Determination of ultra-trace amounts of arsenic(III) by flow-injection hydride generation atomic absorption spectrometry with on-line preconcentration by coprecipitation with lanthanum hydroxide or hafnium hydroxide

Talanta ELSEVIER

Talanta

43 (lYY6)

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Determination of ultra-trace amounts of arsenic(II1) by flow-injection hydride generation atomic absorption spectrometry with on-line preconcentration by coprecipitation with lanthanum hydroxide or hafnium hydroxide’ Steffen Nielsen, Jens J. Sloth, Elo H. Hansen* Received

17 October

1995:

revised

28 November

19Y5: accepted

28 November

I995

Abstract A time-based flow-injection (FI) procedure for the determination of ultra-trace amounts of inorganic arsenic(II1) is described, which combines hydride generation atomic absorption spectrometry (HG-AAS) with on-line preconcentration of the analyte by inorganic coprecipitationPdissolution in a filterless knotted Microline reactor. The sample and coprecipitating agent are mixed on-line and merged with an ammonium buiYer solution. which promotes a controllable and quantitative collection of the generated hydroxide on the inner walls of the knotted reactor incorporated into the FI-HG-AAS system. Subsequently the precipitate is eluted with 1 mol 1 ’ hydrochloric acid. allowing ensuing determination of the analyte via hydride generation. The preconcentration of As(III) was tested by coprecipitation with two different inorganic coprecipitating agents namely La(lll) and Hf(IV). It was shown that As(I11) is more effectively collected by lanthanum hydroxide than by hafnium hydroxide, the sensiti\,ity achieved by the former being : 25% better. With optimal experimental conditions and with a sample consumption of 6.7 ml per assay, an enrichment factor of 32 was obtained at a sample frequency of 33 samples h ‘, The limit of detection (30) was 0.003 /~g 1~ ’ and the precision (relative standard deviation) was l.CUI (n = I I) at the 0.1 erg 1 ’ level. K~~~~1,orrl.s:On-line preconcentration by coprecipitationPdissolution; and hafnium: On-line addition of the coprecipitant; Flow-injection senic(II1) assay

Coprecipitation with hydroxides of lanthanum hydride generation atomic spectrometry; Ar-

1. Introduction *Corresponding

author.

Fax:

+ 45 45

88 31

36:

In 1994. Tao and Hansen [l] for the first time introduced a procedure for on-line flow-injection (FI) preconcentration, in which ultra-trace amounts of the analyte (Se(IV)) were copre-

c-mail:

ehh(+kemi.dtu.dk ’ Presented Flow Injection August

I3

0039-9140 .SSDl

at the analysis

Seventh International Conference (ICFA’95). held in Seattle. WA.

on USA.

cipitated

17, 1995.

96 $15.00

0039-Y

130(96)0

c 1996 1837-g

Elsevier

Science

B.V.

All

rights

rexrled

with

an

inorganic

coprecipitant,

i.e.

868

S. .Yiuhr

rt 01.

Tcrlmrcr

La(OH),. The preconcentration procedure combined on-line coprecipitationdissolution in a knotted Microline reactor and hydride generation (HG) of the dissolved concentrate, the detection being achieved by AAS (quartz tube). The elegant aspect of this procedure is that the inorganic precipitate is easily dissolved in dilute acid (1 mol ll’ HCI), which is also the required medium for the ensuing hydride generation reaction. With such an approach an ordinary FI-HG-AAS system without preconcentration is readily extendable to a time-based FI-HG-AAS system with on-line preconcentration. In their procedure, Tao and Hansen added the coprecipitating agent off-line by spiking each sample before the samples were introduced into the FI system. Recently, this procedure has been improved by facilitating on-line mixing of sample and coprecipitating agent [2]. As was shown, the results with on-line addition of the coprecipitant yielded similar results to those with off-line addition. However, the more automated FI system is obviously preferable as the sample manipulations are significantly reduced. As knowledge of how to optimally control and exploit the method of on-line preconcentration by coprecipitationdissolution is still insufficient, the present methodological study is therefore devoted to obtaining a better understanding of the processes involved, thereby allowing identification of the significant operational parameters and devising means for their optimization. For the very same reasons. aqueous standards were applied throughout the experimental work. The optimization procedure is illustrated for the determination of ultra-trace amounts of As(II1) by coprecipitation with two different agents, namely La(II1) and Hf(IVj. According to the literature [3,4] these copre-cipitants are interesting as they have both been shown to coprecipitate As(II1) quantitatively in batch methods. Further, the hydroxides of these coprecipitants, La(OH), and Hf(OH),, are readily dissolved in dilute acid.

