such an application, it would only be necessary to reduce or eliminate the output system time constant.
CONCLUSIONS The dual-channel background-correcting spectrometer described herein offers several advantages to the spectroscopist. The correction is automatic and instantaneous, enabling the evaluation of background-corrected transient signals. The background can be of changing intensity and can possess either a flat spectral profile or a sloping or peaked one. In addition, the spectrometer output is easy to interpret. At present, there are three shortcomings in the present system. The preamplifiers and subtractor circuits have been developed around inexpensive integrated circuit operational amplifiers which show some thermal and long term drift. Second, the slit width for the background-correcting photomultiplier could be reduced to allow better compensation for nonlinear background profiles-a narrower region on either side of the analyte line would thereby be viewed and any changes in background slope on either side of the line would become less significant. Finally, any spectral line interferences falling within the bandpass of either channel would seriously degrade the accuracy of the determination.
ACKNOWLEDGMENT The authors thank M. Williams and K. Bastin for modification of the monochromator and for assistance in design and construction of the background photomultiplier tube housing. We are also grateful for the use of the nebulizer-burner system supplied by Varian-Techtron.
LITERATURE CITED (1) L. Morren, OpNk, 42, 23 (1975). (2) W. Snelleman, T. C. Rains, K. W. Yee, H. D. Cook, and 0. Menls, Anal. Chem., 42, 394 (1970). (3) M. S . Epstein and T. C. O'Haver, Spectrochim. Acta, Part E, 30, 135 (1975). (4) G. M. HieftJeand R . J. Sydor, Appl. Spectrosc., 26, 624 (1972). (5) R. W. Spillman and H. V. Malmstadt, Anal. Chern., 48, 303 (1976). (6) R. J. Sydor and G. M. Hieftje, Anal. Chem., 48, 535 (1975). (7) R. L. Sellers, G. W. Lowry, and R. W. Kane, Am. Lab., March 1973. ( 8 ) D. W. Brinkman and R. D. Sacks, Anal. Chem., 47, 1723 (1975). (9) K. W. Busch, N. G. Howell, and G. M. Morrison, Anal. Chem., 46, 2074 (1974).
RECEIVEDfor review April 22,1976. Accepted July 14,1976. The authors acknowledge support of this study by the Public Health Service through NIH grant number PHS GM 17905-04 A1 and by the Office of Naval Research.
High-speed Cross-Correlator for Broad-Band Impedance Measurements Dale E. Mathis and Richard P. Buck* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, N.C. 27514
Although many techniques and instruments are currently available to membrane electrochemists for measurement of impedances, perhaps one of the most promising is the dccoupled cross-correlating active bridge. This type of device can exhibit excellent accuracy and resolution of orthogonal components over a large impedance range. An early model of this type of bridge constructed in this laboratory was useful only up to 10 kHz ( I ) , because of the long transition time of the FET switches used in the cross-correlator. Extending the useful range to give reliable measurements up to 1 MHz is necessary for complete characterization of the electrical properties of many membranes. A broad-band instrument would obviate the use of more than one device to span this wide frequency range and eliminate uncertainties thereby introduced. In addition, the large capacitive admittances observed at high frequencies make it possible to measure extremely small capacitances, as small as 1 pF, typical of geometric capacitances in many membrane cells. Finally, high frequency measurements of this type are essential for detection of inductive reactances such as those observed for Ag4RbI5 ( 2 ) . Current developments in the field of CMOS integrated circuit design have resulted in the availability of extremely fast, analog switches, capable of replacing typically used FET switches. We have designed and constructed a dc-coupled cross-correlator based on these new devices and currently use it as the phase-sensitive element of an admittance bridge. Its novel design makes possible the measurement of impedances from 0.001 Hz to 1MHz without compromising the excellent accuracy obtainable by cross correlation (3). We have also been able to improve the design of the gated integrator previously suggested ( I ) through the implementation of these new CMOS devices. The performance of the new cross-correlator has been evaluated and the design and technical details of both new circuits are discussed below.
INSTRUMENTAL The fundamental features of self-balancing,phase-sensitive impedance bridges have been reviewed in a previous publication ( I ) . The important improvements added for high frequency operation are: a F E T input current transducer, compensated to remove the effects of shunt capacitances ( 4 ) , a broad band function generator, such as the Interstate Electronics Co. model F51A used in this study, and the new high speed cross-correlator shown in Figure 1.In this circuit a high speed differential voltage comparator, IC1, is used to provide complementary square waves for the in-phase reference signal. A common-collector amplifier isolates the sine wave from INPUT IK
IK
,FILTER
,INT.
