MChem Master of Chemistry Master of Chemistry: Analytical Chemistry 4 — Questions and Answers
Question 1: In gas chromatography, the Kovats retention index is used to:
- Determine the absolute concentration of an analyte
- Express analyte retention relative to n-alkane standards (Correct answer)
- Measure column efficiency in theoretical plates
- Optimize carrier gas flow rate
Correct answer: Express analyte retention relative to n-alkane standards
The Kovats retention index expresses analyte retention as a linear interpolation between bracketing n-alkanes, making retention data transferable across columns and labs.
Question 2: Which technique directly measures the mass of an adsorbed film with nanogram sensitivity by monitoring changes in resonant frequency?
- Ellipsometry
- Quartz crystal microbalance (QCM) (Correct answer)
- Surface plasmon resonance (SPR)
- Atomic force microscopy (AFM)
Correct answer: Quartz crystal microbalance (QCM)
QCM uses the Sauerbrey equation, which relates frequency decrease linearly to added mass on a piezoelectric quartz crystal oscillator.
Question 3: In 2D NMR, a COSY experiment correlates nuclei that are:
- Directly bonded to a heteroatom
- Scalar (J) coupled to each other through bonds (Correct answer)
- Spatially close via through-space dipolar coupling
- Chemically equivalent by symmetry
Correct answer: Scalar (J) coupled to each other through bonds
COSY (COrrelation SpectroscopY) identifies proton pairs connected through scalar (J) coupling, typically 2-4 bonds apart.
Question 4: What is the purpose of using an internal standard in quantitative analytical measurements?
- To extend the upper limit of the calibration range
- To correct for variations in sample preparation and instrument response (Correct answer)
- To identify unknown compounds by matching retention times
- To shift the analyte signal away from matrix interferences
Correct answer: To correct for variations in sample preparation and instrument response
An internal standard compensates for losses during sample preparation and instrument fluctuations by normalizing the analyte signal to the internal standard signal.
Question 5: In potentiometric pH measurement, a glass electrode develops a potential primarily because of:
- Redox reactions at the glass surface
- Ion exchange of H+ ions at the hydrated gel layer of the glass membrane (Correct answer)
- Precipitation of sparingly soluble salts
- Electron transfer through the glass
Correct answer: Ion exchange of H+ ions at the hydrated gel layer of the glass membrane
The glass membrane potential arises from selective H+ exchange between solution and the hydrated silicate gel layer, generating a Nernstian response.
Question 6: Which dispersive technique uses a grating or prism to separate UV-Vis radiation into wavelengths for multiwavelength detection in a diode array detector (DAD)?
- Interferometry
- Polychromator (Correct answer)
- Monochromator with photomultiplier
- Fourier transform spectrometry
Correct answer: Polychromator
A polychromator disperses light across a diode array simultaneously, enabling full-spectrum acquisition at each time point in HPLC-DAD.
Question 7: In laser-induced breakdown spectroscopy (LIBS), which physical process generates the plasma used for elemental analysis?
- Electrospray ionization of aerosols
- Focused high-power laser pulses ablating and ionizing the sample surface (Correct answer)
- Microwave-induced plasma sustained in a flowing gas
- Glow discharge sputtering of a solid cathode
Correct answer: Focused high-power laser pulses ablating and ionizing the sample surface
LIBS focuses intense nanosecond laser pulses onto the sample, creating a high-temperature microplasma that emits element-specific atomic emission lines.
MChem Master of Chemistry Master of Chemistry: Analytical Chemistry 4
In gas chromatography, the Kovats retention index is used to: