Sample Collection Techniques

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Questions and Answers

What is the typical holding time for electrical conductivity samples?

7 days

4 days (correct)

60 days

48 hours

Which preservation technique is primarily used to minimize reaction rates in biological samples?

Refrigeration or freezing (correct)

Using metal containers

Addition of chemicals

Adding nutrients

What is a primary consideration when selecting sample containers for conventional parameters like pH and hardness?

Avoiding plastics that 'breathe' (correct)

Filling containers to the brim

Collecting in metal containers

Using glass containers

What should be done if the laboratory does not prepare sample bottles?

The sampler must ensure proper cleaning and preparation. Signup and view all the answers

Which of the following analytes can be collected in the same type of plastic container?

Cyanide and trace metals Signup and view all the answers

How should solid samples, such as vegetation, be treated during collection?

Dried as quickly as possible Signup and view all the answers

Which statement is true regarding the collection of organic parameters?

They should only be collected in glass containers. Signup and view all the answers

What is the maximum fill level for sludge samples during collection?

Halfway Signup and view all the answers

What is a key reason for taking samples from different depths when collecting water samples from a lake?

To determine dissolved gas concentrations that may vary Signup and view all the answers

When collecting solid samples, why is a larger gross sample often necessary compared to liquids?

Solids are generally heterogeneous Signup and view all the answers

What is the primary purpose of the cone and quarter technique?

To homogenize and reduce sample volume Signup and view all the answers

Which potential source of error is associated with the cone and quarter technique?

Unequal segregation of heavier materials during processing Signup and view all the answers

How many quarters are produced in the alternate cone technique where the sample size is not reduced?

Four Signup and view all the answers

What is a significant advantage of taking increments from various locations when collecting a solid sample?

It ensures a more representative gross sample Signup and view all the answers

What action should be taken if a composite sample is collected from several increments of soil?

Reduce the sample mass prior to analysis Signup and view all the answers

Why should the cone and quarter technique ideally be used for sample masses greater than 50 g?

To ensure uniformity and reduce potential errors Signup and view all the answers

What should be used to line the lids of containers for storing samples?

Teflon or aluminum foil Signup and view all the answers

Which type of sample represents average conditions over time?

Composite sample Signup and view all the answers

Which statement accurately describes a grab sample?

It is collected at one place and one specific time. Signup and view all the answers

What is a critical factor in the issuance of composite samples?

The number of grab samples and variability of concentration Signup and view all the answers

What is the purpose of sample preparation?

To ensure no analytes are lost and to prepare them appropriately for analysis Signup and view all the answers

Which of the following is NOT a principle of sample preparation?

Minimizing the cost of sample collection Signup and view all the answers

What common issue may occur when using plastic bottles for organic parameters?

They dissolve when exposed to organic solvents. Signup and view all the answers

In sample preparation, what is meant by the recovery of the procedure?

The amount of analyte remaining after preparation compared to the original sample Signup and view all the answers

What is a primary concern when analyzing trace components in a sample during grinding?

Contamination from grinding apparatus Signup and view all the answers

Which tool is suitable for reducing the particle size of large samples?

Jaw crusher Signup and view all the answers

Why is it critical to report analytical results on a 'dry weight' basis?

To mitigate the effects of moisture content on results Signup and view all the answers

What defines essential water in solid compounds?

Water in equilibrium with solid's molecular structure Signup and view all the answers

What is the difference between water of crystallization and water of constitution?

Water of crystallization is related to hydrates, while water of constitution results from thermal decomposition. Signup and view all the answers

What characteristic differentiates nonessential water from essential water in solids?

Nonessential water is physically held and not in stoichiometric proportion. Signup and view all the answers

What happens to a sample's moisture content during the period between collection and analysis?

It can lose an unpredictable amount of moisture. Signup and view all the answers

What process is often used to determine a sample's moisture content for analytical corrections?

Comparing dry and wet weighings Signup and view all the answers

Which type of water is held on the surface of solids and is influenced by factors such as temperature and humidity?

Adsorbed water Signup and view all the answers

What is a characteristic of occluded water in a crystal?

It is trapped inside microscopic pockets. Signup and view all the answers

How does the concentration of water in a solid generally change with increased temperature?

It decreases. Signup and view all the answers

Which type of water can sorb significantly more moisture than adsorbed water in solids?

Sorbent water Signup and view all the answers

What happens to adsorbed water on a solid as the temperature approaches 100°C?

It approaches zero. Signup and view all the answers

Which statement about sorption and adsorption is true?

Sorption may take days or weeks to equilibrate. Signup and view all the answers

What is the relationship between humidity and sorbed water content in colloidal solids?

Higher humidity enables greater sorption of water. Signup and view all the answers

Which of the following statements is correct regarding essential water in some compounds?

