Instrumental Analysis Final Exam questions with
verified answers
(b) Based on your calculations, can you, with 95% confidence, say that
the concentration of lead is below a mandated level of 10.000 ppb? (3
points) Ans✓✓✓ Because the 95% confidence interval for the data set
INCLUDES the 10.000 mandated value, you CANNOT say that the
concentration is below the mandated level.
A version of the Larmor equation is shown below. Given the terms
included in the equation, which relates the resonant frequency
associated with a nuclear spin transition to the magnetogyric ratio and
strength of an external magnetic field, how do you explain the apparent
"chemical shift" that perturbed the physicists who found that the
measured Larmor frequencies for, say, 1H or 13C nuclei depended on
the identity of the molecule being used to make the measurement?
Ans✓✓✓ 1. Shielding experience by a given nucleus is directly related
to the electron density surrounding it. In the absorbance of other
factors its expected shielding would decrease with increasing
electronegativity of adjacent groups. Electrons are attracted by a
magnetic field which affects differences seen in frequencies. Resonance
frequency for a particular nucleus depends on the compound it was
present in chemical shift.
adsorption chromatography Ans✓✓✓ solid stationary phase, liquid or
gas mobile phase
,affinity chromatography Ans✓✓✓ Separation carried out using specific
interaction between solute and an immobilized molecule (eq. protein
and antibody to the protein)
As you have heard a couple of times in class, NMR will be the only
spectroscopy discussed for which the frequency of the radiation used
to excite a transition is dependent both on attributes of the
"chromophore" and the experimental apparatus (namely, the strength
of the external magnetic field). Briefly explain why this is the case for
NMR spectroscopy - why is the transition energy (measured using the
Larmor frequency) dependent on the strength of the external magnetic
field? Ans✓✓✓ a. Higher magnetic field means higher resistance which
means more energy needed to go from parallel to anti parallel.
b. Transition energy is dependent on the strength of the external
magnetic field. Since the NMR flips the parallel spin to anti parallel and
measures the energy used to flip back to the unquantized state. The
flipping is measured by the transition energy. The magnetic field acts as
a resister against the flipping. The resistance is what the transition
energy needs to overcome. Meaning the stronger the magnetic field a
higher resistance state is needed. Which causes a higher transitional
state. Making the transitional energy dependent on the strength of the
external magnetic field.
Assume that you would like to create a diffraction grating for use with
light from a source appropriate for UV-Visible spectroscopy. If the
incident angle (θ) for the light (from the source) is 47°, calculate the
number of lines per centimeter that should be etched into the grating
surface to achieve:
, (a) A second order diffraction angle (Φ) of -23° for 650 nm (visible
region) radiation.
(b) A first order diffraction angle (Φ) of -30° for 150 nm (ultraviolet
region) radiation. Ans✓✓✓
Bias Ans✓✓✓ A report of the systematic (determinate) error of an
analytical method
Both magnetic sector and Fourier-transform ion cyclotron resonance
mass spectrometers use an external magnetic field for separation of
ions based on m/z. How are the two methods similar and how do they
differ? For the magnetic sector instrument, what do you think
influences the resolving power (the ability to distinguish an ion of one
m/z from another) of the approach? Ans✓✓✓ Both use an external
magnetic field to separate the ions based on their mass to charge ratio.
FTIC uses associated cyclotron frequency of the ions within a fixed
magnetic field to measure m/sz magnetic field. Magnetic sector's
resolving power stems from the width of image slit. Increasing reaction
by narrow slides fewer ions.
By now you are all familiar with the Beer-Lambert law (more commonly
known as Beer's Law). In general, the law links the attenuation of
radiation to properties of the medium through which it passes. In
quantitative chemical analysis, there are three parameters that
influence absorbance. State what the three parameters are, and more
importantly, specifically why each should influence the measured
absorbance for a given sample. Ans✓✓✓ a. Extinction coefficient:
verified answers
(b) Based on your calculations, can you, with 95% confidence, say that
the concentration of lead is below a mandated level of 10.000 ppb? (3
points) Ans✓✓✓ Because the 95% confidence interval for the data set
INCLUDES the 10.000 mandated value, you CANNOT say that the
concentration is below the mandated level.
A version of the Larmor equation is shown below. Given the terms
included in the equation, which relates the resonant frequency
associated with a nuclear spin transition to the magnetogyric ratio and
strength of an external magnetic field, how do you explain the apparent
"chemical shift" that perturbed the physicists who found that the
measured Larmor frequencies for, say, 1H or 13C nuclei depended on
the identity of the molecule being used to make the measurement?
Ans✓✓✓ 1. Shielding experience by a given nucleus is directly related
to the electron density surrounding it. In the absorbance of other
factors its expected shielding would decrease with increasing
electronegativity of adjacent groups. Electrons are attracted by a
magnetic field which affects differences seen in frequencies. Resonance
frequency for a particular nucleus depends on the compound it was
present in chemical shift.
adsorption chromatography Ans✓✓✓ solid stationary phase, liquid or
gas mobile phase
,affinity chromatography Ans✓✓✓ Separation carried out using specific
interaction between solute and an immobilized molecule (eq. protein
and antibody to the protein)
As you have heard a couple of times in class, NMR will be the only
spectroscopy discussed for which the frequency of the radiation used
to excite a transition is dependent both on attributes of the
"chromophore" and the experimental apparatus (namely, the strength
of the external magnetic field). Briefly explain why this is the case for
NMR spectroscopy - why is the transition energy (measured using the
Larmor frequency) dependent on the strength of the external magnetic
field? Ans✓✓✓ a. Higher magnetic field means higher resistance which
means more energy needed to go from parallel to anti parallel.
b. Transition energy is dependent on the strength of the external
magnetic field. Since the NMR flips the parallel spin to anti parallel and
measures the energy used to flip back to the unquantized state. The
flipping is measured by the transition energy. The magnetic field acts as
a resister against the flipping. The resistance is what the transition
energy needs to overcome. Meaning the stronger the magnetic field a
higher resistance state is needed. Which causes a higher transitional
state. Making the transitional energy dependent on the strength of the
external magnetic field.
Assume that you would like to create a diffraction grating for use with
light from a source appropriate for UV-Visible spectroscopy. If the
incident angle (θ) for the light (from the source) is 47°, calculate the
number of lines per centimeter that should be etched into the grating
surface to achieve:
, (a) A second order diffraction angle (Φ) of -23° for 650 nm (visible
region) radiation.
(b) A first order diffraction angle (Φ) of -30° for 150 nm (ultraviolet
region) radiation. Ans✓✓✓
Bias Ans✓✓✓ A report of the systematic (determinate) error of an
analytical method
Both magnetic sector and Fourier-transform ion cyclotron resonance
mass spectrometers use an external magnetic field for separation of
ions based on m/z. How are the two methods similar and how do they
differ? For the magnetic sector instrument, what do you think
influences the resolving power (the ability to distinguish an ion of one
m/z from another) of the approach? Ans✓✓✓ Both use an external
magnetic field to separate the ions based on their mass to charge ratio.
FTIC uses associated cyclotron frequency of the ions within a fixed
magnetic field to measure m/sz magnetic field. Magnetic sector's
resolving power stems from the width of image slit. Increasing reaction
by narrow slides fewer ions.
By now you are all familiar with the Beer-Lambert law (more commonly
known as Beer's Law). In general, the law links the attenuation of
radiation to properties of the medium through which it passes. In
quantitative chemical analysis, there are three parameters that
influence absorbance. State what the three parameters are, and more
importantly, specifically why each should influence the measured
absorbance for a given sample. Ans✓✓✓ a. Extinction coefficient:
