On 10% squalane columns, methanol could riot be eluted even a t 100 “ C , but a 2 % Carbowax-1500 10 squalane column was able to elute methanol a t room temperature. For further clarification on this point, namely elution and resolution of highly polar compounds, it may be mentioned that a 20 % Carbowax-150 0 column was able to separate Averill’s polarity mixtures (19) (cyclohexane, benzene, methyl ethyl ketone, and ethanol) in the same order. Olefins, naphthenes, and aromatics were successfully eluted out from both the polar and nonpolar stationary phases as obtained in cases of diatomaceous earth columns. Quantitative data from attapulgite columns were in good agreement with the known data. As indicated earlier, suitable stationary phases on the modified attapulgite column gave satisfactory resolutions for the desired components as expected in cases of conventional standard support materials; for example our studies (20) on hexa-kis(2cyanoeth0xy)hexane (HKH) and tetracyanoethoxy penta-
+
(19) W. Averill, “Gas Chromatography, Third International Symposium, 1961,” N. Brenner, J.-E. Callen, and M. D. Weiss, Ed., Academic Press, New York, N.Y., 1962, p I.
erythritol (TCEP)gave quite sharp peaks for n-paraffins (e.g., C1to CIS), aromatics (e.g., benzene to trimethy1 benzenes), and methyl esters of CS-C9fatty acids. Particular mention can be made of quantitative estimations of aromatics o n TCEP and HKH columns (20). In conclusion we would like to state that our experience with this modified attapulgite suggests that though this material may not be as versatile as the diatomaceous earth materials because of its inherent structure and chemical composition, yet it demonstrates good column efficiency for a good many number of polar and nonpolar stationary phases, thus enabling its use for some types of routine analytical work. More work is in progress wherein we are trying to modify the material further so that the surface is less adsorptive and the concentration of active sites is reduced. RECEIVED for review February 10, 1972. Accepted May 24, 1972. (20) R. K. Kuchhal and K. L. Mallik, unpublished work, 1971.
Programmable Power Supply for Operation of Hollow Cathode Lamps in an Intermittent Current-Regulated High Intensity Mode Emil Cordos and Howard V. Malmstadt Department of Chemistry, School of Chemical Sciences, Uniuersir‘y of Illinois, Urbana, Ill. 61801
HOLLOWCATHODE LAMPS that are operated in a n intermittent mode ( I ) or pulsed mode (2-4) at high currents and high intensities are excellent light sources for atomic fluorescence spectrometry. The intermittent mode of operation requires a programmable power supply which can be controlled by a logic circuit o r by computer in specific precise timing sequences. The power supply must exhibit a fast ON-OFF response to the control signals and provide very precise current regulation to ensure stable light output from the lamps. The characteristics and specifications of a programmable power supply for operating hollow cathode tubes in the intermittent mode are rather specialized, and a t present it is unlikely that suitable power supplies would be available in most laboratories. However, it is possible t o redesign hollow cathode supplies that were originally designed for use only in the dc mode. The design principles and experimental data for the programmable supply are presented in the subsequent sections. The supply can deliver very accurate and precise current pulses (within 0.1%) in the desired timing sequences for hollow cathode tubes. For the application of the power supply in our automated atomic fluorescence spectrometer ( I ) , it was necessary to incorporate a programmable switching system that would be free from electrical noise and provide rapid -___
( 1 ) H. V. Malmstadt and E. Cordos, Paper and Abstract No. 60,
Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland. Ohio, March 1972; and Amer. Lab., 4 (8), 35 (1972). (2) D. G . Mitchell and A . Johansson, Sprctrochini. Acicr, 25B, 175 (1970). (3) Ibid.. 26B. 677 (1971). (4) J. D. Winefordner, Plenary Lecture, Sheffield A A Conference, (1969).
sequencing of a series of hollow cathode lamps into the power circuit. This switch sequence is also described.
