⚗️Analytical Chemistry Unit 3 – Sample Preparation and Handling

Key Concepts and Terminology

• Sample preparation involves steps taken to transform a sample into a form suitable for analysis, including homogenization, extraction, and dilution
• Sample handling encompasses procedures for collecting, transporting, storing, and preparing samples while maintaining their integrity
• Representativeness ensures that the sample accurately reflects the composition of the original material or environment
• Homogeneity refers to the uniform distribution of analytes throughout the sample matrix
• Contamination occurs when unwanted substances are introduced into the sample, potentially interfering with analysis results
• Analyte is the specific substance or chemical component of interest within a sample
• Matrix describes the components of a sample other than the analyte, which can affect the analysis process
• Preconcentration techniques (solid-phase extraction) are used to increase the concentration of analytes in a sample prior to analysis

Sample Collection Techniques

• Grab sampling involves collecting a single sample at a specific time and location, providing a snapshot of the sample's composition
• Composite sampling combines multiple subsamples into a single sample, representing the average composition over a given period or area
• Stratified sampling divides the population into subgroups (strata) based on specific characteristics, and samples are collected from each stratum
• Random sampling selects sample locations or individuals randomly, ensuring each unit has an equal chance of being chosen
• Systematic sampling collects samples at regular intervals (time or space) from a population
• Depth-integrated sampling (water column) involves collecting samples at various depths to account for vertical variations in composition
• Passive sampling uses devices (diffusive samplers) that accumulate analytes over time without active pumping or mixing
• Preservation methods (acidification, refrigeration) are employed immediately after sample collection to maintain the sample's integrity

Sample Preservation Methods

• Refrigeration or freezing slows down chemical reactions and microbial growth, preserving the sample's composition
• Acidification lowers the sample's pH to prevent precipitation, adsorption, or degradation of analytes
• Chemical preservatives (formaldehyde, mercuric chloride) are added to inhibit microbial activity and stabilize volatile compounds
• Drying or lyophilization removes moisture from the sample, preventing chemical and biological changes
• Inert atmosphere storage (nitrogen or argon) prevents oxidation and degradation of sensitive analytes
• Containerization involves selecting appropriate containers (glass, plastic) based on the sample's properties and analytes of interest
• Headspace minimization reduces the air volume above the sample in the container, limiting volatilization and oxidation
• Proper labeling and documentation ensure sample traceability and include essential information (collection date, location, preservatives used)

Sample Preparation Procedures

• Homogenization ensures uniform distribution of analytes within the sample matrix, often achieved through grinding, mixing, or blending
• Subsampling involves selecting a representative portion of the homogenized sample for further analysis
• Extraction separates the analytes of interest from the sample matrix using techniques such as liquid-liquid extraction, solid-phase extraction, or Soxhlet extraction
• Filtration removes particulate matter from liquid samples using membranes or filters with specific pore sizes
• Digestion breaks down the sample matrix, releasing analytes for analysis, and can be achieved through acid digestion, microwave digestion, or enzymatic digestion
• Dilution adjusts the concentration of analytes to fall within the analytical method's working range
• Derivatization modifies the chemical structure of analytes to improve their detectability or separation during analysis
• Spiking involves adding known amounts of analytes to the sample to assess method recovery and matrix effects

Common Equipment and Tools

• Balances are used to accurately weigh samples and reagents, with analytical balances providing high precision for small quantities
• Pipettes and micropipettes deliver precise volumes of liquids for sample preparation and analysis
• Centrifuges separate components of a sample based on their density, using centrifugal force
• Shakers and mixers ensure thorough mixing and homogenization of samples and reagents
• Grinding and milling equipment (mortar and pestle, ball mills) reduce particle size and increase sample homogeneity
• Filtration apparatus (vacuum filtration, syringe filters) remove particulate matter from liquid samples
• Extraction equipment (separatory funnels, Soxhlet extractors) facilitate the separation of analytes from the sample matrix
• Digestion systems (hot plates, microwave digesters) break down the sample matrix and release analytes for analysis

Safety Considerations

• Personal protective equipment (PPE) such as lab coats, gloves, and safety glasses should be worn to protect against chemical splashes and exposure
• Fume hoods provide ventilation and containment when working with volatile or hazardous chemicals
• Proper handling and disposal of chemicals and waste minimize environmental impact and ensure compliance with regulations
• Material Safety Data Sheets (MSDS) provide information on the hazards and safe handling procedures for chemicals used in sample preparation
• Emergency equipment (eyewash stations, safety showers) should be readily accessible in case of accidents
• Proper labeling and storage of chemicals prevent mix-ups and unintended reactions
• Training on safe laboratory practices and emergency response procedures is essential for all personnel involved in sample preparation
• Regular maintenance and calibration of equipment ensure safe and reliable operation

Quality Control and Assurance

• Blanks (method blanks, reagent blanks) are used to assess background contamination and ensure the absence of interferences
• Calibration standards with known concentrations of analytes are used to establish the relationship between instrument response and analyte concentration
• Replicates (sample replicates, method replicates) assess the precision and reproducibility of the sample preparation process
• Certified reference materials (CRMs) with known composition are used to validate the accuracy of the analytical method
• Spiked samples (matrix spikes, surrogate spikes) evaluate the recovery and matrix effects of the sample preparation procedure
• Control charts monitor the stability and performance of the analytical process over time
• Method validation demonstrates that the sample preparation and analytical procedures are fit for purpose and meet specified requirements
• Documentation and record-keeping (standard operating procedures, sample preparation logs) ensure traceability and adherence to established protocols

Troubleshooting and Best Practices

• Identify potential sources of contamination (reagents, glassware, environment) and take steps to minimize or eliminate them
• Use high-purity reagents and solvents to reduce background interference and ensure accurate results
• Properly maintain and calibrate equipment to ensure optimal performance and reliability
• Optimize extraction conditions (solvent selection, temperature, time) to maximize analyte recovery and minimize matrix interferences
• Assess the compatibility of sample preservation methods with the analytical technique to avoid interferences or analyte loss
• Validate sample preparation methods using spiked samples and certified reference materials to ensure accuracy and precision
• Regularly review and update standard operating procedures (SOPs) to incorporate improvements and address any issues encountered
• Collaborate with colleagues and consult literature resources to troubleshoot challenging sample matrices or analytes