Anal. Chem. 1987, 59, 69-74
As a result of theae investigations, the Federal Health Office has recommended avoiding the installation of discarded old sleepers as playground equipment for the prevention of these health risks (18). Registry No. PhOH, 108-95-2;naphthalene, 91-20-3; acenaphthalene, 208-96-8; dibenzofuran,132-64-9; fluorene, 86-73-7; dibenzothiophene, 132-65-0;phenanthrene,85-01-8;anthracene, 120-12-7; cyclopenta[deflphenanthrene, 203-63-4; phenylnaphthalene, 3546571-5; fluoranthene, 206-44-0;pyrene, 129-00-0; chrysene, 218-01-9;triphenylene, 217-59-4;benzo[blfluoranthene, 205-99-2;benzovlfluoranthene,205-82-3;benzo[k]fluoranthene, 207-08-9; benzo[a]fluoranthene,203-33-8; benzo[e]pyrene, 19297-2; benzo[a]pyrene, 50-32-8;perylene, 198-55-0;indenopyrene, 72254-06-9;quinoline, 91-22-5; isoquinoline, 119-65-3;benzo[h]quinoline, 230-27-3;acridine, 260-94-6; azafluoranthene,8912645-4; phthalic anhydride, 85-44-9; 1-naphthol, 90-15-3; 4phenylphenol,92-69-3; benzothiophene, 11095-43-5; carbazole, 86-74-8. LITERATURE CITED (1) Metzner, W.; Bellmann, H. Ullmnns €nzyk/opasdie der technischen Chemk; Verlag Chemle: Weinheim, West Germany, 1976; Vol. 12, pp 865-702.
89
(2) Hunt, G. M.; Garret, 0.Wood Presetvation, 3rd ed.;McGraw-Hill: New York 1967. (3) Rueping, M. U.S. Patent 709799, 1902. See also Wassermann, C. German Patent 138933, 1902. (4) Petrowitz, H. J.; Becker, G. Materklpriifung 1964, 6, 461-470. (5) Petrowitz, H. J.; Becker, 0.Materklpriifung 1965, 7, 325-330. (6) Petrowitz. H. J.; Becker, G. Holz Roh-Werkst. 1976, 3 4 , 315-322. (7) Henningsson, B. Holz Roh-Werkst. 1983, 4 7 , 471-475. (8) Borwitzky, H.; Schomburg, G. J. Chromatogr. 1979, 770, 99-124. (9) Aiben, K. Anal. Chem. 1980, 5 2 , 1625-1628. (10) Nestler, F. H. M.; Anal. Chem. 1974, 4 6 , 46-53. (11) McNeii, D.; Vaughan, G. A. Rec. Annu. Conv. Br. Wood Preserv. ASSOC. 1884, 1-22. (12) Grimmer, G.; Jacob, J.; Naujack, K.-W.; Dettbarn, G. Anal. Chem. 1983, 55, 829-900. (13) Lee, M. L.; Novotny, M.; Bartle, K. Analytical Chemlstfy of Polycyclic Aromatic Compounds; Academic Press: New York, Oxford, 1981. (14) Jacob, I.; Karcher, W.; Wagstaffe, P. J.; Fresenius' Z . Anal. Chem. 1984, 377, 101-114. (15) Heller, I. J. Ind. wg. 1930, 12, 169-197. (16) Flicklnger, C. W.; Lawrence, A. W. Proc. Annu. Meet. Am. WoodPreserv. Assoc. 1982, 11-28. (17) Willeitner, H.; Dieter, H. 0. Holz Roh-Werkst. 1984, 4 2 , 223-231. (18) Public Relations Service of the Federal Health Office, news item of August 9th, 1984.
RECEIVED for review March 10, 1986. Resubmitted August 18, 1986. Accepted August 26, 1986.
