Anal. Chem. 1990, 62,412-414
412
TECHNICAL NOTES Elimination of Power Line Noise from Transient Signals by Selective Triggering Emilie Lasson a n d Vernon D. P a r k e r * Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300 Periodic noise originating from the power line is often troublesome in analytical measurements, especially a t low signal levels. In recent years a great deal of effort has gone into the development of digital filters to obtain signals free of noise ( I ) . Selection of the trigger interval (TI) can be a useful strategy to reduce the effects of periodic noise. In this paper trigger interval refers to the interval between response evoking trigger pulses. It is commonly believed that the trigger interval should not be an integer multiple of the power line frequency (2). However, Bialkowski has shown that there can be advantages to making the experimental cycle synchronous with the line frequency when a baseline subtraction procedure is used (3). Nielsen and Hammerich (4) use a triggering device with an alternating phase shift so that successive triggers are shifted by 180' relative to the line frequency. We have found that averaging signals obtained by triggering at exact selected intervals is very effective in the elimination of noise a t the line frequency and the first overtone. The method is convenient, and the T I can be designed to fit the application at hand. EXPERIMENTAL SECTION Instrumentation and Procedures. Hewlett-Packard HP3314A function generators were used to trigger Nicolet 310 digital oscilloscopes for recording signals. The current follower output from JS Instrument Systems Model J-1600-B potentiostats provided signals with periodic noise. Sine waves were generated with a Princeton Applied Research selective amplifier Model PAR 189. The oscilloscopes were interfaced to personal computers by using IEEE-488 interfaces. Data manipulations were carried out with FORTRAN programs. The procedure for selecting trigger intervals involved averaging two successive oscilloscope recordings of a continuous sine wave and comparing amplitudes before and after averaging. TISwere generated with a resolution of 1 ms below 10 s and 10 ms above 10 s. No difference could be detected when triggering was performed with any of five function generators. All TIS possible with this resolution between 1 and 30 s were evaluated. This range of TI values was selected for voltammetry applications, but others outside of this range can readily be evaluated as well. RESULTS AND DISCUSSION A sample of the data generated during the trigger selection process is shown in Table I. At a trigger interval of 10.36 s the 60-Hz amplitude ratio was about 3000 and that at 120 Hz was nearly 30. A significant feature of the data is that large ratios were observed every fifth 0.01-s increment for the 60-Hz signal while large ratios appear every sixth to seventh 0.01-s increment for the 120-Hz signal. However, it was possible to find TIS, such as that at 10.36 s, where high ratios were observed for both the 60- and 120-Hz signals. For our applications we selected T I values of 1.036, 3.41, 10.36, and 27.6 S.
The effectiveness of the procedure for reducing the amplitude of periodic signals is illustrated in Figures 1 and 2. The amplitude of the sine wave (Figure l a ) was reduced by a factor of about 500 (Figure l b ) when two successive scans were averaged with a trigger interval of 10.06 s. The output of the current follower of a potentiostat at a rest potential of
Table I. Examples of Noise Reduction Factors for Various Trigger Times signal intensity ratioo at trigger time/s
60 Hz
10.26 10.27 10.28 10.29 10.30 10.31 10.32 10.33 10.34 10.35 10.36 10.37 10.38 10.39 10.40 10.41 10.42 10.43 10.44 10.45 10.46
747 1.1 3.3 2.6 1.1 1494 1.1 3.0 2.8 1.1 2986 1.1 3.0 3.0 1.1 1495 1.1 2.8 3.0 1.1 747
120 Hz 1.3
1.0 1.3 3.3 114.8 6.1 1.6 1.0 1.1 2.5 28.1 11.2 2.0 1.1 1.1 1.8 9.3 30.8 2.5 1.2 1.o
"The amplitude of the signal before averaging divided by that after averaging two scans. 0 V vs Ag+/Ag electrode in acetonitrile-Bu4N+PF6-(0.1 M) is shown in Figure 2a. Averaging 20 oscilloscope traces taken at a 3.40-s interval (Figure 2b) reduced the high-frequency noise but left the 60- and 120-Hz periodic noise unchanged. Changing the T I by only 0.01 s to 3.41 s resulted in the essentially complete removal of the periodic noise (Figure 2c). The result for a T I of 3.42 s was nearly identical with that shown in Figure 2b. Measurements over a period of several months have not revealed any significant differences in effective TI. This indicates that any instability in the line noise phase does not significantly affect the utility of the method. It is obvious that large variations in the phase would have a detrimental effect, but we have not experienced problems of this nature. Electroanalytical chemistry is an area where elimination of power line noise can be essential for quantitative work. This is especially true when ultramicroelectrodes, which give rise to exceedingly low currents, are used in voltammetry experiments. The application of the triggering technique to linear sweep voltammetry is illustrated in Figure 3. The voltammogram recorded a t 10 V/s for the oxidation of 9,lO-diphenylanthracene (0.5 mM) in acetonitrile-Bu4NPF6 (0.1 M) measured at a platinum disk electrode (d = 100 pm) is severely distorted by periodic noise (Figure 3a). Averaging 20 scans with a T I of 10.00 s followed by a fast Fourier transform (FFT) treatment ( 4 , 5 )of the data gave a voltammogram in which the 60-Hz component of the noise remained (Figure 3b). The same procedure carried out with a T I of 10.06 s gave a voltammogram free of periodic noise (Figure 3c).
