Flow-through room temperature phosphorescence optosensing for the determination of lead in sea water

Analytica Chimica Acta 395 (1999) 1±9

Flow-through room temperature phosphorescence optosensing for the determination of lead in sea water Blanca San Vicente de la Riva, Jose Manuel Costa-FernaÂndez, Rosario Pereiro, Alfredo Sanz-Medel* Department of Physical and Analytical Chemistry, University of Oviedo. c/Julian Claveria no. 8, Oviedo 33006, Spain Received 21 December 1998; received in revised form 25 February 1999; accepted 31 March 1999

Abstract The chelates formed between the heavy metal ion Pb(II) and the reagents 8-hydroxy-5-quinolinesulphonic acid, 8-hydroxy-7quinolinesulphonic acid and 8-hydroxy-7-iodo-5-quinolinesulphonic acid exhibit strong room temperature phosphorescence (RTP) if retained on the surface of anion exchange resin beads. Based on the on-line formation, in a ¯ow-injection system, of such RTP lead chelates and their transient immobilization on an anion exchange resin, three ¯ow-through optosensing systems are investigated for lead in sea water. Optimum experimental conditions and the analytical performance characteristics of the three optosensors are discussed. Relative standard deviations (RSDs) of the order of 3% are typical at 100 ng mlÿ1 Pb(II) and the active sensing phases can easily be regenerated by passing 500 ml of 6 M hydrochloric acid. A lead(II) detection limit of 0.1 ng mlÿ1 (3background SD, for 2 ml sample injection volumes) was achieved for the optosensor based on 8-hydroxy-7quinolinesulphonic acid. Possible interferences present in sea water, including cations and anions which could affect the sensor response, are discussed in detail. Finally, the selected RTP ¯ow-through optical sensor has been successfully tested for the determination of lead in sea water at a few ng mlÿ1. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Flow-through optosensing; Room temperature phosphorescence; Lead; Sea water

1. Introduction There is a great demand for monitoring low levels of lead in the environment because this dangerous metal is now widespread, contaminating virtually the whole biosphere. The most common procedures used for heavy metal analyses nowadays are based on atomic spectrometry. However, there is an increasing need

*Corresponding author. Tel.: +34-98-510-3474; fax: +34-98510-3125; e-mail: [email protected]

for simple, in situ and continuous techniques for monitoring toxic substances in the environment. The use of environmental optical sensors is especially advantageous as ``on the ®eld'' and remote monitoring of very low concentrations of such toxic metals at comparatively low prices is possible. Unfortunately, only a few papers on optical sensors for lead determination have been published so far [1]: tetrasubstituted aluminium 2,3-naphthalocyanine dyes [2] have been reported as ¯uorescence optical sensors for lead ions; xylenol orange [3,4], dithizone [5], or dithioamine plus protonated Nile Blue [6] have

0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 3 2 3 - 2

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also been employed to develop absorbance/re¯ectance based optical sensors for lead. In any case, optosensors developed for lead determination in real samples suffer from low sensitivity and poor selectivity. Solid surface room temperature phosphorescence (RTP) emissions provide a highly sensitive, low background, selective and time discrimination type of luminescence whose measurement could be used to develop innovative approaches for optical detection [7,8]. Also, ¯ow injection analysis (FIA) strategies can be advantageously exploited in connection with ``¯ow-through optical sensors'' where the packing in the ¯ow cell allows one to simultaneously preconcentrate and detect the analyte [9,10]. This combination of ¯ow-injection techniques with optically active surfaces, in which an indicator is immobilized on a solid support accommodated in a ¯ow-through cell, has proved to offer several important advantages to the optical sensors ®eld. This integrated approach provides the advantages of FIA systems (e.g. it is easy to renew reagents on the solid support, irreversible chemistry can be applied for sensing purposes, sample pretreatment can be integrated in the ¯ow systems, etc.) and allows high sensitivity of analyte detection (which is concentrated in the active surface of the sensor) [11]. The feasibility of the combination of FIA with solid surface RTP optosensing as a detection principle for metal analysis [9] has been previously demonstrated for the determination of aluminium in dialysis concentrates and dialysis ¯uids [12] and gadolinium in synthetic samples [13]. In this paper the development of three ¯ow-through optical sensing systems for lead analysis, based on the formation in a ¯ow-injection system of RTP lead chelates which are immobilized on an anion exchange resin packed in a conventional ¯ow-cell is described. Three chelating agents for lead (8-hydroxy5-quinolinesulphonic acid, 8-hydroxy-7-quinolinesulphonic acid and 8-hydroxy-7-iodo-5-quinolinesulphonic acid) are investigated. The chelates formed between these reagents and Pb(II) exhibit strong RTP emission when immobilized on anion exchange resins, in the absence of oxygen [14]. Practical RTP ¯ow-through optosensors developed and their application to the determination of lead in sea water is discussed.

