427 PHC ATOMIC EMISSION SPECTROSCOPY Direct-Current Plasma A direct-current plasma (DCP) is created by an electrical discharge between two electrodes. A plasma support gas is necessary, and Argon is common. Samples can be deposited on one of the electrodes. Insulating solid samples are placed near the discharge so that ionized gas atoms sputter the sample into the gas phase where the analyte atoms are excited. Direct-Current Plasma Spark and Arc Spark and arc excitation sources use a current pulse (spark) or a continuous electical discharge (arc) between two electrodes to vaporize and excite analyte atoms. The electrodes are either metal or graphite. If the sample to be analyzed is a metal, it can be used as one electrode. Arc and spark excitation sources have been replaced in many applications with plasma or laser sources, but are still widely used in the metals industry and for the analysis of metallic elements in solid samples. For non-conductive materials, the sample is ground with graphite powder to make it conductive. In traditional arc spectroscopy methods, a sample of the solid was commonly ground up and destroyed during analysis and placed into a cup- shaped lower electrode. An electric arc or spark is passed through the sample, heating it to a high temperature to excite the atoms within it. The excited atoms emit light at characteristic wavelengths that can be dispersed with a polychromator and detected. As the spark or arc conditions are typically not well controlled, the analysis for the elements in the sample is qualitative. However, modern spark sources with controlled discharges under an argon atmosphere can be considered quantitative. Spark and Arc Flame emission spectroscopy A sample is brought into the flame as a gas or sprayed solution. The heat from the flame evaporates the solvent and breaks chemical bonds to create free atoms. The thermal energy also excites the atoms into excited electronic states that subsequently emit light when they return to the ground electronic state. Each element emits light at a characteristic wavelength, which is dispersed by a grating or prism and detected in the spectrometer. Application: AES is useful for both qualitative and quantitative elemental analysis. The plasma sources yield significantly better quantitative analytical data than other emission sources do. All metallic elements can be determined by AES. Calibration curves are used for determination of the concentration of trace elements in different samples. What is the difference between atomic emission spectroscopy and atomic absorption spectroscopy? Emission spectroscopy is a spectroscopic technique which examines the wavelengths of photons emitted by atoms during their transition from an excited state to a lower energy state. Each element emits a characteristic set of discrete wavelengths. By observing these wavelengths the elemental composition of the sample can be determined. Absorption spectroscopy measures the loss of electromagnetic energy after it absorbs by the sample under study. For example, if a light source with a broad band of wavelengths is directed at a vapor of atoms, the particles will absorb those wavelengths that can excite them from one energy state to another. AAS AES • preparation of samples 1.Reagents and chemicals: a.Nitric acid conc. b.Hydrochloric acid conc. c.Deionized water (grade 1). d.Hydrogen peroxide (30%). e.Sulpheric acid conc. f.Potassium dichromate solution 5%. B- Equipment and instrument: a.Pipettes 1-5-10-20 ml. b.micro pipettes (10-1000 micro liter). c.Flasks 1000-500-250-200-100-50-10-5 ml flasks. d.Beakers 50-100-250-500 ml. e.Oven up to 700 C. f.Microwave oven for digestion. g.Analytical balance. • PROCEDURE: 1.Receive the sample. 2.Register the data in the Logbook. 3.For open system digestion Weight about 1-2 gram of the sample. 4.For closed system digestion weight maximum 0.5 gram of the sample. 5.The sample should be digested or ashed according to the element been analyzed. 1.Standard Preparation: a. Prepare your different standard concentrations either from stock standard 1000-ppm stored in the refrigerator, or from a suitable salt of the element. b. It is recommended that you prepare 100-ppm STD stock and make the dilutions out of it to minimize the margin errors. 2.Sample Preparation: • open system digestion 1.Take 1-2 gram\s of the sample in 250 ml flask and add 20 ml of concentrated nitric acid. 2.Put it on a burner in the fume hood and heat. 3.Evaporate nitric acid till 3-5 ml is left. 4.Add 2 ml of hydrogen peroxide and continue heating. Notice the white fume evolving (that’s the nitric acid). 5.When the white fume stops, add 20 ml of HCl: H2O 1:3 and heat again till it starts to boil. 6.Leave it for 2 minutes, then filter and complete to volume with de-ionized water. • Ashing: i.Some samples are treated by aching method. ii.Weight one gram and ash the sample in oven at 450ºc for organic samples and up to 800ºc for inorganic samples. iii.Remove the ash from the oven and diluted it with 20 ml of HCl: H2O 1:3 Note: use nitric acid instead of hydrochloric acid for selenium Se i.Filter the sample and complete to volume with deionized water. a.As it is sample: i.Some sample dose does not need to be digested since its elements are free in the aqueous solution. It will be analyzed as it is. If necessary, diluted with de-ionized water.