Chem 550- Ch. 13 UV-Vis Molecular Absorption Spectrometry

Instrumental Design: Double Beam In Time Instruments

- the beams are separated in time by a rotating sector mirror -preferred over in space instruments -100 Carry Program (This is the one is like the one we have in CSL222)

Instrumental Design: Multichannel Instruments

- the dispersive system is a grating spectograph placed after the sample or reference cell. -use multichannel system such as CCD array -good for transient and fast reactions, kinetic studies, liquid chromatography column or capillary column

Instrumental Design- Single Beam Instruments

-require a stabilized voltage supply to avoid error resulting from changes in the beam intensity during the 100% T and sample measurements

Instrument Component -Tungsten Filament Lamp

-visible region (350-2500nm) -spectrum follows black body radiation -Tungsten filament lamps (operated at 2870K) -Tungsten halogen lamps (quartz halogen) --operated at 3500K --enhanced lifetime because sublimated W forms a volatile product WI2 that allows W to be redeposited to the filament LED- white 400-800nm or semi-chromatic Xenon Arc Lamps- 200-1000nm

Limitations to Beer's Law

1. Concentration Limit - beer's law describes the absorption behavior of media at low analyte concentration (less than 10mM) -at high concentration absorptivity of molecule can be impacted by other molecules (nonlinear relationship betwen absorption and concentration) -refractive index change of a solution is not significant 2. Apparent chemical deviations -apparent deviations arise when an analyte dissociates, associates or reacts with a solvent to produce a product with a different absorption spectrum than the analyte. 3. Instrumental deviations due to polychromatic radiation -beer's law strictly only applies when measurements are made with monochromatic source radiation. ---polychromatic sources are used in conjunction with a grating or with a filter to isolate a nearly symmetric band of wavelengths surrounding the wavelength to be used. -when molar absoprtivities are not the same for composing wavelengths, the relationship between absorbance and concentration is no longer linear -Select the band of wavelengths selected for spectrophotomettric measurements such that the molar absorptivities are essentially constant -select a wavelength band near the wavelength maximum absorption -monochromator or filter delta eff less than 1/10 of the absorption band of full width at half maximum 4. Instrumental Deviations in the presence of Stray Radiation - Stray beam: outside the nominal wavelength band scattering reflection off the surfaces of grating, lenses or mirrors, filters and windows -when stray radiation wavelength is different, and is not absorbed by the analyte. But monitored by the dectector.

13-13) Advantages and disadvantages d)photodiodes and photomultiplier tubes

A photodiode consists of a photo-sensitive pn-junction diode that is normally reverse-biased. An incident beam of photons causes a photocurrent proportional to the photon flux. A photomultiplier tube is a vacuum tube consisting of a photoemissive cathode, a series of intermediate electrodes called dynodes, and a collection anode. Each photoelectron emitted by the photocathode is accelerated in the electric field to the first positively charged dynode where it can produce several secondary electrons. These are, in turn, attracted to the next positively charge dynode to give rise to multiple electrons. The result is a cascade multiplication of 106 or more electrons per emitted photoelectron. Photomultipliers are more sensitive than photodiodes, but require a high voltage power supply compared to the low voltage supplies required by photodiodes. Photomultipliers are larger and require extensive shielding. Photodiodes are better suited for small, portable instruments because of their size and ruggedness.

13-13) advantages and disadvantages c) photovaltaic cells and phototubes as detectors for electromagnetic radiation

A phototube is a vacuum tube equipped with a photoemissive cathode and a collection anode. The photoelectrons emitted as a result of photon bombardment are attracted to the positively charged anode to produce a small photocurrent proportional to the photon flux. A photovoltaic cell consists of a photosensitive semiconductor sandwiched between two electrodes. An incident beam of photons causes production of electron-hole pairs which when separated produce a voltage related to the photon flux. Phototubes are generally more sensitive and have a greater wavelength range. Photocells are in general simpler, cheaper and more rugged. Photocells do not require external power supplies

13-13) disadvantages and advantages g)spectrophotometers and photometers

A single-beam spectrophotometer employs one beam of radiation that irradiates one cell. To obtain the absorbance, the reference cell is replaced with the sample cell containing the analyte. With a double-beam instrument, the reference cell and sample cell are irradiated simultaneously or nearly so. Double-beam instruments have the advantages that fluctuations in source intensity are cancelled as is drift in electronic components. The double-beam instrument is readily adapted for spectral scanning. Single-beam instruments have the advantages of simplicity and lower cost. Computerized versions are useful for spectral scanning.

