Sampling and Sample Prep PDF

For review use, not for online uploading Sampling Sampling A procedure whereby a part of a substance is taken to provide for testing a representative sample of the whole or as required by the appropriate specification for which the substance is to be tested. Correct sampling is the key to produci...

For review use, not for online uploading Sampling Sampling A procedure whereby a part of a substance is taken to provide for testing a representative sample of the whole or as required by the appropriate specification for which the substance is to be tested. Correct sampling is the key to producing accurate analysis results. TERMS RELATED TO SAMPLING Population --The whole of the material whose properties the analysts are trying to obtain an estimate. Sample -- a fraction of the population usually selected for analysis. Laboratory Sample --a fraction of the sample actually used in the final laboratory analysis. Lot – quantity of material which is assumed to represent a single population for sampling purposes. Batch – quantity of material )or assumed) to have been produced under uniform conditions. Batch – samples that are prepared or analyzed together with the same process and personnel using the same lot(s) of reagents. Preparation Batch – composed of 1-20 samples of the same quality systems matrix, meeting the above mentioned criteria and processed within 24 hours. Analytical Batch – composed of samples which are analyzed together as a group. They can include prepared samples from various matrices and can exceed 20 samples. Increments – portion of material obtained using a sampling device from lot/batch) Primary or Gross Sample – combination of increments Composite or aggregate sample – combination of primary samples Test (analytical) portion – material actually submitted for analysis Matrix --is the medium containing analyte. Matrix effect -a physical interference, and can either suppress or enhance absorbance signal of analyte. It occurs when components of sample matrix other than the analyte react to form molecular species and sample background. The detector picks up unspecified signals from sample matrix that do not match the absorbance line of the analyte. Steps in analysis  THE ANALYTICAL APPROACH Homogeneous Sample- Same chemical composition throughout(steel, sugar water, juice with no pulp, alcoholic beverages) Heterogeneous Sample- Composition varies from region to region within the sample (pudding with raisins, granola bars with peanuts) - Differences in composition may be visible or invisible to the human eye (most real samples are invisible) - Variation of composition may be random or segregated Sampling is the process by which a sample population is reduced in size to an amount of homogeneous material that can be conveniently handled in the lab in which the composition is representative of the population (unbiased estimate of population mean). Ex. Population : 100 coins Each coin is a sampling unit or an increment Gross sample: 5 coins the collection of individual sampling units or increments Lab sample : the gross sample is reduced in size and made homogeneous - The most important step is the collection of the sample of the material to be analyzed - Sample should be representative of the material - Sample should be properly taken to provide reliable characterization of the material - Sufficient amount must be taken for all analysis Representative Sample - Reflects the true value and distribution of analyte in the original material The fractions of the samples that collected for analyses must be representative of the bulk materials. All 3 steps of sampling, standardization, and calibration require a knowledge of statistics. SAMPLING……. STUDY POPULATION SAMPLE TARGET POPULATION 18  SAMPLING Steps in Sampling Process - Gross representative sample is collected from the lot - Portions of gross sample is taken from various parts of material Sampling methods include - Long pile and alternate shovel (used for very large lots) - Cone and quarter Aliquot - Quantitative amount of a test portion of sample solution  SAMPLING - Care must be taken since collection tools and storage containers can contaminate samples - Make room for multiple test portions of sample for replicate analysis or analysis by more than one technique Samples may undergo - grinding - chopping - milling - cutting Liquid Samples SAMPLING - May be collected as grab samples or composite samples - Adequate stirring is necessary to obtain representative sample - Stirring may not be desired under certain conditions (analysis of oily layer on water) - Undesired solid materials are removed by filtration or centrifugation - Layers of immiscible liquids may be separated with the separatory funnel  SAMPLING Solid Samples - The most difficult to sample since least homogeneous compared to gases and liquids - Large amounts are difficult to stir - Must undergo size reduction (milling, drilling, crushing, etc.) to homogenize sample - Adsorbed water is often removed by oven drying Gas Samples SAMPLING - Generally considered homogeneous - Samples are stirred before portions are taken for analysis - Gas samples may be filtered if solid materials are present Grab samples - Samples taken at a single point in time Composite Samples - Samples taken over a period of time or from different locations SAMPLING Gas Samples Scrubbing - Trapping an analyte out of the gas phase Examples Active Sampling of Gases/Vapors - Passing air through activated charcoal to adsorb organic vapors - Bubbling gas samples through a solution to absorb the analyte Samples may be taken with - Gas-tight syringes - Ballons (volatile organic compounds may contaminate samples) - Plastic bags (volatile organic compounds may contaminate samples) - Glass containers (may adsorb gas components) Passive Sampling of Gases/Vapors  Sampling Errors considerably higher than that in the methodology itself 1. During sampling 2. the sample preparation portion of an analytical procedure Sampling error The total error of an analytical result is the sum of sampling, sample preparation and analytical errors. The large influence on the final analytical result makes it essential that the sampling error is kept to a minimum. The simplest way to achieve this is by increasing the number of sampling sites and sample size of incremental samples within a sampling plan. LOT Primary Secondary Analysis Result µ sample sample s1 s2 s3 sx x (s) Propagation of errors: sx = å i s 2 Example: sx = (5%)2 + (2%)2 + (1%)2 = 30(%)2 = 5.5% GOAL: x = µ Analytical process usually contains several sampling and sample preparation steps Sampling statistics : Total error = sampling error + analytical error sT = ( sS2 + sA2 )1/2 In designing a sampling plan the following points should be considered. 