Modern Analytical Chemistry

Structure determination○

Composition analysis of substances○

Determining purity○

Reasons for using analytical chemistry•

Infrared spectroscopy (bonds)○

Mass spectroscopy (Ar/Mr, arrangement)○

NMR spectroscopy (chemical environment of isotopes)○

Structural determination•

Analytical Techniques2015년 4월 17일금요일 오전 12:04

Modern Analytical Chemistry Page 1

Range of all possible frequencies of light○

Electromagnetic Spectrum•

Uses: NMR Bond Length Bond Type Vis-UV Process: Molecular Rotation Electronic Transition

Lines correlate to frequencies that are absorbed□

Absorbed frequencies excite e- from ground state to excited state□

Spectrum of electromagnetic radiation transmitted through a substance

EM radiation passed through sample□

Incident EM radiation compared to recorded EM radiation□

Production

Absorption Spectrum○

Lines correlate to frequencies that are emitted□

Emitted frequencies are from e- moving from excited state to ground state

□

Spectrum of electromagnetic radiation emitted by a substance

EM radiation measured from excited sample□

Production

Emission Spectrum○

Spectra•

Vibrations○

Rotations○

Electronic rotation○

Absorption•

Spectroscopy Principles2015년 4월 17일금요일 오전 12:08


Allows only radiation of particular wavelength to pass through

Monochromator ○

Splits beam in half to sample and half to reference

Beam Splitter○

Compares the two samples (photomultiplier)

Detector○

Diagram•

Stretching of bonds

Bending of bonds (changes in bond angle)

Types of Vibrational Motions○

Dipole moment is necessary for vibrational motion○

Increases with stronger bond

Increases with greater atomic mass

Energy Required for Vibration○

Wavenumber = 1/λ

Low wavenumber - greater energy required for vibration

Each bond type has characteristic wavenumber○

Bond Stretching/Bending•

Infrared Spectroscopy2015년 4월 17일금요일 오전 12:37


Not all vibrations absorb IR○

For IR absorption, dipole moment must occur○

Vibrations•

H2O/SO2•

CO2•

-CH2-•

Absorption of Radiation2015년 4월 17일금요일 오전 12:48


Human body consists of 70% water○

Water is in different environments

Organs have different water-lipid ratio

Protons in water molecules/proteins/lipids/carbohydrates detected by MRI○

Protons have different environments have varied absorption○

NMR in MRI•

Chemically inert, and does not react with sample

Internal standard is (CH3)4Si○

All protons in are in the same environment, giving strong single peak○

Si has low electronegativity

As CH3 is bonded to Si, TMS absorbs radiowaves much more upfield than protons attached to carbon

○

Tetramethylsilane•

Proton can align with or against the magnetic field○

Number of Adjacent Protons Splitting Pattern Type of Splitting

0 1 Singlet

1 1 1 Doublet

2 1 2 1 Triplet

3 1 3 3 1 Quartet

4 1 4 6 4 1 Quintet

Spin-Spin Coupling•

Nuclear Magnetic Resonance2015년 4월 17일금요일 오전 1:01


Aluminum in blood

Calcium in blood, plants, soil samples, water (hardness)

Iron in plants

Copper in alloys

Chromium in seawater

Detection of metal ions in samples○

Measures low concentration of metals/metal ions○

Uses of AA Spectroscopy•

e- goes from ground state to excited state

When metal atoms are excited by heat, atoms absorb light of certain frequency○

Sample is compared to known calibration curve

Ratio of transmitted/incident light is proportional to concentration of atoms present