1n
'I5 V
Flgure 1. Schematic for cross-correlator and signal generator inter-
face IC1 = National Semiconductor Corp. No. LM 361N differential comparator. IC2 = Analog Devlces, Inc. No. AD7515 CMOS quad spst analog switch. O A l = Burr Brown Corp. No. BB3507J high speed op. amp.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
2033
nru
"I
- IOK
16821
c
I
-0
I I LOyF
4 9 -
e
.4
.e
.e
1.0
Z R ( 8 id6 OHMS)
Flgure 4. Impedance plane plot of the membrane cell: Ag/AgCI/O. 1 KN0~/support/nltrobenzene/support/O.1 KNOJAgCI/Ag
I
OUTPUT
-
d
Flgure 2. Schematic for gated integrator IC1 = Analog Devices, inc. No. AD7513 CMOS dual spst analog switch. OAl = Analog Devices, Inc. No. AD42J FET input op. amp. FFI and FF2 = 7476 dual JKMS flip flop
Supports conslst of 75-mil blaxlally oriented polypropylene with a 9-mil acrylic coating. -2Q and ZR represent the quadrature and real components of the Impedance
admittances. Another circuit for this purpose is shown in Figure 2 which employs only three integrated circuits: (1)7476 dual JKMS flip-lop, (2) AD7513 dual analog gate, and (3) AD42J FET input op. amp. The major advantage of this circuit is that it is significantly more accurate than the earlier version at frequencies exceeding 10 Hz. This improvement allows more overlap in filtered and integrated data to ensure validity. The circuit works as follows: Discharging C' drives &I low which resets the integrator and also drives 8 1 and J 2 high enabling FF2 for clocking. The first negative-going transition of the reference signal drives 8 2 low, resetting FF1 which in turn drives &I high, and unshorts the integrating capacitor. Since Qz is low, integration begins simultaneously. On the second negative-going reference signal transition, one cycle later, FF2 is restored to its initial state with both analog gates open, FF2 disabled, and the value of the integral being held. Discharging C' reinitiates the sequence.
EXPERIMENTAL
IO
20
30
40
I, 50
Z R ( 1 1 0 . OHMS) ~
Figure 3. Impedance plane plot of a 50 k 0 composition resistor in parallel with a 10-pF ceramic disk Capacitor (0)Cross correlator measurements. (0)Hewiett-Packard model 4815A RF vector impedance bridge. -2Q and ZR represent the quadrature and real components Of the impedance
comparator transients, while a constant voltage is applied to the other input to compensate for the voltage developed across the transistor juction. The IEC function generator provides an out-of-phase f 2 - V square wave which is converted to complementary square waves in the same manner. The TTL output from the comparator is made compatible with the CMOS analog gate through a simple resistive divider to the negative supply. The supply current of 10 pA at f 7 . 5 V required for the CMOS analog gate, IC2, is obtained from the f 15-Vop amp power supply with two 7.5-V Zener diodes. The use of an inverted input cross-correlated with a complementary reference signal essentially doubles the sensitivity of the detector. Finally, the output is loaded with a 10-kQ resistor to relax charge induced on the analog gates by inherent capacitive coupling to the reference signal. The average dc component of the cross-correlator output, obtained by an active filter for frequencies above 10 Hz and by integration over a single cycle at lower frequencies, is proportional to one of the orthogonal elements of the cell admittance. In a previous report ( I ) ,a circuit for a gated integrator using discrete components was suggested to measure low frequency 2034
Since the new instrument performs comparably to its predecessor for frequencies less than 10 kHz, the previously published specifications apply (1)and are not demonstrated here. To demonstrate the accuracy of the frequency response from 10 kHz to 1 MHz, measurements were made of a parallel RC network consisting of a 50 kQ f 5% composition resistor (dc value 49.0 kQ) and a 10 pF i 10% ceramic disk capacitor. In addition,the impedance of this network was measured with a HP model 4815A RF vector impedance meter from 400 kHz to 4 MHz for the purpose of comparison. Finally the frequency dependent impedance of a thick nitrobenzene membrane with symmetrical aqueous bathing solutions was also measured to demonstrate the characterization of a real system. The membrane was 50 mm2 by 4 mm thick and 0.1 M KN03 saturated with AgCl was used as a bathing solution. Measurements were made using reversible Ag/AgCl electrodes and AR grade reagents.
