Final Exam Test Review Notes

UV-Vis Spectrometers

• In modern UV-Vis spectrometers, slit width is programmed to change as a function of wavelength to maintain constant spectral resolution across the UV-Vis range.

• At longer wavelengths (lower energy), resolution decreases, so a narrower slit compensates.

• Advantages of increased slit width:

  • Higher signal intensity (better sensitivity)

  • Better signal-to-noise ratio

• Disadvantages of increased slit width:

  • Lower spectral resolution (peaks may broaden or overlap)

  • Potential loss of fine spectral details

Prism Separation in UV-Vis

• A prism separates by refraction,

• GC Detectors: FID vs. TCD

• Flame Ionization Detector (FID)

  • Advantages: High sensitivity (especially to hydrocarbons), wide dynamic range

  • Disadvantages: Destroys sample, not universal (poor response to non-organics)

• Thermal Conductivity Detector (TCD)

  • Advantages: Universal (detects all compounds), non-destructive

  • Disadvantages: Lower sensitivity than FID

Quenchers in Fluorescence

• Visible effect of a quencher on the fluorescence spectrum of quinine: Decreased fluorescence intensity. Emission peaks will be weaker or disappear depending on quencher efficiency.

Beer’s Law in Emission Spectroscopy

• Beer’s Law applies in emission spectroscopy when emission intensity is proportional to analyte concentration.

• This holds at low concentrations where:

  • Self-absorption and inner filter effects are negligible

  • Excitation source is stable

Signal-to-Noise Ratio in Raman Spectroscopy

• Given: Signal to noise ratio (S/N) is 8 after 16 scans

• Find: Number of scans needed to achieve S/N of 100

• Relationship: S/N ∝ \sqrt{number \, of \, scans}

• \frac{100}{8} = \sqrt{\frac{N_{initial}}{16}}

• (\frac{100}{8})^2 = \frac{N_{initial}}{16}

• N_{initial} = 16 * (\frac{100}{8})^2 = 2500

• 2500 scans are needed

Sources for IR and Raman Spectroscopy

• IR spectroscopy source: Globar (silicon carbide), Nernst glower, or heated filament

• Raman spectroscopy source: Monochromatic laser

Plate Height in HPLC

• Relationship: Plate height decreases as particle size decreases

• Smaller particles improve efficiency (more theoretical plates), but increase backpressure

R Groups in Chromatography

• (a) Normal Phase: Polar groups like –SiOH, –NH₂, –CN

• (b) Reverse Phase: Nonpolar/hydrophobic groups like –C₈H₁₇ (C8), –C₁₈H₃₇ (C18)

MS Methods for Biomacromolecules

• Methods that may be used to determine the molecular weight of a large biomacromolecule or polymer:

  • MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization Time-of-Flight)

  • ESI (Electrospray Ionization)

  • FT-ICR (Fourier Transform Ion Cyclotron Resonance) for high resolution

Molecular Ions in CI

• Chemical Ionization (CI): Produces [M + H]⁺ (or [M + other adduct]⁺)

• The molecular ion weight is 1 amu higher than M, due to protonation

n to sigma* Absorption

• The molecules that can have an n to sigma* absorption are NH3 and H2O. Therefore, the answer is (c) both NH3 and H2O.

1H NMR Signals in Aldehydes

• The aldehyde (CH3)3CCH2CHO has three 1H NMR signals because it has three kinds of hydrogen atoms. The answer is (b) three 1H signals.

Multiplicities in 1H NMR of Butanone

• Hs on C1, C3, and C4 are a singlet, a quartet, and a triplet, respectively. The proton signal splits into n + 1 peaks when the adjacent carbon has n equivalent hydrogens. C1 is the isolated methyl, and C3 is the methylene linked to C4 (methyl) in butanone. The correct answer is (c).

NMR Statements

• The statement that is NOT true about the NMR experiment is (a). The energy required to flip the spin of a proton is in the infrared region of the electromagnetic spectrum. It is actually in the radiofrequency region.

Energy Levels for Spin 1/2 Nucleus

• For a spin 1/2 nucleus in a magnetic field, there are 2 energy levels. The answer is (b).

FT NMR Signal Acquisition

• In FT NMR, signal acquisition takes place using the sequence: A 90o pulse - wait time – acquisition. The correct answer is (b).

