Determination of polar aniline derivatives in aqueous environmental samples using on-line liquid chromatographic preconcentration techniques

Summary On-line precolumn sample handling is used to enrich polar aniline derivatives in order to preconcentrate them prior to their separation. Liquid-solid extraction is possible with a cation-exchanger precolumn after acidification of water samples at pH 3 and a clean-up in order to remove the high amounts of inorganic cations present in natural samples. Since inorganic removal cannot be total, overloading of the ion exchanger occurs rapidly. The volume which can be directly percolated through the cation-exchanger precolumn cannot exceed 30 ml and the amount preconcentrated is not sufficient for a determination at the 100 ppt level. A two-step preconcentration procedure is carried out in order to increase the sample volume: the direct percolation of samples through the cation-exchanger precolumn is avoided and the clean-up step is no longer necessary. Aniline derivatives are preconcentrated in their neutral form at pH 7 by a 9-cm long column packed with the copolymer-based PRP-1 sorbent; then, a small volume of water-methanol at pH 2 allows the cationic compounds alone to be desorbed from the PRP-1 column in their protonated form and to be transferred to a 1-cm long cation-exchanger precolumn. This precolumn is then coupled to an analytical C18 column and its content on-line analysed by an acetonitrile gradient. The PRP-1 column acts as a powerful filter to many neutral interferents and aniline derivatives can be thus determined from 150-ml drinking water samples with 10–50 ppt UV detection limits.Dedicated to Roland W. Frei     

Off-Line and On-Line Preconcentration Techniques for the Determination of Phenylureas in Freshwaters

Abstract

Phenylurea herbicides are analysed by reversed-phase liquid chromatography using UV detection at 244 nm after a concentration step in order to determine ppb or sub-ppb levels in drinking and river waters. With an average UV detection limit of 5 ng, a 500 ml sample volume is necessary to reach the 10 ppt level for spiked LC grade water samples and enables easy determination of concentrations below the ppb level for river water samples. Off-line and on-line methods are compared for the concentration step. Off-line concentration consists in a liquid sorption on n-octadecyl silica (C18) and elution by a suitable organic solvent. Polar phenylureas have low retention volumes on C18 silica and consequently the length of the concentration column has to be 10 cm to concentrate them at the ppb level from 100 ml of water and longer for lower levels of detection. Nevertheless, we show that increasing the size of the concentration column does not improve the limits of detection because of the numerous interferences also concentrated when percolating high volumes of water. On-line technology can be used only with short precolumns and requires a sorbent with a great retention for phenylureas. The copolymer-based PRP-1 is found to be an excellent sorbent and it is then possible to apply on-line precolumn technology with preconcentration through two precolumns (10 × 21 mm ID) in series, the first one being packed with C18 silica and the other with the PRP-1 polymer. Interfering compounds are then trapped onto the first precolumn acting as a filter and common phenylurea-breakthrough volumes on the PRP-1 precolumn are higher than 500 ml. Knowing the amounts preconcentrated on both precolumns and using UV and electrochemical detection help the identification of phenylureas in river water.     

Selective Preconcentration and Ion Chromatography of Trace Lead(II) in Environmental Samples Using a Porous Graphitic Carbon Column

Abstract

Highly selective ion analysis of sub-ppm Pb(II) on a porous graphitic carbon (PGC) column, is described using ion chromatography with acidified aqueous mobile phases, post-column detection using PAR and spectrophotometric detection. Sensitivity is enhanced by on-line preconcentration of the metal prior to the chromatography using one millilitre injection volumes applied to a calix[4]arene-tetrahydroxamate based chelating precolumn. The method is applied to water samples collected from a river subjected to an industrial effluent discharge and results obtained compare favourably with those obtained using flame atomic absorption spectroscopy.     

Optimization of An On-Line Precolumn Preconcentration Method for the Determination of Polycyclic Aromatic Hydrocarbons (PAHs) in Water Samples (River and Sea Water)

Abstract

A new on-line precolumn preconcentration method for the determination of EPA priority pollutants (PAHs) in river and sea water has been developed. The retention time for each PAH in the precolumn has been determined for several percentages of organic solvent (acetonitrile) in the sample. This is very important because recoveries show a great dependence on this parameter.