Analytical Potential of Continuous Precipitation in Flow Injection-Atomic Absorption Configurations Pilar Martinez-Jimenez, Mercedes Gallego, and Miguel ValcBrcel*
Department of Analytical Chemistry, Faculty of Sciences, University of Cbrdoba, Cbrdoba, Spain
The incorporation of a continuous precipitation unit built into flow injection manifolds Is presented and discussed, in order to show its analytical potential in the development of indirect automatic atomic absorption methods. The preclpitate is formed by InJecUng an ankn (analyte) into a carrier containing a cation (reagent) and is retained on a stalniess-steel filter. Two unsegmented flow configurations, one of which involves the dissoiutlon of the retained precipitate, have been tested. Three types of precipitates encountered in gravimetric procedures have been considered in this work crystalline (calcium oxalate), curdy (silver chloride), and gelatinous (ferric hydroxide). By continuous precipitation, chloride and oxalate could be determined in the range 3-100 pg/mL and 5-90 pg/mL, respectively, with a relative standard deviation between 2 and 5 %. The sampling frequency ranges between 10 and 50 h-'.
To extend the scope of application of atomic absorption spectrometry (AAS) and to increase the sensitivity achievable for elements directly unsuitable for trace analysis, much attention has been devoted to the development of indirect methods for such elements (1-3). The application of these methods to the determination of nonmetal elements and organic compounds involves carrying out a suitable chemical reaction. One of these is based on the reaction of an anion with a solution of a cation a t an adequate concentration to yield an insoluble compound. The cation is then measured either in the filtrate or in the precipitate. Precipitates of analytical interest have very different physical properties, which determine both their analytical applicability and the 0003-2700/67/0359-0069$0 1.50/0
optimum experimental conditions for their formation. Precipitates are classified as gelatinous or flocculent, curdy, and crystalline (41, and representative examples of these types are ferric hydroxide, silver chloride, and calcium oxalate, respectively. The hydrous oxide of iron is a precipitate that has an indefinite composition and may be represented as Fez03.H20,but for simplicity is usually formulated as Fe(OH)3 and called ferric hydroxide ( 4 ) . No reference has been found in the literature about the application of this preecipitate using U S detection. Pinta (5) had previously determined chloride using a large excess of silver nitrate; chloride in the precipitate was indirectly determined by AAS in the 5-100 pg/mL range. Menache (6) had previously used the calcium oxalate system to determine oxalic acid in urine. In this method, oxalate is precipitated from urine with excess calcium at pH 5. Calcium in the precipitate is determined indirectly by AAS, substracting excess calcium measured in the supernate from the overall amount of calcium present and added to the urine sample. This method permits routine determinations of oxalate between 17 and 186 pg/mL, at a rate of 30 samples/day. The flow injection analysis (FIA) technique is a major alternative to manual methods of analysis (7,8). The advantages of the AAS-FIA association have been recently praised by several authors (9, IO). Indirect methods may also appear more attractive when the necessary chemical and possibly physical manipulation of the sample can be reduced to filling and injecting the contents of a sample loop. Thus, the incorporation of a continuous separation system (ion exchange, liquid-liquid extraction, etc.) offers numerous advantages. We have recently made use of the AAS-FIA association in conjunction with liquid-liquid extraction as a separation technique for the indirect determinations of perchlorate in human 0 1986 American Chemical Society
70
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
urine and serum samples (10,nitrate and nitrite in fwdstuff (12,13),and anionic surfactants in wastewaters (14). An air-segmented flow configuration utilizing a continuous filter has been previously described by Skinner and Docherty (15) for the analysis of potassium in fertilizers. Other system using similar procedures are well-documented in Technicon reports. The results obtained with these configurations are not quantitative; therefore, the use of precipitation reactions with this system designer should be avoided as much as possible (16). This paper describes for the first time the further development of a continuous precipitation system integrated in a flow-injection manifold with atomic absorption detection. Several turbidimetric determinations using an FIA manifold have been reported elsewhere (17-23). The aim of this work is to illustrate the application of a separation method (precipitation) in FIA configurations by making a comprehensive study on the variables involved, the different types of precipitates that can be formed, and the basic types of configurations that can be employed with this purpose. Thus, two unsegmented flow configurations, one of which involves the dissolution of the retained precipitate, have been tested. By use of the dissolution procedure, the problem aasociated with the adsorption of the cations onto all of the precipitates can be avoided by washing the precipitate with a suitable reagent until obtaining a nil signal (the total adsorbed cation is removed). However, although the dissolution procedure should be more accurate in theory, the experimental data refute this assumption. This could be the result of the large number of manipulation steps required by this configuration (washing and dissolution of the precipitates). The application of such a separation method to a real samples is currently in progress in our laboratory.