2. Experimental 2.1. Materials and solutions Analytical reagent-grade chemicals were employed for the preparation of all the solutions. A stock solution of Pb(II) (1000 mg mlÿ1 in 0.5 M HNO3) was obtained from Merck (Darmstadt, Germany). Working Pb(II) standards were prepared daily by appropriate dilution of the stock solution with the carrier solution. The carrier solution consisted typically of a 0.2 M acetic acid/sodium acetate buffer (pH 6.3) and was prepared containing 30 000 mg lÿ1 of NaCl, 3 10ÿ3 M Na2SO3 and 50 mg lÿ1 of 1,10-phenanthroline. Freshly prepared sodium sulphite solution was added to all solutions (at a ®nal concentration of 3 10ÿ3 M) for appropriate chemical deoxygenation [15]. The strongly basic anion exchange resin Dowex 1X2-200 was found most appropriate for the development of an RTP optosensor for aluminium [13]. The same solid support was used in this work for the immobilization of the lead chelates. This Dowex 1X2-200 resin (Sigma, Stainheim, Germany) was cleaned thoroughly before use with 2 M HCl to remove trace metal impurities, then with de-ionized water and ®nally with ethanol to displace air from the pores of the resin and to remove residual monomers and solvents. Freshly prepared ultrapure de-ionized water (MilliQ 3 RO/Milli-Q2 system, Millipore Corp.) was used in all experiments. Special care was taken in the preparation and handling of solutions and containers to minimize any possible risk of lead contamination. Calibrated ¯asks were left overnight in 10% (v/v) HNO3 and then rinsed thoroughly with ultra-pure Milli-Q water before use. 2.2. Instrumentation Fig. 1 illustrates the optosensing FIA manifold used. Phosphorescence emission measurements were made on a Perkin-Elmer LS 50B luminescence spectrometer. The delay time used was typically 0.05 ms and the gate time was also 0.05 ms; instrument excitation and emission slits were set at 10 and 20 nm, respectively. A conventional Hellma ¯ow-cell (Model 176.52) of 25 ml volume was used. At the bottom of the ¯ow cell,

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Fig. 1. Optosensing manifold for lead determination.

a small piece of nylon net was placed to prevent particle displacement by the carrier. The resin was loaded with the aid of a syringe and the other end of the ¯ow cell was kept free [16]. The cell was connected to the ¯ow system and 10 min was allowed for the particles to settle. In order to ensure that the compound ®rst retained by the packing solid material was in the light path, the resin level was maintained 1 mm lower than that of the cell window. The resin packed in this way showed good stability and could be used for two months or even longer with satisfactory phosphorescence readings. A four-channel Gilson (Worthington, OH) Minipuls-2 peristaltic pump was used to generate the ¯owing streams. Omni®t 1106 rotary valves were used for sample and reagent introduction (valves A and B in Fig. 1) and for the elution of the retained compound (valve C in Fig. 1). An Omni®t 2401 mixing T piece, PTFE tubing (0.8 mm i.d.) and ®ttings were used for connecting the ¯ow-through cell, the rotary valves and the carrier solution reservoirs. The pH measurements were made by using a WTW pH meter Model 139 (Wiss. Tech. Werkstates, Weilheim, Germany) and a Radiometer GK-2401-C (Copenhagen) combination glass-saturated calomel electrode. All the measurements were carried out at room temperature (2228C) and at atmospheric pressure. 2.3. General procedure Appropriate volumes of reagents are mixed on-line with ®xed volumes of lead sample, or standards, in a ``merging zones'' FIA system [17], as depicted in Fig. 1. Samples or lead standards (2 ml) are injected via valve A and chelating reagent solution (2 ml)

