5 Sampling and sample preparation

• It is not possible to analyze all the water in a stream for a toxic pollutant.

• It is not possible to analyze all the milk in a tanker to see if it contains a prohibited steroid hormone.

• Sometimes a small sample has to be used because the analytical method is destructive, and we wish to preserve the remainder of the material.

• The analytical sample must be processed in a dependable manner that maintains sample integrity without losing sample or introducing contaminants.

Why perform proper sampling?

Proper Sampling

The process of obtaining a portion of a material that will represent the subject under study.

Gross sample

Consists of several portions of the material to be tested.

Laboratory sample

Small portion of the sample that is actually analyzed.

Aliquot

Measured portion of the volume of a liquid sample taken for the analysis.

Bulk sample (tons) → Gross sample (kilograms) → Laboratory sample (<1 grams) → Analytical sample Aliquot

Order of proper sampling

Representative sample

A sample that is typical of the parent material for the characteristic under inspection.

Homogenous

One phase

Heterogenous

More than one phase

Static system

Composition of the parent material is permanent with respect to position in space and stable in time.

Dynamic conditions

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.

Composite sample

Consists of two or more portions of material (collected at the same time) selected so as to represent the material being investigated.

In situ sample

Allows monitoring the target population without removing individual grab samples.

Grab sample

A portion of the target population is collected at a specific time or location, providing a “snapshot” of the target population.

Characteristics of grab sample

• 1/50th of the total bulk

• Easiest but less reliable

• Satisfactory only for homogenous sample

• Best and easy time for sampling: During transportation

During transportation

Best and easy time for grab sampling

Random sampling

Selected in a way that any portion of the material has an equal and known chance of being chosen; least biased approach to sampling.

Selective (judgmental) sampling

A sample which is deliberately chosen by using a sampling plan that screens out materials with certain characteristics and/or selects only material with other relevant characteristics (directed/focused sampling).

Convenience sampling

A sampling plan in which samples are collected because they are easily obtained.

Systematic sampling

The sample is taken according to a previously arranged regular intervals in time or space.

Systematic-judgmental sampling

A prior knowledge about a system to guide a systematic sampling plan.

Stratified (judgmental-random) sampling

The population being samples is divided into segments (or strata) and a simple random sample is selected from each segment.

Order of taking appropriate sample

Identify population (bulk of material) → Collect sample (gross sample) → Reduce Gross sample to Lab sample (laboratory sample) → Analysis sample (a drop, few mg, few mL) → Sampling → Sample Preservation → Sample Preparation → Analysis

What are the three steps in sampling process?

1. Sample collection (physically removing the sample from its target population)

2. Preserving the sample

3. Preparing the sample for analysis

True

T/F: In the sample collection of liquids, homogenous solutions are easily sampled by siphoning, decanting, or by using a pipet or syringe.

True

T/F: In the sample collection of liquids, manual shaking is often sufficient to ensure homogeneity.

False; sufficient

T/F: In the sample collection of liquids, grab sample is often insufficient.

shaking/stirring

In the sample collection of liquids: If natural diffusion is slow, ________________ is done before sampling.

False; large

T/F: In the sample collection of liquids, small stationary liquid can be sampled using thief sampler.

True

T/F: In a sampling plan for biological fluids, timing for biological samples is important.

grab; Syringes

In case of blood, a _________ sample can be done. _________ are used to collect blood.

True

T/F: Blood can be analyzed as whole blood or separated into serum and plasma.

Serum; plasma

_________ is a fluid that is separated from clotted blood (it does not clot), while, __________ is a fluid that is separated from unclotted blood which contains fibrinogens.

True

T/F: In storing biological samples, preservatives such as NaF for glucose, or anticoagulant (Heparin) are used.

True

T/F: Vacutainers (evacuated tubes with rubber cap) are used to collect samples.

Flow rate and depth

The chemical composition of surface waters, such as streams, rivers, lakes, estuaries, and oceans, is influenced by _____________ and ________.

False; grab samples; submerge the capped bottle below the surface, then remove the cap.

T/F: In getting samples for natural waters, in situ samples are conveniently collected by removing the cap before submerging the bottle below the surface. After the sample bottle is filled, the cap is replaced and the bottle removed.