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2. Experimental

2.1. Appumtus A Perkin-Elmer Model 2100 atomic absorption spectrometer was used in combination with a Perkin-Elmer Model FIAS-400 flow-injection unit (equipped with two individually controlled peristaltic pumps and a five-port FI valve), with hydride generation accessories (the gas-liquid separator used in the chemifold was a PerkinElmer W-configuration unit). An arsenic hollow cathode lamp (S. & J. Juniper. Harlow, UK) was used at a wavelength of 193.7 nm with a spectral bandpass of 0.7 nm, and was operated at 9 mA. The temperature of the quartz atomizer cell was set at 900 “C. Special care was taken to make the conduit between the outlet of the gas-liquid separator and the quartz atomizer cell as short as possible ( = 6 cm). The filterless knotted reactor precipitate collectors were made from 0.5 mm id., 1.8 mm o.d. Microline tubing (cross-linked ethyl vinyl acetate) by tying interlaced knots (the optimal length of the knotted reactor, L,,, was 200 cm). The knots were made with approximately 5 mm diameter loops. All the other reaction coils, connections and conduits in the FI manifold (Fig. 2) consisted of 0.5 mm i.d. PTFE (polytetrafluoroethylene) tubing. The output signals were processed with a time constant of 0.5 s in the peak-height mode and recordings from the graphics screen were printed out by an Epson Model FX-850 printer. The actuation times of the injector valve and the two pumps were programmed with the use of the FI software of the Model 2100 atomic absorption spectrometer. 2.2. Reagents und standurd solutions All the reagents were of analytical-reagent grade, and distilled water was used throughout. Sodium tetrahydroborate solution (1.50% (m/v) in 0.05 mol 1~ ’ sodium hydroxide) was prepared fresh daily. Lanthanum nitrate solution (0.50% (m/v)) was made by dissolving 0.6662 g of lanthanum nitrate hexahydrate in 100 ml of distilled water. Hf(IV) solution (1.00%) was prepared by dissolution of 1.4869 g of HfOCl, in 100 ml of

869

QTA

(900°C)

t

NaB&

MC lL..-1

KR IL,,]

HCl S

I

-

Buffer [Q>PH,

CA

Cl buffer

Buffer ON

b) Fig. I. (a) Schematic diagram of the time-based FI -HG-AAS system for on-line coprecipitation-dissolution as shown in the loading (precipitating) stage. The pumping rates depicted are the eventually optimzed values: S, sample solution; CA, coprecipitation agent; QTA, quartz cell; Ar, argon; MC, mixing coil; RC, reaction coil: KR, filterless knotted reactor; SP, gas-liquid separator: PI and P?, peristaltic pumps; V. valve: W, waste. (b) Identification of the operational parameters which influence the coprecipitation procedure. p. C and L refer to flow rates, concentrations and lengths, respectively.

distilled water. The buffer solution, made freshly every day, was in all instances 0.3 mol 1~~’ ammonium chloride adjusted to the appropriate pH by addition of 0.3 mol I ~ ’ ammonia. Standard solutions of As(III) for calibration were prepared by three-stage aqueous dilutions of a 1000 mg lP ’ stock solution, which was made by dissolving 1.724 g of NaAsO, in 25 ml of 20% (w/v) potassium hydroxide, followed by neutralization with 20% (v/v) sulphuric acid using phenolpthalein as indicator, finally making the sample solution up to 1000 ml with 1% (v/v) sulphuric acid. All glassware was soaked for at least 24 h in 1 M nitric acid, and finally rinsed in distilled water before use. 2.3. Oprrationd

procedurr

The time-based FI-HG-AAS system with online addition of the coprecipitant, La(III), is shown in Figs. 1 and 2, depicting the optimized experimental parameters. In the precipitation procedure (Fig. la), the sample (S), hydrochloric acid and sodium tetrahydroborate solutions were introduced by pump 1, and the buffer and coprecip-

itating agent (CA; either La(III) or Hf(IV)) were delivered via pump 2. During the precipitation sequence both pumps were activated for a period of 100 s. The sample and coprecipitant were premixed in the mixing coil (MC) of length (L& of 6 cm, and subsequently merged with the buffer solution at the entrance to the knotted reactor (KR) which had a length (LKK) of 200 cm. The precipitate, which was formed instantaneously after the merging point of the KR, was collected on the inner walls of the KR. The effluent emerging from the reactor was discarded. Simultaneously, the acid was pumped through the by-pass of the rotor of the valve and directed into the hydride generation system. During this stage, the baseline for the final readout was established. At the end of the precipitation period pump 2 was stopped, and the valve was actuated automatically from the fill mode to the inject mode for a period of 10 s (dissolutionP hydride-generation procedure; Fig. 2a), by which the acid was introduced directly into the KR where the precipitate adhering to the inner walls of the reactor was dissolved. This concentrated plug zone was directed from the KR to the hydride generating

870

QTA

(900°C)

,TA t

RC

I .-

.;

OFF

Fig. 2. (a) Schematic diagram of the time-based FILHG-AAS Identification of the operational parameters which influence symbols and abbreviations. see Fig. I

system, where the analyte was merged with a reducing solution of sodium tetrahydroborate. After passing through a reaction coil (RC) of length LRc. = 60 cm, the gas-liquid mixture was guided into the gas-liquid separator (SP) in which the arsine and the evolved hydrogen were separated from the liquid phase and swept into the atomizer cell by a steady argon carrier flow. The absorption signal was then recorded. The waste from the gas-liquid separator, which included unreacted sodium tetrahydroborate, acid and some argon, was removed by aspiration. The elution stage lasted for 10 s. During this interval the sample solution was interchanged so that the remainder of the previous sample in the sample pump tube could be effectively washed out and the next sample kept ready for precipitation. 3. Results and discussion 3.1. Basis of’ the optimizution

The interpretation

procwhrr

of the peaks recorded by

system as shown the dissolution-hydride

in the dissolution-hydride generation procedure.

