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Automatic precipitation-dissolution in continuous flow systems
trendsin analyticalchemistry,vol. 8, no. l,J989
34 F. V. Bright, G. H. Vickers and G. M. Hieftje, Anal. Chem., 58 (1986) 1225. R. E. Russo and G. M. Hieftje, Anal. Chim. Acta, 134 (1982) 13. J. M. Harris, R. W. Chrisman, F. E. Lytle and R. S. Tobias, Anal. Chem., 48 (1976) 1937. M. J. Wirth and S.-H. Chou, Anal. Chem., 60 (1988) 1882. M. J. Wirth and G. J. Blanchard, in E. H. Piepmeier (Editor), Analytical Applications of Lasers, Wiley, New York, 1986, Ch. 14, p. 477. 8 J. G. Fujimoto, A. M. Weiner and E. P. Ippen, Appl. Phys. Lett., 44 (1984) 832.
9 J. R. Lakowicz, B. P. Maliwal, H. Cherek and A. Butler, Biochemistry, 22 (1983) 1741.
10 M. J., Sanders and M. J. Wirth, Chem. Phys. Lett., 101 (1983) 361. 11 C. V. Shank and E. P. Ippen, Appl. Phys. Lett., 26 (1975) 62.
12 C. V. Shank, E. P. Ippen, 0. Teschke and K. B. Eisenthal, J. Chem. Phys., 67 (1978) 5547. 13 D. Waldeck, A. J. Cross, Jr., D. B. McDonald and G. R. Fleming, J. Chem. Phys., 74 (1981) 3381. 14 M. D. Fayer, Ann. Rev. Phys. Chem., 33 (1982) 63, and references therein. 15 R. S. Moog, M. D. Ediger, S. G. Boxer and M. D. Fayer, J. Phys. Chem., 86 (1982) 4694.
16 P. D. Hyde, D. A. Waldow, M. D. Ediger, T. Kitano and K. Ito, Macromolecules, 19 (1986) 2533. 17 P. Bado, S. B. Wilson and K. R. Wilson, Rev. Sci. Instrum., 53 (1982) 706.
18 L. Andor, A. Lorincz, J. Siemion, D. D. Smith and S. A. Rice, Rev. Sci. Instrum., 55 (1984) 64. 19 J. P. Heritage and D. L. Allara, Chem. Phys. Lett., 74 (1980) 507.
20 B. F. Levine and C. G. Bethea, IEEE J. Quantum Electron,, 16 (1980) 85.
21 G. J. Blanchard
and M. J. Wirth, Anal. Chem., 58 (1986)
532.
22 P. A. Elzinga, F. E. Lytle, Y. Jian, G. B. King and N. M. Laurendeau, Appl. Spectrosc., 41(1987) 2. 23 P. A. Elzinga, R. J. Kneisler, F. E. Lytle, Y. Jiang, G. B. King and N. M. Laurendeau, Appl. Opt., 26 (1987) 4303. 24 P. Debye, Polar Molecules, Chemical Catalog Company, New York, 1929, p. 84. 25 F. Perrin, J. Phys. Radium, 7 (1936) 1. 26 C.-M. Hu and R. Zwanzig, J. Chem. Phys., 60 (1974) 4354. 27 T. J. Chuang and K. B. Eisenthal, J. Chem. Phys., 57 (1972) 5094.
28 G. H. Vickers, R. M. Miller and G. M. Hieftje, Anal. Chim. Acta, 192 (1987) 145.
29 W. A. Wyatt, F. V. Bright and G. M. Hieftje, Anal. Chem., 59 (1987) 572.
30 G. J. Blanchard and C. A. Cihal, J. Phys. Chem., 92 (1988) 5950.
31 J. R. Knutson, L. Davenport and L. Brand, Biochemistry, 25 (1986) 1805.
J. R. Knutson and L. Brand, Biochemistry, 25 (1986) 1811. 33 G. J. Blanchard, J. Chem. Phys., 87 (1987) 6802. 34 G. J. Blanchard and M. J. Wirth, J. Phys. Chem., 90 (1986) 2521. 32 L. Davenport,
Gary J. Blanchard has been a member of technical staffsince September, 1985 at Bell Communications Research, Inc., Red Bank, NJ 07701, U.S.A. He received his Ph.D. in analytical chemistry under M. J. Wirth at the University of Wisconsin-Madison in 1985 and his B.S. in Chemistry from Bates College in 1981.
Automatic precipitation-dissolution in continuous flow systems Miguel Valchrcel and Mercedes Gallego Cbrdoba, Spain The principles and use of precipitate formation and dissolution in continuous flow systems are presented and discussed. Automatic analytical methodologies such as indirect determinations of both organic and inorganic anions andpreconcentration-determination of metal traces using an atomic absorption spectrophotometer are compared with their batch (manual) counterparts.
Introduction
Precipitation is a widely used separation technique in classical analytical chemistry. Despite the extensive developments in automatic methods of analysis, there are few continuous or batch automatic analytical systems based on precipitate formation’. The reasons for this scarcity are: the heteroge0165-9936/89/$03.00.
neous kinetic process, the physical characteristics of the precipitates, their contamination and the need for aging. In addition, weighing is quite a difficult operation to incorporate into automatic systems, except for robotic stations. Our research team has recently approached automatic precipitation-dissolution systems by exploiting the advantages of flow injection analysis (FIA), particularly its simplicity and versatility2’ . Our first efforts in this regard were aimed at the study and application of continuous automatic precipitation-dissolution systems, coupled on-line with a conventional atomic absorption instrument, in implementing different analytical methodologies such as the indirect atomic absorption determination of non-metal species (both organic and inorganic anions) and the determination of traces and sub-traces of metal ions by use of a preconcentration assembly. .CQElsevier Science Publishers B .V.
