Chapter 3. Sampling Flashcards
The analytical process
- select a method
- acquire and process the sample
- dissolve the sample in a suitable solvent
- measure a property of the analyte
- calculate the results
- estimate the reliability of the results
The way you perform an analysis
will depend on your experience,
the equipment available, the
cost, and the time involved
selecting a method
Factors of selecting a method:
- sample type
- size of sample
- sample preparation needed
we can’t analyze the whole thing
obtaining a representative sample
Factors in obtaining a representative sample:
- sample type/homogeneity/size
- sampling statistic/errors
consists of several
portions of the material
to be tested
gross sample
small portion of
gross sample, made
homogeneous
laboratory sample
actually analyzed
analysis sample
a ‘portion of material selected
from a larger quantity of material
❖ physical state (gas, liquid or solid)
❖ homogeneous or heterogeneous
❖ primary sample, laboratory
sample, test sample
sample
1
types of sample (according to sampling plan)
- representative sample
- random sample
- selective sample
- composite sample
a sample that is typical of the
parent material for the characteristic under inspection
representative sample
4 types of representative sample
- homogeneous
- heterogeneous
- static (contained) system
- dynamic system
e.g. a vegetable oil at 40◦C (at this temperature the oil is liquid); a filtered aqueous solution.
homogeneous
e.g. palm oil at 15◦C (this temperature is below the melting
point of the oil); a sample of breakfast cereal, such as
muesli.
heterogenous
the composition of the parent material is permanent with
respect to position in space and stable over the time of
sampling and testing
▸ e.g. oil in a drum; tins of fruit in a warehouse
static (contained) system
the parent material is changing with respect to time
▸ removal of a portion at any instant represents only a ‘snapshot’ of
that moment in time and in that particular location.
▸ e.g. unsaturated and saturated oils being continuously blended;
estuarine water, where the salinity is changing over time
dynamic system
a sample deliberately chosen by
using a sampling plan that screens- out materials with certain characteristics and/or selects only
material with other relevant characteristics
selective sample (directed/focused sample)
Examples of selective sample:
- In food analysis, it may be necessary to locate a
specific adulterated portion of a lot, undiluted by
perfectly good material. - Rodent contamination of flour by hair or urine.
- Toxic gases in a factory atmosphere where the total
level may be acceptable but a localized sample may
contain a harmful concentration.
a sample is selected by a random process to eliminate
problems of bias in selection
and/or to provide a basis for
statistical interpretation of
measurement data.
random sample
3 types of random sampling:
- simple random sampling
- stratified random sampling
- systematic sampling
Any sample has an
equal chance of
selection
simple random sampling
the lot is subdivided/
stratified and a simple
random sample selected
from each stratum.
stratified random sampling
The first sample is
selected at random and then the subsequent
samples are taken
according to a previously
arranged interval,
e.g.
every 5th, 10th or
whatever is appropriate
systematic sampling
consists of two or more portions of
materials (collected at the same
time) selected so as to represent
the material being investigated
taken in proportion to the amount
of the material that they represent
composite sample
review and practice how to identify the appropriate type of sample
2
Obtaining a representative sample
We can’t analyze the whole thing
Factors
▸ Sample type/ homogeneity/ size
▸ Sampling statistics/ errors
study the sampling and analytical operations in ppt
can take a number of forms (a single pile of
grain, soil present in an area of contaminated land or a
shipload of coal.
- not in the form of separate permanently
identifiable units.
bulk materials
comprised of identifiable units, which may
be assigned number.
packaged goods
bulk materials that are partially packaged into
smaller units, such as in bags or drums.
segments
(for both bulk materials and packaged goods)
- a quantity of material transferred on one occasion
and covered by a single set of shipping documents. - may be made up of one or more lots
consignment
a quantity of material which is assumed to represent a
single population for sampling purposes
- may consist of one or more batches
lot
a quantity of material which is known or assumed to
have been produced under uniform conditions.
batch
- portions of the material obtained from the lot/batch
by using a sampling device - often combined to produce a primary or gross sample
increments
portion of material delivered to the
laboratory for analysis
- often obtained from the primary or composite
sample by a series of division and reduction processes (e.g. coning
and quartering, riffling)
laboratory sample
intermediate sample that the analyst
obtains from the laboratory sample (directly for homogenous
samples, or
test/analytical sample
- the quantity of material that is
actually submitted for analysis, taken from the test sample - has to go through a number of treatment steps before the final aliquot is obtained
test portion/analytical portion
describes when, where and how samples are to be taken.
