Instrumental Analysis Exam 2

15. Discuss two fluorometric methods for the determination of inorganic species

*Direct method: formation of a fluorescing chelate and the measurement of its emission. The complex fluoresces. *Indirect method: Diminution of fluorescence resulting from the quenching action of the substance being determined. Have a species that fluoresces with something that quenches fluorescence. You look at the decrease in fluorescence.

6 Why is 70 eV commonly used for EI sources?

70 eV is a good compromise. It causes fragmentation but it won't destroy it. Lower energies won't ionized, and higher energies will completely destroy it.

e. fluorescence lifetime

: Time that passes from excitation to emission, varies within different species, can be used to distinguish species.

b. self absorption

: cause of nonlinearity, wavelength of emission overlaps an absorption peak, excited photon may be absorbed by another before it exits the cell

f. heavy atom effect

: intersystem crossing(spin of electron is reversed) is most common in molecules that contain heavy atoms such as iodine or bromine. The presence of heavy atoms makes a change in spin more favorable

2. What are the advantages of a Fourier transform infared spectrometer compared to a dispersive instrument?

A Fourier transform infared spectrometer is faster because all wavelengths can be done at one time. A dispersive instrument can scan only 1 wavelength at a time. A fourier has only 1 detector and doesn't need a monochrometer. More sencitive and quicker

Chemiluminescence

Chemiluminescence is a process by which radiation is produced as a result of a chemical reaction.

Dissociation

Dissociation occurs when radiation promotes a molecule directly to a state with sufficient vibrational energy for a bond to break.

Photons from the infrared region of the spectrum do what?

Do not have enough energy to cause photoemission from the cathode of a photomultiplier tube

Intersystem crossing

Intersystem crossing is the process in which a molecule in one spin state changes to another spin state with nearly the same total energy (e.g., singlet → triplet).

Iodine does what?

Prolongs the life of the lamp and permits it to operate at a higher temperature. The iodine combines with the gaseous tungsten that sublimes from the filament and causes the metal to be redeposited, thus adding to the life of the lamp.

Which analysis requires narrow slit widths and why?

Qualitative analysis so that fine structure in the spectrum will be resolved.

Quantum yield

Quantum yield is the fraction of excited molecules undergoing the process of interest. For example, the quantum yield of fluorescence is the fraction of molecules which have absorbed radiation that fluoresce.

Quantitative analysis can usually tolerate what?

Rather wide slits because measurements are often made on an absorbtion maximum where there is little change in the absorptivity over the bandwidth. Wide slit widths are desirable because the radiant powers will be larger and the signal to noise ration will be higher.

What affect does conjugation have on UV/Vis spectra and why?

Conjugations cause an increase in the wavelength max. Conjugation is more stable so shifts to longer wavelengths. It also causes the molar absorptivity to double compared to only single bonds because both double bonds absorb.

18. Why is chemluminescence such a sensitive technique?

There is a very low noise level because there is no light source. Excitation is from the chemical reaction. Every single photon hitting the detector is from the reaction.

Draw block diagrams identifying each of the major components of the following types of instruments used for optical spectroscopy:

a) Emission b) Absorption c) Fluorescence d) Single-beam e) Double-beam in Space f) Double-beam in time

d. emission spectrum

fluorescence and phosphorescence spectra involve excitation at a fixed wavelength while recording the emission intensity as a function of wavelength. Shows wavelengths that are emitted by the sample To collect keep the excitation monochrometer steady at a wavelength that causes excitation. Vary the emission monochrometer at a higher wavelength(less energy).

Resonance fluorescence

is observed when an excited species emits radiation of the same frequency at used to cause the excitation.

Fluorescence

is the process in which a molecule, excited by the absorption of radiation emits a photon while undergoing a transition from an excited singlet electronic state to a lower state of the same spin multiplicity (e.g. a singlet -- singlet transition)

What is a photomultiplier?

A vacuum tube consisting of a photoemissive cathode, a series of intermediate electrodes called dynodes, and a collection anode.

7 List the various components of electron impact source and discuss each in detail.

*Filament: made of Re, source of 70 eV electrons. It creates the ion *Target: anode used in association with the filament to produce electrons. Ion travels toward *Repeller: + charge electrode used to push positive ions out of the ionization source (same charge as ion) *Lens stack: series of increasingly more negative electrodes used to accelerater ions to constant kinetic energy. Accelerate and push toward detector. Need all to have same amount of kinetic energy so can determine mass charge. Small will hit before the heavier.

