Sample Collection and Preparation Techniques
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Sample Collection and Preparation Techniques

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Questions and Answers

What is the primary reason for taking multiple sampling increments from different locations in a large container?

  • To minimize the time spent on sampling.
  • To ensure homogeneity of the sample. (correct)
  • To reduce the cost of sampling.
  • To create a larger gross sample.

    • Why are larger gross samples usually required for solids compared to liquids?

  • Solids are generally more heterogeneous. (correct)
  • Solids require more processing time.
  • Solids are more expensive to sample.
  • Solids cannot be easily mixed.

    • What is the best material to line the lids of sample containers for organic parameters?

  • Paper
  • Glass
  • Plastic
  • Teflon or aluminum foil (correct)

    • What is the first step in the cone and quarter technique?

    <p>Pouring the bulk sample into a cone.</p> Signup and view all the answers

    What characterizes a grab sample?

    <p>It is a discretely collected sample at one time and place.</p> Signup and view all the answers

    What potential error can occur during the cone and quarter process?

    <p>Unequal segregation of materials.</p> Signup and view all the answers

    Which sampling method requires more time and is often automated?

    <p>Composite sampling</p> Signup and view all the answers

    When is the cone and quarter technique typically recommended for use?

    <p>When sample masses exceed 50 g.</p> Signup and view all the answers

    What process helps in achieving homogeneity without reducing the sample size?

    <p>Sequentially piling selected quarters of the cone.</p> Signup and view all the answers

    What is a crucial aspect of sample preparation regarding analytes?

    <p>It should not result in loss of analytes.</p> Signup and view all the answers

    What method is commonly used to reduce sample volume in soil preparation?

    <p>Implementing the cone and quarter technique.</p> Signup and view all the answers

    What does the recovery measure determine in sample preparation?

    <p>The efficiency of analytes lost during preparation.</p> Signup and view all the answers

    Which statement about composite samples is true?

    <p>They are formed by mixing discrete samples over a defined time period.</p> Signup and view all the answers

    Which of the following is a potential issue when collecting water samples from different depths in a body of water?

    <p>Concentrations of dissolved gases can vary significantly.</p> Signup and view all the answers

    What type of form must the analytes be prepared in for analysis?

    <p>Solid, liquid, or gas depending on the method</p> Signup and view all the answers

    What can lead to errors in sample preparation?

    <p>The inherent complexity and time-consuming nature of the process.</p> Signup and view all the answers

    What is the required sample throughput per hour to fully utilize the instrument mentioned?

    <p>60 samples</p> Signup and view all the answers

    Why has the use of chemicals in analytical chemistry practices changed?

    <p>To reduce costs associated with chemical purchase and disposal</p> Signup and view all the answers

    What method is commonly used to extract organic compounds from water samples?

    <p>Ether extraction using a separatory funnel</p> Signup and view all the answers

    During the crushing or grinding of solid samples, why is it important not to grind excessively?

    <p>It can alter the composition and cause loss of volatile components</p> Signup and view all the answers

    What happens to the water content of a sample when it is ground?

    <p>It may increase due to increased surface area or decrease for hydrated compounds due to frictional heating</p> Signup and view all the answers

    What is a potential consequence of increased surface area from grinding a solid sample?

    <p>Enhanced reactivity with the atmosphere</p> Signup and view all the answers

    Which method is used to ensure a consistent particle size after grinding samples?

    <p>Screening or sieving the samples</p> Signup and view all the answers

    What is a common substitute for ether extraction in analytical processes?

    <p>Passing water samples through a nonpolar solid</p> Signup and view all the answers

    What is the purpose of grinding when preparing a sample?

    <p>To remove softer and harder particles sequentially.</p> Signup and view all the answers

    What could potentially contaminate a sample during grinding?

    <p>Abrasion from the grinding surfaces.</p> Signup and view all the answers

    Which tool is appropriate for reducing the size of large samples?

    <p>Jaw crusher.</p> Signup and view all the answers

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

    <p>To mitigate the variability in water content in solid samples.</p> Signup and view all the answers

    What type of water is essential and integral to a compound's structure?

    <p>Water of crystallization.</p> Signup and view all the answers

    Which reaction indicates the presence of water of constitution?

    <p>Ca(OH)2(s) ⇄ CaO(s) + H2O(g)</p> Signup and view all the answers

    Which statement describes nonessential water in solid samples?

    <p>It is physically held and not in stoichiometric proportion.</p> Signup and view all the answers

    What happens to a soil sample's water content when collected after rain?

    <p>It increases compared to samples collected in dry conditions.</p> Signup and view all the answers

    What is a potential error during the dissolution of a sample?

    <p>Incomplete dissolution of the sample</p> Signup and view all the answers

    Which gas is likely to volatilize when samples are treated with strong acids?

    <p>H2Te</p> Signup and view all the answers

    Which of the following acids is not suitable for decomposing organic samples?

    <p>Hydrochloric acid</p> Signup and view all the answers

    What is a consequence of introducing analyte impurities from the solvent during sample dissolution?