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through valve B into the ¯ow system. Both solutions, at a ¯ow rate of 1.4 ml minÿ1, are mixed in a T piece and passed through the 0.5 ml reaction coil to ensure chelate formation. The three reagents under study, forming RTP chelates with lead [8-hydroxy-5-quinolinesulphonic acid, 8-hydroxy-7-quinolinesulphonic acid and 8-hydroxy-7-iodo-5-quinolinesulphonic acid, ``ferron''] were assayed for Pb(II) determination. After on-line reaction, the formed anionic lead RTP complex goes through the detection ¯ow cell, D (placed inside the phosphorimeter); here it is retained on the packing of the Dowex 1X2-200 where its RTP intensity is measured at 395 nm for excitation and 595 nm for emission. After measurement, 500 ml of 6 M HCl is injected via valve C (to strip the chelate retained on the solid phase) before proceeding with the next sample injection. Typical ¯ow-injection luminescence signals vs. time are obtained. The observed optimum operating conditions are summarized in Table 1 for each of the three reagents. 2.4. Real sample analysis Standards and sea water samples were prepared as follows. An appropriate aliquot of Pb(II) solution was Table 1 Optimum experimental conditions for lead determination by using the three optimized optosensing systems Buffer

0.2 M sodium acetate/acetic acid‡ 50 mg lÿ1 1,10-phenanthroline‡ 30 000 mg lÿ1 NaCl

pH

6.3a,b 5.7c 310ÿ3 M 1.010ÿ3 Ma 1.310ÿ3 Mb 2.010ÿ5 Mc 30 000 1.4 ml minÿ1 2 ml 500 ml 500 ml of 6 M HCl 0.05 0.05 395/595 10/20

[Na2SO3] Chelating reagent [NaCl] (mg lÿ1) Flow rate Injection loops Reaction coil Stripping reagent Delay time (ms) Gate time (ms) exc/em (nm) Slits, ex/em (nm) a

Chelating reagent: 7-sulphonic oxine. Chelating reagent: 5-sulphonic oxine. c Chelating reagent: ferron. b

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transferred to a 25 ml standard ¯ask and 2.5 ml of 3.010ÿ2 M Na2SO3 (for deoxygenation), 2.5 ml of 2 M buffer and 2.5 ml of 500 mg lÿ1 solution of 1,10-phenanthroline (to prevent interference from possible iron) were added. This solution was diluted to the mark with Milli-Q water. Reagent blanks were prepared following the same procedure without samples. The standards for calibration were prepared similarly except that NaCl was added (to give a ®nal concentration of 30 000 mg lÿ1 NaCl) in order to match sea water salinity. 3. Results and discussion 3.1. Spectral characteristics of the immobilized lead chelates The chelates of lead, formed in the proposed FIA system, exhibit strong RTP when immobilized on the anionic exchanger. Fig. 2 shows schematically the structure of the chelates of lead formed with the reagents under study, as well as the ionic interaction between the chelates and the resin beads of Dowex 1X2-200. The RTP spectra of the three lead chelates retained on a Dowex 1X2-200 resin are shown in Fig. 3. A delay time of 0.05 ms was used in all phosphorescent measurements to ensure that any background ¯uorescence had ceased. When using shorter delay times (i.e., 0.01±0.04 ms) residual ¯uorescence from the complex and the solid support itself could be

