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SAMPLING AND SAMPLING AND SAMPLE PREPARATIONSAMPLE PREPARATION
DEFINITION OF PROBLEM
SOLUTION TO PROBLEM
Information gathering
Select analytical technique or method
Implement analysis of known sample and unknowns
Reduce data, interpret and report results
Important information to provide the analyst:
What is the sample?
What other components are present?
What is the concentration range of the species to be determined?
What degree of accuracy is required?
How many samples are to be analysed?
Concentration range:
- needed to select technique/method for analysis
- for very low concentrations of analyte guard against contamination from reagents/apparatus
Degree of accuracy
- needed to select technique/method for analysis
- bear in mind: time and cost vs accuracy
Sample composition (can do qualitative analysis):
- needed to select method for analysis
- aware of: interferences, may need separations, method/solvent for dissolution, pre-treatment e.g. drying hygroscopic samples
No. of samples:
- could determine approach
- important for planning
PROFFESIONAL ANALYTICAL CHEMISTS IN INDUSTRYPROFFESIONAL ANALYTICAL CHEMISTS IN INDUSTRY
ANALYTICAL CHEMISTS
SOLUTIONTO PROBLEM
COLLECTION OF DATA/DATA INTERPRETATIONS
ABOUT PROBLEM
CHEMISTSENGINEERS
LIFE SCIENTISTSTECH.
REPRESENTATIVE IN FIELD
CLASSICALAPPROACH
A chemical analysis is generally performed on only a fraction of the material.
This fraction must represent the bulk material
Remember:
For solids: Produce a powder that is representative of the bulk
Iron ore sample – showing banded iron formation
Which part of this sample would you analyse?
Sampling
Core drills + cores
Water samplingIce sampling
Sample preparation to produce representative samples:
Crushing:
Jaw crusher
Vertical shaft impactor
Grinding and milling:
Pestle and mortar
Ball Mill
Mixing:
RollersMixing wheel
Considerations during crushing and grinding:
Composition of sample may change:
• loss of volatile components due to heat generated
• change is water content
• increased surface area to react with the atmosphere e.g. Fe2+ oxidised to Fe3+
Differences in hardness of components:
• different size particles
• losses due to dust
• separation of components
Contamination from crushers/mills due to abrasion
STATISTICS OF SAMPLINGSTATISTICS OF SAMPLING
OVERALL VARIANCE =
ANALYTICAL VARIANCE + SAMPLING VARIANCE
2s
2a
2o sss
A chemical analysis can only be as meaningful as the sample!
Sampling – process of collecting a representative sample for analysis
If n particles are randomly drawn, the expected number of A particles will be np
and standard deviation of many drawings will be:
npqn σ
Where does the sampling variance come from?
Consider a powder mixture containing nA particles of type A and nB particles type B.
Probability of drawing A: p =
Probability of drawing B: q =
nA
nA+ nB
nA+ nB
nB = 1 - p
Rearranging Student’s t equation:
µ = true population mean
x = measured mean
n = number of samples needed
ss2 = variance of the sampling operation
e = sought-for uncertainty
Required number of
replicate analyses:
en
tsx s
2
2s
2
e
stn
Since degrees of freedom is not known at this stage, the value of t for n → ∞ is used to estimate n.
The process is then repeated a few times until a constant value for n is found.
How many samples/replicates to analyse?
Example:
In analysing a lot with random sample variation, there is a sampling deviation of 5%. Assuming negligible error in the analytical procedure, how many samples must be analysed to give 90% confidence that the error in the mean is within 4% of the true value?
2
2s
2
e
stn
t =
For 90% confidence:
n
SAMPLE STORAGE Not only is the sampling and sample preparation important, but the sample storage is also critical.
+ LABELLING!!!
The composition of the sample may change with time due to, for example, the following:
• reaction with air
• reaction with light
• absorption of moisture
• interaction with the container
Glass is a notorious ion exchanger which can alter the concentration of trace ions in solution.
Thus plastic (e.g. PPE = polypropylene or PTFE = Teflon) containers are frequently used.
Ensure all containers are clean to prevent contamination.
MOISTURE IN SAMPLES
Moisture may be:
a contaminant or chemically bound in the sample
Varies with temperature, humidity and state of division
Accounted for by:• • •
e.g. adsorbed onto surface
e.g. water of crystallisation
BaCl2·2H2O
DISSOLVING SAMPLES FOR ANALYSISDISSOLVING SAMPLES FOR ANALYSIS
Most analytical techniques require that the samples first be dissolve before analysis.
It is important that the entire sample is dissolved, else some of the analyte may still be in the undissolved portion.
We will consider:• Acid dissolution / digestion• Fusion • Wet ashing• Dry ashing
Inorganic samples
Organic samples
ACID DISSOLUTION
Acids commonly used for dissolving inorganic materials:
Non-oxidising acids – HCl, HF, dilute HClO4, dilute H2SO4, H3PO4
Oxidising acids – HNO3, hot concentrated HClO4, hot concentrated H2SO4
A mixture of acids maybe required, e.g.:
Aqua regia = HCl:HNO3 = 3:1
HCl + HClO4
HNO3 + HClO4 + HF
Note:
Hot concentrated HClO4 is a very strong oxidant! It reacts violently with organic substances. Evaporate samples containing organic substances with HNO3 to dryness first (a few time if necessary) before adding HClO4.
If the solution turns a dark colour when HClO4 is added, remove from heat and add sufficient HNO3 to the solution
…AND RUN!!!!!
NOTE:
Hydrofluoric acid is extremely corrosive and a contact poison. Handled with extreme care!!!
• Symptoms of exposure to HF may not be immediately evident.
• HF interferes with nerve function and burns may not initially be painful. Accidental exposures can go unnoticed, delaying treatment and increasing the extent and seriousness of the injury. • HF penetrates tissue quickly and is known to etch bone • HF can be absorbed into blood through skin and react
with blood calcium, causing cardiac arrest.
HF exposure is often treated with calcium gluconate, a source of Ca2+ that sequesters the fluoride ions.
Vessels for acid digestion manufactured from glass, Teflon, platinum, polyethylene
Further Notes
Do NOT use HF in glass
To prevent loss of volatile species – use teflon-lined bombs (sealed container)
Bombs are frequently manufactured for use in a microwave oven
FUSIONS
Dissolve sample in hot molten inorganic flux.
~10 times more flux than sample (by mass)
Heat crucible to 300 – 1200oC
e.g. platinum, gold, nickel, zirconium
Automated fusion apparatus
Fusion = melting
Crucibles
To dissolve refractory substances
Common fluxes used:
Basic fluxes – Na2O2, Na2CO3, LiBO2, NaOH, KOH
for dissolving acidic oxides of Si and P
Acidic fluxes – Li2B4O7, Na2B4O7, K2S2O7, B2O3
for dissolving basic oxides of Grp I and II metals, lanthanides and Al
Then dissolve in diluted acid solution.
Disadvantages of fusions:
Large concentration of flux contamination
Loss of volatile substances
Large salt content in solution when dissolved
ASHING
Oxidative treatment of organic samples:
C converted to CO2 and H converted to H2O
Wet Ashing
= decomposition of organic samples using strong oxidising agents
e.g. H2SO4 + HNO3
HClO4 + HNO3
Problem: loss of volatile species
Dry Ashing
= decomposition of organic samples by strong heating
Not the most reliable procedure
The solid residue is then dissolved and analysed.