Chapter 6. An Introduction to Spectrometric Methods

Transcription

1 Chapter 6. An Introduction to Spectrometric Methods Spectroscopy: the science that deals with interactions of matter with electromagnetic radiation or other forms of energy acoustic waves, beams of particles such as ions and electrons (SIMS) (AES) Spectrometry: 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.

2 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.

3 Properties of light 1) Wave number = 1 / (cm -1, reciprocal centimeter ; Kayser) = / c = E / hc 2) Particle (energy packets ; photon) E = h = hc / where E is the energy in joules (J) h is Plancks constant ( J s/photon) 1 erg = 10 7 J 1 ev = J Ex. 400 nm x ev? E = h = hc / = ( J s) ( m s 1 ) ( m) ( J/eV) = 3.1 ev

4 Properties of light Sodium-D line: = 589 nm frequency =? = C/ = (3 x10 8 m/s)/(589 nm x 10-9 m/nm) = 5 x s -1 = 5 x Hz Nd:YAG laser: Q: How many photons are there in a typical 1 joule output pulse from the laser whose output light has a wavelength of 1.06 microns? Ans: 1 J output = (Energy per photon) x (# photons) (E per photon) = hc/ = (6.626 x J s photon -1 )(3 x 10 8 ms -1 )/(1.06 x 10-6 m) Thus, # photons = 5 x The intensity (I) is a flux defined as the energy streaming through a unit area per second in the direction of wave propagation. I is related to the square of wave amplitude and is not directly dependent on frequency.

5 Fig. 2. The electromagnetic spectrum showing the colors of the visible spectrum.

6 Superposition of Waves: Force field of waves add linearly in the region where waves are overlap, whether they cross or actually coincide for a distance. y = A 1 sin (ωt + Φ 1 ) + A 2 sin (ωt + Φ 2 ) A 1 and A 2 is maximum amplitudes of the two waves.. ω: angular velocity (=2 ft) * constructive interference: Φ 1 - Φ 2 = 0, 360 o * destructive interference: Φ 1 - Φ 2 = 180 o Fig. 3. Interference of adjacent waves that are a) 0 o, b) 90 o and c) 180 o out of phase.

7 Superposition of complex waveform can be broken into simple component by a mathematical operation called the Fourier Transformation.

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9 Diffraction of Radiation( 회절 ) <wide slit slight diff.> - All type of EM radiation exhibits diffraction - A process in which a parallel beam of radiation is bent as it passes by a sharp barrier or through a narrow opening - Wave property - Diffraction is a consequence of interference <narrow slit big diff.>

10 Diffraction of Radiation: Young s experiment Beam: coherent Angle of diffraction 경로차: CF CF = BCsinӨ = n

11 Types of interaction between radiation and matter 1. Reflection & scattering 2. Refraction & dispersion 3. Absorption & transition 4. Luminescence ( 발광 ) & emission ( 방출 ) Emission or chemiluminescence Refraction Sample Scattering and photoluminescence Reflection Absorption along radiation beam Transmission Fig. 4. Types of interaction between radiation and matter.

12 Dispersion ( 분산 ) The variation in refractive index of a substance with wavelength or frequency dispersion = dn / dλ Normal dispersion: a gradual increase in RI with increasing frequency Anomalous dispersion: a sharp change in RI (prism)

13 Refraction and Reflection of Radiation 1. Refractive index of medium: one measure of its interaction with radiation and is defined by sin θ1 ν1 η2 = = sin θ2 ν2 η 1 2. Reflection occurs whenever radiation is incident upon a boundary between dielectrics across which there is a change in refractive index. reflection If Ө is above 60 o, reflection: big Grazing incidence: Ө almost 90 O

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15 Langmuir-Blodgette or LB films : monolayer or multilayer organized films on solid substrates Molecules - polar head group (-COOH, -OH) -With a long-chain hydrocarbon or fluorocarbon tail Spreading solvent : chloroform Monolayer at the air-water interface

16 What Happens When a Molecule Absorbs Light? Absorption of light: increases the energy of molecule Emission of light: decreases the energy of molecule excited state absorption emission ground state 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

17 Ground state (planar) electronic transition ( n π* (S 1 )) Excited state (pyramidal)

18 Absorption of IR radiation: vibration Absorption of microwave radiation: rotation Nonlinear molecule with n atom: 3N-6 Linear molecule with n atom: 3N-5 (Formaldehyde: 3N-6 = 3x4-6 = 6) Electronic transitions involve simultaneous vibrational and rotational transition IR spectroscopy: good for structural information

19 Absorption

20 Absorption of Radiation Absorption (10-14 ~10-15 s) Excited state (life-time:10-8 s) Vibration (10-15 s)

21 Absorption of Radiation Collisional broadening

22 Emission or Chemiluminescence

23 Emission of Radiation Line spectra: from atoms Band spectra: from molecules For molecules: E = E electronic + E vibrational + E rotational For atoms: E = E electronic

