Abstracts

ORIGINAL PAPERS

Comparison of two validated gas-chromatographic methods for the determination of trihalomethanes in drinking water

Comparação de dois métodos cromatográficos validados para a dosagem de trialometanos em água potável

Maria Yumiko Tominaga^I; Antonio Flavio MídioII, ^* * Correspondence: A. F. Mídio Departamento de Análises Clínicas e Toxicológicas Faculdade de Ciências Farmacêuticas Universidade de São Paulo Av. Prof. Lineu Prestes, 580 05508-900 - S. Paulo - Brasil E -mail: afmidio@usp.br

^ICETESB - Cia de Tecnologia de Saneamento Ambiental, São Paulo,Brasil

^IIDepartamento de Análises Clínicas e Toxicológicas-Toxicologia, Faculdade de Ciências Farmacêuticas, Universidade de São Paulo

ABSTRACT

In this paper the results obtained using two validated gas-chromatographic procedures on drinking water for the determination of trihalomethanes are compared. The volatile compounds, chloroform (CF), bromodichloromethane (BDCM), dibromochloromethane (DBCM) and bromoform (BF) were detected by purge and trap capillary column gas-chromatography with electrolytic conductivity detector ( ELCD) and the simple and rapid gas-chromatographic method by electron capture detector (ECD) after liquid-liquid extraction with n-pentane. For purge and trap ELCD method the response for the volatile compounds was linear for the concentrations of 0.5 to 40 µg/L. For liquid-liquid extraction ECD method the response was linear for the concentrations of 0.5 to 100 µg/L. The comparison of both methods was achieved by analyzing samples of drinking water collected in the city of São. Paulo, Brazil. The ratios of concentrations obtained by the two methods (ECD/ELCD) were as follows: l.l3 ± 0.9 for chloroform; 0.93 ± 0.15 for BDCM and 0.92 ± 0.17 for DBCM. Bromoform was not detected in the drinking water samples. The ratio of 1.08 ± 0.047 for total triahalomethane - THMt ( the sum of the three compounds) shows the equivalence of the compared methods.

Uniterms: Trihalomethanes. Gas-chromatographic detection. Comparison of methods. Drinking water.

RESUMO

São comparados dois métodos cromatográficos validados para a determinação de trialometanos (clorofórmio, bromodiclorometano, dibromoclometano e bromofórmio) em água potável. Os métodos cromatográficos de fase gasosa, a saber: com detetor de captura de elétrons precedido de extração líquido-líquido e com detetor de condutibilidade eletrolítica com "purge and trap" foram comparados em termos de sensibilidade, precisão e recuperação. O estudo demonstrou que os resultados dos dois procedimentos são equivalentes apresentando as mesmas vantagens quando comparados.

Unitermos: Trialometanos. Detecção por Cromatografia A Gás. Comparação de Métodos. Água potável.

INTRODUCTION

Man ingests water in a quantity much bigger than all other foods together and it constitutes his main excretion as well. An adult ingests more than two liters of water a day, about 3% of his weight, which is composed of more than 80% of water (Riedel, 1992).

This extraordinary contact with water justifies and explains how easily macro and microscopic parasites present in water reach and develop in man when some other factors are favorable to their living and multiplication (Tominaga, Mídio, 1999).

On the other hand, man is exposed to chemical contaminants that are present in natural waters. It is supposed that 4.0 billions of cubic meters of chemical contaminants from industrial and domestic effluents as well as agricultural chemicals reach the soil and consequently the water each year (Wang, 1994). A series of chemical and physical transformations including combinations with other chemicals increase their toxicity to men and other living organisms ( Tominaga, Mídio, 1999).

All these risk factors may be controlled by treating natural waters with chemicals which may also generate other public health problems. Chlorine and its compounds are the chemical agents of choice once they can effectively destroy or inactivate parasite organisms. Besides, they are easily employed and detected in water offering residues which protect water from further biological contamination (Dychdala,1983).