1. the number of samples to be taken 2. the size of the sample 3. should individual samples be analysed or should a sample composed of two or more increments (composite) be prepared. Sources of Errors in analysis: 1. Matrix -A limiting factor, different samples can have different components which may interfere with the analytical result, depending on their chemical structure and properties. 2. Environmental factors (ambient temperature during tests/fluactuating) 3. Operators of analytical tests must be properly trained in order to administer the tests accurately. Different test methods require different operator know-how. 4. Equipment  Selection of a limited number of samples for analysis - it allows a reduction in time, expense and personnel required to carry out the analytical procedure, while still providing useful information about the properties of the population. The analysis of a limited number of samples can only give an estimate of the true value of the whole population. Sample selection in Forensic Analysis How to decide? Size of bulk to be sampled Shipload or biological cell? Physical state of fraction to be analysed Solid, liquid, gas Chemistry of the material to be analysed Searching for a specific species? Sampling method is linked to the measurement. Random sampling A sampling technique in which sample units are selected so that all combinations of n units under consideration have an equal chance of being selected as the sample. Statistical sampling- A sampling method in which every possible sample has the same chance of being selected. Random Sampling Random: to eliminate questions of bias in selection. Three types. Simple: any sample has an equal chance of being selected examples Østockpiles of cereals: take increments from surface and interior Øcompact solids: random drilling to sample Ømanufactured products: divide batch (lot) into imaginary segments and use a random number generator to select increments to be sampled Random Sampling Systematic: first sample selected randomly and subsequent samples taken at arranged intervals most commonly used procedure examples Ø solid material in motion (conveyor belt): periodically transfer portion into a sample container Ø liquids: sample during discharge (from tanks) at fixed time/volume increments Ø NOTE: manufactured products: sample more frequently at problematic times (changeover of shift, breaks etc.) Stratified: the lot is subdivided and a simple random sample selected from each stratus. examples Ø scrap metals: sort into metal type before sampling Ø material lots delivered at different times: take proportional weights of material from each lot Ø sedimented liquids: sample from decanted liquid and sediment by proportional weight, proportion the sample on the basis of volume or depth Selective Sampling Selective: screens out or selects materials with certain characteristics examples Øcontaminated foods: attempt to locate the adulterated portion of the lot Øtoxic gases in factory: total level acceptable but a localised sample may contain lethal concentrations A Composite Sample Composite: portions of material selected in proportion to the amount of material they represent. The ratio of the components taken up to make the composite can be in terms of bulk, time or flow. Reduces the cost of analysing large numbers of samples. Not a sampling technique; it is a preparatory technique after the samples have been taken. Subsampling Subsampling of the laboratory sample is done following homogenisation to give subsamples that are sufficiently alike Continuous Monitoring Real-time measurements to provide detail on temporal variability (variability as a function of time) Examples ØIndustrial stack emissions (CO, NO2, SO2) ØWorkplace monitoring (radiation exposure, toxic gases etc.) ØSmoke, heat and CO detectors ØWater and air quality monitoring Correct sample delimitation Incorrect sample delimitation  Identify the population Collect a gross sample Reduce the gross sample to a lab sample Steps in obtaining a lab sample ( a few grams ~ a few hundred grams). Lab sample may constitute as little as 1 part in 107 or 108 of the bulk material.  Sampling particulate solids Identify the population to be analyzed Randomly collect N particles to give a gross sample Reduce particle size of gross sample and homogenize No Randomly collect N particles Is this sample of a suitable size for the lab Store the lab sample Remove portions of the lab sample for analysis  Obtain a representative of the sample - the normal lab sample weighs less than 1 gram, yet the analysis results of the sample are used to characterized the entire material from which the sample was taken. - it is essential that the lab sample accurately represents the composition of the bulk material. -sampling bulk material is easy, the composition is uniform throughout. Methods in Sampling Bulk Materials: Acceptable methods American Society for Testing Materials (ASTM) National Bureau of Standards Association of Official Analytical Chemists (AOAC) American Oil Chemists Society (AOCS) Exact procedures used vary depending on the nature of the sample, but they are compromised by 3 steps: 1. Obtaining a gross sample 2. Reducing the gross sample to a transportable size 3. Obtaining a laboratory sample  - sampling heterogenous material is more difficult generally requiring carefully prescribed procedures that are desired to ensure proper representation in the final sample.  