24 Emission of Radiation <line spectra> <band spectra>

25 Photomluminescence

26 Photoluminescence IC: raditionless transition between states with the same quantum state (S 1 S 0 ) ISC: raditionless transition between states with different quantum state (S 1 T 1 ) S 1 : singlet excited state T 1 : triplet excited state Life time: s Life time: s S 0 : singlet ground state

27 Fluorescence vs Phosphorescence Fluorescence: S 1 S 0 Phosphorescence: T 1 S 0 (very rare) ISC (T1 S0)can occur before phosphorescence: cooling required

28 Fluorescence Spectrometer Luminescence is observed at 90 o to the incident light Emission spectrum: constant ex and variable em Excitation spectrum: constant em and variable ex

29 Excitation and Emission Spectra Emission spectrum: constant ex and variable em Excitation spectrum: constant em and variable ex An excitation spectrum looks very much like an absorption spectrum

30 Excitation and Emission Spectra The does not exactly overlap: In the emission spectrum, o comes at slightly lower energy than in the absorption spectrum o (absorption) < o (emission)

31 Absorption of Radiation in Analytical Chemistry 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/P = 0.1, A= - log T=1) Absorption spectrum: absorbance vs wavelength

32 Absorption of Radiation: 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

33 - Ultraviolet detector: most common - Refrative index (universal) - Fluorescence - Electrochemical - Conductivity (ion-exchange C) - Mass spectrometry - Chemi-(electrochemi-)luminescence HPLC Detector

34 Detectors in Capillary Electrophoresis Absorbance Methods: Capillary small inside diameter short path length for absorbance low absorption In order to improve sensitivity of absorbance measurement increase path length (silver coating)

35 Luminescence in Analytical Chemistry Relation of emission intensity to concentration: I = kp o C I: emission intensity P o : radiant power of incident light C: concentration of emitting species In FL: Higher radiation power higher intensity better detection In Absorbance: Higher radiation power no change in absorbance (Laser-induced fluorescence; LIF) good for the detection of trace amount Emission intensity is not proportional to analyte concentration at high concentration, or in the presence of significant amount of absorbing species Self-absorption

36 Derivatization H 3 C N CH 3 H H 3 C N CH 3 O S Cl O + H 2 N C R COOH O R S N C O H H Dansyl Chloride Amino Acid COOH Dansyl Amino Acid Chromophore & fluorophore Not detected by UV-Vis or FL.

37 Chemi- and Bioluminescence

38 Chemiluminescence A + B C* Catalyst C* + D C + light CL light intensity = Quantaum Yield x Rate of Reaction CL Intensity = photons emitted per second Quantum Yield = photons emitted per molecule reacting Rate of Reaction = molecules reacting per second Characteristics of CL High sensitivity Wide dynamic ranges Simple apparatus Safe Reagents (as use of labels) Sensitive to CL reaction conditions

39 Gas Phase Chemiluminescence CL with ozone: SO, NO Liquid Phase Luminol Peroxyoxalate Dioxetanes Lucigenin Acridinum esters Ru(bpy) 3 2+ Bioluminescence Luminescence at living systems Firefly Bacterial BL

40 Bioluminescence Firefly BL (ATP BL) luciferase Luciferine + ATP + O > oxyluciferine + AMP + light Application: immunoaasay, detection of bacterial growth and substrates producing ATP CK Creatine Phosphate + ADP > Creatine + ATP Bacterial BL luciferase FMNH 2 + O 2 + RCHO > FMN + RCOOH + H 2 O + light Application: toxicity sensor, immunoassay, detection of substrates producing NADH dehydogenase Substrate + NAD > NADH + product NADH + FMN + H + oxidoreductase > NAD + + FMNH 2

41 (a) O Luminol Chemiluminescence O NH NH OH -, H 2 O 2, catalyst * O - O - 3-APA + ligh (425 nm blue emission) NH 2 O NH 2 O (b) O 3-aminophtalate* (3-APA*) Catalysts: Heme proteins (HRP, hemoglobin..) transition metal ions (Co 2+, Cu 2+, Fe 2+,...) H 2 N C 2 H 2 N (CH 2 ) 4 O aminobutylethylisoluminol (ABEI) NH NH Application: -H 2 O 2 detection at submicromolar concentration: H 2 O 2 producing substrates using oxidase enzymes - Immunoassay using HRP, isoluminol, or ABEI as labels - Amino acid detection using isoluminol or ABEI as labels - Transition metal detection - BOD sensor (HRP used) - Forensic science (blood trace)

42 Sensors based on Luminescence Quenching Emission intensity [emitting species] Absorption : M + h M* Rate = d[m*] /dt = k a [M] k a illumination intensity absorptivity of M Rate constant Emission : M* M + h Rate = -d[m*] /dt = k e [M*] Deactivation : M* M + heat Rate = -d[m*] /dt = k d [M*] Rate of disappearance Quenching

43 Luminescence Quenching Quenching : The excited molecules can transfer energy to a different molecule, a quencher (Q), to promote the quencher to an excited state (Q*) M* + Q M + Q* Rate = -d[m*] /dt = k q [M*] [Q] Under constant illumination, the system soon reaches a steady state in which the concentrations of M* and M remain constant