35
trendsin analytical chemistry, vol. 8, no. 1,1989
Basic components The usual operation of a continuous precipitation system involves introducing the sample containing the analyte into a carrier solution which includes the precipitating reagent (usually a cation). Mixing both solutions results in the formation of a precipitate that is retained on a suitable filter, which is located prior to the measuring instrument and allows passage of the filtrate, which then proceeds on to the detector. Alternatively, the precipitate can be washed and the analyte determined after dissolution, There are thus two essential elements in a continuous precipitation system, namely the precipitation coil (usually a helically coiled piece of PTFE tubing of 0.3-0.7 mm I.D. and 100-300 cm in length) and the filter. The filters used in continuous separation techniques implemented in FIA systems were originally designed as cleaning devices in high-performance liquid chromatography (HPLC). We have assayed two types of stainless-steel filter: cylindrical and planar. The cylindrical filter, with a higher filtration area (3 cm2) and a chamber inner volume larger than 300 ~1, provides the better results. For a given filter geometry, variation in the pore size from 0.5 to 2.0 ,um has no influence on the analytical signal. The conventional batch precipitation procedure and that performed in a FIA manifold are compared in Fig. 1. Solutions are mixed in the injection zone. The precipitate is formed and allowed to settle in the precipitation coil and is filtered on the continuous filter. Types of flow injection configurations The indirect determination of the analyte in continuous precipitation systems can be carried out
either in the filtrate (by measuring the difference between the initial concentration of the precipitating reagent in the carrier and the concentration in the filtrate) or in the precipitate, once dissolved. Thus, there are two main types of configurations according to the heterogeneous processes involved: without and with dissolution of the precipitate formed. The first alternative can be implemented by normal and reversed FIA, and the second with and without injection. Withoutprecipitate dissolution The simplest possible configuration is shown in Fig. 2a (normal FIA mode). The sample containing the analyte is injected into a reagent cation stream. Precipitation takes place in the coil and the precipitate is retained on the filter. Since the concentration of the reagent cation decreases in the reaction plug zone, after passing to the filter, a negative peak is obtained upon arrival of the analyte at the detector. An identical volume of water is also injected and a small FIA peak is obtained and used as blank. The difference between these peaks is proportional to the analyte concentration. As an excess of precipi-
ANALY TE (sample)
WASHING SOLUTION DISSOLVING SOLUTION
PUMP
PUMP
I
n
I
I
1
ANION (sample)
TIME
I
REAGENT
b)
CATION
WASTE ANALVTE (sample) REAGENT
PRECIPITATION
FILTER
PUMP
TIME
Fig. 1. Comparison between continuous and manual precipitation. IV = injection valve, W = waste.
Fig. 2. Precipitation manifolds withoutprecipitate dissolution: (a) normal FIA mode, (b) reversed FIA mode. IV = injection valve, SV = selecting valve, AAS = atomic absorption spectrometer.
trends in analytical chemistry, vol. 8, no. I, I989
36
tating cation is required, which corresponds to a concentration that falls outside of the detector linear range, a water stream is incorporated into the flow system to dilute the cation solution prior to the nebulizer . When continuous precipitation is implemented in a reversed flow-injection configuration (Fig. 2b), it is necessary to use a further selecting valve to perform blank measurements. First, the reagent cation is injected into a water stream and a high FIA peak is obtained. The selecting valve is then switched and the sample is continuously pumped into the system; another identical injection of the reagent causes the analyte to form a precipitate, which is retained on the filter. The FIA peak obtained decreases with an increase of the analyte concentration in the sample. This configuration does not always require the incorporation of a water stream for dilution of the reagent cation prior to the nebulizer as this is already diluted in the carrier. With precipitate dissolution
The three basic operations involved, namely precipitate formation, washing and dissolution, can be performed in the configuration depicted in Fig. 3a. a)
ANALYTE (sample) REAGENT CATION
WASHING SOLUTION DISSOLVING SOLUTION
PUMP
I
1
TIME
b) WASTE ANALYTE
PRECIPITATION
FILTER
REAGENT
“:z~’
Three valves (one for injection and two for selection purposes) are required. In the precipitation step, the sample is injected into a reagent cation stream, the precipitate formed is retained on the filter and the reagent stream is sent to waste through selecting valve 2. In the washing step, selecting valve 1 allows for introduction of a washing solution stream which is led to the filter through the second selecting valve. The analytical signal yielded tends to raise the baseline. In the third step, a dissolving solution is passed through the precipitate retained on the filter. The rapidly dissolved precipitate yields a positive FIA peak, the height of which is proportional to the analyte concentration. Another configuration involving continuous precipitation and dissolution without injection for the continuous preconcentration of metal traces is shown in Fig. 3b. In the preconcentration step, both the sample and the reagent are continuously pumped into the system and the precipitate formed is retained on the filter. In the dissolution step, the selecting valve is switched to pass a stream of dissolving solution through the precipitate which, once dissolved, gives a positive FIA peak proportional to the amount of analyte present in the sample volume introduced. In Table I the advantages and disadvantages of the two main types of configurations are summarized. The most favourable aspects of continuous precipitation processes are their simplicity and higher sampling rate (decantation, centrifugation and dilution steps are compulsory in batch methods) and low cost per determination. The problem arising from the adsorption of interferents on the precipitate is overcome by using an FIA system involving a washing step prior to the precipitate dissolution. The reversed FIA mode, where the carrier never contains the cation, is free from the risk of adsorption of cations on the precipitate. The manifolds employed in precipitation-dissolution processes are generally more complicated and involve a larger number of steps, to the obvious detriment of precision and sampling frequency. They also call for a suitable reagent to effect the rapid dissolution of the precipitate. General features
PUMP
Types ofprecipitate y_-ii_~ TIME
Fig. 3. Precipitation manifolds with precipitate dissolution: (a) with sample injection, (b) without sample injection. For description see text. N = injection valve, SV = selecting valve, AAS = atomic absorption spectrometer.
We have assayed the continuous formation and dissolution of the three most representative types of precipitates4: gelatinous (also known as flocculent), curdy and crystalline, represented by ferric hydroxide, silver chloride and calcium oxalate, respectively. All three have different physico-chemical characteristics; however, all of them are suitable for the de-
37
trends in analytical chemistry, vol. 8, no. I,1989
TABLE
I. Advantages and disadvantages of continuous precipitation
Continuous
precipitation
systems Continuous precipitation-washing-dissolution
Advantages
Disadvantages
Advantages
Disadvantages
Simplicity High sampling frequency High precision
Filter washing after n injections Blank measurements
Decreased contamination No blank or filter washing Wider scope of application - simultaneous determinations - preconcentration
More complicated manifold Rapid dissolving reagent Low precision and sampling rate
velopment of automatic continuous methodologies provided that they are adapted to the features of the particular precipitate, Flow injection variables
The efficiency of a continuous precipitation system is directly influenced by the flow-rates, injected volume, geometric characteristics of the precipitation coil and type of filter. This study was carried out by the univariant method because we believe that the simplex method is not recommendable when a new approach is under development and one wants to establish the influence of each individual variable. When a configuration without precipitate dissolution is used, the injected volume affects the peak width, but not the peak height, as a result of the increased reaction zone and longer filtrate plug. When the precipitate is dissolved, the signal increases with the amount of precipitate and hence with the injected volume; it is thus necessary to control the sample volume, whether injected (Fig. 3a) or introduced in a continuous fashion (Fig. 3b). The reactor length significantly influences the precipitation efficiency. Thus, with short coils (usually
Filters should ideally be cleaned with the same reagent used for precipitate dissolution, in ultrasonic batches for 5 min. Continuous precipitation systems with precipitate dissolution require no filter cleaning whatsoever. Analytical features: sensitivity and selectivity
Continuous precipitation systems considerably increase the sensitivity of methods developed by conventional techniques. For a given chemical system and precipitating cation concentration, the sensitivity is greater when the cation is inserted into a sample carrier stream (reversed FIA mode) than when the sample itself is injected (normal FIA mode)5. This is probably because when the sample is used as carrier and the reagent is injected into the flow, the amount of sample in the reagent zone increases with increasing dispersion, thereby resulting in a greater extent of mixing and precipitation at lower sample concentrations. There are no significant differences in the sensitivity achieved with systems involving precipitate dissolution provided that low volumes are inserted. For several years the selectivity enhancement capabilities of flow injection analysis (kinetic discrimination or enhancement in favour of the analyte of interest) have been praised in the literature6. We have shown that the determination of chloride ion by preANALYTE CONCENTRATION
Filter washing
Continuous precipitate build-up resulting from successive sample injections in sequential determinations may clog the filter when the methodology used does not involve precipitate dissolution. Fig. 4 shows the maximum number of consecutive injections of the same sample that can be made before the filter requires changing or washing. Crystalline precipitates (calcium oxalate) are more compact and clog the filter pores after only 20 injections, whereas the other two types of precipitate (curdy or gelatinous) allow up to about 300 consecutive injections.
