Validation of an environmental friendly segmented flow method for the determination of phenol index in waters as alternative to the conventional one

Abstract

Phenolic compounds are a sort of common pollutants in water. Phenol index becomes an expedite indicator for the evaluation of the contamination level of water samples, in spite of the knowledge of the individual phenol and its derivatives are also important.

In this work, an environmental friendly method for the determination of phenols, using a segmented flow system based on the conventional method's reactions without the liquid–liquid extraction step, was validated.

Three linear dynamic ranges using C₆H₅OH: 1–10 μg l⁻¹, 10–200 μg l⁻¹ and 0.2–2.5 mg l⁻¹, with a coefficient of variation lower than 2%, were obtained. Several method's performance parameters were determined: limits of detection, limits of quantification, precision through duplicate analysis and trueness using the reference materials purchased from LGC Promochem, RTC no. QCI-043-2 Lot:P1. Measurement uncertainty was evaluated using an interlaboratory approach based on proficiency testing data. Relative combined uncertainty for phenols in water samples, $u_c^{rel}({\gamma }_{sample})$, of 0.054 were obtained, in according to those imposed by the Portuguese Legislation: target $u_c^{rel}({\gamma }_{sample})=0.58$ for 1 μg l⁻¹ of phenol (surface waters) and target $u_c^{rel}({\gamma }_{sample})=0.06$ for 500 μg l⁻¹ of phenol (wastewaters).

A high efficiency reduction and elimination of reagents and wastes, reduction of analysis time and exposition of the analyst were also obtained.

Introduction

Phenolic compounds are organic pollutants, which occur frequently in natural and wastewaters. They can have industrial origin from different activities such as production of synthetic plastics, paints, drugs, polymers, pesticides, detergents, deodorants, paper and cellulose [1], [2]. These compounds appear in a list of dangerous substances of the Environmental Protection Agency (EPA) due to their toxicity, bioaccumulation, persistency in the environment and carcinogenic properties [3]. They affect taste and the odour of drinking water and fish. Numerous phenolic compounds show toxic effects in animals and plants since they easily penetrate into skin and cellular membrane [1].

Several analytical procedures have been proposed for detection of individual phenolic compounds either chromatographic methods or spectrophotometric ones [4], [5]. Although the knowledge of individual phenolic compounds is very important, the fast and simple attainment of the results obtained are equally important in the environmental monitoring point of view. Therefore, analytical methods that give direct information about the total phenol compound contents are very suitable once that reduction of time and costs occurs.

The reference method for the determination of phenols stipulated by the Portuguese Legislation (DL no. 236/98) [6] is based on the reaction between phenols and 4-aminoantipyrine to form antipyrine dyes. This methodology involves a distillation step followed by a liquid–liquid extraction using enormous quantities of sulfuric acid and chloroform with a great environmental impact. On the other hand it is a time-consuming procedure, requires high manual labour and a lot of laboratory glassware.

The development of flow techniques capable of being implemented in routine laboratories has brought a new element to the analytical chemistry, allowing the measurements to be carried out faster, reducing both analysis time and consumption of reagents, production of less wastes and minimum intervention of the analyst, constituting an advantageous alternative to the traditional method.

Segmented flow analysis was one of the first techniques widely imposed in the laboratories requiring a large volume of analysis. Samples are sequentially aspirated and, between them, air bubbles are located which separate the flow established, including also a washing cycle. The air bubbles are eliminated before reaching the detector flow cell.

In this work, a validation and quality assurance program was developed for implementation of a segmented flow system used to determine phenols, as alternative to the conventional method, fulfilling the requirements of the DL no. 236/98 for natural and wastewaters. This program includes the control of: (i) the calibration function characteristics namely test of the variances homogeneity, for the working range boundaries, and Mandel's test, for linearity; (ii) limit of detection (LOD) and limit of quantification (LOQ); (iii) stability of calibration functions; (iv) interferences; (v) precision through duplicate analysis; (vi) trueness using reference materials. The uncertainty measurement was estimated using an interlaboratory approach.

The procedure was based on the oxidative coupling of phenols with 4-aminoantipyrine (4-AAP) in alkaline medium as described by Eq. (1) [2]. Notice that phenol, ortho-, meta- and para-substituted phenols could be detected under proper pH conditions, but para-substituted by aryl, alkyl, nitro, benzoyl compounds do not react efficiently or do not react at all with 4-AAP [2]. The products of the reaction are generally red compounds having absorbance of 505 nm [7], [8]:

When reporting phenols content using C₆H₅OH for calibration, the results represent the minimum amount of phenolic compounds in the real sample. For that, in this work we decided to use the term phenol index in spite of phenols.