Scientific Sampling Techniques

Scientific Techniques to Separate and Determine Purity

Sampling Techniques

• Sampling is employed for analysis after a representative piece of material is extracted
• Special care must be taken to collect a representative sample to avoid contamination
• Ambient sampling of the atmosphere provides analytical data on seasonal or other trends
• Near ground monitoring sites give the data required to asses air quality for pollution control designs
• Groundwater-monitoring wells are used to sample aquifers providing data to ensure water quality
• Rivers and streams are sampled to track pollution coming from industry, agriculture, sewers, and cities
• The ocean is sampled to study carbon cycle amounts on earth, and seafloor processes

Sampling Process

• The Sample that arrives at the lab is converted by a set of operations to the test sample before analysis
• Homogenization maybe be required to make the sample uniform
• Particulate solids might require conversion to a solution
• Low analyte concentrations or interfering substances may necessitate isolation or concentration via separation and purification
• Additives are used to mask interference, or the analyte must be chemically converted to another form
• The sampling plan represents the distribution of one or multiple analytes of interest: either spatial dimensions or dynamically changing systems
• The success of the sampling plan depends on how accurately a much larger system is represented in the microcosm of the laboratory sample

Homogenization

• The laboratory sample needs to be further reduced and processed to what is frequently the test sample
• If the analyte is associated with one or more constituents, it is possible to grind the laboratory sample to reduce the average particle size
• The average particle size is reduced until the analyte can be regarded as a pointlike component of the entire laboratory sample
• The point like particle size is called the liberation size and varies with the analyte and the type of material.
• Mixing of the laboratory sample is another critical operation which can be done manually or automated
• Liquid samples or even gases need thorough blending by stirring, agitating or gently warming the storage vessel

Labeling and Storage

• Sample labels provide appropriate details, including the batch number, container number, amount taken, and purpose
• Labels are applied at the time of sampling
• The container is labeled with sample type, material name, identification code, batch/lot number, quantity, date of sampling, storage conditions, handling precautions, and container number
• The storage container should not interact with the sampled material to prevent contamination.
• The container should protect from light, air and moisture per pharmaceutical or product requirements
• As sealed containers should be tamper-evident.
• Samples of loose materials and liquids get placed in appropriate clean containers or bottles with screw tops and inert liners

Sampling for solids, liquids and gases

• Gas sampling is performed by collecting particulates via glass/quartz/fiber or membrane filters for analysis
• The analysis is done by collecting and extracting particulates from the air via filters
• The filters are then dissolved and analysed.
• Air samples can also be fully contained in special bags allowing for lab analysis via gas chromatography or other equipment
• Air samples are moved into sample bags using pumps

Water sampling

• Shallow water samples can be easily taken using sampling bottles
• Deeper water samples can be taken using an electronic instrument called a CTD rosette with attached Niskin bottles
• CTD rosettes contain a CTD and Niskin bottles to measure conductivity, temperature, depth, and to hold the separate water samples
• The Niskin bottles are sent down open, and triggered to close electronically at a specific depth
• This way, water can be sampled at many different depths throughout the water column, and kept separate from each other

Water Sample pre-treatment

• Most water samples require pre-treatment via filtration through a 0.45 µm filter
• Liquid samples may require immediate addition of analyte-specific preservatives to stop any chlorine reactions
• The added preservative should not interfere during measurements
• Trace metal analyses often require cooling

Soils sampling

• The sampling of soils, dusts, sludges, or sediments poses difficulties due to inhomogeneities
• Using a soil corer is typically employed by walking in a zig-zag pattern to accumulate cores
• The greater the number of collected cores mixed together, the better the resulting sample represents the average area condition
• Sediment sampling is carried out using an Ekman Bottom Grab, designed to collect samples free from vegetation and intermixtures of sand, stones & other coarse debris

Separation Techniques:

• Preparation and Isolation of inorganic compounds, precipitation, filtration, drying, and crystallization are used to separate substances
• Filtration is used to separate an insoluble solid from a liquid by using small pores to sepearat molecules

Crystallization

• Crystallization is used to produce solid crystals from a solution by removing most of the solvent via evaporation

Immiscible liquids Separation

• Two immiscible liquids can be put in separation funnel and a beaker. As one opens it one can pour out the lower layer of liquid for example water carefully into the beaker.

Analytical techniques to determine purity

• Melting points and boiling points are used to characterize organic and inorganic crystalline compounds and ascertain their purity

Melting and boiling point

• Pure substances melt at a highly-defined temperature with a small temperature range of 0.5 – 1 °C
• Impure substances exhibit a large melting interval & melt at lower temperatures
• Melting and boiling points are important QC parameters for pharmaceutical substances
• Impurities in a crystalline solid disrupt forces that hold solids together, requiring less energy to melt the solid surrounding the impurity
• Using a mortar, pestle and capillary tube one can work the testing substance into the tube
• Heating in a tube in small increments allows one to properly measure the solids melting points
• The Vapour pressure equals the applied pressure during measurement of the Boling point of liquids

Refractometry

• A refractometer measures the degree to which light is bent as it passes from air into a sample
• Refractometers are used to determine the index of refraction of liquid samples
• The refractive index is light's speed in two mediums
• Refractive index is commonly determined as part of the characterization of liquid samples, similar to solid compounds and melting points are to each other,
• Used to identify or confirm a sample by comparing the refractive index to known values.

Polarimetry

• Polarimetry is of interest to chemists because the ability of a substance to affect polarized light is closely related to its chemical structure
• Polarimetry is a non-destructive technique to measure the optical activity exhibited by inorganic and organic compounds.
• A polarimeter relies on the specific structure of inorganic and organic compounds in water

Like Dissolves Like

• Polar substances WILL dissolve in polar substances
• Non-polar substances WILL dissolve in nonpolar substances
• Non-polar substances WILL NOT dissolve in polar substances
• Like dissolves like dictates that polar dissolves polar, so water dissolves salt

Solubility

• Ethanol dissolves in water, a polar solvent, but the left side of the molecule has non-polar properties
• The molecules alcohol functional group (OH) allowing molecules to be dissolve in solvent
• Different organic compounds types all display different solubility properties

Separation techniques

• Common separation techniques:
  • Solvent Extraction
  • Distillation
  • Simple Distillation
  • Fractional Distillation
  • Thin Layer Chromatography
  • High Performance Liquid Chromatography
  • Gas Chromatography
• These techniques depend on separation from either solvent extraction or extraction using fractioning

Thin layer chromatography and chromatography

• Thin layer chromatography uses a thin, uniform layer of silica gel or alumina coated onto a piece of glass, metal or rigid plastic via mobile phase
• Molecules with high affinity remain a longer time
• Molecules that have affinity move a faster with movement allowing molecules to separate in different fractions
• A “retention factor” is applied after drawing a line near bottom, used to determine ratio of molecule travel via liquid over mobile