Aq
IO NUMBER
20
30 OF
200
LO
IDENTICAL
SAMPLES
Cl
300
Loo
INJECTED
Fig. 4. Variation of the apparent analyte concentration as a function of the number of identical samples injected by using a continuousprecipitation system without precipitate dissolution.
trends in analytical chemistry, vol. 8, no. I, 1989
38
TABLE II. Comparison of the selectivity levels achieved in the indirect determination of chloride by precipitation with silver Abbreviations used: a = conventional procedure; b and c = in flow injection manifolds without and with precipitate dissolution
Selectivity factoP
Ion
IBr10; ClOCN-, SCNAsO:-, BrO;, AsO;, CrO:-
Indirect atomic absorption determination of anions
b/a
c/a
2.0 7.0 6.0 12.0 8.0 5.0
300.0 70.0 15.0 14.0 8.0 5.0
’ Tolerated ratio in continuous ual method.
method/tolerated
ratio in man-
cipitation with silver on a non-dissolution .precipitation manifold is more tolerant to potential interferents (mainly anions such as I-, Br- and SCN-) than its manual counterpart. This increased selectivity can be further enhanced by dissolving the precipitate prior to the determination of the analyte since the species coprecipitating with silver may not be soluble in the reagent used to dissolve the precipitate. Such is the case with iodide ion; this forms an insoluble halide with silver which, unlike that of chloride, is ammonia-insoluble, which allows the determination of chloride in the presence of high iodide concentrations. Table II lists some of the most representative interferents in the determination of chloride. Applications Precipitation and filtration systems and FIA have seldom been used in conjunction, probably because of the still short life of this flow technique. As a detailed description of each procedure is beyond the scope of this article, we give a critical overview of the results obtained, which are then compared with those provided by the corresponding manual procedures in four basic aspects: sensitivity, selectivity, precision and rapidity. Table III lists the most rele-
TABLE
vant features (configuration employed, analyte concentration range, sampling frequency and precision) of the analytical applications of these systems reported to date.
In the determination of chloride in water? by the precipitation alternative and the two FIA modes, the linear range is similar in the normal FIA mode with and without precipitate dissolution. The reversed FIA mode has better features with respect to the normal FIA mode: higher sensitivity, and sampling frequency (because of the higher flow-rate of the carrier solution) and lower detection limit. However, normal flow injection has two advantages over reversed flow injection without precipitate dissolution: low sample volume consumption (in reversed FIA the sample circulates continuously through the system while in normal FIA the sample is injected) and slightly higher selectivity. The flow techniques have clear, advantages over conventional procedures: sensitivity, detection limit, selectivity and sampling frequency are more favourable in automatic precipitation. The results found in the determination of chloride in different types of water by the proposed procedures were contrasted with those obtained by the classical potentiometric method. As can be seen from Table IV, there is very good agreement between the two sets of results. Sequential multideterminations based on the use of a continuous precipitation-dissolution system can be implemented by merely finding two species precipitating jointly with the same cation when one of the precipitates obtained is selectively soluble in a given solvent. This novel application of FIA precipitation systems is of great interest as there are no literature references to indirect atomic absorption spectroscopy multideterminations involving precipitation reactions. Our team recently proposed the simultaneous determination of chloride and iodide based on the different solubility of their silver precipitates in
III. Analytical methodologies
Analyte
Sample
Configuration
Concentration range @g/ml)
Sampling frequency (h-l)
Chloride
Waters
Precipitation (nFIA) Precipitation (rFIA) Precipitation-dissolution with injection (nFIA) Precipitation (nFIA) Precipitation-dissolution Precipitation (rFIA) Precipitation (rFIA)
3-100 0.3-10 8:l to 1:60
200 10
2.0 3.5 2.3-4.7
5-90 0.001-1.5 2.5-35 2.0-30
20 l-15 100 100
5.0 1.0-3.6 2.0 0.6
Chloride/iodide
Foodstuff
Oxalate Lead Sulfonamides Local anaesthetics
Waters Pharmaceuticals Pharmaceuticals
50
Relative standard deviation (%)
trendsin
analytical chemistry, vol. 8, no. I,1989
TABLE
IV. Determination
Sample Tap water
Well water
River water
1 2 3 1 2 3 1 2 3
39
Preconcentration of metal-ion traces and subtraces
of chloride in waters
Continuous precipitation (mg/ml)
Potentiometry (mtiml)
15.8 42.4 85.6 49.6 107.1 179.7 13.3 22.6 35.8
15.5 42.6 85.0 48.5 110.0 178.1 14.7 22.7 35.1
* 0.3” zk 0.6” + 0.7“ rf: 0.8’ + 0.5b + o.5b + o.2b + 0.2’ k 0.6b
f 0.5 f 0.4 f 0.4 k 0.2 ztr0.5 + 0.6 k 0.1 + 0.2 + 0.4
a Normal FIA mode. b Reversed FIA mode.
ammonia7. In the first step, the precipitation takes place in a normal flow injection manifold, and a peak proportional to the concentration of the sum of the two anions is obtained. A nitric acid stream is then introduced into the system and the precipitate is washed. When the baseline is reached, a selecting valve is switched and an ammonia stream selectively dissolves the silver chloride in the precipitate retained on the filter. A positive FIA peak appears as a result and chloride is determined alone. Iodide is determined by the difference7. Mixtures of these anions at ,@ml levels can be determined for chlorideto-iodide ratios from 8:l to 1:60. This procedure has been satisfactorily applied to the determination of these anions in foods and drinks, e.g. wine, cheese, bread, coffee and eggs. The determination of oxalate is based on the manual procedure developed by Menache’ for the analysis of oxalic acid in urine. By use of a continuous precipitation system, the author’s team4 carry out this determination by injecting 50 ,LL~ of sample solutions into a 60 pug/ml solution of calcium in acetic acidacetate buffer. Owing to the sluggish precipitation, the sample plug must be stopped at the reactor for 1 min and heated at 60°C. Unlike the manual method, this is quite fast as it allows up to 20 determinations per hour. The manual method entails allowing the samples to cool for 24 h in order to ensure complete precipitation of calcium oxalate, which results in a sample throughput of only 30 samples per day. The procedure involving precipitate dissolution (with a 0.2 M solution of EDTA in 7 M ammonia) which, in principle, should be more accurate than the non-dissolution method on account of the intermediate washing operation included to remove any cation vestiges adsorbed on the precipitate, has in fact a higher relative standard deviation (9% vs. 5%). This can be attributed to the larger number of steps involved in the procedure with precipitate dissolution, which also results in decreased sampling frequency.
