Anal. Chem. 1980, 52, 1385-1387
during the first several minutes. This is not true with larger quantities of other kinds of organic matter using less powerful and rapid oxidants than 72% perchloric acid ( 4 ) . If determination of thorium isotopes, protactinium, or the transuranium elements simultaneously with that of polonium is desired, omit the cerium, replace the sodium sulfate with an equal quantity of potassium sulfate, and separate the other a emitters except uranium from polonium by precipitation with barium sulfate in the presence of hydrochloric acid as described in the fourth paragraph under “Determination of Thorium-230 in Uranium Ores and Mill Tailings” (6). Use only 3 drops of freshly-prepared 20% ferrous ammonium sulfate hexahydrate in 1% sulfuric acid to reduce neptunium, plutonium, and americium prior to precipitation rather than the 1 mL of 25% potassium metabisulfite recommended previously (6) to lessen the possibility of reducing polonium to the elemental state. Polonium can then be separated from the filtrate by either extraction into trilaurylamine or precipitation with tellurous acid and stannous chloride ( 4 ) . The uranium in the filtrate can be reduced with titanium trichloride, precipitated with barium sulfate, and extracted into Aliquat 336 from 10 M hydrochloric acid as described in the second paragraph under “Uranium-232” ( I ) . The barium sulfate can also be dissolved in aluminum nitrate for the extraction (6) but the blanks will be significantly higher because of the uranium present in the aluminum nitrate. The separated fractions can then be treated further as necessary for electrodeposition and a spectrometry ( I , 3,6, 7). If determination of the other a emitters without polonium is desired, the filter can more simply be burned off in a platinum dish and the residue fused with potassium fluoride (6).
1385
As noted previously ( 3 ) ,the presence of hydrochloric acid is necessary to prevent precipitation of polonium with barium sulfate or the double rare earth sulfates, but slows the rate of precipitation of the barium sulfate significantly. This is not very serious when the precipitation is used only as a method of separation and a t least four separate 1-mL portions of barium chloride are used (6). Under these conditions,the precipitaticin is self-seeding. However, when only two 1-mL portions of barium chloride are used to permit direct a counting of the precipitate, a seeding suspension is necessary to restore the rate and efficiency of the precipitation, particularly in the presence of both hydrochloric acid and certain heavy metals. This material will be described in a subsequent publication (7). LITERATURE CITED Sill, C. W. Anal. Cbem. 1974, 4 6 , 1426-1431. Sill, C. W.; Wiliis, C. P. Anal. Cbem. 1977. 4 9 , 302-306. Sill, C. W. Anal. Chem. 1978, 50, 1559-1571. Bernabee, R . P.; Sill, C. W. “Radiochemical Determination of Polonium210 in Ores and Environmental Samples”, Radiological and Environmental Sciences Laboratory, Department of Energy, -. Idaho Falls, Idaho, 1979, unpublished document. (5) Sill, C. W.; Williams, R . L. Anal. Cbem. 1989, 4 7 , 1624-1632. (6) Sill. C. W. Anal. Cbem. 1977. 49. 618-621. i 7 j Sill, C. W. “Determination of Gross Alpha, Plutonium, Neptunium, and/or Uranium by Gross Alpha Counting on Barium Sulfate ’, Anal. Cbem., in press.
(1) (2) (3) (4)
RECEIVED for review February 5 , 1980. Accepted April 28, 1980. Use of commercial product names is for accuracy in technical reporting and does not constitute endorsement of the product by the United States Crovernment.