3. What type of detectors are used in IR spectroscopy? (sensitive thermometer)

*Thermal transducer: heat capacity of absorbing element must be low, thermal noise is a major issue. Cooling the detector decreases noise. The beam is chopped. *Thermocouples: 2 pieces of metal fused to each end of a dissimilar metal. Dissimilar(difference in electron transfer ability makes it more sensitive) Results in a current from one to the other *Bolometer: 2 thin strips of metal, resistance changes with temp. *Pyroelectric transducer: crystal between 2 electrodes. 1 electrode is IR transparent. The crystal is heated and the charge changes, charge detected as a current. More sensitive than thermocoupler. Common for FTIR *Photoconducting transducer: film of semiconductor material on nonconducting surface. Decreases the electrical resistance. Measure change in resistance. Hg/Cd/Te offer superior response time (need to be cooled with liquid nitrogen)

What variables influence absorbance?

*Variables that affect absorbance pH-atom or species respond differently at different pH Nature of solvent: is it polar/nonpolar, cause red/blue shift Temperature: can change equilibrium Electrolyte concentration: more electrolyte may cause interaction with analyte Presence of interfering species

What are some of the factors that lead to deviations in Beer's Law and explain why?

-Chemical interactions: another species is interfering: example: oxidation reaction, an analyte dissociates, associates, or reacts with a solvent to produce another product with a different absorption spectrum than the analyte. -Stray radiation: radiation from the instrument that is outside the nominal wavelength ban chosen for the determination (from gratings, lenses, mirrors, filters, windows, etc.) the radiation may not pass through the sample and most likely differs from the chosen radiation. This is a big problem at wavelength extremes where the sample doesn't absorb well or the detector doesn't detect well. - non-monochromatic radiation (polychromatic): the relationship between absorption and concentration may not be linear with the polychromatic light -High concentration: the particles are too close, self absorption may occur, dipole moments may occur changing the distribution of electrons so the molecule absorbs light differently.

Describe the difference in spectra observed in the gas, polar solvent, and nonpolar solvent. Why are the spectra different?

-Gas-phase: Are used for volatile compounds. Can be obtained by allowing a drop or two of the pure liquid to evaporate and equilibrate with the atmosphere in a stoppered cuvette. -Polar solvents: like water, alcohols, esters, and ketones tend to obliterate spectral fine structure arising from vibrational effects. -Nonpolar Solvent: like hydrocarbons, are similar to gas-phase, The electronic transitions can be observed but the vibrational and rotational structure have been lost.

4. What effect does temperature, concentration, and solvent viscosity have on fluorescence and why?

-Temperature: Lower temperatures decrease the movement of the molecules. This decreases collisions of the molecules. Lower temperature increases the probability of fluorescence. Higher temperatures allow movement of the molecules or collision of molecules. Higher temperatures decrease the quantum efficiency of fluorescence -Concentration: at low concentrations fluorescence is linear but is starts to lose linearity at high concentrations. Linearity is lost due to self quenching and self absorption which are greater at higher concentrations. -Solvent viscosity: high viscosity decreases the movement of molecules which decrease the collision of the molecules. Therefore higher solvent viscosity promotes fluorescence. Low viscosity solvents increase the number of collisions which decreases the probability of fluorescence. Temperature and viscosity are pretty much the same

What does a single-beam spectrophotometer do?

A single-beam spectrophotometer employs one beam of radiation that irradiates one cell.

singlet state

A singlet state is one in which the spins of the electrons of an atom or molecule are all paired so there is no net spin angular momentum

The emission spectrum is what?

A spectral continuum

A triplet state

A triplet state is one in which the spins of the electrons of an atom or molecule are unpaired so that their spin angular moments add to give a net non-zero moment.

What factors lead to the absorption of UV/Vis radiation by inorganic molecules?

Absorption in inorganic molecules involves transitions between filled and unfilled d/f orbitals (splitting of d and f orbital).

2 Discuss the advantages and disadvantages of chemical ionization sources.

Advantage: gives a high response for molecular ion, can get molecular weight of species, it can be more sensitive Disadvantage: Can't use for fragmentation pattern, not reproducible so can't compare with library record. The line production is dependent on the reagent and sample reagent pressure which both vary too much. You lose information that could be obtained from the fragmentation.

