663
Anal. Chem. 1986, 58, 663-664
New Configuration for Construction of pH Gradients in Flow Injection Analysis Angel Rios, M. D. Luque de Castro, and Miguel Valc&rcel* Department of Analytical Chemistry, Faculty of Sciences, University of Cbrdoba, Cbrdoba, Spain The simultaneous determination of metal species based on the construction of p H gradients in flow injection analysis (FIA) ( I ) was suggested by Betteridge and Fields in some interesting and ingenious papers (2-5), but it has not had subsequent development. The establishment of these pH gradients in FIA a t the two carrier-sample interfaces is accomplished by injecting a large sample volume a t (pH), into a carrier at (pH),. The magnitude of these gradients is a function of the injection sample volume and the residence time of this sample within the reactor. When the sample contains two cations, M, and M, that react with the same reagent, R, in the carrier, but over different and nonoverlapping pH intervals, the established p H gradient makes possible the existence of three reacting zones in the injected plug: central zone a t (pH),, where the reaction with MI takes place and two zones a t the ends at (pH),, where M2 reacts. Three peaks are obtained when the plug passes through the detector; the two extreme ones related to the concentration of Mz and the central one to the concentration of MI. In principle Betteridge and Fields applied this approach to the determination of Pb(I1) and V(V) with 4-(2-pyridylazo)resorcinol (PAR). These cations react with PAR yielding nonoverlapping absorbance vs. p H curves for V(V) below pH 3.0 and for Pb(I1) above p H 8.9. Calibration curves are linear for lead in the presence of variable amounts of vanadium. Vanadium (central peak) however cannot be determined accurately because in order to achieve a good resolution of the peaks, it is necessary to work with high vanadium concentrations that fall outside the PAR/V(V) ratio suitable for obtaining linear calibration curves. We think that the major shortcoming of this configuration is the impossibility of obtaining a reagent concentration high enough for the vanadium determination a t the center of the plug, since the access of PAR to the central zone also implies the access of the buffer of p H 9.9 in which PAR is dissolved. This results in an increase in the pH in this zone and, hence, poses an interference for the reaction PAR/Pb(II). T o circumvent this problem, a new configuration is suggested to establish pH gradients in FIA that ensures the optimum p H and the suitable reagent concentration in the central zone of the sample plug. It corporates a second injection valve containing PAR a t p H 2 into the loop of the sample valve (as is shown in Figure 1). The simultaneous injection of the two valves provides a reacting plug with three zones under adequate conditions for the resolution of these mixtures.
EXPERIMENTAL SECTION Reagents. An aqueous solution of M PAR (pH 10.2) was prepared as follows: 0.0638 g of PAR, 3.05 g of NH4C1,and 17.5 mL of NH, (25%) were diluted to 250 mL with distilled water. An aqueous solution of M PAR (pH 2.1) was prepared as follows: 0.638 g of PAR, 3.05 g of NH4Cl,and HCl to pH 2.1 were combined in a 250-mL volumetric flask. The concentration of stock solutions of Pb(I1) and V(V) was 1.000 g/L. Apparatus. A Pye Unicam SP6-500 spectrophotometer connected to a Radiometer REC 80 recorder, a Gilson Minipuls-2 peristaltic pump, a Hellma 178.12QS flow cell (inner volume 18 pL), and two Tecator L100-1 injection valves were used. Manifold. As shown in Figure 2, the manifold consists of a single channel of PAR at pH 10.2 in which the sample injection valve, Vs, is inserted and whose loop accommodatesa second valve, V,, the loop of which in filled with a PAR solution of pH 2.1. The
A)
8) INJECTION
Flgure 1. Formation of a quadruple carrier,-sample-carrier,-sample-carrier, interface by use of injection valve, V,, inserted into the loop of V,: (A) filling position of loops of sample and carrierpand (6)
schematic situation of the zone with four interfaces at the moment of the simultaneous injection of both valves. I , and I, are the two interfaces created.
PAR IpH:10,2I
530 nm
q i 2 8 mL min”
1
Flgure 2. Manifold with dual injection valve for simultaneous injection of sample, V,, and reagent, V,: V , = 950 pL, V , = 30 ,uL.
simultaneous injection of both provides three reacting zones. The pH of the central one is close to 2, whereas that of the extremes is approximately 10. The reactions developed in L (PAR/Pb(II) and PAR/V(V)) result in three peaks that are measured at 540 nm.