The proposed procedure, combined with HPLC and spectrofluorimetric detection, reaches very low detection limits (subnanograms per liter) and it has been applied to river and sea water samples with good results.     

Determination of Benzidine and its Congeners by Liquid Chromatography with Electrochemical Detection

High performance liquid chromatography coupled with electrochemical detection (HPLC-EC) method has been developed for the determination of benzidine and its related congeners in wastewater at the low ng/mL level. With a µ-Bondapak C₁₈ column, 53% acetonitrile (pH 4.7), and electrochemical detection at +1.0 V applied potential, the detection limits range from 3 ng/mL for benzidine to 10 ng/mL for 3,3′-dichlorobenzidine, assuming a 100 µL injection. The detection limits can be further lowered to the sub-ng/mL level by incorporating a short precolumn into the HPLC system for on-line sample enrichment. Recoveries are generally greater than 80% — except for benzidine and 3,3′-diaminobenzidine which give 50–70% recoveries. Industrial wastewater samples from a coke-plant and a plant producing 3,3′-dichlorobenzidine based dyes were successfully studied via this method.     

Determination of lead in natural waters using flow injection with on-line preconcentration and flame AAS detection

A rapid and sensitive method has been developed for the determination of lead in water samples by flame atomic absorption spectrometry using on-line preconcentration on a microcolumn packed with silica gel treated with a mixture of Aliquat 336 and nitroso-R-salt. The lead is retained at pH 5.5. The preconcentrated lead is directly eluted from the column to the nebulizer-burner system using 150 L of 0.1 mol/L hydrochloric acid. The optimum preconcentration conditions are given and the retention efficiency achieved is higher than 80%. The enrichment factor is 37 and 100 for sample volumes of 5 and 30 mL, respectively. The limits of detection are 10.0, 6.0 and 4.0 ng/mL when 5, 10 and 30 mL of water is preconcentrated.     

Trace Enrichment and HPLC Analysis of Chlorophenols in Environmental Samples,Using Precolumn Sample Preconcentration and Electrochemical Detection

Abstract

A HPLC method has been developed for trace analysis of chlorophenols in the 0.2–2 ppb range from spiked water samples. Simple liquid-liquid extraction followed by on-line preconcentration of total mono- and dichlorophenols has been performed using a divinylbenzene-styrene copolymeric sorbent (PRP₁) as packing material for the precolumn. The chlorophenols have been eluted from the precolumn on an analytical column (5μm LiChrosorb RP-18, 12.5 cm × 4 mm) by use of a switching valve system followed by separation. Detection was carried out with an electrochemical detector. The linearity of the detector response has been proved over two orders of magnitude. The detection limit of chlorophenols by means of the electrochemical method is in the lower picogram range. The recoveries of the isomeric chlorophenols from spiked river water samples having initial concentrations of 2ppb are usually 70–90%. The procedure has been applied to drinking water and river water.     

Use of direct injection precolumn techniques for the high-performance liquid chromatographic determination of the retinoids acitretin and 13-cis-acitretin in plasma.

A previously developed highly sensitive high-performance liquid chromatographic method for the determination of retinoids, using direct injection of large plasma volumes, on-line solid-phase extraction and ultraviolet detection, was improved and fully validated for the determination of acitretin and 13-cis-acitretin in plasma samples. The addition of acetonitrile to improve the recovery was performed on-line by a T-piece, avoiding any cis-trans isomerization which could occur when acetonitrile was added prior to storage in the autosampler. About 30 injections could be made onto one precolumn despite the large injection volume (1 ml of plasma containing the internal standard). Full automation was attained by the use of automated precolumn replacement. In addition, forward- and back-flush purging of the precolumn enhanced the longevity of the analytical column. This consisted of three coupled C18 columns of 125 mm length each. The quantification limit was 0.3 ng/ml, using ultraviolet detection at 360 nm, and the mean inter-assay precision was 3.8% for the two compounds.     