generation stage. For explanation

(b) of

the AAS instrument formed the basis for an effective optimization of the FI preconcentration procedure, as the peaks reflect the effect and efficiency of the prconcentration technique incorporated into the FI system, that is, the coprecipitation-dissolution process, and, following elution. the chemical and/or physical modifications of the analyte in the concentrate into a detectable species via hydride generation and subsequent gas-liquid separation. However, in order to exploit a method completely and thereby reach the limit of detection of the detector concerned, the optimization procedure must be performed most systematically. Thus the optimization was controlled by estimating the gradient of every single peak, that is, the ratio between the area (I) and the absorbance (A) of the signals was evaluated. As the maximum effect of preconcentration was desired. resulting in maximum peak areas, the FI procedure was optimized so as to obtain the lowest Il.4 ratios. To achieve this end it was imperative that the final

optimization of the FI procedure was completely controllable and reproducible. Since the coprecipitation and dissolution/hydride generation processes are completely separate and individual, they will also be treated as such in the following, where the optimization procedure will be described by firstly setting the present investigation into context with current knowledge, and secondly by identifying and detailing the individual parameters/factors important to the two individual processes and the overall operation. The optimization procedure is thus based on a factorial approach, where the response function was the minimization of the I/A ratio. 3.2. Introduction coprecipitation

to the procedure of’

First of all, as the time available for the coprecipitation process under dynamic conditions is limited, typically from a few seconds to a few tenths of a second, this process must happen extremely fast to allow a quantitative recovery of the analyte. Furthermore, this requires a close and compatible interaction between the precipitate collector and the precipitate generated, i.e. the character of the precipitate. From previous work [2] the authors achieved some very important experiences with on-line coprecipitation in KRs, the geometrical design of which leads to the generation of a strong secondary flow pattern, which promotes quantitative coprecipitation although the KR is filterless. Thus, the material of the KR is a very important factor. In order to achieve quantitative coprecipitation of the analyte, the KR has to be tightly knotted, the material of its construction must have a high affinity to the precipitate formed, and the inorganic precipitate must in turn be gelatinous to facilitate the entrapment of the analyte species. Unfortunately, the number of commerically accessible tubular polymer materials with a high hydrophilic character and suitable dimensions (preferably of an inner diameter (id.) of about 0.5-0.7 mm [5]) is rather restricted. However, in the experimental work with on-line coprecipitation of Se(IV), where the coprecipitant and sample were mixed on-line, the results strongly indicated that the collection of the

analyte in La(OH), in a knotted Microline reactor (i.d. KK - 0.50 mm) was quantitative below a sample flow rate of Qs = 6.4 ml min ’ [2], although the hydrophilic character of Microline (vinyl acetate) is not particularly high. Since no better material appears to be available. this material was also selected for use herein to make the KRs. Another important factor, as identified previously [2], is the total flow rate through the KR. Hence, it is not sufficient to optimize the capacity of the KR. This is due to the fact that if the flow rate through the KR is too high the efficiency of adherence of the precipitate on the inner walls of the KR decreases condsiderably, and with that the enrichment factor (EF) and the concentration efficiency fator (CE) decrease too [2]. With this experience in mind a sample flow rate of Qs = 4.0 ml min ~ ’ seemed to be a rational choice. Further, it is essential that the developed FI procedure should possess practical applicability, and therefore the preconcentration period (r,) was limited to T, = 100 s, which in turn determined the obtainable value of EF. Thus, at T, = 100 s and with a subsequent elution period (Tn) of 10 s, which is sufficient for cleaning the system between each analytical cycle, the sampling frequency cf) became33h ‘. From a practical point of view the FI procedure should be able to handle samples in acidic media. since environmental samples are typically preserved by addition of acid to a pH level of = 3. Therefore all aqueous standards were prepared at a pH of 3. 3.3. Optitnizution of the experinlentul purameters the on-line preconcentrcltion by coprecipitation of As(Itl) M’ith Lu(III)

.fbr

The experimental parameters which have an influence on the coprecipitation process are identified in the inset of Fig. 1, (part b, where brackets refer to the individual parameters), and comprising the concentration of the coprecipation agent La(II1) (C,-,A). the pumping rate of this constituent (QcA), the characteristics of the buffer and the lengths of the mixing (PH, C,, and Qhutfer, coil (L,,.) and KR (L,,). To achieve an effective preconcentration procedure and to exploit the

872

S. Nir1.w

et 01.

Tulu~m

capacity of the KR, these parameters must be optimized very carefully, because the process of coprecipitation is very sensitive under the dynamic conditions prevailing, and because the residence time of the coprecipitation media in KR is only of the order of a few seconds. Consequently, every single liquid element must be accounted for during the process of coprecipitation. As AS(II1) is to be determined at ultra-trace levels through coprecipitation under weakly alkaline conditions by mixing the sample with large excesses of La(II1) and ammonium buffer, it is preferential if the flow rates of these constituents (QCA and QB) can be kept as low as possible in order to avoid excessive dilution of the sample and to limit the flow rate through the KR. Experimentally it was found that conditions were optimal for QcA and QB z 0.550.6 ml min ~ ‘. Since the sample and the coprecipitant are mixed on-line prior to entering the KR, it might be expected that it is necessary to utilize a certain pathlength in order to effect this, but, as was also verified in previous work [2], the shortest possible length of MC (here corresponding to 6 cm) was, in fact. sufficient. This is not surprising considering that the ratio of the flow rates of the coprecipitant and the sample was l/8. Using Q(.,, = QB = 0.5 ml min ~ ’ and a sample flow rate Q, of 4.0 ml min ‘, the residence time (T,,) of the coprecipitating media in a KR of L kK 200 cm and i.d.,, 0.5 mm will be z 4.7 s. Considering this very short residence time, the effects of the pH value of the buffer (the concentration of which was fixed at C,, = 0.3 M in order to maintain a high buffering capacity) and the La(II1) concentration (Cc,) were closely investigated. The results are shown in Fig. 3. As can be seen, optimal conditions for preconcentrating As(II1) by collecting the analyte in the generated lanthanum hydroxide are found in a fairly narrow pH interval, i.e. 9.50 < pH, < 9.70. while the concentration of the coprecipitating agent should be in the range 90 < Cc, < 180 ppm. This clearly demonstrates that the process or mechanism of coprecipitation is critical under the dynamic conditions, particularly the pH, value, which for off-line applications has been found to be optimal over a much wider range, i.e. 9- 11 [3].