observed. In the absence of oxygen, the excitation spectra of the three complexes studied showed intense absorption bands at about 395 nm. On the other hand, the RTP emission spectra showed maxima at about 595 nm for the three lead chelates. The excitation and emission wavelengths cited above were selected for subsequent experiments. 3.2. Optimization of the experimental conditions Optimization studies of the different experimental variables in¯uencing the RTP measurements were carried out by injection of Pb(II) standards (100 ng mlÿ1 Pb(II)). First, the in¯uence of the buffer pH was carried out; Fig. 4 shows the observed effect of the pH of the acetic acid/sodium acetate carrier on the RTP signals for the three chelates. Optimum conditions are observed in the pH interval 6.0±6.5 for the 7-sulphonic oxine and the 5-sulphonic oxine. Therefore, a pH of 6.3 was selected for further experiments for these two reagents. When using the iodo derivative ``ferron'' a pH of 5.8 was selected (see Fig. 4). Sodium acetate concentration in the buffer (ionic strength) was also studied. Only small variations of the RTP signals with increasing buffer concentration were observed and a buffer concentration of 0.2 M was ®nally selected. The observed effect of the concentration of the chelating reagents on RTP signals is shown in Fig. 5 while low concentrations of the chelating reagents did not allow the ef®cient formation of the chelate, very high concentrations decreased RTP

Fig. 2. Interaction between the chelates and the resin beads of Dowex 1X2-200. Reagents used: (a) 8-hydroxy-7-quinolinesulphonic acid; (b) 8-hydroxy-5-quinolinesulphonic acid; (c) 8-hydroxy-7-iodo-5-quinolinesulphonic acid.

B. San Vicente de la Riva et al. / Analytica Chimica Acta 395 (1999) 1±9

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Fig. 3. RTP excitation (a) and emission spectra (b) for the lead chelates immobilized in Dowex 1X2-200 resin beads in the absence of oxygen. (±

±) 7-sulphonic oxine; (ÐÐÐ) 5-sulphonic oxine; (...) ferron.

signals (due to self-absorption problems in the solid phase and also to a displacement effect of the chelate in the support by the free reagent in excess). A well known requirement to obtain sensitive analytical RTP signals is the elimination of oxygen throughout the ¯ow system. Two modes of deoxygenation were tested: the ®rst was based on the bubbling of an inert gas (Ar) to displace dissolved oxygen; the second consisted of chemical deoxygenation by the addition of sodium sulphite [15]. Fig. 6 shows that higher RTP signals were obtained when Na2SO3 was used as oxygen scavenger, at a concentration of 3.010ÿ3 M. Around twice the sensitivity was observed compared to Ar bubbling. Therefore,

Na2SO3 was selected to eliminate dissolved oxygen through the system. Apart from the sodium acetate/acetic acid buffer (pH 6.3) and the oxygen scavenger Na2SO3 addition, the carrier and the lead standards were made to contain 30 000 mg lÿ1 NaCl. As can be seen in Fig. 7, the presence of NaCl, trying to match sea water composition in the sample, enhances the RTP signal (up to a plateau observed for 5000 mg lÿ1 NaCl). Higher salt concentrations did not noticeably affect the RTP signals. Therefore, 30 000 mg lÿ1 NaCl (as usually present in sea water [18]) was routinely added to the lead calibration standards. Considering this effect of NaCl, further studies of this effect were carried out by comparing RTP signals after adding 30 000 mg lÿ1 NaCl either to the carrier,

Fig. 4. Effect of the pH of the carrier on the RTP intensity for injections of 100 ng mlÿ1 of Pb(II) when using one of the three chelating reagents: (*) 7-sulphonic oxine; (^) 5-sulphonic oxine; (*) ferron.

Fig. 5. Effect of the concentration of the chelating reagent on the RTP intensity for injections of 100 ng mlÿ1 Pb(II): (&) 7sulphonic oxine (10ÿ3 M); (~) 5-sulphonic oxine (10ÿ3 M); (*) ferron (10ÿ6 M).