EXPERIMENTAL SECTION Reagents. AU reagents were of analytical reagent grade, and solutionswere prepared with distilled water. An ironml) solution was prepared by dissolving 1.00 g of metal iron in 50 mL of concentrated nitric acid (1 + 1) and subsequently was diluted to 1 L with water. A silver(1) solution was prepared by dissolving 1.574 g of silver nitrate in 1 L of 1% (v/v) nitric acid. A ealcium(rI) solution (0.500 g/L) was made by dissolving 1.249 g of calcium carbonate,previously dried at 110 "C, in 60 mL of 3 M acetic acid. The carbon dioxide was removed by boiling, and subsequently the solution was diluted to 1 L with water. Chloride and oxalate stock solutions (Loo0 g/L) were prepared in distilled water from sodium chloride and diammonium oxalate monohydrate, previously dried at 110 "C. Apparatus. A Perkin-Elmer 380 atomic absorption spectrometer equipped with suitable hollow cathode lamps and an adjustablenebulizer was used throughout. The instruments were set and the air-acetylene flame adjusted following standard recommendations. The spectrometer output was connected to a Radiometer REC-80 servograph recorder. Gilson minipulsd pump, Rheodyne 5301 and 5041 selecting valves, and a Tecatar L 100-1 injection valve were used. A Scientific System 0.5-105 column provided with a removable sueen-type stainless-steelfdter (pore size,0.5 -; chamber inner volume, 580 p b filtration area, 3 em*, aproximately) which was originally designed as a cleaningdevice for HPLC, was employed for filtration purposes. Manifold Design. (I) Without Precipitate Dissolution. The manifold used is illustrated in Figure 1A. The sample containing the anion to be determined was injected into a carrier solution including an excess of the precipitatingcation. The precipitation reaction took place in the precipitation coil (L),and the precipitate formed was retained in a column furnished with a stainless-steel filter. Since the concentration of the cation decreases as the precipitate is formed, its absorbance diminishes and yields the FIA peaks that allow the anion to be determined. Owing to the need for an excess of precipitating cation, which corresponds to a concentration falling out of the linearity range when an atomic absorption spectrometer is used, a water stream was incorporated
A
ll'(lDa
Y
Figure 1. Precipitation manifolds: (A) wilhout dissoiution and (E) wlth dissoiutlon. The optimum values of m e FIA variables are summarized in Table I; (W) washing solution and (D) dissolving solution: recordings: (1) base line, (2)cation, (3)water injection. (4) anion injection, (5 and 6) signals corresponding to the precipitate washing and dissolution, respectively.
Table I. Optimum Values Obtained in the Study of the FIA Variables variables mL/min, precipitating cation mL/min, diesolving reagent mL/miu, water dilution V,: pL, sample loop size L cm, precipitation coil length m: mm. oreeioitation coil diameter q:
q: q:
ammonia chloride oxalate 1.3 3.0 2.5 110
11.3
53
300 0.5
200 0.5
200 0.5
0.1
3.8 3.8
3.0 3.0
3.1
into the flow system to effect the cation dilution prior to the nebulizer. Distilled water was used as blank. This configuration entailswashing the column fdter periodically in an ultrasonic bath. (2)With Precipitate Dissolution. In this configuration(Figure 1B) the precipitate is retained on the filter and subsequently dissolved with a suitable reagent after washing. Valve M selects the washing solution (W) or the reagent stream (D) for the precipitate dissolution. Valve S, located in front of the filter, is a key piece in this confiation. It is an ordinary four-way injection valve transformed into a selecting one for channels 1and 2. In one of its positions, it allows channel 1 to meet the precipitate and wastes the stream flowing along channel 2. In the other position the precipitate is first washed and then dissolved. The positive peak height obtained from the signal yielded by dissolved precipitate is directly proportional to the anion concentration in the sample solution. In this configuration no water dilution stream is necessary. This configurationrequires neither washing of the column filter in an ultrasonic hath nor using distilled water as blank. Procedure. With the manifold shown in Figure 1A and the recommended FIA variables listed in Table I, aliquots of chloride (3-100 pg/mL), oxalate (5-90 pg/mL), or ammonia (2-50 pg/mL) are injected into a reagent carrier solution of 80 pg of Ag(I)/mL, 60 pg of Ca(II)/mL or, 25 pg of Fe(III)/mL, respectively. The precipitating cation Ag(I), Ca(II), or Fe(II1) is continuously monitored, and a transient decrease of the signal (directlyrelated to the anion concentration in the sample) is recorded in the detector as chloride, oxalate, or ammonia is injected as a result of the precipitate formation. High temperatures favor the precipitation of oxalate with calcium; thus, both the precipitation coil and the sample and carrier solutions are thermostated at 60' C. This manifold calls for the use of distilled water as blank.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
-
a x
w
V
2
2
-100
a
-100..
