Transcript Sample Preparation and Results

Sample Preparation and
Results
Results
 Mean, Median, Average
 Deviation from the mean
 Absolute and Relative Measures:
Relative = Absolute/Mean Value
 Precision
 Accuracy
Precision and Accuracy
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High Precision
High Accuracy
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High Precision
Low Accuracy
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Low Precision
Low Accuracy
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Low Precision
High Accuracy
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True Value
Methods
Signal
Sensitivity – ability to discriminate between
small differences in analyte concentration
Amount
Methods
Detection Limit – minimum
concentration or mass of analyte that
can be detected at the known
confidence level
Signal have to be 5 times bigger than
background noise
Methods
Signal
Dynamic Range (DR)
DR
Concentration
Analytical Process
Systematic Error
Random Error
Standards, Blanks
Results
Rounding Off (5 round to nearest even
digit)
Significant digits (Error of measurement)
 0.1%
- 0.001 x value
 Accuracy of measurement steps –
defines significant digits
Samples and Results
Sampling
 Representative
 Homogeneous (Particles Size)
 Number of Samples – Error of
measurement
Sample preparation
Solid – Liquid Extraction
Matrix, solvent and analyte
Solubility of analyte
Interaction between solvent and
analyte
Soxhlet Extractor
1 – solvent flask
2 – extractor
3 – solvent collector
4 - condenser
Extraction
Factors affecting extraction:
 Interaction between solvent and analyte
 Distribution coefficient:
KD = (mi/V)Solvent /(mi/V)Matrix
Where: mi – mass of analyte
V – volume of solvent and matrix
 Volume
of solvent vs. multiple extraction
Multiple Extraction (Oilseed)
Solvent Volume
Single extraction
100 mL
Multiple extraction
20 mL
20 mL
20 mL
20 mL
20 mL
Amount extracted Extracted (%)
0.75 g
75
0.70 g
0.26 g
0.028 g
0.009 g
0.0015 g
70
96
98.8
99.7
99.85
Extraction
Factors affecting extraction:
 Interaction between solvent and analyte
 Distribution coefficient:
KD = (mi/V)Solvent /(mi/V)Matrix
Where: mi – mass of analyte and V – volume of solvent
 Volume
of solvent vs. multiple extraction
 Particles size
Liquid-Liquid Extraction
Distillation - extraction
GC – Sample Extraction with Solvents
Methylene Chloride
Ethyl Ether
Carbon Disulfide
SPME – Solid Phase Microextraction
SPME - Fused silica fibres (1 to 2 cm long)
coated with various polymeric phase
Equilibrium forms between the three phases:
Fibre coating to sample phase
Headspace to sample phase
Fibre coating to headspace
SPME – Solid Phase Microextraction
SPME Fibres
The commercial polymeric phases available are:
Polydimethylsiloxane(PDMS)
Polydimethylsiloxane/divinylbenzene(PDMS/DVB)
Polyacrylate (PA)
Carbowax/divinylbenzene (CW/DVB)
Polydimethylsiloxane/carboxen (PDMS/CAR)
Polydimethylsiloxane/divinylbenzene/carboxen1006
(PDMS/DVB/CAR)
Carbowax/Templated resin (CW/TPR)
SPME Adsorption and Desorption
http://www.chemsoc.org/exemplarchem/entries/2004/westengland_smith/ExempWeb/SPME.htm
SPE
Solid Phase
Extraction Cartridges
Sample Preparation - SPE
GC – Derivatization with Silyl Reagents
END
Diagram of SPME Extraction
Direct sampling SPME
Headspace SPME
SPME - Adsorption