Transcript Slide 1

SAMPLING AND
SAMPLE PREPARATION
DEFINITION OF PROBLEM
Information gathering
Select analytical technique or method
Implement analysis of known sample and unknowns
Reduce data, interpret and report results
SOLUTION TO PROBLEM
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?
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
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
No. of samples:
- could determine approach
- important for planning
PROFFESIONAL ANALYTICAL CHEMISTS IN INDUSTRY
ANALYTICAL
CHEMISTS
CLASSICAL
APPROACH
COLLECTION OF DATA/
DATA INTERPRETATIONS
ABOUT PROBLEM
CHEMISTS
ENGINEERS
LIFE SCIENTISTS
TECH.
REPRESENTATIVE
IN FIELD
SOLUTION
TO PROBLEM
Remember:
A chemical analysis is generally performed on only a
fraction of the material.
This fraction must represent the bulk material
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
Ice sampling
Water sampling
Sample preparation
 to produce representative samples:
Crushing:
Jaw crusher
Vertical shaft impactor
Grinding and
milling:
Ball Mill
Pestle and mortar
Mixing:
Mixing wheel
Rollers
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 SAMPLING
A chemical analysis can only be as meaningful as the
sample!
Sampling – process of collecting a representative
sample for analysis
OVERALL VARIANCE =
ANALYTICAL VARIANCE + SAMPLING VARIANCE
so
2
2
 sa  ss
2
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:
nA
p = nA+ nB
Probability of drawing B:
nB
q=
nA+ nB = 1 - p
If n particles are randomly drawn, the expected number
of A particles will be np
and standard deviation of many drawings will be:
σ
n

npq
How many samples/replicates to analyse?
Rearranging Student’s t equation:
Required number of
replicate analyses:
  x 
ts s
n
2
n 
2
t ss
e
2
e
µ = true population mean
x = measured mean
n = number of samples needed
ss2 = variance of the sampling operation
e = sought-for uncertainty
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.
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 2
n 
For 90% confidence:
t =
n 
t ss
e
2
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
e.g. adsorbed
onto surface
e.g. water of
crystallisation
Varies with temperature,
humidity and state of
division
BaCl2·2H2O
Accounted for by:
•
•
•
DISSOLVING 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 Inorganic samples
• Fusion
• Wet ashing
Organic samples
• Dry ashing
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.
Further Notes
Vessels for acid digestion manufactured from
glass, Teflon, platinum, polyethylene
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
Fusion = melting
To dissolve refractory substances
Dissolve sample in hot molten inorganic flux.
~10 times more flux than sample (by mass)
Heat crucible to 300 – 1200oC
Crucibles
Automated fusion apparatus
e.g. platinum, gold,
nickel, zirconium
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
Problem: loss of volatile species
Wet Ashing
= decomposition of organic samples using strong
oxidising agents
e.g.
H2SO4 + HNO3
HClO4 + HNO3
Dry Ashing
= decomposition of organic samples by strong heating
The solid residue is then
dissolved and analysed.
Not the most reliable procedure