The type of hydrate present varies with humidity. Signup and view all the answers

Study Notes

Sample Collection and Preparation

Liquid and gas samples are often homogeneous, requiring smaller gross samples.
Larger containers like steel drums or bodies of water may require multiple sampling units taken from different locations.
Solid samples are often heterogeneous, requiring larger gross samples.
Composite samples – combining multiple sample increments from different locations – are useful for obtaining representative samples.
Cone and quarter method is a technique used to reduce the mass of a solid sample while creating a homogeneous sample:
- Bulk sample is poured into a cone.
- Cone is flattened and divided into four equal sections.
- Two opposing sections are removed.
- The remaining two sections are combined and the process is repeated, leading to a reduction in sample mass.
Potential errors in the cone and quarter technique:
- Unequal segregation of heavier materials during flattening and coning.
- Loss of fine material due to dust production or static cling.
Another variation of the cone and quarter method ensures homogeneity without sample reduction:
- Cone is divided into four equal sections, numbered 1 through 4 clockwise.
- Section 1 is removed and set aside.
- Sections 3, 2, and 4 are combined sequentially onto section 1.
- This process is repeated six times.
Holding times for various parameters:
- Electrical conductivity: 4 days
- Cyanide: 7 days
- Formaldehyde: 48 hours
- Residual chlorine: immediately analyzed
- Methanol: 7 days
- Nitrate: 5 days
- Metals: 60 days
Sample preservation techniques:
- Addition of a chemical to stabilize the analyte, preventing degradation.
- Refrigeration or freezing can also preserve samples.
- Biological parameters often require special preservation methods to ensure accurate analysis.
Sample containers:
- Container selection is important for minimizing analyte change.
- Containers should be clean and properly prepared, including pre-labeling, pre-charging with preservative, and capping tightly.
- Lab-prepared containers should be tested for contamination.
- Containers should be chosen based on the volume required for analysis.
General guidelines for container selection:
- Conventional water parameters (pH, hardness, nutrients): 500 mL plastic containers are suitable.
- Avoid breathing plastic containers when analyzing ammonia.
- Sludge samples should be filled no more than halfway to allow for gas formation.
- Trace metals and cyanide: Collect in the same plastic containers used for conventional water parameters.
- Solid samples for conventional or trace metal parameters: Wide-mouth plastic or glass jars or plastic bags.
- Vegetation: Dry quickly to prevent decay.
- Organic parameters: Collect in glass containers with Teflon or aluminum foil linings. Avoid plastic bottles due to contamination risks.
- Volatile parameters: Collect in containers with no headspace.
Types of samples:
- Grab sample: Individual sample taken at one location and time.
- Composite sample: Mixture of grab samples taken over time or a continuous portion of the flow. The number of grab samples depends on the variability of concentration and flow. Represents average conditions for a given duration, typically 24 hours.
Sample preparation:
- Essential steps to prepare samples for analysis.
- Often the most time-consuming, error-prone, and labor-intensive part of an analysis.
- Requires chemical expertise.
- Five general principles apply to all sample preparations.
Principles of sample preparation:
- Minimize analyte loss during preparation. Recoveries should be determined if loss occurs.
- Transform analytes into the best chemical form for the analysis.
Sample size reduction:
- Jaw crusher: For large samples.
- Disk pulverizer: For large samples.
- Ball mill: For medium-sized samples.
- Mortar and pestle: For small samples.
Moisture in solid samples:
- Variable water content in solid samples can affect analysis results.
- Analytical results are typically reported on a "dry weight" basis.
- Separate portion of the sample is used to determine moisture content, which is then used to correct other analytical results.
Forms of water in solids:
- Essential water: Integral part of the molecular or crystalline structure of a solid compound.
  - Water of crystallization: Present in hydrates, for example, CoCl2·6H2O.
  - Water of constitution: Formed when a pure solid is decomposed by heat or chemical treatment, for example, calcium hydroxide when heated.
- Nonessential water: Water that is physically held by a solid, not in stoichiometric proportion.
  - Adsorbed water: Held on the surface of solids, amount depends on temperature, humidity, and surface area.
  - Sorbed water: Seen in colloids like starch, protein, and silica gel, held in the interspaces of the colloidal solid, amount depends on temperature and humidity.
  - Occluded water: Trapped in microscopic pockets in solid crystals, its amount is independent of humidity.
Effect of temperature and humidity on water content:
- Concentration of water in a solid generally decreases with increasing temperature and decreasing humidity.
- The specific effect depends on the types of water present.
- Essential water: Compounds may form multiple hydrates, the specific hydrate present depends on humidity.
- Adsorbed water: Increases with increasing humidity, decreases with increasing temperature.
- Sorbed water: Higher potential for water sorption than adsorption, slower equilibration than adsorption.
- Occluded water: Not in equilibrium with the atmosphere, amount is independent of humidity.
Dissolving and decomposing:
- Many inorganic analytical methods require the sample to be in aqueous solution.

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Description

Explore various methods for collecting and preparing liquid, gas, and solid samples. This quiz covers the cone and quarter method, composite sampling, and potential errors in sample preparation. Enhance your understanding of how to obtain representative samples in different scenarios.