chemical and physical properties: practical session
DESCRIPTION
Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.TRANSCRIPT
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Lab 6: Saliva PracticalBeer-Lambert Law
University of Lincoln presentation
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This session….
• Overview of the practical…• Statistical analysis….• Take a look at an example control
chart…
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The Practical
• Determine the thiocyanate (SCN-) in a sample of your saliva using a colourimetric method of analysis
• Calibration curve to determine the [SCN-] of the unknowns
• This was ALL completed in the practical class
• Some of your absorbance values may have been higher than the absorbance values of your top standards… is this a problem????
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Types of data
QUALITATIVENon numerical i.e what is present?
QUANTITATIVENumerical: i.e. How much is present?
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Beer-Lambert LawBeers Law states that absorbance is
proportional to concentration over a certain concentration range
A = cl
A = absorbance = molar extinction coefficient (M-1 cm-1 or mol-1 L cm-1)c = concentration (M or mol L-1)l = path length (cm) (width of cuvette)
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Beer-Lambert Law
• Beer’s law is valid at low concentrations, but breaks down at higher concentrations
• For linearity, A < 1
1
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Beer-Lambert Law
• If your unknown has a higher concentration than your highest standard, you have to ASSUME that linearity still holds (NOT GOOD for quantitative analysis)
• Unknowns should ideally fall within the standard range
1
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Quantitative Analysis
• A < 1– If A > 1:
• Dilute the sample• Use a narrower cuvette
– (cuvettes are usually 1 mm, 1 cm or 10 cm)
• Plot the data (A v C) to produce a calibration ‘curve’
• Obtain equation of straight line (y=mx) from line of ‘best fit’
• Use equation to calculate the concentration of the unknown(s)
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Quantitative AnalysisCalibration curve showing absorbance as
a function of metal concentration
y = 0.9982x
R2 = 0.9996
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1
Concentration (mg L-1)
Abso
rbanc
e ( no
uni
ts)
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Statistical Analysis
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Mean
The mean provides us with a typical value which is representative of a distribution
nsobservatio of (N) number thensobservatio theall of (å) sum the
Mean
nsobservatio of (N) number thensobservatio theall of (å) sum the
Mean
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Normal Distribution
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Mean and Standard Deviation
MEAN
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Standard Deviation
• Measures the variation of the samples:– Population std ()– Sample std (s)
= √((xi–µ)2/n)
• s =√((xi–µ)2/(n-1))
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or s?
In forensic analysis, the rule of thumb is:
If n > 15 use If n < 15 use s
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Absolute Error and Error %
• Absolute ErrorExperimental value – True Value
• Error %100%
value True Value True– value alExperiment
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Confidence limits
1 = 68%
2 = 95%
2.5 = 98%
3 = 99.7%
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Control Data
• Work out the mean and standard deviation of the control data– Use only 1 value per group
• Which std is it? or s?
• This will tell us how precise your work is in the lab
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Control Data
• Calculate the Absolute Error and the Error %– True value of [SCN–] in the control = 2.0 x 10–3 M
• This will tell us how accurately you work, and hence how good your calibration is!!!
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Control DataPlot a Control Chart for the control
dataQuality Control Chart
1.00E-03
1.50E-03
2.00E-03
2.50E-03
3.00E-03
3.50E-03
4.00E-03
1 6 11 16 21 26 31
Measurement number
Con
trol
thio
cyanate
con
centr
ation
(m
ol/L
)
Control valueinner limitinner limitouter limitouter limitgroup values
2.5 2
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Significance
• Divide the data into six groups:– Smokers– Non-smokers– Male– Female– Meat-eaters– Rabbits
• Work out the mean and std for each group ( or s?)
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Significance
• Plot the values on a bar chart
• Add error bars (y-axis) – at the 95% confidence limit – 2.0
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Significance
Variation in [SCN-] in Saliva for Various Groups of Forensic Science Students (not REAL data)
0123456789
Smokers Non-Smokers
Male Female Lions Rabbits
Mea
n [S
CN
-] (M
)
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Identifying Significance
• In the most simplistic terms:
– If there is no overlap of error bars between two groups, you can be fairly sure the difference in means is significant
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Acknowledgements
• JISC• HEA• Centre for Educational Research and
Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org