43 (1996)

867-880

As is apparent from Fig. 3, the efficiency of collection decreases at higher concentrations of La(II1) and/or higher values of pH,. A reasonable explanation might be that large particles are formed under these conditions, whereby the efficiency of adherence to the inner walls of KR is reduced as larger particles are dislodged and carried to waste (W2, Fig. 2) [l]. Thus, it is essential that the inorganic precipitate is generated in the form of small, curdy particles which willingly adhere to hydrophilic Microline tube. During the following investigations pH, = 9.60 and Cc, = 100 ppm were used. The optimization of the capacity of the KR is very important. Its i.d. must necessarily be small, i.e. id.,, z 0.50-0.70 mm, in order to allow for the creation of a strong secondary flow pattern within it. Settling for an i.d. of 0.5 mm, the optimal capacity was therefore determined by optimizing the length of the reactor. L,,. For L,, < 175 cm, the collecting capacity of KR was found to increase almost linearly, while at lengths exceeding 175 cm it remained practically constant (Fig. 4). These results contain some very interesting information about the coprecipitation process. Thus, by depicting the normalized absorbances (A/A,,) for a given length of KR. where A,,,., (here A = 0.229) is the average value of 0.5 1

~tpH=9.40

I f

0.25 4

0

50

iw

tpti=9.50

II-

pH’9.60

150

2w

-A-

[La(NO&l

1

tpH=9.70

250

300

350

4al

(PPm)

Fig. 3. The influence of the La(II1) concentration on the efficiency of the coprecipitation process at ditrerent pH values as measured by the recorded output signals. All measurements were made on As(lI1) standards at a concentration of 0.50 pg I ’ at a sample loading time (T,) of 100 s and for L,, = 200 cm. Operational conditions are otherwise as given in Figs. I and 2.

873

0,24

0

50

100

150

Length Fig. and

300

of KR (cm)

4. Etkct of the length of the KR on the recorded output signal as determined L,, = 200 cm. Operational conditions are otherwise as given m Figs. I and

the absorbances obtained for L,, 2 175 cm, an indication for the concentration distribution of the analyte within the KR at the end of the coprecipitation period can be obtained, provided that it can be assumed that the composition of the precipitate is generated uniformly throughout any cross-section of the KR-an assumption which seems reasonable according to the flow pattern in the KR, and becuse the coprecipitation takes place on-line. Such a concentration distribution curve is shown in Fig. 5 for a reactor of length 200 cm (other experimental parameters are shown in the Figure legend). The highest A ,A,, value (concentration) was obtained at a length of r 50 cm, and in fact 57% of all the precipitate is collected within this part of the reactor, while only a minor fraction is contained in that part of the reactor which exceeds 100 cm. This observation strongly indicates that the length of MC (6 cm) is sufhcient for mixing sample and coprecipitant on-line, and that the coprecipitation process takes place very quickly. Furthermore, as the signals are constant for L,, 2 175 cm. the coprecipitation is obviously quantitative for a reactor of 200 cm. Consequently, the affinity between the Microline reactor and the precipitate La(OH), is adequate. To keep the KR as short as possible,

250

200

for

a 0.20

i/g I

’ As(lI1)

standard.

T,. = 100 s

2.

and thereby avoid unneccesary back pressure during the period of coprecipitation and in the moment of dissolution, L,, = 200 cm was selected. Back pressure in the FI system might possibly cause fluctuations resulting in non-reproducible results. but with the conditions used no such problems were encountered, which confirms that lanthanum hydroxide is readily dissolved in 1 M HCI. The last parameter of the coprecipitation sequence to be optimized was the sample loading period (TV). Obviously, for increasing values of T,. the amount of precipitate increased: thus it was found that the preconcentration effect increased almost linearly up to T,. = 150 s, which indicates that the collection is quantitative. However, long sample loading times will-for a fixed value of the sample flow rate, which, as stipulated earlier. for practical reasons was fixed at 4.0 ml min ’ -lead to lower sampling rates. A T,. value of 100 s was therefore adopted. although only two-thirds of the capacity of the reactor was thus exploited. The time that the coprecipitating media takes to reach equilibrium, TEq. may be found by extrapolating the curve for absorbance (A) vs. T,. to A = 0 or by calculating the line of regression.

874

100

Length

150

of KR (cm)

Fig. 5. Calculated concentration distribution of the analyte (As(ll1)) in a KR of 200 cm at the end of the coprecipitation using an As(II1) concentration of 0.20 ,ug I ‘. Operational conditions are otherwise as given in Fig. 4.

Using the latter approach TEq was determined to be - 2.4 s. Of course, a negative value of TEq is impossible, but this value again emphasises that the coprecipitation happens very quickly and virtually instantaneoulsy.

