CCIL Sample Management and Preparation 3 — Questions and Answers
Question 1: Which preservation chemical is added to water samples collected for nitrate/nitrite analysis?
- Sodium thiosulfate
- Sulfuric acid (Correct answer)
- Sodium hydroxide
- Ascorbic acid
Correct answer: Sulfuric acid
Sulfuric acid lowers the pH to ≤2, inhibiting microbial conversion of nitrate and nitrite in water samples.
Question 2: What does the term 'matrix spike' refer to in sample preparation quality control?
- Adding a preservative to the sample matrix in the field
- Adding a known concentration of analyte to a sample to assess recovery efficiency (Correct answer)
- Spiking the calibration standards with matrix components
- Introducing an internal standard during instrument analysis
Correct answer: Adding a known concentration of analyte to a sample to assess recovery efficiency
A matrix spike involves fortifying a sample with a known analyte amount to evaluate method recovery in that specific matrix.
Question 3: When subsampling a heterogeneous solid sample such as contaminated soil, which technique best ensures representativeness?
- Taking material only from the top 2 cm of the container
- Cone and quarter or riffle splitting after thorough mixing (Correct answer)
- Selecting the most visually contaminated portion
- Removing the sample with a spoon from one side of the container
Correct answer: Cone and quarter or riffle splitting after thorough mixing
Cone and quarter or riffle splitting reduces heterogeneous bulk samples to representative subsamples with minimal bias.
Question 4: A laboratory must analyze water for mercury. Which container material is MOST appropriate?
- Standard HDPE plastic bottle
- Glass bottle with Teflon-lined cap (Correct answer)
- Polypropylene centrifuge tube
- Amber glass with rubber septum
Correct answer: Glass bottle with Teflon-lined cap
Mercury adsorbs to plastic and can permeate through standard containers, so borosilicate glass with Teflon-lined caps is required.
Question 5: What is the function of an equipment blank (also called a rinsate blank) in field sampling?
- To verify the purity of reagent water used in the laboratory
- To confirm that sampling equipment was adequately decontaminated between samples (Correct answer)
- To measure analyte concentrations in ambient air around the sampling site
- To test whether sample containers are free of manufacturing residues
Correct answer: To confirm that sampling equipment was adequately decontaminated between samples
Equipment blanks use analyte-free water rinsed through sampling gear to detect cross-contamination from inadequate decontamination.
Question 6: Which holding time applies to water samples collected for biochemical oxygen demand (BOD) analysis according to standard methods?
- 6 hours at 4°C
- 48 hours at 4°C (Correct answer)
- 7 days at 4°C
- 28 days at 4°C
Correct answer: 48 hours at 4°C
BOD samples must be analyzed within 48 hours of collection when stored at 4°C to prevent significant microbial changes.
Question 7: In the context of laboratory sample management, what is 'cradle-to-grave' tracking?
- Monitoring a sample from its environmental source through final disposal (Correct answer)
- A method for dating geological samples using isotopic ratios
- The lifecycle of a reagent from purchase to expiry
- Tracking analyst training records from hire to retirement
Correct answer: Monitoring a sample from its environmental source through final disposal
Cradle-to-grave tracking documents a sample's complete journey from collection through analysis to final disposal for full accountability.
CCIL Sample Management and Preparation 3
Which preservation chemical is added to water samples collected for nitrate/nitrite analysis?