a
0
Ca C 2 0
-
--
u
V
z m a
-50
71
-150
t c
I
0
m m Q
-150.-
1 20
40
60
80
100
Temperature ('C)
100
200
ern
300
Flgure 3. Influence of precipitation coil length on signal.
Figure 2. Effect of temperature on absorbance.
In manifold 1B the precipitate is first washed with a suitable reagent until obtaining a nil signal in the detector (the total adsorbed cation is removed). Then the precipitate is dissolved, and the cation contained in the precipitate yields a positive peak, the height of which is directly proportional to the anion concentration in the sample.
4 00
I
RESULTS We have assayed the continuous formation of three types of precipitates, mentioned above in the introduction, in flow injection configurations. The criterion followed for optimization of these determinations was to obtain the greatest possible absorbance increment between the signals yielded by the sample and the blank, which ultimately results in the maximum yield achievable in the precipitation process. The most significant results obtained are discussed below. Gelatinous Precipitates. The precipitation of ferric hydroxide takes place upon injection of aqueous solutions at different pH (adjusted with NH3 or NaOH) into a carrier solution of 25 fig of Fe(III)/mL. The signal response depends critically on the pH of the sample and the iron solution (which must be fixed between 3.0 and 3.5). Since this precipitation is favored by heating in the batch procedure, we studied the influence of the temperature on the sample and carrier solutions as well as on the precipitation coil. As can be observed in Figure 2, this variable scarcely affects the analytical signal. The values of the optimized FIA variables are summarized in Table I. The effect of varying the injected volume has been studied. The absorbance decreases with increasing injected volume of blank because of the carrier dilution; however, this effect is analogous to that resulting from the use of increased injected volumes for the same sample, as the amount of precipitate formed is greater, so that tthe difference between the peak heights (hsample - hblank) remains constant between 27 and 450 fiL. Owing to the wide range of variation of this variable, we chose 110 fiL for this study. The effect of the precipitation coil length was studied a t a constant flow rate. As can be observed in Figure 3, the curve obtained can be divided into three distinct parts: left, flat, and right. Left Zone. When short reactors are used, the plug stays only for a short time in them. In addition to a small extent of mixing, the reaction is incomplete, so some cation remains unprecipitated and yields the correspondingresponse in the detector. Flat Zone. Precipitation is virtually complete within the bolus and hardly any cation is free, so the minimum response is obtained on passage of the fiitered bolus through the detector. Right Zone. On account of the increased residence time for the bolus and once the minimum time required for complete precipitation has elapsed, it can be assumed that the cations in the bolus diffuse into the precipitate through the interfaces involved (especially that at the trailing end of the bolus) so
10
20
io pg/ml
40
ammonia
Flgure 4. Calibration curves for ammonia obtained with different concentrations of hydrochloric acid as a dissolving solution: (A and B) for 1 and 0.1 M of hydrochloric acid, respectively;washing solution, hot water. that, once filtered, the precipitate gives rise to a signal attributable to the diffused cation. This phenomenon becomes more apparent with increase in the reactor length. Therefore, the left and the right zones in Figure 3 are both due to the occurrence of free cation within the precipitation bolus, although for different reasons. The shapes of the left and right zone are also a function of concentration; thus the system can be calibrated. The precision of this method was checked on 11 samples injected on different days, at the same pH. The relative standard deviation found was 2.2%. Ferric hydroxide, on account of its gelatinous nature, tends to adsorb Fe(II1) on its surface. In order to avoid errors in the determination of precipitated Fe(III), this is washed with hot water (in which it is scarcely soluble) until obtaining a