A typical application of the Kalousek switch is demonstrated by an experiment (Figure 4) in which a solution of Cd2+ in 0.1M KC1 was examined. When the electrode was alternately switched to -0.7 V (SCE) (where the limiting diffusion current is just reached) and to the ramp voltage (more positive), first a n anodic current (curve 2) is observed due to the oxidation of the Cd amalgam formed during the period, when the auxiliary voltage was applied. When the ramp voltage brings the potential to values near - 0.7 V, only the reduction current of the same value is obtained as in a classical polarographic curve without snitching. For comparison, the classical polarographic curvewas also recorded (curve 1 in Figure 4). When recording curve 3, the current was measured during the period in which the electrode was polarized by the auxiliary voltage. Even in this case, the current must reach the same value as in the classical curve when the ramp voltage becomes equal to the auxiliary voltage. Curves 2 and 3 were recorded using the circuit for elimination of the charging current. The difference between the Kalousek curves recorded with and without elimination of the charging current is given in Figures 5 and 6. The curve with elimination of charging current was recorded at approximately 4 times higher switching frequency than the curve in Figure 6 which would be totally distorted at such a frequency.
The described Kalousek switch circuit can be modified for use as a simple square-wave polarograph by removing the analog switch between R3and the input of OA 2. The amplitude of the superimposed square-wave voltage (of the order of millivolts) is set by the potentiometer. By alternative turning of Sz to the ON and OFF states, the electrode o n the output of OA 2 is polarized to the voltage E or E AE. A capacitor (2 pF) is placed between the diode clipping circuit and RIO,t o eliminate the dc component of the signal. Some imperfection of this circuit used as a square-wave polarograph is caused by the fact that the D M E is polarized during its whole drop life and that the current is measured through this whole period, not only at the end of the drop time. Nevertheless, this set-up enables determinations at concentrations down to 10+M with potentiostatic conditions. Thus work can be carried out even in dilute (10-*M) supporting electrolytes, which is advantageous for elimination of the influence of impurities therein. More details are given in (9).
+
RECEIVED for review January 31, 1972. Accepted May 25, 1972.
(9) R. Kalvoda and I. Holub, Chem. Lisry, in press.
Switched Resistor Modulation of Microwave Excited Electrodeless Discharge Lamps Donald Alger, R. M. Dagnall, M. D. Silvester, and T. S . West Department of Chemistry, Imperial College of Science and Technology, London, SW7 2A Y , U.K.
RECENTLY, Browner, Dagnall, and West ( I , 2 ) and later Thompson and Wildy (3, 4 ) described methods of electrically modulating microwave excited EDLs (electrodeless discharge lamps) by transformer-coupling a small voltage ac signal into the magnetron anode circuit. The disadvantages of this method of modulation have been explained by Aldous et a / . ( 5 ) and have been greatly reduced by the use of stabilization modulation, as described by Dagnall, Silvester, and West (6). However, although this form of stabilization has been found satisfactory, it has two major limitations. First, a voltage amplifier capable of giving an ac output of about 200 volts peak-to-peak is required for injection to the error sensing point of the stabilization circuit. Second, as this amplifier is by necessity ac-coupled to the error sensing point, the modulation can only be met about a mean power level. This, because the current modulation of the system is limited to 63 (67,limits the powers between which the magnetron can be modulated, thus making pulsing of the microwave power inpractical. To overcome these two problems a new type of modulator has been built, which relies on the value of the magnetron anode load resistor being periodically changed. Its perform(1) F. R. Browner, R. M. Dagnall, and T. S. West, Anal. Chim. Acra., 45, 163 (1969). (2) Ibid., 46, 207 (1969). (3) P. C . Wildy and K. C. Thompson, AtiuIyst, 95, 562 (1970). (4) h i d . , p 776. (5) K. M. Aldous, D. Alger, R. M. Dagnall, and T. S. West, Lab. Pract., 587 (1970). (6) R. M. Dagnall, M. D. Silvester, and T. S . West, T u / r ~ ~ t18, u, 1103 (1971).