0003-2700/90/0362-04 12$02.50/0 0 1990 American Chemical Society
I4NALYTICAL CHEMISTRY, VOL. 62, NO. 4, FEBRUARY 15, 1990
413
I
a
V
__._(
l b
v
o
time (ms)
Figure 1. (a) Continuous sine wave (60 Hz) recorded on a digital oscilloscope. (b) Average of two consecutive recordings as in a with a trigger interval of 10.06 s. Note the 100-fold scale change.
V ~~
1.200
0.400
E (V
Ag*/Ag)
Figure 3. (a) Linear sweep voltammogram for the oxidation of 9,lOdiphenylanthracene (0.5 mM) in acetonitrile-Bu,NPF, (0.1 M) at a voltage sweep rate of 10 VIS. (b) Average of 20 scans with a trigger interval of 10.00 s after treating the data with an FFT procedure. (c) As in b with a trigger interval of 10.06 s.
Table 11. Frequency Content of Linear Sweep Voltammograms V
O
sweep rate, V/s 1 -0
10
I
--0
20
a
Lo
10 100 1000
no. of cycles, 60 Hz/scan
freq range: Hz
filter cutoff,b Hz
240 24 2.4 0.24
2-500 20-5000 200-50000 2000-500000
40 400 4000 40000
"The frequency content of the linear sweep voltammogram using 512 points for a 1 V scan in the FFT procedure. *Low-pass filter cutoff used in the FFT Drocedure. C
as well. The frequency ranges of the voltammetric signals in column 3 are from (l/NT) to (1/277 where N is the number of points (512) in the FFT procedure and T is the time per point. The last column gives the FFT filter cutoff. The FFT filter is effective for voltage sweep rates of 1 V/s or less, and 60-Hz noise is not very significant at 100 V/s or higher sweep rates. The problem area is from about 1 to 50 V/s where the FFT filter cannot eliminate the noise due to significant 60-Hz components in the voltammograms. Under these circumstances, a suitably selected T I eliminates the effect of the power line interference. Trigger intervals that give 180' phase shifts for successive scans may be calculated from eqs 1and 2 where x and y are ideal trigger interval (60 Hz) = x(1/60) + 1/120 (1)
o3 (0
t i mx, e (ms)
a
Lo
Figure 2. (a) Output from a potentiostat current follower at a constant potential in the absence of Faradaic current. (b) Average of 20 scans with a trigger Interval of 3.40 s. (c) Average of 20 scans with a trigger interval of 3.41 s.
The frequency content of linear sweep voltammograms as a function of voltage sweep rate is summarized in Table 11. To emphasize the effect of power line noise interference, the number of 60-Hz cycles that appear in a 1-V scan are included
ideal trigger interval (120 Hz) = y(1/120)
+ 1/240
(2)
Anal. Chem. 1990, 62,414-416
414
integers. Equating the right-hand sides of the equations shows that y = 2x + l j 2 . I t is obvious that ideal T I for the two frequencies cannot be achieved simultaneously, so it is n e e essary to compromise. We therefore believe that our empirically based procedure for finding suitable TI cannot be greatly improved upon. We conclude that the use of precise selected TI for repetitive recording of transient signals is a highly effective method of eliminating periodic noise. The method is especially attractive because of its simplicity and convenience of application.
ACKNOWLEDGMENT We thank Dr. Stephen Bialkowski for helpful discussions.
LITERATURE CITED (1) Bialkowski, S.E. Anal. Chem. 1988, 60, 355A, 403A. (2) Heiftie, G. M.: Anal. Chem. 1972, 4 4 , 69A, 81A. (3) Bialkowski, S.E. Rev. Sci. Instrum. 1987, 58, 667. (4) Nielsen, M. F.; Hammerich, 0.; Laursen, S.A. Acta Chem. S a n d . , in
press. (5) Hayes, J. W.; Glover, D. E.; Smith, D. E.;Overton, M. W. Anal. Chem. 1973, 4 5 , 277.
