Chemical Sampling & Analysis Techniques Flashcards

A Marine Chemist must verify that a tank atmosphere is below 10% LEL before hot work. The CGI reads 8% LEL. What is the correct action?

Answer: Do not issue the certificate; the space must be further ventilated until readings are consistently below 10% LEL at all sampling points

A single reading of 8% LEL is at the threshold and may not be representative of all areas; consistent readings below 10% LEL at all required points must be confirmed before issuing a hot work certificate.

Which analytical technique is most commonly used in a laboratory to confirm the identity and quantify trace concentrations of chlorinated solvents in a tank atmosphere sample collected on a sorbent tube?

Answer: Gas chromatography with electron capture detection (GC-ECD)

GC-ECD provides excellent sensitivity and selectivity for halogenated compounds, making it the standard method for confirming chlorinated solvent identities and concentrations.

During hydrogen sulfide (H2S) sampling in a sludge tank, the detector reads 0 ppm at the hatch but a longer sampling probe at 6 feet depth reads 200 ppm. What does this demonstrate?

Answer: H2S can stratify near tank surfaces and sediments; sampling at multiple depths is essential

H2S, being heavier than air, can accumulate at lower levels near sludge layers while near-hatch readings appear safe, demonstrating the necessity of multi-depth sampling.

What is the purpose of a zero-air calibration check performed on a photoionization detector (PID) before atmospheric sampling?

Answer: To confirm the instrument reads zero in clean, hydrocarbon-free air and establish a baseline

A zero-air check ensures the PID is not drifting or contaminated from previous use, establishing a clean baseline before measurements in the field.

When a Marine Chemist collects an air sample using activated charcoal sorbent tubes, the sample must be sealed and shipped to the laboratory under what condition?

Answer: Capped, labeled with sampling data, stored cold (4°C), and shipped promptly with chain-of-custody documentation

Activated charcoal tubes must be capped immediately after sampling, refrigerated, and accompanied by chain-of-custody records to prevent desorption losses and ensure sample integrity.

Which method is used to determine the oxygen-deficient threshold in marine confined spaces under OSHA and NFPA standards?

Answer: Electrochemical cell sensor measuring oxygen partial pressure, with deficiency defined as below 19.5% O2 by volume

OSHA defines an oxygen-deficient atmosphere as below 19.5% O2 by volume, measured using an electrochemical sensor calibrated against ambient air.

A cargo tank previously carrying crude oil is to be gas-freed for man-entry. During sampling, the PID detects 5 ppm benzene at the breathing zone level. The OSHA PEL for benzene is 1 ppm TWA. What is the correct finding?

Answer: The space is not safe for unprotected entry; additional ventilation or respiratory protection is required

Even though 5 ppm benzene is well below the IDLH, it exceeds the OSHA PEL of 1 ppm TWA, making unprotected entry unsafe without additional controls.