The preconcentration and determination of traces of metal ions is a very interesting application of these automatic configurations. Trace preconcentration is generally carried out by coprecipitation with a carrier precipitate (collector) as direct separations based on precipitation reactions yield only small amounts of precipitate and their manipulation is thus cumbersome. This preconcentration technique requires large sample volumes, is rather time-consuming (i.e. it has a low sample throughput) and occasionally provides irreproducible results as a result of the manipulation preceding the analytical measurements. On the other hand, automatic preconcentration requires no collector as the direct separation is feasible however small the amount of precipitate obtained may be, thanks to the absence of manipulations in continuous precipitation systems. In the manifold employed for the determination of lead traces in water at the ng/ml level’, the sample is continuously aspirated into the system and merged with an ammonia stream. Precipitation is instantaneous and the basic salt formed is retained on a filter of suitable size. In the second step, the precipitate is dissolved in a nitric acid solution and the transient signal yielded by dissolved lead is measured by AAS. The method affords a concentration factor of up to 700. The sensitivity of the method depends on the sample volume used (Table III) and the selectivity is quite good. Indirect atomic absorption determination of pharmaceuticals
The latest application developed to date is the determination of pharmaceuticals. In general, organic pharmaceuticals can be determined through reaction with metals forming either a precipitate or an extractable ion-association compound or other complex, the metal content of which is measured by AAS*‘. We are currently working on the application of continuous precipitation systems to the analysis of active major constituents in pharmaceutical preparations. One of the achievements of this work is the development of an indirect determination for sulfonamides based on their precipitation with copper or silver ions and the use of a reversed FIA configuration”. Unlike its manual counterpart, which requires 100 mg of sulfonamide and a large excess of silver or copper, the FIA methods are quite sensitive ‘(Table III) and feature a detection limit of 1 pg/ml sulfonamide. Some substances (glucose, lactose, sucrose, ethylene glycol, glycerol, vanillin, talc power, etc.) that might be found in pharmaceutical samples as excipients and diluents, commonly used in the preparation of tablets, syrups, injections, drops and
trendsin analyticalchemistry,vol. 8, no. 1,1989
40
ointments, do not interfere at all. The copper method is more selective than the silver method and can be applied to the determination of sulfonamides in urine. Local anaesthetics can also be determined on a flow-injection precipitation configuration12. The method is based on the precipitation of these alkaloids with cobalt containing solutions (official method for identification of lidocaine proposed by the European and U.S. Pharmacopeias). Lidocaine, tetraCaine and procaine were used as samples to test the efficiency of the method. Some substances potentially found in pharmaceuticals and several ions commonly found in biological fluids did not interfere. The experimental results indicate satisfactory specificity, linearity, repeatability and sensitivity. The avobtained in pharmaceuticals erage recoveries ranged from 96.9 to 103.8% and the relative standard deviations were between 0.5 and 2.1%, which indicate a high accuracy and precision. Trends The implementation of automatic continuous methodologies based on precipitate formation (and dissolution in some instances) is a novel approach to non-chromatographic continuous separation techniques, one of the clearest trends in FIA. There is a host of potential applications based on roughly the same principles as those developed so far: indirect determination of organic and inorganic anions and preconcentration of metal traces (e.g. using organic collectors). There are also new interesting potential approaches such as: l Development of analytical procedures relying on continuous homogeneous precipitation, a Use of other types of detectors (e.g. photometric, fluorimetric),
Application of ultrasound to improve the physical properties of the precipitate in the coil or to facili-, tate its dissolution in the filter, Use of surfactants to dissolve precipitates and to create organized media with interesting analytical properties. Acknowledgement The authors gratefully acknowledge the aid of P. Martinez-JimCnez and R. Montero, and the financial support of the CICYT (grant No. PA86-0146).
References 1 M. Valcarcel and M. D. Luque de Castro, Automatic Methods of Analysis, Elsevier, Amsterdam, 1988. 2 M. Valcarcel and M. D. Luque de Castro, Flow Injection Analysis, Principles and Applications, Ellis Horwood, ChiChester, 1987. 3 J. Ruzicka and E. Hansen, Flow Injection Analysis, Wiley, New York, 2nd ed., 1988. M. Gallego and M. Valcarcel, Anal. 4 P. Martinez-Jimenez, Chem., 59 (1987) 69-74. M. Gallego and M. Valcarcel, J. Anal. 5 P. Martinez-Jimenez, At. Spectrorn., 2 (1987) 211-215. 6 G. E. Pacey, presented at the IV International Conference on Flow Analysis, Las Vegas, NV, U.S.A., April I7-20,1988 . M. Gallego and M. Valcarcel, Anal. 7 P. Martinez-Jimenez, Chim. Acta, 193 (1987) 127-135. 8 R. Menache, Clin. Chem., 20 (1974) 1444-1446. M. Gallego and M. Valcarcel, Analyst 9 P. Martinez-Jimenez, (London), 112 (1987) 1233-1236. 10 S. S. M. Hassan, Organic Analysis Using Atomic Absorption Spectrometry, Ellis Horwood, Chichester, 1984. 11 R. Montero, M. Gallego and M. Valcarcel, J. Anal. At. Spectrom., 3 (1988) 725-729. 12 R. Montero, M. Gallego and M. Valcarcel, Anal. Chim. Acta, (1988) in press. Miguel Valc&cel and Mercedes Gallego are at the Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, 14004 Cckdoba, Spain.
An Introduction to Applications of Light Microscopy in Analysis, by D. Simpson and W. G. Simpson,
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34 F. V. Bright, G. H. Vickers and G. M. Hieftje, Anal. Chem., 58 (1986) 1225. R. E. Russo and G. M. Hieftje, Anal. Chim. Acta, 134 (1982) 13. J. M. Harris, R. W. Chrisman, F. E. Lytle and R. S. Tobias, Anal. Chem., 48 (1976) 1937. M. J. Wirth and S.-H. Chou, Anal. Chem., 60 (1988) 1882. M. J. Wirth and G. J. Blanchard, in E. H. Piepmeier (Editor), Analytical Applications of Lasers, Wiley, New York, 1986, Ch. 14, p. 477. 8 J. G. Fujimoto, A. M. Weiner and E. P. Ippen, Appl. Phys. Lett., 44 (1984) 832.
