ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2011, том 66, № 8, с. 870-876
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
UNCERTAINTY IN THE QUANTIFICATION OF PENTACHLOROPHENOL IN WOOD PROCESSING WASTEWATERS BY SPME-GC-MS © 2011 I. Brás", N. Ratola*, A. Alves*
aEscola Superior de Tecnología de Viseu — Instituto Politécnico de Viseu Repeses, 3505-510 Viseu — Portugal bLEPAE — Departamento de Engenharia Química — Faculdade de Engenharia da Universidade do Porto Rua Dr. Roberto Frias, 4200-465 Porto — Portugal Received 02.04.2010; in final form 30.12.2010
Pentachlorophenol is a trace contaminant with toxicological effects in environment. When dealing with the analysis and quantification of trace amounts in complex matrices, procedures such as extraction/pre-con-centration are often needed and chromatographic determination are often operated close to the limit of detection, which contribute to the increase of the uncertainty of the measurement. This matter is of crucial importance when the results obtained approach the legal limits and is frequently ignored in scientific work. The aim of this study was the estimation of the global uncertainty associated to the determination of pentachlo-rophenol (PCP) in aqueous samples, by gas chromatography with mass spectrometric detection (GC-MS), after solid-phase microextraction (SPME). It was concluded that, while in the range of 5 to 40 p.g/L the uncertainty did not exceed 20%, in the vicinity of the limit of detection (0.75 p.g/L) it raised up to 64%.
Keywords: analytical uncertainty, pentachlorophenol, SPME, GC-MS.
Pentachlorophenol (PCP) is considered as a dangerous substance by the European Union [1] and the United States Environmental Protection Agency [2], given its toxicity, persistence and potential for bioaccumulation [3]. Being a chlorinated hydrocarbon insecticide and fungicide, it is primarily used as a wood preservative since 1936, but may also act as a pre-harvest defoliant in cotton, a wide spectrum herbicide, and as a biocide in industrial water systems [4]. As a result of over 50 years of use as a pesticide, PCP residues are ubiquitous and harmful to the environment and humans [5, 6]. The latter are exposed to PCP in the workplace, in treated homes, indoor and outdoor air, drinking water and food [7]. It can enter the body by inhalation, ingestion of contaminated water or food and skin contact with treated wood [4]. According to European and Portuguese legislation, the maximum admissible concentration of PCP in inland surface, estuary, internal coastal and territorial waters is 2 ^g/L [8].
Several methods have been employed to assess PCP levels in numerous matrices. In waters, PCP was first estimated by Deichmann and Schafer [9], by spectrophotometry. Other quantification technologies include gas chromatography (GC) with electron capture detection (ECD) [10-13], flame ionization detection (FID) [1416], Fourier-transform infrared spectroscopy (FTIR) [17] and mass spectrometry (MS) [18-21]; high performance liquid chromatography (HPLC) with electrochemical detection (ED) [22] and MS [23]; capillary electrophoresis [24, 25]; supercritical fluid chromatography [26] and flow injection analysis (FIA) [27].
The reference methods for the determination of PCP in aqueous samples have been employing HPLC or GC after sample clean-up by liquid-liquid extraction (LLE) [8]. Traditionally, LLEs require extensive training, high organic solvent and time consumption, often being responsible for increased uncertainties in the quantification of the analyte [28]. Solid-phase extraction (SPE), with similar drawbacks, was also reported [23, 26].
Solid-phase microextraction (SPME) has important advantages in sample preparations, such as quickness, sensitivity and versatility [29]. Furthermore, no solvents or complicated apparatus are required and the problems related with solvent disposal and operator safety are abolished. Several authors employed SPME in PCP determination in aqueous samples, mainly using 85-^m poly-acrylate (PA) fibers [11, 12, 14, 18, 20, 22, 30, 31]. Li et al. [16] studied an alternative calix[4]arene coating. Other microextraction techniques such as stir-bar sorptive extraction (SBME) or single-drop microextraction (SDME) were also recently reported in literature [32, 33].
The quantification of trace compounds in naturally contaminated samples extracted by SPME is usually done by matrix-match calibration, where standards are extracted in the same conditions as the samples. For such chemicals, the reliability ofthe final results is crucial, and in this sense, so is the estimation of the global uncertainty associated to those results.