PRINCIPLES OF OPERATION A simplified schematic diagram of the power supply circuit is given in Figure 1 to illustrate the basic principles of operation and to outline the parameters that determine the hollow cathode load current. The high voltage high current unregulated raw supply, ERS,is connected in series with the hollow cathode tube, H.C., a power transistor Q1 and a small accurate resistor, R,. Any change in hollow cathode load current iL, because of fluctuations in the raw voltage supply or in lamp resistance causes a brief change in the voltage drop e, across the high precision reference resistor R,. The change in e, causes the operational amplifier (OAl) control circuit to vary the base current of power transistor Q1 which causes the load current t o readjust t o its preset value. The operation of the control circuit and a n understanding of the significant parameters available for presetting the hollow cathode lamp current are best understood from the basic equations. In the simplified diagram of Figure 1, the precision resistors R, and R,z are shown in parallel because A, is grounded and R Z 2is held a t cirtual ground by OA2 which is connected as an inverter with unity gain. The inverter is necessary to provide the correct polarity for e, which is connected in series with the voltage drop el across the feedback resistor of O A l . The parallel resistors R, and R z 2form an effective resistance R, in series with the hollow cathode lamp. The same load current, iL, flows through the lamp and R, and causes a voltage drop, e,, across R, as shown in Equation 1 ; iLRD= e,
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
(1) 2407
To FET smtch
‘ r
driver
Rf
n
Figure 1. Simplified schematic diagram of the programmable current-regulated power supply
n I HC. 500 V
+
itn
c-
2R; 0 0 K 0,
0-Ei
-
v
Pi
:1
P
I1 I
DI 0 2
F
substituting R,for R, in Equation 6 gives
//%’A
1 ERS
500 V
Within the limits of the pass transistor Q1, the operational amplifier control circuit, the supply \oltage, and the firing voltage of the lamps, the load current does not depend on the drifts in ERS or the voltage drop across the load. The load current can be adjusted by changing the resistor values included in Equation 7 or by varying the input reference voltage,
-0
15 IJ K
111-
I
=
E ,uF
lN4166A
et.
T
The FET switch, QA, is operated by the control logic to turn e , (and therefore iL) ON and OFF. When a logic “0” is applied to the switch driver, QAis “OFF”, the output of O A l is zero, and transistor Q1 does not conduct. For a logic “1” a t the switch driver, the voltage e , is applied to the OA input and transistor Q1 conducts and the hollow cathode current is determined by the preset values for the resistors and reference voltage as calculated from Equation 7. In this way, the I number and the length of “ON” and “OFF” periods of the I -15 V lamp can be easily controlled by the TTL logic circuit of the FET switch driver. Figure 2. Modified high voltage raw supply for load circuit and voltage supplies for control circuits
*
EXPERIMENTAL
The sum of the voltages between ground and virtual ground (summing point) of O A l must be zero, so that e,
=
(2)
e,
and i f R f = e,
=
e,
(3)
Substituting from Equation 3 into Equation 1, one obtains : iLRp = i f R f
(4)
The feedback current, if, is essentially equal to the OA input current ii, which is determined by the values of the input reference voltage, e,, and input resistor, Ri, so that; if = eJR,
(5)
Therefore substituting for if in Equation 4, the expression for load current becomes :
For the circuit of Figure 1, the parallel resistance R , equals R,R,?/(R, R,?), and since R,,equals 103R,, R, R,, and
+
2408
The Heath EU-703-62 current-regulated dc power supply for hollow cathode lamps was rebuilt for the high current programmable intermittent mode operation described under the section on principles. Only a few modifications in the original rectifier circuit are necessary to obtain the required raw high voltage supply, E R S ,and the other circuit operating voltages. However, the control circuit is completely modified, but mounted in the original chassis. The rectifier circuits with modifications are shown in Figure 2. To obtain a stable -10-V reference supply, the original S 2091 transistor is replaced with a 2N702 pnp transistor. The control section of the programmable power supply is shown in Figure 3 and is similar to the basic circuit of Figure 1. The input reference voltage is preset by means of a 1pole, 10-position witch. The input current is thus obtained by selecting an input reference voltage from about 0-10 volts which is applied through a 200-K resistor and a FET switch (QA)to the OA summing point. The “ON” resistance of the F E T switch is about 20 ohms and is very small in comparison to the 200-K precision input resistor. The general purpose OAs (Heath EU-900-NC card) can not provide sufficient base current for transistors Q1 to QlO (DTS 413, Delco Semiconductors), so a booster operational amplifier, B (Health EU900-CA-circiiii card), is connected in the feedback loop. A series of switches provided by reed relays (S1 to S10) allows sequential operation of the hollow cathode lamps.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
