- Email: [email protected]
Pentachlorophenol preconcentration using quinolin-8-ol immobilized on controlled-pore glass and flow spectrophotometric determination
ANALYTICA CHIMICA ACTA ELSEVIER
Analytica Chimica Acta 288 (1994) 259-264
Pentachlorophenol preconcentration using quinolin-8-ol immobilized on controlled-pore glass and flow spectrophotometric determination C. Moreno-Romfin, M.R. Montero-Escolar, M.E. Le6n-Gonzfilez L.V. P6rez-Arribas, L.M. Polo-Diez
*,
Departamento de Qu(mica Analltica, Facultad de Ciencias Qubnicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
(Received 6th June 1993)
Abstract
A pentachlorophenol preconcentration method using a microcolumn (20 x 2.5 mm i.d.) filled with quinolin-8-ol (oxine) immobilized on controlled-pore glass (CPG) is proposed. The method is based on retention at pH 1 and elution at pH 3 with acetonitrile-water (80 + 20). Maximum retention of pentachiorophenol was 180/~g per gram of oxine-CPG. The breakthrough volume was 100 ml. The good ability for retention and elution allows the flow spectrophotometric determination of microgram amounts of pentachlorophenol. The calibration graph was linear from 4.0 to 25 /~g for a sample volume of 100 ml, the repeatability using a single column for 12 /zg of pentaehlorophenol was 1.2% (R.S.D., n = 4), and the reproducibility between five different microcolumns for the same concentration level was 1.6%. The 3tr detection limit was 2.5/~g and a preconcentration factor of 250-fold was obtained. Common cations and other phenols do not interfere significantly. Key words: Flow systems; UV-Visible spectrometry; Pentaehlorophenol; Preconcentration; Waters
1. Introduction
Phenols in general are an important group of compounds to be monitored in surface water, waste water and leachate, and an automated online system would give valuable data in m a n y situations. In particular, chlorophenols and nitrophenols are used as pesticides, bactericides, wood preservatives and synthetic intermediates. Moreover, chlorophenols are also byproducts of the
* Corresponding author.
chlorine bleaching process used in pulp and paper mills. The efficiency of a bio-oxidation waste water treatment plant can be monitored by observing how it handles low-level concentrations of pentachlorophenol; this phenol is more difficult to biodegrade than most other phenols, and its increase in the plant effluent may indicate decreased efficiency and a potential problem [1]. Moreover, chlorophenols have been investigated as an indicator p a r a m e t e r for polychlorinated dibenzodioxins and furans from municipal waste combustion [2]. Several on-line determinations of phenols have b e e n reported recently [1,3].
0003-2670/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD! 0003-2670(93)E0616-F
260
C Moreno-Romdn et aL / Analytica Chimica Acta 288 (1994) 259-264
The US Environmental Protection Agency recommends a maximum level of 1 ~g m1-1 of total phenolic compounds in water supplies [4] for analysis at this concentration level it is necessary to preconcentrate the sample before detection [1,5] for sensitivity enhancement. The use of solid materials for the retention of analytes or interferences in a liquid sample passing through them is a common alternative to sample pretreatment on account of the advantages offered. Quinolin-8-ol (oxine) has been used in immobilized form for the preconcentration of trace levels of metal ions [6-8]; other applications of immobilized oxine include the liquid chromatographic separation of metal ions [9] and the metal-assisted separation of phenols [10]. Controlled-pore glass (CPG) has been the support of choice in some of these applications because immobilization reactions on the glass surface are relatively simple [11]. Also, CPG exhibits the good mechanical strength and swelling stability required for flow systems. The purpose of this work was to study the retention of phenols, and especially pentachlorophenol, for preconcentration on oxine immobilized covalently on CPG, and spectrophotometric detection in a continuous-flow system.