Trace-level determination of polar phenolic compounds in aqueous samples by high-performance liquid chromatography and on-line preconcentration on porous graphitic carbon

The use of porous graphitic carbon (PGC) was investigated for the trace enrichment and the on-line liquid chromatographic separation of polar phenolic compounds (phenol, di- and trihydroxybenzenes, aminophenols, etc.) from aqueous samples. Comparison between retentions obtained with PGC and with the copolymer-based sorbent PRP-1 showed similar variations of the capacity factors with the mobile phase composition, but an inverse retention order. The capacity factor of a very polar analyte, such as 1,3,5-trihydroxybenzene (phloroglucinol), is 1000 in pure water, whereas this analyte is not retained by C₁₈-silica and is poorly retained by PRP-1 (k′ = 3 in water). A precolumn packed with PGC can be coupled to a PGC analytical column for simple separation in the reversed-phase mode. This methodology has been applied to the direct determination of pyrocatechol, resorcinol and phloroglucinol below the 0.1 μg/1 level in a 50-ml sample.     

Evaluation of an on-line coupled continuous flow liquid membrane extraction and precolumn system as trace enrichment technique by liquid chromatographic determination of bisphenol A

An on-line coupled continuous flow liquid membrane extraction (CFLME) and C₁₈ precolumn system was developed for sample preconcentration in liquid chromatography determination. After preconcentration by CFLME, which is based on the combination of continuous flow liquid–liquid extraction and supported liquid membrane, bisphenol A (BPA) was enriched in 960 μl of 1 mol l⁻¹ NaOH used as acceptor. This acceptor was on-line neutralized and transported onto the C₁₈ precolumn where analytes were absorbed and focused. Then the focused analytes were injected onto a C₁₈ analytical column for separation and detected at 220 nm with a diode array detector. CFLME related parameters such as flow rates, pH of donor and acceptor, and enrichment time were optimized. The proposed method presents a detection limit of 0.03 μg l⁻¹ (S/N=3) when 60 ml samples was enriched with an enrichment time of 30 min. Compared with C₁₈ based column-switching procedure, this proposed procedure presents similar sample throughput and lower detection limits. The proposed method was successfully applied to determine BPA in tap water, river water, and municipal sewage effluent samples.     

Liquid chromatography of phenolic compounds in natural water using on-line trace enrichment

Summary Two packing materials, C18 and PLRP-S, are studied for on-line trace enrichment of phenolic compounds in water. Various precolumns of different internal diameter are also tested and the addition of an ion-pair reagent to increase retention and thus, breakthrough volumes of phenolic compounds, is studied. Best results are obtained when a PLRP-S precolumn is coupled on-line with a C18 analytical column and DAD detector. Addition of TBA considerably increases breakthrough volumes. In contrast, when a C18 precolumn is used, breakthrough volumes are lower and it is impossible to determine TCP and PCP, under the experimental conditions used, because of interference of other nonpolar compounds in the samples. The performance of the system is evaluated with river and tap water and the preconcentration of 10 ml of sample in a PLRP-S precolumn involves a linear range between 1 g 1^–1 and 20 l^–1 and limits of determination between 0.5 g l^–1 and 1 g l^–1 are obtained.     

Rapid and highly sensitive determination of low-molecular-weight carbonyl compounds in drinking water and natural water by preconcentration HPLC with 2,4-dinitrophenylhydrazine.

The aim of this research was to develop a simple procedure for a highly sensitive determination of low-molecular-weight (LMW) carbonyl compounds in drinking water and natural water. We employed a preconcentration HPLC system with 2,4-dinitrophenylhydrazine (DNPH) for the determination of LMW carbonyl compounds. A C-18 reverse-phase preconcentration column was used instead of a sample loop at the sample injection valve. A 0.1 - 5.0 mL portion of the derivatized sample solution was injected with a gas-tight syringe, and a 15% acetonitrile aqueous solution was pushed through the preconcentration column to remove the unreacted excess DNPH, which caused serious interference in the determination of formaldehyde. The detection limits were 1 - 3 nM with a relative standard deviation of 2 - 5% for 20 nM standard solutions (n = 5). The calibration curves were essentially unaffected by coexisting sea salts. Applications to commercial mineral water, tap water, river water, pond water and seawater are presented.     