Chlorinated waters, however, may present by-products which represent a certain risk of health significance to human population (Singer, 1993).They are produced by the reaction of free chlorine used in the disinfection and certain organic substances (humic and fulvic compounds) present in natural waters. Table I shows the main halogenated by-products present in chlorinated waters and among them the most important ones named trihalomethanes (THM) (Singer, 1993).

It was only at the beginning of the seventies that chloroform and other trihalomethanes (bromodichloromethane, dibromochloromethane and bromoform) were detected in drinking water and considered, by far, the most important by-products in the chlorination of natural water due to the discovery of the carcinogenicity of chloroform in a bioassay of the National Cancer Institute (Singer, 1993). In 1979 the United States Environmental Protection Agency (USEPA) proposed a maximum allowable concentration of 100 mg of total trihalomethanes (THMt) per liter of potable water (USEPA, 1979; Singer, 1993; Richardson, 1998). More recently, however, the maximum contaminant level was lowered to 80 mg/L (Pontius, 1997).

Epidemiological studies have been designed, specially in the United States, to evaluate the presence of THM in drinking water and its correlation with cancer production in experimental animals and humans (Page et al., 1976; Bull, 1982; Young et al., 1987; Cech et al., 1993; Craun, 1993). However, little is known about the real effect of these compounds in low concentrations in drinking water (µg/L, ng/L) to the human population. Experimental studies have shown that the sub-products of water chlorination are carcinogenic to animals in concentrations much higher than those found in the water for human consumption (WHO,1994; WHO,1996). Nevertheless the theory that there is a high probability of the relationship between cancer of urinary bladder, colon and rectum and the exposure to THM in drinking water is well accepted (Menzer, 1991; WHO,1993; WHO,1994; Richardson,1998; WHO,1996).

Trihalomethanes have been determined in drinking water by liquid-liquid extraction and gas-chromatography with electron capture detection (ECD), static headspace system and dynamic headspace (purge and trap) with ECD, electrolytic conductivity and mass-spectrometry (Oliver, 1989). Purge and trap technique is the most employed in the United States while in Europe the static headspace system is mostly used (USEPA,1979; APHA, 1995).

Despite of the obvious advantages of the large number of analytical procedures the volatility of the compounds are still prone to problems associated with sample preparation, quality of reagents, laboratory devices, special equipment together with analyte losses and potential errors.

The purpose of this work was to evaluate the advantages in sensitivity and accuracy of two validated chromatographic methods associated with the determination of trihalomethanes in drinking water.

MATERIAL AND METHODS

MATERIAL

Instrumentation

A Hewlett Packard gas-chromatograph, model 5730, equipped with electron capture detector (ECD), Ni⁶³; a Hewlett Packard integrator, model 3390 and a 6 feet glass column (2 mm id), packed with 10% esqualane in Chromossorb WAW (80-100 mesh) were used for method A.

A Varian gas-chromatograph, model Star 3400 CX, equipped with a Tremetrics electrolytic conductivity detector (ELCD), model 1000; a Teckmar purge and trap system (automatic sampler model 2016 and a sample concentrator model 3000); a Supelco pre-concentration column; a Hitachi integrator model D-200 and a J&W Scientific DD-624 capillary column (30 m, 0.53 mm id and 3 mm film thickness) were used for method B.

So as not to contaminate all glassware and any other material used in the analyses (made of pyrex and/or silicon) were washed with diluted nitric acid solution, rinsed with distilled deionized water freed from organic interfering compounds by boiling for 60 min and purged with He for 15 min.

Reagents

All reagents were of analytical grade. Methanol (Carlo Erba) and n-pentane (Fischer) were HPLC grade. Anhydrous sodium sulphite (Synth) was pro-analysis grade. Trihalomethane mixture: 1 mL ampoule containing 200 µg of each compound (Supelco - catalog N-4-8746). Standard solutions of trihalomethanes were prepared with a mixture of chloroform (CF), bromodichloromethane (BDCM), dibromochloromethane (DBCM) and bromoform (BF) containing 20 µg/mL of each compound in methanol.