nil signal for iron, in approximately 5 min. Once washed, the precipitate is dissolved by passing hydrochloric acid through the filter (a 1M concentration is sufficiently high to ensure complete dissolution). The risk of obtaining erroneous results can be discarded on the grounds of (a) the low solubility of the ferric hydroxide precipitation in hot water and (b) the fact that dissolution is complete when using 1 M HC1, as shown in the shape of the calibration curve obtained by plotting the absorbance as a function of the ammonia concentration;when 0.1 M HC1 is used, dissolution is incomplete from a given amount of precipitate, above which the signal obtained is
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
Table 11. Analytical Features of FIA Procedures for Ammonia, Chloride, and Oxalate linear range, pg/mL
element ammoniab ammoniaC chlorideb chloride' oxalateb
3-45 3-100 3-100
oxalateC
5-60
2-50
detection limit, RSD," 9%
pg/mL
50 10
2.2
1.0 2.0 1.3 2.1 3.0 4.0
5-90
sampling frequency, h-'
6.0
50 10 20
2.0 5.2 5.0 8.7
6
Relative standard deviation. *Using manifold 1A. Using manifold 1B. a
20
2 Q
-
40
60
80
loo pg/ml
CL-
Figure 6. Calibration curves for chloride obtained with different concentrations of nitric acid as a washing solution: (A, B, and C) for 1 X 5 X lo-,, and 10 X M of nitric acid, respectively: dissolving solution, 6 M NH,. 3001
-5001
20
40
60
chloride,
BO
100
YgimL
Flgure 5. CaEbration c w e s for c M o r i determination at three different silver concentrations: (A, B, and C) for 30, 50, and 80 pg/mL of silver(I), respectively.
virtually constant. These behaviors are shown in Figure 4. The analytical features of FIA procedures for ammonia are summarized in Table 11. The sensitivity achieved, given as the slope of the calibration curve (absorbance vs. pg/mL), was 0.0044 mL/pg and 0.011 mL/pg for precipitation and dissolution of ferric hydroxide, respectively. Curdy Precipitates. The precipitation of silver chloride using two flow configurations has been affected by several variables. The influence of the pH of the injected chloride solution was studied by the FIA technique, the peak height found being maximum between pH 1.3 and 6.2. The effect of pH of the silver carrier stream was similar, the optimum range extending to pH 2.4-7.1. The adjustment of the pH of silver carrier and chloride solutions was carried out with diluted nitric acid or sodium hydroxide. The ionic strength of the sample (adjusted with KN03) did not affect the atomic absorption signal, at least up to 0.2 M. As can be observed in Figure 2 the temperature did not affect the signal. Therefore, it was advisable to work at room temperature. The indirect analytical signal of chloride is strongly affected by the concentration of silver in the carrier solution. Figure 5 shows the effect of the concentration of this cation on the signal corresponding to chloride ion. The sensitivity given as the slope of the calibration curve is highest for an 80 pg of Ag(I)/mL carrier solution (-0.005 mL/wg), as can be observed in Figure 5. The optimum values found in the study of the FIA variables are summarized in Table I. The effect of varying the injected volume is similar to that seen with ferric hydroxide. The
optimum injected volume ranges between 27 and 450 pL. The influence of the precipitation coil length on the absorbance is illustrated in Figure 3; this effect is similar to that of the ferric hydroxide precipitate. The AgCl precipitate is a coagulated colloid adsorbing Ag(1) on its surface. Therefore, it is necessary to remove adsorbed Ag(1) prior to dissolving the precipitate in order to avoid systematic errors. With this purpose we have tried various washing solutions: water, potassium nitrate, and diluted nitric acid. Prior to dissolution, a water stream was passed to remove Ag(1) from the carrier solution (80 pg/mL) present in the system. When the precipitate is washed with water or 1-2 M potassium nitrate, the signal obtained for Ag(1) is virtually negigible; however, the introduction of the solvent solution (6 M NH3)results in an intense signal, though not proportional to the amount of precipitated chloride. These two washing solutions were consequentely discarded for use. As can be observed in Figure 