chromatography. Flashcards
What is Chromatography ?
Chromatography is an important analytical technique because it allows chemists to separate substances in complex mixtures. There are a variety of types of chromatography, which can be used in different contexts.
What is Qualitative analysis?
Qualitative analysis means any method of identifying the chemicals in a sample.
Quantitative analysis means working out how much of a particular chemical there is in a sample.
Any analysis must be carried out on a sample of a substance that is representative of the bulk of the substance. This means that the results from the analysis can be used to draw conclusions about the rest of the substance.
It is important to mix solutions thoroughly before testing, and to take many samples at random from the solution when taking repeat readings.
It is important to follow standard procedures when analysing chemicals. This ensures that the results are reliable and valid.
What is Quantitative analysis?
means working out how much of a particular chemical there is in a sample.
WHy is it important to follow standard procedures when analysing chemicals.
This ensures that the results are reliable and valid
Any analysis must be carried out on a sample of a substance that is representative of the …. of the substance.
Any analysis must be carried out on a sample of a substance that is representative of the bulk of the substance. This means that the results from the analysis can be used to draw conclusions about the rest of the substance.
It is important to mix solutions thoroughly before testing, and to take many samples at random from the solution when taking repeat readings.
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This means that the results from the analysis can be used to draw conclusions about the rest of the substance.
It is important to mix solutions thoroughly before testing, and to take many samples at random from the solution when taking repeat readings.
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What else should you remember when sampling?
It is important to mix solutions thoroughly before testing, and to take many samples at random from the solution when taking repeat readings.
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Standard procedures cover what 3 things?
Standard procedures cover the following:
collecting samples
storing samples
preparing samples for analysis
In chromatography, substances are s…………… as they travel in a ……………phase which passes through a …………. phase. Different substances travel at ………… speeds, so some move ……………. than others in a given time
In chromatography, substances are separated as they travel in a mobile phase which passes through a stationary phase. Different substances travel at different speeds, so some move further than others in a given time
In paper chromatography, the stationary phase is what? .
In paper chromatography, the stationary phase is paper. The mobile phase may either be an aqueous (water-based) liquid or a non-aqueous organic (carbon-based) solvent. An example of an organic solvent is propanone - which is the main chemical in nail varnish remover.
The mobile phase may either be an …………………. Give an example
The mobile phase may either be an aqueous (water-based) liquid or a non-aqueous organic (carbon-based) solvent. An example of an organic solvent is propanone - which is the main chemical in nail varnish remover.
For each chemical in the sample, there is a dynamic equilibrium between the……………..phase and the…………phase. The overall separation depends upon how strongly………….the chemicals are to the mobile and the stationary phases.
For each chemical in the sample, there is a dynamic equilibrium between the stationary phase and the mobile phase. The overall separation depends upon how strongly attracted the chemicals are to the mobile and the stationary phases.
What is Thin layer chromatography (TLC)?
Thin layer chromatography (TLC) is similar to paper chromatography but instead of paper, the stationary phase is a thin layer of an inert substance (eg silica) supported on a flat, unreactive surface (eg a glass plate).
What are some advantages of TLC over paper chromatography. Give an example.
TLC has some advantages over paper chromatography. For example:
the mobile phase moves more quickly through the stationary phase
the mobile phase moves more evenly through the stationary phase
there is a range of absorbencies for the stationary phase
What does TLC produce that is more useful than paper chromatography,
TLC tends to produce more useful chromatograms than paper chromatography, which show greater separation of the components in the mixture - and are therefore easier to analyse.
A chromatogram can be compared with a chromatogram of a ………… ……………material to identify which …………….the mixture contained.
A chromatogram can be compared with a chromatogram of a standard reference material to identify which chemicals the mixture contained.
Sometimes the substances being separated are colourless. In this case, what can be used to show where the spots are?
Locating agents bind to the chemicals in the spots. Sometimes, another chemical is then added, which reacts with the locating agent to produce a coloured spot, or the chromatogram is put under ultraviolet light and the locating agent glows to show where the spots are.
Locating agents
What do Locating agents do?
Locating agents bind to the chemicals in the spots. Sometimes, another chemical is then added, which reacts with the locating agent to produce a coloured spot, or the chromatogram is put under ultraviolet light and the locating agent glows to show where the spots are.
What is Rf value?
Rf value
The movement of a substance during chromatography, relative to the movement of the solvent, is measured by calculating its retardation factor (Rf).
Rf value = distance travelled by substance/distance travelled by solvent
What is the formula for Rf value?
Rf value = distance travelled by substance/distance travelled by solvent
Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.
Calculating the Rf value allows chemists to identify what?
Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.
What is Gas chromatography?
In gas chromatography (GC), the mobile phase is an inert gas (eg helium).
The stationary phase is a very thin layer of an inert liquid on an inert solid support - such as beads of silica packed into a long thin tube (this flexible tube is coiled many times inside a thermostatically-controlled oven to keep it at a constant temperature).
What is GC used for?
GC is used to separate complex mixtures. It is much better at this than thin-layer or paper chromatography. This is because it is more sensitive - allowing the determination not only of what chemicals are in the mixture, but also how much of each chemical there is.
What does a gas chromatogram show ?
A gas chromatogram might show the time along the x axis and the strength of response along the y axis.
What is retention time?
The amount of time that a substance takes to pass through the column is called it retention time. The retention time of an unknown substance can be compared with standard reference data to help to identify it.
What three main pieces of information can be gathered from a gas chromatogram:
Three main pieces of information can be gathered from a gas chromatogram:
the number of compounds in the mixture - represented by the number of peaks
how much of each compound is present - represented by the height of the peak (higher = more)
the retention time - indicated by the position of the peak