3.4. Optirnizution of’ the cxperiiwmtul puranwter.s for the on-line dissolution of thr preripitutr in the KR und hydride genrrution ?!!I’As(III) To achieve a high sensitivity via the ensuing hydride generation procedure, the precipitate in the KR must be dissolved immediately to obtain a plug zone of concentrate. Thus the approach of dissolution-hydride generation will be virtually identical to the common FI-HG-AAS system without preconcentration. Therefore the flow rate, Qnc,, and the concentration of the acid, C,,.,, are very important factors. Apart from the fact that the geometrical design of the KR promotes the conditions for achieving an effective preconcentration, it also, via the inherent secondary flow pattern, facilitates effective dissolution. Consequently, the degree of dispersion and the extent of inhomogeniety of the concentrate formed by the dissolution process are both limited, which in turn

period,

promotes the conditions for achieving low I/A values via effective hydride generation and strict control of the FI processing of the concentrate from the KR to the quartz tube atomizer. The parameters which are of importance for the dissolutionhydride generation procedure are identified in Fig. 2b. Optimization of this part of the assay sequence is well described elsewhere [5,6], and therefore only a few comments will be linked to the hydride generation part of the procedure. As it was found previously [I] that 1 M HCl readily dissolved the La(II1) precipitate quickly and effectively, even if elevated flow rates were used (around S-9 ml min ‘), this medium was also used herein. Higher concentrations did not have any significant effect. However. the efficiency of the hydride generation is very much dependent on the concentration of the sodium tetrahydroborate. Thus it was found that the optimal concentration of this constituent was 1.50% (m/v). Pumped at a rate of 1.50 ml min ‘, this means that the concentration of sodium tetrahydroborate in the final reaction medium is 0.22% (m/v) while that of HCl is 0.085 M. These conditions caused a vigorous evolution of hydrogen in the RC. The evolved hydrogen therefore undoubtedly

contributes considerably to the stripping of the analyte from the RC to the gas-liquid separator (SP), which explains why a rather low optimal flow rate of argon, QAr = 29 ml min ~ ‘, sufficed. In spite of the relatively large pressure fluctuations generated by the discontinuous release of hydrogen, the gas segmentation actually does limit the dispersion of the concentrate in the RC. which, for the same reasons as for the KR, is knotted. The length of RC was optimized to L,, = 60 cm (i.d.,, = 0.50 mm), which indicates that the hydride generation happens immediately. By prolonging the reaction time, i.e. LRC > 60 cm. the signals obtained were very reproducible, indicating that the analyte is transported smoothly from the RC to the quartz-tube atomizer via the gas-liquid separator. However, to achieve maximum signals, it is extremely important to control the flow rate of the liquid withdrawal from the gas-liquid separator, Qw,. By varying this parameter, it was noticed that the signals dropped significantly, i.e. up to lo- 15%, if Qw, > Qw,.Opt, ( z 22 ml min ~’ with the ultimately optimized flow rates). This shows that unless the waste flow of Wl (Qw,) is very conscientiously adjusted, there is a possibility of considerable loss of analyte into the waste. 3.5. The influence of .wn~plejlo~~~rute on the perfomance of the FI&HG-AAS system As mentioned earlier, the collection efficiency gradually decreased at higher sample flow rates which was the reason why it was fixed at 4.0 ml min ~ ‘. However. if the sample flow rate was varied at levels below 4.0 ml min ’ while keeping the total amount of analyte introduced constant. virtually identical calibration curves were obtained. Thus, when comparing two calibration runs where the aspirated sample volume (I’,) of As(II1) in both cases was 6.7 ml, but where the sampling time ( TC) and the sample flow rate ( Qs) in the first instance were 100 s and 4.0 ml min ’ respectively and 200 s and 2.0 ml min ’ in the second one, the linear slopes of the two calibration graphs deviated by less than 10% from each other (the slope being highest for the lower flow rate). If the difference in the two calibration runs

is beyond the experimental error, it does, however, show that it is a reasonable assumption to consider the coprecepitation quantitative up to a sample flow rate of 4.0 ml min’ and with T, equal to 100 s. The characteristics of performance under these conditions are given in Table 1. As mentioned above, the ratio between the area (I) and the absorbance (A) of the individually recorded peak signals was a guiding parameter in the optimization procedure. Since the area reflects the effect of the coprecipitation, it is not surprising that it remained practically constant for aspiration of identical amounts of As(III), but as the optimization progressed the absorbances increased, i.e. the IIA ratio decreased progressively. By calibration at (T,, Qs) = (100 s, 4.0 ml min ~ ‘), the average value of the Z/A ratio, (l/A),,, in the concentration range of As(II1) 0.0550.30 pg l- ‘, approached a value of around 1.91, which seemed to be the optimal efficiency obtainable. By comparison, calibration at (T,, Q,) = (200 s, 2.0 ml min ‘) yielded an (Z/A),, value of 1.93. Thus, these results show that (1) La(OH), effectively collects As(II1) on the inner walls of a knotted Microline reactor under the dynamic conditions prevailing, and (2) the dissolution of La(OH), is effective and so is the hydride generation reaction. Apparently the concentration distribution of the analyte in the KR (Fig. 5) promotes appropriate conditions for dissolution of the coprecipitate. In the concentration range investigated (i.e. O0.20 ppb As(III)), the presence of As(V) did not influence the signal provided that the concentration was below 0.20 ppb. Whether higher concentrations of this species can be tolerated in the coprecipitation reaction with La(II1) or Hf(IV) can only be estimated by optimising the dissolution ~ hydride generation conditions to eliminate, if possible, the differences in kinetic behaviours of As(II1) and As(V). 3.6. Ewluution

of‘ the preconcentration

eflect

To estimate the effect of the preconcentration obtained the same FIIHG-AAS system was used to determine As(II1) without preconcentration. The optimal parameters obtained by optimizing the elution/hydride generation procedure with

876 Table I Characteristics

for

the FIIHG-AAS

system

with

on-line

Parameter

dissolution

Analyte Se( IV)’