1 Discuss the advantages and disadvantages of electron impact sources.

Advantages: Convenient/easy, high currents resulting in good sensitivity, reproducible fragmentation pattern allows identification of analyte. Very reproducible, good for qualitative, can compare to a library frag pattern. Disadvantages: Extensive fragmentation may cause loss of molecular ion peak, limited to easily "volatized" samples if it can't go into the gas phase then you cant use electron impact, limited to analytes with molecular weights less than 10^3 daltons (1000 amu). If the molecular weight gets too high it is no longer volatile

2. Explain the similarities and differences between molecular fluorescence, phosphorescence and chemluminescence.

All of them are the emission of radiation -Fluorescence: a pathway by which an excited atom/molecule relaxes to its ground state, emission of radiant energy, excitation brought on by absorption of a photon. (Singlet to singlet) -Phosphorescence: similar to fluorescence except has a sig. longer lifetime. Involves change in electron spin (singlet to triplet) -the emission of energy as electromagnetic radiation during a chemical reaction, excited species is formed during the course of a chemical reaction

What is an auxochrome and what affect does it have on UV/Vis spectra and why?

An auxochrome is a functional group that does not absorb UV/Vis but shifts peak to longer wavelengths (Red Shift) and increases intensity.

8 What is the detector for a GC/MS?

An electron multiplier NOT a mass analyzer or mass spec

Molecules have vibrational and rotational states along with the electronic states but what do atoms have?

Atoms do not have the vibrational and rotational states. The spectra exhibits some strong individual peaks.

What is the best spectrum?

Depends on what you want. A narrow slit width is necessary for more spectral detail. If quantitation data is desired a broader slit width should be used.

5 How do the spectra for Electron Impact (EI) and Chemical Ionization (CI) sources differ from one another?

EI spectra: no molecular ion present, lots of fragmentation, reproducible fragmentation pattern. CI spectra: molecular ion peak, less fragmentation, also reduced amount of structural info, not reproducible

External conversion

External conversion is a radiationless process in which a molecule loses electronic energy while transferring that energy to the solvent or another solute.

14. Discuss the effect of concentration on fluorescence intensity.

Fluorescence is also related to absorbance, at low concentrations is linear, but as concentrations rises it begins to lose linearity. Higher concentration causes self-quenching admits a photon that is admitted by and self-absorption energy is absorbed by a photon which case a loss of linearity

19. How are fluorescence and phosphorescence different in terms of intensity, lifetime, and energy?

Fluorescence is higher energy, lower intensity, and a shorter lifetime. Phosphorescence is lower energy, higher intensity, and longer lifetime

Internal conversion

Internal conversion is the intermolecular process in which a molecule crosses to a lower electronic state with emitting radiation.

17. Why are fluorescence methods more sensitive and selective than absorption techniques?

In absorption your looking at everything coming in, if only a few absorb you have very high background noise. Multiple species in the solution may absorb. In fluorescence you are not seeing the source light. You are looking into the darkness for a few photons. There is no source interference. Can go much lower and stay linear leading to single photon detection. Can look at multiple species in a solution and only one may fluoresce. It is very selective to that one species. Only a few compounds will fluoresce: selective

What affects does stray (scattered) radiation have on spectra and explain why?

It can lead to deviation from the linear relationship between path length and absorbance. It can also lead to negative absorbance errors. Stray light limits the absorbance.

9. Discuss the advantages and disadvantages of molecular luminescence over absorption techniques.

Molecular luminescence has higher sensitivity(can go to much lower concentrations), large linear concentration range, more specificity, there is also less background noise. The disadvantage is that with the higher specificity only a few compounds can be measured via molecular luminescence and is subject to serious interference effects from the matrix at lower concentrations. Molecular luminescence is less applicable since fewer species fluoresce.

7. Describe some of the structural features of a molecule that affect fluorescence.

Most unsubstituted aromatic compounds fluoresce in solution. Efficiency of fluorescence increases with the number of rings and their condensation. Fused ring structures also exhibit fluorescence. Substitutions on the benzene ring cause shifts in wavelength of fluorescence and also the efficiency of fluorescence. The greater the number of resonance structures the greater the probability of fluorescence. Rigidity increases the probability of fluorescence. Rigidity and the number of resonance structures increase the probability of fluorescence. Metero atoms(non carbon and hydrogen) change the fluorescence.

What factors led to the absorption of UV/Vis radiation by organic molecules?

Organic molecules absorption deals with the valence electrons that can be excited to higher energy levels. Single bonds: are not in the VAC-UV region because the energy associated with the electrons is so high. (n to sigma*) This is typically not performed because of the difficulty with the VAC-UV. Double bonds: (n/pi to pi*) the energy required (200-700) nm is with the UV-Vis region. Chromophores (unsaturated functional groups as pi group) are required.