Sub-Picoampere Current-to-Frequency Converter John Yun-Kuang Huang, James D. Defreese,” and Paul W. Gilles Department of Chemistry, University of Kansas, Lawrence, Kansas 66045
In our high-temperature vaporization studies, an EA1 Quad 200 quadrupole mass spectrometer (QMS) is used to measure the ion intensities via pulse counting of several vapor species under various experimental conditions. The system works well with a low signal rate but starts losing pulse counts a t signal rates higher than 10 kHz. We have experimentally characterized t h e system and found that our counting system is paralyzable with a maximum output frequency of about 120 kHz and has a deadtime of 3.5 f 0.5 ps. Theories (2-3) for correcting the counting loss due to pulse overlap are available, and we have applied them to our system but only with limited success due t o uncertainties in characterizing some system parameters. We therefore concluded that this sytem was not suitable for vaporization studies on substances with relatively high vapor pressures. Besides, the presence of rf noise in the vicinity of the counting system arising from the 2-MHz oscillator required for operating the quadrupole mass spectrometer is an ever-present problem that occasionally results in counting failure. It was clear that we needed either a faster counting system, i.e., one with shorter deadtime, or a different analog-to-digital signal conversion scheme. Current-to-frequency converters based on the charge balance principle appeared to be a sensible solution to our problem. The principle of such converters has been discussed ( 4 ) and designs have been presented ( 5 , 6). However, these circuits are quite involved and tedious t o build. Therefore, we decided to design and construct a device based on commercially available modular units which use the charge balance principle. T h e result is a low cost, high performance, and easily constructed current-to-frequency converter (IFC) which is described here. 0003-2700/80/0352-1385$01.OO/O
INSTRUMENTATION A block diagram of the mass spectrometer with its control and data acquisition system is shown in Figure 1. The quadrupole assembly is installed inside a stainless steel vacuum system which also accommodates a heating assembly for high temperature vaporization studies. High vacuum, normally lo4 Torr, is achieved through a sorption pump and two ion pumps. When the QMS is operated in its “automatic” mode, the mass spectra can be displayed on an H P 13OC oscilloscope or recorded cln a strip chart recorder via a Keithley 417 picoammeter, which can measure currents in the A range. In addition, if the QMS to 3 X is operated in its “manual” mode, the electron multiplier (EM) output current at each mass can also be measured with the pulse counting system. In this case, the HP2116B minicomputer and its peripheral devices perform the mass scanning and other data acquisition tasks. A Systron Donner SD1037 counter is used as a readout device. The items enclosed in the dashed lines have been replaced by the IFC shown in Figure 2. The converter circuit consists of the following Analog Devices modular units a 310K varactor bridge operational amplifier (OA),a 460L voltage-to-frequencyconverter (VFC), and a 902 dual power supply. The choice of the feedback resistor Rf for the 310K OA depends on the desired current range. It may be a high-valued single resistor or may consist of a voltage divider circuit built from several lower-valued resistors so as to provide a high equivalent resistance (7). In either case, Rf must be high precision and have a low temperature coefficient (TC). For measuring current in the 10-’4-10-8 A range, we use a 1000-Mn glass resistor with fl% precision and a 0.07%/°C TC. Offset adjustment for the 310K OA is provided by a 50-kCl potentiometer R1 padded on each end by resistances of 25 kfi. The 460L VFC has a transfer function of 1 MHz/lO V and provides TTL/DTL compatible output pulses that can be fed 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
EA1 Q U A D
Table I. Components for Current-to-Frequency Converter
Z O O QMS KEITHLEY 417
CHART
item
description
source
1 310K, Varactor bridge
I
L . . . . _ . . . . _ . . . . . _ _ . . ’
R E P L A C E D WITH
IFC
I
460L
i MODCOH
n/5 nu8
VFC
-
CURRENT-TO-FREQUENCY CONVERTER I I F C l
COMPUTER
Figure 1. Quadrupole mass spectrometer with data acquisition and control system
directly into suitable counters. External components for the unit consist of only two trimming potentiometers, Rz and R3,for full-scale and input-offset adjustments, respectively. The 902 modular power supply provides f15 V dc a t 100 mA to power the 310K OA and 460L VFC. All of the external components are connected to the mating sockets of the 310K and 460L. The component side of the 310K socket has a metal shield and both units are shielded from the power supply with an aluminum plate. Shielded coaxial cable, type RG-58, is used in the signal path. The entire device is installed inside an aluminum utility cabinet and is very compact and portable. A complete list of the components for the IFC and their sources is given in Table I.
CALIBRATION AND CHARACTERIZATION Calibration of the 460L. A voltage reference source (VRS) (Heath EU-80A) was used for voltage input to the VFC. The input offset was compensated by applying +(10.00 f 0.01) mV to the VFC input and adjusting R3 until the frequency output was 1000 Hz as measured on the SD1037 counter. Full-scale adjustment was accomplished by applying a voltage of +(9.998 f 0.001) V from the VRS to the VFC input and adjusting Rzuntil the frequency output was (9.999 f 0.001) X lo5 Hz. Calibration of the IFC. The EM output a t mass 20 was used A, the for the current source. For currents less than 6.5 X “electron energy” and “emission current” control of the QMS ionizer were adjusted to obtain various current magnitudes. For currents greater than 6.5 X 10‘’’ A, one of the ion pumps had to be turned off to increase the background pressure inside the vacuum system. Ten minutes was allowed for the pressure to stabilize.
