introduction “analytical chemistry deals with methods for
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Introduction
What is Analytical Chemistry (Instrumental Analysis) ?
“Analytical Chemistry deals with methods for determining the chemical composition of samples of matter. A qualitative method yields information about the identity of atomic or molecular species or the functional groups in the sample: a quantitative method in contrast, provides numerical information as to the relative amount of one or more of these components.” – Skoog, Holler, Nieman, Instrumental Analysis, 5th ed.
There are Analytical Methods and Classical Methods
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Classical methods (wet chemistry) – Crude separations: precipitation,
distillation – Gravimetric analysis – Titrimetric analysis – Rely on chemical reactions and measured
quantities (e.g. mass, volume) – …these are old methods,
nevertheless accurate
Instrumental methods
– Very efficient separations – Exploits physical and chemical properties as absorption of light; behavior in a magnetic field; tendency to move across a membrane – Analyte electrical signal – Computer control and data acquisition – Constantly evolving (recent Nobel prizes) –
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…a “sneak peek” at instrumental methods
Chemical and Physical Properties are Employed
Characteristic Properties Instrumental Methods Emission of radiation Emission spectroscopy (X-ray, UV,
visible, electron, Auger); fluorescence, phosphorescence
Absorption of radiation X-ray, UV, Visible, IR; photoacoustic spectroscopy,
NMR and Electron spin resonance
Scattering of radiation turbidimetry; Raman spectroscopy
Refraction of radiation refractometry; interferometry Diffraction of radiation X-Ray and electron diffraction
methods Rotation of radiation polarimetry; optical rotary
dispersion, circular dichroism Electrical potential Potentiometry;
chronopotentiometry Electrical charge Coulometry Electrical current Amperometry; polarography
Electrical resistance conductometry Mass gravimetry (quartz crystal
microbalance) Mass-to-charge ratio mass spectrometry
Rate of reaction kinetic methods Thermal characteristics thermal gravimetry and
titrimetry; differential scanning colorimetry; differential thermal
analyses ,
Light
Electro-magnetism
Mass
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What is the general instrumental approach to gathering information?
Data Domains refers to the various ways of encoding information.
Instrument Energy Source Data Domain Readout
Any human sense organ
light, pressure, chemical, acoustic
electrical currents via
nerves
brain response…sense of
sensephotometer UV-Vis,
IR,light from various
lampselectrical current current meter
atomic emission spectrometer
flames voltage voltage meter
pH meter electrodes voltage voltage meter
other domains… time, frequency, color, visual, digital
Electrical versus Non-electrical Domains
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An example (fluorescence) of the progression through other domains !
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Selecting an Analytical Method: There is always more than one way to ‘skin a cat’…but usually
one way is best!
1) What accuracy and precision is required ? 2) How much sample is available ? 3) What is the concentration range of the analyte ? 4) What interferences are/could be present? (is my method selective ?) 5) What is the phase of the sample ? 6) How many samples need to be analyzed ?
(What time and money are available)
Performance Criteria impacts which experimental technique to use…
Precision, Bias, Sensitivity, Detection Limits, Dynamic Range, Selectivity, Recover, Speed,
Ease & Convenience, Skill required, Cost and availability of equipment, Ruggedness
(Let’s deal with each of these in turn)
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Precision
Given a normal distribution (Chem 350) of measurements,
Mean value = u0 ‘true value’ (no error!)
St. Dev. = σ measure of the ‘spread’ of results
Precision only has meaning with respect to a number of measurements…
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Bias
Bias: (Systematic Error): The ‘direction’ of error (magnitude and sign) remains the same if the measurements are repeated under identical conditions Imprecision (or Random Error): The sign and magnitude of the error changes randomly between measurements.
Bias = (measured value – true value)
Sensitivity
Broadly speaking, it is the ability to measure small changes in concentration and is related to the slope of a calibration curve and its associated
precision. Analytical sensitivity = γ = slope/std. dev.
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Detection Limits
Detection limits are also broadly defined. They are usually quoted with respect to a given procedure.
LOD = Limit of detection
S = Signal response; N = Noise
SLOD = Sblank + 3(St. Dev.)blank
Given a typical calibration curve, we know that…
S = slope × [C]
[C]min = (SLOD – Sblank)/slope
technically valid, but if you were an instrument manufacturer how would you be sure what the
LOD was?
Limit of Quantification: Concentration at which signal is provides either a linear response or statistically meaningful response.
Usually, SLOQ = 10×(Std. Dev)blank
a minimum concentration that can be detected at a certain confidence level.
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Is the Benzo(a) pyrene peak above the LOD? LOQ? What would be the
estimated LOD of B(a)P in terms of mass?
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Dynamic Range of Instrument
The linear range of the calibration curve
Linearity usually stops at higher and
lower concentrations Why? …depends on specific instrument (high
end reasons) 1) detector saturation 2) absorption of light (e.g. fluorescence) 3) space charge effects
What are the consequences of this?
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Selectivity The degree to which a method does/does not respond to a given component in a sample. Can be defined mathematically for ion-selective electrodes.
Is Infrared (IR) Spectroscopy selective? Is NMR selective ?
What is fluorescence selective for? This parameter is strongly considered in gas
chromatographic methods !
GC detectors (more to come…) PID (photoionizable) vs FID (combustable)
vs ECD vs XSD (Halogens) …
Let’s Quickly discuss each of these
– Speed – Ease and convenience; Operator skill – Cost and availability of the instrument
– Cost per sample (overhead) – Ruggedness
A city police department's radar
speed violation tickets were legally invalidated in court
after somebody proved the
calibration process for the radar guns
wasn't traceable to national standards
Calibration (3 basic types)
"Honestly officer, battery-powered milk floats can't do 75
miles per hour.... even downhill."
The basic idea is to ‘train’ the instrument to convert a measured signal to a concentration
…just like in Gen Chem
1) External Calibration (most common)
•Standards are of known concentration• Plot signal vs. concentration
•To find unknown concentration: interpolate between known points
y = 2xR² = 1
0
5
10
15
20
25
0 2 4 6 8 10 12
Inst
rum
en
t R
esp
on
se
X-Data (Arbitrary Units)
Calibration Example Plot
1 2
2 4
3 6
4 8
5 10
6 12
7 14
8 16
9 18
10 20
y = 0.0388x + 0.22
0
0.2
0.4
0.6
0.8
1
1.2
-10 0 10 20 30
Vs (mL)
Ab
so
rba
nc
e2) Standard Addition Calibration
•Useful when interferences present in sample
•Samples are spiked with various amounts ofstandard (known)
•All solutions in the same “matrix”…accounts for artifacts from mixture
Extrapolation from “zero” added material determines how much analyte was in original sample
The beakers below have increasing amounts of
analyte as per a ‘standard’ calibration
1 2 3 54
How would the calibration ideally
look?
concentration
Instrument response
Internal Calibration
concentration
Instrument response
Blue: ideal ResponseRed: Actual Response
The actual (red) response of the instrument
is hardly linear and is hence problematic!
What causes this?
For example, instruments that use a flame (atomic absorption, FID) have a high degree of variability
in their response. ..flames naturally flicker and don’t have a
steady ‘condition’.
The way to overcome this problem is to spike each of the
standards with a compound similar (chemically speaking) to
the one under study
Analyte of interest (e.g. Cocaine)
Spiked internal reference (e.g. morphine, codeine)
concentration
Blue: ideal response
Red: actual response
Black: internal
reference
What can we say about the ratio of
red:black as a function of conc. ?
If we plot the ratio ‘Analyte/Reference’ to
the ‘Concentration of the Analyte’, we get a
linear response… (from your book)