2014 학년도 1 학기 분석화학실험. 담당교수 : 이원용 ( 연구실 : 과 443-C, 전화 : , 전자우편 : 분광분석 / / 분리분석 / 전기분석화학

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1 2014 학년도 1 학기 분석화학실험 담당교수 : 이원용 ( 연구실 : 과 443-C, 전화 : , 전자우편 : wylee@yonsei.ac.kr) 분광분석 / / 분리분석 / 전기분석화학

2 Instrumental Analysis General concept Spectrophotometric experiment I A Lamp Monochromator Electrochemical experiment Optical cell with sample Phototube E Power supply i i t t

3 Chapter 18: Fundamentals of Spectrophotometry Spectroscopy: the science that deals with interactions of matter with electromagnetic radiation or other forms energy acoustic waves, beams of particles such as ions and electrons Spectrophotometry: a more restrictive term, - any procedure that uses light to measure chemical concentrations. - the quantitative measurement of the intensity of electromagnetic radiation at one or more wavelengths with photoelectric detector.

4 18-1. Properties of Light Electromagnetic radiation ; EM wave ; radiation ; radient ray ; ray ; light One linearly (or plane) polarized and consists of a single frequency, that is, is monochromatic.

5 18-2 Absorption of light Absorption of light: increases the energy of molecule (the molecule is promoted to an excited state) Emission of light: decreases the energy of molecule Ground state: lowest energy state of a molecule Excitation Relaxation M + h υ M* (life time: 10-6 ~10-9 S) M* M + light (fluorescence, phosphorescence) or M* M + heat

7 Absorption Single-beam spectrophotometric experiment Light source Wavelength selector (monochromator) P o Sample P Light Detector b When light is absorbed by a sample the radiant power of the beam of light is decreased Radiant power (P): the energy per second per unit area of the light beam Transmittance (T): T = P/P o (T = 0 ~ 1) Absorbance (A), or optical density: A = log (P o /P) = -log T (if 90% light is absorbed, 10% transmitted: T = 0.1P o /P o = 0.1, A= - log T=1) Absorption spectrum: absorbance vs wavelength

8 Absorption: Beer s Law The part of molecule responsible for light absorption: chromophore Absorbance is directly proportional to the concentration Beer-Lambert law: A = εbc ε : molar absorptivity (extinction coefficient) characteristic of a substance that tells how much light is absorbed at a particular wavelength b: path length c: concentration Beer s law works for monochromatic radiation passing through a dilute solution < 10 mm Colorimetry: a procedure based on absorption of visible light

9 Chapter 21: high temp Atomic Spectroscopy ( K)

10 21-1. An Overview Most compounds Atoms in gas phase high temp ( K) (AES) (AAS) sample Mass-to-charge (ICP-MS) (AFS)

11 Atomic Absorption experiment Flame in Atomic Spec. Cuvette in Mol. Spec. (Path length in Flame: 10 cm)

12 Flames Premix burner: fuel, oxidant, and sample are premixed. Nebulization: formation of a small droplets Aerosol: a fine suspension of liquid (solid) particles in a gas Nebulizer: create an aerosol from the liquid sample aerosol reaching the flame contains only about 5% of initial sample Pneumatic nebulizer

Hollow-Cathode Lamp in AAS Monochromators cannot isolate lines narrower than 10-3 10-2 nm.

13 Hollow-Cathode Lamp in AAS Monochromators cannot isolate lines narrower than nm. To get narrow lines of the correct frequency, Use of hollow cathode lamp containing the same element that being analyzed Filled with Ne or Ar at a pressure of 130 ~ 700 Pa High voltage (~300V) is applied between the anode and cathode Filler gas is ionized and positive ions are accelerated toward the cathode Accelerated positive ions strike the cathode with enough energy to sputter metal atoms from the cathode into the gas phase Free atoms are excited by collisions with high-energy electrons: photon emission Atomic radiation has the same frequency as that absorbed by the analyte atoms

14 Chapter 14: Fundamentals of Electrochemistry Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effects. The study of chemical changes caused by passage of an electric current and production of electrical energy by chemical reactions.

14-2. Galvanic Cells (Voltaic Cells) A galvanic cell : uses a spontaneous chemical reaction to generate electricity To accomplish this: 1. One reagent must be oxidized 2. The other must be reduced 3.

