IA test 2
7. Describe some of the structural features of a molecule that affect fluorescence
Aromatic rings, resonance structures, and structure ridgidity (decreases the amount of vibration rotation states) which increases the probabilrty to fluoresce. Hetero atoms will decrease the probability of fluorescence.
7. Why have single bond spectra of organic compounds not been widely exploited for analytical purposes when studying absorption
Because they can easily be ruptured under vacuum uv and are rarely absorbed. Too much energy.
3 Describe the purpose of the quadrupole mass spectrometer and how it works
Filters ions by mass to charge ratio Contains two poles, x axis poles filters ions w/ too high mass to charge ratio and the two y axis poles filters ions w/ too low mass to charge ratio. Only ions with proper mass to charge ratio values can go through successfully.
16. Why are fluorescence methods more sensitive and selective than absorption techniques
Fluorescence has decreased background noise, due to it only measuring photons emitted by the sample which enhances the signal to noise ratio Its selective because many molecules will be absorbed but only very few will fluoresce.
4. What effect does temperature, concentration, and solvent viscosity have on fluorescence and why
High viscosity and low temp: lower collision and more fluorescence Low viscosity ang high temp: high collision and less fluorescence Concentration: Has to be high but not too high because it can cause self-absorption. Self absorption- a molecule that absorbs a photon emitted by another molecule Self quenching- molecules collide with another molecule and transfers energy.
9 Be able to describe how an ion trap mass spectrometer works
Ionization source turns on and off - Sample gets ionized fully at fixed intervals in the trap - Ions get trapped because of the positive potential of all of the electrodes - By reducing positive potential of the bottom electrode, ions fall out of the trap and be detected. - Based on the size of the ion pulse, adjust size of the second ionization - Acquire spectrum
Stray radiation
Light that does not pass through the sample but strikes the detector.
Molecular orbital theory
a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms.
Ligand
a molecule that binds to another molecule.
Emission spectra
fix excitation, vary emission
Heavy atom effect
increases the lifetime and causes phosphorescence
Ligand field theory
ligand attaches to a molecule that absorbs at a certain wavelength and causes splitting of D orbitals.
UV Cutoff
range of wavelength where the solvent also absorbs light. You want to use a solvent that absorbs at a lower wavelength than what your solute needs.
Antibonding orbitals
sigma* and pi*
Ground state
lowest energy state of an atom
Transition metals
metals that are in between the main 8 columns. Absorption peaks are broad and strongly influenced by chemical environmental factors.
Pauli exclusion principle
no two electrons in an atom can have the same set of four quantum numbers. No more than two electrons can occupy the same orbital and those two must have opposite spins.
Stokes shift
the emitted radiation has a lower frequency or wavelength, than the radiation that exited the fluorescence. Shift in wavelength
Self absorption
wavelength of emission overlaps an absorption peak
15. What is an auxochrome and what affect does it have on UV/Vis spectra and why
A funstional group that does not absorb uv vis red shifts peaks to longer wavelengths and increases intensity.
1 Discuss the advantages and disadvantages of electron impact sources
Advantages- better chance in finding out what the sample is. Convenient and easy to use, provides high currents so high sensitivity. Disadvantages- a lot of fragmentation cannot distinguish molecular weight. Limited samples that easily volatile. Extensive fragmentation means that the molecular ion peak is easily lost.
9. Discuss the advantages and disadvantages of molecular luminescence over absorption techniques
Advantages- higher sensitivity, large linear concentration ranges, more specificity Disadvantages- quatitative methods can be subject to interference effects from the matrix since it has a higher sensitivity. Since its highly specific only few molecules work with it.
8 What is the detector for a GC/MS
An electron multiplier tube (made up of electro emissive surface) because an ion hits the surface, electrons get kicked off.
15. Why is it not necessary to have a wavelength selector when performing a chemluminescence experiment
Because the only source of radiation is between the analyte and the reagent.