Uncertainty is a parameter associated with the result of a measurement that estimates a range ofvalues that could be reasonably attributed to that result. This definition is
different from the term "error", which represents the difference between the result and the true value [34]. Uncertainty derives from several sources such as sampling, interferences and matrix effects, environmental conditions, measuring equipment, reference values, approximations and assumptions incorporated in the method and calculation procedure, and random variation [34]. Often neglected, this parameter represents the variability and the confidence of the analytical method and accounts for most error sources inherent to a given method, each one in the form of a "partial" uncertainty. It allows not only the best interpretation of the results but also the diagnosis and control ofsuch sources. Hence, it contributes to a decision about the fitness of the methodology employed for the intended analysis. In fact, for values close to the limits ofde-tection attained currently by the sharpest apparatus, the uncertainty ofthe results can be unsuitably high [35—39].
When an analytical methodology is implemented, validation is required. Therefore, a set of parameters that include characterization (specificity and practicability), quantification (linearity and detection limits) and reliability (precision, accuracy and robustness), are obtained for that purpose. A different number ofapproaches are possible [40], but the global uncertainty (U) can be estimated using the method performance parameters obtained from validation [34]. Depending on the specific methodology, the parameters suspected to contribute the most are: the best available estimate of overall precision; the value obtained for the average analytical recovery; the data related with the method linearity (calibration curve), including the errors (inaccuracy) of the measuring equipment used in the preparation of the standard solutions.
This study illustrates how to calculate the global uncertainty associated to the quantification of PCP in wood processing wastewaters, employing gas chromatography with mass spectrometric detection (GC-MS) after SPME.
EXPERIMENTAL
Chemicals. Pentachlorophenol was obtained from Su-pelco (Bellefonte, PA, USA). Sodium sulfate anhydrous p.a. and sulfuric acid 95—97% p.a. were purchased to Merck (Darmstadt, Germany). An aqueous effluent with unknown concentration of PCP was collected in a wood-processing factory.
Preparation of standard solutions. PCP solutions from 0.8 to 40 ^g/L were made from a 2400 mg/L stock solution, which was prepared by accurately weighing 0.06 g of PCP and diluting in a 25 mL volumetric flask with 0.1 M sodium hydroxide. An intermediate stock solution was prepared with 100 ^L of the stock solution diluted in a 50 mL volumetric flask with the same solvent. The working standards were then made with different aliquots ofthe intermediate stock solution in 50 mL volumetric flasks, using a saturated solution (pH 2) as solvent. This solution was prepared by adding sodium sulfate (Na2SO4) until
saturation of the deionised water, then controlling the pH with 1 M sulfuric acid.
Experimental procedure. The quantification of PCP was attained with an 6890 Series gas chromatograph from Agilent (Santa Clara, CA, USA) equipped with a 5973 N Series mass spectrophotometer selective detector in scan mode, ranging from 15.0 to 500 amu (atomic mass unit). The interface temperature was 160°C and the ion source (electron ionization) was set at 230°C with electron energy of 70 eV whilst the quadrupole mass filter was kept at 150°C. The capillary column was a Hewlett-Packard 5MS (30 m x 0.25 mm x 0.25 |m) (Avondale, PA, USA). Helium (99.9999% purity) was the carrier gas, at a constant 1 mL/min flow through the column. The oven was initially set at 80°C, and then raised to 260°C at 15°C/min. The injector was in splitless mode at 250°C, closed for 3 min before purging with helium at 20 mL/min.
For sample preparation, a 85-|m PA fiber (Supelco) and the respective SPME sampling manual holder (Su-pelco) were employed. The fiber was pre-conditioned at 300°C for 2 h in the GC injection port. For the extraction of PCP, 2 mL of the standard solution or sample were measured into a 4 mL amber vial and fiber was immersed in the solution for 30 min at room temperature (25 ± ± 2°C), with rapid and constant stirring. After this period, the fiber was removed and desorbed in the GC injection port for 3 min.
Global uncertainty calculation. Equation 1 can calculate the estimation ofglobal uncertainty — U, expressed as a relative standard deviation [34]:
U = U
2 + U22 + U32 + UÀ
(1)
The quantification of the partial relative uncertainties U1, U2, U3 and U4 is henceforth explained. The uncertainty associated with the preparation of standard solutions, U1, is equal to the highest individual uncertainty ofprepa-ration of each standard, wst, which is given by
Ust =
S
AZ
Zi
(2)
where A% is the inaccuracy associated to the measurement equipment and z is the value measured by the respective equipment. In other terms, the uncertainty associated to preparation of the standards is the sum of the uncertainties of each measurement necessary to prepare the standards. Uncertainty is expected to rise with decreasing qua
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