Batch injection with potentiometric detection

Analytica Chimicu Actn, 252 (1991) 215-221 Elsevier Science Publishers B.V., Amsterdam

21s

Batch injection with potentiometric detection Joseph Wang * and Ziad Taha Department

o/ Chemistv,

New Mexico State University, Los Cruces, NM 88003 (USA) (Received 29th November

1990)

Abstract The characteristics and advantages of employing ion-selective electrodes (ISEs) as detectors for batch injection (Bl) systems are evaluated. Sharp potential response peaks are obtained for the injection of microliter samples onto the nearby ISE surface. Such dynamic measurements performed in batch systems result in high sample throughput and reproducibility, similar lo those of established flow-injection operations. For example, pH measurements can be performed at rates of up to 720 samples per hour. There is no observable carry-over (between samples of low and high concentrations) and the precision is typicatly l-29 (relative standard deviations). The applicability of BI-ISE system to chloride and fluoride measurements and for assays of real samples is also illustrated. The effecls of various experimental variables on the BI potentiometric response are described. Such use of highly specific detectors offers many prospects for future use of BIA systems. The absence of pumps, injection valves and ffow cells greatly reduces the instrumentation costs for rapid automated measurements of discrete ions. Keywords:

Potentiometry;

Batch injection;

Foods; Ion-selective

The increasing demand for rapid assays of large numbers of samples has resulted in the development of high-speed automated systems with good precision and accuracy [l-4]. Analytical flow systems based on air-segmented continuous-flow [2] and non-segmented flow-injection 133 methods have received most of the attention in automated analysis. Other innovative concepts are desired to meet the growing trend and need for automated methods. The recently introduced batch injection analysis (BIA) represents a departure from the traditional way of performing automated assays [5,6]. This non-flow technique involves the injection of a sample plug (from a micropipette tip) toward a nearby detector, immersed in a large-volume blank solution. Transient response peaks, similar to those of the well-established flow-injection analysis (FIA), are observed on passage of microliter samples over the detector, Such dynamic batch operation results in many of the attractive features 0003-2670/91/$03SO

electrodes;

Waters

(sample throughput and size, sensitivity, reproducibility and simplicity) of FIA, while eliminating the need for expensive pumps or valves and associated tubing. The BI concept was illustrated earlier [5,6] using amperometric detection at ordinary electrodes, as well as at biologically and chemically modified surfaces. The purpose of this work was to explore the suitability of potentiometric ion-selective electrodes (ISEs) as detectors in BIA. Potentiometric ISEs have been widely used in automated (air-segmented and FIA) systems for the rapid determination of discrete ions in numerous matrices [3]. Many effective ISEs, suitable for such determinations, have been introduced in the past two decades. The inherent selectivity of these electrodes permits potentiometric measurements with little or no sample pretreatment. Such high specificity makes ISEs very attractive for BI operation, which lacks a solution-handling capability and relies on the use of active or selective detectors. With highly

0 1991 - Elsevier Science Publishers B.V. All rights reserved

216

J. WANG

hole

AND

2. TAHA

surements were made with a laboratory-built amplifier, with a gain range of 2.5-1000, and the millivolt output was recorded on an Omniscribe strip-chart recorder.

Reagents stirring bar _

Fig. 1. Large-volume cell for BI with potentiometric detection.

selective detectors, such as ISEs, BI provides an effective means for rapidly transporting samples to the sensing surface. The requirements, performance characteristics, advantages and utility of the BI-ISE combination are examined, evaluated and illustrated in this paper.

EXPERIMENTAL

Apparatus The BI electrochemical cell is shown in Fig. 1. The inverted ISE was introduced through a hole drilled at the bottom comer. A hole, in the cell cover, located exactly opposite the center of the ISE, was used for accommodating. a standard Eppendorf micropipette. The pipette tip was kept at a fixed distance (usually 2 mm) from the center of the ISE. Reproducible sample injections onto the sensing surface were accomplished via controlled dispensation, (in accordance with the manufacturer’s recommendations). Another hole in the cover served for introducing the Ag/AgCl reference electrode (BAS, Model RE-1) during fluoride and chloride measurements. The cell was placed on a magnetic stirrer. Other details and dimensions of the BI cell are given elsewhere [5]. Spherical (Beckman, Model 39831) and flat (Markson, Model 989B) pH electrodes were used for pH measurements. Fluoride and chloride electrodes were obtained from Orion (Models 940900 and 941700, respectively). In all instances, the electrode diameter was 12 mm. Potentiometric mea-

All chemicals were of analytical-reagent grade. Standard solutions and working buffers were prepared with distilled, deionized water. Standard buffer solutions (pH 2-12) were prepared using buffer tablets (Metrepak pHydrion buffers). Stock solutions of fluoride and chloride were prepared using sodium fluoride and potassium chloride (Baker), respectively. The cell solution for the pH measurements was 0.1 M phosphate buffer containing 0.25 M potassium chloride (pH 7.00). Chloride measurements were made in potassium dihydrogenphosphate solution (0.05 M) containing 0.25 M sodium nitrate (Baker); the pH was adjusted to 6 using sodium hydroxide. Fluoride experiments were done in a solution containing 0.2 M sodium acetate, 0.17 M acetic acid, 0.35 M sodium chloride and I g 1-r l,Zdiaminocyclohexane-N, N, N, N-tetraacetic acid (DCTA) (Aldrich). Some fluoride measurements were made in the presence of 0.5 mM aluminum sulfate (Fisher). Bovine serum albumin was obtained from Sigma. Juice samples (Campbell’s tomato juice and Spicy Hot V8 vegetable juice) were purchased in a local store and were filtered through a 0.45-pm filter before measurement. A coffee sample was prepared by dissolving 0.3 g of Mountain Blend coffee in 25 ml of distilled water. Tap water samples were collected in the laboratory.

RESULTS

AND DISCUSSION

BIA employs a stirred blank solution, ;antaining an appropriate detector, into which highly reproducible sample volumes are injected [5]. Transient peaks r esult from the passage of the sample zone over the nearby detector surface. Because of the short injector-detector distance, the so’lution-handling capability is negligible, and highly reactive or selective detector surfaces are desired. The inherent specificity of ISEs make them very attractive for batch-injection measure-

BATCH INJECTION

WITH POTENTIOMETRIC

DETECTION

217

ments of discrete ions. Three common ISEs were employed to assess and illustrate this capability. pH measurements Figure 2 shows representative strip-chart recordings for sequential triplicate injections of 20-~1 solutions of different pH (4 and 10). Despite the enormous (106) difference in the hydrogen ion concentration, the BI system shows no observable carry-over. The rapid increase and decrease in the potential response results in sharp, baseline-resolved peaks. These indicate effective transport and replenishment of the sample zone to and from the surface. As a result, the analytical readout is obtained within few seconds following the injection and the BI-pH system can be operated at very high sampling rates. For example, Fig. 3 illustrates pH measurements at rates of (A) 720 and (B) 320 samples h -‘. The peak width (at 0.6C,,,,,) is extremely short (2.5 s; note the different time scales). Such injection rates compare favorably with those employed in analogous FIA

3min

El

TiMEb) Fig. 2. Typical recorder tracing of sequential triplicate injections of solutions of (A) low (4) and (B) high (10) pH values. Sample volume, 20 ~1; electrode-tip distance, 2 mm; stirring rate, 300 ‘pm. Cell solution, 0.1 M phosphate buffer-O.25 M potassium chloride (pH 7.0).

>

I&J set

ET

-

a,I

SOsee

e

TIME t-1

Fig. 3. Replicate injections of samples of pH 10 at rates of (A) 720 and (B) 360 samples h-‘. Conditions as in Fig. 2.

systems. Further speed improvements may be attained by using smaller sample volumes. Critical to the success of the BIA operation, particularly to the reproducibility of the response, is the use of a flat ISE surface. Figure 4 shows typical BIA peaks for 40 repetitive injections of 20 ~1 of solution of pH 1.0, as obtained with (A) spherical and (B) planar pH electrodes. The latter yielded a very reproducible response during this prolonged series Irelative standard deviation (R.S.D.) = 1.3%&l. In contrast, inferior reproducibility (R.S.D. = 6.8%) was observed with the spherical pH sensor. Apparently, a planar, “walljet”-like, detector design is essential for the reproducible transport pattern of the sample plug over the surface (and particularly for its “wash-out” via radial dispersion). The high reproducibility of the BI-pH measurements (observed with the flat sensor) compares with that of FIA-pH measurements, and may be further improved through automation of the injection process (particularly via robotics). Another prolonged experiment was aimed at examining the effect of surface-active

J. WANG

218

materials on the BI-potentiometric response. Figure 4C shows the resulting peaks for a solution of pH 4 containing 10 mg 1-l of bovine serum albumin. A highly reproducible response is observed over the entire series (R.S.D. = 1.3%), indicating that the presence of the surface-active protein does not affect the BI data. Such behavior is attributed to the short residence time of the surfactant solution over the surface and to its rapid dilution in the cell solution. The strong buffer capacity of the cell solution prevents memory effects associated with a gradual “build-up” of samples in the cell. This point is illustrated in Fig. 5, which shows three injections of 20 ,ul of solution of pH 10. After these injections, at points indicated by the arrows, 6 ml of solution of pH 2 were added to the cell solution (from a hole in its cover). Each such addition is equivalent to 300 injections. Despite the large

2% TIME Fig. 4. Repetitive BI-pH measurements. Forty injections of samples of pH 10 obtained with (A) spherical and (B) planar pH electrodes. (C) Fifty injections of sample of pH 4 containing 10 mg I-’ bovine serum albumin. Other conditions as in Fig. 2.