Phosphorescence

Phosphorescence is the process in which a molecule, excited by the absorption of radiation, emits a photon while undergoing a transition from an excited triplet state to a lower state of a different spin multiplicity (e.g., a triplet → singlet transition).

What affect does solvent polarity have on UV/Vis spectra and why?

Polar solvent: cause blue shift (shift to shorter wavelengths) in n to pi*/sigma (higher energy) a polar solvent causes a red shift in pi to pi* because attractive forces between the solvent and the absorber lower the energy levels of the unexcited and excited states. It lowers the energy of the excited state by more which decreases the energy gap. This results in a red shift (longer wavelengths) as polarity decreases red shift

Predissociation

Predissociation occurs when a molecule changes from a higher electronic state to an upper vibrational level of a lower electronic state in which the vibrational energy is great enough to rupture the bond.

3. Compare resonance fluorescence to Stokes shift.

Resonance fluorescence is typical of atomic species but rarely occurs in molecular species. In resonance fluorescence the same photons that are absorbed are emitted (same energy) Molecules rarely undergo resonance fluorescence because the lose energy with the molecules in vib/rot states or transfer to other areas of the molecule Stokes shift if typical of molecular species. The energy emitted is less than that absorbed because some energy is lost within the molecule.

What factors must be considered in the development of conditions that would yield reproducible (preferably linear) results between absorbance and analyte concentration?

Slide 39 *Selection of the wavelength: a wavelength corresponding to an absorption peak, a flat region, high sensitivity at wavelength max. *Variables that affect absorbance: pH-atom or species responds differently at different pH Nature of solvent: is it polar/not polar, cause red/blue shift Temperature: can change equilibrium Electrolyte concentration: more electrolyte may cause interaction with analyte Presence of interfering species going from ionized to the non-ionized form *Clean/matched cells/tubes *Calibration curve (don't use literature molar absorptivity) Solvent choice: don't use a wavelength lower than solvent limit or it will absorb If changes the distribution of electrons it will change how it interacts with the light.

Be able to match various types of sources, detectors, and optical materials with the appropriate wavelengths ranges they are most commonly used for.

Sources: -Deuterium/Hydrogen Lamps: UV region, output range of 160-800 nm, in UV region (190-400 nm) a continuum spectrum exist above 400 nm the spectra consists of lines and bands - Tungsten Filament Lamps: Visible and near-infrared, energy is similar to blackbody radiation so is dependent on temperature, useful for wavelength region 350-2500 nm - Light Emitting Diode: 375-1000 nm, white leds 400-800 nm -Xenon Arc Lamps: 2000-1000 nm, peak intensity around 500 nm

Describe that process in detail and explain the differences in this process between atomic and molecular species.

Step 1. electron excitation, a collision has to occur and the differences in this process between atomic and molecular species. Step 2. Relaxation, the energy excitation must be lost. this could be done through heat, formation of new species, fluorescence, or phosphorescence, etc

8. Discuss the various factors that affect fluorescence and phosphorescence

Structure: unsubstituted aromatic compounds increase fluorescence with the increasing number of rings. Double bonds increase fluorescence. The number of resonance structures increase fluorescence. Increased rigidity increases fluorescence. pH: Temperature: Low temperature results in a higher probability of fluorescene. At higher temperatures there are a greater number of collisions. Solvent: Increased solvent viscosity increases probability of fluorescence by limiting the number of collisions. Heavy atoms: Decrease fluorescence due to increase in the rate of triplet formations. This same reason causes an increase in phosphorescence. Dissolved Oxygen: will interfere with fluorescence may cause intersystem crossing and conversion to triplet state. Get rid of oxygen to increase fluorescence. Concentration: High concentration will cause a loss of linearity in fluorescence due to self quenching and self absorption.

What are the differences, advantages, and disadvantages between Single-Beam and Double-Beam Instruments?

The advantage of double-beam instruments is that they compensate for all but the most short term fluctuations in the radiant output of the source as well as the drift in the transducer and amplifier. They also compensate for wide variations in source intensity with wavelength. Double-beam design is good for continuous recording of transmittance or absorbance spectra. Double-beam instruments involve the splitting of beam. the double-beam in time is preffered to double-beam in space because the difficulty in matching the two detectors needed for the double beam in space. The double-beam is less sensititve because of the splitting of light. The double beam in time the light spends part of the time in the sample and part of the time in the blank. In the double beam in space the sample and blank recieve only 50% of the light. The single beam has the potential to be the more sensitive instrument, but usually it is cheaper material. The disadvantage of the single beam is that it takes more time. You measure the sample and the black at different times. You can't compensate for fluctuation. The single beam is more sensitive because your not splitting the light.

