化学专业英语之分析化学.pdf

Analytical Chemistry Lecture 1 Introduction Measurement basics Spectroscopy Lecture 2 Optical methods Mass spectrometry Electroanalytical Techniques Lecture 3 Separation methods Microscopic and surface analysis Polymer analysis SL 2014 Analytical Chemistry SL 2014 What is Analytical Chemistry? Analytical Chemistry seeks ever improved means of measuring the chemical composition of natural and artificial materials. The techniques of this science are used to identify the substances which may be present in a material and to determine the exact amounts of the identified substances. SL 2014 Analytical chemists work to improve the reliability of existing techniques to meet the demands for better chemical measurements which arise constantly in our society. They adapt proven methodologies to new kinds of materials or to answer new questions about their composition. They carry out research to discover completely new principles of measurement and are at the forefront of the utilization of major discoveries such as lasers and microchip devices for practical purposes. They make important contributions to many other fields as diverse as forensic chemistry, archaeology, and space science. analytical chemistry serves the needs of many fields. In medicine, analytical chemistry is the basis for clinical laboratory tests which help physicians diagnose disease and chart progress in recovery. Environmental quality is often evaluated by testing for suspected contaminants using the techniques of analytical chemistry. In industry, analytical chemistry provides the means of testing raw materials and for assuring the quality of finished products whose chemical composition is critical. Many household products, fuels, paints, pharmaceuticals, etc. are analyzed by the procedures developed by analytical chemists before being sold to the consumer. The nutritional value of food is determined by chemical analysis for major components such as protein and carbohydrates and trace components such as vitamins and minerals. Indeed even the calories in a food are often calculated from its chemical analysis. Classification of Analytical Methods Classical methods gravimetric titrimetric Instrumental methods spectroscopy electroanalytical chromatography SL 2014 Types of Instrumental Methods SL 2014 Instruments for Analysis Components of a typical instrument Signal generator Input transducer or detector Analytical signal Signal processor Electrical or mechanical input signal 12.301 Output signal Meter or Scale Recorder Digital unit SL 2014 4 Basic Components of Instruments for Chemical Analysis signal generators detectors (input transducers) signal processors (circuits Field-Effect Transistor) Power Supplies and Regulators Transformers Rectifiers and Filters Voltage Regulators SL 2014 Readout Devices Oscilloscopes/Cathode Ray Tubes Recorders Analog Meters Alphanumeric Displays nixie tubes LEDs (light emitting diode ) LCDs (Liquid Crystal Display ) Computers SL 2014 Signal to Noise Ratio (S/N) signal - the information about the analyte that is of interest to the chemist noise - extraneous information that is unwanted because it degrades the accuracy and precision of an analysis and also places a lower limit on the amount of analyte that can be detected SL 2014 Sources of Noise in Instrumental Analyses Chemical Noise - results from uncontrollable variables that affect the chemistry being investigated Instrumental Noise Thermal Noise Shot Noise Flicker Noise Environmental Noise SL 2014 Signal-to-Noise Enhancement Some Hardware Devices for Noise Reduction Grounding and Shielding Difference Amplifiers Analog Filtering Modulation Signal Chopping; Chopper Amplifiers Lock-in Amplifiers SL 2014 Properties of Electromagnetic Radiation Electromagnetic Radiation energy radiated in the form of a WAVE caused by an electric field interacting with a magnetic field result of the acceleration of a charged particle does not require a material medium and can travel through a vacuum SL 2014 wavelength x frequency = velocity wavelength is measured in units of length meters millimeters (mm) = 0.001 m = 10-3m micrometers (m) = 0.000001 m = 10-6m frequency is expressed in units of inverse time (1/sec) called Hertz (1 Hz = 1/sec) Visible light can be split into its components by a prism. SL 2014 radiation Radiation can be absorbed, reflected, scattered, refracted, or transmitted. Absorbed: energy is retained by a substance. Reflected: the surface returns a portion of the energy. Scattered: the electromagnetic waves are changed from propagating in one direction to all directions. Refracted: the electromagnetic waves are changed from propagating in one direction to another direction. Transmitted: energy passes through space or the media. Transmission of Radiation Transmission rate at which radiation passes through a transparent material is less than through a vacuum depends upon the kinds and concentrations of atoms, ions, and molecules in the medium radiation must interact with material interaction must not undergo permanent energy transfer SL 2014 Dispersion variation in refractive index of a substance with wavelength or frequency Refraction of Radiation Snells Law SL 2014 Reflection of Radiation reflection always occurs when radiation passes from one medium to another reflection greatest when two materials have the greatest difference on their refractive indices SL 2014 Scattering of Radiation blue sky results from greater scattering of shorter wavelength visible light Types of Scattering Rayleigh Scattering scattering involving molecules considerably smaller than the wavelength of radiation SL 2014 Raman Scattering part of the radiation undergoes quantized frequency changes Absorption of Radiation energy of exciting photon must equal energy difference between ground state and 1 excited state for absorption to occur SL 2014 Absorption Methods, Transmittance T = P/Po whereT = transmittance P = power of transmitted radiation Po = power of incident radiation %T = (P/Po)*100 where%T = percent transmittance SL 2014 Absorption Methods, Absorbance A = - log10T = - log10 (P/Po) whereA = absorbance Absorption Methods, Beers Law A = abc = ebc wherea = absorptivity b = path length c =concentration e =molar absorptivity SL 2014 Atomic Absorption absorption occurs with only a few well- defined frequencies electronic excitation SL 2014 Molecular Absorption E = Eelectronic+ Evibrational+ Erotational SL 2014 Emission of Radiation radiation resulting from the relaxation to lower energy states of excited particles SL 2014 Components of Optical Instruments SourceWavelength SelectorSampleDetector Signal Processor Readout Spectroscopic Methods SL 2014 UV-Visible-Near IR Region H2or D2 160 - 375 nm, must use Quartz windows and cuvettes Sources Sources UV-Visible-Near IR Region Xe arc lamps 250 - 600 nm, max I at 500 nm W filament 320 - 2500 nm, needs close V control SL 2014 Sources IR Region Nernst glower - rare earth oxides globar - silicon carbide rod incandescent wire - nichrome wire SL 2014 Sources Line Sources metal vapor - Hg and