trace micro wk 5

• Sample analysis workflow

  1. Sample collection and storage

  2. Sample preparation

  3. Detection

  4. Data analysis
• Objective

  Identification? Quantification?
• Analytical technique


  To measure the parameter
• Sample collection and storage

  1. What is the sample being collected

  2. Sampling (different range of procedures depending on sample)
• Things to consider for sample storage

  • Storage conditions (temp)
  • Length of storage time (max time frame before it undergo changes)
  • Storage vessel (material used does not react w sample)
  • Addition of preservatives or stabilizers (EDTA added to prevent clotting in blood samples)
• Potential errors for sample collection and storage

  • Uneven sampling
  • Loss of analytes due to evaporation,decomposition or adsorption onto vessel materials
  • Contamination (sampling tools, airborne contamination, container material)
• Need for sample preparation

  • Interference from matrix
  • Very low concentrations difficult to analyze
  • Extraction of trace elements
  • Overall improves sensitivity and determination of trace amounts
• Factors affecting choice of sample preparation

  • Type of matrix
  • Conc of sample
  • Volume of sample
  • Type of analysis
• Factors affecting sample preparation


  • Environment (purify dust particles through HEPA filters)
  • High purity reagents (Most used analytical reagent: Milli-Q water (ultra pure water), Ultra pure solvent must be used to avoid contamination: Trace elements and anhydrous)
• Different methods of sample preparation

  1. Filtration (require ideal filter: not clog easily, uniform and reproducible pore size, not shed fibers)

  2. Depth filters (formed from randomly distributed matrix of fibers, no well defined pore size, separation relies on physical trapping and surface contact, thicker filter = smaller particles which passes through, e.g. glass fiber filters)

  3. Barrier filters (better defined pore size, filter gets easily saturated, commonly material: aluminum oxide)

  4. Choice of filter (depends on requirements on analysis, cleaning of filters, depends on analyte and filter material)

  5. Homogenisation (solid samples: ensure material is homogenized, reduce particle size, increasing total surface area to aid in digestion process, e.g. cold grinder)

  6. Digestion by solvent (addition of solvent which extract analytes of interest, solvents should not react with analytes of interest)

  7. Wet decomposition (wet ashing: reagents: minerals and oxidizing agents, effective for inorganic and organic materials, destroys samples matrix, can be done in open or closed system, analyze specific minerals)

  8. Microwave decomposition (heats solvent via dipole rotation and ionic conduction, speeds up acid dissolution by fast and homogenous heating, advantages: minimal solvent use, excellent recovery, disadvantages: thermal liable analysis might be decomposed)

  9. Combustion (dry ashing: no solvents used, heat sample in furnace to remove organic substances (500-600 degrees celsius), disadvantages: contamination, slow ashing of some materials due to no solvents used)

  10. Drying

  11. Cartridge extraction
• Dipole rotation and ionic conduction

  Polar molecules align dipole molecules w microwave electric field, constant rotation of molecules cause them to collide with other molecules to generate heat
• Separation techniques for liquid samples

  1. Liquid-liquid extraction using 2 immiscible solvents

  2. Evaporation

  3. Solid phase micro extraction
• Solid Phase Micro Extraction (SPME)

  Solvent free sampling and preparation technique, extracting phase can be liquid or solid, extracts different kinds of analytes from either gas or liquid sample
• Sources of errors for extraction and decomposition


  • Incomplete extraction/digestion
  • Loss of analytes due to evaporation, decomposition
  • Contamination from atmosphere
• Detection methods

  • Mass spectrometry (at trace level, low concentration)
  • Chromatography
  • UV-VIS spectroscopy (measure how much light a chemical substance absorbs)
• Types of errors in analysis data

  • Systematic errors (give rise to constant and proportional error)
  • Random error (give rise to random variations in analytical results)
• Why are spectrophotometer used

  Ability to disperse light that makes up white light, dispersed colors allow for quantitative (concentration) and qualitative (identify) analysis
• Spectroscopy overview

  Interaction between matter and electromagnetic radiation, detection and estimation of the number of food constituents by measuring amount of radiation absorbed or emitted
• 3 Parts to spectroscopy

  • Absorption
  • Mass spectrometry
  • Emission
• Absorption in spectroscopy

  Radiative energy is absorbed by material, measures energy that is not absorbed, widely used in food analysis (e.g. estimation of proteins, carbs, minerals, additives)
• Emission in Spectroscopy

  Radiative energy is released by material, used to estimate certain minerals
• UV-VIS absorption spectrophotometry


  Measures the amount of light absorbed after it passes through a sample, transmitted light = light not absorbed, the lower the wavelength = the higher the energy
• Inside a spectrophotometer

  1. Lamp: shines light through sample

  2. Lens: disperses light

  3. Monochromator: filter amount of wavelength that can pass through

  4. Sample cuvette: analyte absorbs wavelength

  5. Detector: unabsorbed light will be amplified to get a readout
• Sample Receptacle

  • Cuvettes (2 transparent + 2 frosted side, disposable plastic (350-800 nm wavelength), quarts (wide wavelength range: 200-800 nm))
  • Capillary tubes (used for small sample)
• Need for blanks in UV-VIS spectroscopy

  To off set solvent in diluent
• Absorbance and transmittance

  100% transmittance = 0% absorbance, spectrophotometer measures transmittance but translates into absorbance values, absorbance max value: 1
• Colors in spectroscopy

  • Qualitative analysis
  • Quantitative analysis
• Qualitative analysis

  Compounds have specific spectrum that can be used for identification purpose, look at peaks of the graph for maximum wavelength absorbed by sample
• Quantitative analysis (conc of analyte)

  Beer-Lambert law: absorbance of a material in solution is directly dependent on the concentration of that material, absorbance is proportional to concentration
• Deviations from Beer-Lambert law (if abs >1)

  • Stray light at high absorbance light
  • Stray light due to instrument malfunction
  • Reaction of solvents
• What to do if abs>1

  Too concentrated, dilute further
• Steps to conducting quantitative analysis

  1. Set standards (known and varying conc of sample)

  2. Plot standard curve to determine conc of unknown

  3. Plot standard curve (absorbance versus wavelength), Y = mx + c, Y = absorbance, X = concentration (M), R2 must be less than 1
• Importance of sample storage

  • Prevent changes in analyte between sampling and analysis
• Analyte undergone change between sampling and analysis = analysis becomes useless
• Some sample constituents change rapidly = need to be measured in situ