Molecular Absorption Spectroscopy

Molecular Spectroscopy (based on ultraviolet, visible, and infrared radiation) The identification and determination of a number of inorganic, organic, and biochemical species.

Ultraviolet/visible absorption spectroscopy  primarily for quantitative analysis Infrared absorption spectroscopy  for determining the structure of both inorganic and organic compounds

Ultraviolet and Visible Molecular Absorption Spectroscopy 1. absorbing species. Absorption of u.v/visible radiation by molecules. Band the transition of an electron from the ground state to one of the many vibrational and rotational energy states of excited electronic energy state.

Solvent effect A in B solvent A in C solvent

Absorption by organic compounds 1) Absorption of radiation by organic molecules (180-780nm) : results from interactions between photons and those electron in bond. 2) The wavelength depends on how tightly its several electrons are bound.

The shared electrons in single bond(C-H, C-C)  vacuum ultraviolet region(below 180nm)  Single bond spectra not widely used for analytical purpose  quartz and atmospheric components absorb in this region (It require the evacuated spectrophotometer with lithium fluoride optics).

4) Electrons in double and triple bonds of molecules  easily excited  species with unsaturated bonds generally exhibit useful absorption peaks.

5) Unsaturated organic functional group that absorb in the u. v 5) Unsaturated organic functional group that absorb in the u.v. or visible region chromophores. 6) Saturated organic compounds containing heteroatoms (oxygen, nitrogen, sulfur or halogen) contain nonbonding electrons (excited by radiation in the 170-250 nm range)

Absorption by inorganic species. The ions and complexes of elements in the first two transition series absorb broad bands of visible radiation  transitions between filled and unfilled d-orbitals.

Charge-Transfer Absorption For quantitative purpose, charge-transfer absorption is important due to large molar absorptivities(>10,000)  High sensitivity ex) A charge- transfer complexes.  phenolic complex of iron(III), the 1,10-phenanthroline complex of iron(II)

2. Typical Instrument. Spectrophotometer : a grating or a prism monochromator advantage  the wavelength used can be varied continuously  obtain entire absorption spectra.

Photometer : an absorption filter or an interference filter advantage  low cost, simplicity, ruggedness, portability, and ease of maintenance disadvantage  less versatility, their inability to generate entire spectra.

Figure 1. Single-beam photometer for absorption measurements in the visible reion.

* Uitraviolet/Visible Spectrophotometers Single-Beam Instrument (spectronic 20) 2) Double-Beam Instrument 3) Multichannel

Single-Beam Instrument (spectronic 20) The spectral range : 340-625 nm Effective bandwidth: 20 nm For quantitative absorption measurement at a single wavelength Advantage: simplicity, low cost, ease of maintenance Instruments range from 190 ( 210) nm to 800 (1000) nm

Figure 2. Its optical diagram. (Courtesy of Milton Roy Company, Analytical Products, Rochester, NY)

Figure 3. A double-beam recording spectrophotometer for the ultraviolet and visible regions; the Perkin-Elmer Series. (Courtesy of Coleman Instruments Division, Oak Brook, IL 50421.)

Multichannel Instrments Multichannel, or diode array, spectrometers are products of modern optoelectronic technology It makes it possible to record an entire ultraviolet or visible spectrum rapidly. All wavelengths can be monitored simultaneously and data for an entire spectrum collected in a second or less

Figure 4. Diagram of a multichannel spectrophotometer based on a grating and photodiode detector.

3. Qualitative Application of UV/Vis Spectroscopy ; Ultraviolet spectra lack sufficient fine structure to allow unambiguous identification of an analyte. Solvent - A solvent for uv/vis spectroscopy must be transparent and should dissolve a sufficient quantity of the sample.

-Polar sovent(water, alcohols and ketone) tend to obliterate vibration spectra. Nonpolar solvents(cyclohexane) provide better spectra The polarity of the solvent often influences the position of absorption maxima.