15 14-2. Galvanic Cells (Voltaic Cells) A galvanic cell : uses a spontaneous chemical reaction to generate electricity To accomplish this: 1. One reagent must be oxidized 2. The other must be reduced 3. The two reagents must be physically separated electrons are forced to flow through external circuit to go from one reagent to the other High input impedance potentiometer (voltmeter) (+) Anode reaction : oxidation Cathode reaction : reduction Cd(s) Cd 2+ (aq) + 2 e - 2 AgCl(s) + 2 e - 2Ag(s) + 2Cl - (aq) Cd(s) + 2 AgCl(s) Cd 2+ (aq) +2Ag(s) + 2Cl - (aq) When electrons flow from the left electrode to the right electrode : positive voltage When electrons flow from the right electrode to the left electrode : negative voltage

16 14-3. Standard (reduction) Potentials (activities of all species = 1) A quantitative description of the relative driving force for a half-cell reaction. A relative quantity vs standard hydrogen electrode assigned to zero volt. E 0 (SHE)=0 H + (aq, A = 1) + e- ½ H 2 (g, A = 1) E 0 (SHE)=0

17 Reduction : spontaneous SHE Oxidation: Spontaneous

18 14-4. Nernst Equation (activities of all species = 1) Le Chatelier s principle: increasing reactant concentrations drives the reacting to the right The net driving force of the reaction is expressed by the Nernst equation The Nernst equation tells us the potential of a cell whose reagents are not all unit activity Nernst Equation for a Half-Reaction aa + ne - bb R: gas constant = J/Kmol T: temperature (K) E E o RT nf ln A A b B a A (14.13) ΔG = ΔG o + RT lnq (Q; reaction quotient) -nfe = -nfe o + RT lnq ( 양변을 nf 로나누어준다 ) E = E o (RT/nF) lnq

17-1. Fundamentals of Electrolysis Dipping Cu and Pt electrodes into a solution of Cu2+ and forcing electric current through to deposit Cu metals at the cathode and to liberate O2 at the anode

19 17-1. Fundamentals of Electrolysis Dipping Cu and Pt electrodes into a solution of Cu2+ and forcing electric current through to deposit Cu metals at the cathode and to liberate O2 at the anode Cathode: Cu e - Cu(s) E = (0.0592/2)log([1/[Cu 2+ ]) Anode: H 2 O ½ O 2 + 2H + + 2e - E = (0.0592/2)log(1/pO 2 1/2 [H + ] 2 ) H 2 O + Cu 2+ Cu(s) + ½ O 2 + 2H + E = E o (cathode) E o (anode) = = V 0.2 M Cu2+ and 1.0 M H+ and liberates O2 at a pressure of 1.00 bar E = E o (cathode) E o (anode) = = V This voltage would be read on the potentiometer if there were negligible current not spontaneous A power supply is needed to force the reaction occur (electrolysis) the reaction of interest occur

20 Potential, V vs SCE Cyclic Voltammtry Forward scan Backward scan (reverse scan) E pc : cathodic peak potential, E pa : anodic peak potential i pa : anodic peak current, i pc : cathodic peak current Time, s Scan rate = V/s : 1.0 V/20 s = 50 mv/s Figure. Cyclic voltammetric excitation signal used to obtained voltammogram Figure. Cyclic voltammogram for 6.0 mm K 3 Fe(CN) 6 in 1.0 M in KNO 3.

21 Cyclic Voltammtry(CV) reversible E p = E pa -E pc = 59 mv/n 1 mm O 2 irreversible 0.06 mm 2-nitropropane Reversible means the reaction is fast enough to maintain equilibrium concentrations of the reactant and product at the electrode surface

22 Cyclic Voltammtry(CV) Peak current for a reversible system the working electrode (Randles-Sevcik equation) i p = (2.69x10 5 ) n 3/2 AD 1/2 C*v 1/2 A : electrode area, D : diffusion coefficient C* : bulk concentration (mol cm -3 ) v : scan rate (V s -1 ) i p v 1/2 from slope D can be calculated i p v 1/2

23 Cyclic Voltammtry(CV): Scan Rate Effect Figure (a) Effect of variation of scan rate on cyclic voltammograms and (b) plot of ip versus v 1/2.

24 Chapter 23: Introduction to Analytical Separations

25 In real analytical problems, we must identify and quantitate one or more components from a complex mixture Separation of mixture into each component is the first step in analysis Sample (mixture) <separation> Component 1, 2, 3, --- Detection Optical (absorbance, FL, CL) Electrochemical (voltammetry) Mass-to-charge

26 23-2. What is Chromatography? Martin and Synge: Nobel Prize in Chromatography operates on the same principle as extraction, but one phase is held in place while the other moves past it. Mobile phase Gas: gas chromatography Liquid: liquid chromatography Sample Injection Stationary phase Solid: GSC, LSC Liquid: GLC, LLC (partition chromatography) Detector

Chromatography (LSC) Solute A has a greater affinity than solute B for the stationary phase: (A is more polar) Solute A is more strongly adsorbed than solute B on the solid particles Solute A spends

27 Chromatography (LSC) Solute A has a greater affinity than solute B for the stationary phase: (A is more polar) Solute A is more strongly adsorbed than solute B on the solid particles Solute A spends a more time in stationary phase solute A moves down the column more slowly than solute B (longer retention time) Column packing (stationary phase): solid particles (silica: polar) filled with solvent Solvent (mobile phase): Non-polar organic solvent Fluid entering the column: eluent Fluid emerging from the end of column: eluate The process of passing liquid or gas through a chromatography column is called elution

28 Types of Chromatography Adsorption chromatography Stat. phase: solid Mobile phase: gas/liquid Partition chromatography Stat. phase: liquid Mobile phase: gas/liquid

