Sampling Techniques PDF

 Analytical Process Steps q An analysis provides chemical or physi­cal information about a sample Problem definition & Method selection Sample collection ( ) Sample preservation Sample preparation...

 Analytical Process Steps q An analysis provides chemical or physi­cal information about a sample Problem definition & Method selection Sample collection ( ) Sample preservation Sample preparation Analyte separation Analysis Data processing and reporting Ex: Food Safety Analysis  Introduction § Sample: This is a small part drawn from the population (portion of a population) § Population vs Sample The population is the entire group The sample (portion of a population)  What is sampling? § Sampling can be defined as the process or technique of selecting a suitable sample for the purpose of determining parameters or characteristics of the whole population. § Sampling is the process to get a representative and homogeneous sample. Representative i.e., content of analytical sample reflects content of bulk sample. Homogeneous i.e., the analytical sample has the same content throughout. § Sampling covers all the necessary steps prior to measurement, including the preparation and storage of the samples § Sampling error: is the differences between sample values and the population value.  Terminology ØTypes of analysis: 1- Qualitative (what) 2- Quantitative (how much?) ØAnalyte and Matrix Component of interest in the sample is the Analyte Remainder of the sample is the Matrix Ø A lot is the total amount of material that we take our sample from. Ø The bulk sample is a large sample that is first taken from the lot. Ø The gross sample is several small portions of the sample. Ø This is reduced to provide a laboratory sample. Ø An aliquot of this sample is taken for the analysis sample.  Steps involved in sampling bulk material Identify the population from which the sample is to be obtained Collect a gross sample that is truly representative of the population being sampled Reduce the gross sample to a laboratory sample that is suitable for analysis Example  Terminology Ø Sample Size and Analyte Level Techniques for handling very small samples are quite different from those for treating macro samples  Importance of Sampling and Sample Preparation Ø Sampling depends on: properties of the analyte nature of the sample Ø Purpose of sampling Obtaining correct and informative results Ø Inappropriate sampling causes irreversible damage Environmental samples, such as soil, air, and water: Representative sampling right at the point of collection Define that the entire sample or a portion will be analyzed Sampling depends on the location, depth, and time of the year Ø Sample contamination during sample collection = invalid data Sample Preparation Steps  Sampling Plan Ø What is sampling plan? It is a comprehensive protocol that specifies the following i) Number, location, and size of the samples ii) Instructions for altering, processing or reducing samples iii) How many samples must be run iv) Which method or instrumentation to use v) How to report results  Where to Sample the Target Population (Sample types) Ø Sample composition is not identical to its target population ≡ Sample error Ø Homogeneous (well­stirred oil tank: where to sample x) Ø Heterogenous samples (where to sample √ ) Examples: ­ Oral suspension ­ Clinical sample, such as blood or urine (A patient’s blood glucose level) ­ Concentration of dissolved O in a lake 2 Sampling Methods  Quiz Sample error: with A or B?  Random sampling Ø There are different types of random sampling Judgmental sampling Wastewater sampling  What type of sample to collect? Ø Common methods for obtaining (collecting) samples Ø Grab sample we collect a portion of the target population at a specific time and/or location) For homo­geneous target population → random grab samples For heterogeneous target population → systematic grab samples (properties change over time and/or space) Ø Composite sam­ple A set of grab samples that we combine into a single sample before analysis (target population’s average composition over time or space) Examples: wastewater treatment plants Determination of polychlorinated biphenyls (PCBs) in fish  What type of sample to collect? Ø In situ sample For continuous monitoring of a time­dependent change in the target population without removing individual grab samples Example: monitoring the pH of a solution moving through an industrial produc­tion line using a pH electrode  Error Sources Prior to Total Element Determination Ø Contamination of sampling tools and vessels, Ø ­ Adsorption of the analytes on the surface of the collection and storage tools Complex matrices e.g., blood, urine, human milk, wet precipitation, sea water, fresh water, soil solution and some basic food materials ­ Dissection of tissues with trace element levels (µg/kg) (tools?) Ø Time of sampling for body fluids and fresh waters Ø In environmental samples, e.g., plants and soils, waste and industrial materials (trace metals in mg/kg) (representative analytical subsamples not contamination) Ø Place of sampling (environmental and industrial materials) Ø Careful selection of number and mass of individual samples, which depends on analyte’s distribution and concentration Error Sources Prior to Total Element Determination  Implementing a sampling plan Ø Common steps: 1. Physically removing the sample from its target population (device must be inert and clean to …..) 