CMC Chemical Sampling & Analysis Techniques 2 — Questions and Answers
Question 1: When collecting an air sample from a confined space using a detector tube, what is the correct procedure for ensuring accurate results?
- Draw the specified volume of air through the tube at the rate indicated by the manufacturer (Correct answer)
- Draw air as fast as possible to minimize exposure time
- Break both ends of the tube but only draw half the specified volume
- Reuse the detector tube if the color change appears minimal
Correct answer: Draw the specified volume of air through the tube at the rate indicated by the manufacturer
Detector tubes must be used strictly per manufacturer specifications, including drawing the exact air volume at the correct rate, to ensure accurate readings.
Question 2: A Marine Chemist is sampling ballast water for oxygen content. Which instrument provides a real-time continuous measurement rather than a grab sample reading?
- Electrochemical oxygen analyzer with in-line probe (Correct answer)
- Winkler titration kit
- Colorimetric oxygen test kit
- Passive diffusion badge sampler
Correct answer: Electrochemical oxygen analyzer with in-line probe
Electrochemical analyzers with in-line probes continuously monitor oxygen levels, whereas titration and colorimetric kits analyze discrete grab samples.
Question 3: Which sampling method is most appropriate for determining the time-weighted average (TWA) concentration of benzene in a cargo hold over an 8-hour period?
- Personal passive diffusion badge worn throughout the work shift (Correct answer)
- Single grab sample collected at the start of the shift
- Multiple detector tube readings averaged manually
- Photoionization detector spot readings taken hourly
Correct answer: Personal passive diffusion badge worn throughout the work shift
Passive diffusion badges integrate exposure over the entire wearing period, providing a true TWA concentration for comparison to occupational exposure limits.
Question 4: When using a flame ionization detector (FID) to measure hydrocarbon vapors in a tank, the instrument reads in units of ppm as what reference gas?
- Methane or hexane equivalent, depending on calibration (Correct answer)
- Carbon dioxide
- Oxygen
- Hydrogen sulfide
Correct answer: Methane or hexane equivalent, depending on calibration
FIDs measure total hydrocarbons referenced to a calibration gas, commonly methane (CH4) or hexane (C6H14), and results are expressed as ppm of that equivalent.
Question 5: A sample collected from the bottom of a cargo tank is found to have a different composition than one collected from the top. This stratification is most likely caused by:
- Differences in vapor density causing heavier gases to sink and lighter gases to rise (Correct answer)
- Instrument drift from repeated sampling
- Temperature equalization throughout the tank
- Cross-contamination from the sampling line
Correct answer: Differences in vapor density causing heavier gases to sink and lighter gases to rise
Gases with higher molecular weights are denser and tend to accumulate at lower levels, while lighter gases rise, creating concentration gradients within a confined space.
Question 6: Which of the following is a key limitation of colorimetric indicator tubes used for shipboard atmospheric testing?
- Many tubes have known cross-sensitivities to other chemicals that can cause false readings (Correct answer)
- They cannot detect concentrations above 1 ppm
- They require electrical power to operate
- They must be calibrated against certified gas standards daily
Correct answer: Many tubes have known cross-sensitivities to other chemicals that can cause false readings
Colorimetric tubes can react with interfering gases, producing stain lengths that over- or underestimate the target compound's true concentration.
Question 7: When obtaining a representative bulk liquid sample from a marine cargo tank containing a homogeneous liquid, the preferred sampling point is:
- Mid-depth of the liquid column to capture the most representative stratum (Correct answer)
- Only at the surface to avoid sediment
- Exclusively at the tank bottom to capture any water accumulation
- At the vapor space above the liquid
Correct answer: Mid-depth of the liquid column to capture the most representative stratum
For homogeneous liquids, a mid-depth or all-levels composite sample best represents the bulk cargo; surface and bottom-only samples may miss stratification effects.
CMC Chemical Sampling & Analysis Techniques 2
When collecting an air sample from a confined space using a detector tube, what is the correct procedure for ensuring accurate results?