○

Principles of AA spectroscopy•

Produces light of same frequency as is absorbed by specie being detected

Metal being detected is used as cathode of lamp

Monochromatic Light Source (Hollow Cathode Lamp)○

Liquid sample converted into small droplets

Evaporates and atomizes solvent

Atomizer○

e- excited at flame

Flame○

Only detects radiation of same/certain frequency

Monochromator○

Converts photons into electric current/signal

Detector○

Diagram•

Atomic Absorption Spectroscopy2015년 4월 17일금요일 오전 1:03


Separation of mixtures○

Identification of compounds in mixtures○

Determine purity of a substance○

Chromatography Uses•

Components adsorb onto stationary phase

Stationary phase does not move○

Components dissolve in mobile phase

Mobile phase moves through the stationary phase○

Adhesion of atoms/ions/molecules of gas/liquid/dissolved solid to a surface

Adsorption○

Depends on different abilities of the components to adsorb and dissolve

Separation○

Stationary and Mobile Phases•

Chromatography2015년 4월 17일금요일 오전 1:23


Water within fibres of paper

Stationary Phase○

Water or ethanol

Mobile Phase○

Paper Chromatography•

Silica or Alumina supported on glass/plastic

Stationary Phase○

Water or ethanol

Mobile Phase○

Thin-Layer Chromatography•

Ratio of distances moved by solute and solvent○

Rf = Distance Travelled by Component / Distance Travelled by Solute○

Rr Values•

Both used for separation

Similarities○

Gives better separation, as particles are finer than pores in paper□

Withstands stronger solvents□

Separation is more easily recoverable□

TLC

Sugars and H2SO4 reagent

May need to be developed using UV/Iodine/Acid□

Paper

Differences○

Paper vs. TLC•

Silica or Alumina

Stationary Phase○

Eluent

Mobile Phase○

Stationary phase packed on top of glass wool

Eluting solvent saturates powdered stationary phase

Sample, dissolved in eluent, is added at the top of the column

More eluting solvent is added at the top of the column

Collected separately as they leave the column□

Components move at different rates and have different retention times

Eluent is removed by evaporation

Procedure○

Column Chromatography•

Paper, Thin-Paper, and Column Chromatography2015년 4월 17일금요일 오전 1:25


Silica or Alumina

Stationary Phase○

Liquid (e.g. water or alcohol)

Mobile Phase○

Mobile phase forced through column under pressure at constant temperature

Sample placed in liquid stream/components separate as they pass through tube

Relative attraction to stationary/mobile phases determines retention time

□

Components reach detector at different times

Detected by absorption of UV/fluorescence/conductivity

Procedure○

High-Pressure Liquid Chromatography•

Liquid (long-chain alkane/hydrocarbon) coated solid support

Stationary Phase○

Inert gas (H2 or N2)

Mobile Phase○

Sample injected into mobile phase

Sample vaporized at high temperatures

Sample is carried by inert gas through column

Change of detector signal with corresponding time is recorded□

Detector at end of column

Components have different retention times□

Dependent on mass, polarity, and volatility □

Components are separated by partition between alkane and gas

Speed of separation is temperature dependent

Area under peak proportional to quantity of component

Procedure○

Gas-Liquid Chromatography•

Gas-Liquid Chromatography vs. High-Pressure Liquid Chromatography•

Gas-Liquid Chromatography High-Pressure Liquid Chromatography

Identifies volatile/stable compoundsAnalysis of urineUnderground mine gas analysisBlood-alcohol level determination

Analyzes temperature-sensitive compoundsOil analysisAlcoholic beveragesAntioxidants/sugars/vitamins in foodPharmaceuticalsPolymersInsecticide/Herbicide quality controlBiotechnology and Biochemistry research

High-Pressure Liquid and Gas-Liquid Chromatography2015년 4월 17일금요일 오전 1:34


Repulsion between d electrons and non-bonding electrons on ligandInteractions of different orbitals of ligands vary

3 low energy : 2 high energy split

Charged and polar ligands cause d-orbitals to split○

Different ligands and extent of split (spectrochemical series)○

Cl- H2O NH3

Ligands and d-orbital Splitting•

○ Result of e- transition between d-orbital splitIncreased d-orbital splitting leads to higher frequency radiation absorption○

Complementary color of color absorbed is observed○

Transition Metal Color•

[Mn(H2O)6]2+ is almost colorless, and [Fe(H2O)]3+ is yellow-brownElement being considered (especially nuclear charge)○

[Fe(H2O)6]2+ is pale-green and [Fe(H2O)6]3+ is yellow-brown [Ar] 3d6 [Ar] 3d5

Charge of the ion, which affects d-orbital splitting due to numbers of e- present○

[CuCl4]2- is yellow, [Cu(H2O)6]2+ is pale blue, and [Cu(NH3)]2+ is dark blueIdentity of ligand○

Stereochemistry○

Factors Affecting Transition Metal Color•

Part of molecule responsible for absorbing radiation

Chromophore○

Alternated double and single bonds

More likely to absorb longer wavelengths

Delocalization of pi bonds over large areas absorb light in far UV/Visible light region

Conjugated System○

Phenolphthalein in acid vs. alkali

Retinol (UV) vs. Retinal (Visible)

sp2 hybridization enables delocalization○

Double Bonds and Absorption•

○ Direct relationship between absorption and concentration

ε = molar absorption coefficient c = concentration l = distanced travelled by light

Absorbance = log (I0/I) = ε * c * l○

○ Transmittance = (Io/I) * 100

Beer-Lambert Law•

Wavelength Absorbed Wavelength Emitted

400 nm (Violet) 560 nm (Green-Yellow)

490 nm (Blue-Green) 620 nm (Red)

580 nm (Yellow) 430 nm (Dark Blue)

650 nm (Red) 520 nm (Green)

Visible Spectroscopy and UV Spectroscopy2015년 4월 17일금요일 오전 1:33


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Modern Analytical Chemistry