CIH Air Sampling and Instrumentation 2 — Questions and Answers
Question 1: Which sampling method is most appropriate for collecting a time-weighted average (TWA) sample of a volatile organic compound over an 8-hour shift?
- Impinger with liquid sorbent
- Passive diffusion badge sampler (Correct answer)
- Midget impinger with distilled water
- Real-time photoionization detector
Correct answer: Passive diffusion badge sampler
Passive diffusion badge samplers are designed for TWA sampling over full work shifts without requiring a pump.
Question 2: A personal air sampler pump calibrated at 2.0 L/min drifts to 1.85 L/min by end of shift. What is the most appropriate corrective action for future sampling?
- Average the start and end flow rates for calculation
- Discard the sample as invalid
- Recalibrate the pump before and after each sampling period (Correct answer)
- Increase the sampling duration to compensate
Correct answer: Recalibrate the pump before and after each sampling period
NIOSH recommends calibrating pumps before and after sampling to detect drift and ensure data validity.
Question 3: What does the term 'face velocity' refer to in the context of a laboratory fume hood?
- The velocity of air at the exhaust duct
- The inward air velocity measured at the hood sash opening (Correct answer)
- The velocity of turbulent airflow inside the hood
- The average airspeed across the entire hood face including sash
Correct answer: The inward air velocity measured at the hood sash opening
Face velocity is the inward air velocity measured at the plane of the open sash, typically 80–120 fpm for adequate containment.
Question 4: Which instrument principle is used in a flame ionization detector (FID) for measuring total hydrocarbons?
- Ultraviolet light absorption by organic molecules
- Ionization of compounds in a hydrogen flame creating a measurable current (Correct answer)
- Infrared absorption at characteristic wavelengths
- Electrochemical oxidation at a catalytic electrode
Correct answer: Ionization of compounds in a hydrogen flame creating a measurable current
FIDs burn sampled air in a hydrogen flame; organic compounds produce ions that generate a current proportional to carbon concentration.
Question 5: When sampling for respirable crystalline silica using a 10-mm nylon cyclone, the recommended sample flow rate is:
- 1.0 L/min
- 1.7 L/min (Correct answer)
- 2.5 L/min
- 3.5 L/min
Correct answer: 1.7 L/min
The 10-mm nylon cyclone must be operated at 1.7 L/min to achieve the ISO/ACGIH respirable particle size-selective criterion.
Question 6: A colorimetric detector tube shows a stain length of 35 mm for toluene. The tube calibration scale reads 50 ppm at 35 mm but the sample was taken at half the recommended strokes. What is the corrected concentration?
- 25 ppm
- 50 ppm
- 100 ppm (Correct answer)
- 70 ppm
Correct answer: 100 ppm
Halving the number of strokes means half the required air volume was sampled, so the true concentration is twice the read value (50 × 2 = 100 ppm).
Question 7: Which characteristic of a sorbent tube most directly determines its ability to retain sampled contaminants without breakthrough?
- Tube diameter and wall thickness
- Sorbent capacity and affinity for the target analyte (Correct answer)
- Color of the indicating layer
- Length of the front section only
Correct answer: Sorbent capacity and affinity for the target analyte
Breakthrough occurs when the sorbent's capacity or affinity for the analyte is exceeded, allowing contaminant to pass through unretained.
CIH Air Sampling and Instrumentation 2
Which sampling method is most appropriate for collecting a time-weighted average (TWA) sample of a volatile organic compound over an 8-hour shift?