Q&A Flashcards
What is analytical chemistry
obtaining, processing, and communicating information about the composition and structure of matter
Areas where analytical chemistry is regularly employed
Government institutes : research in food/water/minerals
Public analyst laboratories: Consumer protection
Hospitals: Analysis specimens for diagnosis
Water authorities: Checking for pollution/sewage levels/bacteria
Quality control departments: Check specifications of products / raw materials
Product development departments: New inci/stability
What are some analytical service providers & why may they be used
Expensive resource for smaller companies, outsource to provide regular testing.
Eurofins scientific
Uk national measurements laboratory and design
What are classical analytical methods (3)
- Gravimetric analysis, converting the analyte into an insoluble form (precipitate) which can be
filtered, dried and weighed. - Volumetric analysis, use of standard solutions (known volumes of known concentrations) to react
with the analyte in a titration to a defined ‘end point’, e.g., a colour change. - Electrical methods, based on electrochemistry and may involve measurement of current, voltage,
resistance or conductance. pH is perhaps the best known here
What are instrumental analytical methods
Spectroscopy
Chromatography
Mass spectroscopy
NMR
What is meant by separative methods
require the analyte to be extracted or isolated from the sample. This may be because the sample contains other components which behave similarly to the analyte of interest. Separation may also be used as a pre-treatment before final analysis by another technique
What is meant by non-separative methods
methods usually rely on the fact that a particular property can be observed or measured regardless of whether other components are present in the sample
Examples of separative methods
Chromatography
Solvent extraction
Distillation
Centrifugation
Electrophoresis
Filtration
Examples of non-separative methods
Spectroscopy
Radio-isotope determination
Electrochemical determination
NMR
Immunoassay
What are the steps in an analytical sequence
Sample : Bulk supply
Representative sample : Homogenous portion
Pre treatment: Provide extract free from interferences
Concentration: Maximise reactions for analytes
Analysis: Classical / instrumental
Results : Repeat results
Interpretation : Evaluation or use of software
Importance of Representative Samples
essential that the sample is representative of
that bulk material.
sample represents as near as possible
the original nature of the bulk material and that no contamination is introduced, or
loss of volatile material allowed to occur
Pre treatment how is it carried out
remove interference from other constituents in the sample. include filtration, precipitation, centrifugation and extracting the analyte of interest from the
sample using an organic solvent. Other treatments may include removing moisture by drying, ashing
the sample by heating to 600°C to remove organic material and leaving inorganic residue, or dissolving
the sample in an appropriate solvent
What is the draw back of using more steps in quantitive analysis , how can reduce this
losing some of the analyte.
determine the ‘recovery’ of the analyte after a particular stage. This is carried
out by subjecting samples with a known concentration of the analyte to the pre-treatment, and then determining whether the pre-treatment has affected the quantity of analyte originally present. If the pretreatment had no result on the content, the recovery should be 100%.
What does the analysis constitute
‘measurement’ of the analyte. In
modern instrumental techniques, this stage may well be carried out automatically thereby reducing the role of the analyst to one of setting up and calibrating the instrument
Sources of error in analytical chemistry
Systematic errors - determinate
Random errors - indeterminate
Gross errors - total
Systematic errors - determinate , what are they due to
their influence on the correct result is well
known and can be removed or reduced by adopting good experimental practice. These errors may
arise because the analyst does not follow the accepted method exactly, uses non-specified or
impure reagents, fails to allow reactions to proceed to completion or relies on instruments that are
faulty, poorly maintained or non-calibrated
How can you determine
their influence (systematic errors)
use of control samples containing a known quantity of analyte, comparing the results of different, but appropriate, methods, or subtracting the value of blank samples
Random errors - indeterminate , what are they due to
arise from slight variations in all methods
and tend to be unpredictable and thus difficult to detect and quantify. They may arise through
slight variations between different batches of reagents, slight temperature changes during
a reaction or different analysts identifying a different end-point (such as a colour change) in a
reaction. Generally, the effect of random errors on a result is small and in a set of replicate analyses
the statistical mean value will correspond to the true value
Effect of gross errors and cause
make the final result meaningless. They may be caused by selecting an entirely inappropriate method, eliminating a key stage or using an instrument incorrectly or one that is fault
What occurs at the stationary phase
solutes progress through at different rates depending upon the degree of interaction that the solutes have with the stationary phase
How does the solute react with the stationary phase (mechanisms)
a) solute dissolves in the stationary phase and moves through as if in solution. This is called partition. (The stationary phase must be a liquid.)
b) solute is adsorbed by the stationary phase and moves through the solid particles by surface effects.
This is called adsorption or affinity. It requires a solid stationary phase.
c) solute reacts chemically with the stationary phase or with ions attached to it. This is called ion
exchange. (This has a solid or liquid stationary phase.)
d) the stationary phase is designed to have a porous nature and only the smallest solute molecules can enter the pores. Larger molecules will be washed through rapidly. This is called gel permeation or size exclusion. (The stationary phase usually a gel.) Thus, during separation, largest molecules will separate first and the smallest molecules will appear last.
When is chromatography used
To check purity of raw materials and compare results with pure samples
How do the separated components appear
as a series of peaks on the chart record which is called a chromatogram.
What is the difference with chromatography and gas chromatography
mobile phase is a gas, comprises both gas-liquid
chromatography (GLC) and gas-solid chromatography (GSC