Analog Devices, operational amplifier Norwood, Mass. Analog Devices, 2 mating socket, AC1017 Norwood, Mass. 3 metal shield, AC1118 Analog Devices, Norwood, Mass. 4 460L, voltage-to-frequency Analog Devices, converter Norwood, Mass. 5 mating socket, AC1016 Analog Devices, Norwood, Mass. 6 902, dual power supply Analog Devices, Norwood, Mass. 7 trim pot, 500 n, 67F3659 Newark Electronics 8 trim pot, 50 k n , 6733685, Newark Electronics 2 ea. 9 resistor, l o 9 n t 176, Victoreen, O.O7%/”C TC, RX-1 Cleveland, Ohio 1 0 aluminum utility cabinet, Bud Radio Inc., 4 X 5 X 6 inches Willoughby, Ohio The IFC offset was adjusted using the base-line current I b of the mass spectrum between mass 4 and mass 1 2 where no peak exists. R1 on the 310K OA was adjusted until the frequency output for base-line current F b was about 5 Hz. Then, before each set of measurements, the picoammeter and chart recorder were zeroed and Zb was recorded. The mass spectrum was then focused on mass 20 and the peak current I, recorded for about 2 min. The range switch of the picoammeter was employed to keep the recorded current between 20 and 70% of full scale. Next, the E M was connected to the IFC and approximately 20 frequency readings were taken from the SD1037 counter using a 1-s gate time. For the low current measurements, F b was also taken for subtraction from Fp. The average of the currents recorded before and after the frequency measurements was considered to correspond to the measured Fp. After correcting the peak current and frequency measurements for the base-line values, such data from 19 sets of measurements and 2.97 a t mass 20 with currents ranging between 7.5 X X A and frequencies between 8 Hz and 2.932 X lo5 Hz were fit by least squares to a linear equation of the form In F = a In I + b. The calibration equation thus obtained was F = [(8.66 & 0.40) X 1013]I(o~9920 *O.oozO) The result indicates that the nonlinearity of the IFC is less than
TER
Flgure 2. Component wiring diagram for current-to-frequency converter. Rf, 1000 MQ; C,, 2 2 pF; R,, 50 KQ (offset): R,, 500 R (full scale): R,, 50 KQ (offset): R4, R5, 25 KQ; Re, R,, 33 Q: C,, C,, 1 WF
Anal. Chem. 1980, 52. 1387-1389
1% over a dynamic range of lo5.
1387
trometric measurements, the stability is very satisfactory. ( 5 ) Low Cost, Easy to Construct, and Compact. The total cost of all components in the IFC is below $375.00, less than one fourth of the list prices for commerically available instruments. The IFC can be constructed in two days and occupies a space of only 4 X 5 X 6 :inches, making it highly portable. (6) Wide Applicability. The IFC has been successfully applied to our mass spectrometric vaporization studies for over a year. Other areas of application 'could easily include pyrometry, spectrometry, chromatography, electrochemistry, etc. Specifically, the IFC is being used as a measurement device for phototube currents and for electrochemical studies with microelectrodes. Devices similar to this IFC but with different input, e.g., positive current, and output characteristics can easily be constructed from other commercially available modules.