6, we have run three calibration curves by using HN03 at different concentrations as a washing solution and 6 M ammonia as a solvent solution. The use of a M concentration results in incomplete washing, and hence the calibration curve obtained after dissolution of the precipitate M nitric acid concenhas a nonzero intercept. A 10 X tration ensures complete washing, but gives rise to the full dissolution of concentration below 20 pg of Cl-/mL, which therefore yields no signal after the subsequent dissolution with ammonia. According to the above considerations, we chose a nitric acid M, in order to achieve the best concentration of 5 x possible results in the washing step and to avoid the precipitate dissolution. The dissolution of the precipitate was tested with 2-7 M ammonia (it was complete above 5 M). A concentration of 6 M ammonia was thus chosen. These experiments were performed with 80 pg/mL silver(1)in the carrier and 60 pg/mL injected chloride solutions. The sensitivity of both methods, with and without dissolution, was similar in both instances. Table I1 summarizes the analytical features of the determination of chloride with both manifolds. Crystalline Precipitates. The use of the FIA technique has resultated in a significant simplification in the determination of oxalate by precipitation with calcium solution. The influence of pH of the injected sample and carrier solutions
ANALYTICAL CHEMISTRY,
VOL. 59, NO. 1, JANUARY 1987
73
Table 111. Influence of Foreign Ions on the Determination of 40 pg/mL Oxalate tolerance ratio (ion/oxalate) 100 40 20
5 1
foreign ion
IOf, MOO:-
so:co:-,
As0:-
~ 0 ~ 3 -
F-
9 - 150 was studied in the range 2-13. The absorbance was minimum and constant throughout the ranges 4-10 and 4-11 for calcium and oxalate solutions, respectively. To control the pH and aid precipitation, 10 mL of 1 M acetic acid/sodium acetate buffer (pH 4.75) was added to 100 mL of carrier solution. The optimum calcium concentration for the complete precipitation of 40 pg of oxalate/mL was 40-70 pg/mL. A concentration of 60 pg of Ca(II)/mL was adopted. The ionic strength did not affect the signal up to 0.1 M KNOB. The values of the optimized FIA variables are summarized in Table I. The effect of varying the injected volume and tube length was similar to that found for the above precipitates. As can be observed in Figure 2, peak height increases (and the absorbance decreases) with increasing temperature. The oxalate and calcium carrier solutions, as well as the precipitation coil, were heated at 60 "C taking into account that higher temperatures resulted in the formation of bubbles in the solutions that disturbed the analytical signal. The sample bolus had to be automatically stopped at the precipitation reactor at least for 1 min on account of ita slow precipitation. These experiments were performed with a Tecator 5020 flow injection automatic analyzer. Table I1 summarizes the most important analytical features of this method as implemented with manifold 1A and 1B. Calcium is adsorbed on the precipitate surface as a result of the continuous passage of its carrier solutions through it. Adsorbed calcium is removed by continuous washing with cold water until the calcium(I1) signal becomes zero (in approximately 5 min). EDTA (0.2 M) in 7 M ammonia was used for the precipitate dissolution. The linear range of the method is narrowed when calcium incorporated into the precipitate is determined by its self-dissolution because the injection of concentrations above 60 pg/mL oxalate results in a plateau (instead of a FIA peak) in the dissolution process, which is rather slow. The plateau amplitude is proportional to the amount of precipitate obtained. The influence of several other anions, acting as precipitants of the calcium ion, on the precipitation of calcium oxalate has been investigated with the aid of manifold 1A. The tolerance ratio of each foreign species was taken as the largest amount yielding an error less than *5% in the peak height for 40 pg/mL of oxalate. As can be observed in Table 111, the FIA method is very selective; anions such as sulfate, molybdate, iodate, and arsenate are tolerated at high ratios, and only fluoride perturbs seriously at a foreign ion/oxalate ratio above 1.