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Fig. 6. Effect of the deoxygenation mode on the RTP signals for injections of 100 ng mlÿ1 Pb(II) (7-sulphonic oxine used as chelating reagent).

or to the sample, or both to the sample and carrier. Also, the RTP signals without NaCl (in samples and in carrier) were measured. The results obtained are plotted in Fig. 8 which shows that the best results were achieved when sodium chloride was present in both the carrier and sample solutions. This effect could be attributed to the fact that the chloride ions could act by removing the retained free reagent excess from the resin (thus avoiding inner ®lter problems perhaps allowing for a more ef®cient retention of the chelate on the solid support). The in¯uence of the carrier ¯ow-rate was also studied over the interval 0.5±2.1 ml minÿ1, but no substantial alteration in the Pb(II) RTP peak height was observed. In order to avoid back-pressure operational problems from the solid support, a ¯ow-rate of 1.4 ml minÿ1 was selected for further experiments. Finally, the optimum gating time (tg) for the RTP lead determination was investigated (in the gating time interval from 0.02 to 0.4 ms using a ®xed delay time of

Fig. 7. Effect of the concentration of NaCl in the sample on the RTP signals for injections of 100 ng mlÿ1 of Pb(II) (7-sulphonic oxine used as chelating agent).

Fig. 8. Influence of the presence of 30 000 mg lÿ1 NaCl in either the carrier or the sample or in both.

0.05 ms). As can be seen in Fig. 9 the signal to background noise ratio of the RTP decreased drastically for gating times >0.05 ms; therefore a 0.05 ms setting was selected for further experiments. 3.3. Analytical performance characteristics Following the general procedures, the effect of foreign ions (typically present in sea water) on the RTP signal of lead was studied. Potential interferences were added to a standard lead solution containing 100 ng mlÿ1 analyte and their effect on the RTP signal of lead was investigated. The results are summarized in Table 2. As can be seen, most species did not interfere. Moreover, even for those elements causing interference, the effect is not important when analyzing sea water (the concentration levels of metals there is clearly much lower [18]). However, small concentrations of Fe(III) caused a severe interference, but it can be avoided by adding

Fig. 9. Effect of the integration time on the signal to background noise ratio for 100 ng mlÿ1 lead.

B. San Vicente de la Riva et al. / Analytica Chimica Acta 395 (1999) 1±9 Table 2 Study of interferences of different species on RTP lead optosensing (using 7-sulphonic oxine as chelating reagent) of 100 ng mlÿ1 lead Element

Concentration

Intensity (%)

± Cd

± 200 mg lÿ1 1 mg lÿ1

100 108 107

Mg

1 mg lÿ1 2 mg lÿ1 150 mg lÿ1

94 88 88

Al

250 mg lÿ1 1 mg lÿ1

100 91

Hg

200 mg lÿ1 400 mg lÿ1

103 94

Zn

400 mg lÿ1 1 mg lÿ1

97 92

Cu

200 mg lÿ1 400 mg lÿ1

90 77

Co

200 mg lÿ1 400 mg lÿ1

101 86

Fe

±

±a

Mn

100 mg lÿ1

79

Ca

5 mg lÿ1 86 mg lÿ1

99 173

a

Addition of 50 mg lÿ1 1,10-phenanthroline in the buffer eliminates interference of 5 mg lÿ1 Fe (III).

1,10-phenanthroline (to the carrier and lead samples) as shown in Table 2. In an evaluation of the effect of the sample volume injected on the analytical lead signal, as expecte, the relative net signal intensity observed in the interval

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studied was higher when the sample volume was higher. However, considering that higher sample volumes give rise to longer response times, a compromise sample volume of 2 ml was chosen. Analytical performance characteristics of the three proposed optosensors were evaluated under selected optimum conditions, collected in Table 1. Table 3 summarizes comparatively the results obtained for the three chelating reagents studied. Calibration graphs were prepared from the results of triplicate 2-ml injections of Pb standard solutions of increasing concentration and proved to be linear up to 2 ng mlÿ1 Pb(II). The detection limit, calculated as the concentration of lead which produced an analytical signal three times the standard deviation of the background signal, was 0.1 ng mlÿ1 Pb(II) for the most sensitive reagent, 7-sulphonic oxine. The precision (repeatibility) of the proposed ¯ow-through RTP optosensors, evaluated as the relative standard deviation of ®ve replicates of a sample containing 100 ng mlÿ1 Pb(II), was ca. 3% for the three reagents. The response time for full signal change was 5 min and no hysteresis effects were observed. The triplet lifetimes of the RTP of the different metal chelates immobilized on the resin were also measured (Table 3) showing that the immobilized lead(ferron)2 chelate has the longest triplet lifetime of the three chelates under study. This higher value could be due to the internal heavy atom effect produced by the iodine atom. Lead(7-sulphonic oxine)2 exhibits a longer triplet lifetime than lead(5-sulphonic oxine)2, probably because the sulphonic group which binds to the solid support is further away form the metal chelating bonding groups in the 5-sulphonic oxine than in the 7-sulphonic oxine (see Fig. 2). Thus, less rigidity of the phosphorophor should result with the former reagent.