13-13)advantages and disadvantages e) double beam in space and double beam in time spectrophotometers

Both types of spectrophotometers split the beam into two portions. One travels through the reference cell and one through the sample cell. With the double-beam-inspace arrangement, both beams travel at the same time through the two cells. They then strike two separate photodetectors where the signals are processed to produce the absorbance. With the double-beam-in-time arrangement, the two beams travel at different times through the cells. They are later recombined to strike one photodetector at different times. The double-beam-in-time arrangement is a little more complicated mechanically and electronically, but uses one photodetector. The double-beam-in-space arrangement is simpler, but requires two matched photodetectors.

13-5) A solution containing 7.35ppm KMnO4 had a transmitance of 0.145 in a 1.00 cm cell at 520nm. Calculate the molar absorptivity of KMnO4 at 520nm. 13-7) A solution containing the complex formed between Bi(III) and thiourea has a molar absorptivity of 9.32 x 10^3 L/mol *cm at 470nm a)what is the absorbance of 4.25 x 10^-5 M solution of the complex at 470nm in a 1.00 cm cell? b) what is the percent transmittance of the solution described in (a) c)what is the molar concentration of the complex in a solution that has the absorbance described in (A) when measured at 470nm in a 2.50cm cell? 13-9) a 2.50mL aliquot of a solution that contains 6.4ppm iron(III) is treated with an appropriate excess of KSCN to form the Fe(SCN)2+ complex diluted to 50,0mL. What is the absorbance of the resulting solution at 580nm in a 2.50cm cell?

Convert C into molarity A = -log (T) Molar absorptivity e= absorbance/b (path length) c(concentration)

13-13) Disadvantages and advantages b) filters and monochromators as wavelength selectors

Filters provide low resolution wavelength selection often suitable for quantitative work, but not for qualitative analysis or structural studies. Monochromators produce high resolution (narrow bandwidths) for both qualitative and quantitative work.

13-13) disadvantages and advantages a) hydrogen and deuterium discharge lamps as sources for UV radiation

Hydrogen and deuterium lamps differ only in the gases that are used in the discharge. Deuterium lamps generally produce higher intensity radiation.

13-13) advantages and disadvantages h) conventional and multichannel spectrophotometers

Multichannel spectrophotometers detect the entire spectral range essentially simultaneously and can produce an entire spectrum in one second or less. They do not use mechanical means to obtain a spectrum. Conventional spectrophotometers use mechanical methods (rotation of a grating) to scan the spectrum. An entire spectrum requires several minutes to procure. Multichannel instruments have the advantage of speed and long-term reliability. Conventional spectrophotometers can be of higher resolution and have lower stray light characteristics.

Instrumental Design: Double Beam In Space Instruments

Simultaneous measurment of the reference and sample with two match detectors. -The ration of the two outputs are determined

13-13)advantages and disadvantages f) spectrophotometers and photometers

Spectrophotometers have monochromators or spectrographs for wavelength selection. Photometers generally have filters and may use an LED source for wavelength selection. The spectrophotometer can be used for wavelength scanning or for multiple wavelength selection. The photometer is restricted to one or a few wavelengths.

Measurement of Transmittance and Absorbance

To compensate attenuation effects, the transmittance of the analyte solution is compared with that of the solvent in an identical cell.

Instrument Component - D2 and H2 Lamps

To measure absorption spectrum, the UV-Vis spectrometer requires contiuum source whose radiant power does not sharply change over a considerable range of wavelengths -UV region (190-400nm) -electrical discharge lamp -formation of excited molecular deuterium followed by dissociation into atomic deuterium produces various wavelengths of UV range

He said this in class : What is molar absorption a fucntion of???

Wavelength