RESULTS AND DISCUSSION The variables were optimized in order to achieve good separation of the peaks, thus ensuring the existence of clearly defined p H zones. As the rate of both reactions is fast, the optimum reactor length is only 60 cm and the flow rate 2.8 mL/min. The injected volumes are key factors in achieving the separation between peaks. The sample valve, V,, inserts
0003-2700/86/0358-0663$01.50/0 0 1986 American Chemical Society
664
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
1
Table I. Simultaneous Determination of Pb(I1) and V ( V ) (@g/mL)in Synthetic Samples"
A
added
-
I
C
lmin.
V(V)
Pb(I1)
V(V)
6.0 6.0 8.0 10.0 6.0 8.0 6.0
0.5
6.0 6.0 7.9 9.8 6.0 8.0 5.9
0.6
1.0 1.0 1.0 2.0 2.0 3.0
1.1 1.1 1.1 2.0 2.0
3.1
Average o f t r i p l i c a t e injection.
B
1I
found
Pb(I1)
followed for measuring the maximum absorbances, also makes it necessary to consider the zone of negative absorbance (Figure 3). The first peak does not satisfy the additivity condition, possible due to the short interval elapsed between injection and detection, which does not provide suitable pH for the reaction between PAR and Pb(I1). In order to overcome this problem, the length of reactor L must be increased. In this way, however, the absorbance of the other two peaks diminishes because the dispersion is increased. We have opted for working with the second and third peaks. The equations for the calibration curves corresponding to vanadium (second peak) and lead (third peak) are
ir A'
AV = 0.202[V5+]
C'
+ 0.067
C for 0.3 C [V5+] 2 . 5 ~ g / m L( r = 0.9994)
APb = 0.061[Pb2+] - 0.231 for 6.0
C
[Pb2'] C14.0 yg/mL ( r = 0.9997)
B'
(i Recordings obtained with the configuration shown in Figure 2 for injection by means, V,, of (A, A') Pb(I1) sample, (B, B') V(V) sample, and (C, C') mixture of both cations. The concentrations of Pb(I1) are 8.0 and 6.0 wg/mL and of V(V) are 1.0 and 2.0 kg/mL for Figure 3.
upper and lower recordings, respectively. a large volume (950 wL) at pH 2.1, establishing two reacting zones in the carrier-sample interfaces at a pH sufficiently alkaline (>8.9) to allow development of the reaction PAR/ Pb(I1). Valve V, injects into the sample a small volume (30 HL)of reagent at pH 2.1, which ensures the occurrence of an exclusive reacting zone for V(V) at the center of the sample plug. As a matter of fact, the proposed configuration combines the normal and reversed FIA modes, the reversed mode corresponding to injection of the reagent into the sample stream. This fact makes it necessary to consider two different base lines: one originated by the stream of PAR at pH 10.2 (colored solution), which must be taken into account for the extreme peaks, and the other due to the sample (colorless), to which the central peak must be referred. This criterion,
and show higher sensitivity for determination of vanadium, The main advantage of this configuration is the possibility of determining V(V) and Pb(I1) simultaneously by applying the above formulated equations. In Table I are shown some synthetic samples analyzed by use of the proposed injection approach. The relative standard deviations obtained for 11 samples (triplicate injection) containing 2.0 and 6.0 pg/mL V(V) and Pb(I1) were f2.8% and +=LO%, respectively. These results can be considered acceptable. We wish to point out that the chemical system is not totally suitable for the simultaneous determination of vanadium and lead since the reagent has several acid-base forms with different colors, which can cause interference (6). The proposed configuration ensures the establishment of the suitable pH gradient and reagent concentration at the center of the injected sample. The versatility of the suggested configuration is of additional interest because the use of the second injection valve allows any modifying species to be included with the sample in a suitable manner. Registry No. Pb, 7439-92-1;V, 7440-62-2;PAR, 1141-59-9.
LITERATURE CITED ( 1 ) Valcircel, M.; Luque de Castro. M. D. "Flow Injection Analysis: Principles and Applications"; Ellis Horwood: Chichester, in press. (2) Betteridge, D.; Fields, B. Anal. Cbem. 1978, 50, 654. (3) Betteridge, D.; Dagless, E. L.; Fields, 5.; Sweet, P.; Deans, D. R . Anal. Proc. (London) 1981, 26. (4) Baban. S.; Beetlestone, D.; Betteridge, D.; Sweet, P. Anal. ' Chirn. Acta 1980, 774, 319. (5) Betteridge. D.; Fields, B. Anal. Chlm. Acta 1981, 132, 139. (6) Sandell, E. B.; Onishi, H. "Photometric Determination of Traces of Metals. General Aspects"; Wiley: New York, 1979; pp 481.
RECEIVED for review May 30, 1985. Accepted September 17, 1985. This work was supported by the CAICyT under Grant 2012-83.
