Technique
Principal underlying analysis
Procedure
Typical
analytes
Comments
Gravimetric
analysis
Ion under investigation in solution is caused to
precipitate out of solution by adding a reactant. The
precipitate is filtered, washed to remove
contaminants and dried to remove water.
Weigh sample (sausage), dissolve and filter
out insoluble particles. Add excess of reactant
to ensure all the ion is precipitated and filter
again. Wash and dry the precipitate. Weigh the
precipitate and calculate the amount (moles)
convert to % mass in sample
H2O (water
content), Cl-, SO42, Ag+
Errors associated with contaminant ions not
being washed from precipitate, not drying.
Use last weight not average weight.
Volumetric
analysis
An unknown chemical is analysed by measurement
of the volume of a standard solution. The standard
solution is prepared from a solid that is pure, does
not react with the atmosphere, stable etc. In a back
titration an excess of a standard is added initially
and the titration measures how much excess remains
after the reaction with the unknown
Standard solution is prepared by weighing out
a mass of primary standard and dissolving it in
a volumetric flask. The unknown is reacted
with the standard until an endpoint is reached.
A chemical equation(s) is used to determine
the amount of unknown.
Acids/bases,
oxidants/reductants
that are soluble
Concordant results are important here, make
sure you check. Sometimes you need to
write equation i.e. acid/base or partial ionic
equations to get full equation. Use of
appropriate indicator in acid/base titrations’.
Remember must be on the vertical line near
equivalence point.
Thin layer
chromatography
(TLC)
Separation of components is achieved because
different chemicals have different attractions for
either the mobile or stationary phase (Alumina
Al2O3). The process involves adsorption to the
stationary phase and desorption into the mobile
phase.
Place the sample on the origin above the level
of the solvent. Allow the solvent to travel up
the plate and mark where the solvent front
reaches. Record the distance travelled by each
component.
Rf= SD/SFD
Dyes in foodstuffs,
amino acids, drug
preparations
Samples need to be coloured or visible
under UV light. Compare Rf values with
standards for identification.
Gas
chromatography
Because different components have different boiling
points they will condense and vaporise in the
column by varying degrees. Larger molecules
generally have higher boiling points and will
therefore condense and adsorb to the stationary
phase more than smaller molecules taking a longer
time to elute. (Higher Rt ). An inert gas such as
nitrogen is used as the mobile phase.
Substance must be volatile. Inject into column,
record the time each component elutes (come
out) the column to identify (retention time,
Rt). Compare to Rt of standard. Area under the
peak determines concentration, again must be
compared to standards.
Low molecular
mass compounds,
gases, must be
volatile
compounds e.g.
ethanol,
hydrocarbons
Rt of standard is used to identify
component, area under the peak is used to
quantify unknown. Smaller components
emerge first. Rt dependant on length of
column, type of stationary and mobile
phases used.
Technique
Principal underlying analysis
Procedure
Typical
analytes
Comments
High Performance
liquid
chromatography
Separation of components is achieved because
different chemicals have different attractions for
either the mobile or stationary phase (column
packed with small, tightly packed particles) The
process involves adsorption to the stationary phase
and desorption into the mobile phase. Larger
molecules tend to be slower.
Dissolve and filter sample and inject into
column, record the time each component
elutes (come out) the column to identify
(retention time, Rt). Compare to Rt of
standard. Area under the peak determines
concentration, again must be compared to
standards.
Medium to high
molecular mass
organic
compounds can’t
be volatilised.
Proteins, DNA,
pesticides,
Rt of standard is used to identify
component, area under the peak is used to
quantify unknown. Smaller components
emerge first. Rt dependant on length of
column, type of stationary and mobile
phases used.
Flame test
The sample gives off light because thermal energy
causes electrons in the atoms to move further away
from the nucleus (excited), when they return to
ground state they give off light.
Place sample on nichrome wire (wire doesn’t
interfere) and place in blue flame of Bunsen
burner
Very few metal
ions, Na, Ca, K
etc.
Not a very powerful technique because only
a few metal ions have characteristic colours
and it relies on the eye to monitor. Only
qualitative.
Atomic emission
spectroscopy
The sample gives off light because thermal energy
causes electrons in the atoms to move further away
from the nucleus (excited), when they return to
ground state they give off light. The light is passed
through a prism and an emission spectrum is
obtained
A hollow cathode lamp is used to produce a
spectrum of light for an individual element. This
light is shone through individual atoms in a flame,
only atoms of that element will absorb this light.
