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.