Sample Collection Techniques: Containers, Preservation, and Holding Times

Why Sample Integrity Matters

A water or wastewater sample is only useful if it still represents the water at the moment it was collected. From the second you fill a bottle, chemical reactions, microbial growth, and temperature changes begin to alter the sample. Choosing the wrong container, skipping preservation, or waiting too long to deliver the sample to the lab can produce results that are completely wrong — leading to permit violations, public health risks, or costly repeat testing. State licensing exams and regulators expect operators to know these rules cold.

Containers: Why the Material and Condition Matter

Different analytes require different container materials.

• Plastic (polyethylene or polypropylene) bottles are used for most inorganic analyses, including nutrients, metals (with acid preservation), and general chemistry such as chlorine residual. Plastic can leach organic compounds, so it is not suitable for organics.
• Amber glass bottles block light and are required for analytes that degrade under UV, including BOD (biochemical oxygen demand), oil and grease, and many organic compounds. Glass does not absorb or leach organics.
• Sterile containers — either sterile plastic Whirl-Pak bags or sterile glass/plastic bottles pre-dosed with sodium thiosulfate — are used exclusively for microbiological (bacteriological) samples. The sodium thiosulfate neutralizes residual chlorine so it does not continue killing bacteria after collection, giving a false low count.

All containers must be clean, appropriately pre-treated, and kept closed until the moment of sampling.

Preservation Methods

Preservation slows the chemical and biological changes that begin immediately after collection.

• Ice/refrigeration (4 °C): The most common preservation for many analytes, including BOD, TSS, and fecal coliform. Samples must be kept cold from the moment of collection — place them in a cooler with ice immediately.
• Chemical preservatives: Added before or at the time of collection.
• Sulfuric acid (H₂SO₄) is used for nutrients such as ammonia, nitrate, and total Kjeldahl nitrogen (TKN), and for BOD when combined with refrigeration per the required method.
• Sodium thiosulfate neutralizes chlorine in bacteriological bottles.
• Other preservatives (ascorbic acid, zinc acetate, etc.) apply to specific analytes as listed in Standard Methods, Method 1060.
• No preservation / analyze immediately: Chlorine residual, dissolved oxygen, pH, and temperature must be measured in the field or within minutes of collection. These parameters cannot wait.

Always add preservative before adding sample when the method requires it — many bottles come pre-dosed from the lab.

Holding Times

Holding time is the maximum time allowed between sample collection and laboratory analysis. Exceeding it invalidates the sample. Common examples (always verify with your state lab and current Standard Methods):

| Analyte | Holding Time (typical) |

|---|---|

| Chlorine residual | Analyze immediately |

| pH | Analyze immediately or within 15 minutes |

| Fecal coliform / total coliform | 6 hours (refrigerated) |

| BOD | 48 hours (refrigerated) |

| Nutrients (ammonia, nitrate) | 28 days (acid-preserved, refrigerated) |

| TSS | 7 days (refrigerated) |

Record the exact time of collection on the field sheet. Your lab will calculate whether the holding time was met.

Aseptic Technique for Microbiological Samples

Contaminating a bacteriological sample gives a false high count and can trigger a boil-water advisory unnecessarily — or miss real contamination. Follow these steps:

1. Pre-flush the tap: open fully and flush for at least 2–3 minutes to clear standing water from the service line.

2. Do not flame the tap in modern practice — use a clean tap in good condition.

3. Open the sterile bottle only when ready. Do not touch the inside of the bottle, cap, or neck. This is the no-touch method.

4. Hold the bottle below the tap, fill without overflowing (leave headspace if indicated), and cap immediately.

5. Place on ice in the cooler at once.

6. Label the bottle before or immediately after collection — never leave it unlabeled.

Chain of Custody and Field Data Sheets

Chain of custody (COC) is the unbroken, documented record of who had possession of a sample from collection through analysis. It protects the legal defensibility of results. Every field data sheet must record:

• Sample location and description
• Date and exact time of collection
• Collector's full name
• Container ID or label number
• Preservative added and amount
• Holding time deadline (calculated from collection time)
• Condition at time of collection (e.g., ice temperature)

Any gap in the COC can make the result inadmissible in an enforcement action.

Safety

• Acid preservatives (H₂SO₄): Wear chemical-splash goggles and acid-resistant gloves. Add acid to sample, not sample to acid (lab pre-dosing eliminates this risk). Know the location of the eyewash station.
• Biological samples: Wastewater samples may contain pathogens. Wear nitrile gloves, avoid splashing, and wash hands thoroughly after sampling.
• Coolers and fieldwork: Inspect cords on any field equipment. Work with a buddy near open water or confined spaces — follow your facility's confined space and fall-protection procedures even during routine sampling.
• Label all chemical containers clearly. Never store preservative acids next to or near food or drink.