Sources of uncertainty for the Determination of Chlorpyrifos by Gas Chromatography Equipped with Flame Photometric Detector

Available online at www.ilcpa.pl International Letters of Chemistry, Physics and Astronomy 6 (2014) 48-55 ISSN 2299-3843 Sources of uncertainty for the Determination of Chlorpyrifos by Gas Chromatography Equipped with Flame Photometric Detector O. A. Zalat*, M. A. Elsayed, M. S. Fayed, M. K. Abd El Megid Egyptian Armed Force, Cairo, Egypt *E-mail address: ABSTRACT Analysts are increasingly being required to evaluate the uncertainty associated with methods. Estimating the uncertainty of an analytical result is an essential part of quantitative analysis. This paper discusses the sources of uncertainty of chlorpyrifos determination by gas chromatography equipped with flame photometric detector (GC-FPD). The analysis was performed on HP-5 MS, 30 m x 0.32 mm capillary column with a 0.25 µm stationary film thickness using ultra pure nitrogen (99.9999 %) as a carrier gas at 25 psi constant pressure. The method has been optimized. Factors affecting quantization of chlorpyrifos such as injector temperature, carrier gas inlet pressure, air to hydrogen ratios and initial temperature program have been studied to get the best sensitivity, minimum delectability. The liner range of the detector was from 0.15 ng/ml to 1200 ppm, the minimum detection limit was 0.15 ng/ml and the relative standard deviation was 0.839. Keywords: Sensitivity; Minimum detectability; Chlorpyrifos; Injection 1. INTRODUCTION Chlorpyrifos is an organophosphorous insecticide widely used for pest control in agriculture and with minimum degree for indoor applications. The structure of chlorpyrifos is shown in Figure 1. Wide range of harmful effects of the organophosphates on humans are observed [1]. The immediate effect of an acute exposure is the accumulation of acetylcholine at the receptors, giving rise to the characteristic symptomatology of the acute organophosphorous poisoning [2]. Chlorpyrifos is stable in air (nonvolatile) and it is not sensitive to ultra violet radiation. It is stable to neutral and weakly acidic solution, but it is hydrolyzed by strong bases. The rate of chlorpyrifos hydrolysis increases with both pH and temperature, its structure is shown in Figure 1 [2]. International Letters of Chemistry, Physics and Astronomy 6 (2014) 48-55 Figure 1. Chlorpyrifos structural formula. Chlorpyrifos lethal dose in rats is (LD50) 202 mg/kg. Safety measures for farm workers are very poor. Many of them do not strictly follow the manufacturer’s directions for use of the commercial formulations containing 48 % (w/v) of chlorpyrifos marketed which strongly recommend that it is not to be applied by cold fogging or atomizing [2]. Chlorpyrifos analysis is carried out conventionally by gas chromatography (GC) or by high-performance liquid chromatography (HPLC) [3-8]. Flame photometric detector detects compounds by burning it in a flame and sensing the increase of light emission from the flame during that combustion process [9]. Therefore, the FPD is a flame optical emission detector comprised of a hydrogenair flame, an optical window for viewing emissions generated in the flame, an optical filter for spectrally selecting the wavelengths of light detected [9]. The aim of this search is to optimize and discuss the factors affecting detector response to get the best determination results of chlorpyrifos with gas chromatography technique equipped with FPD. 2. EXPERIMENTAL 2. 1. Materials and method Chlorpyrifos was purchased from sigma Aldrich; purity >97 %. Stock standard solutions were prepared by weighing about 1 mg of pure material. Chlorpyrifos was dissolved in chloroform and diluted to a volume 25 mL. The solutions were then transferred into TFEfluorocarbon-sealed screw-cap vials, stored at 4 °C and protected from light. A set of test samples were Prepared by dilution from stock standard solutions to cover a wide range of concentrations (0.0015-1200 ppm) of the test substance. 2. 2. Instrumentation Agilent, 7890A gas chromatograph, Auto sampler (7693-series) with a split/ splitless injector system and a flame photometric detection with sulphur filter were used. Ultra pure nitrogen (99.9999 %) at 25 psi constant pressure after passing through a molecular sieve trap was used as carrier gas. The injection port was held at 250 °C and used in the splitless mode. Separation was carried out on a HB-5, 30 m x 0.32 mm capillary column with a 0.25 µm stationary film thickness (Agilent Technologies). Operating conditions were as follows: detector temperature, 250 °C and hydrogen was used as detector at a flow of 150 mL/min. The flow of zero air (99.999 %) for FPD was 110 mL/min. The column temperature was maintained at 60 °C for 1 min and then programmed at 10 °C /min to 250 °C, and held for 1 49 International Letters of Chemistry, Physics and Astronomy 6 (2014) 48-55 min. The total analysis time was 11 min. The volume of sample injected in splitless mode was 1.00 µL. 2. 3. Analytical procedure Injector temperature was studied in the range from 150 to 250 °C while fixing the detector temperature at 250 °C. The used temperature program was set at an initial temperature at 60 °C (hold 1 min), final temperature at 250 °C (rate 20 °C/min, hold 7 min). The detector response for was then measured by injecting 1 µl of 4 ppm of chlorpyrifos. By using the same standard solution prepared above the initial temperature program was varied in the range of 50-100 °C while fixing other conditions. The change in retention time and detector response was then measured against initial temperature. The FPD response was also studied with the change in carrier gas flow inlet pressure (Nitrogen in this case) from 10 to 35 psi. The detector response was then measured and tabulated. The effect of changing the air to hydrogen ratio supplied to the detector from 0.2 to 1.2 volumetric ratio was also studied . 3. RESULTS AND DISCUSSION 3. 1. Injector temperature Injector temperature was studied in the range of 120 - 250 °C while fixing the detector temperature at 250 °C and using suitable temperature program. Initial temperature was set at 60 °C (hold 1 min), final temperature at 250 °C with increasing rate 20 °C/min, hold 7 min(. Figure 2 shows the relation between injector temperature and detector response (peak area). The figure summarized that the highest peak area of all samples were obtained at 250 °C, this could be attributed to the highest volatility of the samples introduced in the column. 2,5 Area x 10 5 2 1,5 1 0,5 0 0 50 100 150 200 250 300 Injector tempereture °C Figure 2. Effect of injector temperature on peak area using splitless mode injection volume (1µl) detector temperature 250 °C. 50 International Letters of Chemistry, Physics and Astronomy 6 (2014) 48-55 3. 2. Temperature programming For the purpose of this optimization the initial temperature was varied in the range of 50 - 100 °C while fixing the injector temperature 250 °C and the detector temperature at 250 °C. The results show that as the i (...truncated)