RESULTS AND DISCUSSION Figure 3 depicts the impedance plane plot of the frequency-dependent impedance of the test circuit. Both cross-correlator and vector bridge data are displayed. In addition, a perfect semicircle with a fixed intercept of (0,O) and forced agreement with the low frequency points has been superimposed over the data set. Since all the data deviate from ideality above 300 kHz, it is hypothesized that this is a real effect attributable to an inherent inductive reactance in series with the capacitance. Agreement between the two instruments is excellent and the relative error in the impedance is less than 4% up to 700 kHz. Although relative deviation for the crosscorrelator method increases rapidly above 700 kHz, the mean error over the common frequency range for the two methods is still less than 10%. Analysis of the cross-correlator data yields a value of 48.9 kQ for the resistor and 10.9 p F for the parallel capacitor. Both of these figures are in good agreement with the known values of the circuit components although the
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
measured capacitance is a t the upper limit of the manufacturer'stolerances. Blank measurements indicated that the geometry of the component holder introduces roughly an extra picofarad of capacitance in parallel, making the component's capacitance more closely 10 pF. The factors limiting the high frequency accuracy of the cross-correlator are well understood. The two most important sources of error are: (1) timing errors caused by the finite transition time of the comparator and the analog gate. These delays result in a constant 40-ns error in the time of gating. The magnitude of the resulting phase error depends on the phase of the signal being gated, but this error can never be more than 7.2" a t 1 MHz; (2) frequency dependent background signals caused by feedthrough of the reference signal to the switch junction. The reference signal is a square wave; the coupling is capacitive and the relaxing load is constant so the integrated magnitude of the feedthrough for a single gate transition is constant. The value of this constant is small and a t low and intermediate frequencies represents a negligible part of the total signal. However, a t frequencies above 500 kHz, where the duration of this transient represents a significant portion of the excitation period, high background levels are introduced and must be measured and corrected for. The uncertainty associated with this correction and the signal distortion introduced by these transients severely restricts the upper frequency limit of the instrument. The empirical impedance plane plot of a constrained nitrobenzene membrane with symmetrical, aqueous, bathing solutions is shown in Figure 4. The presence of two distinct circles is indicative of two rate processes for ionic transport. The high frequency process is characteristic of the bulk re-
sistance of the nitrobenzene phase and the geometrical capacitance of the membrane. Measurement of these bulk membrane properties would not have been possible with a more limited bridge. The second process obsirved is probably caused by slow interfacial transport kinetics coupled with double layer charging (5) but more experiments are required for a conclusive assignment. Involvement of the support, acrylic coated polypropylene, as an equivalent surface resistance is indicated here, since a second semicircle is not observed when hydrophilic supports are used. It is clear from the above example that broad band capability can increase the utility of impedance measurements for the study of membrane transport phenomena. However the use of this new cross-correlator need not be restricted to membrane electrochemistry. In fact, any application in which dc-coupled phase-sensitive detection up to 1MHz is required could potentially benefit from application of this circuit. Similarly, the gated integrator proposed here could also be used in other applications, with its simplicity, accuracy, and capability for direct computer control as significant advantages. LITERATURE CITED A. J. Bentz, J. R. Sandifer, and R. P. Buck, Anal. Cbem., 46, 543 (1974). R. D. Armstrong and K. Taylor, J. flectroanal. Cbem., 63, 9 (1975). M. Hieftje, Anal. Cbem., 44 (7), 69A (1972). P. G. Cath and A. M. Peabody, Anal. Chem., 43 ( I I ) , 91A (1971). (5) J. R. Macdonald, J. Cbem. Phys., 61, 3977 (1974).
(1) (2) (3) (4)
RECEIVEDfor review May 24,1976. Accepted August 16,1976. This work was supported by the National Science Foundation Grant MPS 7500970 and MRL Grant DMR7203024.
Solvent for Extraction of fucose Spots f ram Paper Chromatograms Bipin Rasiklal Mehta Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
Generally sugars are separated on paper chromatograms using a mixture of n-butanokacetic acidwater (4:1:5) (I), and spots are revealed by aniline hydrogen phthalate (I).The aldopentose gives a bright red spot and aldohexose, deoxy sugars, and uronic acids give various shades of green and brown (2). For the quantitative estimation, the extraction of sugar hexose from the chromatogram is generally carried out using 0.7 N hydrochloric acid in 80% ethanol (3). Complete extraction of L-fucose by this solvent takes prolonged time. Table I. Extraction of Fucose from Paper Chromatograms Composition of solvent, % v/v Serial Ethyl No. alcohol Waterb Ether Acetone
Hence, for quickening the extraction process, modification of the known solvent has been carried out and is reported here. EXPERIMENTAL Mixed sugar solution was prepared by dissolving 100 mg each of the galactose, glucose, mannose, xylose, fucose, and rhamnose in distilled water and making up the volume to 100 ml. The mixed sugar solution, 0.05 ml, was spotted on one end of a
Aciditya of solvent,
% extraction after
2h
4h
6h
24 h
62.50 65.00 20.00 22.50 24.00 25.00 95.00 96.25
80.00 85.00 33.30 33.00 34.00 33.30
100.00 100.00
100.00 100.00
100.00 100.00 100.00 100.00
N 3
80 80 70
4
70
5 6 7
60 60 70 70 60 60
1
2
8
9 10
20 20 20 20 20 20
20 20 20 20
...
...
10. 10
20 20
... ... ... ...
... ...
...
... ... ...
0.7 1.0 0.7
10
1.0 0.7 1.0 0.7 1.0
20 20
0.7 1.0
10
36.25 45.00
12.50 12.50 12.00 13.00 77.50 86.25 96.25 96.25
100.00 100.00
100.00 100.00
40.00 40.00 42.00 35.00
Hydrochloric acid was used for the purpose. Water includes the water content of the hydrochloric acid used for adjusting the aciditv of the solvent. a
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13,
NOVEMBER 1976
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