ance was compared to that of the stabilization modulator when used to operate several EDLs. EXPERIMENTAL
The microwave generator and the spectrophotometric system used in this investigation have been described previously ( 6 ) . The circuit of the switched resistor modulator, shown in Figure 1, consists of a variable resistor (similar to the magnetron power control) connected across the magnetron power control in series with a transistor. When the transistor is turned on with a signal generator (Hewlett-Packard, Model 3310A), this modulation resistor is connected in parallel with the magnetron power control, which lowers the magnetron load resistance and results in an increase in magnetron microwave power output. When the modulation resistor is “out of circuit,” the magnetron load resistor controls the power (which is the modulation minima) and when the modulation resistor is “in circuit” it controls, in conjunction with the magnetron load resistor, the power (which is the modulation maxima). This system is capable of modulating the microwave power between any two levels in the power range of 15100 W, which gives a maximum modulation depth of 85%. It was only capable of square wave modulation, but any mark : space ratio could be used. The minimum time width to which the transistor would respond was 20 psec (equivalent to a square wave of 25 kHz with a mark: space ratio of 1). However, its response started to drop off at time widths of 25 psec (equivalent to a square wave of 20 kHz with a mark:space ratio of 1). These frequencies would easily be increased, if desired, by using a different transistor. RESULTS AND DISCUSSION
The effect of the frequency of modulation on the intensity of emitted resonance lines was investigated for several EDLs-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
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Table I. Comparison of Noise, Drift, and Signal Magnitude for Several EDLs Using Both Stabilization and Switched Resistor Modulation Upper Lower Upper Lower Drift, Drift, Signal power, power, RMS noise, Signal power, power, RMS noise, magnitude W W x x hr magnitude W W % hr
z
Cd Hg
20 16 Zn 22 Sb/Iy 16 Se 16 18 As/Ir Pb/In 17 N.B. All modulation was at 20 kHz 50 35 54 36 35 38 43
0.4 0.6 0.4 0.5 0.7 0.4 0.9
-1.0 -1.2 -1.7 -1.3 -1.3 +0.2 -2.2
I
C
h Y -
I Figure 1. Circuit diagram of switched resistor modulator =
to magnetron power supply
B = to magnetron anode C and D = modulator monitor point R1 = 22 kCi R? = 10 Sl RV1 = 5 kn RV2 = 7.5 kfi M = 0-100 W incident power meter Tr.1 = M J 423 Motorola 1/P = Modulation input
ciz., Cd, Hg, Zn, Sb, Se, As, and Pb. All of these sources gave a similar result to that obtained for the stabilization modulation (6), and a maximum output was obtained in all instances for a n operating frequency of 20 KHz. As in the case of stabilization modulation, this maximum probably would have occurred at a higher frequency, but for a drop-off in the transistor response at this frequency. The appearance of slight phase shifts, which started to occur at 25 kHz, probably because of capacitance effects in the lead from the photomultiplier to the preamplifier, made it undesirable to investigate higher frequencies by changing the switching transistor. Various mark: space ratios were investigated, but in every
2256
56 36 61 38 36 41 46
15 15 15 15 15 15 15
0.3 0.5 0.3 0.5 0.6 0.5 0.7
-1.1 -1.3 -1.5 -1.4 -1.4 -0.1 -1.6
8.2 6.1 8.1 4.9 5.2 6.2 8.3
square wave, 1 : 1 mark space ratio ( i . e . , optimum conditions).
I
A
4.2 5.6 4.8 4.6 4.5 5.7 6.3
instance a ratio of 1 yielded the largest ac component of the resonance line radiation. The two resistors (Le., the anode load resistor and the modulation resistor) were adjusted so that the ac component of the resonance radiation emitted from the EDLs was at a maximum value. Then the short term noise, the drift, and the ac signal magnitude of the resonance radiation emitted by the EDLs, when modulated by the switched resistor modulation and the stabilization modulator, were compared for various EDLs (Table 1). The short term noise was about 2 5 z less, the drift was about the same, and the signal magnitude was about 30-80z higher using the switched resistor modulation as compared with the stabilization modulator. A brief investigation was carried out on the feasibility of pulsing EDLs from low power operation to high power oueration. This could prove useful for EDLs which are easily self-reversed by obtaining high electrical power (during the pulse) while maintaining a low overall thermal condition. The modulator operated very successfully in this mode for a mercury EDL. This EDL was usually operated (Le., continuously) at a mean power of 38 W. With the modulator, it was operated a t 15 W for three quarters of the time and was pulsed to 100 W for the other quarter; the same average power was then obtained as continuous operation. When this procedure was used, the radiation output during the pulsed period was 15 times as intense as that obtained during continuous operation. Further work is being carried out on the possible uses of this technique in conjunction with gated amplifiers. CONCLUSION
The switched resistor modulator described in this communication was both simpler and cheaper to construct and more versatile in operation than the stabilization modulator used in a previous study (6). Additionally, the switched resistor modulation gave lower noise levels and higher ac signal levels. It may also be used to pulse the microwave power and thus increase riidiation intensities emitted by EDLs. The optimum frequency of operation is 20 kHz, using a square waveform witb a mark: space ratio of 1.
RECEIVEC for review February 29, 1972. Accepted May 30, 1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
