below. Computer and Interface (CI). The computer and interface consist of a Hewlett-Packard 9825A calculator, a Molectron DL 242 interface, and a Hewlett-Packard 9872A plotter. The 9825A is equipped with the String Variables-Advanced Programming and Extended Input/Output-Plotter ROMs and is programmed in the BASIC-like HPL language. The control program easily fits into the minimal 7K configuration. IEEE-488, serial, and parallel interfaces provide communication with the plotter, printer, and the Molectron Interface, respectively. The DL-242 contains a four-channel multiplexed ADC and three channels of multiplexed DAC. All have 0-10 V ranges (12 bit resolution). All ADC channels can be strobed by software, and channel 1 can be triggered externally. The SR line is used to strobe channel 1. The DAC outputs are latched with an analog sample and hold and must be updated periodically. The interface lacks digital control lines, and we use a DAC output to supply a trigger pulse for the TG. Signal Conditioner (SC). The analog output from the SO ranges from 0.4 to 0.9 V corresponding to the bottom and top of the oscilloscope screen, respectively. The SC shifts and expands the SO’S output to fully utilize the ADC’s range. A simple twostage operational amplifier with offset was used. A schematic is available from the authors upon request. Trigger Generator (TG). The TG (Figure 2) permits the PMT or the computer to trigger the SO. The PMT generates a suitable trigger pulse which is routed directly to the SO’Strigger input. The interface pulse is conditioned to match the PMT pulse shape before it is injected into the SO trigger input. The interface output drives transistor Q1 which discharges C1 creating a suitable spike. R1 is adjusted to set the spike’s level. Amplitude Fluctuation Correction Algorithm. Most boxcar integrators collect the input signal in one pass along the time axis. If N transients are averaged, then the external sweep is set to correspond to the first time and N points are collected and averaged. The sweep is then advanced to the next sampling time and the process is repeated until the entire transient is acquired. This normal procedure works well if the signal amplitude is stable. If the transient varies in amplitude (but not shape) during data
-
Figure 2. Schematic diagram of the trigger generator: R1, 50K 10-turn trlmpot; R2, 1K; Q1, 2N3904; C1, 1000 pF Mica. +V is 5 V. Inputs are from the trlgger PMT and from the Molectron DL242 interface. The output goes to the oscilloscope trigger. collection, however, severe signal distortions can result. For example, in a luminescence lifetime measurement the amplitude, but not the inherent shape of the signal, changes if the excitation intensity varies during the experiment or if the sample decomposes thermally or photochemically. Thus, if the sample intensity decreases over the experiment, a decay recorded in the above manner appears to decay too quickly, and one calculates too short a lifetime or postulates too complex a decay scheme. To minimize this error we adopted another averaging technique in which one datum point per sampling window was collected through the entire transient. This process of collecting and accumulating entire transients is repeated until the desired number of averages is collected. For a stable signal, results are the same as for the normal algorithm. However, if there is amplitude drift, the correction algorithm minimizes error since the time required to collect an entire transient is minimized. Thus, in the fiial average, every point contains contributions taken over the entire data acquisition period, and the amplitude fluctuations are spread over the entire wave form rather than strongly skewing the entire acquisition. Of course, neither algorithm preserves true signal amplitudes but, in cases where only shape is important (e.g., decay times),this loss of amplitude information is inconsequential. Software. In our standard control program, the operator can set the number of points to be recorded on a transient and the number of averages to be acquired. These data points are divided evenly across the time axis of the SO and can vary from 4 to 4096. Usually 64 to 100 points give good curve definition. A calibration dialogue is included for system calibration. The program then automatically collects the specified aumber of base line corrected points and transients. Results can be plotted and saved on tape and data reduction routines are easily added. A program listing and description of the calibration is available on request. Chemicals, Tris(2,2’-bipyridine)ruthenium(II)chloride ([Ru(bpy),]Cl,) from G. Frederick Smith was purified by recrystallization from water. Solutions were made up in aerated distilled water.