29 23-3. Chromatogram unretained species - Retention time for each component: t r - Dead time for unretained species: t m - Adjusted retention time (t r ) = t r t m - Capacity factor (k ) = (t r t m )/t m = t r /t m - Relative retention (α) for any two components (A, B) = (t r ) B / (t r ) A = (k ) B / (k ) A Selectivity factor = K B / K A (partition coefficient)

Band Broadening & Efficiency of Separation Plate theory: theoretical plates (1941, Martin & Synge) Rate theory: Van Deemter (1956) One main cause of band

30 Band Broadening & Efficiency of Separation Plate theory: theoretical plates (1941, Martin & Synge) Rate theory: Van Deemter (1956) One main cause of band broadening is diffusion Definition of diffusion coefficient (D): Flux (mol/m 2 s) = J = - D dc/dx concentration gradient Standard deviation of band : σ = (2Dt) 1/2

31 Chapter 24: Gas Chromatography

32 Gas Chromatography Mobile phase (carrier gas): gas (He, N 2, H 2 ) - do not interact with analytes - only transport the analyte through the column Analyte: volatile liquid or gas Stationary phase: - solid (GSC) or non-volatile liquid (GLC) GSC (gas-solid adsorption chromatography) - semi-permanent retention of active or polar molecules - severe tailing of elution peaks GLC (gas-liquid partition chromatography) - non-volatile liquid is coated on the inside of the column or on a fine solid support - In 1955, the first commercial apparatus for GLC appeared on the market

33 24-1. The separation process in gas chromatography Temp of a sample injector port: 50 o C above the b.p. of least volatile component of the sample rapidly evaporates (2-50 m) (thermostated) The column should be hot enough to provide sufficient vapor pressure for analyte to be eluted in a reasonable time.

34 Open Tubular Columns Thin coating: small C-term (decreased H) : Compared with packed columns, OTC offers higher resolution, shorter analysis time, greater sensitivity, lower sample capacity Length: m

35 Liquid Sta. Phase Choice of liquid phase for a given problem: like dissolves like - Nonpolar columns: best for nonpolar solutes - Polar columns for polar solutes - As a column ages, stationary phases bakes off surface silanol groups (Si-OH) are exposed peak tailing (polar analyte) Therefore, stationary phase is covalently attached to silica surface

36 The Retention Index Non-polar column Polar column (retention time: hydrocarbon<ketone<alcohol) Dipole interaction H-bonding Column oven temp = 70 o C

37 Temperature Programming Temp of column (oven) increases Solute vapor pressure increase decrease retention time Isothermal at 150 o C Temp programming: o C at 8 o C/min Precaution: at too high temp. thermal decomposition of analyte

38 Sample Injection in GC Liquid samples are injected into GC by syringe through a rubber septum into a heated port Gaseous samples use gas-tight syringe <Sample size> Packed column: sub L 20 L, Capillary column: 10-3 L (split injection) Spilt injection delivers only 0.2-2% of the sample to the column

39 Quantitative and Qualitative Analysis by GC Qualitative analysis: - retention time (GC-FID, TCD, ECD ): comparison with authentic sample - mass (GC-MS) Quantitative analysis: - peak area or peak height

40 Thermal Conductivity Detector (TCD) <Advantages of TCD> - simple system - wide linear dynamic range (~ 10 4 ) - general response to organic and inorganic species - non-destructive <Limitation of TCD> - relatively low sensitivity

41 Chapter 25: High-Performance Liquid Chromatography

42 HPLC Mobile phase: liquid Analyte: non-volatile liquid Stationary phase: - solid (GSC) or non-volatile liquid (GLC) HPLC; uses high-pressure pump to deliver liquid mobile phase <HPLC system> Mobile phase High-pressure pump injector column detector

44 Elution Process In adsorption chromatography, solvent molecules compete with solute molecules for sites on the stationary phase Elution occurs when solvent displaces solute from the stationary phase

45 Normal- vs Reversed-Phase Chromatography Normal-phase chromatography (e.g. adsorption chromatography based on silica gel) Stationary phase: polar (e.g. silica) Mobile phase: non-polar (hexane, i-propylether) Reversed-phase chromatography Stationary phase: non-polar (hydrocarbon) or weakly polar Mobile phase: more polar (water, methanol, acetonitrile) <Normal Phase C> Polarity: A>B>C <Reversed- Phase C> Mobile phase: low polarity MP: high polarity C B A A B C time MP: medium polarity MP: medium polarity C B A A B C

46 - Ultraviolet detector: most common - Refractive index (universal) - Fluorescence - Electrochemical - Conductivity (ion-exchange C) - Mass spectrometry - Chemi-(electrochemi-)luminescence Detectors in HPLC

Chapter 27: Gas Chromatography

Chapter 27: Gas Chromatography Gas Chromatography Mobile phase (carrier gas): gas (He, N 2, H 2 ) - do not interact with analytes - only transport the analyte through the column Analyte: volatile liquid