Sample

coprecipitation

flow

rate

(ml min

’)

(Cc, (~~m):~Hlo,,,,<,i Calibration range (jig I-‘) Regression equation in calibration range (6 standards, n = 3. Ci in log I- ‘) concentration range (I/A ) A,rrape (analyte 0.0550.40 ppb) Sample volume per assay (ml) (loading 100 S) Sampling frequency, j (samples h ‘) Relative standard deviation (n= II, 0.10 {‘g I-‘) (‘!%I) Limit of detection (30) (/tg I ‘) Enrichment factor (EF) Concentration efficiency (CE = EF 1860) ’ Coprecipitation b Coprecipitation ‘ Not determined.

with with

4.8 210:9.10 0.01~0.30 0.510 c+o.o05 (r = I .OOO) N.D’ time,

As(II1)”

As(li1)”

4.0 IOO:‘9.60 0.005 0.30 I.170 C+0.017 (r = 0.998) I.91

4.0 100/s. IO 0.005. 0.40 0.906 C+O.O17 (r = 0.996) 2.11

7.9

6.1

6.1

33 0.5

33 I.0

33 0.3

0.006 30 16.5

0.003 32 Il.4

0.003 25 13.5

La(OH),. Hf(OH),.

La(II1) were reused except that the argon flow rate had to be readjusted. A sample volume of 100 ,ul was employed. which in turn required an optimal argon flow rate of 43 ml min ‘. Consequently, the elutionhydride generation procedures with and without preconcentration were almost identical, which again indicates that the dissolution of La(OH), is very effective. With a calibration series comprising As(II1) standards in the range 2.0- 10.0 pug 1~ ‘, the (Z/A),, value was found to be approximately 2.28, that is somewhat higher than the value found for the optimized system with on-line preconcentration ( 1.9 1). The characteristics of performance are given in Table 2.

element, in fact, is named after the capital of Denmark). To compare the efficiencies of La(II1) and Hf(IV), the coprecipitation procedure with the use of Hf(IV) was optimized according to the same principles as described above for La(II1). Thus, in the FI preconcentration procedure for collection of As(II1) in Hf(OH), the parameters of the elution/hydride generation procedure as employed for La(II1) were reused. The results obtained are given in Table 1, to which the following comments should be added. Table 2 Characteristics centration Sample

3.7. On-line preconcentrution of As(III) by coprecipitation-dissolution \vitlz Hf(IV) as coprecipitant As mentioned earlier. Hf(IV) has proved to be an effective coprecipitating agent for As(II1) in batch procedures, and therefore it would be highly interesting to estimate its potential capacity on the FI-HG-AAS approach (also because this

volume.

for

the FIIHG-AAS

V, (III)

Calibration range (p& I ‘) Regression equation in calibration range (7 standards, n = 3. C,, in /ig I-‘) Sampling frequency, f (samples h -‘) Relative standard deviation (n = I I; 5 /Jg I-‘) (‘!
system

without

100 0.50- 10.0 0.037 C+O.O06 (r = 0.999) I20 0.4 0.08 I

precon-

By comparison with the results attained by using La(III), the investigation with Hf(IV) yielded a surprising outcome as optimal results could be achieved within wider ranges of the values of the critical parameters. Thus, similar responses were found for 9.10 I pH, < 9.40 and 180 I CH1.~,v’I 300 ppm. Therefore the coprecipitation of As(II1) seemed. in fact, to be more robust using Hf(IV) rather than La(II1) under the dynamic conditions in the FI system. During the ensuing investigations pH, was fixed at 9.10 and [Hf(IV)] at 220 ppm. Analogous to the results obtained by La(II1) similar indications can be drawn. Thus the concentration distribution of the analyte achieved using Hf(IV) was almost identical to the distribution in Fig. 5. By varying T,- the results obtained indicate that the coprecipitation of As(lI1) is quantitative or almost quantitative until T<.= 175 s using the optimal length of the KR, L,, = 200 cm, whereby only z 60% of the capacity of collection of the KR is exploited at T,. = 100 s. The results obtained by varying T, are interesting. Thus while the collection efficiency of La(II1) decreased slightly for T,- 2 50 s, it remained constant in the range 75 I T, I 175 s for Hf(IV), which indicates that the coprecipitation is quantitative or almost quantitative during that part of the coprecipitating period. Two calibrations were performed by coprecipitation with Hf(IV) using equal amounts of As(II1). where the aspirated sample volume (V,) was 6.7 ml and (T,, Q,) was (100 s, 4.0 ml min ~ ‘) and (200 s, 2.0 ml min ~ ‘). Contrary to the observation made for collection of As(II1) in La(OH),, the highest sample flow rate for Hf(IV) exhibited the steepest slope of the calibration curve ( =: 5% higher than for the lower flow rate), although the integrated signals were of the same order of magnitude, i.e. the effect of preconcentration was identical. This indicates that the efficency of dissolution is reduced by preconcentrating equal sample volumes at increasing T,.. By calibration at (T,, Q,) of (100 s, 4.0 ml min’) and (200 s, 2.0 ml min ~ ‘) the (//A),, values were 2.11 and 2.29 respectively, in the concentration range of As(II1) 0.05-0.40 pg I- ‘, This effect will be explained in the following section. The characteristics of performance for Hf(IV) are given in Table 1 at

calibration min ~~‘).

