Created with MindGenius Business 2005® 3. Sampling (ELO) “A defined procedure whereby a part of a substance, material or product is taken to provide for.

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Presentation transcript:

Created with MindGenius Business 2005® 3. Sampling (ELO) “A defined procedure whereby a part of a substance, material or product is taken to provide for testing or calibration a representative sample of the whole. Sampling may also be required by the appropriate specification for which the substance, material or product is to be tested or calibrated.” ie The sample(s) must be REFLECTIVE of the TRUE situation Background Reading: Quality Assurance in Analytical Chemistry, Chapter 3 For an example relevant to your analyses, see BS EN ISO 707:2008 Milk and milk products – Guidance on sampling

Created with MindGenius Business 2005® Sampling Strategies (1) Depend on: Types of parent material Homogeneous or heterogeneous Static (stable and contained) or dynamic (cannot be resampled) Concentration Trace (prone to heterogeneity and contamination) Principal component (% uncertainty much lower) Packaging Bulk (eg a silo) Packaged (eg cornflakes) Items (eg tablets) Results required Quantitative (how much) Qualitative or compliance (yes/no answer) – “acceptance sampling”

Created with MindGenius Business 2005® Sampling Strategies (2) Criteria? attributes (x% of items must conform) variables (a specified average and sd) Selective sampling OK? (eg fruit but not stone)? Overall strategy determined by purpose: Client requirements Statutory requirements (legal obligations) Trade definitions (contractual)

Created with MindGenius Business 2005® Types of Sampling 100% sampling (all items monitored) Probability sampling (statistical chance of being representative) Judgement (guided by the reason for analysis) Quota (stratified judgement sampling) Convenience (as and when available) Non-probability sampling (selective)

Created with MindGenius Business 2005® Sample size and number Must be specified as part of the method. Sample Size Enough for method Enough to be reflective (esp for trace analysis) Sample number enough to reduce uncertainty (esp that from sampling) to acceptable levels

Created with MindGenius Business 2005® Sampling Uncertainty Overall uncertainty in analysis is composed of: Measurement uncertainty (quantifiable using standards) Sample uncertainty which is caused by Population uncertainty (real wobble in the system) Sampling uncertainty (due to the process of collection) Sampling uncertainty must be reduced until: it does not obscure population uncertainty the resource required outweighs the risk of an incorrect result More samples and smaller particles reduce sampling uncertainty See also CH250:1 Causes of uncertainty

Created with MindGenius Business 2005® How many samples are needed? Number of samples needed can be estimated by rearranging the equation for confidence intervals: n is the number of samples t is Student’s t (routinely approximated to 2 for 95% confidence) s is the standard deviation for the method (measured using reference standards, and inter-laboratory trials) E is the size of the effect that must be measurable (in the same units as s).

Created with MindGenius Business 2005® Number of samples - worked eg Cadmium concentrations in soil were measured at a brownfield site during preliminary surveying. The mean ± standard deviation were (16 ± 4) ppm. a) Calculate the number of samples required to obtain a total uncertainty of 20%. b) How many samples would be required to reduce this uncertainty to 10%?

Created with MindGenius Business 2005® Number of samples - Answer Cadmium concentrations in soil were measured at a brownfield site during preliminary surveying. The mean ± standard deviation were (16 ± 4) ppm. a) Calculate the number of samples required to obtain a total uncertainty of 20%. Between 6 and 7, assuming t=2 b) How many samples would be required to reduce this uncertainty to 10%? 25, assuming t=2

Created with MindGenius Business 2005® There are various statistical calculations based on a) the proportion of analyte present and b) the particle size to approximate sample mass needed. The simplest of these is as follows: Where Ks is the sampling constant, m is the mass in g and %CV is the Coefficient of Variation (which must be calculated from measurements on several test portions). How big should the sample be?

Created with MindGenius Business 2005® How big should the sample be? Once Ks is known the equation can be rearranged to calculate: a) The test portion mass (m) required to achieve a specified %CV b) The likely %CV from a given test portion mass