DISCUSSION The time required for the formation of nuclei of a size sufficient to continue growth is known as "induction period". It varies in length according to the particular precipitate and the conditions set; it is somewhat short for silver chloride and ferric hydroxide and long for calcium oxalate. This behavior was also encountered in the FIA procedure, in which to achieve the precipitation of calcium oxalate, the sample bolus must be stopped in the precipitation coil, in contrast to the methods for silver chloride and ferric hydroxide. In the batch procedure the precipitate of silver chloride must be protected from light, since it is sensitive to and readily
110
20
30
40
150
400
450
Figure 7. Variation of absorbance as a function of the number of
identical samples injected. reduced by this. This FIA technique is not affected by this problem because silver chloride is located inside the steel filter, which is safely light-tight. Since crystalline precipitates are characterized by readily filterable particles that are relatively pure, a batch-type precipitation is more easily carried out with crystalline precipitates. Conversely, when the FIA technique is applied, curdy and gelatinous precipitates are preferred on account of the ease of filtration. This is due to the fact that although gelatinous precipitates have smaller particle sizes, the colloid particles agglomerate to form larger aggregates of filterable size, less compact and permeable to the solution, so that they do not clog the filter pores. On the other hand, crystalline precipitates (CaC204),more compact and less permeable to the solution, occupy the filter pores and block them easily, thus hindering the smooth flowing of the solution, which eventually results in the disengagement of the FIA connections (this compels the worker to clean the filter more often than for crystalline precipitates). This effect can be observed in Figure 7 , where we have plotted the variation of the absorbance as a function of the number of identical samples injected. The column filter was washed in an ultrasonic bath, with the same reagent used for the dissolution of the precipitates. The problem posed by the cation adsorption on the precipitates has been solved at the washing stage in all three examples studied in this work. this is corroborated by the obtainment of calibration curves of zero intercept at the dissolution stage. The shortcomings of the dissolution of the precipitates are more complicated as this should be instantaneous in order to obtain a transient signal (FIA peak) and not a plateau (as with CaC204at concentrations above 60 pg/mL of oxalate). Therefore, the indirect determination of anions with the dissolution manifold has several disadvantages with respect to manifolds involving no dissolution: (a) it is necessary to find a reagent ensuring the complete dissolution of the precipitate, and (b) owing to the rather large number of manipulation steps required, both precision (as relative standard deviation) and sampling frequency are significantly reduced. On the other hand, it features two major advantages: no blank or filter washing step is necessary. However, the combination of both manifolds allows one to carry out simultaneous determinations by just finding two anions that precipitate with a mutual cation and subsequently solubilizing one of these with a suitable reagent. This approach is currently under study in our laboratory both by normal and by reversed FIA. The simplicity, rapidity, and versatility of the FIA technique endow it with greater practical interest than its batch counterpart. The following advantages have been found: (1) Inherent in the FIA technique: small sample volume and low sample and reagent consumption, ease of automation, higher sampling frequency (decantation, centrifugation, and dilution
74
Anal. Chem. 1987, 59, 74-79
steps are compulsory in batch methods), and low cost per determination. (2) Specific advantages: wide pH range and higher sensitivity, selectivity, and precision (relative standard deviation). Moreover, water can be used as blank (manifold 1A). Registry No. C1-, 16887-00-6;NH3, 7664-41-7;oxalate, 33870-5; calcium oxalate, 563-72-4;silver chloride, 7783-90-6;ferric hydroxide, 1309-33-7. LITERATURE CITED (1) Kirkbright, G. F.; Johnson, H. N. Talsnta 1973, 2 0 , 433. (2) Garcia-Vargas, M.;Mllla. M.;PBrez-Bustamante, J. A. Ana/yst (London) 1983, 108, 1417. (3) Hassan, S. S. M. Organic Analysis Using Atomic Absorption Spectromehy; Ellis Horwood: Chbhester, 1984. (4) Kenner, C. T. AnalLNcal Separations and Determlnations ; Macmillan: New York, 1971; p 108. (5) Pinta, M. Methodes phvs. Anal. 1970, 6(3), 268. (6) Menache. R. Clln. Chem. 1974, 20, 1444. (7) Ruzicka, J.; Hansen, E. H. Flow Injection Analysis; Wiley: New York, 1981
(8) V i i i r c e l , M.;Luque de Castro, M. D. Flow Injection Analysis. frinciples and Applications : EHIs Horwood: Chichester, in press.
