## 1 Introduction
Thus it is necessary to monitor drinking water sources for THM contamination. The US Environmental Protection Agency (EPA) and European Union (EU) (respectively) have set maximum allowable (drinking water) limits for total THMs at 80 and 100 ppb (ug: L\({}^{-1}\)), where >90 ppb can have adverse effects on liver and kidney functions in humans [14; 15]. However, prolonged  
Figure 1: Example of a possible haloform reaction of 3,5-hydroxybenzoic acid at pH > 7. THM intake concentration (exposure) limits have not been agreed upon. Currently, the Newfoundland and Labrador provincial government of Canada monitors 473 public water supplies in the island part of the province, where THM ranges from negligible to 300 ppb (\(\mu\)g\(\cdot\) L\({}^{-1}\)) [3]. Current analytical techniques include: LLE, purge-and-trap and photometric detection methods [15], as in EPA501.2 (using liquid-liquid extraction (LLE)), and THM monitoring methods, as in EPA501.3 (using gas chromatography-mass spectrometry (GC/MS)), are typical. Samples must be analyzed within 10 days of sample collection [5] and, hence, it is important to utilize detection methods that are fast and robust for representative determination of THM concentrations in the natural waters. In most occasions, spot (or grab) sampling is usually employed to sample natural waters, where the samples are analyzed in batch analysis later in the lab. Although spot sampling with subsequent THM extraction and analysis can give reasonable indicative data, the technique acquires information of an analyte concentration only at a particular point in time at a specific location. Although a large amount of spot water samples can be obtained, the extraction analysis cannot account for the effects of physical conditions such as episodic contamination or gradient flow. Comparatively, time weight averaging (TWA) sampling in a passive mode may be utilized to extract small amounts of analytes over longer time intervals, which can represent long-term concentrations more accurately than spot sampling. The literature has shown that use of solid phase microextraction (SPME) technique affords the possibility of sampling THM analytes by either method (spot [16; 17; 18] and TWA [19; 20; 21]) with easy incorporation to subsequent GC/MS analysis. Work by [PERSON] [22] and [PERSON] [23] have shown the utility of headspace- (HS-) SPME with GC/MS for measurement of THMs from swimming pools and chlorinated drinking waters. SPME is based upon analytes in the sample interacting with a selected polymeric fiber in a multiphase equilibration process, with analytes migrating along the phases until equilibrium is reached. This method of sampling can be used to extract analytes from a sealed vial by HS- or by direct immersion- (DI-) SPME sample extraction. As such, the mass of an analyte extracted by the SPME device fiber is related to the overall equilibrium of the analyte in the phases. Because the total mass of an analyte should be constant throughout the extraction, the mass balance formula for the analyte is given in Equation (4) [24]. \[C_{o}V_{s}=C_{f}V_{f}+C_{h}V_{h}+C_{s}V_{s} \tag{4}\]  
where \(C_{o}\) is the initial concentration of analyte in the sample, \(C_{f}\), \(C_{h}\), and \(C_{s}\) are the equilibrium concentrations of the analyte in the fiber coating, headspace, and sample, respectively, and where \(V_{f}\), \(V_{h}\), and \(V_{s}\) are the volumes of the fiber film coating, headspace, and sample. For HS-SPME extraction, if the coating/gas distribution constant is taken to be \(K_{fh}=C_{f}/C_{h}\), and the gas/sample matrix distribution constant is taken to be \(K_{hs}=C_{h}/C_{s}\), the mass of the analyte absorbed by the coating can be expressed [24] in Equation (5). \[n=\frac{K_{fh}K_{hs}V_{f}C_{o}V_{s}}{K_{fh}K_{hs}V_{f}+K_{hs}V_{h}+V_{s}} \tag{5}\]  
For DI-SPME extraction, there will be a single distribution constant to describe the equilibrium between the fiber and the sample, \(K_{fs}\).