Instrumental Analysis Exam 2

• UV-Vis is defined by the wavelength of light used to measure the absorbance of a sample. Qualification is determined by Beer's Law and characterization by the functional groups involved.
• Molecular absorption spectroscopy is based on the measure of transmittance or the absorbance of a specific wavelength. This is represented by Beer's law.

  $A=$$-logT=$$log(Po/P)=$$Ebc$
• Absorbance is proportional to concentration. A or T is not determined by a single parameter and high concentration does not make A automatically high
• Single beam UV-Vis set-up

  • filter or monochromator - light selector vs pure light
  • sample cell - determines the character
  • references cell - the blank
  • Po - filtered light; very difficult to remain at the same intensity throughout
• The double beam method has better accuracy than the single beam because it eliminates errors due to fluctuations in the source intensity
• The differences between a single slot and a double beam spectrometer is that the double beam contains a sector mirror which removed the major sources of noise. This is controlled by the motor. The other different is in the photodetector.
• UV-Vis light source, D2 or H2 lamps are kept at lower pressure with a transparent sample holder. The photon goes from the ground state to the excited state then to a higher state emmitting a photon. The photon energy is always changing
• The limitations of Beer's law are that it does not take into account the concentration of the reactants and the absorptivity depends on the refractive index of the medium.
• Polychromatic radiation is the emission of light by a black body at all wavelengths and is a main source of continuous radiation.
• Stray radiation in UV-Vis is commonly from scattering and reflection off surfaces such as grates, lenses, and mirrors. This causes significant deviation from the linear relationship. The band would be represented earlier than it should in an A vs. wavelength graph
• In most cases, the noise related to UV-Vis is thermal noise. There is difficulties differentiating which is light and which is noise causing a spike to occur in the graph.
• When the absorbance is high, there is less light to absorb so there is no additional electron flow
• The slit width is very important. The smaller slit will develop more narrow peaks. As the slit width increase, the character of the peaks will be removed and the peaks will no longer be present aka a loss of resolution
• The relative magnitude from the graph can be determined by the highest peak divided by the next peak over. For example, the max peak is 63.4 which is divided by the second peak 23.4, yielding 3.
• False peaks appear from scattered radiation at extreme wavelengths. This is normally seen at smaller wavelengths despite being caused by higher ones.
• When producing a UV-Vis spectrum, the phase of the compound is very important. If the solution is in the gaseous phase, peaks will form since the compound is able to transition freely (electrically, vibrationally, and rotationally). In a nonpolar solution, the electronic structure is the only one remaining which produces less peaks with minimal resolution. In a polar solution, all characteristic is lost
• The most successful transition between energy states is that of alkenes and alkynes. This is the transition from pi to pi*. This is a very small gap
• For organic species, higher conjugation leads to shifting the absorbance to longer wavelengths and red-shifting.
• For inorganic species, they are less absorbing and can experience transitions between filled and unfilled d orbitals. This normally characterized by group 1 and 2 elements and generates wavelengths within the visible spectrum. Cr2O7^2- is the most efficient at picking up electrons.
• Charge transfer absorption occurs because molar absorbtivities are really high which causes high sensitivity. This occurs in rare elements where one is electron donating and another is electron gaining. There is intense absorption because metals are already coordinated molecularly and they have the tendency to lose electrons.
• Beer's law for mixtures is the addition of all of the absorbances or the addition of all of the wavelengths. The wavelengths of the substances need to be as different as possible. The resulting peak will be a combination of the substances.
• luminescence is the emission of light by an object that has been exposed to a source of energy. There are three types of luminescence which are photofluorescence, phosphorescence, and chemiluminescence.
• Luminescence is released from a sample following incident radiation by chemical reactions. luminescence intensity is proportional to the number of photons emitted.
• In comparison to UV-Vis, molecular luminescence is more sensitive and can be used to measure small amounts of molecules. The instrumentation is arranged in an angled configuration which prevents radiation from reaching the detector. The signal that is produced is based on the photocurrents and the photons rather than the absorbance. Finally chemiluminescence is highly sensitive
• For photoluminescence, the source sends incoming light into the spectrum. The incoming light is not uniform and needs to be quantified and filtered to calibrate. This is why there is a wavelength selector before reaching the sample. This equipment falls into the category of a fluorometer and a spectrofluorometer.
• For photoluminescence, the spectrofluorometer contains a lot of parts. the light will be sent into the system and experience grating before being diluted for the sample. After reaching the sample, the beams will be split by a beam splitted before experiencing grating again at the excitation monochromator.
• Electron spin can be described by quantum mechanics as s + or - 1/2. U spin is + 1/2 and down spin is the inverse. This is important for determining singlet, doublet, and triplet excited states
• As electrons spin between energy levels photoexcitation occurs. A ground state singlet can be described as two opposite facing arrows on the same level. An excited singlet is opposite arrows on different levels. This is antiparallel which represents emission to fluorescence. An excited triplet state is shown as two arrows pointed upwards on different levels. This is parallel which represents emission to photofluorescence. There will be a greater energy for an excited state singlet.
• The efficiency can be determined by the ratio of the rate constant that is being solved for over the sum of all of the rate constants.

  • kf - rate constant for fluorescence
  • kic - RC for internal conversion
  • kec - RC for external conversion
  • kisc - RC for intersystem crossing
  • kdis - RC for photo-dissociation
• The rate constant of external conversion can act as an external to quench the rate of internal conversion
• Photo-dissociation is when the electrons fall apart or decompose and no longer yield the starting materials.
• There are 4 different types of fluroescence spectra

  1. UV-Vis absorption spectrum
  2. Fluorescence excitation spectrum
  3. Fluorescence emission spectrum
  4. Some luminophores as well but not as produced
• In terms of fluorescence, the transition from the ground to the excited state allows light to be absorbed developing the UV-Vis spectrum
• The fluorescence excitation and emission spectra are developed while holding a single parameter constant. The excitation spectrum often resembles the absorption spectrum in its general features. They are not interchangeable with UV-Vis because they are much more sensitive and do not provide identical information.
• Fluroescene emission spectrum is often on the long-wavelength side of fluorescence excitation spectrum. This is because of red shift/ stokes shift. This is the shift from the original emission spectrum to the fluorescence spectrum.
• Supposedly emission and fluorescence are the same but there is some energy lost between excitation and emission. when the photon is going up, there is only one thing acting on it as it comes down which is the solvent.
• Simple heterocyclic compounds do not produce fluorescence. This is because they often reside in the n orbital. As they shift from the pi orbital to the n orbital, they skip the pi* orbital which produces the fluorescence
• Fused aromatic rings and unsubstituted aromatic hydrocarbons both produce fluorescence. the more structurally rigid they are, this normally helps generate better fluorescence.
• By secondary absorption, higher concentrations within the inner filters will produce a lower fluorescence.
• With phosphorescence, this occurs at longer wavelengths causing it to not be visible to the naked eye. THis occurs at a much longer timescale which means the longer that the system waits, the higher possibility to lose energy and develop a triplet. Triplets require the loss of energy.