conditions

of (T,., Q,) = (100 s, 4.0 ml

3.8. Compm+~g tlw @icirnq~ of La(OH), and Hf(OH), us collecting agents f&r coprecipituting AsjIll) in II knotted Microline rructor. By comparing the results of the calibration curves that were achieved using La(II1) and Hf(IV), the difference in the collection efficiencies of As(II1) in the two hydroxides can be estimated because the conditions of the elutionjhydride generation procedures were identical for the two calibrations The comparison is described for the conditions (T,, Qs) = (100 s, 4.0 ml min ~ ‘). By collecting As(II1) the effect and efficiency of preconcentration obtained are highest using La(OH),, as the integrated signals are higher and the Z/A ratios are lower than by using Hf(OH),. The efficiency ratio of Hf(OH), to La(OH), was = 77.4’%). as calculated from the respective slopes. Consequently, As(II1) is more effectively collected in La(OH), than Hf(OH), using a knotted Microline reactor. As both La(II1) and Hf(IV) coprecipitate As( III) instantaneously, the explanation for the less effective collection of As(II1) by Hf(IV) could be ascribed to a less effective adherence of the coprecipitate and/or collection of the analyte in Hf(OH),. These possibilities were evaluated by a simple experiment. where As(II1) was coprecipitated in the batch mode with the two agents. It was observed that the precipitates of both La(OH), and Hf(OH), were generated very fast but. while La(II1) formed small, fine hydroxide particles, the precipitate of Hf(OH), appeared very bulky. Such a precipitate might reduce the efficiency of adherence to the inner walls of the knotted reactor and/or promote the dislodgement of particles from it. Furthermore, the efficiency of adsorption of the analyte on the precipitate might be reduced as the very bulky precipitate of Hf(OH), limits the surface area of the precipitate on which the analyte can be adsorbed. Thus the formation of a bulky precipitate of Hf(OH), explains why the efficiency of the FI procedure decreased when longer sampling periods (T,-) were employed. This caused the genera-

878

S. Yieiwn

0 o,ooo1

0,001

CI al.

Tdanru

0,Ol

43 (1996)

Ql

Concentration Fig. 6. The influence on the recorded and Se(lV). Operational conditions

output signal of a 0.20 /~g I ’ aspirated are otherwise as given in Figs. I and 2.

tion of a disproportionately large amount of Hf(OH),. i.e. the ratio of analytejprecipitate decreased with increasing TV although identical sample volumes were aspirated. This deteriorated the ensuing dissolution process which in turn led to broadening of the recorded peak signals. Hence, a suitable coprecitating agent should possess the ability to form small. gelatinous particles, which willingly adhere to the hydrophilic Microline tube and easily dissolve. From a thermodynamic point of view, the chance of obtaining many, smaller particles at the expense of fewer larger ones is much higher when the mixing occurs fast and abruptly, and this is quite feasible in a FI system. This is also probably the explanation why the optimal operational conditions under the dynamic conditions of FIA are different from those of batch assays, and also why it is by no means a trivial exercise to transform a batch assay into a FI procedure [I]. 3.9. Inarstigutions

qf‘ intet-ferences

If the FI-HG-AAS system with on-line preconcentration is to have any practical utility, e.g. for assays of natural water samples, it is essential that it can tolerate the presence of pertinent ions and at the concentration levels at which they may be

867-880

10

1

100

(ppm) As(lI1)

sample

at increasing

concentrations

of Cu(II)

found in such samples. In this work, which by no means was intended to be a thorough investigation of interferences, the work was therefore centered on two representative interferents, i.e. Cu(I1) and Se(IV). The former is known to generally interfere in the hydride generation reaction [7], while the latter has been shown to be coprecipitated with La(OH),. To investigate the extent of these two interferences, 0.20 ,ug 1 - ’ As(II1) samples were spiked with increasing levels of these two constituents. The results are shown in Fig. 6. It appears that concentrations of Cu(I1) up to 1 ppm, i.e. at 5000 times excess, could be tolerated without any interference, yet at higher concentrations the recorded As(II1) signal decreased. Thus, at a Cu(I1) level of 10 ppm. the signal was reduced z 30% compared to the signal without addition of Cu(I1). In comparison, Tao and Hansen [I] found that by preconcentration of Se(IV), using a similar FI system but with off-line addition of the coprecipitating agent to the sample, the signal for a Se(IV) sample of 0.50 pg 1-l was reduced by 36% at a Cu(I1) level of 0.50 ppm. Consequently, the determination of As(II1) is much more tolerable of Cu(I1). In determinations of 10 ilg 1- ’ As(II1) samples with the FIIHGAAS system without preconcentration and by spiking samples off-line with Cu(I1) at a level of

40 ppm it was found that the signals were only reduced by 19%. This strongly indicated that the decreasing signals for Cu( II) concentrations above 1 ppm are mainly due to the fact that the conditions of the coprecipitation process are impaired in the presence of Cu(I1). As for the investigation concerning Se(N). it is seen from Fig. 6 that the interference of Se(IV) is much stronger than that exhibited by Cu(II). Already, at a Se(IV) concentration of 5 pg 1~ ’ the signal for the 0.20 pg I- ’ As(II1) sample is reduced by 15%. For comparison, Tao and Hansen found that for the determination of Se(IV), at a level of 0.50 pg I ~ ‘, the signal was reduced by less than 5% at an As(II1) concentration of 50 jig 1~ ’ [l]. Again, the interfering effect of Se(N) in the determination of As(II1) is probably due to the fact that the conditions of the coprecipitation process are impaired by the presence of Se(IV). Yet, the Cu(I1) as well as the Se(N) interference are non-signal yielding interferences, and therefore they can readily be compensated for by applying the standard-addition approach, which under any circumstances would be the option of choice when assaying natural samples.

tr, :

Emrug: 42 FlAS : Run

Ua”ele”9th:

193.7 SIEP I of

0.oil4

Slit: 0.7 L Ln*p. 1 current: 9 RR 3 TEtlP.: VOO’C Ills: 20 5 RS ICC:

Fig. 7. Typical peak readout for a 0.20 (cg I- ’ aspirated As(IIl) sample as obtained by the FILHG-ASS system depicted in Figs. I and 2. Note that the area under the peak up to the peak maximum (which is reached I .4 s after initiation of the peak) corresponds to approximately 30% of the total area, while the remaining 70’!/;) is ‘distributed’ over the remaining 5 6s.