with oxine immobilized on CPG, a HewlettPackard HP 8452A diode-array spectrophotometer equipped with a l-era path length silica flow cell and a PTFE reaction coil. The spectrophotometer was interfaced to an HP Vectra AT computer and an HP Think Jet printer. 2.2. Reagents
2. Experimental
All chemicals were of analytical-reagent grade and purified water was obtained using a Milli-Q apparatus (Millipore). Pentachlorophenol (99% pure, Carlo Erba), 2,4,6-triehlorophenol (99% pure, Carlo Erba), 2,4-dichlorophenol (99% pure, Aldrich Chemic), 4-chloro-3-methylphenol (99%, Aldrich Chemie), 2-nitrophenol ( > 99% pure, Fluka), 4-nitrophenol ( > 99% pure, Fluka), 3nitrophenol (Carlo Erba), 2,4-dinitrophenol (99.4% pure, Chem Service), 4,6-dinitro-2-methylphenol (98% pure, Chem Service) and 2,4-dimethylphenol (90% pure, Merck) were used for preparing 125 rag 1-1 stock standard solutions in 1.0 X 10 - 3 M sodium hydroxide solution. (Aminophenyl)trimethoxysilane (95% pure, ABCR) was supplied as mixed isomers and stored under refrigeration. Controlled-pore glass (CPG240-200, Sigma) was boiled in 5% nitric acid for 30 min, filtered on a glass filter, washed with deionized water and dried in an oven at 95"C.
2.1. Equipment
2.3. Immobilization procedure and storage
The system consisted of a Wiz programmable peristaltic pump connected to a column filled
Immobilization on CPG was achieved following the procedure described by Marshall and
achoo0h nor ___
1 4 ml/rnin
0 4 mlimin
~
I
I
0ETEoTo 1 802
nrn
'
COMPUTER
PRINTER
Fig. 1. Flow system for determination of pentachlorophenoL
i
waste
C Moreno-Romdn et al./Analytica Chimica Acta 288 (1994) 259-264
Mottola [11]. The product was packed in a glass microcolumn (20 x 2.5 mm i.d.) so that the length of the immobilized oxine zone was 15 ram. The column filled with the immobilized oxine was stored in a refrigerator at 4°C and pH 1. The dried material was stored in a desiccator. The efficiency of the immobilization procedure was evaluated indirectly. The difference between the initial oxine concentration added to the diazonium salt-CPG and the final concentration in the waste was measured spectrophotometrically in a buffered solution (pH 9, 0.1 M NH4CI-NH 3) at 360 nm in the presence of 1.0 x 10 - 2 M A I 3+.
3. Results and discussion
3.1. Immobilization efficiency In order to determine the amount of oxine immobilized on CPG, the waste liquid from the immobilization procedure was collected. The concentration of oxine was evaluated by formation of the aluminium-oxinate complex at pH 9.2 and spectrophotometric measurement at 360 nm. The amount of oxine immobilized was about 130 mg per gram of CPG.
3.2. Stability
2.4. Flow determination of pentachlorophenol
The hydrolytic stability of the immobilized oxine-CPG was studied in the pH range 1-12. The material is very stable in strongly acidic solutions. A slight coloration of the effluent is observed at pH > 4.8. At pH > 9 the yellow effluent becomes dark yellow with a maximum at 380 nm that may be due to the hydrolysis of the azo bond (the oxine absorbance maximum at this pH is at 308 nm). Regarding stability, no significant change in the retention of phenols has been observed over a 5-month storage period.
Calibration was achieved by pumping aliquots of solutions containing pentachlorophenol in the 4 - 2 5 / , g in 100 ml volume range at 1.4 ml min -1 through the microcoltmm filled with oxine-CPG at pH 1. Once pentachlorophenol had been retained on the column, after selection of the second channel in the pump, elution was effected with an acetonitrile-water (80:20) at pH 3, the flow-rate being 0.4 ml rain-1. Spectrophotometric measurements of the peak height were made at 302 nm against a blank solution (Fig. 1).
Table 1 Results of optimization of working conditions for the flow system Variable
Range studied
Value selected
pH (retention) How-rate (retention)(ml min-1) Eluent
1.0-12.8 0.4-2.9 Acetonitrile-water Methanol-water Ethanol-water Acetone-water 50 + 50-80 + 20 2-12 0.4-1.3 10-500 1-200
1.0 1.4 Acetonitrile-water
Eluent composition (v/v) pH (elution) Flow-rate (elution) (ml rain- i) Sample volume (ml) Pentachlorophenol per gram of oxine-CPG (~g)
261
a Maximum at which the column is saturated with pentachlorophenol.