4. Quantitative Application of UV/Vis Spectroscopy - One of the most useful tools for quantitative analysis Wide applicability : Enormous numbers of inorganic, organic, and biochemical species High sensitivity : Typical detection limit 10-4 -10-5 M

3) Moderate to high selectivity 4) Good accuracy : the relative error range 1% - 5% 5) Ease and Convenience <Procedure detail> Spectrometric analysis : yield a reproducible relationship between absorbance and analyte concentration.

Wavelength selection Variables that influence absorbance Cleaning and handling of cells Determination of the relationship between absorbance and concentration.

Standard Addition method Adding one or more increments of a standard solution to sample aliquots of the some size Each solution is then diluted to a fixed volume before measuring its absorbance.

ex) * Several identical aliquots Vx of the unknown solution with a concentration Cx  transferred to volumetric flask having a volume Vt * To each of these flask is added a variable volume Vs mL of a standard solution of the analyte(concentration Cs) * Each solution is diluted to volume.

A plot of As as a function of Vs straight line of the form As = mVs + b (m: kCs, b=kVxCx)

* In the interest of saving time and sample, it is possible to perform a standard addition analysis using only two increments of sample.  A single addition of Vx mL of standard would be added to one of the two sample.

Analysis of Mixture : The total absorbance of a solution at any given wavelength is equal to the sum of the absorbance of the individual components in the solution

5. Photometric and Spectrophotometric Titration - Useful for locating the equivalence points of titration The application of absorption Require that one or more of the reactants or product absorb radiation Titration Curve - A plot of absorbance as a function of titrant volume.

· Each molecular species has a unique infrared Infrared Absorption Spectroscopy IR Spectrophotometry  One of the most powerful tools for identifying pure organic and inorganic compounds because almost all molecular species absorb infrared radiation · Each molecular species has a unique infrared absorption spectrum. · The identification of an analyte By an exact match between the spectrum of a compound of know structure and that of an analyte.

· a typical infrared spectrum for a liquid consists of 1. I.R Absorption Spectrum Vibrational absorption occurs in the infrared region with liquid or solid ( rotation is often hindered or prevented thus, the effects are not detected.) · a typical infrared spectrum for a liquid consists of a series of vibrational peaks 2. Instrumentation for IR spectroscopy · Three type of I.R Dispersive spectrometer r.s sample monochromator D

· Disadvantage ; the complexity of the instruments and their high cost · Advantage ; any scattered radiation generated in the cell compartment is largely removed by monochromator 2) Fourier- transform(FT-IR) spectrometer · Advantage ; Unusually high sensitivity, resolution and speed of data acquisition · Disadvantage ; the complexity of the instruments and their high cost · No dispersing element. all wavelength are detected and measured simultaneously

· To separate wavelengths It is necessary to modulate the source signal in such a way that it can subsequently be decoded by a Fourier Transformation, a mathematical operation that require a high-speed computer. 3) Filter photometer Monitoring the concentration of air pollutants such as carbon monoxide, nitrobenzene, vinyl chloride and pyridin = Interference filter

3. Qualitative Application of I.R Peaks useful for the identification of functional groups (in the shorter wavelength region of the infrared) Identification of the functional groups in a molecules is seldom sufficient to permit positive identification of the compound The entire spectrum from 2.5 to 15 µm must be compared with that of known compounds.

· I.R cells are required to permit the transmission of 4. Quantitative Infrared photometry and spectophotometry · I.R cells are required to permit the transmission of measurable intensities of radiation through pure samples or very concentrated solution of analyte. · Measurements of dilute analyte solutions are frequently precluded by the lack of good solvents that transmit over appreciable regions of the infrared spectrum · A reference absorber is often dispensed with entirely In qualitative infrared work.

· Applications of quantitative I.R spectroscopy - IR ; Potential for determining an unusually large number of substances because nearly all molecular species absorb in this region. - The uniqueness of an infrared spectrum provides a degree of specificity - The specificity has particular application to the analysis of mixtures of closely related organic compounds Government regulations The variety of atmospheric pollutants can be determined with a simple, portable filter photometer equipped with a separate interference filter.