Determination of Seven Sodium Alkyl Sulphates in Environmental Water by Ion-pair Chromatography with Suppressed Conductivity Detection

Feb 01, 2014

A simple and fast analytical method using ion-pair chromatography with suppressed conductivity detection was developed to simultaneously detect and quantify sodium decylsulphate (C10), sodium undecylsulphate (C11), sodium dodecylsulphate (C12), sodium tridecylsulphate (C13), sodium tetradecylsulphate (C14), sodium cetylsulphate (C16), and sodium octadecylsulphate (C18) in environmental water. The effect of the mobile phase parameters (NH4OH, CH3CN, and Na2CO3 concentrations) on the retention factors has been studied in detail, and the separation of the seven analytes was achieved using gradient elution.



Surfactants are widely used in personal and home care products including household detergents, shampoos, and cleaning products. They are also used in oil, pesticides, and metal processing (1–3). The annual worldwide consumption of surfactants is steadily increasing. Surfactants are classified into four groups according to their chemical characteristics: Cationic, anionic, non-ionic, and amphoteric (4). Anionic surfactants account for more than 40% of the total surfactants. Once in effluent sewage water, these anionic surfactants produce foams, emulsify, and suspend. They prevent the oxygen exchange between air and environmental water, resulting in the deterioration of water quality and potential harm to aquatic ecosystems (5–6). Recently, surfactants have been receiving more attention because of their growing occurrence in environmental water (7–8).

Several methods have been reported for the determination of anionic surfactants by spectrophotometry (9), liquid chromatography (LC) (10–12), and by electrochemical means (13). The spectrophotometry method is used for detections at low µg/L levels and suffers from chemical interferences. The LC method can detect the substances that contain a UV chromophore; and this method has been reviewed by Sohrabi (9). The surfactants which have to be derived can be detected by LC–UV. The electrochemical method can detect the compounds with oxidation and reduction. It is unsuitable for anion surfactants without oxidation and reduction potential.

This work opted for the suppressed conductivity detection based on the intrinsic properties of anion surfactants. Some papers have been published on ion chromatography investigations. Levine studied sodium dodecylsulphate in wastewater (14). Porter detected quantitatively poly disperse polyethylene oxide (PEO) and sodium dodecylsulphate (SDS) in water (15). In J. Weiss's publication, only five sodium alkyl sulphates have been detected (16). The focus of these studies were directed towards the traditional method and a limited number of analytes (17–18).

As a result, the aim of this work was to develop an ion chromatography method for the separation and precise quantification based on ion-pair chromatography with suppressed conductivity detection. This method offered three advantages: Firstly, it should be noted that the separation of seven species was achieved in less than 20 min. The effect of mobile phase parameters (NH4OH, CH3CN, and Na2CO3 concentration) and the optimization of a gradient elution on separation were studied. To the best of our knowledge, this is the first report on the separation of these seven analytes; secondly, the environmental water samples were injected into the ion chromatography instrument without any pretreatment; finally, suppressed conductivity is known to detect simple cationic, anionic compounds such as F or Cl-. Few ion-pair methods with suppressed conductivity detection for the separation of sodium alkyl sulphates are available. As a result of these advantages, this method can become a technique capable of the simultaneous determination of seven sodium alkyl sulphates in environmental water and can potentially be applied to environmental monitoring.

Experimental

Instrumentation: The ion chromatography (IC) system consisted of a 850 professional IC separation centre, a 872 gradient pump, a 771 IC compact interface, a 858 professional sample injector equipped with a 20 µL sample loop, and a suppressed conductivity detector (Metrohm). Conductivity suppression was performed by an anion chemical suppressor with a carbon dioxide suppressor (Metrohm). The chemical suppressor was operated in the external chemical mode, with 3 mM sulphuric acid as a regenerant. All instrument control and data collection was performed by MagIC Net 2.2 for Windows chromatography software (Metrohm).

The analytical column was a neutral polymer column, 4 mm × 250 mm, IonPacNS1 (Dionex). The resin in the columns was composed of ethyl vinyl benzene cross-linked polystyrene-divinylbenzene substrate (EVB-DVB) (Dionex).

Reagents: Sodium decylsulphate (C10), sodium undecylsulphate (C11), sodium dodecylsulphate (C12), sodium tridecylsulphate (C13), sodium tetradecylsulphate (C14), sodium cetylsulphate (C16), and sodium octadecylsulphate (C18) were obtained from Alorich (purity>99%).

HPLC-grade acetonitrile and analytical-grade ammonia were used (Merck). Deionized 18.2 MΩ water (Watsons) was used for eluent and standard solution preparation.

Preparation of Standard Solutions: Stock solutions and standard solutions for calibrations were prepared with deionized water. A mixed working solution at a concentration of 2500 mg/L was prepared by dissolving an accurately weighed quantity of 0.25 g of seven sodium alkyl sulphates into deionized water and diluting to 100 mL. The stock solution was serially diluted with deionized water to the concentrations of 200 mg/L, 150 mg/L, 100 mg/L, 50 mg/L, 30 mg/L, and 10 mg/L as the working solutions. They were kept in glass vials at 4 °C.

The Source of Samples: Samples of river water and sewage were collected from various locations in Lanzhou (China). The river samples were collected at surface level from discrete sections of the Yellow River in Lanzhou. Sewage samples were collected from living areas, industrial parks, and university areas. A total of 11 samples were obtained.


Figure 1: Optimized separation chromatogram of seven anions spiked in a 200 mg/L standard solution.
Chromatographic Separation: The separation column was a 4 mm × 250 mm, NS1 column (Dionex). A gradient elution was adopted by eluent A and eluent B. Eluent A was composed of a 5 mmol/L ammonium hydroxide +22% acetonitrile +0.3 mmol/L sodium carbonate; eluent B was 5 mmol/L ammonium hydroxide +80% acetonitrile. The following gradient programme was used throughout the experiment: 100% eluent A was increased to 100% B by a concave 2 mode at 20 min. The column oven was maintained at 30 °C. The sample injection volume was 20 µL. The flow rate was 1 mL/min. At 23 °C, the pH of eluent A and eluent B was 10.63 and 8.82, respectively. The chromatogram of an optimized separation of seven anions spiked in a 200 mg/L standard solution is shown in Figure 1.