RECEIVED for review August 7, 1989. Accepted October 23, 1989. The National Science Foundation (CHE-8803480) is thanked for generous support of this work.
Real-Time Monitoring of Iodine In Process Off-Gas by Inductively Coupled Plasma-Atomic Emission Spectroscopy Toshihiro Fujii,* Takashi Uehiro, and Yukihiro Nojiri National Institute for Environmental Studies, Tsukuba, Ibaraki 305, Japan
Yoshihiro Mitsutsuka and Hitoshi Jimba Department of Chemistry, T h e Meisei University, Hodokubo, Hino 191, Japan During nuclear fuel reprocessing and waste immobilization, iodine monitors are required on-line to ensure that discharged iodine is within regulation limits (I). Reliable methods are needed that are capable of measuring iodine-129 in real time at or below the maximum permissible concentration of 0.1 part per million (ppm) (v/v) in the off-gas (2). A detection capability of 0.1 ppm would make it possible to attest whether more than 99.6% radioiodine was removed. This would meet the regulation requirement of abatement facilities of the radioiodine in the nuclear fuel reprocessing plants ( 3 ) . There have been a number of methods for developing such monitors (4). Fernandez et al. ( 5 ) developed a technique that includes cryogenic sample collection, chemical form separation, quantitation by gas chromatography (GC), and isotope dilution mass spectrometry (IDMS). This technique employs negative surface ionization. They concluded that (1)less than 1ng of CH31can be measured by GC, but iodine molecules must be measured by IDMS and (2) isotopic ratios of 1271/1291 as large as can be successfully measured on samples as small as 80 ng of iodine. However, this method is not a real-time on-line analysis. Some on-line measurement instruments based on laser spectrometry have been tested on a laboratory scale. Baronvavski's group (6) used a He-Ne laser to detect lZgIby an extracavity fluorescence configuration. They detected a concentration of lZ9I in air in the range of 2 X lo-' mol/dm3 (4.48ppm, (v/v)) and claimed an lZgIdetection limit of about mol/dm3. The main disadvantage of this method is interference from nitrogen oxides (NO,) that are present in the off-gas streams. Intracavity absorption spectroscopy has been used to detect very weak absorption lines in a number of atomic and molecular species (7). Hohimer and Hargis (8)used a continuous wave (CW) dye laser to detect 12'1 and lZgIa t 488 nm by intracavity absorption. They detected iodine in the concentration range to mol/dm3 (22.4 to (2.24 X lo3) ppb (v/v)). Goles et al. (9) used a CW dye laser at 580 nm and detected an lZgIconcentration of about mol/dm3.
* Author to whom correspondence should
be addressed.
0003-2700/90/0362-0414$02.50/0
There is no doubt that a gas analyzer based upon electron impact quadrupole mass spectrometry would be very helpful to determine the amount of specific compounds known to be present in air environments. The first application of I2detection has been reported by Matsuoka's group ( I O ) . Their results indicate a straight calibration curve for I2 peaking at m / z 256 down to a 0.1-ppm (v/v) level of the standard gas sample without noticeable interferences. As a means of atomizing an analytical sample, inductively coupled plasma (ICP) has been shown to have a remarkable adaptability for the determination of analyte concentrations with a variety of spectroscopic techniques (11). In recent years ICP has found applications in commercial instruments for atomic emission spectrometry (AES), atomic fluorescence spectrometry (AFS), and mass spectrometry (MS). Particularly the ICP-AES, the earliest of these, has come to be a comparatively mature technique due to the accumulated understanding of its function and behavior. According to this knowledge, in contrast to most elements, iodine has its most sensitive spectral lines in the vacuum ultraviolet region. Therefore, spectral lines in the vacuum region were proposed by various authors (12),the first being Kirkbright et al. (13). They purged the monochromator and optical path between the plasma torch and the entrance slit with nitrogen gases and determined a detection limit of 50 ppb for the iodine solution. These considerations led to ICP-AES being a candidate method for iodine monitoring. The advantage of ICP-AES, in principle, is that it is able not only to determine the total concentrations of both the organic and molecular radioiodines in the off-gas stream, but also to offer the potential for a real-time determination. This paper describes a method for monitoring the gas phase of iodine by ICP emission spectroscopy. The technique uses the spectral lines in the vacuum ultraviolet region with the aid of a vacuum spectrometer. The gas sample containing iodine molecules is fed directly into the plasma.
EXPERIMENTAL SECTION ICP. The equipment used will be discussed with reference to the schematic diagram shown Figure 1. The ICP atomic emission spectrometer (14) used was a vacuum instrument equipped with 45 channels for simultaneous detection. The vacuum polychro0 1990 American Chemical Society