Ethyl acetate for the desorption of a liquid chromatographic precolumn on-line into a gas chromatograph

The feasibility of using ethyl acetate for the desorption of trace pollutants from a liquid chromatographic precolumn on-line into a diphenyltetramethyldisilazane-deactivated retention gap and, subsequently analysis by means of capillary gas chromatography has been demonstrated. First 5% of methanol are added to the water sample to prevent sorption of analytes onto parts of the preconcentration system. About 1 ml of this aqueous sample is injected onto a precolumn containing a polymeric stationary phase, using water–methanol (95:5, v/v) for transport and clean-up. The precolumn is desorbed with ethyl acetate and a fraction of 75 μl is injected on-line into the retention gap; separation is then achieved on a capillary CP Sil 19 column. No breakthrough of the test compounds was observed in the preconcentration step. The recovery was quantitative and the response obtained with flame ionization detection was linear in the range 0.1–100 ng/ml. The effect of varying the sorption flow rate on the recovery was studied. The system was applied to the analysis of river water.     

Liquid chromatographic determination of trimethylamine in water

A method for the selective determination of trimethylamine (TMA) in aqueous matrices by liquid chromatography is reported. The proposed procedure is based on the derivatization of the analyte with 9-fluorenylmethyl chloroformate (FMOC) in a precolumn (Hypersil C18, 30 microm, 20 mm x 2.1 mm i.d.) connected on-line to the analytical column (LiChrosphere 100 RP18, 5 microm, 125 mm x 4 mm i.d.). Gradient elution was performed with a mixture of acetonitrile-water-0.05 M borate buffer (pH 9.0). The method has been applied to the direct determination of TMA in water within the 0.25-10.0 microg/ml concentration interval, and can also be adapted to the determination of TMA over the range 0.05-1.0 microg/ml by incorporating a preconcentration stage with C18 solid-phase extraction (SPE) cartridges. Good linearity, reproducibility and accuracy was achieved within the tested concentration intervals. The limits of detection at 262 nm were 50 and 5 ng/ml for the direct method and for the method involving preconcentration, respectively. The proposed conditions allowed the selective determination of TMA in the presence of other primary and secondary short-chain aliphatic amines. The utility of the described procedure has been tested by determining TMA in different water samples.     

Liquid chromatographic determination of azidothymidine in human plasma using on-line dialysis and preconcentration on a silver(I)-thiol stationary phase

An automated method is described for the routine determination of 3′-azido-2′,3′-dideoxythymidine (AZT), the best known drug against acquired immunodeficiency syndrome (AIDS). The method is based on on-line dialysis to remove matrix macromolecules, followed by selective preconcentration and clean-up with a silver(I)-thiol stationary phase. After desorption of the solute with a small plug of perchloric acid, chromatography is applied using an octadecyl-modified silica column. Using UV absorbance detection at 269 nm, the minimum detectable concentration in plasma is 20 ng ml^?1 (600-μl sample). The within-day reproducibility at the 20 ng ml^?1 level is 4.4% and at least 128 samples can be analysed unattendedly without exchanging the dialysis membrane, the precolumn or the analytical column.     

Use of Hypercrosslinked Polystyrene for the Determination of Pyrocatechol, Resorcinol, and Hydroquinone by Reversed-Phase HPLC with Dynamic On-line Preconcentration

Adsorption isotherms of pyrocatechol, resorcinol, and hydroquinone on hypercrosslinked polystyrene MN-200 are described by the Langmuir equation; distribution coefficients of these compounds are 1500, 550, and 370 mL/g, respectively; breakthrough capacities are 15.3, 11.2, and 11.3 mg/g, respectively. The comparison of hypercrosslinked polystyrene with other sorbents in the efficiency of the dynamic preconcentration of hydroxyphenols demonstrated its advantages. A procedure is proposed for the determination of pyrocatechol, resorcinol, and hydroquinone in water by high-performance liquid chromatography with dynamic on-line preconcentration on a concentrating column instead of the injector loop, which provides the determination of these compounds at concentrations down to 2–3 g/L. The accuracy of the procedure was verified by the added–found method with a water sample.     