2 Discuss the advantages and disadvantages of chemical ionization sources
Can find the molecular weight of the species, less fragmentation. Also gets better detyection limit because it has less info. Not reproducible. Disadvenatges- harderr to find out what the sample is
6 Why is 70 eV commonly used for EI sources
Used since 70 ev works best with volatile gases
5. What is the main advantage of charge-transfer absorption over other types of absorption phenomena
Charge transfer leads to higher molar absorptivity and makes it much easier to detect at lower concentrations. AA charge on a part of a molecule moves to another part of the molecule
18. What is a chromophore and what effect do multiple choromophores have on spectra
Chromophore is part of a molecule that absorbs photons. The more chromosphore the better molar absorbtivit it has
14. Discuss two fluorometric methods for the determination of inorganic species
Direct method- the formation of a fluorescing chelate (complex) and the measurement of its emission Indirect method- the diminution of fluorescence resulting from the quenching action of the substance being determined.
8. What are the differences, advantages, and disadvantages between Single-Beam and Double-Beam-Instruments
Double beam instruments has two beam of lights(qualitative), one passing through the sample and one through a reference sample (blank). Single beam is quantitative. Double beam instruments are faster and produce better results while single beem are less expensive, slow and only get quantitative results. Double beam better for collecting spectra. Double beam in space has two detector, while double beam in time only has one.
3. Compare resonance fluorescence to Stokes shift
Emitting at the same wavelength as absorbed and is emitting at a longer wavelength Stokes shift- gap between the absorption and emission band. Occurs between molecular species Resonance fluorescence- same amount of energy absorbed, is the same amount off energy emitted. Happens between atom species.
10. What factors lead to the absorption of UV/Vis radiation by organic molecules
Excitation of bonding electrons like pi to pi* or n to sigma*
7 List the various components of electron impact source and discuss each in detail
Filament- source of 70 eV electrons Target- Repeller- positively charged, repels analyte towards lens stack Lens stack-
1. How can nonabsorbing analytes be determined photometrically and what are some of the requirements for successful application
For a compound that doesn't fluoresce, you must make a fluorescent complex by attaching a chelate. This reagent must be in excess in order to force completion of the absorbing complex. Successful application usually requires the color forming complex be forced to near completion
16 What are the uses of a mass spec
For quantitative purpose, provides molecular weight of a sample, forms ions to study ,ass to charge ratio
2. What are the advantages of a Fourier transform infrared spectrometer compared to a dispersive instrument
Fourier gives a better signal noise and ccollects spectra quickly and better wavelength accuracy
2. What are some of the factors that can lead to deviations in Beer's Law and explain why
High concentrations- particles are too close and cause self-quenching and self-absorption Chemical interactions- analyte associates, dissociates, or reacts with the solvent to produce a product with different absorption characteristics than an analyte Non-monochromatic radiation- anything that radiates that is not the analyte Stray light- light that does not pass through the sample but strikes the detector.
13 What is meant by the term "base peak" in when referring to a mass spectrum
Highest level of absorptivity and highest abundance, and everything is in reference to it. Most abundant, highest peak.
4. Compare the purposes of UV/Vis and IR absorption spectroscopy
IR- used to determine functional groups (qualitative purposes) UV/VI- used for quantitative purposes, can be used for qualitative purpose but not likely.
1. Compare the purposes of UV/Vis and IR absorption spectroscopy
IR iderifies fuctional groups by bsorbing vibration rotational levels (primarily used for qualitative) UV vis is primarily used for quantitative
13. What affect does solvent polarity have on UV/Vis spectra and why. Explain in terms of types of transitions
If the solvent is going from non-polar to polar while the electron is being excited from non bonding orbital to an antibonding orbital then a blue (hypsochromic) shift occurs and the energy gap between them inceases. This also happen when going from a polat to non polar while the electron being excited jumps from bonding to antoboning. If the solvent is going from polar to noinpolar while the electron being excited is going from nonbonding to antibonding then a red (bathochromic) shift occurs and the energy gap between the two states decrease. This also happens when it goes from non-polar to polar while the electron being excited goes from bonding to antibonding.
13. Discuss the effect of concentration on fluorescence intensity
Increasing the concentration, makes the intensity higher and it self quenches and absorbs. The self quenching and absorbing is due to collisions between molecules which in turn lowers the intensity.