9 J. R. Lakowicz, B. P. Maliwal, H. Cherek and A. Butler, Biochemistry, 22 (1983) 1741.
10 M. J., Sanders and M. J. Wirth, Chem. Phys. Lett., 101 (1983) 361. 11 C. V. Shank and E. P. Ippen, Appl. Phys. Lett., 26 (1975) 62.
12 C. V. Shank, E. P. Ippen, 0. Teschke and K. B. Eisenthal, J. Chem. Phys., 67 (1978) 5547. 13 D. Waldeck, A. J. Cross, Jr., D. B. McDonald and G. R. Fleming, J. Chem. Phys., 74 (1981) 3381. 14 M. D. Fayer, Ann. Rev. Phys. Chem., 33 (1982) 63, and references therein. 15 R. S. Moog, M. D. Ediger, S. G. Boxer and M. D. Fayer, J. Phys. Chem., 86 (1982) 4694.
16 P. D. Hyde, D. A. Waldow, M. D. Ediger, T. Kitano and K. Ito, Macromolecules, 19 (1986) 2533. 17 P. Bado, S. B. Wilson and K. R. Wilson, Rev. Sci. Instrum., 53 (1982) 706.
18 L. Andor, A. Lorincz, J. Siemion, D. D. Smith and S. A. Rice, Rev. Sci. Instrum., 55 (1984) 64. 19 J. P. Heritage and D. L. Allara, Chem. Phys. Lett., 74 (1980) 507.
20 B. F. Levine and C. G. Bethea, IEEE J. Quantum Electron,, 16 (1980) 85.
21 G. J. Blanchard
and M. J. Wirth, Anal. Chem., 58 (1986)
532.
22 P. A. Elzinga, F. E. Lytle, Y. Jian, G. B. King and N. M. Laurendeau, Appl. Spectrosc., 41(1987) 2. 23 P. A. Elzinga, R. J. Kneisler, F. E. Lytle, Y. Jiang, G. B. King and N. M. Laurendeau, Appl. Opt., 26 (1987) 4303. 24 P. Debye, Polar Molecules, Chemical Catalog Company, New York, 1929, p. 84. 25 F. Perrin, J. Phys. Radium, 7 (1936) 1. 26 C.-M. Hu and R. Zwanzig, J. Chem. Phys., 60 (1974) 4354. 27 T. J. Chuang and K. B. Eisenthal, J. Chem. Phys., 57 (1972) 5094.
28 G. H. Vickers, R. M. Miller and G. M. Hieftje, Anal. Chim. Acta, 192 (1987) 145.
29 W. A. Wyatt, F. V. Bright and G. M. Hieftje, Anal. Chem., 59 (1987) 572.
30 G. J. Blanchard and C. A. Cihal, J. Phys. Chem., 92 (1988) 5950.
31 J. R. Knutson, L. Davenport and L. Brand, Biochemistry, 25 (1986) 1805.
J. R. Knutson and L. Brand, Biochemistry, 25 (1986) 1811. 33 G. J. Blanchard, J. Chem. Phys., 87 (1987) 6802. 34 G. J. Blanchard and M. J. Wirth, J. Phys. Chem., 90 (1986) 2521. 32 L. Davenport,
Gary J. Blanchard has been a member of technical staffsince September, 1985 at Bell Communications Research, Inc., Red Bank, NJ 07701, U.S.A. He received his Ph.D. in analytical chemistry under M. J. Wirth at the University of Wisconsin-Madison in 1985 and his B.S. in Chemistry from Bates College in 1981.
Automatic precipitation-dissolution in continuous flow systems Miguel Valchrcel and Mercedes Gallego Cbrdoba, Spain The principles and use of precipitate formation and dissolution in continuous flow systems are presented and discussed. Automatic analytical methodologies such as indirect determinations of both organic and inorganic anions andpreconcentration-determination of metal traces using an atomic absorption spectrophotometer are compared with their batch (manual) counterparts.
Introduction
Precipitation is a widely used separation technique in classical analytical chemistry. Despite the extensive developments in automatic methods of analysis, there are few continuous or batch automatic analytical systems based on precipitate formation’. The reasons for this scarcity are: the heteroge0165-9936/89/$03.00.
neous kinetic process, the physical characteristics of the precipitates, their contamination and the need for aging. In addition, weighing is quite a difficult operation to incorporate into automatic systems, except for robotic stations. Our research team has recently approached automatic precipitation-dissolution systems by exploiting the advantages of flow injection analysis (FIA), particularly its simplicity and versatility2’ . Our first efforts in this regard were aimed at the study and application of continuous automatic precipitation-dissolution systems, coupled on-line with a conventional atomic absorption instrument, in implementing different analytical methodologies such as the indirect atomic absorption determination of non-metal species (both organic and inorganic anions) and the determination of traces and sub-traces of metal ions by use of a preconcentration assembly. .CQElsevier Science Publishers B .V.
35
trendsin analytical chemistry, vol. 8, no. 1,1989
Basic components The usual operation of a continuous precipitation system involves introducing the sample containing the analyte into a carrier solution which includes the precipitating reagent (usually a cation). Mixing both solutions results in the formation of a precipitate that is retained on a suitable filter, which is located prior to the measuring instrument and allows passage of the filtrate, which then proceeds on to the detector. Alternatively, the precipitate can be washed and the analyte determined after dissolution, There are thus two essential elements in a continuous precipitation system, namely the precipitation coil (usually a helically coiled piece of PTFE tubing of 0.3-0.7 mm I.D. and 100-300 cm in length) and the filter. The filters used in continuous separation techniques implemented in FIA systems were originally designed as cleaning devices in high-performance liquid chromatography (HPLC). We have assayed two types of stainless-steel filter: cylindrical and planar. The cylindrical filter, with a higher filtration area (3 cm2) and a chamber inner volume larger than 300 ~1, provides the better results. For a given filter geometry, variation in the pore size from 0.5 to 2.0 ,um has no influence on the analytical signal. The conventional batch precipitation procedure and that performed in a FIA manifold are compared in Fig. 1. Solutions are mixed in the injection zone. The precipitate is formed and allowed to settle in the precipitation coil and is filtered on the continuous filter. Types of flow injection configurations The indirect determination of the analyte in continuous precipitation systems can be carried out
either in the filtrate (by measuring the difference between the initial concentration of the precipitating reagent in the carrier and the concentration in the filtrate) or in the precipitate, once dissolved. Thus, there are two main types of configurations according to the heterogeneous processes involved: without and with dissolution of the precipitate formed. The first alternative can be implemented by normal and reversed FIA, and the second with and without injection. Withoutprecipitate dissolution The simplest possible configuration is shown in Fig. 2a (normal FIA mode). The sample containing the analyte is injected into a reagent cation stream. Precipitation takes place in the coil and the precipitate is retained on the filter. Since the concentration of the reagent cation decreases in the reaction plug zone, after passing to the filter, a negative peak is obtained upon arrival of the analyte at the detector. An identical volume of water is also injected and a small FIA peak is obtained and used as blank. The difference between these peaks is proportional to the analyte concentration. As an excess of precipi-
ANALY TE (sample)
WASHING SOLUTION DISSOLVING SOLUTION
PUMP
PUMP
I
n
I
I
1
ANION (sample)
TIME
I
REAGENT
b)
CATION
WASTE ANALVTE (sample) REAGENT
PRECIPITATION
FILTER
PUMP
TIME
Fig. 1. Comparison between continuous and manual precipitation. IV = injection valve, W = waste.
