Sampling and Sample Preparation Flashcards
Process of obtaining a portion of a material
that will represent the subject under study
Proper sampling
Differentiate gross, laboratory, and aliquot sample
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
Differentiate representative and composite sample
Representative sample - sample that is typical of the parent material for the characteristics under inspection
Composite sample - consists of two or more portions of material (collected at the same time) selected so as to represent the material being investigated
Differentiate in situ and grab sample
In Situ sample - allows monitoring the target population without removing individual grab samples
Grab sample - portion of the target population is collected at a specific time or location, providing a “snapshot” of the target population
composition of the parent material is permanent with respect to position in space and stable in
time
Static 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 particular location
Dynamic condition
Define 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
Define selective or 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)
Define convenience sampling
a sampling plan in which samples are collected because they are easily obtained
Define systematic sampling
the sample is taken according to a previously arranged regular intervals in time or space
Define systematic-judgmental sampling
a prior knowledge about a system to guide a
systematic sampling plan
Define stratified or judgmental-random sampling
population being sampled is divided into segments (or strata) and a simple random sample is selected from each
Describe the steps in sampling process
- Sampling > Sample Preservation > Sample Preparation > Analysis
Describe the steps in sample collection
Identify the population > collect sample > reduce gross sample to lab sample > analysis sample
Describe sampling plan and type of sample for homogeneous solution.
Grab sample is sufficient and sampled by siphoning, decanting, or by using pipet or syringe
How to ensure homogeneity of liquid?
Manual shaking (done before sampling if natural diffusion is slow)
Large stationary liquid can be sampled
using _____
thief sampler
Describe sampling plan for biological fluids (e.g. blood)
Sampling after 12h of fasting and collected (grab sample) by syringes for blood
Differentiate serum and plasma
Serum - fluid separated from clotted
blood, serum does not clot
Plasma - fluid separated from unclotted blood, contains fibrinogens
How to store a blood sample?
Use preservatives such as NaF (glucose) or anticoagulant (heparin) and store in a vacutainer
Chemical composition of surface waters, such as streams, rivers, lakes, estuaries, and oceans, is influenced by ____ and ____
flow rate and depth
How to get grab sample of natural waters?
- Submerging a capped bottle below the surface and removing the cap (palm under the bottle, hand not upstream)
- After the sample bottle is filled, the cap is replaced, and the bottle is removed
The _____, which may be enriched with heavy metals or contaminated with oil, is avoided when collecting the sample
air-water interface
Wells used for collecting groundwater samples must be ___ until the water’s temperature, pH, or specific conductance are constant
purged
What type of containers must be used when collecting pesticides, oil and grease, and organics?
Glass containers
Why plastic bottles are preferred when collecting sample of trace metals?
glass surfaces easily absorb metal ions
How to preserve natural waters sample? (3)
- Control pH and temperature
- Limit exposure to light and atmosphere
- Add chemical preservatives
Describe the sample collection for gases (workplace gas)
- Gas is drawn onto a collection medium using a specially designed sampling pump (will pull the gas into the container)
- Flush the container for a predetermined time
- Seal the container
Disadvantage of gas sampling
Tendency for some gases to adsorb the container’s wall (analytes at concentrations too low to detect may change the chemical composition of sample)
Usual sampling method for gases is ____
displacement of liquids (not reacting with or solubilizing the sample)
Sampling of gases which changes the sample from gaseous to a liquid state
cryogenic cooling
Describe sampling of gases that utilizes chemical reaction and its type of sample
A bottle is used in which a reagent is dissolved in an appropriate media (pretreated with reagent).
In situ sample
Differentiate sampling with and without enrichment
Sampling with enrichment - gas is adsorbed into a solid collection phase or adsorbed into a solution
Sampling without enrichment - used to determine organic gases that occur permanently in the air of the workplace
Sample container that is generally used for direct sampling of gas (3)
- gas storage vessels
- gas-tight syringes
- gas sample bags pumped at 1-3 L/min flow rate
special glass tubes designed to collect airborne
contaminants by bubbling the sample air at a high flow rate into a specific absorbing liquid inside
impingers (bubblers)
In the sampling process using impingers, air drawn into the impinger is formed through a ____, which is covered by a
liquid such as _____. The pollutant dissolved in the liquid is subsequently analyzed, usually by
____
nozzle; high purity water; colorimetric technique
The chemical composition of a gas sample is usually stable when it is collected using a ____ (3)
- solid sorbent
- a filter
- by cryogenic cooling
O2 or CO2 dissolved in liquid (e.g. blood) is considered as ____sample
liquid
Typical examples of gaseous sample
- automobile exhaust
- emissions from industrial smokestacks
- atmospheric gases
- compressed gases
- solid-aerosol particulates
Why solid samples are often the most difficult to sample?
Due to inhomogeneity, variation of particle size, and variation within particles
Large particulate solids, such as coals and
ores, can be sampled by randomly collecting samples with ___ or by ___
a shovel or by riffling
Sediments from the bottom of streams, rivers, lakes, estuaries, and oceans are collected with a ___ or ___
bottom grab sampler or with a corer
equipped with a pair of “jaws” that close when they contact the sediment, scooping up sediment in the process
Grab samplers
Disadvantage of using grab sampler in collection of sediments
- tendency to lose finer grained sediment particles as water flows out of the sampler
- loss of spatial information both laterally and with depth due to mixing of the sample
Describe a cylindrical coring device and its advatange/disadvantage
Dropped into the sediment, collecting a column of sediment and the water in contact with sediment.
Advantage: maintain vertical profile (changes in the sediment’s composition with depth are preserved)
Disadvantage: only small surface area is sampled
a thin-walled steel tube which retains a core sample when it is pushed into the soil and
removed
soil punch
Soil sample collected at depths of up to 30
cm is easily collected with ___ or ___ but ____
scoops or shovels but the sampling variance is generally high
How to collect soil samples at greater depth?
Digging and collecting samples
How to use soil sampler?
- Push the sampler into the product
- Rotate the handle
- Remove sampler
- Discharge rod to push sample out
Two types of sampling of powder.
Static Sampling
1. Scoop
2. Thieving
3. Cone and quartering
Dynamic sampling
1. Table sampling
2. Chute splitting
3. Spin riffling
widely used and consists of plunging a scoop into a heaped batch
scoop sampling
Describe thieving method
- 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 allows material to be collected
______ from _____
simultaneously from several locations
accomplished by a combination of crushing and grinding the gross sample
coning and quartering
Discuss the process of coning and quartering
- gross sample piled into a cone
- flattened
- divided into quarters
- separation of quarters
- discarding opposite quarters
- repeated
superior methods of powder sampling are obtained by using procedures where the sample is removed from a moving powder bulk
dynamic sampling
Discuss table sampling method
- involves powder flow along an inclined table in which there are a series of holes
- some powder fall through the holes and is discarded
- the powder remaining on the incline place passes onto the next row of prism with another holes and more is removed
- repeated until the powder at the end of table is sampled
Describe chute splitter and chute sampling.
Chute splitter - 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
Chute sampling - sample is poured into the chute splitter and repeatedly halved until a sample of desired size is obtained
Describe spin riffling
Uses a device that 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
(Powder) samples stored at _____ are less prone to biodegradation and the loss of volatile material
reduced temperature
How to prevent loss of volatile material in (powder) sample?
The sample completely fills its container without leaving a headspace where gases can collect
_____ allows the same number of particles to be sampled with a smaller, more manageable combined mass
Reducing the sample’s average particle size
The majority of analytical techniques, particularly those used for a quantitative analysis, require that the analyte be ____
in solution
Metals/alloys can be obtained by ___ (3)
- Sawing
- Milling
- Drilling
With some metal materials, a representative sample can be obtained by ____ across the piece at ___
sawing; random intervals
When sampling a metal in the form of a sheet, random samples can be collected with ____
a metal punch
Samples can be obtained from a metal wire by _____
randomly cutting off pieces of an appropriate length
Larger pieces of metal, such as bars or
bricks, are best sampled by ____ and collecting __
sawing through the metal at randomly selected points and collecting the “sawdust”
How to sample biological tissue? (2)
- If entire organ is removed: homogenized before smaller portions are taken for analysis
- If not: small potions maybe combined to form a composite sample, and the composite sample is homogenized before analysis
most commonly used for taking samples from bags because they are relatively cheap and sample quickly
Simple bag sampling spears (sack samples)
To obtain a good cross section of sample, the sampling spear should be ___ in length
40-45 cm
_____ type of sampling spears penetrates bag easily
tapered type
Differentiate the solvent used in sample dissolution based on the material.
distilled water - inorganic salts
methanol, chloroform, toluene - organic materials
If the samples are difficult to dissolve, the first approach is to ____
try digesting the sample with acid or base
Many digestions are now carried out in closed containers using _____ as a source of energy for heating the solution
microwave radiation
Advantages of microwave digestion
- Higher temperature and pressure range
- May be accomplished in less than 30 minutes 3. Closed container prevents the loss of volatile gases
Inorganic samples that resist decomposition by digestion with acid or bases often can be brought into solution by fusing with a large excess of an alkali metal salt, called a ____
flux
Discuss dry ashing digestion method and goal
- sample is placed in a suitable crucible and heated over a flame or in a furnace
- C -> CO2; H -> H2O; S -> SO2; N -> N2
- the goal is the removal of organic material, leaving behind an inorganic residue, or ash, that can be further analyzed
To achieve a separation there must be at least ____ of the analyte and interferent
one significant difference between the chemical or physical properties
The goal of an analytical separation is to ___
remove either the analyte or the interferent from the sample matrix
More often than not, the accuracy and precision of an analysis is limited by ___ rather than the measurement step
the sampling
If the sampling imprecision is relatively large, it is better to ___ and ___
use a rapid, lower precision method and analyze more samples
We can improve variance due to sampling by
- collecting more samples of proper size
- increasing the number of times we analyze each sample improves the variance