80 + 20 3 0.4 100 180 a
262
c. Moreno-Romdn et al. / Analytica ChimicaActa 288 (1994) 259-264
3.3. Selection o f working conditions
Hydrodynamic and chemical variables affecting the flow system were studied by UV-visible absorbance measurements between 190 and 800 nm with a diode-array spectrophotometer. Table 1 shows the range and characteristics tested and the values selected for pentachlorophenol. The retention of pentachlorophenol from aqueous solutions in the microcolumn was evaluated at different pH values. At low pH (between 1 and 3), the retention was > 97%, whereas at pH 7 only 7% of the pentachlorophenol was retained. At pH > 9, the effluent on becomes yellow-orange with an absorbance maximum at 380 nm and pentachlorophenol is not retained in the microcolumn. Under the acidic retention conditions used, the phenols are not ionized and at least three types of interactions may be involved in the retention of the phenolic compounds: hydrogenbonding, ~--rr and dispersion interactions. The influence of the flow-rate on the retention of pentachlorophenol was evaluated between 0.4 and 2.9 ml min-1. At flow-rates higher than 1.4 ml rain-1 the retention decreased, being only of about 88% at 2.9 ml min -1. For flow-rates between 0.4 and 1.4 ml min -1 the retention was higher than 98%; therefore, 1.4 ml min -1 was chosen for further studies. Once the pH and flow-rate conditions for retention had been established, several parameters affecting the elution were studied. Several organic solvent-water mixtures were evaluated as eluents for pentachlorophenol. Ethanol-water and methanol-water mixtures produced a yellow effluent with maximum absorbance at 380 nm, which interfered, making the spectrophotometric evaluation of the eluate difficult. Acetone-water mixtures produced small bubbles in the system and hence gave rise to irreproducible results. Several acetonitrile-water mixtures were tested; a 50 + 50 mixture yielded only a recovery of 56%, whereas an 80 + 20 mixture yielded recoveries > 92%. The effect of pH on elution with acetonitrilewater (80 + 20) was studied in the range 3-12. At pH < 3 the recovery was low, and at pH above 7 the effluent again became yellow-orange with
maximum absorbance at 380 nm. At pH 3 the highest recovery (ca. 92%) was achieved. The elution flow-rate was studied between 0.4 and 1.3 ml rain-1; for flow-rates higher than 0.4 ml rain-1 the recovery decreased, so this value was selected for further studies. In the elution process a well defined peak appeared, showing the good elution characteristics. The maximum amount of pentachlorophenol retained by the column per gram of oxine-CPG was established as 180 /~g. The breakthrough volume was determined by studying spectrophotometrically the recovery of 10 /~g of pentachlorophenol in volumes between 10 and 500 ml. The volume of 100 ml was chosen as optimum because higher volumes yielded lower recoveries (71% was obtained for 250 ml). To study the stability of the pentachlorophenol retained on the column, 100 ml of solution containing 20/~g of pentachlorophenol at pH 1 were pumped through the column. The column with the retained pentachlorophenol was kept in a refrigerator for 4 days; subsequently, pentachlorophenol was eluted with acetonitrile-water (80 + 20) at pH 3 and the recovery was > 98%. Under the conditions established, it was possible to use the column to make about ten determinations of pentachlorophenol with recoveries of ca. 96%. This behaviour shows the reversibility of the retention-elution process. 3.4. Analytical characteristics
The linear calibration range was 4.0-25/zg for a sample volume of 100 ml. The 3o- detection limit was 2.5/zg. The repeatability for use of one microcolumn, expressed as relative standard deviation, was 1.2% for 12 /~g of pentachlorophenol (four determinations). The reproducibility between five different microcolumns for the same amount (12 /.tg) was 1.6%. A preconcentration factor of 250-fold was obtained for an initial sample volume of 100 ml (the final volume was about 0.4 ml). 3.5. Interferences
The effect of several metal ions on the peak height in the elution of pentachlorophenol was
C. Moreno-Romdn et al. /Analytica Chimica Acta 288 (1994) 259-264
263
