Chemical Analysis Qualitative Analysis Quantitative Analysis Determination “Analyze” a paint...
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Transcript of Chemical Analysis Qualitative Analysis Quantitative Analysis Determination “Analyze” a paint...
Chemical Analysis
Qualitative Analysis
Quantitative Analysis
Determination
“Analyze” a paint sample for lead
“Determine” lead in a paint sample
Bulk Material
↓
Sample
↓
Analytical Sample
↓
Analytical Matrix
↓
Analyte + Concomitants
BLANK
Same concomitants
No analyte
Difficult if not impossible to acquire a true blank
INSTRUMENTAL ANALYSIS
1) Electroanalytical Chemistry
2) Spectrochemical Analysis
3) Chromatographic Separations
A Typical Instrument
Analytical Sample
Signal Generator Signal
Transducer
Signal Processor
i
Output
V
Types of Signals
1. Emission of Radiation
2. Absorption of Radiation
3. Scattering of Radiation
4. Refraction of Radiation
5. Diffraction of Radiation
6. Rotation of Radiation
7. Electrical Potential
8. Electrical Current
9. Electrical Resistance
10. Mass-to-charge Ratio
11. Reaction Rate
12. Thermal Properties
13. Mass
Signal Sources
1) Analytical Signal
2) Blank Signal
3) Background Signal
4) Dark Signal
Measured Signal: A combination
of these
Analytical Figures of Merit
“Indicate a characteristic of an instrumental technique for a given analyte”
“7”Accuracy, Precision, Signal-to-Noise Ratio
Sensitivity, Limit of Detection
Linearity, Linear Dynamic Range
Accuracy
Indicates how close the measured value is to the true analytical concentration
Requires a Standard Reference Material (SRM) of other official measure
NIST: National Institute of Standards and Technology
Accuracy
Most commonly reported as percent error
│Cm - Ct│
Ct
where:
Cm = measured concentration
Ct = true concentration
x 100%
Precision
Indicates the reproducibility of repetitive measurements of equivalent samples
May be expressed as:
1. Standard Deviation (s or σ)
2. Relative Standard Deviation (RSD)
3. Confidence Limits
Precision
Standard Deviation
For an infinite number of measurements (σ)
For a finite number of measurements (s)
Standard Deviation
Note that both s and σ have the same units as the original values
How many values should be obtained?
Rule of thumb: 16
0
5
10
15
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35
40
45
96.0 97.0 98.0 99.0 100.0 101.0 102.0 103.0 104.0
Value
Po
pu
lati
on
Total Population = 1000
0.00%
0.10%
0.20%
0.30%
0.40%
0.50%
0.60%
0.70%
0.80%
0.90%
1.00%
0 5 10 15 20 25 30 35 40 45 50
Number of Samples
Err
or
in M
ea
n
How far is the measured mean from the true value?
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
0 5 10 15 20 25 30 35 40 45 50
Number of Samples
Err
or
in S
td. D
ev
.
How far is s from σ?
Short Cut: σ ≈ 1/5 (peak-to-peak noise)
Relative Standard Deviation
RSD = σ/mean
Where the mean may be the signal or the analyte concentration. RSD is a unit-less value, so σ must have the same units as
the mean.
RSD is often reported as %RSD, and may be used to compare different techniques.
Confidence Limits
Define an interval that encloses
the true value (Ct) with aspecified level of confidence.
1. Cm ± σ 66.7% Confidence Level
2. Cm ± 2σ 95% Confidence Level
3. Cm ± 3σ 99.0% Confidence Level
Signal to noise Ratio (S/N)
S/N = Sm/σ = 1/RSD
Notes:
1. N = noise (σ)
2. S/N is unitless
3. Always try top maximize S/N
4. S/N is used to compare instruments
5. A plot of S/N versus an instrumental parameter reaches a maximum at the optimum value for that parameter
Sensitivity
Experimental slope of a calibration curve
m = ΔS/ΔC
Sensitivity is almost always specific for one particular instrument.
0
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0 2 4 6 8 10 12
Concentration (ppm)
Sig
na
l (V
)
m
LOD
LDR
Limit of Detection
The analyte concentration yielding an analytical signal equal to 3 times the standard deviation in the blank signal.
LOD = 3 x σbl / m
By definition, the LOD has just one significant figure!!
Linearity
Measure of how well the observed data follows a straight line.
SA = mC
SA = Analytical Signal
m = calibration sensitivity
Remember SA = Stot - Sbl
Linearity
Plot log(S) versus log(C)
log(SA) = log(m) + log(C)
The slope of this plot should be 1.00
A calibration curve is defined as linear if the
log-log plot has a slope in the range 0.95-1.05
Linear Dynamic Range
The concentration range over which the calibration curve is linear
Lower End → LOD
Upper End
Analyte Concentration where the observed signal falls 5% below the extrapolated line
LDR Units are
“orders of magnitude”
or
“decades”
of analyte concentration
LDR is easiest to observe on log-log plot
If linearity is poor, define an analytically useful range (AUR)
Other figures of merit may be calculated, but these 7 are sufficient.
Selectivity and Resolution may be useful in cases where more than one analyte is
determined in the same sample.