RESULTS AND DISCUSSION Figure 3 demonstrates the determination of the photoluminescence lifetime of [Ru(bpy),]Cl,. The base line corrected average of 32 complete transients, along with the corresponding semilogarithmic plot of intensity vs. time, is shown. The measured lifetime of 420 ns agrees well with previously reported values of 411 (4), 376 (5), and 405 (6) ns. Figures 4-6 illustrate instrument performance in signal averaging, autozeroing, and source fluctuation compensation. These data are exponential decays derived from the decay of a pulsed RC network. Figure 4 is a weak, noisy decay curve. The dotted line is a single sweep. The smooth solid transient is the average of 100 transients. The SIN enhancement is about the expected 10 and clearly permits more accurate decay parameter evaluation. Figure 5 demonstrates autozero performance for severe base
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
'
BASELINE CORRECTION
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Flgure 3. Autozero base line corrected decay curve (-) for [Ru( b ~ y ) ~ ]in " aerated water aind the corresponding semilogarithmic decay curve (- - -) vs. time. The early rise in the signal is due to the finite excitation pulse width and 'the response time of the electronics. The data fitting is done only prist the peak where the data are reliable. Other than the offsets of the SO and the SC amplifier, the PMT background was negilgible.
AVERAGING TECHNIQUES
SIGNAL AVERAGING .
:
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:
,
1 transient 108 transients
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Flgure 5. The effect of operation of the autozero base line correction circuit. Without autozeroing, the "RAW SIGNAL" (upper -) was collected. Using the autozero circuit the actual "BASELINE" (lower --) was simultaneously collected in real time. The "CORRECTED SIGNAL" (-) which was indistinguishable from the true signal was obtained by subtracting the base line from the raw signal. 800
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TRUE SIGNAL ONE TRANSIENT/SWEEP AVERAGE INTENSITY/POINT - - -
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Flgure 4. Resut )f signal (averagingon a noisy, repetitive wave .-rm. The dotted line is the acquisitionofa single transient. The solid line is the result of averaging '100 transients.
line drift. Drift was simulated by turning the base line control on the sampling oscilloscope. The raw signd, the upper dotted line, is the signal collected without the autozeroing circuit. The apparent nonexponentiality is caused by base line drift (lower dotted curve). The real-time base line, collected simultaneously with the signal using the autozero circuit, was subtracted from the raw signal to yield the corrected decay (solid line). This decay faithfully reproduces the true one. A base line collected before or after the data collection clearly would be unacceptable. Figure 6 shows an exponential decay in which the amplitude is decreasing over time mimicking severe sample decomposition during collection. Intensity variation was simulated by varying the variable gain control on the SO. The true undistorted decay shape iij given by the dotted line. With distortion, normal boxcar integrator acquisition gives the dashed line while the amplitude correction algorithm gives the solid line. The true sample lifetime is 44.5 ps. The lifetime of the curve collected by the standard method is 20.7 ps (54% error). The corrected decay yields 36.4 ps (18% error). The superior performance of the correction algorithm is evident.
Flgure 6. The effect of the different collection algorithms on the acquisition of transients whose intensities decreased with time: undistorted wave form (-); normal algorithm (- - -); source fluctuation correction algorithm (-).
LITERATURE CITED (1) Plumb, I. C.; Cooper G. H.; Heap D. G. J. Phys. E 1977, 10, 744. (2) Taylor D. G.; Turley, T. J.; Rodgers M. L.; Peterson Steven H.; Demas J. N. Rev. Scl. Instrum. 1980, 51, 855. (3) Malrnstadt, H. V.; Enke, C. G.; Crouch, S. R.; Horlick G. Instrumentation for Scientlsts Series: Optimization of Electronic Measurements, Module 4"; W. A. Benjamin: Menlo Park, CA. 1974;
Chapter 5. (4) Turley, T. J. Master Thesis, University of Virginia, Charlottesville, VA, 1980. (5) Demas, J. N.; Flynn, C. M., Jr. Anal. Chem. 1976, 48, 353. (6) Bolletta, F.; Maestri, M.; Moggi, F. J. Phys. Chem. 1973, 77, 861.
RECEIVED for review January 6,1982. Accepted June 7,1982. We gratefully acknowledge support by the Air Force Office of Scientific Research (Grant 78-35901, the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Department of Energy for Grant DE-FG02-CS84063;however, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE. All calculations were performed on the University of Virginia laser facility purchased in part through National Science Foundation Grant CHE 77-09296.