(9) Gallego, M.; Luque de Castro, M. D.;Valcircel, M. At. Specfrosc. 1985, 6 , 16. (10) Tyson, J. F.; Adeeyinwo, C. E.; Appleton, J. M. H.; Bysouth, S. R., Idris, A. B.; Sarkissian, L. L. Analyst(London) 1985, 710, 487. (11) Gallego, M.; Valcircel, M. Anal. Chim. Acta 1985, 169, 161. (12) Gallego, M.;Silva, M.; Valcircel, M. Fresenius' Z . Anal. Chem. 1986, 323, 50. (13) Silva, M.; Gallego, M.; Valcircel, M. Anal. Chim. Acta 1986, 779, 341. (14) Gallego, M.; Silva, M.; Valcircel, M. Anal. Chem. 1986, 58, 2265. (15) Skinner, J. M.; Docherty, A. C. Talsnta 1987, 14, 1393. (18) Foreman, J. K.; Stockwell, P. B. Automatic Chemical Analysis; Ellis Horwocd: Chichester, 1975; p 300. (17) Krug, F. J.; Bergamin, H. F.; Zagatto, E. A. G.; Storgaard Jorgensen, S.Analyst (London) 1977, 702, 503. (18) Van Staden, J. F.; Basson, W. D. Lab. fract. 1980, 2 9 , 1279. (19) Baban, S.; Beetlestone, D.;Betterldge, D.; Sweet P. Anal. Chim. Acta 1980. 174, 319 (20) Van Staden, J. F. Fresenius' 2.Anal. Chem. 1982, 310, 239. (21) Krug, F. J.; Zagatto, E. A. G.; Reis, B. F.; Bahia, F. 0.; Jacintho, A. 0. Anal. Chim. Acta 1983, 145, 179. (22) Zaitsu, T.; Maehara, M.; Toei, K. Bunseki Kagaku 1984, 3 3 , 149. (23) Moller, J.; Winter, B. Fresenius' 2. Anal. Chem. 1985, 320, 451.
RECEIVED for review December 9, 1985. Resubmitted April 15, 1986. Accepted June 27, 1986.
Potentiometric Detector for Fast High-Performance Open-Tubular Column Liquid Chromatography Andreas Manz and Wilhelm Simon*
Department of Organic Chemistry, Swiss Federal institute of Technology (ETH),CH-8092 Zurich, Switzerland
A detector for open-tubular column llquld chromatography, based on an lon-selective microelectrode cell assembly, has been used In cspillaries of 14 and 3.5 pm 1.d. For metalnsd sample components, 10' theoretical plates are obtalned in 220 s u#llzlng a column of 130 cm length (3.5 pm 1.d.). This Is possible only with a detector cell volume of 60.2 X lo-'* L. The detection llmit of 5 X lo-' M iodide in a background of lo-' M KCI leads to a mlnlmum detectable quantity of 7.0 X lo-'' g of Iodide.
Over the last 9 years considerable effort has been focused on liquid chromatography in open-tubular columns (1-38). The advantage claimed over conventionally packed HPLC systems is short analysis time at low pressure drop for a given number of theoretical plates (6, 7,11,12,15). Theoretically, the zero retention time is proportional to the square of the open-tubular column inner diameter, d: (6, 15). A further advantage is due to a lower detection limit (concentration) caused by inverse proportionality between the component concentration maximum in the elution profile and the column inner diameter for a given injected sample concentration (39). If realistic analysis times are sought, open-tubular column diameters
