Sample Prep 3

Question 1

What are the stages involved in SPE?

Answer 1

1. Washing
2. Conditioning
3. Eluting
4. Drying

Expensive as each cartridge is disposed after one extraction

Question 2

What is the principle of SPME?

Answer 2

Based on partitioning the analyte between a phase immobilised on a fused-silica SPME fibre and the matrix

It is a solvent free extraction that incorporates sample extraction, concentration and sample introduction into a single procedure.

Question 3

What are the 2 types of extraction mode for SPME?

Answer 3

1. Direct immersion of SPME fibre in to liquid sample matrix
2. Head space extraction in which the liquid sample matrix is heated in vial to volatilise the analytes and the fibre is placed just above the sample matrix

Question 4

What is the extraction procedure?

Answer 4

1. Pierce septum on sample container
2. Expose SPME fibre/extract analytes
3. Retract fibre/withdraw needle

Question 5

What conditions have to be optimised for extraction?

Answer 5

1. Sample pH
2. Sample volume
3. Sample agitation/stirring speed
4. Extraction temp
5. Extraction time
6. Change of activity coefficient

Question 6

What is the desorption procedure?

Answer 6

1. Pierce septum in GC inlet or introduce needle into SPME/HPLC interface
2. Expose fibre/desorb analytes
3. Retract fibre/withdraw needle

The analytes are thermally desorbed by exposing the fibre in the injection port of a GC or redissolving in an organic solvent

Question 7

What does derivatisation do for the SPME method?

Answer 7

Increases the volatility and reduce the polarity of some analytes improving extraction efficiency, selectivity and subsequent GC detection

Question 8

What is direct derivatisation?

Answer 8

The derivatisation reagent is added into the sample matrix, the SPME fibre then extracts the derivatives analytes in solution or HS and delivers them to the GC

Eg. Propyl chloroformate derivatives amino group on amphetamines in urine

Question 9

What is derivatisation on SPME fibre?

Answer 9

Employed after the extraction procedure - extracted compounds on the fibre are exposed to the derivatising reagent in the vapour phase for a given time

Question 10

What is derivatisation in the GC injector port?

Answer 10

Amphetmaines after extraction are derivatives in the liner by injection of heptafluorobutyric anhydride to form amide derivatives

Question 11

What are the advantages of SPME?

Answer 11

1. Fast technique
2. No solvent
3. Little equipment
4. Easy handling
5. Ease of automation
6. Good linearity and sensitivity

Question 12

What are the disadvantages of SPME?

Answer 12

1. Matrix and volatility of target analytes have to be taken into account
2. Very slow extraction compared to LLE and SPE with packed bed columns
3. Desorption needs more time than the injection of liquid extracts after LLE and SPE
4. Carryover effects occur very easily in SPME because of repeated use of one fibre
5. Recoveries lower compared to LLE and SPE

Question 13

What is stir bar sorptive extraction?

Answer 13

Uses a stir bar coated with PDMS layer (0.5-1mm) however it has low extraction efficiency (only extracts 0.5 micro/l volume) but has 100-1000 times more sensitivity than SPME

Question 14

What are the advantages of SBSE?

Answer 14

1. Simplicity
2. High sensitivity
3. Low cost
4. Simple for automation
5. Can handle range of sample volumes

Question 15

What is the principle of SBSE?

Answer 15

Extraction of the solutes from the aqueous phases into the extraction medium (PMDS) which is controlled by the partition coefficient (Ko/w) of the solutes between the silicone phase and the aqueous phase

For non polar analytes