Non-Scanning Mass Analyzer

• The non-scanning mass analyzer is TOF (Time-of-Flight). Therefore, the answer is (b).

Resolution of FT Instrument

• Resolution of an FT instrument depends on the time over which the interferogram is acquired. The answer is (c).

Solvent Effects on Absorption

• The compound has an absorption at 600 nm in water, 620 nm in methanol (CH3OH), and 640 nm in decane. There is a hypsochromic shift in the wavelength on going to more polar solvents.

Vibrations of CO2

• The symmetric and asymmetric stretch vibrations and the bending mode vibration of CO2 are Raman active, IR active, and IR active, respectively.

NMR Components

• Main components of NMR:

  • Magnet: Provides strong, stable magnetic field

  • RF transmitter/receiver coil: Sends/receives radiofrequency pulses

  • Sample probe: Holds NMR tube in the field

  • Shims: Adjust field homogeneity

  • Computer/console: Controls pulses, acquires and processes data

NMR Probe

• The coil emits the RF pulse and reads the signal from the sample.

Spin-Lattice vs. Spin-Spin Relaxation

• T1 (Spin-lattice relaxation): Converts the excess energy into translational, rotational, and vibrational energy of the surrounding atoms and molecules (the lattice).

• T2 (Spin-spin relaxation): Transfers the excess energy to other magnetic nuclei in the sample.

Equivalent Protons in Meta-Xylene

• Meta-xylene (1,3-dimethylbenzene) has three sets of equivalent protons. The answer is (b).

Protons in a Magnetic Field

• When an external magnetic field is applied, some protons align with the field and some align opposite to it. The correct answer is (c).

Deshielded Protons

• The compound with the MOST deshielded protons is CH3Cl. The answer is (a).

Matching 1H NMR Spectrum

• Since there are no signals at about 9 ppm for the aldehyde hydrogens in the spectra, the spectrum is not for compound butaldehyde.

Van Deemter Equation

• Define A, B, and C terms in the Van Deemter equation. Which term does not contribute to MPLE?

R Groups in Chromatography

• What types of R groups are used for (a) normal phase and (b) reverse phase chromatography?

Bulk Property Detector

• The bulk property detector is the refractive index detector. The answer is (c).

HPLC Separation by Molecular Size

• Gel permeation chromatography separates sample components by molecular size. The answer is (a).

Van Deemter Plot Information

• A Van Deemter plot can be used to determine the optimum mobile phase flow rate. The answer is (b).

Retention in Chromatography

• The compound eluted last and retained more in Normal and Reversed phase are Polar and non-polar, respectively. The answer is (a).

Theoretical Plates

• Theoretical plates are used to estimate the efficiency of a column. The answer is (a).

Stationary Phase in Reverse Chromatography

• In reverse chromatography, the stationary phase is made non-polar. The answer is (a).

Gel Permeation Chromatography Elution

• In Gel permeation chromatography, small molecules will be eluted last. The answer is (a).

Brand Broadening in Gas Chromatography

• The major contribution to the band broadening in gas chromatography is Longitudinal diffusion

Column Efficiency

• The efficiency of the column to increase when Plate number becomes greater and plate height becomes smaller. The answer is (a).

Mass Flow-Dependent Signals

• The detectors that give mass flow-dependent signals are flame ionization detector and Thermal conductivity detector. Therefore, the answer is (d) All of the above.

Carrier Gas Preference

Helium is generally preferred as carrier gas over nitrogen and hydrogen because:
Nitrogen is heavier at higher flow rates which causes a pressure drop within the column and that result in longer analysis times, broad peaks. In contrast to hydrogen, helium is safer, non-flammable, and will not cause any explosion.

EI and CI Applications

• EI and CI are best used to analyze molecules with boiling points less than 500 °C and weighs less than 1,000 Da. The answer is (a).

Mass Spectra of 1-Decanol

• 1. Identify the ion source in each.

• 2. Identify the base peak in each spectrum.

• 3. Why does spectrum (a) contain more peaks than spectrum (b)?

Michelson Interferometer Intensity

• In Michelson Interferometer, if the reflected and transmitted beams are in phase at the beam splitter, then maximum intensity will reach the detector. The answer is (a) True.