Table 3 Analytical performance characteristics for the RTP optosensors of lead by using the three chelating reagents

a

ÿ1

Detection limit (mg l ) Response time (min) Precision (RSD%, nˆ5)b Lineal range Triplet lifetime (ms) a b

7-Sulphonic oxine

5-Sulphonica oxine

Ferron

0.1 5 3% Up to 2 mg lÿ1 0.071

0.2 5 3% Up to 2 mg lÿ1 0.028

3.2 5 ± ± 0.141

Calculated as three times the standard deviation of the background signal. At 100 mg lÿ1 Pb.

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7-sulphonic oxine as chelating reagent provided the best overall analytical performance characteristics for lead determination in sea water samples, therefore this reagent was selected in further experiments.

were obtained, as demonstrated by the results given in Table 4. 4. Conclusions

3.4. Analysis of real samples The usefulness of the proposed optosensing system was evaluated for the determination of traces of lead in sea water samples by following the general procedure, using the 7-sulphonic oxine reagent. Some chemical species present in the sea water samples can also form chelates with 7-sulphonic oxine. Therefore, a concentration of 310ÿ3 M 7-sulphonic oxine was used in order to ensure enough chelating reagent for complete lead complexation. Sea water samples from GijoÂn bay (Asturias) were ®rst analyzed without any other previous treatment of the samples except ®ltration through a 0.45 mm ®lter (Millipore Millex-HV). As no Pb(II) RTP signals were obtained from such waters, they were spiked in the laboratory with Pb(II) at different concentration levels and then analyzed by the proposed RTP ¯ow-through optosensing system. The results showed poor recoveries (<10% for the different additions of lead). At the same time, a darkening of the solid active phase was observed after passing the sea water sample. However, when the spiked sea water samples were ®ltered by passing them through an Omni®t column of 50 mm length and 3 mm inner diameter, packed with the anionic exchanger resin Dowex 1X2-200 at a ¯ow rate of 0.4 ml minÿ1 in order to remove oils and other organic impurities present in such sea waters, these problems were overcome. The resin-®ltered sea water samples were analyzed and good Pb(II) recoveries

The marriage of FIA strategies with RTP measurements [9] seems to provide an excellent approach to tackle the problem of development of environmental optical sensors. Three RTP chelates for lead are shown to offer some distinct merits for analytical use, including favourable analytical wavelengths, with excitation maximum at ca. 400 nm and emission maximum at ca. 600 nm, a large singlet±triplet splitting of 200 nm and relatively long triplet lifetimes. The detection limit for lead determination in sea water by using the selected RTP system with the 7sulphonic oxine reagent was 0.1 ng mlÿ1 (injected sample volumes of 2 ml). To our knowledge, this system provides the most sensitive optical sensor for lead described in the literature. The good Pb(II) recoveries observed for the analysis of sea waters with the proposed RTP ¯ow-through optical sensor, the low price of the required instrumentation and the possibility of automation of analysis offer very good prospects for its implementation in ®eld measurement campaigns (e.g. laboratories installed on ships).

Acknowledgements Financial support from European Union (DG XII Science, Research and Development) through Project ref. MAS3-CT97-0143 (``MEMOSEA'') is gratefully acknowledged.

Table 4 Recovery study of spiked Pb(II) in sea water samples Sea water sample no.

Pb(II) spiked (ng mlÿ1)

Pb(II) recovereda (ng mlÿ1)

Recovery (%)

1 2 3 4 5 6 7

6 15 30 40 50 75 100

6.51 15.13 30.35 41.25 48.03 70.53 96.21

109 100 101 103 96 94 96

a

MeanSD (nˆ5).

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