Transcript Analytical chemistry

Spectroscopic Analysis
Part 1 – Introduction
Chulalongkorn University, Bangkok, Thailand January 2012
Dr Ron Beckett
Water Studies Centre
School of Chemistry
Monash University, Melbourne, Australia
Email: [email protected]
Water
Studies
Centre
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Course Objectives
• To gain a qualitative understanding of: the scope
and methodology of Analytical Chemistry, EMR,
specified spectroscopic experiments and analysis
methods
• Be able to perform certain calculations related to
EMR and calibration of spectroscopic analyses
• The examinable material will be defined by the
handout notes and problems performed in class
(NOT the Skoog et al. textbook, Chapters 1, 24-28,
which should be used as reference material to
assist in your understanding)
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Assessment Scheme
• 75% for a 2 hour exam based on the lecture notes
and problems done in class
• 10% for ONE homework assignment and general
contribution to discussions in class
• 15% for THREE tests done in class
• Attendance: 1 mark deducted for each absence or
late appearance in any class session without
acceptable reason. Advise lecturer in advance of
expected absence
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Analytical Chemistry
Analytical chemists perform two main tasks:
1. Qualitative Analysis identifies the
elements and compounds present in a
sample
2. Quantitative Analysis determines the
relative amounts of these components
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Importance of Analytical Chemistry
Analytical chemistry is used to collect
necessary data to solve problems in many
fields of science such as:
Chemistry, Biology, Environmental Studies,
Agriculture, Medicine, Materials Science,
Archeology, Forensic Science, Geology,
Physics, Engineering, etc
(see Figure 1.1 on page 5 of textbook)
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Methods of Chemical Analysis
• To estimate the concentration of an analyte may
require both measurement of the mass or volume of
the sample and some physical quantity that is
related to the concentration of the element or
compound.
• This quantity can be classified as:
1. Gravimetric – mass of a precipitate
2. Volumetric – volume of a titration
3. Electroanalytical – voltage, current, amount of charge
4. Spectroscopic – absorbance, fluorescence, emission
5. Miscellaneous – radioactivity, reaction rate, refractive index
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Steps in Quantitative Analysis
1. Choosing a Method
• What is the concentration range (detection limit)
• What accuracy is required (depends on the problem)
• What elements or compounds are involved
• How complex is the mixture (selectivity, separation)
• How many samples are involved (analysis time)
• What methods are available to you !!!
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Steps in Quantitative Analysis
2. Obtaining the Sample
• Golden Rule – an accurate chemicial
analysis may be rendered useless if the
sample analysed does not have the average
composition of the whole batch
• How would you collect a representative 1 g
sample from a 10 tonne truck load of rock
containing a small amount of the precious
metal silver, given the particle size varies
from 0.1 mm to 10 mm ? (2 m x 2 m x 1 m)
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Steps in Quantitative Analysis
2. Obtaining the Sample
• Replicate Samples – how many samples are
required and where, when and how are they to
be collected?
• How would you collect a water sample in
order to estimate the annual load of copper
(tonnes of Cu per year) being transported
down the Chaopraya River in Bangkok. Note
both the water flow rate and the copper
concentration must be measured.
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Steps in Quantitative Analysis
3. Processing the Sample
• Preparing and Storing Samples –
SOLIDS - grinding, mixing, storage to avoid
changes (water content, oxidation). Selecting the
subsamples for analysis
LIQUIDS – solvent & analyte evaporation, settling
of solids, storage to minimise chemical changes
(refrigeration, freezing, deoxygenate, dark)
• Preparing Solutions – how to dissolve the
sample (acid, base, oxidant, reductant, organic
solvent)?
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Steps in Quantitative Analysis
4. Eliminating Interferences
• Impurities may contribute to (or decrease)
the spectroscopic signal used to calculate
the concentration e.g. Si produces the same blue
colour as PO43- with molybdenate reagent
• These interferences must be identified and
their effect eliminated which can present a
difficult problem.
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Steps in Quantitative Analysis
5. Callibration
• Accurate standards are usually used to plot a
calibration line e.g. Absorbance vs Concentration
A4
A3
Ax
A=elc
A2
A1
See Fig 1F-2
on Page 15
of Text
A0
C0
C
1
C
2
C
3
Cx
C
4
Concentration
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Steps in Quantitative Analysis
5. Callibration
• Straight line calibration graphs are preferred
but may not be always possible
• Standard compounds used to make the
calibration solutions must be chemically
stable e.g. metals should not oxidise, salts should not
gain or lose water
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Steps in Quantitative Analysis
6. Calculating Results and Estimating Reliability
• Concentration of unknown samples is calculated
using the calibration line equation
• Precision is estimated using the standard deviation
or standard error using replicate measurements
• Accuracy is best evaluated by including some
verified standard materials close in nature and
composition to the unknown samples
The checking of precision and accuracy is very
important in analytical chemistry and is referred to
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as quality control or quality assurance