● gas supply and controls;
● sample injection port;
● column housed in a thermostatically-controlled oven;
● detection and recording system;
● microcomputer with control and data processing software.These are described in the following sections.Mobile phase The mobile phase is known as the carrier-gasbecause its sole purpose is to
transport solutes through the column, thus not contributing to chromatographic
selectivity. It should be inert, non toxic, non flammable and preferably cheap.
Heliumand nitrogenare invariably used routinely, the former with capillary
(open tubular) columns and the latter with packedcolumns (vide infra). Helium
gives better chromatographic efficiency (reduced band broadening) due to faster
mass transfer (Topic D2). The carrier gas must be purified by passing it through
suitable adsorbents so as to avoid undesirable chemical changes to sample
components and stationary phases, or adverse effects on detector performance.
The most common contaminants and the means of removing them are:
● Air or oxygenat levels above about 10 ppm, which can oxidize sample
components and liquid stationary phases, especially at high column tempera-
tures. These can be removed by a cartridge containing molecular sieve.
● Hydrocarbons, which affect detector performance by contamination or
producing a large and constant background signal. These can be removed by
a cartridge containing activated carbon.
● Water vapor, which can affect some solid and bonded-liquid stationary
phases, and the performance of some detectors. This can also be removed
with molecular sieve.The carrier gas is supplied from a cylinder via a pressure-reducing valveat
10–45 psi (0.7–3 bar) which provides flow rates of between 1 and 50 cm^3 min-^1
depending on the type of column in use. A mass-flow controllerensures
constant flow rates regardless of back-pressure and temperature (the viscosity of
gases increases with temperature).Sample injection Samples should preferably be injected into the flowing mobile phase rapidly so
as to occupy the smallest possible volume when vaporized. This ensures a
narrow initial sample band that maximizes column efficiency and resolution.
There are a number of methods of injection and designs of injection port
available, and the choice is determined by the type of column in use and the
nature of the sample. Small volumes of liquids or solutions (0.1–10ml) are gener-
ally injected into a heated injection port, through which the carrier gas continu-
ously flows, from a calibrated microsyringe used to pierce a self-sealing
silicone-rubber septum. Gases are introduced via a gas sampling valve or gas
syringe, and solids with volatile components as solutions. Septa must be
replaced regularly to avoid leakage and can be a source of contamination by
previously injected samples or the bleeding of plasticizers into the gas stream,
especially when operating at very high temperatures. A separate septum-purge
gas stream vented to the atmosphere, or septumless valves can overcome the
latter problem. Capillary (open tubular) columns require specially designed
injection ports to prevent overloading them with samples, which can severely
impair efficiency and resolution.
D4 – Gas chromatography: principles and instrumentation 139