Sampling'

1
Sampling.
Objective to take from an object (the
population) an amount of material (the sample)
that can be analysed in a laboratory in such a
way that the analytical result for the sample is
of such quality that it represents the
concentration of the analyte in the object.
object
homogeneous
heterogeneous
discrete
continuous
discontinuous
Well mixed gases, liquids or solutions, pure
metals.
Ores in soils or rocks, particles in
suspension.
Reactive solutions or gases settling
suspensions.
Change as a function of distance from source
(plumes)
examples
2
Sampling an object.
object
Quality control samples
Parts of the object
increments
sample

• Field
• co-located (replicates)
• Spiked in field
• Field blanks
• Control site sample(s)

Representative parts of the object
Gross sample
Representative parts of the sample.
Sub-samples
storage
analyses

• Laboratory
• duplicates
• Spiked
• blanks
• RMs

(x y)units
The analytical result(s) can only be as good as
the sample(s) collected.
3
Types of Errors.
Gross Errors arise from mistakes dont make
them. Systematic Errors lead to poor accuracy
can be identified and allowed for. Random
Errors lead to poor precision cant be avoided
but can be minimized.
Errors in sampling
Random errors - sample is not representative
too small and thus dont contain all of the
components of the object.

• Systematic errors
• biased towards certain components of the object
• eg sediment from a river where fine particles
  have been lost while collecting sample,
• Samples taken from within concentration gradients
  (object boundary effects such as haloclines in
  coastal waters, sides of rivers, surfaces of
  piles, tops of barrels )
• ?? poor definition of the object ??,
• changes that occur after sampling
• eg redox reactions, evaporation, biological
  changes, separation of particles, adsorption to
  surfaces (particles of containers)
• contamination of the collected sample
• eg from preserving agents, dust, container
  rinsing solutions, sampling equipment,

4
Sampling errors.
For systematic errors (biases) Bt B1 B2 B3
(detect and correct) For random errors Vt
V1 V2 V3 V s2 , the variances.
(minimize)

• Consider the random errors arising from two
  activities of an analysis
• sampling (ss) and measurement (sm).
• Neither sm or ss are significant - where the
  precision of the result is not of concern.
• sm is but ss is not significant - where the
  sample is stable and homogeneous and/or the
  analytical method is not very precise.
• sm is not but ss is significant - the major
  contribution to the total error comes from
  sampling. The most common scenario frequently
  ss 3sm.
• Both sm and ss are significant - the sampling
  error is under control.

Total error st2 ss2 sm2. Which errors
should we try to minimize, if any, to ensure
that the result (x y) units is
appropriate? Eg If ss 9 and sm 3 then st
v(45 9) 7.3 If ss 6 and sm 3 then st
v(36 9) 6.7 If ss 9 and sm 1.5 then
st v(45 1) 6.9 Improve precision by
focusing on the least precise component usually
sampling.
5
Sampling Protocols
ISO 17025 5.7.1. The laboratory shall have a
sampling plan and procedures for sampling .
Sampling plans shall, whenever reasonable, be
based on appropriate statistical methods. The
sampling plan shall address the factors to be
controlled to ensure the validity of
results. 5.7.3. The laboratory shall have
procedures for recording relevant data and
operations relevant to sampling Shall means it
must be documented.
The data generated from the samples must be
accurate and adequately precise so that the
hypothesis can be tested.
1. sample type and the analytes
3. sample containers cleaning methods
2. sampling equipment cleaning methods
The protocol must define
5. sampling locations
7. amount of each sample
6. number of samples
4. time of sampling
8. sample collection procedures
9. sample labeling
10. sample preservation
12. required recording of data observations
11. quality control samples
13. sample transport
14. training needs for the samplers
(?) sample pretreatment
6
Sampling Protocols cont.

• sample type and the analytes
• soil- horizon,
• water- surface, depth, tap, ground, river,
  marine, rain, vent
• air- inside, outside, plume, ambient,
• Blood, urine, sweat,

• sampling equipment cleaning methods
• Equipment use recommended procedures if
  possible - soil corers, air pumps, sediment
  traps, water bottles, commercially available
• Clean - to avoid contamination with the analyte
  rinse with de-ionized water, rinse with sample
  water, brush with a clean brush, sterilized
  stainless steel needles,

• sample containers cleaning methods
• Containers plastic bags for soils, plants
  plastic or glass bottles for liquids vapour
  losses samples often in containers for
  considerable times.
• Clean to avoid analyte additions or losses
  adsorption to surfaces, particles or liquids from
  container surfaces soak in acids (1N HNO3, 1N
  HCl, ) and the DI water, dry in ovens, cap, seal
  in bags for storage, rinse filters with DI water
  and oven/furnace dry,

• time of sampling
• Depend on hypothesis and sampling demands
• Pb in air a function of traffic,
• O2 in water a function of photosynthesis,
  pollution from industry, sewage treatment, ..
• Na in canned meat a function of production
  schedules,

7
Sampling Protocols cont.

• sampling locations and points of sampling

Defined by regulations urine or blood for drug
testing well studied systems

• Random (statistical) where all parts of the
  object have an equal chance of being sampled.
  Used when prior knowledge is lacking.
• Divide the object into a fixed number of parts,
  number them and then pick a predetermined number
  using a random number generator.

eg soil in a farmers cultivated lot when
determining available P 1 hectare (100m x 100m),
divide into 100 100m2 parts and randomly select
20. Also need to determine where within the
10m x 10m part to take the sample (use the same
random numbering method).
or Pb in street dusts, contamination of coastal
waters, a shipment of pimento berries in bags,
Need to define action to take if cant sample
at predetermined position eg rock in the soil.
8
Sampling Protocols cont.

• sampling locations and points of sampling cont.

Systematic where the object has a known
structure or behavior a plume from a point
source entering a river through a halocline in
an estuary parts of a plant (roots, stems,
leaves, flowers), Sample at fixed distances,
salinities, defined parts.
Sequential when there is some regularity to the
object. Products on a production line (take ever
100th or take a predetermined set of randomly
number samples)
Selecting sampling sites randomly or sequentially
must be done prior to sampling but systematic
sampling site selection will be done following
observations of the object.
Ad hoc sampling as opportunity arrives
forensic, only small amounts are available.
The interpretation of the data generated from a
sampling programme must be interpreted while
bearing in mind how the samples were selected.