Factors influencing the use of solid phase microextraction techniques for environmental analysis of industrial waste water
The goal of an analytical laboratory is to have a high sample throughput, quick turnaround time and reduced operating costs. In most of the cases, a sample preparation step is necessary to prepare the sample for the specific analysis. Generally, a sample preparation step is incorporated in the analysis to eliminate interferences from the sample and to result in a "cleaner" sample that produces a signal with an enhanced signal response. In chromatographic analysis, the sample preparation step is in most cases the rate determining step. This is because most of the sample preparation steps are outdated, time- and labour-intensive, contain multiple steps that can lead to the loss of analytes and involve the use of toxic organic solvents. It should also be noted that the possibility of errors increases significantly if the number of steps are increased. The errors can be due to human, systematic or contamination inconsistencies. A sample preparation technique should be reproducible and cost effective. SPME meets these requirements. It is quick, sensitive and versatile; and no solvents or complicated apparatus are required. SPME consists of only two steps, namely the concentration and extraction of analytes out of the sample matrix, followed by desorption using a GC or HPLC. The successful analytical extraction of small polar organic compounds from water samples is very difficult and the commonly used liquid-liquid extraction techniques are often not suitable for some polar analytes. The required recovery of target compounds out of the aqueous phase can not be achieved because of low partition coefficients. Continuous direct injections of water samples into a GC lead to the removal of the stationary phase of the gas chromatographic column and ultimate deterioration of the column, which increases maintenance costs (Chapter 1). Direct injections are sometimes not sensitive enough for the very low level of organic pollutants in waste water streams, an aspect that will be demonstrated in the presentation of data in later chapters. SPME is a potential alternative technique for the sampling of these pollutants in environmental waste water, particularly because of the diverse range of possible organic contaminants (Chapter 2). An investigation was launched to compare two methods for the analysis of organic analytes in industrial waste water. The current method in use is a direct injection NAC method. Data from this technique has been unsatisfactory and this has led to the development of a SPME method, specific to the needs of an industrial client (Chapter 3). Comparisons between these two methods show that SPME gives a more effective picture of the pollution status. The effectiveness of SPME depends on the efficiency of extraction. The various factors that have an influence on the extraction were investigated, in particular fibre choice, extraction duration, temperature and sample pH. It was found that fibre selection is dependent on the molecular weight, polarity, functionality and concentration level of the analyte of interest. According to the results, extraction time before equilibrium is achieved, affects the extraction process significantly. The amount of analyte extracted after equilibrium has been reached will remain similar, but for pre-equilibrium extraction, it is crucial to keep the extraction time constant. Associated with equilibrium effects is the variation of temperature and its influence on the extraction concentrations of the analytes. The higher the sampling temperature, the greater the extraction efficiency of the analytes will become, until a maximum is reached. The pH of the sample influenced the amount of acidic and basic analytes sorbed; basic compounds partitioning towards the fibre when the matrix had a high pH and similarly, acidic compounds partitioning towards the fibre when the pH was low. Polar analytes were driven out of the solution when the ionic strength of the sample was increased (Chapter 4). Before SPME can be quantitatively used in analysis, all these factors should be optimized for maximum sensitivity, precision and ruggedness. Quantification with SPME can be done using either an internal standard, or external standards with calibration graphs. For simple matrices, an internal standard can be used, provided the compound chosen is similar in characteristic to the target analytes. In this study it was found that the internal standard approach was not an appropriate method for quantification, simply because the range of organic compounds in the waste water was so diverse that no internal standard could be chosen to represent the group in response. It was concluded that SPME was a suitable method for qualitative analysis, because more analytes could be detected. For quantitative analysis, it is important to optimize and control the extraction process. This makes the procedure sensitive to subtle changes that can affect data and therefore may not be rugged enough for routine analysis.
- ETD@PUK