AND Z. TAHA

TIME(+) Fig. 5. Detection peaks for Injections were followed by solution (to the cell, through the arrows). Conditions as in

injections of samples of pH 10. additions of 6 ml of the pH 10 its cover, at points indicated by Fig. 2.

volume added and the enormous difference in the proton concentration, the BI peak potentials for the pH 10 samples remain essentially the same, and no baseline drift is observed. It is clear from these data that BI-pK measurements do not exhibit memory effects even after several hundred injections. Similarly to analogous FIA measurements, it is important to consider the formation of a concentration gradient due to a pH titration occurring between the sample and the buffer background solution. The resulting interfacial concentration profile depends primarily on the buffer capacity of the background and sample solutions, and also on the dispersion of the sample zone. Under the limited-dispersion conditions existing in the BI system, such effects are not significant and do not affect the quantification (i.e., pH measurements). For example, BI detection peaks for injections of 20-1.11samples of pH 4.solution decrease by 8 and 11% on increasing the phosphate buffer concentration (of the background solution) from 0.01 to 0.1 and 0.5 M, respectively. The neutralization process of the pH titration reaction has a profound effect on the rep!enishment of the solution from the surface. The higher the buffer capacity, the more effective is the sample removal. For example, the peak width at 0.6C,,,,, (for 20-~1 samples of pH 4 solution) decreased from 6.3 to OS s on

l3ATCH

INJECTION

WITH POTENTIOMETRIC

DETECTION

increasing the phosphate buffer concentration from 0.01 to 0.5 M. This effect becomes greater in the absence of solution stirring to facilitate the sample removal (with peak widths of 20 and 0.5 s for buffer concentrations of 0.01 and 0.5 M, respectively). Figure 6 shows a typical strip-chart recording for a series of standard hydrogen ion solutions over the pH range 2-11. Samples were injected in triplicate, alternating between low and high pH levels. The BI-pH system responds rapidly to these sharp pH changes. The resulting plot of peak potential vs. pH was linear, with a slope of 65 mV pH-’ (correlation coefficient 0.998; not shown). Similar deviations from a Nemstian behavior were reported for FIA-ISE systems [‘/,8]. Figure 7 illustrates the application of the BI-pH system to pH measurements of various drinks, The four samples (a-d) and two standards (e and f) yielded sharp potential peaks at a rate of 180 samples h- ‘. The resulting pH values [(a) 4.2, (b) 7.3, (c) 4.1 and (d) 5.01 were in excellent egree-

300

219

C

a

OE I Ql

3min

III

III III e TIME c’-Y

Fig. 7. Continuous BI-pH monitoring of (a-d) various drink samples and buffer standards of (e) pH 10 and (f) pH 4. Samples: (a) Campbell tomato juice, (b) tap water. (c) Spicy Hot VS vegetable juice and (d) coffee. Conditions as in Fig. 2.

ment with those obtained in analogous assays using the conventional (non-injection/stationaryj approach.

TIME(+)

Fig. 6. BI-pH calibration data. Samples were injected in triplicate and had the pH values shown from 2 to 11. Conditions as in Fig. 2.

Chloride and fluoride measurements Figure 8 examines carry-over effects using (A) chloride and (B) fluoride ISEs. Well defined and sharp peaks, with no apparent carry-over, are observed (despite the use of widely different concentration levels). The faster response of the chloride electrode permits the use of a higher injection rate. The fluoride electrode, in contrast, is more sensitive and allows convenient quantification at the micromolar level. The absence of carry-over effects and the rapid fall-off to the baseline level are attributed to the attractive “wash-out” nature of the “wall-jet”-like configuration (both electrodes had a flat surface). Such a configuration also ensures that the incident sample zone is actually being active at the surface, with negligible contributions from the outside solution. Coupled with the huge dilution factor offered by the largevolume BI ceh, the system can tolerate numerous

J. WANG AND 2. TAHA

220 Volume, pl 50

d 3min E IO

I

I TiME(+)

d Pm Fig. 10. Effect of (A) sample volume and (B) tip-electrode distance on the BIA response for 1 mM fluoride solution. Other conditions as in Fig. 8B.