    <p>Positive bias in results</p> Signup and view all the answers

    Why is hot concentrated nitric acid used in wet ashing processes?

    <p>To dissolve metals and destroy organic components</p> Signup and view all the answers

    At what concentration is concentrated hydrochloric acid usually found?

    <p>12 M</p> Signup and view all the answers

    What error can occur due to contaminants from vessel walls during high temperature treatments?

    <p>Addition of unwanted analytes</p> Signup and view all the answers

    Which elements are typically unaffected by hot concentrated nitric acid during dissolution?

    <p>Al and Cr</p> Signup and view all the answers

    Study Notes

    Sample Collection

    • Liquid or Gas: Samples can be homogeneous, requiring smaller gross samples
      • Heterogeneous Examples: Steel drums, lakes
    • Solids: Often heterogeneous requiring larger gross samples
      • Composite Samples: Mix sampling increments for representation
      • Sample Reduction: May be needed after composite sampling

    Sample Preparation

    • Cone and Quarter Technique: A common homogenization and sample volume reduction method
      • Error Potential: Unequal segregation of heavier materials, loss of finer materials, static charge
    • Grab vs. Composite Samples: Types of samples depend on information sought, purpose, and flow patterns
      • Grab Sample: An individual sample collected at one location and time
      • Composite Sample: Mix of grab samples collected over time or a continuous proportion of the flow
      • Composite Utility: Represents average conditions over a period, often 24 hours
      • Composite Collection: Requires automation and more time

    Sample Preparation Principles

    • No Analyte Loss: Sample preparation should maintain analyte, or loss should be quantified (recovery is the measure of the analyte remaining)
    • Optimal Chemical Form: The analyte(s) should be in the best chemical form for the analytical method
    • Sample Throughput: Sample preparation needs to keep pace with modern analytical methods
      • Contemporary Techniques: Much faster analysis times compared to traditional wet-chemistry methods
    • Solvent Use: Minimization of solvent use and chemical waste is important

    Sample Preparation - Solids

    • Particle Size Reduction: Crushing or grinding to ensure homogeneity and reagent reactivity
      • Caution: Grinding can change composition by altering sample surface area, potentially causing loss of volatile components, and affecting water content
    • Screening: Ensures consistent particle size after grinding
      • Process: Grind, remove softer particles(repeat until desired particle size)
    • Grinding Apparatus Contamination: Ensure grinding surfaces do not contaminate, especially for trace component analysis

    Tools for Particle Size Reduction

    • Large Samples: Jaw crushers, disk pulverizers
    • Medium Samples: Ball mill
    • Small Samples: Mortar and pestle

    Moisture in Solid Samples

    • Water Content Variability: Solid samples often contain water in equilibrium with the atmosphere, leading to variable water content.
      • Consideration: The sample may lose unpredictable moisture between collection and analysis
    • Dry Weight Basis: Analysis results are often reported on a dry weight basis to eliminate water variability
      • Moisture Correction: Requires a separate portion of the sample to determine water content, with a correction factor applied to all other analytical results.

    Forms of Water in Solids

    • Essential Water: Integral to a solid's molecular or crystalline structure
      • Crystallization Water: In hydrated compounds
      • Constitution Water: Formed during decomposition, e.g. calcium hydroxide
      • Stoichiometric Proportion: Essential water is present in a fixed ratio
    • Nonessential Water: Physically held by the solid, not in a stoichiometric proportion
      • Solution Requirement: Many samples need to be dissolved with reagents

    Errors in Decomposing and Dissolving

    • Incomplete Dissolution: Not all of the sample or analyte may dissolve
    • Volatilization: Loss of analyte during dissolution
      • Consideration: Volatile compounds may be lost from acid or base treatments
    • Solvent Contamination: Contamination from impurities in the dissolving solvent
    • Vessel Contamination: High temperature reactions could lead to contamination from vessel walls

    Decomposing with Acid or Base

    • Process: Heat a sample suspension in acid or alkali metal hydroxide until dissolved

    Hydrochloric Acid

    • Common Use: Dissolution of inorganic samples, not as effective for organic samples
    • Dissolving Properties: Dissolves many metal oxides and metals oxidized more easily than hydrogen
    • Concentration: Concentrated HCl is about 12 M, boiling leads to gas loss and formation of a 6 M constant boiling mixture

    Nitric Acid

    • Strong Oxidizer: Dissolves common metals, except for Al and Cr
    • Organic Sample Decomposition: Used to determine trace metal content, this process is called wet ashing
      • Wet Ashing: Destroys organic components to analyze inorganic constituents

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    Related Documents

    Sample Preparation Guide PDF

    Description

    This quiz focuses on the various methods involved in sample collection and preparation, including techniques for homogeneous and heterogeneous samples. You'll explore concepts like grab and composite sampling, as well as the cone and quarter technique for sample volume reduction. Test your understanding of these essential practices in laboratory settings.

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