Page 402 - Instrumentation Reference Book 3E

Detectors 385
the components of the previous sample have been sessed by a body by virtue of the motion of the
eluted from the column. It is therefore advanta- molecules of which it is composed. Raising the
geous to arrange the analytical conditions so that temperature of the body increases the energy of
the sample is eluted as quickly as possible, con- the molecules by increasing the velocity of the
sistent with adequate resolution of the peaks of molecular motion.
interest. Two techniques commonly used in pro- In solids the molecules do not alter their position
cess gas chromatography, and now finding more relative to one another but vibrate about a mean
applicatiomn in the laboratory, are “heart cut” and position, while in a liquid the molecules vibrate
“backflush.” Both techniques rely on an about mean positions, but may also move from
understanding of the components being analyzed one part of the liquid to another. In a gas the
and their elution times on various phases. With molecular motion is almost entirely translational:
these techniques, the analytical chemist chooses the molecules move from one part of the gas to
to analyze only those components in which he or another, only impeded by frequent intermolecular
she is interested and vents the balance to “waste.” collisions and collisions with the walls vessel. The
Heart cut is the fastest way to separate trace collisions with the walls produce the pressure of the
level concentrations of components when they gas on the walls. In a so-called “perfect gas” the
elute on si tail of a major component. Using two molecules are regarded as being perfectly elastic so no
columns, the heart cut valve diverts the effluent of energy is dissipated by the intermolecular collisions.
the heart cut column to either to vent or to the Consideration of the properties of a gas which
analysis column for further separation. Flow of follow as a consequence of the motion of
carrier gas in both columns is maintained the its molecules is the basis of the kinetic theory.
same by using restriction orifices. Normally, the Using this theory Maxwell gave a theoretical
effluent of the heart cut column is diverted to verification of laws which had previously been
vent, but when a component of interest appears, established experimentally. These included
it is diverted to the analysis column and then Avogadro’s law, Dalton’s law of partial pressures,
returns to its venting position. In this way a and Graham‘s law of diffusion.
“cut” containing only the component(s) of interest Since heat is the energy of motion of the gas
and a narrow band of the background compon- molecules, transfer of heat, or thermal conductiv-
ent are introduced to the analytical column. ity, can also be treated by the kinetic theory. It
Reversing the flow of carrier gas in the direc- can be shown that the thermal conductivity K of
tion opposite that of the sample injection is called component S is given by
backflushing. Therefore, backflushing a column
results in any components still in the column Ks = tpVXC,
being swept back to the point of injection in where p is the gas density, Vis the mean molecular
approximately the same amount of time it took velocity, X is the mean free path, and C, is the
to flow to ;their present location in the column. specific heat at constant volume. Thus, thermal
Components will “flush” from the column in the conductivity depends on molecular size, mass,
reverse order to which they appear on the col- and temperature.
umn, meaning that in many cases the heavy com- The quantity VX is the diffusion coefficient D of
ponents will flush back to vent first. This can be the gas; and the thermal conductivity can be written
used effectively to in some cases reduce flush the
light components to vent and direct the heavy Ks = 4 DpC,
components to the detector, thus significantly
decreasing ithe cycle time. Alternately, cycle time According to this treatment, the thermal
can be reduced by not having to flush the entire conductivity of the gas is independent of pressure.
sample through the colurnn(s), also resulting in This is found to be true over a wide range of
increased column life. pressures. provided that the pressure does not
become so high that the gas may no longer be
regarded as being a perfect gas. At very low pres-
18.3 Detectors sures, the conductivity of the gas is proportional
to its pressure, and this is the basis of the oper-
18.3.1 Thermal conductivity detector (TCD) ation of the Knudsen hot-wire manometer or
The thermal conductivity detector is among the Pirani gauge (see Chapter 10).
most commonly used gas detection devices. It It can be shown that the conductivity KT of a
measures the change in thermal conductivity of pure gas at absolute temperature T varies with
a gas mixture, caused by changes in the concen- temperature according to the equation
tration of tlhe species it is desired to detect.
All matter is made up of molecules which are in
constant rapid motion. Heat is the energy pos-