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
