slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
SAMPLING AND SAMPLE PREPARATION PowerPoint Presentation
Download Presentation
SAMPLING AND SAMPLE PREPARATION

Loading in 2 Seconds...

play fullscreen
1 / 29

SAMPLING AND SAMPLE PREPARATION - PowerPoint PPT Presentation


  • 261 Views
  • Uploaded on

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:

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'SAMPLING AND SAMPLE PREPARATION' - brygid


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide2

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

slide3

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?

slide4

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

slide5

PROFFESIONAL ANALYTICAL CHEMISTS IN INDUSTRY

ANALYTICAL

CHEMISTS

CHEMISTS

ENGINEERS

LIFE SCIENTISTS

TECH.

REPRESENTATIVE

IN FIELD

CLASSICAL

APPROACH

COLLECTION OF DATA/

DATA INTERPRETATIONS

ABOUT PROBLEM

SOLUTION

TO PROBLEM

slide6

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

slide7

Iron ore sample – showing banded iron formation

Which part of this sample would you analyse?

slide8

Sampling

Core drills + cores

slide9

Ice sampling

Water sampling

slide10

Sample preparation

 to produce representative samples:

Crushing:

Jaw crusher

Vertical shaft impactor

slide11

Grinding and milling:

Ball Mill

Pestle and mortar

slide12

Mixing:

Mixing wheel

Rollers

slide13

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

slide14

STATISTICS OF SAMPLING

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

slide15

Consider a powder mixture containing nA particles of type A and nBparticles type B.

Probability of drawing A: p =

Probability of drawing B: q =

nA

nA+ nB

nB

= 1 - p

nA+ nB

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:

slide16

How many samples/replicates to analyse?

Rearranging Student’s t equation:

Required number of

replicate analyses:

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.

slide17

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?

For 90% confidence:

t =

slide18

SAMPLE STORAGE

Not only is the sampling and sample preparation important, but the sample storage is also critical.

+ LABELLING!!!

slide19

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.

slide20

MOISTURE IN SAMPLES

Moisture may be:

a contaminant or chemically bound in the sample

e.g. adsorbed onto surface

e.g. water of crystallisation

BaCl2·2H2O

Varies with temperature, humidity and state of division

  • Accounted for by:
slide21

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

slide22

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

slide23

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!!!!!

slide24

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.

slide25

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

slide26

Fusion = melting

FUSIONS

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

e.g. platinum, gold, nickel, zirconium

Automated fusion apparatus

slide27

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

slide28

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

slide29

Dry Ashing

= decomposition of organic samples by strong heating

The solid residue is then dissolved and analysed.

Not the most reliable procedure