2. Preserving the sample (why?) (what about in situ sampling) 3. Preparing the sample for analysis Ø The initial sample is called the primary or gross sample Ø Reducing the sample’s particle size to ­ convert the sample into a more readily analyzable form ­ improve its homogeneity.  Commerci al solvents Suspensio ns; such as Beverages, those such as found in milk or many oral fruit juice medicatio ns Natural waters, Bodily including fluids, such lakes, as blood streams, and urine seawater and rain  Sample Collection Ø Surface water ­ < 5 m (Capped bottle) ­ > 5 m (Niskin bottle) Ø Purging ­ Groundwater ­ Municipal water Ø What type of sample for Municipal wastewater treatment plants? Industrial discharges? Ø Types of sample containers: ­ Glass (advantages and disadvantages?, when? ) ­ Plastic (advantages and disadvantages?, when? ) Ø Sample container ­ Mouth ­ Cap  Sample Preservation and Preparation Ø Purpose of preservation? (how?) Ø Additional preparation (which?) 26  Gaseous samples Automo bile exhaust Emission s from Aerosol industria particula l tes smokest acks Compres Atmosph sed eric gases gases 27  Sample Collection Ø Sample container: Stainless steel canister or a Tedlar/Teflon bag ­ advantages and disadvantages? ­ Cryogenic cooling Ø Trapping using solid sorbents or filtration (When?) Ø Sample volume: ­ volatile gases (2–100 L) ­ semi­volatile gases (2–500 m3) Ø Type of sorbents: ­ inorganic sorbents (disadvantage?) (silica gel, alumina, magnesium aluminum silicate, molecular sieves) ­ organic polymers ( - 2,4-diphenyl-p-phenylene oxide, styrene-divinylbenzene, - polyurethane foam) ­ carbon sorbents (advantage and disadvantage?) Ø Filtration: glass fiber filters for non­volatile compounds Ø In situ sampling (when?)  Sample Preservation and Preparation Ø There is little need (why?) Ø How to release gaseous compounds from solid sorbents ? Ø How to determine gaseous analyte in a solid sorbent, which is selective to this analyte ?  large particula tes (in smaller tissues ores) particula (biologic tes (soils al and specime sedimen ns) ts) tablets, sheet pellets, material and s capsules (polymer (pharma s, rolled ceutical metals) animal s) feeds Ø Why solid samples must be collected carefully?  Sample Collection Ø Sediments - A bottom grab sampler ­ How? ­ Advantages and disadvantages? - A corer ­ How? ­ Advantages? Ø Soil samples (depth) up to 30 cm: scoop or shovel or soil punch > 30 cm: a trench or an auger Ø Particulate materials ­ Large (ores): riffle (Describe) ­ small (powders): sample thief (Describe)  Sample Preservation Ø Why may solid sample undergo a change in composition without preservation? Ø Storing samples at lower temperatures (Advantages and disadvantages?) Ø How to minimize the loss of volatiles? Ø Why do contact of air with anaerobic sediments must be prevented?  Sample Preparation Ø Solid samples usually need some processing before analysis 1­ Reducing particle size A heterogeneous material with large particulates: (Ex: Ni­bearing ore) (why?) ­ to collect the same number of particles with a smaller, more manageable mass and reduce standard deviation 2­ Many analytical techniques require that the analyte be in solution  Reducing Particle Size Crushing and grinding the gross sample Ø Ways of crushing and grinding? - Jaw crushers: reduce large particles to diameters of a few millimeters. - Ball mills, disk mills, and mortars and pestles: to further reduce particle size Ø Disadvantages of crushing and grinding? ­ Increasing the surface area ­ losing volatile components ­ oxidation may alter the gross sample’s composition ­ Contamination  Reducing Particle Size Ø State the techniques of obtaining subsamples. Ø What are the steps that can be carried out before quartering procedures? Ø Describe the method of coning and quartering. Coning and quartering  Bringing Solid Samples Into Solution (Sample Dissolution) A) Easily dissolved samples: inorganic salts (water) and organic compounds (organic solvents) B) Difficulty dissolved samples: i) Digestion with an acid or a base (wet ashing) ii) Microwave digestion iii) Fusion using fluxes iv) Dry ashing  Bringing Solid Samples Into Solution i) Digestion with an acid or a base Ø How is digestion carried out with an acid or a base ? Ø What are the advantages and disadvantages of digestion with an acid or a base? Bringing Solid Samples Into i) Digestion Solution with an acid or a base  Bringing Solid Samples Into Solution ii) Microwave digestion Ø How is microwave digestion carried out? Ø Describe the vessels for microwave digestion? Ø What are the advantages and disadvantages of microwave digestion?  Bringing Solid Samples Into Solution iii) Fusion using fluxes Ø When is sample digested by fusion and how is it carried out? Ø What are the disadvantages of digestion by fusion?  Bringing Solid Samples Into Solution iv) Dry ashing Ø How is dry ashing carried out? Ø What is the goal of dry ashing? Ø How can the concentration of the organic material be determined using dry ashing?  Separating the Analyte from Interferents Ø When is analyzing samples a relatively simple task? Ex. Glucose in honey ­ In the absence of interferents: Ssamp= kA CA ­ In the presence of interferent (I): Ssamp= kA CA + kI CI ­ Method selectivity is measured by the selectivity coefficient (KA,I ) KA, I = kI / kA Ssamp= kA CA + KA, I * kA * CI Ssamp= kA (CA + KA, I * CI) - No interference when (KA, I * CI )