Instrumental Analysis Exam 2 questions with
verified answers
1 Discuss the advantages and disadvantages of electron impact sources.
Ans✓✓✓ 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
1. Compare the purposes of UV/Vis and IR absorption spectroscopy.
Ans✓✓✓ 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. Ans✓✓✓ 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.
,14. Discuss the effect of concentration on fluorescence intensity.
Ans✓✓✓ 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
15. Discuss two fluorometric methods for the determination of
inorganic species Ans✓✓✓ *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.
17. Why are fluorescence methods more sensitive and selective than
absorption techniques? Ans✓✓✓ 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
, 18. Why is chemluminescence such a sensitive technique? Ans✓✓✓
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.
19. How are fluorescence and phosphorescence different in terms of
intensity, lifetime, and energy? Ans✓✓✓ Fluorescence is higher
energy, lower intensity, and a shorter lifetime. Phosphorescence is
lower energy, higher intensity, and longer lifetime
2 Discuss the advantages and disadvantages of chemical ionization
sources. Ans✓✓✓ 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.
2. Explain the similarities and differences between molecular
fluorescence, phosphorescence and chemluminescence. Ans✓✓✓ 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)
verified answers
1 Discuss the advantages and disadvantages of electron impact sources.
Ans✓✓✓ 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
1. Compare the purposes of UV/Vis and IR absorption spectroscopy.
Ans✓✓✓ 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. Ans✓✓✓ 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.
,14. Discuss the effect of concentration on fluorescence intensity.
Ans✓✓✓ 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
15. Discuss two fluorometric methods for the determination of
inorganic species Ans✓✓✓ *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.
17. Why are fluorescence methods more sensitive and selective than
absorption techniques? Ans✓✓✓ 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
, 18. Why is chemluminescence such a sensitive technique? Ans✓✓✓
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.
19. How are fluorescence and phosphorescence different in terms of
intensity, lifetime, and energy? Ans✓✓✓ Fluorescence is higher
energy, lower intensity, and a shorter lifetime. Phosphorescence is
lower energy, higher intensity, and longer lifetime
2 Discuss the advantages and disadvantages of chemical ionization
sources. Ans✓✓✓ 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.
2. Explain the similarities and differences between molecular
fluorescence, phosphorescence and chemluminescence. Ans✓✓✓ 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)