Zone Broadening

• A decrease in the selectivity factor is not a result of zone broadening. The answer is (b).

WCOT Columns

• WCOT columns may be constructed of metal, glass, plastic, or fused silica. The answer is (a) True.

GC Detectors: FID vs. TCD

• What are the advantages and disadvantages of flame ionization detection and thermal conductivity detection in GC? Name one other type of detection discussed in class and tell what it is used for.

Flame Ionization Detector

• Organics with alcohol, carbonyls, halogens, and amine functional groups are easily detected by a flame ionization detector. The answer is (b) False.

Raman Activity

• For a molecule to be Raman active, it must undergo a change in polarizability upon vibration. The answer is (c).

Laser in FTIR

• What is the purpose of a laser in an FTIR instrument?

Hard vs. Soft Ion Sources in MS

• What is the difference between a hard and a soft ion source in MS? Give two examples of each.

MS Analyzers

• What are the 3 types of MS analyzers? Which are scanning and which are non-scanning?

Gel Filtration Chromatography Exclusion Limit

• What is the exclusion limit in gel filtration chromatography? Illustrate with a plot of MW vs retention time.

Van Deemter Plot Comparison

• Compare the van Deemter plot for HPLC to that for GC. Which terms in the van Deemter equation do not contribute to HPLC?

Stationary Phase in Chromatography

• What matrix is used for the stationary phase in ion exchange and gel filtration chromatography? (b) How is the pore size controlled?

Carrier Gas in Gas Chromatography

• The most suitable gas to use as a carrier gas in gas chromatography is He (Helium). The answer is (b).

Normal Phase HPLC

• In normal phase HPLC:

  • the mobile phase is nonpolar, and the stationary phase is polar.

  • eluent strength is increased by adding a more polar solvent to the mobile phase. Therefore, the answer is (c).

Ion-Exchange Chromatography

• The type of chromatography wherein the analyte ions in the mobile phase are attracted to the counter ions in the stationary phase is Ion-Exchange chromatography. The answer is (c).

Separation of Polar Molecules

• Ion-exchange chromatography is used for the separation of polar molecules. The answer is (a).

Liquid Chromatography Separation

• This type of liquid chromatography separation depends on interactions between solute, mobile phase, and immobilized liquid stationary phase, is Partition. The answer is (b).

Siloxane Film Structure

• Draw the general structure of the siloxane film used on the stationary phase in GLC and HPLC. Use R groups to represent hydrophobic or hydrophilic substituents. Give an example of an R group.

Retention Factor

• What does the retention factor, k', describe?

Column Efficiency

• The factor that is not determining the column efficiency in liquid chromatography is Detector response. The answer is (c).

Gel Permeation Chromatography Elution

• In Gel Permeation Chromatography small molecules will be eluted last. The answer is (a).

Gel Filtration

• Gel filtration or gel permeation chromatography work by the same principles of Size exclusion.

EI and CI Applications

• EI and CI are best used to analyze Molecules with boiling points less than 500 °C and weighs less than 1,000 Da.

Ion Detection in MS

• In MS ions are usually detected using Electron multiplier.

State of Matter in Mass Spectrometry

• Mass spectroscopy is being performed in the gaseous state of matter. The answer is (c).

GC-MS Systems

• GC- MS has been developed for which of the following systems, Capillary column. The answer is (c).

Reciprocating Pump Disadvantage

• The disadvantage of reciprocating pump used in liquid chromatography is Produces pulsed flow. The answer is (a).

Columns in Liquid Chromatography

• The columns that are not used in liquid or high-performance liquid chromatography are Capillary column. The answer is (d).

Hard vs. Soft Ion Sources in MS

• What is the difference between a hard and a soft ion source in MS. Identify to which category each of the following belong.

MS Methods for Biomacromolecules

• Which MS methods may be used to determine the molecular weight of a large biomacromolecule or polymer?

Longitudinal Diffusion in Chromatography

• Longitudinal diffusion affects the chromatograph by Causing band broadening.

HPLC Elusions

• What are the two types of elutions typically used in HPLC, Gradient and Isocratic.

Mass Spectra Source Identification

• Identify the source for the mass spectra below. What do the labels on some of the peaks refer to?