18. How are fluorescence and phosphorescence different in terms of intensity, lifetime, and energy
Intensity depends on the experiment. Phosphorescence has a longer lifetime while fluorescence has higher intensity
3. 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
Inner transition metal absorption peaks are very narrow. Less influenced, sharper, and more defined. Closer to the nucleus so less sensitive to environmental perturbations. Transition metal peaks are very broead compared to inner Splitting of d orbitals in transition metals
5 How do the spectra for Electron Impact (EI) and Chemical Ionization (CI) sources differ from one another
Introduce a reagent gas (more concentrated with analyte) with the sample molecule, the reagent gas gets excited and ionized and collides with the analytes, therefore energy will be internalized in the gas and less energy is transferred to the molecule causing less fragmentation.
17. Why is chemluminescence such a sensitive technique
It has low noise level.
9. Describe the difference in spectra observed in the gas, polar solvent, and nonpolar solvent. Why are the spectra different
Its different in the way the lines look within the graph Gas phase has higher resolution, non-polar solvent has wider peaks, polar solvents have super wide peaks Polar solvent reduce due to the of vibration rotational energy. Polar solvents caused peaks to blend into one hump. With nonpolar only electronic states can be detected.
Electronic orbitals
Only with O, N, S molecules
10. Discuss how the Pauli Exclusion Principle is related to fluorescence and phosphorescence
Pauli Exclusion principle- when going up an energy orbital it cannot come down unless it flips the spins, when electrons are trying to occupy the same orbital.
4 What is the purpose of chemical ionization and how is this done
Produce some fragmentation, used to measure molecular weight of species.
11 What is meant by the terms "hard" and "soft" ionization techniques and how that is realted to elctrom imact and chemscal ionization tecniques
Relates to the fragmentation patterns. Ei is an example of hard ionization and causes a lot of fragmentation. Ci is related to soft ionization, less fragmentation
5. List and explain the various ways an excited molecule can lose its energy (relaxation or deactivation processes)
Relaxation (radiatelty) - emission of photons Non-radiative- collisions vibrations
Bonding orbitals
Sigma, pi
2. Explain the similarities and differences between molecular fluorescence, phosphorescence and chemluminescence
Similarities- emission techniques are the same Differences- the sources of excitiation. Fluorescence and phosphorescence soure of excitation is absorption of a photon and chemluminescence is a chemical reaction. Phosphorescence lowere energy, longer lifetime, and fluorescence has higher energy intensity can be manipulated Depending on the experiment for ntoh fluorescence and phosphorescence. Chemluminescence very sensitive because of very low noise level, emission technique
12. Explain the relationship between vibrational relaxation and fluorescence
Since the lifetime of vibrational relaxatyion is so short this causes fluorescnence to always invlvves a transition from the lowest vibrational level of an excited electronic state.
2. Why are electrons in double or triple bonds more easily excited by electromagnetic radiation compared to single bonds
Single bonds are in vacuum uv region, so single bonds absorb at a lower photon frequency, lower energy and longer wavelength. Doble bonds are less likely for bond rupture. Double bonds Takes less energy to excite than single bonds Have more molar absorptivity
8. What variables influence absorbance
Solvent polarity- pH molecular substituent- auxochromes, shift wavelength and/ or intensity. Anything that changes the distribution of electrons, changes the way the species absorbs light polarity, concentration
11. What factors lead to the absorption of UV/Vis radiation by inorganic molecules
Splitting between d and f orbitals. Jumps from d orbitals
4. What affect does stray (scattered) radiation have on spectra and explain why
Stray radiation is light that doesn't pass through the sample but hits the detector and this causes deviation in beers law. This can cause false positives (false peaks) because the detector doesn't detect very well at the wavelength the stray radiation is from.
12. The absorption of UV or visible light by an atomic or molecular species can be considered a two-step process. Describe that process in significant detail and explain the differences in this process between atomic and molecular species
The part is excitation. Photons being absorbed. Second part is relaxation. A release of energy that is either radiative or non radiative. Radiative is fluorescence and phosphorescence.
