CMC Chemical Sampling & Analysis Techniques 3 — Questions and Answers
Question 1: 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?
- Do not issue the certificate; the space must be further ventilated until readings are consistently below 10% LEL at all sampling points (Correct answer)
- Issue the certificate since 8% is below 10%
- Average with a prior reading of 12% LEL and issue the certificate
- Assume the reading is within instrument error and issue the certificate
Correct 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.
Question 2: 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?
- Gas chromatography with electron capture detection (GC-ECD) (Correct answer)
- Infrared spectroscopy of the raw sorbent tube
- pH titration of the sorbent extract
- Flame photometric detection without separation
Correct 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.
Question 3: 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?
- H2S can stratify near tank surfaces and sediments; sampling at multiple depths is essential (Correct answer)
- The instrument is malfunctioning and must be replaced
- H2S is lighter than air and accumulates at the top of tanks
- The hatch reading represents the true average concentration
Correct 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.
Question 4: What is the purpose of a zero-air calibration check performed on a photoionization detector (PID) before atmospheric sampling?
- To confirm the instrument reads zero in clean, hydrocarbon-free air and establish a baseline (Correct answer)
- To measure the oxygen content of ambient air
- To test the lamp intensity at its maximum output
- To verify the battery charge level of the instrument
Correct 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.
Question 5: 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?
- Capped, labeled with sampling data, stored cold (4°C), and shipped promptly with chain-of-custody documentation (Correct answer)
- Stored at room temperature in sunlight to preserve the charcoal
- Left uncapped for 24 hours to allow equilibration before sealing
- Shipped without labeling to avoid tampering concerns
Correct 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.
Question 6: Which method is used to determine the oxygen-deficient threshold in marine confined spaces under OSHA and NFPA standards?
- Electrochemical cell sensor measuring oxygen partial pressure, with deficiency defined as below 19.5% O2 by volume (Correct answer)
- Colorimetric indicator paper turning blue below 18% O2
- Combustible gas indicator reading above 25% LEL as a proxy for O2 depletion
- pH paper test on condensation droplets
Correct 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.
Question 7: 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?
- The space is not safe for unprotected entry; additional ventilation or respiratory protection is required (Correct answer)
- 5 ppm benzene is acceptable since it is below the IDLH of 500 ppm
- Issue a certificate since benzene is below the LEL
- The PID result can be disregarded because benzene is heavier than air
Correct 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.
CMC Chemical Sampling & Analysis Techniques 3
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?