Internal standards: 1-bromobutane ( Merck ) and 1-chlor-2-bromopropane (100 µg/mL) in methanol (Supelco catal.N4-8713).

Blank water

The production of blank water for the analyses of volatile organochloride compounds in drinking water was achieved by distilling, deonizing and boiling it for 60 min and finally purging it with He for 15 min.

Sampling

The samples used in this work were obtained in the following manner: 20 samples of drinking water were collected at different times and conditions from several parts of the city of São Paulo, Brazil. To collect the water the tap was open wide and the water was let to flow freely till the temperature was constant. The flow was then adjusted to 500 mL/min and the water was received in a 125 mL borosilicate glass vial containing about 0.2 g of anhydrous sodium sulfite. The vials were closed hermetically with no headspace formation and maintained at 4 °C until analyses took place.

Methods

Method 1. Determination of trihalomethanes by means of liquid-liquid extraction and electron capture detection (ECD) gas-chromatography

The water sample was transferred to a separatory funnel and 5 µL of internal standard (1-bromobutane 1.015 mg/L) were added. A 25 mL flask with a polyethylene cap was totally filled with the water sample. With a hypodermic syringe 5 mL of the sample were drained. With another hypodermic syringe 4 mL of n-pentane were transferred to the flask. The mixture was shaken thoroughly for 1 min and the layers were let to separate. The organic phase (2 mL) was then injected in the chromatographic system with ECD in the following conditions: temperatures: oven = 70 °C (isothermal); injection port = 100 °C and detector = 250 °C. Nitrogen carrier flow of 20 mL/min.

• Validation of the method

Calibration curves and linearity were performed in blank water samples in which trihalomethanes and internal standard (1.015 µg/L) were spiked to obtain concentration ranges from 0.5 to 10 µg/L and from 10 to 100 µg/L. Curves were constructed by plotting peak-area ratios of trihalomethanes to internal standard as a function of the compounds concentrations. The equations of the calibration lines were calculated by least-squares linear regression and was used to calculate the concentration of each trihalomethane in the unknowns. Routine quality control was assessed by use of in-house trihalomethane controls analyzed in each run. Quality control (QC) samples used to assess within and between-day precision were freshly prepared and assayed with each calibration curve.

Precision, accuracy and recovery tests were determined in triplicate by spiking trihalomethane-free water samples with known amounts of the compounds at concentrations of 2.0; 4.0; 10.0; 20.0 and 40.0 µg/L and internal standard. The samples were extracted and analyzed in triplicates as described above.

Method 2. Determination of trihalomethanes by purge and trap capillary column gas-chromatography with electrolytic conductivity detector (ELCD)

The water sample was transferred to a 50 mL volumetric flask in which 5 µL of the internal standard solution (1-chloro-2-bromopropane) was added. By means of a high performance syringe for gases (Teflon-coated plungers) 5 mL of the mixture was transferred to the automatic sampler of the purge and trap system. The sample was purged for 11 min and dessorbed at 180 °C for 4 minutes to the gas chromatograph with ELCD in the following conditions: temperatures of the oven: 40 °C (initial) and 200 °C (final); temperature of the injector = 200 °C; temperature of the detector = 250 °C; temperature of the reactor = 840 °C . Electrolyte (n-propanol) flow = 0.6 mL/min; hydrogen flow =30 mL/min; helium flow in the column = 6 mL/min and 30 mL/min in the detector.

• Validation of the method

Calibration curves and linearity were performed in 5 mL blank water samples in which trihalomethanes and internal standard (1-chloro-2-bromopropane = 10 µg/L) were spiked to obtain concentration ranges from 0.1 to 40 µg/L. They were constructed by plotting peak-area ratios of trihalomethanes to internal standard as a function of the compound concentrations. The equations of the calibration lines were calculated by least-squares linear regression and were used to calculate the concentration of each trihalomethane in the unknowns. Routine quality control (QC) was assessed by use of in-house trihalomethane controls analyzed in each run. Quality control (QC) samples used to assess within and between-day precision were freshly prepared and assayed with each calibration curve.