studied. Metal ions such as aluminium, calcium and magnesium do not interfere in the determination of 20/~g of pentachlorophenol under the conditions established up to a concentration level of 100/~g m1-1. The influence of the presence of other phenols was studied; Table 2 shows the percentage retention of 20 gg of each phenol by the column under the conditions optimized for pentachlorophenol. The relationship between the structure of particular phenols and the retention is not clear, but the presence of chlorosubstituents results in higher retention; hence the retention of pentachlorophenol is higher than that of 2,4,6-trichlorophenol, and the retention of the latter is higher than that of 2,4-dichlorophenol. Retention of nitrophenols depends on the number of substituents and their relative positions in the aromatic ring. Nitrosubstituents in the ortho or para position results in substantially increased retention relative to 3-nitrophenol. However, an increase in the number of nitrosubstituents produced a decrease in retention. Further, the presence of a methyl group resulted in a greater electron density in the aromatic ring and hence •r-~r interactions, and also dispersion interactions, were enhanced. Indeed, 4,6-dinitro-2-methylphenol was more strongly retained than 2,4-dinitrophenol. To study the selectivity, the effect of 10/~g of different chlorophenols, methylphenols and nitrophenols on the absorbance of 10 /~g of pen-
Table 3 Influence of different phenols on the spectrophotometric determination of pentachlorophenol
Table 2 Retention of different phenols in the o x i n e - C P G column
4. Conclusions
Phenol 2,4,6-Trichlorophenol 2,4-Dichlorophenol 2-Nitrophenol 3-Nitrophenol 4-Nitrophenol 2,4-Dinitrophenol 4-Chloro-3-methylphenol 4,6-Dinitro-2-methylphenol 2,4-Dimethylphenol
A(max)
Retention
(nm)
(%) ~
290 286 348 326 282 262 280 272 276
84.2 6.1 44.4 9.3 43.8 20.9 14.0 46.7 24.0
a U n d e r the same conditions as pentachlorophenol.
Phenol
2,4,6-Trichlorophenoi 2,4-Dichlorophenol 4-Chloro-3-methylphenol 2,4-Dinitrophenol 4,6-Dinitro-2-methylphenol 4-Nitrophenol 3-Nitrophenol 2-Nitrophenol 2,4-Dimethylphenol
Interference (%) Without column
In the proposed flow m e t h o d
8.0 62 37 481 83 66 -
6.0 15 14 18 -
tachlorophenol at 302 nm was studied; the results were compared with those obtained by mixing solutions containing 10/.~g of pentachlorophenol and 10/~g of the other phenols in a final volume of 10 ml under the chemical conditions proposed for the flow determination of pentachlorophenol (Table 3). Except for 2,4,6-trichlorophenol, the level of interference is clearly decreased owing to the different retentions of the individual phenols and the different absorption maxima, as can be seen in Table 2. Phenols such as 4-nitrophenol and 2-nitrophenol produce very a high absorbance but, owing to their retention by the column of oxine-CPG, the interference in the determination of pentachlorophenol is very low.
Oxine immobilized on CPG shows a good ability to preconcentrate pentachlorophenol. OxineCPG can be packed in a microcolumn and used in flow systems, allowing preconcentration and further spectrophotometric determination of pentachlorophenol at low concentrations. The stability of the material makes the off-line sampling and later determination in the laboratory possible. In addition, the immobilization procedure is inexpensive and simple.
264
C. Moreno-Romdn et al. / Analytica Chimica Acta 288 (1994) 259-264
5. A c k n o w l e d g e m e n t s T h e financial s u p p o r t o f t h e S p a n i s h D G I C Y T , p r o j e c t PB89-109, is gratefully acknowledged.
6. References [1] R.G. Melcher, D.W. Bakke and G.H. Hughes, Anal. Chem., 64 (1992) 2258. [2] T. Oberg and J. Berstr6m, Chemosphere, 19 (1989) 337. [3] E. Rodrlguez-Gonzalo, J.L. P~rez-Pav6n, J. Ruzicka and G.D. Christian, Anal. Chim. Acta, 259 (1992) 37.
[4] Environmental Protection Agency, Manual of Methods for Chemical Analysis of Water and Wastes, Office of Technology Transfer, Washington, DC, 1974. [5] G.W. Patton, LL. McConnell, M.T. Zaranski and T.F. Bidleman, Anal. Chem., 64 (1992) 2858. [6] G. Persaud and F.F. Cantwell, Anal. Chem., 64 (1992) 89. [7] P.Y.T. Chow and F.F. Cantwell, Anal. Chem., 60 (1988) 1569. [8] M.R. Weaver and J.M. Harris, Anal. Chem., 61 (1989) 1001. [9] M.-S. Kuo and H.A. Mottola, Anal. Chim. Acta, 120 (1980) 255. [10] G.J. Shahwan and J.R. Jerozek, J. Chromatogr., 256 (1983) 39. [11] M.A. Marshall and H.A. Mottola, Anal. Chem., 55 (1983) 2089.