IUPAC: A predetermined procedure for the selection, withdrawal,
preservation, transportation and preparation of the portions to be
removed from a population as samples.
Sampling plan
may be laid down in national or international standards or in a set of
guidelines
- Codex Alimentarius Commission general guidelines on sampling
- sampling procedures for monitoring water quality (e.g. ISO 5667-1)
- sampling procedures for inspection by attributes (ISO 2859 series of standards)
- sampling procedures for inspection by variables (ISO 3951)
Sampling plan
should specify the number and size of primary samples to be
obtained from the lot/batch
▸ should include the number, location and size of the portions to be
taken, and instructions for the extent of compositing and reduction
of these portions to produce the laboratory sample
▸ should address whether the process of sampling should be a ‘one-off’ or whether it should be repeated and if so, how often
Sampling plan
sampling plan with
regular requirement for analysis
sampling scheme/sampling schedule
describes a combination of procedures
where several related sampling schemes are combined
sampling programme
…poor sampling may result in the sample submitted to the
laboratory for analysis being unrepresentative of the bulk material
from which it was taken. This could result in substandard batches of
material being accepted or perfectly good batches being rejected. In
environmental analysis, poor sampling may result in invalid decisions
being made about how to treat an area of contaminated land. This, in
turn, could impact on public health.
read on the legal and statutory requirements
the process of selecting a portion of material, in some manner, to
represent or provide information about a larger body of material
sampling
ISO/IEC 17025: 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.
sampling
▸ allows a statistical evaluation to
be applied to the data.
▸ used when a representative
sample is required.
probability sampling (random sampling)
3 Types of Probability Sampling
- simple random sampling
- stratified sampling
- systematic sampling
involves taking increments from the bulk
material in such a way that any portion of the bulk has an equal probability of being sampled.
used when little information is available about the material
that is being sampled, or
sampling from a batch or consignment of manufactured
product for quality control purposes
simple random sampling
▸ requires the consignment to be subdivided
into groups (strata) according to predefined
criteria. A simple random sampling plan is then applied to
each stratum. The number of samples taken from each
stratum is proportional to its size (e.g. weight or volume).
▸ aims to produce a more representative sample than would
be obtained by simple random sampling.
stratified sampling
▸ most commonly used sampling technique.
▸ involves taking increments from the bulk material at
predetermined intervals, as defined by the sampling
plan.
systematic sampling
▸ used when a representative
sample cannot be collected or is
not appropriate
▸ the correct sampling approach
to use to produce a selective
sample
non-probability sampling
3 Types of Non- Probability Sampling
- judgement sampling
- quota sampling
- convenience sampling
▸ use of knowledge
about the material to
be sampled, and the
reason for sampling, to select specific samples
judgement sampling
▸ requires the consignment to
be subdivided into groups
(as for stratified sampling)
then judgement sampling is
used to select samples from
each group
quota sampling
▸ involves selecting
samples on the basis
of availability and/or
accessibility.
convenience sampling
▸ involves the taking of a sample
from material which does not
consist of discrete, identifiable
or constant units
▸ bulk material may be gaseous,
liquid or solid.
bulk sampling
▸ involves the application of a
predetermined sampling plan to
decide whether a batch of goods
meets the defined criteria for
acceptance
acceptance sampling
aims to see that the customer gets the quality required, while
remembering that financial resources are limited and that the cost of the
article must reflect the cost of inspection, as well as the cost of
production.
acceptance sampling
the item in the batch of product either conforms or not.
The number of nonconformities in the batch is counted and if this
reaches a predetermined figure, the batch is rejected.
by attributes
the characteristic of interest is measured on a continuous
scale. If the average meets a predetermined value and the variation in the
characteristic being measured is within an acceptable standard
deviation, the batch is accepted.
by variables
read on Example: Cornflakes are sold in packets
of 500 g and watch youtube video on sampling
study schematic diagram of the typical measurement process, the uncertainty and decision chain, examples of measurement objectives, some sources of uncertainty in sampling and sample preparation, fish bone diagram of possible sources contributing to the uncertainty, uncertainty contributions divided into random and systematic effects
including sample splitting/preparation and handling (combined contribution of random effects)
analytical variability
combined effect of bias sources
analytical bias
dominated by heterogeneity and operator variations
sampling variability
combined effect of selection bias, operator bias, etc.
sampling bias
read on “How can knowledge on the type of sampling error help in
designing the sampling?”
write in flashcard the sampling uncertainty formulas
study Example of calculating the standard deviation
for sampling and write on notebook
study the Examples of tools for the estimation of uncertainty contributions