Precision, accuracy and recovery tests were determined in triplicate by spiking trihalomethane-free water samples with known amounts of the compounds, at concentrations of 2.0; 4.0; 10.0; 20.0 and 40.0 µg/L and internal standard. The samples were analyzed in triplicates as described above.

RESULTS AND DISCUSSION

Both methods showed good linearity in the concentration ranges: 0.5 - 10 and 10-100 µg/mL for all trihalomethanes for Method 1 and 0.5-40 µg/mL for Method 2. Limits of detection (LOD) and limits of quantification (LOQ) for both methods were obtained according to Miller and Miller (1988) and are presented in Table II.

Accuracy expressed as bias (%) precision and recovery for Methods 1 and 2 are presented in Tables III and ^IV respectively. For gas-chromatographic purposes accuracy (bias) up to 20% of the real value and a precision (coefficient of variation- CV) lower than 15% are accepted (Causon, 1997).

Values for CV obtained when Method 1 was applied (Table III) showed good precision for the detection of the four trihalomethanes. The higher value of CV obtained was 3.6% for chloroform spiked in the concentration of 4 µg/L. The method was also considered accurate with the only exception of bromoform in the concentration of 10 µg/L when a bias of + 23% was found. Recoveries for Method 1 were around 100% for all THM studied in all spiked concentrations.

When Method 2 was used, (^{Table IV} ) capillary gas-chromatography produced obviously better separation of the compounds. Values obtained for CV showed that the purge and trap technique and detection with ELCD presented good precision for the determination of the four THM. The higher CV obtained (10.2%) was for chloroform spiked in the sample in the concentration of 2.00 µg/L. This method is also accurate; the higher bias between spiked and found concentrations was + 6,0% for bromodichloromethane in the concentration of 20 µg/L.

Trihalomethanes were analyzed by both methods in 20 samples of drinking water of different sources distributed to the population of the city of São Paulo, Brazil. The samples were collected, preserved (with sodium sulfite) and stored at 4 °C until analyzed (Ho,1988). The samples showed to be stable for 35 days or more if stored in the temperature cited above.

The ratios of THM concentrations were determined to assess the comparison of the two methods. Data from the analyses are summarized in Table V. Bromoform was not detected in any sample when analyzed by the two methods.

The ratio of chloroform concentrations detected by both validated methods (mean of 20 samples) was 1.13 ± 0.09 with a coefficient of variation of 8.27%. The limit of quantification (LOQ) found for chloroform when Method 1 was used was 1.37 µg/L. For method 2 it was 0.74 µg/ L (Table V).

The same parameters were 0.93 ± 0.15 with a CV of 16.41% for bromodichloromethane. The limits of quantification were 3.90 µg/L for Method 1 and 0.40 µg/L for Method 2. It can be observed that 50% of all BDCM concentrations when ECD method was used are lower than the LOQ.

For dibromochloromethane the mean value of the ratios was 0.92 ± 0.17 for a CV of 18.32%. LOQ values for ECD and ELCD methods are 2.26 µg/L and 0.70 µg/L respectively. When ECD method was used to analyze the samples 70% of the results were lower than the LOQ.

The Brazilian legislation on drinking water quality as well as in many other countries proposes the determination of THM and the expression of the results in terms of the sum of the concentrations of the four compounds known as total trihalomethanes (THMt).

The sum of the results of each THM in the samples, analyzed by both methods and the ratio ECD/ELCD are expressed in Table VI. A ratio of 1.08 ± 0.05 and a CV of 4.43% indicate that both methods can be considered equivalent.

This way the choice of the analytical method is a matter of availability of the equipment. Electron capture detection is much easier to be found in any laboratory and is not so much time consuming as ELCD. On the other hand in the ELCD method liquid-liquid extraction is not necessary avoiding the analyst to be exposed to organic solvents.

In the validation and comparison of these two methods all the possibilities of use were not exhaustively studied. The equivalence of both and the advantages that each one may present were stressed.

Recebido para publicação em 08 de agosto de 2002.

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