CPC Chemical Instrumentation & Calibration 3 — Questions and Answers
Question 1: In HPLC, what is the van Deemter equation used to describe?
- The relationship between mobile phase pH and retention factor
- The dependence of column plate height on mobile phase linear velocity (Correct answer)
- The effect of temperature on stationary phase selectivity
- The relationship between detector response and analyte concentration
Correct answer: The dependence of column plate height on mobile phase linear velocity
The van Deemter equation (H = A + B/u + Cu) describes how band broadening (plate height H) varies with mobile phase velocity, identifying optimal flow rate.
Question 2: Which phenomenon causes baseline drift in a differential refractometer (RI) detector during gradient HPLC elution?
- Increased UV absorption of the stronger solvent
- Changes in mobile phase composition alter the bulk refractive index (Correct answer)
- Column bleed contaminating the detector cell
- Stationary phase dissolution raising solvent viscosity
Correct answer: Changes in mobile phase composition alter the bulk refractive index
RI detectors respond to the overall refractive index of the column effluent; solvent composition changes during gradient elution cause continuous baseline drift.
Question 3: In FTIR spectroscopy, what is the function of the Michelson interferometer?
- To disperse IR radiation into individual wavelengths using a grating
- To generate an interferogram by combining beams with varying path difference (Correct answer)
- To focus IR radiation onto the ATR crystal surface
- To filter out water vapor absorption bands from the spectrum
Correct answer: To generate an interferogram by combining beams with varying path difference
The Michelson interferometer splits the IR beam, introduces a variable path difference with a moving mirror, and recombines the beams to create an interferogram that is Fourier-transformed into a spectrum.
Question 4: What does the term 'linearity of calibration' mean in method validation?
- The ability of the method to produce responses proportional to analyte concentration over a defined range (Correct answer)
- The ability to detect the analyte in the presence of interfering substances
- The closeness of a measured value to the true value
- The ability to reproduce results under identical conditions
Correct answer: The ability of the method to produce responses proportional to analyte concentration over a defined range
Linearity demonstrates that instrument response is directly proportional to analyte concentration across the working range, typically verified by the coefficient of determination (R²).
Question 5: Which mass spectrometry ionization technique is most suitable for intact large biomolecules like proteins?
- Electron ionization (EI)
- Chemical ionization (CI)
- Electrospray ionization (ESI) (Correct answer)
- Fast atom bombardment (FAB)
Correct answer: Electrospray ionization (ESI)
ESI is a soft ionization technique that produces multiply charged intact molecular ions from large polar biomolecules without fragmentation.
Question 6: When calibrating a volumetric flask using gravimetric water delivery, the measured mass is corrected for buoyancy. What standard density value is typically used for air buoyancy correction?
- 0.0012 g/mL (Correct answer)
- 0.00120 g/mL
- 0.001205 g/mL
- All three values are equivalent; the correction is negligible for Class A glassware
Correct answer: 0.0012 g/mL
Air density is approximately 0.0012 g/mL (1.2 mg/mL) at standard conditions; this value is used to calculate the buoyancy correction for weighing water in air.
Question 7: In a quadrupole mass filter, ions are selected by:
- Their time of flight through a drift tube
- Their deflection angle in a magnetic sector
- Their stable oscillating trajectories under specific DC and RF voltage ratios (Correct answer)
- Their energy loss in a collision cell filled with inert gas
Correct answer: Their stable oscillating trajectories under specific DC and RF voltage ratios
A quadrupole mass filter passes only ions with a specific m/z that have stable trajectories under the combined DC and RF fields applied to four rod electrodes.
CPC Chemical Instrumentation & Calibration 3
In HPLC, what is the van Deemter equation used to describe?