I
100 p F
Figure 3. Programmable current-regulated power supply with multiplexing circuit for sequencing a series of hollow cathode lamps
200 K
6
mA 17.4 41.4 64.6 88.4 112.0 135.8 159.8 186.1 208.4 215.3
iL,
Table I. Independence of Load Current from Series Power Transistor and Load Resistance Transistor Q1 Transistor QY Transistor Q 8 Transistor Q4 Transistor QL load: 1000 load: lo00 load: 2000 load: loo0 load: 1000 _ _ _ _ ~ ___ -~ Rei std Re1 std Rei std Re1 std Re1 std Av,mV dev, Av,mV dev, Av,mV dev, Av,mV dev, Av,mV dev, 252.3 0.25 0.63 252.8 0.46 252.3 0.31 252.3 0.41 252.5 609.0 0.00 609.0 0.13 609.8 0.17 609.2 0.11 609.0 0.800 964.9 0.03 965.0 0.00 964.6 0.135 964.9 0.03 964.2 0.04 0.09 0.09 1322.6 1322.4 1322.6 0.06 1321.8 0.08 1322.6 0.03 1677.1 0.09 1679.0 0.06 1678.1 1678.1 0.06 1678.1 0.06 0.136 2037.9 0.01 2038.9 0.03 2038.1 0.05 2038.1 0.05 2038.8 0.133 2397.4 0.04 2398.6 0.04 2397.8 2398.8 0.04 2398.0 0.04 0.114 2762.5 0.03 2762.2 0.04 2762.0 0.04 2762.4 0.05 2667.5 0.05 3130.9 0.03 3130.9 0.04 3131.1 3131.5 0.02 0.05 3249.0 0.07 3351.4 0.07 3506.0 0.02 3274.2 0.25
z
z
z
The reed relay drivers (not shown in Figure 3) can be connected to a ring counter or any multiplexing device so the specific sequence in which the lamps are fired can be controlled by a logic circuit. The voltage drop e, across R, is inverted by a unity gain operational amplifier OA2, so that the polarity is of correct phase to use the npn power transistors that were available and the -10-volt input reference voltage. By using the inverter, the hollow cathode load current iL is split between R, and R,?so that the effective resistance R , for Equation 6 equals R,R,ri(R, R,:). Since R, = 10 ohms and R,? = 10 K , the effective resistance R, = 9.99 ohms. A small feedback capacitor (100 pf) is used to prevent oscillation which otherwise might appear when the control pulse is applied to the F E T switch.
+
RESULTS AND DISCUSSION The response of the power supply to the applied control pulse is illustrated in Figure 4. The lower square waveform represents the applied control pulse and the upper waveform is lhe voltage e,yacross the output of the inverter. This voltage is proportional to iL as indicated in Equation 1. It can be seen in Figure 4 that the current through the load resistor rises to the peak value in less than 200 psec which is satisfactory
z
Transistor QJ load: 2000 Re1 std Av, mV dev, 252 5 0 38 608 6 0 15 964 6 0 09 1323 1 0 03 1678 0 0 06 2038 5 0 02 2398 3 0 02 2654 8 0 04
when the lamps are operated with pulses in the millisecond range. However, this rise time can be varied by choosing different values for the feedback capacitor of O A l and the emitter resistor of Q1. The power supply has been tested in several ways to demonstrate that the load current iL is independent of several variable components. The data in Table I illustrate that the characteristics of the series power transistor Q1 and the value of the load resistance (effective resistance of hollow cathode lamps) d o not influence iL. The results in Table I were obtained by the dual channel synchronous integration measurement system (5). A fixed number of control pulses (usually 10) are applied to the FET switch and the integrated voltage across R s is obtained. The results in Table I are for selected peak load currents from about 17-215 mA, where each of 10 current pulses is ON for 10 msec. The integrated voltage values given in Table I for each current are the average of 10 separate integrations of 10 pulses each. The per cent relative standard deviations of the 10 integrations for each condition at each current show that for the current values from 41-208 m A the precision is ( 5 ) E. Cordosand H . V . Malmstadt,AhuaL.C~~h1.,44,2277(1972).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
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Table 11. Independence of Load Current on Type of Hollow Cathode Lamp and on Series Power Transistor Transistor QI Transistor Qz Transistor Q3 Transistor Q4 load: Mglamp load: Ca lamp load: A1 lamp load: multiple element lamp Re1 std Re1 std Re1 std Re1 std i L , mA Av, mV dev, Z Av, mV dev, Z Av, mV dev, Z Av, mV dev, 252.5 0.42 17.4 252.7 0.45 252.5 0.38 252.7 0.52 608.6 0.15 41.4 609.5 609.8 0.17 0.17 609.4 0.22 964.2 0.04 965.4 0.09 64.6 964.6 0.09 964.8 0.11 1322.2 0.03 1323.2 0.03 1323.0 0.00 1322.4 88.4 0.06 1678.0 0.00 1678.0 0.00 1678.0 1678.0 0.00 112.0 0.00 2037.4 0.02 2038.8 0.08 2037.8 0.02 2038.4 135.8 0.04 2397.8 0.02 2398.0 0.00 2398.0 2398 .O 0.00 159.8 0.00 2762.2 0.05 2762.0 0.05 2762.6 0.02 2763.6 184.1 0.03 3131.8 0.03 3131.6 0.03 0.05 3132.4 3133.0 0.02 208.4 3236.8 0.18 3447.4 0.19 3508.0 0.00 3348.0 215.3' 0.18 Power supply >(oltageshould be increased for currents at this or higher values.
z
u
Table III. Comparison of Theoretical with Measured Values of Integrated Hollow Cathode Current Integrated ei, Integrated es measd, mV _ _ _mV _ _ ~ Re1 Difference Re1 std Integrated std from theoiL, dev, e, calcd, dev, retical mA Av, mV Z mV Av, mV Z value, 255.0 252.5 0.07 -0.98 17.4 1020.0 0.00 613.3 -0.68 609.1 0.06 41.4 2453.2 0.02 967.6 964.8 0.03 -0.28 3870.6 0.03 64.6 -0.12 88.4 5296.0 0.01 1324.1 1322.5 0.03 -0.02 112.0 6714.1 0.02 1678.5 1678.1 0.03 +0.07 8167.3 0.03 2036.8 2038.2 0.02 135.8 +0.12 159.8 9580.1 0.02 2395.0 2398.0 0.02 +0.10 184.1 11,038.5 0.01 2759.6 2762.4 0.02 +0.14 208.4 12,508.4 0.01 3127.2 3131.6 0.02 245.3 12,993.1 0.01 3848.2 3365.0 3.20 -12.68 from 0.01-0.17 % regardless of series transistor or value of load resistance from 1-2 K. Also note that the absolute integrated voltage values across each row of Table I are well within 0.1 for almost all conditions. At the lowest current, iL = 17.4 mA, the standard deviation is higher because of measurement errors at the low voltages due to OA fluctuations and some noise pickup. However, even here the averages across the top row are also within 0.1 %. At the highest current, iL = 215.3 mA, the absolute values are not constant for different transistors because of the current limiting in the OA control circuit. With the 2 K load, the voltage drop across the load was too large at the two highest currents to provide current regulation. Therefore, if load currents of about 200 mA or more are to be regulated with loads of about 2 K, it is necessary to increase the magnitude of the raw supply voltage ERS, In Table I1 the data illustrate that the hollow cathode load current is independent of the type of hollow cathode lamp as well as the series power transistor. Again the per cent relative standard deviations are about 0.01 to 0.1 % for most current values, except at the extremes for the reasons cited for Table I. Again note the excellent reproducibility of the absolute value of the average across a row in Table I1 regardless of lamp type. A final check on the operation of the power supply is to compare the calculated theoretical values of iL with the measured values, as illustrated in Table 111. The calculated values are obtained by using Equation 6 and the conversion factor for the integration lock-in measurement system. The measured integrated signal for the dpplied input voltage et is given in 2410
O
I
i
3 4
5 msec
Figure 4. Oscilloscopic traces of control pulse (lower) and currentregulated pulse (upper) in load circuit column 1. This value is used for the calculations utilizing Equations 1 and 6 to obtain the theoretical values in Table 111. The differences between theoretical and measured values are within about i O . 1 for current values between 88-208 mA. The deviation increases to about 1 % at the low currents, probably because of small offsets in the OAs. The large deviation at the highest current is because of current limiting in the control circuit. The per cent relative standard deviations of about 0.02 to 0.03% are particularly noteworthy. They are based on an average for all of the measured values of e, obtained under all conditions (different lamps and transistors) in Tables I and 11. Measurements of individual current pulses rather than the integrated averages of 10 pulses were also made. A sampleand-hold amplifier was used to hold the information for one 10-msec pulse. The per cent relative standard deviations for the individual pulses were also in the range of 0.01 to 0.1 %. CONCLUSIONS
The programmable power supply has been used for over 1 year for the operation of hollow cathode lamps in an intermittent current regulated mode in our automated A F spectrometer. The supply can be readily assembled by modification of existing dc power supplies and use of basic circuit cards. RECEIVED for review May 24, 1972. Accepted July 31, 1972. Presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1972. Work supported in part by NSF Grant GP 18910.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