Drift. To test the stability, a reference current of approximately -5 X IO-' A was generated through a 10-MR resistor with a VRS voltage of -(50.42 h 0.02) mV. Four sets of measurements of both the VRS voltage output and the IFC frequency output were taken during a period of 21 h. Each set consisted of 10 readings during a 30-s period. Values of 9.7071 f 0.0011, 9.7026 f 0.0011, 9.7022 f 0.0006, and 9.7046 f 0.0026 MHz/V were obtained. Thus, no change greater than that attributable to the stability of the current source was observed. DISCUSSION The features of the IFC described here can be summarized as follows. (1)High Sensitivity. The transfer function of this device is about 1 X Hz/A. For mass spectrometric measurements using an electron multiplier with a gain of lo5, signals corresponding to four ions per second can be detected with a signal-to-noise ratio of about two. (2) Good Linearity. The results of in-situ calibrations using the ion current of residual gas a t mass 20 indicate that the nonlinearity of this device is less than 1%. With a more stable current source of wider range, we are confident that the nonlinearity figure could be shown to be even better. ( 3 ) Wide and Variable Dynamic Range. The present calibration shows excellent linearity over a dynamic range of IO5. Variable current ranges can be achieved simply by substitution of appropriate resistors in the 310K OA feedback loop. (4) Low Noise and High Stability. The maximum input bias current for the 310K is specified to be 1 x A. With proper installation, this limit can be achieved easily. Because of the integrating nature of the VFC, it is also quite immune to the 2-MHz rf noise from the QMS. Based on the 21-h drift test and our experience in applying the IFC t o mass spec-
ACKNOWLEDGMENT The authors gratefully acknowledge W. R. White of the University of Kansas Electronics Design Laboratory, R. Dalle-Molle, and I. R. Bonnell for many helpful discussions. LITERATURE CITED (1) Hayes, J. M.; Schoeller, D. A. Anal. Chem. 1977, 49, 306-311. (2) Hayes, J. M.; Matthews, D. E.; Schoeller, D. A. Anal. Chem. 1978, 50, 25-32. (3) Ingle, J. D., Jr.; Crouch, S. R. Anal. Chem. 1972, 4 4 , 777-784. (4) Malmstadt, Howard V.; Enke, Christie G.; Crouch, Stanley R. "ADDBOOK ONE: Experiments in Digital and Analog Electronics", E & L Instruments, Inc.: Derby, Conn., 1977; Experiment 5-2. (5) Woodruff, T. A.; Malmstadt, H. V. Anal. iChem. 1974, 46. 1162-1 170. (6) "Operating Manual for the ATC Model 151 I-to-F Converter"; Analog Technology Corp., Irwindale, Calif., 1977. (7) Malmstadt, H. V.; Enke, C. G.; Crouch, S. R.; "Electronic Measurements for Scientists"; Benjamin: Menlo Park, ICaIif., 1974; p 424.
RECEIVED for review March 17, 1980. Accepted April 28,1980.
Stability of Extracted Bis(dibuty1dithiocarbamate) Copper (11) Complexes K. S. Tung and
D. H. Karweik"
Depadment of Chemistry, Wayne State University, Detroit, Michigan 48202
Because of the number, variety, and importance of amines in both living systems and commerce, there has been considerable work reported on the detection and determination of amines ( I ) . One group of methods for the determination of secondary amines involves the isolation of the secondary amines through their reaction with carbon disulfide in the presence of a base such as pyridine or ammonia to form a dialkyldithiocar bamate.
this method indicated that with some samples the color formation was not reproducible and that the color faded with time in less than a predictable manner. The fading color was also accompanied by the formation of a light yellow, finely divided precipitate. I t is the purpose of this paper to describe the source of the irreproducibility and to present a simple modification of the procedure which eliminates the cause of the irreproducibility. Further, this report presents the skeleton of a proposed mechanism which may be involved in the decomposition of the complex.
r
',>NH
R
+
CS2
+
NH3
-
'>N-C-S
/I
- NH4
(1)
R'
Several options are available to quantitatively determine the amount of the dithiocarbamate (DTC) formed ( I , 2). Although m a y of the methods cited in Refs 1and 2 involve destruction of the &thiocarbamic acid (3),an alternative of utilizes the formation of a Cu(II)-bis(dialkyldi~hiocarb~ate) complex (Cu(DTC),) which can be extracted into an organic solvent (4-6).T h e concentration of the extracted complex is determined spectrophotometrically. Recently an improved version of the methods of Dowden ( 4 ) and Umbreit ( 5 ) was reported in which ammonia was used as the base and chloroform as the extracting solvent (6). Further examination of 0003-2700/80/0352-1387$01 .OO/O
EXPERIMENTAL Apparatus. Spectrophotometric measurements were made in matched 1.00-cm cells with a Cary 14 spectrophotometer. A Perkin-Elmer 137 infrared spectrometer and Hewlett-Packard 700 G.C. equipped with a flame ionization detector were used to determine chloroformpurity ad quantitate the levels Of present. A 6-foot, 0.085-inch i.d. column packed with Porapak for 6 h at 200 oc. (80-100 mesh) was used after Reagents. All solutions were prepared from water which had been doubly distilled from alkaline permariganate. Stock solutions were prepared as previously described ( 6 ) ,with the exception of the dibutylamine which was freshly distilled before the stock
C 1980 American Chemical Society
