Een samenvatting met alle informatie benodigd voor het tweedejaars tentamen Analytische Chemie m.b.t. het onderdeel 'Chromatografie' gebaseerd op het boek Principles of Instrumental Analysis door Douglas A. Skoog et al.
GAS CHROMATOGRAPHY
COLUMNS
Long columns (compared to LC)
2-50m
PACKED
Glass, metal or Teflon tubes
Length is 2-3m
di =2-4 μm
CAPILLARY
Wall coated open tubular (WCOT)
Capillary tubes coated with a thin layer of the stationary phase
Support-coated open tubular (SCOT)
Inner surface of the capillary is lined with a thin film (≈30μm) as a support
material
Holds more stationary phase as does a WCOT thus has a greater sample capacity
Fused-silica open tubular columns (FSOT)
Type of WCOT column
Increased flexibility due to purified silica with an outside protective polyimide
coating
260 μm <di>320μm are most widely used
Megabore columns
di≥530 μm
Properties and characteristics of typical GC columns: p. 713, table 27-1.
DETECTORS
Characteristics for the ideal GC detector:
1. Adequate sensitivity
2. Good stability and reproducibility
3. Linear response to solutes that extends over several orders of magnitude
4. Temperature range from room temperature to at least 400°C
5. Short response time independent of flow rate
6. High reliability
7. Easy to use
8. Nondestructive of sample
9. Similarity in response toward all solutes.
FLAME IONIZATION DETECTORS (FID)
The most applicable detector for GC. The effluent from the column is mixed with H2 and air,
which is ignited electrically. At the temperature of the flame, most organic compounds produce
ions and electrons that conduct electricity through the flame. A potential charge of a few
hundred volts is applied across the burner tip and a collector electrode located above the flame.
The resulting current is amplified and measured. It is most useful for organic samples.
, THERMAL CONDUCTIVITY DETECTORS (TCD)
Based upon the changes in the thermal conductivity of the gas stream brought by the presence of
analyte molecules. An electrically heated element (fine platinum, gold, or tungsten wire) whose
temperature at constant electrical powers depends upon the thermal conductivity of the
surrounding gas. The resistance of the wire gives a measure of the thermal conductivity of the
gas.
ELECTRON-CAPTURE DETECTORS
The effluent from the column is passed over a β-emitter (Ni-63). An electron from the emitter
causes ionization of the carrier gas (N) and the production of a burst of electrons. In the absence
of organic species, a constant standing current between electrodes results from this ionization
process. Organic molecules tend to capture electrons, thus the current decreases. It is sensitive
to functional groups such as halogens, peroxides quinones and nitro groups. Insensitive to
amines, alcohols and hydrocarbons.
NITROGEN PHOSPHORUS DETECTOR
The nitrogen phosphorus detector (NPD) is a selective detector for nitrogen and phosphorus-
containing compounds. Its detection principle is not fully understood but appears to be based on
the electronic conductivity of a flame in presence of an alkali metal salt and changes therein
caused by nitrogen or phosphorus in the compounds entering the detector. A special bead
(containing rubidium) is mounted in a silica lattice between the jet and the polarization
electrode. This bead is fused onto a platinum wire. The bead is heated, independent from the
usual detector body heating. Due to the high temperature of the bead, rubidium ions interact
(in an unknown way) specifically with N- and P-containing sample molecules. The ions
formed are collected and the ion amount is measured with an amplifier.
MASS SPECTROMETRY
1. Ionization- Gaseous atoms of a particular element are bombarded with electrons fired from an
electron gun. These electron particles will give the gaseous atoms of the specific element a
charge. This charge may vary; +1 or some atoms may have no charge at all (the atoms with no
charge will not be deflected at all by the 'Deflection' later on).
2. Acceleration- The atoms, now charged, are called ions and because these ions are charged
they can be accelerated by an electric field.
3. Deflection- The charged ions of the particular element, now being accelerated through the
deflector, will now be deflected by a magnetic or electric field.
4. Detection- By the charged ions being deflected, they will hit a detection material-either
electric or photographic.
QUALITATIVE ANALYSIS
In theory, retention times should be useful for the identification of components in mixtures. In
fact, however, the applicability of such data is limited by the number of variables that must be
controlled in order to obtain reproducible results. Nevertheless, GC provides an excellent means
of confirming the presence or absence of a suspected compound in a mixture.
RETENTION INDEX
The retention index is an index to identify solutes from chromatograms. By definition, the
retention index ( ) for a normal alkane is equal to 100 times
the number of carbons in the compound.
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