Analytica Chimica Acta 288 (1994) 259-264
Pentachlorophenol preconcentration using quinolin-8-ol immobilized on controlled-pore glass and flow spectrophotometric determination C. Moreno-Romfin, M.R. Montero-Escolar, M.E. Le6n-Gonzfilez L.V. P6rez-Arribas, L.M. Polo-Diez
*,
Departamento de Qu(mica Analltica, Facultad de Ciencias Qubnicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
(Received 6th June 1993)
Abstract
A pentachlorophenol preconcentration method using a microcolumn (20 x 2.5 mm i.d.) filled with quinolin-8-ol (oxine) immobilized on controlled-pore glass (CPG) is proposed. The method is based on retention at pH 1 and elution at pH 3 with acetonitrile-water (80 + 20). Maximum retention of pentachiorophenol was 180/~g per gram of oxine-CPG. The breakthrough volume was 100 ml. The good ability for retention and elution allows the flow spectrophotometric determination of microgram amounts of pentachlorophenol. The calibration graph was linear from 4.0 to 25 /~g for a sample volume of 100 ml, the repeatability using a single column for 12 /zg of pentaehlorophenol was 1.2% (R.S.D., n = 4), and the reproducibility between five different microcolumns for the same concentration level was 1.6%. The 3tr detection limit was 2.5/~g and a preconcentration factor of 250-fold was obtained. Common cations and other phenols do not interfere significantly. Key words: Flow systems; UV-Visible spectrometry; Pentaehlorophenol; Preconcentration; Waters
1. Introduction
Phenols in general are an important group of compounds to be monitored in surface water, waste water and leachate, and an automated online system would give valuable data in m a n y situations. In particular, chlorophenols and nitrophenols are used as pesticides, bactericides, wood preservatives and synthetic intermediates. Moreover, chlorophenols are also byproducts of the
* Corresponding author.
chlorine bleaching process used in pulp and paper mills. The efficiency of a bio-oxidation waste water treatment plant can be monitored by observing how it handles low-level concentrations of pentachlorophenol; this phenol is more difficult to biodegrade than most other phenols, and its increase in the plant effluent may indicate decreased efficiency and a potential problem [1]. Moreover, chlorophenols have been investigated as an indicator p a r a m e t e r for polychlorinated dibenzodioxins and furans from municipal waste combustion [2]. Several on-line determinations of phenols have b e e n reported recently [1,3].
0003-2670/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD! 0003-2670(93)E0616-F
260
C Moreno-Romdn et aL / Analytica Chimica Acta 288 (1994) 259-264
The US Environmental Protection Agency recommends a maximum level of 1 ~g m1-1 of total phenolic compounds in water supplies [4] for analysis at this concentration level it is necessary to preconcentrate the sample before detection [1,5] for sensitivity enhancement. The use of solid materials for the retention of analytes or interferences in a liquid sample passing through them is a common alternative to sample pretreatment on account of the advantages offered. Quinolin-8-ol (oxine) has been used in immobilized form for the preconcentration of trace levels of metal ions [6-8]; other applications of immobilized oxine include the liquid chromatographic separation of metal ions [9] and the metal-assisted separation of phenols [10]. Controlled-pore glass (CPG) has been the support of choice in some of these applications because immobilization reactions on the glass surface are relatively simple [11]. Also, CPG exhibits the good mechanical strength and swelling stability required for flow systems. The purpose of this work was to study the retention of phenols, and especially pentachlorophenol, for preconcentration on oxine immobilized covalently on CPG, and spectrophotometric detection in a continuous-flow system.
with oxine immobilized on CPG, a HewlettPackard HP 8452A diode-array spectrophotometer equipped with a l-era path length silica flow cell and a PTFE reaction coil. The spectrophotometer was interfaced to an HP Vectra AT computer and an HP Think Jet printer. 2.2. Reagents
2. Experimental
All chemicals were of analytical-reagent grade and purified water was obtained using a Milli-Q apparatus (Millipore). Pentachlorophenol (99% pure, Carlo Erba), 2,4,6-triehlorophenol (99% pure, Carlo Erba), 2,4-dichlorophenol (99% pure, Aldrich Chemic), 4-chloro-3-methylphenol (99%, Aldrich Chemie), 2-nitrophenol ( > 99% pure, Fluka), 4-nitrophenol ( > 99% pure, Fluka), 3nitrophenol (Carlo Erba), 2,4-dinitrophenol (99.4% pure, Chem Service), 4,6-dinitro-2-methylphenol (98% pure, Chem Service) and 2,4-dimethylphenol (90% pure, Merck) were used for preparing 125 rag 1-1 stock standard solutions in 1.0 X 10 - 3 M sodium hydroxide solution. (Aminophenyl)trimethoxysilane (95% pure, ABCR) was supplied as mixed isomers and stored under refrigeration. Controlled-pore glass (CPG240-200, Sigma) was boiled in 5% nitric acid for 30 min, filtered on a glass filter, washed with deionized water and dried in an oven at 95"C.
2.1. Equipment
2.3. Immobilization procedure and storage
The system consisted of a Wiz programmable peristaltic pump connected to a column filled
Immobilization on CPG was achieved following the procedure described by Marshall and
achoo0h nor ___
1 4 ml/rnin
0 4 mlimin
~
I
I
0ETEoTo 1 802
nrn
'
COMPUTER
PRINTER
Fig. 1. Flow system for determination of pentachlorophenoL
i
waste
C Moreno-Romdn et al./Analytica Chimica Acta 288 (1994) 259-264
Mottola [11]. The product was packed in a glass microcolumn (20 x 2.5 mm i.d.) so that the length of the immobilized oxine zone was 15 ram. The column filled with the immobilized oxine was stored in a refrigerator at 4°C and pH 1. The dried material was stored in a desiccator. The efficiency of the immobilization procedure was evaluated indirectly. The difference between the initial oxine concentration added to the diazonium salt-CPG and the final concentration in the waste was measured spectrophotometrically in a buffered solution (pH 9, 0.1 M NH4CI-NH 3) at 360 nm in the presence of 1.0 x 10 - 2 M A I 3+.
3. Results and discussion
3.1. Immobilization efficiency In order to determine the amount of oxine immobilized on CPG, the waste liquid from the immobilization procedure was collected. The concentration of oxine was evaluated by formation of the aluminium-oxinate complex at pH 9.2 and spectrophotometric measurement at 360 nm. The amount of oxine immobilized was about 130 mg per gram of CPG.
3.2. Stability
2.4. Flow determination of pentachlorophenol
The hydrolytic stability of the immobilized oxine-CPG was studied in the pH range 1-12. The material is very stable in strongly acidic solutions. A slight coloration of the effluent is observed at pH > 4.8. At pH > 9 the yellow effluent becomes dark yellow with a maximum at 380 nm that may be due to the hydrolysis of the azo bond (the oxine absorbance maximum at this pH is at 308 nm). Regarding stability, no significant change in the retention of phenols has been observed over a 5-month storage period.
Calibration was achieved by pumping aliquots of solutions containing pentachlorophenol in the 4 - 2 5 / , g in 100 ml volume range at 1.4 ml min -1 through the microcoltmm filled with oxine-CPG at pH 1. Once pentachlorophenol had been retained on the column, after selection of the second channel in the pump, elution was effected with an acetonitrile-water (80:20) at pH 3, the flow-rate being 0.4 ml rain-1. Spectrophotometric measurements of the peak height were made at 302 nm against a blank solution (Fig. 1).
Table 1 Results of optimization of working conditions for the flow system Variable
Range studied
Value selected
pH (retention) How-rate (retention)(ml min-1) Eluent
1.0-12.8 0.4-2.9 Acetonitrile-water Methanol-water Ethanol-water Acetone-water 50 + 50-80 + 20 2-12 0.4-1.3 10-500 1-200
1.0 1.4 Acetonitrile-water
Eluent composition (v/v) pH (elution) Flow-rate (elution) (ml rain- i) Sample volume (ml) Pentachlorophenol per gram of oxine-CPG (~g)
261
a Maximum at which the column is saturated with pentachlorophenol.