Fig. 2. Precipitation manifolds withoutprecipitate dissolution: (a) normal FIA mode, (b) reversed FIA mode. IV = injection valve, SV = selecting valve, AAS = atomic absorption spectrometer.
trends in analytical chemistry, vol. 8, no. I, I989
36
tating cation is required, which corresponds to a concentration that falls outside of the detector linear range, a water stream is incorporated into the flow system to dilute the cation solution prior to the nebulizer . When continuous precipitation is implemented in a reversed flow-injection configuration (Fig. 2b), it is necessary to use a further selecting valve to perform blank measurements. First, the reagent cation is injected into a water stream and a high FIA peak is obtained. The selecting valve is then switched and the sample is continuously pumped into the system; another identical injection of the reagent causes the analyte to form a precipitate, which is retained on the filter. The FIA peak obtained decreases with an increase of the analyte concentration in the sample. This configuration does not always require the incorporation of a water stream for dilution of the reagent cation prior to the nebulizer as this is already diluted in the carrier. With precipitate dissolution
The three basic operations involved, namely precipitate formation, washing and dissolution, can be performed in the configuration depicted in Fig. 3a. a)
ANALYTE (sample) REAGENT CATION
WASHING SOLUTION DISSOLVING SOLUTION
PUMP
I
1
TIME
b) WASTE ANALYTE
PRECIPITATION
FILTER
REAGENT
“:z~’
Three valves (one for injection and two for selection purposes) are required. In the precipitation step, the sample is injected into a reagent cation stream, the precipitate formed is retained on the filter and the reagent stream is sent to waste through selecting valve 2. In the washing step, selecting valve 1 allows for introduction of a washing solution stream which is led to the filter through the second selecting valve. The analytical signal yielded tends to raise the baseline. In the third step, a dissolving solution is passed through the precipitate retained on the filter. The rapidly dissolved precipitate yields a positive FIA peak, the height of which is proportional to the analyte concentration. Another configuration involving continuous precipitation and dissolution without injection for the continuous preconcentration of metal traces is shown in Fig. 3b. In the preconcentration step, both the sample and the reagent are continuously pumped into the system and the precipitate formed is retained on the filter. In the dissolution step, the selecting valve is switched to pass a stream of dissolving solution through the precipitate which, once dissolved, gives a positive FIA peak proportional to the amount of analyte present in the sample volume introduced. In Table I the advantages and disadvantages of the two main types of configurations are summarized. The most favourable aspects of continuous precipitation processes are their simplicity and higher sampling rate (decantation, centrifugation and dilution steps are compulsory in batch methods) and low cost per determination. The problem arising from the adsorption of interferents on the precipitate is overcome by using an FIA system involving a washing step prior to the precipitate dissolution. The reversed FIA mode, where the carrier never contains the cation, is free from the risk of adsorption of cations on the precipitate. The manifolds employed in precipitation-dissolution processes are generally more complicated and involve a larger number of steps, to the obvious detriment of precision and sampling frequency. They also call for a suitable reagent to effect the rapid dissolution of the precipitate. General features
PUMP
Types ofprecipitate y_-ii_~ TIME
Fig. 3. Precipitation manifolds with precipitate dissolution: (a) with sample injection, (b) without sample injection. For description see text. N = injection valve, SV = selecting valve, AAS = atomic absorption spectrometer.
We have assayed the continuous formation and dissolution of the three most representative types of precipitates4: gelatinous (also known as flocculent), curdy and crystalline, represented by ferric hydroxide, silver chloride and calcium oxalate, respectively. All three have different physico-chemical characteristics; however, all of them are suitable for the de-
37
trends in analytical chemistry, vol. 8, no. I,1989
TABLE
I. Advantages and disadvantages of continuous precipitation
Continuous
precipitation
systems Continuous precipitation-washing-dissolution
Advantages
Disadvantages
Advantages
Disadvantages
Simplicity High sampling frequency High precision
Filter washing after n injections Blank measurements
Decreased contamination No blank or filter washing Wider scope of application - simultaneous determinations - preconcentration
More complicated manifold Rapid dissolving reagent Low precision and sampling rate
velopment of automatic continuous methodologies provided that they are adapted to the features of the particular precipitate, Flow injection variables
The efficiency of a continuous precipitation system is directly influenced by the flow-rates, injected volume, geometric characteristics of the precipitation coil and type of filter. This study was carried out by the univariant method because we believe that the simplex method is not recommendable when a new approach is under development and one wants to establish the influence of each individual variable. When a configuration without precipitate dissolution is used, the injected volume affects the peak width, but not the peak height, as a result of the increased reaction zone and longer filtrate plug. When the precipitate is dissolved, the signal increases with the amount of precipitate and hence with the injected volume; it is thus necessary to control the sample volume, whether injected (Fig. 3a) or introduced in a continuous fashion (Fig. 3b). The reactor length significantly influences the precipitation efficiency. Thus, with short coils (usually
Filters should ideally be cleaned with the same reagent used for precipitate dissolution, in ultrasonic batches for 5 min. Continuous precipitation systems with precipitate dissolution require no filter cleaning whatsoever. Analytical features: sensitivity and selectivity
Continuous precipitation systems considerably increase the sensitivity of methods developed by conventional techniques. For a given chemical system and precipitating cation concentration, the sensitivity is greater when the cation is inserted into a sample carrier stream (reversed FIA mode) than when the sample itself is injected (normal FIA mode)5. This is probably because when the sample is used as carrier and the reagent is injected into the flow, the amount of sample in the reagent zone increases with increasing dispersion, thereby resulting in a greater extent of mixing and precipitation at lower sample concentrations. There are no significant differences in the sensitivity achieved with systems involving precipitate dissolution provided that low volumes are inserted. For several years the selectivity enhancement capabilities of flow injection analysis (kinetic discrimination or enhancement in favour of the analyte of interest) have been praised in the literature6. We have shown that the determination of chloride ion by preANALYTE CONCENTRATION
Filter washing
Continuous precipitate build-up resulting from successive sample injections in sequential determinations may clog the filter when the methodology used does not involve precipitate dissolution. Fig. 4 shows the maximum number of consecutive injections of the same sample that can be made before the filter requires changing or washing. Crystalline precipitates (calcium oxalate) are more compact and clog the filter pores after only 20 injections, whereas the other two types of precipitate (curdy or gelatinous) allow up to about 300 consecutive injections.