Trace analysis of sulfonylurea herbicides in water by on-line continuous flow liquid membrane extraction--C18 precolumn liquid chromatography with ultraviolet absorbance detection

An on-line system that consists of continuous-flow liquid membrane extraction (CFLME), C18 precolumn, and liquid chromatography with UV detection was applied to trace analysis of sulfonylurea herbicides in water. During preconcentration by CFLME, five target compounds, including metsulfuron methyl, bensulfuron methyl, tribenuron methyl, sulfometuron methyl, and ethametsulfuron, were enriched in 960 microl of 0.5 mol l(-1) Na2CO3-NaHCO3 (pH 10.8) buffer used as acceptor. This acceptor was on-line neutralized and transported to the C18 precolumn where the analytes were absorbed and focused. Then the focused analytes were injected onto a C18 analytical column for separation and detection at 240 nm. The proposed method was applied to determine sulfonylurea herbicides in water, river, and reservoir water with detection limits of 10-50 ng l(-1) when enriching a 120-ml sample. Throughput is typically one sample per hour.     

Ion Chromatographic determination of trace anions and cations in high-purity water

An ion chromatographic measuring system for the off-line and on-line determination of some trace anions and cations in high-purity water is presented. The ng/L level of anions and cations in 20–130 mL high-purity water can be analyzed after preconcentration on ion exchange columns. The concentrated solutes are eluted by eluents from the trap column and separated using a Dionex analytical column. The quantification of each ion is achieved using the suppressor technique and conductivity detector. The influence of various parameters on the results is discussed. The detection limits of cations and anions are between 10 and 30 ng/L for chloride, bromide, nitrate, phosphate, sulphate, sodium, ammonium, potassium, magnesium and calcium ions.     

Cr(III) and Cr(VI) on-line preconcentration and determination with high performance flow flame emission spectrometry in natural samples

Methods for the on-line chromatographic preconcentration of Cr(III) and Cr(VI) have been developed. Cr(VI) has been preconcentrated on an RP C18 silica based column with tetrabutylammonium-bromide (TBABr) as ion-pairing agent. Specially for Cr(III) a new and effective preconcentration technique based on the sorption of Cr(III)-ions in a C18 column in presence of KH-phthalate has been developed. The efficiency of sample introduction into the atomic emission spectrometer could be improved by hydraulic high pressure nebulization. For the detection of chromium the acetylene/N₂O flame has been used as a powerful emission spectrometric source. Applying these steps the detection limit (3) could be improved to 25 pg/mL for Cr(III) and to 20 pg/mL for Cr(VI). The method has been applied for the chromium speciation in natural water samples.     

Analysis of chlortetracycline by high performance liquid chromatography with postcolumn alkaline-induced fluorescence detection.

A high performance liquid chromatographic method for the analysis of chlortetracycline (CTC) using postcolumn fluorescence detection has been developed. After chromatographic separation of CTC on a polystyrene-divinylbenzene copolymer column, a highly fluorescent derivative isochlortetracycline (iso-CTC) was formed postcolumn in an on-line reaction coil with the addition of 25% NaOH (w/v). Chromatographic separation was achieved on a PRP-1 column, 15 cm x 4.6 mm, with 27:73 acetonitrile:0.2% perchloric acid (v/v), at 1.0 mL/min. Fluorescence derivatization was achieved by the on-line addition of 25% NaOH (w/v), at a flow rate of 0.2 mL/min, into the column eluant in a post-column reaction coil. The reaction coil was 9 m of teflon (1/16 in o.d., 0.3 mm i.d.) knitted into a six-sided coil. The fluorescent derivative was detected at lambda ex 355 nm and lambda em > 389 nm. Using this method after a simple sample cleanup, CTC can be detected in milk at 0.04 micrograms/mL, which is comparable to that obtained by microbiological assays. The detection method was linear between 0.02 micrograms/mL and 4 micrograms/mL. Because of the chromatographic separation, the method is more selective than microbiological assays and more sensitive than ultraviolet detection. With the chromatographic system described, the keto tautomeric forms of CTC and 4-epi-CTC are separated in a system which minimizes their formation on-column. In acidic aqueous organic solutions, the keto tautomer of CTC is the only product formed to any significant amount.