80 + 20 3 0.4 100 180 a
262
c. Moreno-Romdn et al. / Analytica ChimicaActa 288 (1994) 259-264
3.3. Selection o f working conditions
Hydrodynamic and chemical variables affecting the flow system were studied by UV-visible absorbance measurements between 190 and 800 nm with a diode-array spectrophotometer. Table 1 shows the range and characteristics tested and the values selected for pentachlorophenol. The retention of pentachlorophenol from aqueous solutions in the microcolumn was evaluated at different pH values. At low pH (between 1 and 3), the retention was > 97%, whereas at pH 7 only 7% of the pentachlorophenol was retained. At pH > 9, the effluent on becomes yellow-orange with an absorbance maximum at 380 nm and pentachlorophenol is not retained in the microcolumn. Under the acidic retention conditions used, the phenols are not ionized and at least three types of interactions may be involved in the retention of the phenolic compounds: hydrogenbonding, ~--rr and dispersion interactions. The influence of the flow-rate on the retention of pentachlorophenol was evaluated between 0.4 and 2.9 ml min-1. At flow-rates higher than 1.4 ml rain-1 the retention decreased, being only of about 88% at 2.9 ml min -1. For flow-rates between 0.4 and 1.4 ml min -1 the retention was higher than 98%; therefore, 1.4 ml min -1 was chosen for further studies. Once the pH and flow-rate conditions for retention had been established, several parameters affecting the elution were studied. Several organic solvent-water mixtures were evaluated as eluents for pentachlorophenol. Ethanol-water and methanol-water mixtures produced a yellow effluent with maximum absorbance at 380 nm, which interfered, making the spectrophotometric evaluation of the eluate difficult. Acetone-water mixtures produced small bubbles in the system and hence gave rise to irreproducible results. Several acetonitrile-water mixtures were tested; a 50 + 50 mixture yielded only a recovery of 56%, whereas an 80 + 20 mixture yielded recoveries > 92%. The effect of pH on elution with acetonitrilewater (80 + 20) was studied in the range 3-12. At pH < 3 the recovery was low, and at pH above 7 the effluent again became yellow-orange with
maximum absorbance at 380 nm. At pH 3 the highest recovery (ca. 92%) was achieved. The elution flow-rate was studied between 0.4 and 1.3 ml rain-1; for flow-rates higher than 0.4 ml rain-1 the recovery decreased, so this value was selected for further studies. In the elution process a well defined peak appeared, showing the good elution characteristics. The maximum amount of pentachlorophenol retained by the column per gram of oxine-CPG was established as 180 /~g. The breakthrough volume was determined by studying spectrophotometrically the recovery of 10 /~g of pentachlorophenol in volumes between 10 and 500 ml. The volume of 100 ml was chosen as optimum because higher volumes yielded lower recoveries (71% was obtained for 250 ml). To study the stability of the pentachlorophenol retained on the column, 100 ml of solution containing 20/~g of pentachlorophenol at pH 1 were pumped through the column. The column with the retained pentachlorophenol was kept in a refrigerator for 4 days; subsequently, pentachlorophenol was eluted with acetonitrile-water (80 + 20) at pH 3 and the recovery was > 98%. Under the conditions established, it was possible to use the column to make about ten determinations of pentachlorophenol with recoveries of ca. 96%. This behaviour shows the reversibility of the retention-elution process. 3.4. Analytical characteristics
The linear calibration range was 4.0-25/zg for a sample volume of 100 ml. The 3o- detection limit was 2.5/zg. The repeatability for use of one microcolumn, expressed as relative standard deviation, was 1.2% for 12 /~g of pentachlorophenol (four determinations). The reproducibility between five different microcolumns for the same amount (12 /.tg) was 1.6%. A preconcentration factor of 250-fold was obtained for an initial sample volume of 100 ml (the final volume was about 0.4 ml). 3.5. Interferences
The effect of several metal ions on the peak height in the elution of pentachlorophenol was
C. Moreno-Romdn et al. /Analytica Chimica Acta 288 (1994) 259-264
263
studied. Metal ions such as aluminium, calcium and magnesium do not interfere in the determination of 20/~g of pentachlorophenol under the conditions established up to a concentration level of 100/~g m1-1. The influence of the presence of other phenols was studied; Table 2 shows the percentage retention of 20 gg of each phenol by the column under the conditions optimized for pentachlorophenol. The relationship between the structure of particular phenols and the retention is not clear, but the presence of chlorosubstituents results in higher retention; hence the retention of pentachlorophenol is higher than that of 2,4,6-trichlorophenol, and the retention of the latter is higher than that of 2,4-dichlorophenol. Retention of nitrophenols depends on the number of substituents and their relative positions in the aromatic ring. Nitrosubstituents in the ortho or para position results in substantially increased retention relative to 3-nitrophenol. However, an increase in the number of nitrosubstituents produced a decrease in retention. Further, the presence of a methyl group resulted in a greater electron density in the aromatic ring and hence •r-~r interactions, and also dispersion interactions, were enhanced. Indeed, 4,6-dinitro-2-methylphenol was more strongly retained than 2,4-dinitrophenol. To study the selectivity, the effect of 10/~g of different chlorophenols, methylphenols and nitrophenols on the absorbance of 10 /~g of pen-
Table 3 Influence of different phenols on the spectrophotometric determination of pentachlorophenol
Table 2 Retention of different phenols in the o x i n e - C P G column
4. Conclusions
Phenol 2,4,6-Trichlorophenol 2,4-Dichlorophenol 2-Nitrophenol 3-Nitrophenol 4-Nitrophenol 2,4-Dinitrophenol 4-Chloro-3-methylphenol 4,6-Dinitro-2-methylphenol 2,4-Dimethylphenol
A(max)
Retention
(nm)
(%) ~
290 286 348 326 282 262 280 272 276
84.2 6.1 44.4 9.3 43.8 20.9 14.0 46.7 24.0
a U n d e r the same conditions as pentachlorophenol.