Aq
IO NUMBER
20
30 OF
200
LO
IDENTICAL
SAMPLES
Cl
300
Loo
INJECTED
Fig. 4. Variation of the apparent analyte concentration as a function of the number of identical samples injected by using a continuousprecipitation system without precipitate dissolution.
trends in analytical chemistry, vol. 8, no. I, 1989
38
TABLE II. Comparison of the selectivity levels achieved in the indirect determination of chloride by precipitation with silver Abbreviations used: a = conventional procedure; b and c = in flow injection manifolds without and with precipitate dissolution
Selectivity factoP
Ion
IBr10; ClOCN-, SCNAsO:-, BrO;, AsO;, CrO:-
Indirect atomic absorption determination of anions
b/a
c/a
2.0 7.0 6.0 12.0 8.0 5.0
300.0 70.0 15.0 14.0 8.0 5.0
’ Tolerated ratio in continuous ual method.
method/tolerated
ratio in man-
cipitation with silver on a non-dissolution .precipitation manifold is more tolerant to potential interferents (mainly anions such as I-, Br- and SCN-) than its manual counterpart. This increased selectivity can be further enhanced by dissolving the precipitate prior to the determination of the analyte since the species coprecipitating with silver may not be soluble in the reagent used to dissolve the precipitate. Such is the case with iodide ion; this forms an insoluble halide with silver which, unlike that of chloride, is ammonia-insoluble, which allows the determination of chloride in the presence of high iodide concentrations. Table II lists some of the most representative interferents in the determination of chloride. Applications Precipitation and filtration systems and FIA have seldom been used in conjunction, probably because of the still short life of this flow technique. As a detailed description of each procedure is beyond the scope of this article, we give a critical overview of the results obtained, which are then compared with those provided by the corresponding manual procedures in four basic aspects: sensitivity, selectivity, precision and rapidity. Table III lists the most rele-
TABLE
vant features (configuration employed, analyte concentration range, sampling frequency and precision) of the analytical applications of these systems reported to date.
In the determination of chloride in water? by the precipitation alternative and the two FIA modes, the linear range is similar in the normal FIA mode with and without precipitate dissolution. The reversed FIA mode has better features with respect to the normal FIA mode: higher sensitivity, and sampling frequency (because of the higher flow-rate of the carrier solution) and lower detection limit. However, normal flow injection has two advantages over reversed flow injection without precipitate dissolution: low sample volume consumption (in reversed FIA the sample circulates continuously through the system while in normal FIA the sample is injected) and slightly higher selectivity. The flow techniques have clear, advantages over conventional procedures: sensitivity, detection limit, selectivity and sampling frequency are more favourable in automatic precipitation. The results found in the determination of chloride in different types of water by the proposed procedures were contrasted with those obtained by the classical potentiometric method. As can be seen from Table IV, there is very good agreement between the two sets of results. Sequential multideterminations based on the use of a continuous precipitation-dissolution system can be implemented by merely finding two species precipitating jointly with the same cation when one of the precipitates obtained is selectively soluble in a given solvent. This novel application of FIA precipitation systems is of great interest as there are no literature references to indirect atomic absorption spectroscopy multideterminations involving precipitation reactions. Our team recently proposed the simultaneous determination of chloride and iodide based on the different solubility of their silver precipitates in
III. Analytical methodologies
Analyte
Sample
Configuration
Concentration range @g/ml)
Sampling frequency (h-l)
Chloride
Waters
Precipitation (nFIA) Precipitation (rFIA) Precipitation-dissolution with injection (nFIA) Precipitation (nFIA) Precipitation-dissolution Precipitation (rFIA) Precipitation (rFIA)
3-100 0.3-10 8:l to 1:60
200 10
2.0 3.5 2.3-4.7
5-90 0.001-1.5 2.5-35 2.0-30
20 l-15 100 100
5.0 1.0-3.6 2.0 0.6
Chloride/iodide
Foodstuff
Oxalate Lead Sulfonamides Local anaesthetics
Waters Pharmaceuticals Pharmaceuticals
50
Relative standard deviation (%)
trendsin
analytical chemistry, vol. 8, no. I,1989
TABLE
IV. Determination
Sample Tap water
Well water
River water
1 2 3 1 2 3 1 2 3
39
Preconcentration of metal-ion traces and subtraces
of chloride in waters
Continuous precipitation (mg/ml)
Potentiometry (mtiml)
15.8 42.4 85.6 49.6 107.1 179.7 13.3 22.6 35.8
15.5 42.6 85.0 48.5 110.0 178.1 14.7 22.7 35.1
* 0.3” zk 0.6” + 0.7“ rf: 0.8’ + 0.5b + o.5b + o.2b + 0.2’ k 0.6b
f 0.5 f 0.4 f 0.4 k 0.2 ztr0.5 + 0.6 k 0.1 + 0.2 + 0.4
a Normal FIA mode. b Reversed FIA mode.
ammonia7. In the first step, the precipitation takes place in a normal flow injection manifold, and a peak proportional to the concentration of the sum of the two anions is obtained. A nitric acid stream is then introduced into the system and the precipitate is washed. When the baseline is reached, a selecting valve is switched and an ammonia stream selectively dissolves the silver chloride in the precipitate retained on the filter. A positive FIA peak appears as a result and chloride is determined alone. Iodide is determined by the difference7. Mixtures of these anions at ,@ml levels can be determined for chlorideto-iodide ratios from 8:l to 1:60. This procedure has been satisfactorily applied to the determination of these anions in foods and drinks, e.g. wine, cheese, bread, coffee and eggs. The determination of oxalate is based on the manual procedure developed by Menache’ for the analysis of oxalic acid in urine. By use of a continuous precipitation system, the author’s team4 carry out this determination by injecting 50 ,LL~ of sample solutions into a 60 pug/ml solution of calcium in acetic acidacetate buffer. Owing to the sluggish precipitation, the sample plug must be stopped at the reactor for 1 min and heated at 60°C. Unlike the manual method, this is quite fast as it allows up to 20 determinations per hour. The manual method entails allowing the samples to cool for 24 h in order to ensure complete precipitation of calcium oxalate, which results in a sample throughput of only 30 samples per day. The procedure involving precipitate dissolution (with a 0.2 M solution of EDTA in 7 M ammonia) which, in principle, should be more accurate than the non-dissolution method on account of the intermediate washing operation included to remove any cation vestiges adsorbed on the precipitate, has in fact a higher relative standard deviation (9% vs. 5%). This can be attributed to the larger number of steps involved in the procedure with precipitate dissolution, which also results in decreased sampling frequency.