Radiant Energy

• The wavelength of light with the highest radiant energy is 200 nm. The answer is (a).

Spectrophotometer Component

• A component of a spectrophotometer that isolates radiant energy of a specific wavelength and excludes that of other wavelengths is called a Monochromator. The answer is (c).

Sample Cuvette Material

• Method requiring the measurement of absorbance in the UV region of the electromagnetic spectrum require that the sample cuvette be made of, Quartz. The answer is (c).

Atomic Absorption Spectroscopy Principle

• The principle of Atomic Absorption Spectroscopy is Radiation is absorbed by non-excited atoms in vapour state and are excited to higher states. The answer is (a).

Sample Introduction Probes

• The probes that are used for the introduction of the sample are Graphite. The answer is (c).

Sputtering in Hollow Cathode Lamp

• The process of 'sputtering' that occurs in Hollow Cathode Lamp is Positive ions collide with cathode surface and metal atoms from cathode are ejected. The answer is (a).

Absorption of Radiation Instrument

• The missing block in the block diagram for Absorption of Radiation Instrument is Detector. The answer is (d).

Spectroscopy

• What are "optically thin conditions" and to what form of spectroscopy do they apply?

Fluorescence Quenching

• What is quenching of fluorescence. (a) What visible effect would a quencher have on the fluorescence (emission) spectrum?

Wave Number of Near Infrared Spectrometer

• The wave number of near infrared spectrometer is 12500 – 4000 cm-1. The answer is (c).

UV-Vis Spectrometer Lamps

• A conventional bench top UV-Vis spectrometer uses a deuterium lamp as a UV source and a tungsten lamp as a source in the visible region.

Monochromator Dispersion

• How might one decrease the reciprocal linear dispersion of a monochromator, Increase the spectral bandwidth of the source and decrease the grating order

Michelson Interferometer Intensity

• In Michelson Interferometer, if the reflected and transmitted beams are in phase at the beam splitter, then maximum intensity will reach the detector. The answer is (a) True.

Atoms in the Gas Phase

• Atoms in the gas phase produce discreet line spectra. The answer is (c).

Radiation in Atomic Absorption Spectroscopy

• In Atomic Absorption Spectroscopy, the generally used radiation is Hollow cathode lamp. The answer is (d).

PMT Function

• A PMT turns radiant energy into electrical current. The answer is (a) True.

Highest Energy Radiation

• The type of radiation that has the highest energy is X-ray. The answer is (d).

Transmittance Equation

• Transmittance is given as T = P/Po. If Po is the power incident on the sample, P represents Radiant power transmitted by the sample. The answer is (a).

Flame Photometer Component

• The item not a component of the emission system in Flame photometer is Chopper. The answer is (d).

Spectroscopies

• For each brief description below, describe the relevant spectroscopy as infrared, Raman, UV-Visible, photoelectron, fluorescence

  • A photon of λ=280 nm is absorbed by a molecule, which then emits a photon of λ=630 nm: Fluorescence

  • A molecule absorbs a photon at 2175 cm-1, producing a v = 0 to v = 1 transition in the symmetric stretch normal mode of CO2; then the photon is re-emitted with no change in the dipole moment: Raman

  • The absorption band is at 2130 cm-1: IR

Photon of 550 nm Light

• A photon of 550 nm light calculates: Wavenumber, Frequency and energy.

  • Wavelength, λ = 550 \times 10^{-9} m

  • Speed of light, c = 3.00 \times 10^8 m/s

  • Planck’s constant, h = 6.626 \times 10^{-34} J

  • Wavenumber \bar{v} = \frac{1}{λ} = \frac{1}{550 \times 10^{-9}} = 1.818 \times 10^{6} m^{-1}

  • Frequency v = \frac{c}{λ} = \frac{3.00 \times 10^8}{550 \times 10^{-9}} = 5.45 \times 10^{14} Hz

  • Energy E = h v = (6.626 \times 10^{-34}) (5.45 \times 10^{14}) = 3.61 \times 10^{-19} J

Collision Quenching

• Collision quenching increases as temperature increases. The answer is (d).

Stokes Shift

• The shift to longer wavelength is called the Stokes shift.

