Chapter 0 The Analytical Process
As an illustration of how the analytical process works, the books starts with an
example of the steps taken by a pair of analytical chemists who want to determine the
caffeine content in a chocolate bar. This is a nice example, and you should read
through it. Rather than recap their story here, I want to point out several key terms that
appear in the narrative that you need to be aware of.
0-1 Analytical Chemist’s Job
Sampling
Real analysis usually starts with an ugly sample that really isn’t well
suited for analysis. Thus while this course will concentrate on the nice
clean analysis of a simple neat sample, real analytical chemists usually
spend a great deal of time getting their sample ready for the analysis and
making sure that their preparation hasn’t actually ruined their sample
The material of interest may be
Homogenous- the material has one uniform composition
Heterogenous - The composition varies form one part to another
If the material is homogenous then getting a sample is easy, and any old
piece will do (Maybe - see example in book about analyzing chocolate).
If, however, the material is heterogeneous, then care must be taken to
make sure the sample obtained truly represents the entire material
Sample Preparation
Once you have a sample, you need to quantitatively get it into a form that
you can analyze. The steps taken here will vary from sample to sample
and method to method.
Decanting - Pouring an upper layer off of a 2 phase mixture.
(Best done in a Separatory Funnel)
Analytes - The substance you wish to measure.
Quantitative transfer – Complete transfer of a material from one vessel
or form to another.
Slurry - Suspension of a solid in a liquid
Supernatant liquid - A liquid above a packed solid.
Chemical Analysis
Chromatography - any method that separates different components in a
mixture.
Qualitative Analysis - Identification of what the unknown is
Quantitative Analysis - Identifying how much of the unknown is present.
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Calibration curves
One tool that is used frequently is plot the response of some instrument
vs the amount of one of the chemicals we are analyzing
When we use a set of standards to calibrate the response of the machine,
this plot is called the Calibration curve or standard curve.
Interpreting Results
Your final result is a number, and some measure of that number’s
uncertainty. Will spend a chapter on statistics for this.
Quality Assurance
Now you have a set of results, how reliable are you? In an industrial or
commercial setting you need to follow a set of practices called Quality
Assurance to prove and document that the results you obtain are
reliable.
0-2 Steps in a chemical analysis
The analysis of a sample contains many steps:
1. Formulating the questions
2. Searching the literature
3. Sampling - obtaining a representative sample
-extracting the smaller lab sample from the bulk sample
4. Sample Preparation - converting the lab sample into something suitable for
analysis
-May include concentration, dilution, or chemical reaction of analyte
-Removing or masking other things that can interfere with the analysis
5. Actually performing the analysis on several samples
6. Final interpretation of the results
7. Drawing a conclusion
The bulk of this book deals with step 5. You can see that it is a small part of an
analytical chemist’s job.
Now go back to the example given in 0-1 and see if you can see where each of these
steps were performed.