Electrons are promoted from the ground state to an
excited state (opposite to atomic emission).
Sample is atomised in flame, light is emitted
and passed through prism. The greater the
intensity of the light the higher the
concentration in solution. Monochromator
selects strongest wavelength. Need to compare
to set of standards to measure concentration
Sample is atomised in flame, light from
appropriate hollow cathode lamp is shone
through the flame. The quantity of light
absorbed is proportional to the concentration
of metal ions present in sample. Compare to
standards
Many metals
Electrons within molecules, ions or atoms are
promoted from low energy to higher energy by UVvisible light. The strongest wavelength absorbed by
the chemical under investigation is selected. The
concentration of the unknown is compared to
standards
UV or visible light is shone through the
sample. The amount absorbed is proportional
to the concentration of the unknown. The
unknown is compared to a set of standards.
Low molecular
mass organic
molecules e.g.
glucose, aspirin,
quinine
Not as accurate as AAS, generally
examiners will show emission spectrum
from a mixture and you have to identify
possible components by comparing with
emission spectra of individual elements,
look for bands in common.
Very accurate, can determine ppm or even
ppb. Very specific because light can only be
absorbed by that particular element. Most
questions follow this pattern: Read
unknown off graph to find concentration
then multiply by volume then find % mass
etc. Check units!!!!!!!!
Use standard curve to measure unknown.
Check that the wavelength used is not
interfered with by another component of the
mixture.
Atomic absorption
spectroscopy
(AAS)
UV-visible
Spectroscopy
68 different metals
Technique
Principal underlying analysis
Procedure
Typical
analytes
Comments
Infrared
Spectroscopy
IR measures the absorption of energy by molecules
as they vibrate, rotate and stretch. Different bond
types take different amounts of energy to get them
to vibrate, rotate and stretch.
Infrared radiation is split in two and
simultaneously passed through the sample cell
and a reference cell. The reference cell is used
to discount the effect of the solvent, water or
carbon dioxide as they may absorb the
radiation. Hence the deference between these
two cells is recorded on the spectrum.
Organic molecules
Use data sheet to find bond types.
Wavenumber = 1/ wavelength
Shows transmittance not absorbance.
Nuclear Magnetic
Resonance (NMR)
1H
Organic molecules
Use data book to find environments. Check
peak splitting to find neighbouring groups.
Remember peak height shows number of
hydrogens in that environment (add split
peaks together)
Mass
spectrometry
Molecules or atoms are ionised and accelerated in
an electric field that is perpendicular to a magnetic
field. The magnetic field deflects particles according
to their mass to charge ratio, the smaller the mass to
charge ratio the more strongly it will be deflected.
Molecules can also be fragmented in the mass
spectrometer and form a finger print that can be
used for identification
Use tetramethylsaline (TMS) to determine the
zero point on the spectrum. Dissolve sample in
a solvent that does not give a signal e.g. D2O
not H2O, because deuterium has two nucleons
and hence no overall magnetic moment. Place
sample in glass tube that spins so the test
material is subjected to a uniform magnetic
field. A radio receiver coil detects the radio
frequencies emitted as nuclei relax to a lower
energy level. Computer records the data as a
spectrum. Match chemical shifts to
environments; area under peak is a measure of
the number of hydrogens. In high resolution
1H NMR spectroscopy a peak is split by the
number of neighbouring hydrogens plus one.
Gaseous sample is ionised by a beam of high
energy electrons. The positive ions formed are
accelerated by an electric field. The ions enter
a magnetic field perpendicular to their path
and are deflected. A collector measures the
current due to the ions reaching the detector
and the data is recorded as a spectrum. Peak
height measures abundance and distance from
carbon-12 measures relative isotopic mass.
Hugh range of
elements and
organic molecules
must be able to be
volatilised.
Peak height shows abundance. Isotopes can
be seen. Fragments must be able to be made
from the parent ion. Base peak is most
common fragment.
NMR (and 13C NMR) spectroscopy uses radio
waves to align hydrogen nuclei with an applied
magnetic field. The energy promotes the protons to
a higher spin level, when they relax protons release
this energy and it is recorded as a spectrum. The
amount of nuclear shielding provided by other
atoms near the proton determines how much energy
is needed to change the spin. Protons therefore have
different environments that result in different peaks
on the spectrum. The distance from TMS is called
the chemical shift.