1

Fig. 8. Carry-over for sequential injections of (A) chloride solutions of low (5 mM) and high (100 mM) concentrations and (8) fluoride solutions of low (SO CM) and high (1 mM) ccxentrations. Samples were injected in triplicate. Sample volume, SO ~1; electrode-tip distance, 2 mm; stirring rate, (A) 300 and (B) SO0 rpm. Cell solutions: (A) 0.05 M KH,PO,-0.25 M NaNOJ; (B) 0.2 M CH,COONa-0.17 M CHJOOH-0.35 M NaCI, 1 g I-’ DCTA.

injections with little or no memory effects. For example, Fig. 9 examines the effect of adding, between the injections, large volumes of the chloride and fluoride solutions (through the cell cover

I

I

20

I

TIME@-)

Fig. 9. Detection peaks for injections of (A) 100 mM chloride and (B) 0. 5 mM fluoride solutions_ Arrows indicate additions of 2 ml of (A) 100 mM chloride solution and (8) 6 ml of 0.5 mM fluoride solution. Conditions as in Fig. 8, but the cell solution in (B) contains also 0.5 mM aluminum sulphate and sample volumes of 20 PI.

at points indicated by the arrows). Such additions (each of which corresponded to a few hundred injections) had a negligible effect on the BI peaks. Note the small step in the potential baseline associated with the large chloride additions (A). Such flat “ramps” do not effect the superimposed BI potential peaks. Similarly, the fluoride BIA response (B) is not affected by two similar additions (corresponding to a total of 600 injections). This is due to the presence of aluminum ions in the cell solution, which prevent a build-up of the free fluoride ion concentration and assures a steady baseline potential. Figure 10 examines the effect of (A) the injection volume and (B) the tip-electrode distance on the BI-ISE response. The peak potential increases rapidly with increasing volume up to about 50 ~1 and then starts to level off. A similar volume effect was observed in BI-voltammetric measurements [5]. The use of an Eppendorf micropipette assures reproducible sample volumes, as indicated by the high precision obtained throughout. The response increases sharply with increasing tipelectrode separation up to 2 mm, after which it decays rapidly. As in FIA, the key issue in BI is the reproducible transport of the sample zone toward the detector. Figure 11 illustrates the high reproducibility of the BI-ISE operation. Two series of fifty and twenty repetitive injections of (A) chloride and (B)

BATCH

INJECTION

WITH

POTENTIOMETRIC

DETECTION

I IMk I+-)

Fig. 11. Repetitive injections of (A) 25 mM chloride and (B) 1 mM fluoride solutions. Other conditions as in Fig. 8.

fluoride solutions, respectively, yielded a highly stable response with R.S.D. 2.4% and 1.4%. The stable response demonstrates again that BID can tolerate the presence of analyte in the cell even after numerous injections. Notice also the sharp peaks and rapid “wash-out” using high injection rates of 100 (chloride) and 50 fluoride samples h--l. Overall, the performance shown in Fig. 11 compares favorably with that of analogous FIAISE systems. Repetitive 50-pl injections chloride and fluoride standard solutions of increasing concentration, over the ranges OS-10 mM and 10-5000 PM, respectively, were used to assess the linearity and detection limits (not shown). The resulting plots of peak potential vs. log (concentration) were linear [slopes of 57.8 (chloride) and 56.8 (fluoride) mV per decade]. Detection limits of 0.1 mM (0.18 pg) of chloride and 2 PM (2 ng) of fluoride ions were estimated based on a signal-tonoise ratio of 3.

221

In conclusion, the above experiments illustrate the compatibility and ease with which BIA and ISEs can be combined. Because of their high selectivity, ISEs are particularly suitable for the BI operation. This is analogous to FIA operation with limited dispersion (which serves mainly for rapid sample introduction toward selective detection). In the absence of such specificity, the BI operation should rely on the use of reactive detector surfaces, i.e. heterogeneous conversion of the analyte to a detectable species. Owing to its simplicity, speed and reproducibility, the BI-ISE system should find wide applications in numerous disciplines. This approach should also be valuable for testing rapidly the performance of ISEs. It is not possible, however, to perform multi-ion analysis. While the BI-ISE system was illustrated with three common electrodes, many other commercially available ISEs can be accommodated in the BI cell. The operation of BIA is not limited to electrochemical detectors. Other detection schemes can be adapted for use in this approach. Hence many innovative possibilities are expected in the near future.

REFERENCES ’I W.B. Furman, Continuous Flow Analysis. Theory and Praclice, Dekker, New York, 1976. L.T. Skeggs, Am. J. Pathol., 28 (1957) 311. J. Ruzicka and E.H. Hansen, Flow Injection Analysis, Wiley, New York, 1981. L.R. Snyder, Anal. Chim. Acta, 114 (1980) 3. J. Wang and Z. Taha, Anal. Chem., 63 (1991) 1053. J. Wang, L. Chen and 1.. Angnes, Electroanalysis, (1991) in press. J. Slanina, W.A. Lingerak and F. Bakker, Anal. Chim. Acta, 117 (1980) 91. M.L. Balconi, F. Sigon, R. Ferraroli and F. Realini. Anal. Chim. Acta, 214 (1988) 367.