44 Luminescence Quenching In the steady state : the rate of appearance of M* = the rate of destruction of M* - rate of appearance of M* = d[m*]/dt = k a [M] - rate of disappearance of M* = k e [M*] + k d [M*] + k q [M*][Q] k a [M] = k e [M*] + k d [M*] + k q [M*][Q] Quantum Yield : the fraction of absorbed photons that produce the desired process (0 Φ 1 ) The quantum yield for emission from M* in the absence of quencher Φ 0 = photons emitted per second / photons absorbed per second = emission rate / absorption rate

45 Luminescence Quenching The quantum yield for emission from M* in the absence of quencher k e [M*] Φ 0 = k e [M*] / k a [M] = k e [M*] + k d [M*] + k q [M*][Q] 1) When Q = 0 Φ 0 = k e / (k e + k d ) 1) When Q = 0 Φ Q = k e / (k e + k d + k q [Q]) I 0 Φ 0 k e + k d + k q [Q] k q = = = 1 + [Q] I Q Φ Q k e + k d K e + k d

46 Luminescence Quenching Stern-Volmer equation I 0 = 1 + (constant) * [Q] I Q I 0 : emission intensity in the absence quencher I Q : emission intensity in the presence quencher

47 Luminescence in Analytical Chemistry Anal. Chem. October 1, A

48 Luminescence in Analytical Chemistry

49 Luminescence in Analytical Chemistry

50 Electrophoresis Electrophoresis: separation method based on differential rate of migration of charged species in a buffer solution under the influence of an electric filed First developed by the Swedish chemist Arne Tiselius in the 1930s: 1948 Nobel Prize Analytes: - inorganic anions, cations - amino acids - carbohydrates - peptides, proteins - nucleic acids, polynucleotides Special strength of electrophoresis: separation of charged macromolecules - proteins (enzymes, hormons, antibodies) - nucleic acids (DNA, RNA)

51 Basis for Electrophoresis v= μ e E v: migration velocity of an ion (cm s -1 ) μ e : electrophoretic mobility (cm 2 V -1 s -1 ) E: electric field (V cm -1 ) Electrophoretic mobility: - Proportional to the ionic charge on the analytes - Inversely proportional to frictional retarding force determined by (1) size and shape of the ion (2) the viscosity of the medium in which analyte migrates For the same size: The greater the charge the greater the driving force faster migration For the ions of the same charge: Smaller ion smaller frictional force faster migration Therefore, the ion s charge-to-size ratio determines the electrophoretic mobility

52 Capillary Electrophoresis: Capillary Electrophoresis (CE) - Use of fused silica (SiO 2 ) capillary tube (50 cm long, inner diameter: m) - Electric field: 30 kv (silica: good heat dissipation) - High speed, high-resolution separations - Exceptionally small sample volumes ( nl) Migration rate: v = μ e (V/L) v: migration rate V: applied voltage in volts L: length over which the voltage is applied Higher voltage higher separation speed

53 Capillary Electrophoresis (CE) <Plate Height in Capillary Electrophoresis> - No stationary phase in CE H = A + B/U x + CU x Open tubular column N = μ e V/2D No stationary phase D: diffusion coefficient High voltage higher resolution separation (20,000 60,000 V can be applied) N 100, ,000 One or two order-of-magnitude better performance than HPLC

54 Electroosmosis The inside of a fused silica wall is covered with silanol (Si-OH) groups with a negative charge (Si-O - above ph =2) immobile layer Electro-osmotic flow

55 Electroosmotic velocity: v eo = μ eo E Electroosmosis electroosmotic mobility: proportional to surface charge density (higher ph faster) inversely proportional to the square root of ionic strength Electroosmotic flow : flat flow Hydrodynamic flow: Parabolic velocity profile

56 Capillary Zone Electrophoresis v electroosmotic + - v electrophoretic v total = v electroosmotic + v electrophoretic

57 Micellar Electrokinetic Capillary Chromatography Micells: act as the pseudo-stationary phase in solution The more time the neutral molecules spends inside the micelle, the longer is its migration time Micelle: sodium dodecyl sulfate + + Anode - Order of elution: cation > neutral hydrophilic > neutral hydrophobic > micelle > anion

58 Sample Introduction

59 Chiral Separation in CE Cyclodextrin in buffer: optically active cavity for chiral separation

60 Detectors in Capillary Electrophoresis Highly sensitive detector is required in CE (extremely small sample amount) - UV detector : most general - Fluorescence detector (laser-induced fluorescence): good sensitivity - Amperometric detector - Conductivity detector - Chemiluminescence detector - Mass spectrometer

61 Detectors in Capillary Electrophoresis

62 Lab-on-a-Chip: 초소형전자동분석시스템 개념 장점 D. J. Harrison, et. al., Science, 1993, 261, Highly integrated system (sample pretreatment, reaction, separation, and detection all on-a-chip) High speed analysis (few second or few minutes) Ultra-low volume - minimal reagent or sample consumption (1-10 pl) Highly parallel - many samples at once: high-throughput Small and possibly portable system Potentially disposable assay systems

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