Table I. Peak Alternating Currents as a Function of Concentrations Concn C, moles/liter
2.50 X 5.00 X
Ba
1.00 X 2.00 X 2.50 X
I
C
A\
I
- a15
1
I
- 0.35
I
- 0.55
-
v
-a175
Figure 1. Ac anodic stripping voltammograms
+
Curve A: Urine in 0.2M.HC104; Curve E : as A 1.00 X 10-6M TI(I); C u r v e C : a s B + 5.00 X 1 0 - 6 M P b ( l l ) ; C u r v e D : a s E i n c . d .
tion of the concentration range from 2.50 X 10-7M to 2.50 x 10-6M with Pb(I1) interference to 5.00 X 10W6M. The lead concentration of normal urines, about 10 pg/l. does not show a peak, and only a slight increase of the base current after the thallium wave has been observed. At a higher lead concentration, a wave appears 124 mV anodic of the thallium wave, and the broadness of the Pb(I1) wave indicates the irreversibility of its electrode process. Resolution of TI(I) and Pb(I1) is possible a t a ratio of 1 to 20. Earlier studies have shown that Tl(1) is excreted in a directly reducible form (2). Consequently, with the use of an internal standard, a quantitative method to determine
-
Av peak height,a/p pA
0.107 0.216 0.428 0.859 1.100
IP
- lib.c
( x 10-5)
2.14 2.16 2.14 2.15 2.20 Av 2.16 f 0.02
Std dev.O %
2.5 1.1 0.6 0.5 0.3
Average and average deviation of 5 replicate determinations on same urine. b Preelectrolysis of 2 minutes.
thallium traces in clinical cases of poisoning can be proposed a t the 0.05- to 0.5-ppm level. Sample urines with albuminuria up to 0.1% gram and treatments effected with chelating agents and phenothiazines (10) do not affect results. A sample of urine supplied by the “Doctor Josi. M . M. Fernandez” Hospital (Rosario University) from a patient with thallium poisoning was analyzed by the method proposed, thus confirming the validity of the analysis.
CONCLUSIONS Using anodic stripping ac voltammetry, a rapid and direct method for the determination of thallium in urine is established whose sensitivity is enhanced by the low background current with a reproducibility of 2.5% for 0.05 ppm thallium in urine. The Tl(1) wave has a symmetrical shape and its ac peak is a linear function of the concentration in the micromolar range without interference from overlapping waves. The method avoids pre-treatment by dilution, cell exchange, use of complexing agents, degasification, and foaming, and would maintain its simplicity. Further extensive application of this powerful electroanalytical technique in biochemical research is in progress. Received for review June 2, 1972. Accepted December 19, 1972. ( 1 0 ) J. E. Camoirano, Actas BioqGimicas. 7, 8 ( 1 9 6 6 ) .
I AIDS FOR ANALYTICAL CHEMISTS Averaging Polarograph with Digital Output Basil H. V a s s o s l Department of Chemistry, Seton Hall University, South Orange. N.J. 07706
The potentialities of sampling in polarography have been recognized for some time (1,2) but its importance has become much greater in connection with the use of digital computers for laboratory data processing 13-51. There are several types of modern digital electroanalytical instruments. Some sophisticated and expensive units use a computer for control and data processing (6,7). Other instruments (8) attempt to fulfill in one complex unit all the needs of electroanalysis. Between these elaborate units and the old-fashioned po1Present address, D e p a r t m e n t of C h e m i s t r y , Colorado S t a t e U n i v e r s i t y , F o r t Collins, Colo. 80521.
1292
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
larograph there is a vast gap; it is hoped that the design presented below will fill this gap. The basic design philos(1) K. Kronberger, H . Strehiow. and A. W. Ebel, Poiarogr. Eer . 5, 62
(19 5 7 ) . (2) E . R . Brown, D. E. Smith, and D. D. DeFord, Ana/. Chem . 38, 1131 (19 6 6 ) . (3) G . Lauer. R. Abel. and F C. Anson, Anal. Chem.. 39, 765 (1967) (4) G . Lauer and R. A. 0steryoung.Anab Chem , 40, ( i o ) ,30A (1968) ( 5 ) S . P. Perone,Ana/. Chem.. 43, 1288 ( 1 9 7 1 ) . (6) S. P. Perone, J. W. Frazer. and A. Kay, A n a / . Chem. 43, 1485 (1971). (7) H. E. Keller and R . A . Osteryoung.Ana/. Chem , 43,342 ( 1 9 7 1 ) . (8) L. Ramaley and G. S. Wiison, Ana/. Chem.. 42, 606 (1970)
,f
SPRING
\c
STEEL
LUCITE
---hp///////A
I
HOLE FOR
TI
PIVOT
C
L_
E SAMPLE S 3
~1
e l
F
IN e
SAMPLE S2
l
Z
N
-
”
l
5
(a) (b)
15V
;iT
FROM GATE
TC022pF
2N3904
Figure 2. Knocker system: ( a ) mechanical construction; the coil is taken from a 12-V DC power relay. ( b ) driving circuit; the 25-V zener (1N2821) protects against switching transients. If the mechanical power is insufficient, a larger zener voltage can be used, since the duty cycle is low
-
2 13 SEC
( b)
Figure 1. ( a ) Main timing system. Transformer TI gives a 25 VAC output, 21 is a 1N3826 Zener diode. The flip-flops (FF) are halves of Motorola MC 856 P integrated circuits. The integrated circuit gates are: AND = half of MC3026L, AND/NOR = half of MC3020. The inverters are all one-transistor power units ( b ) Detail of the transistor inverters. Note that the logic state 1 becomes about f 1 5 V . The RC input circuit increases the rise time of the transition, which improves the operation of the FET gates
- - , ~
R E S E T INTEGRATOR AND A D V A N C E 1
,
- -:INTEGRATE
SAM!L_E
C
INSTRUMENTATION Digital circuitry is used for the main timing sequence, and andog elements for the potentiostat, ramp generation, and sampling. While it would have been possible to generate a “step-ramp’’ by digital means, the potentiostat and sampling circuits are much simpler in analog than digital form. The digital circuitry shown in Figure 1 uses a 60-Hz line-derived square wave to control a timing sequence (9). The outputs from the power inverters, shown in ( b ) of the same figure are logic pulses between + l 5 V and ground, of 133 msec duration, and in the particular sequence necessary to operate the gated analog circuitry. These logic signals from the clocking system are designated for convenience by the letters A-F. Provision is made (output D) for knocker signals (Figure 2 describes the knocker). Other outputs control the stepwise advance of the cell voltage (A), and sampling (C, E , and F ) . Signal B governs a 1.06-second sampling of the DME current. (9) 6 .H. Vassos and G. W. Ewing, “Analog and Digital Electronics for Scientists,” J. Wileyand Sons, New York, N . Y . , 1972, p 238.
r I
LT---,--
D
ophy adopted sought: moderate cost, improved results over classical polarographs, step output, and a synchronizing signal to permit direct interfacing with digital devices. The advantages of both analog and digital circuits, preferably in integrated form, were to be used as appropria t e.
1
LJ
_
-
s
- -
DRO_PI(WX -
--
-
-_
I
,
L..
_
- - -
-
I
Lj ____________
E-,? I
’
LJ
------
- - 1 I
1
-