True

T/F: The air–water interface, which may be enriched with heavy metals or contaminated with oil, is avoided when collecting the natural water samples.

True

T/F: Wells used for collecting groundwater samples must be purged until the water’s temperature, pH, or specific conductance are constant.

False; glass containers

T/F: Plastic containers are always used when collecting samples for the analysis of pesticides, oil and grease, and organics.

False; plastic bottles

T/F: Since glass surfaces easily adsorb metal ions, glass bottles are preferred when collecting sample for the analysis of trace metals.

Sample preservation

The process of preserving a sample by controlling its pH and temperature, limiting its exposure to light or to the atmosphere, or adding chemical preservatives.

True

T/F: In active sampling procedure, workplace air is drawn onto a collection medium using specially designed sampling pumps.

True

T/F: In the sampling plan for gases, a pump is used to pull the gas into the container, and, after flushing the container for a predetermined time, the container is sealed.

The tendency for some gases to adsorb to the container’s walls, analytes at concentrations too low to detect may change the sample’s chemical composition with time.

Disadvantages in sampling plan for gases includes?

True

T/F: Usual sampling method for gases is displacement of liquids (not reacting with or solubilizing the sample)

Cryogenic cooling

A sampling technique which changes the sample from a gaseous to a liquid state (>150 K using He, H₂, or N₂)

True

T/F: Sampling plan for gases involves a sampling that utilizes chemical reaction (using a bottle in which a reagent is dissolved in an appropriate media (pretreated with a reagent)).

True

T/F: The reaction between the gas and the reagent takes place in situ during sampling

Sampling with enrichment

Gas is adsorbed onto a solid collection phase or adsorbed into a solution.

Sampling without enrichment

Used for determining organic gases that occur permanently in the air of the workplace (e.g. propane, propene)

• a gas storage vessels

• gas-tight syringes

• gas sample bags pumped at 1-3 L/min flow rate.

3 equipment/containers generally used for direct sampling of gases

Impingers (bubblers)

Special glass tubes designed to collect airborne contaminants by bubbling the sample air at a high flow rate onto a specific absorbing liquid inside.

False; high purity water

T/F: Air drawn into the impinger is formed through a nozzle, which is covered by a liquid such as low purity water.

Colorimetric technique

The pollutant dissolved in the liquid is subsequently analyzed, usually by ___________________ technique.

False; stable

T/F: The chemical composition of a gas sample is usually unstable when it is collected using a solid sorbent, a filter, or by cryogenic cooling.

• Automobile exhaust

• Emissions from industrial smokestacks

• Atmospheric gases

• Solid aerosol particulates

• Compressed gases

5 typical examples of gaseous samples

True

T/F: O₂ or CO₂ dissolved in liquid (e.g. blood) is considered as liquid sample.

False; most difficult

T/F: Solids samples are often the easiest to sample due to inhomogeneity, variation of particle size, and variation within particles.

True

T/F: Solids come in variety of forms, each of which is sampled differently.

False; large particulate solids

T/F: Small particulate solids, such as coals and ores, can be sampled by randomly collecting samples with a shovel or by riffling.

• Sediments

• Surface Soil

• Metals or Alloys

• Powder

• Biological tissue

Examples of solid samples

Bottom grab sampler

A sampling device equipped with jaws that close when they contact sediment, scooping up the sediment in the process.

True

T/F: In the collecting sediments from the bottom of streams, rivers, lakes, estuaries, and oceans, a bottom grab sampler or a corer is used.

Corer

A cylindrical sampling device used to collect a column of sediment and the water in contact with the sediment.

Grab samplers

These are quipped with a pair of “jaws” that close when they contact the sediment, scooping up sediment in the process.

• The tendency to lose finer grained sediment particles as water flows out of the sampler

• The loss of spatial information, both laterally and with depth, due to mixing of the sample.

Disadvantages in the collection of sediments

True

T/F: In the cylindrical coring device, the corer is dropped into the sediment, collecting a column of sediment and the water in contact with the sediment.

False; vertical profile

T/F: Samples collected with a corer maintain their horizontal profile. As a result, changes in the sediment’s composition with depth are preserved.

Only a small surface area is sampled

Main disadvantage of cylindrical coring device

Soil punch

A thin-walled steel tube which retains a core sample when it is pushed into the soil and removed.

scoops or shovels

Soil samples collected at depths of up to 30 cm are easily collected with _________ or __________, although the sampling variance is generally high.

True

T/F: Soil samples collected at greater depth are obtained by digging a trench and collecting samples.

1. Scooping

2. Thieving

3. Cone & quartering

3 basic methods in static sampling

Dynamic sampling

Superior methods of powder sampling are obtained by using procedures where the sample is removed from a moving powder bulk.

1. Table sampling

2. Chute splitting

3. Spin riffling

3 basic methods in dynamic sampling

Scoop sampling

Widely used and consists of plunging a scoop into a heaped bath.

Thieving

It consists of plunging a capture device (comprising one or more separate sample chambers) into the bulk material to retrieve several small aliquots of the powder. The sample chamber(s) can be opened and closed by an operator via controls at the top of the device.

Sample thief

It allows material to be collected simultaneously from several locations.

Coning and quartering

Accomplished by a combination of crushing and grinding the gross sample.

True

T/F: In coning and quartering, to ensure that all particles are reduced to a uniform size, the sample is intermittently passed through a sieve.

1. Gross sample piled into a cone

2. Flattened

3. Divided into quarters

4. Separation of quarters

5. Discarding of opposite quarters

6. Process is repeated

Process in coning and quartering

Table sampling

A method of powder sampling where powder flows along an inclined table with series of holes. Some powder falls through the holes and is discarded while the powder remaining on the incline plane passes onto the next row of prisms and holes and more is removed. This process is continued and the powder that is collected at the end of the table is the sample.

Chute splitting

A method of powder sampling where a chute splitter consisting of a V-shaped trough and chutes is used to repeatedly halve the sample until a desired size is obtained.

Chute splitter

It consists of a V-shaped trough along the bottom of which is a series of chutes alternately feeding two trays placed on either side of the trough.

Spin riffling

A method of powder sampling where a spinning riffler device uses mechanical (vibrational) energy to provide a constant flow of material from its holder. The steady flow passes through a divider head that rotates at a constant speed, thus, minimizing segregation.

Advantage: Good for powders with poor flow characteristics
Disadvantage: Very operator-dependent

Advantage/s and disadvantage/s of cone & quartering

Advantage: Reliable for homogeneous and non-flowing powders
Disadvantage: Particle segregation and non-flowing powders

Advantage/s and disadvantage/s of scoop sampling

Advantage: Able to separate large quantity of material

Disadvantage: Very dependent upon initial feed

Advantage/s and disadvantage/s of table sampling

Advantage: Can reduce powder sample

Disadvantage: Operator bias by 50% in one pass

Advantage/s and disadvantage/s of chute splitting

Advantage: Reliable for free-flowing powder samples

Disadvantage: Not efficient at handling large samples of powder

Advantage/s and disadvantage/s of spin riffling

True

T/F: Sampling of metal or alloys can be obtained by sawing, milling, or drilling.

False; it is necessary

In sampling of metal or alloy, it is not necessary to obtain materials from both the surface and interior.

False; by sawing across the piece

T/F: With some materials, a representative sample of metals or alloys can be obtained by milling across the piece at random intervals.

False; random samples

T/F: When sampling a metal in the form of a sheet, composite samples can be collected with a metal punch.

True

T/F: Samples can be obtained from a metal wire by randomly cutting off pieces of an appropriate length.

False; best sample by sawing

Larger pieces of metal, such as bars or bricks, are best sampled by drilling through the metal at randomly selected points and collecting the “sawdust”

Sampling of biological tissue

Is done by removing the entire organ, which is then homogenized before smaller portions are taken for analysis.

False; composite sample

In sampling of biological tissue, alternately small portions maybe combined to form a grab sample. That sample is then homogenized before analysis.

Simple bag sampling spears (sack samplers)

These are most commonly used for taking samples from bags because they are relatively cheap and sample quickly.