Section C Acquiring the Sample A railroad car containing 25 tons of silver ore. Buyer and seller must agree on a price, based primarily on the silver content of the shipment. The ore itself is heterogeneous, of many lumps that vary in size as well as in silver content. Explanation of procedure The assay will be performed on a sample that weighs about 1 g. This small sample must have a composition that is representative of 25 tons (or 22.700,000 g) of ore in the shipment. a difficult undertaking requires a careful, systematic manipulation of the entire shipment Sampling involves obtaining a small mass of a material whose composition accurately represents the bulk of the material being sampled. (Skoogs, West) The gross sample is obtained by gross selection: - sampling heterogenous material is more difficult generally requiring carefully prescribed procedures that are desired to ensure proper representation in the final sample. Ex. Cone and quarter method: - unloading of ore from a shipment * require crushing to obtain a uniform size * removal of a fixed sample of the gross sample by piling into cone and flattening followed by quartering. *opposite quarters are retained, mixed and the procedure repeated until appropriate sample (until 5 to 25 lb) is obtained.  QUARTERING SAMPLES Quartering Method (using a shovel) Mix samples thoroughly. Pour it onto a Save Discard large flat surface. Divide the sample into four equal parts. Discard Save Save the 2 opposite quarters. Be sure to save the fine material at the bottom of the saved quarter. If the sample is still to large, divide the sample again.  Soil Sampling Points X X X X X X X X X X Steps in sampling and measurement of salt in a potato chip. Step 1 introduces the sampling variance. Steps 2 to 4 introduce the analytical variance. Making Samples Homogeneous It is usually necessary to make samples homogeneous before they are analyzed, otherwise it would be difficult to select a representative laboratory sample from the sample. Homogenization can be achieved using mechanical devices (e.g., grinders, mixers, slicers, blenders), enzymatic methods (e.g., proteases, cellulases, lipases) or chemical methods (e.g., strong acids, strong bases, detergents). Mechanical Homogenizers Reducing Sample Size Once the sample has been made homogeneous, a small more manageable portion is selected for analysis. This is usually referred to as a laboratory sample, and ideally it will have properties which are representative of the population from which it was originally selected. Sampling plans often define the method for reducing the size of a sample in order to obtain reliable and repeatable results. Preventing Changes in Sample Once the sample is selected ensure that it does not undergo any significant changes in its properties from the moment of sampling to the time when the actual analysis is carried out, e.g., enzymatic, chemical, microbial or physical changes. There are a number of ways these changes can be prevented.  Lipid Protection. Unsaturated lipids may be altered by various oxidation reactions. Exposure to light, elevated temperatures, oxygen or pro-oxidants can increase the rate at which these reactions proceed. Necessary to store samples that have high unsaturated lipid contents under nitrogen or some other inert gas, in dark rooms or covered bottles and in refrigerated temperatures. Providing that they do not interfere with the analysis antioxidants may be added to retard oxidation.  Enzymatic Inactivation. Many foods contain active enzymes they can cause changes in the properties of the food prior to analysis. If the action of one of these enzymes alters the characteristics of the compound being analyzed then it will lead to erroneous data and it should therefore be inactivated or eliminated. Freezing, drying, heat treatment and chemical preservatives (or a combination) are often used to control enzyme activity, with the method used depending on the type of food being analyzed and the purpose of the analysis.  Microbial Growth and Contamination. Microorganisms are present naturally in many foods and if they are not controlled they can alter the composition of the sample to be analyzed. Freezing, drying, heat treatment and chemical preservatives (or a combination) are often used to control the growth of microbes in foods.  Physical Changes. A number of physical changes may occur in a sample, e.g., water may be lost due to evaporation or gained due to condensation; fat or ice may melt or crystallize; structural properties may be disturbed. Physical changes can be minimized by controlling the temperature of the sample, and the forces that it experiences. Sample Quality The chain of events from the process of taking a sample to the analysis is no stronger than its weakest link. Each sample should be registered (have a unique barcode) and all details recorded including the storage conditions and chain of contact. details to consider: sample properties (e.g. volatility, sensitivity to light) appropriate container (e.g. glass is not suitable for inorganic trace analyses, low molecular weight polyethylene is not suitable for hydrocarbon samples) length of holding time and conditions (e.g. cream separates out from milk samples when left standing, sedimentation of particles in liquids occurs) amount of sample required to perform the analysis. Chemical analyses use only a small fraction of the available sample. Sample Preservation and Holding Time Recommended Environmental Sample Preservation and Holding Times Alternative storage conditions are acceptable if analyte stability within a matrix can be demonstrated and data quality objectives are not compromised. The analyte requiring the most preservation treatment as well as the shortest holding time should dictate the preservation treatment of the sample overall. Sample Storage - Samples are stored if cannot be analyzed immediately - Sample composition can be changed by interaction with container material, light, or air - Appropriate storage container and conditions must be chosen - Organic components must not be stored in plastic containers due to leaching - Glass containers may adsorb or release trace levels of ionic species Sample Storage - Appropriate cleaning of containers is necessary - Containers for organic samples are washed in solvent - Containers for metal samples are soaked in acid and deionized water - Containers must be first filled with inert gas to displace air - Biological samples are usually kept in freezers - Samples that interact with light are stored in the dark Sample Storage - Some samples require pH adjustment - Some samples require addition of preservatives (EDTA added to blood samples) - Appropriate labeling is necessary - Computer based Laboratory Information Management Systems (LIMS) are used to label and track samples General Rules for Samples Place in ice immediately after sampling, cool to