Chemical Sampling & Analysis Techniques Flashcards
7 cards from real CMC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Chemical Sampling & Analysis Techniques flashcards as text
What is the primary reason a Marine Chemist should purge the sampling line before collecting an atmospheric reading in a deep tank?
Answer: To replace the air trapped in the line with the actual tank atmosphere, ensuring the reading reflects conditions inside the tank
Dead air in the sampling hose dilutes or displaces the tank atmosphere; purging by drawing several hose volumes ensures the reading represents actual in-tank conditions.
Which of the following correctly describes the difference between a grab sample and an integrated sample in atmospheric monitoring?
Answer: A grab sample captures a discrete moment in time, while an integrated sample accumulates exposure over a defined period
Grab samples reflect instantaneous conditions, while integrated samples (such as badges or pumped sorbent tubes) average concentrations over the sampling duration for TWA calculations.
A Marine Chemist is testing for carbon monoxide (CO) in an engine room using an electrochemical sensor. The instrument suddenly reads much lower than expected after the sensor was exposed to high CO concentrations in the previous tank. This effect is known as:
Answer: Sensor inhibition or depression caused by overexposure, requiring fresh-air recovery or sensor replacement
Electrochemical CO sensors can be temporarily inhibited or permanently damaged by exposure to very high concentrations, causing falsely low readings until the sensor recovers or is replaced.
When using a multi-gas detector in a confined space that contained inert gas (nitrogen) for tank blanketing, what reading pattern would indicate the space is still hazardous?
Answer: O2 below 19.5%, CO and H2S at 0 ppm, LEL at 0% — the low oxygen itself indicates an inert hazard
A nitrogen-blanketed tank may show zero LEL and zero toxic gas while remaining immediately dangerous due to oxygen displacement below the 19.5% deficiency threshold.
Which ASTM standard method governs the sampling of petroleum products from marine cargo tanks using a thief sampler or beaker sampler?
Answer: ASTM D4057, Standard Practice for Manual Sampling of Petroleum and Petroleum Products
ASTM D4057 provides standardized procedures for manually collecting representative samples of petroleum products from tanks, pipelines, and marine vessels.
During hot work preparation, a Marine Chemist finds that a CGI equipped with a catalytic bead sensor reads 0% LEL in a tank previously loaded with halogenated solvents. What concern should this raise?
Answer: Halogenated compounds can poison catalytic bead sensors, causing falsely low or zero LEL readings even when flammable vapors are present
Halogenated hydrocarbons poison catalytic combustion sensors by deactivating the bead, leading to dangerously false-low LEL readings; a PID or FID should be used instead.
What information must always be recorded on the sample label when a Marine Chemist collects a sorbent tube sample for laboratory analysis?
Answer: Tank or space location, date and time, sample volume pumped, flow rate, sampler's name, and the target analyte
Complete sample metadata—location, time, volume, flow rate, target compound, and sampler identity—is required for accurate laboratory calculations and defensible chain of custody.