lecture 1- sampling and sample preparation

general steps in chemical analysis

1. Formulating question

2. Designing analytical method

3. Sampling

4. Sample preparation

5. Analysis

6. Assessing data

7. Method validation

8. Documentation

why is good sample preparation important

1. ensure analysis of representative sample

2. avoid damage to analytical instrumentation

3. separate analyte from interferents

4. increase conc of analyte so it can be detected

5. ensure physical conditions of sample are optimal for analysis

steps to take in sampling process

1. take care as collection tools and storage containers may contaminate samples

2. make room for multiple test portions for replicate analysis

3. ensure enough sample taken to give representative sample

4. biological samples such as tissue must be homogenised

factors affecting storage

1. safety of sample

2. form of sample

3. how long does it need to be stored

4. how long it will last before degradation

5. will the container affect sample

6. how much sample present

extraction techniques

inorganic= ashing, acid dissolution and digestion

organic= solvent extraction, liquid-liquid extraction, solid phase extraction

ashing

- removes organic material from sample before analysis by incinerating sample

- uses furnace oven (450-550 degrees)

- organic components form CO2 and H20 vapour, leaving inorganic compounds behind as solid oxides

- dry ashing where solid decomposes on its own

- wet ashing where solid decomposed in acid

-remaining residue dissolved in acid

- some metals volatile so may be lost from sample

- applicable for AAS and ICPMS

acid dissolution and digestion

- dissolves metals, alloys, ores, glass, trace elements in organic materials

- concentrated acid added to sample and heated in microwave extractor to speed up extraction

- common acids used: HCL, HNO3, H2SO4

- applicable for AAS

solvent extraction

- uses acid to dissolve sample

- acid added to sample and heated

- solution filtered and made to certain volume for analysis

- special microwave oven used to heat sample to speed up extraction

continuous solvent extraction

- extracts soluble analyte from insoluble sample matrix

- removes soluble matrix from insoluble analyte

1. sample placed in thimble

2. thimble put in main chamber

3. solvent placed in round flask at bottom

4. solvent boiled and condensed into main chamber

5. when solvent reaches a level, solvent syphons back down to flask, taking analyte with it

liquid-liquid extraction

- transfer of analyte from one liquid phase to another

- 2 phases immiscible (do not form homogenous mixture when mixed)

- 3 main types: single drop micro extraction, dispersive liquid-liquid micro extraction, hollow fibre liquid phase micro extraction

single drop micro extraction

- uses microliter syringe to form solvent drop above solution to extract volatile analytes

- submerged into aqueous phase

- analyst can inject sample into instrument after retracting drop back up

dispersive liquid-liquid extraction

- dispersion of fine droplets of extraction solvent into aqueous phase

- fast extraction due to high SA between droplets and aqueous phase

- high extraction rate and low solvent consumption

hollow fibre liquid phase extraction

- hollow fibre allows analyte to enter through pores in membrane to organic phase

- longer extraction time of 30-60 mins

- hollow fibre is hydrophobic

- solvent must have high boiling point

solid-phase extraction

- separation of analyte from liquid phase using solid phase material

- 3 types: solid phase extraction, solid phase microextraction, dispersive

solid phase extraction: bind and elute strategy

- transfer of analyte in liquid phase to solid phase through absorption

- sample matrix can be washed through filter using solvent

- sample eluted off in elution solvent

solid phase extraction: removal trapping strategy

- transfer of dissolved matrix in liquid phase to solid phase through absorption

- analyte washed through column

- matrix components retained on solid phase

solid phase extraction: molecularly imprinted polymers

- can selectively bind to analyte

- polymers formed round a template

- when template removed, small cavity left that can rebind to analyte

- can be made as bulk solids, thin films, powders and nanoparticles

solid phase extraction: headspace and direct immersion

- solid phase is a retractable microfibre coated with solid phase material

- placed or suspended above sample for extraction of volatile components

- directly inject microfibre into a GC or GC-MS

dispersion solid phase extraction

- solid phase is a sorbent powder

- high extraction affinity due to high SA of solid phase

- easy to add different chemical groups

quick, easy, cheap, effective, rugged and safe (QuEChERS)

- extracts pH dependent analytes

- homogenise sample

- Add organic solvent (acetonitrile)

- add sachet containing salts MgSO4 (induces phase separation between water and acetonitrile) and NaOAc (buffers sample)

- further dispersive extraction using sorbents primary and secondary amines, C18

- commercially available and easy to perform

immunoaffinity extraction

- using antibodies to recognise and purify biomolecules

- allows specific separation of biomolecule

- lower yields compared to other extraction techniques

- highly specific to analyte