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

CHEMISTS ENGINEERS LIFE SCIENTISTS TECH. REPRESENTATIVE IN FIELD COLLECTION OF DATA/ DATA INTERPRETATIONS ABOUT PROBLEM 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

Water sampling Ice sampling

Sample preparation



to produce representative samples:

Crushing: Jaw crusher Vertical shaft impactor

Grinding and milling: Pestle and mortar Ball Mill

Mixing: Rollers Mixing 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. Fe 2+ oxidised to Fe 3+ 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 s o 2



s a 2



s s 2

Where does the sampling variance come from?

Consider a powder mixture containing n A type A and n

B

particles type B.

particles of Probability of drawing A: p = n A n A + n B Probability of drawing B: q = n B n A + n B = 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 e n



t 2 s 2 s e 2 µ = true population mean x = measured mean n = number of samples needed s s 2 = 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?

n



t 2 e s 2 2 s For 90% confidence: t



=

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 reaction with air 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 Varies with temperature, humidity and state of division e.g. water of crystallisation BaCl 2 ·2H 2 O

• • •

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

• • •

Fusion Wet ashing Dry ashing Inorganic samples Organic samples

ACID DISSOLUTION

Acids commonly used for dissolving inorganic materials:

Non-oxidising acids

– HCl, HF, dilute HClO 4 , dilute H 2 SO 4 , H 3 PO 4

Oxidising acids

– HNO 3 , hot concentrated HClO 4 , hot concentrated H 2 SO 4 A mixture of acids maybe required, e.g.: Aqua regia = HCl:HNO 3 = 3:1 HCl + HClO 4 HNO 3 + HClO 4 + HF

Note:

Hot concentrated HClO 4 is a very strong oxidant! It reacts violently with organic substances. Evaporate samples containing organic substances with HNO 3 to dryness first (a few time if necessary) before adding HClO 4 .

If the solution turns a dark colour when HClO 4 is added, remove from heat and add sufficient HNO 3 the solution to

…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 Ca 2+ 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 – 1200 o C Automated fusion apparatus Crucibles e.g. platinum, gold, nickel, zirconium

Common fluxes used:

Basic fluxes



– Na 2 O 2 , Na 2 CO 3 , LiBO 2 , NaOH, KOH for dissolving acidic oxides of Si and P

Acidic fluxes

– Li 2 B 4 O 7 , Na 2 B 4 O 7 , K 2 S 2 O 7 , B 2 O 3



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 CO 2 and H converted to H 2 O Problem: loss of volatile species Wet Ashing = decomposition of organic samples using strong oxidising agents e.g.

H 2 SO 4 + HNO 3 HClO 4 + HNO 3

Dry Ashing = decomposition of organic samples by strong heating The solid residue is then dissolved and analysed.

Not the most reliable procedure