3 Describe the purpose of the quadrupole mass spectrometer and how it works.

The analyzer consists of four rods. The rods operate in pairs each carrying a voltage. Ions of the proper M/Z value successfully traverse the entire filter. High pass rods filter out ions with too low of an M/Z. The low pass rods filter out ions with too high of an M/Z. At a set of conditions only ions of a specific M/Z successfully travel through, the rest are drawn into the rods. Mass to charge ratio determines stable trajectory.

Why have single bond spectra of organic compounds not been widely exploited for analytical purposes when studying absorption?

The excitation of energy associated with electrons forming single bonds is sufficiently high so absorption must occur in the vacuum-uv region. These transitions involve the excitation of nonbonding n electrons to sigma* orbitals. The molar absorptivities of this transition is low to intermediate. This is typically not done due to the experimental difficulty vacuum-uv region. 1. Energy is too high leading to bond rupture. 2. Have to work in a vacuum to get rid of atmospheric compenents.

What is the difference between the absorption spectra of inner transition elements (the lanthanide and actinide series) to transition metals? What molecular process is responsible for the absorption of UV/Vis radiation for these elements/compounds? Which ones are more sensitive to environmental perturbations and why?

The inner transition elements (lanthanides) produce very narrow well defined and characteristic peaks. If ligands are attached they have very little effect. The inner transition elements deal with the inner electrons like the 4f and 5f which are shielded or screened from external influences. Transition metals absorb at one if not all of their oxidation states. The absorption peaks are broad and strongly influenced by chimerical environmental factors. The d and f orbitals are not as shielded. Ligands have drastic shift on the spectrum. The stronger the ligand the more the spectrum will shift (to a shorter wavelength). there is a greater effect in transition metals. Inorganic absorb light because of the splitting of the d and f orbitals once there split then absorbance can occur. the splitting of the orbitals from a lower energy to a higher energy orbital. The transition metals are more subject to deal with environmental perturbations because electrons of the d orbital are farther from the nucleus. d is more sensitive.

What happens in a deuterium lamp?

The lamp energy from the power source produces an excited deuterium molecule that dissociates into two atoms in the ground state and a photon and the energies of the two atoms. The latter can vary from nearly zero to the energy of the excited molecule. Therefore the energy of the radiation, which is the difference between the quantized energy of the excited molecule and the kinetic energies of the atoms, can also vary continuously over the same range.

Why are electrons in double or triple bonds more easily excited by electromagnetic radiation compared to single bonds?

The single bonds require sigma to sigma* transitions, this is a very large energy gap. A double/triple bond only requires a pi to pi* transition, this is a much lower energy gap.

Why is fluorescence seldom observed from * transitions?

The transition is too energetic. If occurs it often causes bond ruptures

The absorption of UV or visible light by an atomic or molecular species can be considered a how many step process?

Two step process

1. Compare the purposes of UV/Vis and IR absorption spectroscopy.

UV/Vis- quantitative, how much IR- qualitative, functional groups. IR- organic material absorbs IR because of the covalent bonds. Ionic materials do not have covalent bonds so they do not absorb IR. Salts are typically used.

13. Explain the relationship between vibrational relaxation and fluorescence.

Vibrational relaxation is a nonradiative form of relaxation, while fluorescence is radiative relaxation. There is no emission of a photon in vibrational relaxation, it is relaxation to a lower vibrational state. This vibrational energy loss always occurs first and very fast compared to the emission of a photon. Due to this fluorescence always occurs from the excited ground state.

Vibrational relaxation

Vibrational relaxation is the process by which a molecule loses its excess vibrational energy without emitting radiation.

a. self quenching

collision between excited molecules. Ex; benzene collides with benzene causing a transfer of energy

Be able to draw a theoretical mass spectra for a simple compound such as water, carbon dioxide methane, etc. and discuss how this spectra was generated in the mass spectrometer and discuss each of the peaks in your theoretical spectra.

draw it!

c. excitation spectrum

obtained by measuring luminescence intensity at a fixed wavelength while the excitation wavelength is varied. Is often identical to an absorption spectra taken under the same conditions. Shows which wavelengths absorbed actually cause excitation. Is a subset of the absorption spectra. Not every wavelength absorbed causes excitation To collect keep the emission monochrometer steady at a known fluorescence wavelength. Vary the excitation monochrometer at a shorter wavelength (higher energy)