8. Discuss the various factors that affect fluorescence and phosphorescence
The structure, ridgity, and resonance Concentration, temp, viscosity (see question 4) Heavy atom effect- increases thye probability of intersystem crossing and conversion leading excited molecule to triplett effect to increase phosphorescence.
3. What type of detectors are used in IR spectroscopy
Thermal couple- measures change in voltage Bolometer- measures change in resistance Pyro electric transducer- measures change in voltage Photoconductiond transducer- measures change in resistance
14. What affect does conjugation have on UV/Vis spectra
They absorb at longer wavelengths, and increase at molar absorptivity. It decreases the enery gap because it makes it more stabke and takes less energy to absorb. Depends on structural stability
12 What is the peurpose of the lens stack in the electron impact source
To focus the ion beam and keep a constant kinetic energy
10 Why must mass specs systems being under a vaccum
To minimize collisions between molecules. Collisions make it difficult to determine what the mass to charge ratio is.
6. Why is fluorescence seldom observed from sigma* sigma transitions
Too high of energy. Causes bond to rupture (basically like a single bond) Doesn't absorb due to rupturing
15 What is meant by the term "tuning" in mass spec and how is tuning accomplished
Tuning introduces a standard that calibrates is read, it also needs to be tuned so the correct wavelength is used. To tune, a reference sample is used
4. What types of molecular transitions are associated with IR absorption
Vibration rotation changes
6. What factors must be considered in the development of conditions that would yield reproducible (preferably linear) results between absorbance and analyte concentration
Wavelength selection Control variable that effect absorbance such as pH, solvent, matrix, etc. Clean handling of cells Relationship between absorbance and concentration.
14 What is a mass spectrum and what are the y and x axis in a mass spectrum
X-axis is the mass to charge ratio and the y-axis is the relative abundance of ions
3. Be able to discuss the effect of slit width on absorbance measurements
You get a more quantitative absorbance measurement if the slit width is wider. You get a more qualitative absorbance measurement if te slit width is narrower.
Auxochrome
a functional group that does not absorb photons, but shifts peaks to longer wavelengths and increases intensity.
Excited state
any energy state higher than ground state
Vacuum UV
below 200 nm. refers to the wavelength range where a vacuum apparatus is ogten used, because the light is strongly absorbed in air
Self quenching
collisions between excited molecules
Photoconducting transducer
consists of a thin fil, of semiconductor material deposited on a non-conducting glass surface
17 What are the components of a mass spec system and describe the role of each component and how each component works
inlet system- introduces sample Ion Source- ionize sample Mass analyzer- quadrupole/ ion trap Detector- electron multiplier tube made up of electromissive material signal processor and readout- computer
Indirect method
involves the dimension of fluorescence resulting from quenching action of the substance being determined
Direct method
involves the formation of a fluorescing chelate and the measurement of its emission
Nonbonding orbitals
n
Excitation spectra
obtained by measuring luminescence intensity at a fixed wavelength while the excitation wavelength is varied. Fix the emission and vary the excitation
Chromophore
part of the molecule that absorbs
Interferogram
pattern formed by wave interference
Pyroelectric transducer
pyroelectric crystal sandwhiched between two electrodes one of which is IR transparent. Changes in temperature are detected when IR alters the charge distribution across the crystal which is detected as a current
Inner transition metals
row 6 and row 7 lanthanides and actinides.
Spectral resolution
the ability to resolve spectral bands into their separate components.
Wavelength extremes
the wavelengths where the detector does not respond very well or the source does not emit very well.
Bathochromic shift
this is also called a red shift happens at higher wavelengths. n to Sigma* or pi*, red shift occurs only when going from polar to nonpolar. Sigma or pi to Sigma* or pi*, red shift occurs only when going from nonpolar to polar. Decrease in energy gap
Hypsochromic shift
this is also called as a blue shift happens at lower wavelengths. n to Sigma* or pi*, blue shift occurs only when going from a non polar to polar. Sigma or pi to Sigma* or pi*, blue shift occurs only when going from polar to nonpolar. Increase in energy gap
Thermocouple
two pieces of metal fused to either endd of a dissimilar metal. A potential difference develops with a difference in temp.
Bolometer
two thin strips of metal whose resistance changes greatly with temp.