ICP-AES Advantages

• Advantages of ICP-AES over flame AE:

  • Increasing sensitivity of the older flame methods

  • Good emission results from nearly all elements under a single set of conditions

  • Spectra for a wide range of elements can be recorded simultaneously

  • High temperature limit chemical and spectral interference and permit the determination of nonmetals such as chlorine, bromine and sulfur

  • Lower limits of detection are usually in the ppb region compared to the ppm

  • region for flame AES

Fluorescence and Phosphorescence of Phenanthroline

• The excitation, fluorescence and phosphorescence of phenanthroline is shown. Label the peaks which correspond to these three processes.

IR Absorbance Band

• An IR absorbance band in a peptide is found to be a 1500 cm-1. What is the band frequency? what is its wavelength?

  • Wavenumber, v = 1500 cm^{-1}

  • Speed of light, c = 3.00 \times 10^{10} cm/s

  • Frequency v = c \bar{v} = (3.00 \times 10^{10} cm/s) (1500 cm^{-1}) = 4.5 \times 10^{13} Hz

  • Wavelength λ = \frac{1}{\bar{v}} = \frac{1}{1500} = 6.67 \times 10^{-4} cm

Spectroscopies Order

• According to the Boltzmann distribution order these types of spectroscopies from the inherently most sensitive to the least sensitive: infrared, UV-Vis absorbance, NMR

Terms Definition

• Define the following (a) internal conversion (b) intersystem crossing (c) quenching (d) fluorescence lifetime

  • When two singlet states overlap, energy is usually passed to the lowest vibrational state of the lowest electronic state by a process called internal conversion.

  • Intersystem crossing is a process in which the spin state of an electron is reversed and energy from an excited singlet state is passed on to an excited triplet state.

  • Deactivation of an excited electronic state may involve transfer of energy to the solvent. In this case, no photon is emitted. o The process is called external conversion or collision quenching.

Monochromator Dispersion

• Which factors might decrease the reciprocal linear dispersion of a monochromator? Decrease distance (d) and increase the focal length (F)

Fluorescence and Phosphorescence

• What is the difference between fluorescence and phosphorescence?

  • Fluorescence: from an excited singlet electronic state

  • Phosphorescence: from an excited triplet state

Singlet and Triplet State

• What is the difference between a singlet and a triplet state?

  • Electronic states:

    • Singlet state: contain no unpaired electrons

    • Triplet state: contain two electrons with the same spin

Monochromators Position

• Two monochromators positioned at right angles is a characteristic of Fluorometer. The answer is (c).

Photon Absorption

• A photon is absorbed by a vibrational band at 3610 cm-1. What is the photon Frequency (Hz), Wavelength (nm) and Energy (J)

  • Wavenumber, v = 3610 cm^{-1}

  • Speed of light, c = 2.998 \times 10^{10}cm/s

  • Planck’s constant, h = 6.626 \times 10^{-34} J

  • Frequency v = v \cdot c = (3610 cm^{-1}) (2.998 \times 10^{12}cm/s) = 1.08 \times 10^{14}Hz

  • Wavelength λ = \frac{c}{v} = \frac{2.998 \times 10^{12}m/s}{1.08 \times 10^{14}Hz} = 2.77 \times 10^{-3}m \rightarrow 2770nm

  • Energy E = h v = (6.626 \times 10^{-34}) (1.08 \times 10^{14}Hz) = 7.18 \times 10^{-20} J

Slit Width in UV-Vis Spectrometers

• In most modern UV-Vis spectrometers slit width is programmed to change as a function of wavelength. Why? (b) What are the advantages and disadvantages of increased slit width?

  • Advantage: power increase

  • Disadvantage: wider bandwidth

Prism

• A prism separates UV or visible light by (d) All of the above

Absorption Measurement

• Method requiring the measurement of absorbance in the UV region of the electromagnetic spectrum require that the sample cuvette be made of Quartz. The answer is (c).

Spectrometer Source of Light

• Which of the following is a source used in Raman spectroscopy? The answer is (a) Laser

UV-Vis Absorption

• UV-Vis absorption spectra result from (b) Excitation of electron

Fluorescence

• The wavelength of light fluoresced by molecules is longer than that absorbed because (c) Some of the energy absorbed by the molecules is lost to molecular vibrations before the fluorescence is produced, which results in a longer wavelength.