The preconcentration and determination of traces of metal ions is a very interesting application of these automatic configurations. Trace preconcentration is generally carried out by coprecipitation with a carrier precipitate (collector) as direct separations based on precipitation reactions yield only small amounts of precipitate and their manipulation is thus cumbersome. This preconcentration technique requires large sample volumes, is rather time-consuming (i.e. it has a low sample throughput) and occasionally provides irreproducible results as a result of the manipulation preceding the analytical measurements. On the other hand, automatic preconcentration requires no collector as the direct separation is feasible however small the amount of precipitate obtained may be, thanks to the absence of manipulations in continuous precipitation systems. In the manifold employed for the determination of lead traces in water at the ng/ml level’, the sample is continuously aspirated into the system and merged with an ammonia stream. Precipitation is instantaneous and the basic salt formed is retained on a filter of suitable size. In the second step, the precipitate is dissolved in a nitric acid solution and the transient signal yielded by dissolved lead is measured by AAS. The method affords a concentration factor of up to 700. The sensitivity of the method depends on the sample volume used (Table III) and the selectivity is quite good. Indirect atomic absorption determination of pharmaceuticals
The latest application developed to date is the determination of pharmaceuticals. In general, organic pharmaceuticals can be determined through reaction with metals forming either a precipitate or an extractable ion-association compound or other complex, the metal content of which is measured by AAS*‘. We are currently working on the application of continuous precipitation systems to the analysis of active major constituents in pharmaceutical preparations. One of the achievements of this work is the development of an indirect determination for sulfonamides based on their precipitation with copper or silver ions and the use of a reversed FIA configuration”. Unlike its manual counterpart, which requires 100 mg of sulfonamide and a large excess of silver or copper, the FIA methods are quite sensitive ‘(Table III) and feature a detection limit of 1 pg/ml sulfonamide. Some substances (glucose, lactose, sucrose, ethylene glycol, glycerol, vanillin, talc power, etc.) that might be found in pharmaceutical samples as excipients and diluents, commonly used in the preparation of tablets, syrups, injections, drops and
trendsin analyticalchemistry,vol. 8, no. 1,1989
40
ointments, do not interfere at all. The copper method is more selective than the silver method and can be applied to the determination of sulfonamides in urine. Local anaesthetics can also be determined on a flow-injection precipitation configuration12. The method is based on the precipitation of these alkaloids with cobalt containing solutions (official method for identification of lidocaine proposed by the European and U.S. Pharmacopeias). Lidocaine, tetraCaine and procaine were used as samples to test the efficiency of the method. Some substances potentially found in pharmaceuticals and several ions commonly found in biological fluids did not interfere. The experimental results indicate satisfactory specificity, linearity, repeatability and sensitivity. The avobtained in pharmaceuticals erage recoveries ranged from 96.9 to 103.8% and the relative standard deviations were between 0.5 and 2.1%, which indicate a high accuracy and precision. Trends The implementation of automatic continuous methodologies based on precipitate formation (and dissolution in some instances) is a novel approach to non-chromatographic continuous separation techniques, one of the clearest trends in FIA. There is a host of potential applications based on roughly the same principles as those developed so far: indirect determination of organic and inorganic anions and preconcentration of metal traces (e.g. using organic collectors). There are also new interesting potential approaches such as: l Development of analytical procedures relying on continuous homogeneous precipitation, a Use of other types of detectors (e.g. photometric, fluorimetric),
Application of ultrasound to improve the physical properties of the precipitate in the coil or to facili-, tate its dissolution in the filter, Use of surfactants to dissolve precipitates and to create organized media with interesting analytical properties. Acknowledgement The authors gratefully acknowledge the aid of P. Martinez-JimCnez and R. Montero, and the financial support of the CICYT (grant No. PA86-0146).
References 1 M. Valcarcel and M. D. Luque de Castro, Automatic Methods of Analysis, Elsevier, Amsterdam, 1988. 2 M. Valcarcel and M. D. Luque de Castro, Flow Injection Analysis, Principles and Applications, Ellis Horwood, ChiChester, 1987. 3 J. Ruzicka and E. Hansen, Flow Injection Analysis, Wiley, New York, 2nd ed., 1988. M. Gallego and M. Valcarcel, Anal. 4 P. Martinez-Jimenez, Chem., 59 (1987) 69-74. M. Gallego and M. Valcarcel, J. Anal. 5 P. Martinez-Jimenez, At. Spectrorn., 2 (1987) 211-215. 6 G. E. Pacey, presented at the IV International Conference on Flow Analysis, Las Vegas, NV, U.S.A., April I7-20,1988 . M. Gallego and M. Valcarcel, Anal. 7 P. Martinez-Jimenez, Chim. Acta, 193 (1987) 127-135. 8 R. Menache, Clin. Chem., 20 (1974) 1444-1446. M. Gallego and M. Valcarcel, Analyst 9 P. Martinez-Jimenez, (London), 112 (1987) 1233-1236. 10 S. S. M. Hassan, Organic Analysis Using Atomic Absorption Spectrometry, Ellis Horwood, Chichester, 1984. 11 R. Montero, M. Gallego and M. Valcarcel, J. Anal. At. Spectrom., 3 (1988) 725-729. 12 R. Montero, M. Gallego and M. Valcarcel, Anal. Chim. Acta, (1988) in press. Miguel Valc&cel and Mercedes Gallego are at the Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, 14004 Cckdoba, Spain.
An Introduction to Applications of Light Microscopy in Analysis, by D. Simpson and W. G. Simpson,
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Review
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Gas and Liquid Chromatography in Analytical Chemistry, by R. M. Smith, Wiley, Chichester, 1988,
v + 402 pp., ISBN O-471-90980-7, f 60.00. Chromatography ‘871 Proceedings of the Advances of Liquid Chromatography (Balatonsz$lak, 1986) and the Budapest Chromatography Conference (Budapest 1987), edited by H. Kal&z and L. S. Ettre,
Chromatography,
edited by F. A. A. Dallas, H. Read, R. J. Ruane and I. D. Wilson, Plenum Press, New York, 1988, v. + 247 pp., ISBN o-306-43934-9, US$ 59.50.
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Vandecasteele, Ellis Horwood, Chichester, + 171 pp., ISBN o-7458-0175-7, f 29.95.
1988, 6
Ion Chromatography in Water Analysis, by 0. A. Shpigun and Yu. A. Zolotov, Ellis Horwood, ChiChester, 1988, 6 + 188 pp., ISBN o-7458-0020-3, f 32.50. Kinetic Metho& in Analytical Chemistry, by s. Perez-Bendito and M. Silva, Ellis Howood, Chi_ Chester, 1988, 6 + 330 pp., ISBN o-7458-0105-6, f 49.50.