Phenol
2,4,6-Trichlorophenoi 2,4-Dichlorophenol 4-Chloro-3-methylphenol 2,4-Dinitrophenol 4,6-Dinitro-2-methylphenol 4-Nitrophenol 3-Nitrophenol 2-Nitrophenol 2,4-Dimethylphenol
Interference (%) Without column
In the proposed flow m e t h o d
8.0 62 37 481 83 66 -
6.0 15 14 18 -
tachlorophenol at 302 nm was studied; the results were compared with those obtained by mixing solutions containing 10/.~g of pentachlorophenol and 10/~g of the other phenols in a final volume of 10 ml under the chemical conditions proposed for the flow determination of pentachlorophenol (Table 3). Except for 2,4,6-trichlorophenol, the level of interference is clearly decreased owing to the different retentions of the individual phenols and the different absorption maxima, as can be seen in Table 2. Phenols such as 4-nitrophenol and 2-nitrophenol produce very a high absorbance but, owing to their retention by the column of oxine-CPG, the interference in the determination of pentachlorophenol is very low.
Oxine immobilized on CPG shows a good ability to preconcentrate pentachlorophenol. OxineCPG can be packed in a microcolumn and used in flow systems, allowing preconcentration and further spectrophotometric determination of pentachlorophenol at low concentrations. The stability of the material makes the off-line sampling and later determination in the laboratory possible. In addition, the immobilization procedure is inexpensive and simple.
264
C. Moreno-Romdn et al. / Analytica Chimica Acta 288 (1994) 259-264
5. A c k n o w l e d g e m e n t s T h e financial s u p p o r t o f t h e S p a n i s h D G I C Y T , p r o j e c t PB89-109, is gratefully acknowledged.
6. References [1] R.G. Melcher, D.W. Bakke and G.H. Hughes, Anal. Chem., 64 (1992) 2258. [2] T. Oberg and J. Berstr6m, Chemosphere, 19 (1989) 337. [3] E. Rodrlguez-Gonzalo, J.L. P~rez-Pav6n, J. Ruzicka and G.D. Christian, Anal. Chim. Acta, 259 (1992) 37.
[4] Environmental Protection Agency, Manual of Methods for Chemical Analysis of Water and Wastes, Office of Technology Transfer, Washington, DC, 1974. [5] G.W. Patton, LL. McConnell, M.T. Zaranski and T.F. Bidleman, Anal. Chem., 64 (1992) 2858. [6] G. Persaud and F.F. Cantwell, Anal. Chem., 64 (1992) 89. [7] P.Y.T. Chow and F.F. Cantwell, Anal. Chem., 60 (1988) 1569. [8] M.R. Weaver and J.M. Harris, Anal. Chem., 61 (1989) 1001. [9] M.-S. Kuo and H.A. Mottola, Anal. Chim. Acta, 120 (1980) 255. [10] G.J. Shahwan and J.R. Jerozek, J. Chromatogr., 256 (1983) 39. [11] M.A. Marshall and H.A. Mottola, Anal. Chem., 55 (1983) 2089.