Chemical Sampling & Analysis Techniques Flashcards

When collecting an air sample from a confined space using a detector tube, what is the correct procedure for ensuring accurate results?

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.

A Marine Chemist is sampling ballast water for oxygen content. Which instrument provides a real-time continuous measurement rather than a grab sample reading?

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.

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?

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.

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?

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.

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:

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.

Which of the following is a key limitation of colorimetric indicator tubes used for shipboard atmospheric testing?

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.

When obtaining a representative bulk liquid sample from a marine cargo tank containing a homogeneous liquid, the preferred sampling point is:

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.