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:
Select analytical technique or method
Implement analysis of known sample and unknowns
Reduce data, interpret and report results
SOLUTION TO PROBLEM
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?
- needed to select method for analysis
- aware of: interferences, may need separations, method/solvent for dissolution, pre-treatment e.g. drying hygroscopic samples
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
COLLECTION OF DATA/
A chemical analysis is generally performed on only a fraction of the material.
For solids: Produce a powder that is representative of the bulk
Which part of this sample would you analyse?
Core drills + cores
to produce representative samples:
Vertical shaft impactor
Pestle and mortar
Contamination from crushers/mills due to abrasion
A chemical analysis can only be as meaningful as the sample!
Sampling – process of collecting a representativesample for analysis
OVERALL VARIANCE =
ANALYTICAL VARIANCE + SAMPLING VARIANCE
Consider a powder mixture containing nA particles of type A and nBparticles type B.
Probability of drawing A: p =
Probability of drawing B: q =
= 1 - p
Where does the sampling variance come from?
If n particles are randomly drawn, the expected number of A particles will be np
and standard deviation of many drawings will be:
Rearranging Student’s t equation:
Required number of
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.
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?
For 90% confidence:
Not only is the sampling and sample preparation important, but the sample storage is also critical.
The composition of the sample may change with time due to, for example, the following:
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 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
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.
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
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
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
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
To dissolve refractory substances
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
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
Oxidative treatment of organic samples:
C converted to CO2 and H converted to H2O
Problem: loss of volatile species
= decomposition of organic samples using strong oxidising agents
e.g. H2SO4 + HNO3
HClO4 + HNO3
= decomposition of organic samples by strong heating
The solid residue is then dissolved and analysed.
Not the most reliable procedure