3.10. T/w gus

liquid

separator

As each of the experimental parameters was systematically optimized. whereby it was possible to identify any limiting factors of the FI procedure with the incorporated quartz-tube atomizer, it became of interest to investigate if the technical conditions could somehow be improved. Attention was especially focused on the gasliquid separator. As detailed in Section 2, the separator used herein was the one commercially available from Perkin-Elmer; the so-called W model, which has a ‘dead volume’ of 3 ml. Recently an alternative gas-liquid separator has been developed by Fang [6] and such a unit was placed at the disposal of the authors at his proposal. This separator. which has a dead volume of 4.5 ml, proved unfortunately to have an inferior performance, which clearly shows that in order to develop an improved gas-liquid separator-at least for measuring analyte concentrations below 1 j’gl ‘p attention must be focused on minimizing its dead volume. The following observations indicate that the sensitivity can indeed be improved by minimizing the dead volume of the Perkin-Elmer W model separator. The dissolution of the precipitate in the knotted reactor happened instantaneously, which was readily visible by the ensuing l-2 s of vigorous gas liberation/foaming in the gas-liquid separator. With the dimensions selected, the hydride generation reaction itself was completed within z 1 s. and from the readout on the monitor it was evident that the essential part of the signal (the peak height maximum) was available after z 1.4 s, while the time required to record all of the signal peak was about 6.5 s (Fig. 7). These feats indicate that an essential part of the gas concentrate became delayed by approximately 2-3 s by the passage of the gas-liquid separator or, more specifically. that the argon stream and the hydrogen developed caused a pronounce dilution of the concentrate in this unit. To minimize this effect it would be very interesting to test a Perkin-Elmer W model with a dead volume of less than 3 ml.

880

S. Nielsrn

et al.

Talmta

4. Conclusion With the results obtained in previous work [1,2], and the work reported in this paper, it is apparent that the FI approach for preconcentration of trace levels of elements by on-line coprecipitation-dissolution in inorganic media offers itself as an attractive and effective option. The coprecipitation process is very sensitive under the dynamic conditions prevailing, especially regarding the optimal value of the pH of the buffer used and the concentration of the coprecipitating agent. This is amply demonstrated in the difference of the optimal values of these two parameters for the preconcentration of As(II1) and Se(IV), although the very same coprecipitating agent, La(III), was used in the two cases (Table 1). Although the coprecipitation process proceeds extremely fast using either La(N) or Hf(IV), it was found that La(OH), collected As(II1) more effectively than Hf(OH), using a knotted Microline reactor. This is probably due to the fact that the precipitate of Hf(OH), was generated as bulky particles, which impaired both the conditions for the on-line coprecipitation process and also the ensuing on-line dissolution of the precipitate. This, in turn, proved that an ideal precipitating agent should give rise to the formation of small, gelatinous particles, which promote conditions for willing adherence to the hydrophilic Microline tube and subsequently give rise to ready dissolution. From the experience gained by on-line preconcentration of As(II1) and Se(N) via coprecipitationdissolution in inorganic media it is expected that subjecting other hydride-forming elements to a similar procedure should not pose any serious problems. Further, as the FI procedure presented herein seems to have reached completion, it should in the future be of great interest to investi-

43 (I 996)

867-880

gate the possiblities to include on-line speciation of As, Sb, Se and Te. Whether the on-line reduction should be incorporated in the period of preconcentration or in the period of elution remains to be answered. In this context it must be borne in mind that the threshold of tolerance of the coprecipitation process towards interfering species was low. Therefore the on-line reduction of the analyte might preferably be executed during the period of elution, which requires that the on-line preconcentration and the sensitivity of the analyte species measured are identical. Similar considerations must be made for achieving a selective determination of the analyte at the lowest oxidation state, as the same method of hydride generation is to be used. Presently, work is in progress along these lines, with As, Se and Sb as analytical species.

Acknowledgements The authors extend their appreciation to Julie Damm’s Foundation (Denmark) for partial financial assistance of this research programme. Thanks are also due to Dr. Vagn Gundersen of the RIS0 Research Center for donation of HfOCl,.

References [I] G.H. Tao and E.H. Hansen. Analyst, 119 (1994) 333. [2] S. Nielsen. J.J. Sloth and E.H. Hansen. Analyst, 121 (1996) 31. [?] 0. Kujirai, M. Kohri. K. Yamada and H. Okochi. Anal. Sci. 6 (1990) 379. [4] J. Ueda and S. Kagaya, Bull. Chem. Sot. Jpn., 65 (1992) 1496. [i] Z.L. Fang, Flow Injection Separation and Preconcentration. VCH, Weinheim. 1993. [h] Z.L. Fang. Flow Injection Atomic Absorption Spectrometry, Wiley. Chichester. UK, 1995. [7] B. Welz and M. Melcher, Analyst. 109 (1984) 569.