Page 355 - Instrumentation Reference Book 3E

338 Chemical analysis: spectroscopy
ions having energies within a restricted band reduced to a minimum. The system can be operated
are brought to a focus at the monitor slit at temperatures up to 350 “C and incorporates
which intercepts a portion of the ion beam. They its own controlled heating and temperature-moni-
then enter the electromagnetic analyzer which toring facilities. It includes both large and small
gives direction and mass focusing. This double reservoirs to enable a wide range of quantities of
focusing results in ions of all masses being liquid or solid samples to be introduced.
focused simultaneously along a given plane. The To introduce less volatile and solid samples
ions can be recorded photographically on a plate into the ion chamber a probe may be used. The
over a period of time to give a very high sensitiv- sample is loaded onto the tip of the probe, which
ity and reduction of the effects of ion-beam fluc- is inserted into the ion source through a two-stage
tuation. vacuum lock.
Alternatively, the accelerating or deflecting The probe may be heated or cooled indepen-
field may be arranged so that ions of a given mass dently of the ion chamber as required from -50
are focused on a detector which may consist of a to +350”C. The temperature is measured by a
plate or, if initial amplification of the charge is platinum resistance thermometer, forming part
required, onto an electron multiplier or scintilla- of the temperature control system, which enables
tion detector. By arranging the deflecting field to the temperature to be set from the instrument
change in a predetermined manner, the instru- control panel.
ment may be arranged to scan a range of masses Effluents from a gas chromatograph column
and so record the abundance of ions of each usually flow at about 50cm3/min and consist
particular mass. Such a record is known as a mainly of carrier gas. In order to reduce the flow,
“mass spectrum” and mathematical analysis of the gas is passed through a molecular separator
this mass spectrum enables the composition of designed to remove as much as possible of the
the sample to be determined. Mass spectra carrier gas but permitting the significant compo-
obtained under constant conditions of ionization nents to pass into the mass spectrometer.
depend upon the structure of the molecules from
which the ions originate. Each substance has its 16.7.3 Ion sources
own characteristic mass spectrum, and the mass
spectrum of a mixture may therefore be analyzed In the system shown, the ions are produced by a
in terms of the spectra of the pure components, spark passed between electrodes formed from the
and the percentage of the different substances in sample by applying a controlled pulsed r.f. vol-
the mixture calculated. tage. Positive ions representative of the sample
Analysis of the mass spectrum of a mixture may are produced in the discharge and are accelerated
involve the solution of a large number of simulta- through a simple ion gun. This beam is defined by
neous equations, which can be accomplished using resolving slits before it passes into the analyzer
a microprocessor or a small computer. section.
Other methods may be employed in order to
produce ions of the sample which are impelled
16.7.2 Inlet systems
towards the exit slit by a small positive potential
The mode of introduction of the sample into the in the ion chamber. These methods involve
ion source is dependent upon the nature of the increasing the energy of the sample by some form
sample and, in particular, its volatility. of radiation. Organic compounds require
The simplest system designed to introduce photons of energy up to 13 eV to produce ioniza-
reference compounds into the ion source includes tion so that a high energy beam of short wave-
a 35cm3 reservoir into which the compound is length radiation is sufficient. Where energies
injected through a septum. Flow into the ion greater than 11 eV are required, window materials
source is through a molecular leak, and a shut- become a problem, so that the photon source has
off valve is provided. Facilities for pumping out to emit radiation directly into the ion source. A
the system and obtaining temperatures up to helium discharge at 21.21 eV provides a conveni-
100 “C are provided. ent source of photons capable of ionizing all
Relatively volatile gases and liquids may be organic compounds.
introduced by a probe attached to a small reser- Electrons emitted by a heated filament and
voir into which the sample is injected and from accelerated by about 10eV and directed across
which it flows to the ion source at a controlled the ion chamber may also be used to ionize many
rate. The temperature of the system may be con- substances. While 70 eV produces the maximum
trolled between ambient and 150 “C. ion yield, any voltage down to the ionization
For less volatile substances an all-glass heated voltage of the compound studied may be used.
system may be used. Glass is used for the system The electric field production near a sharp
so that catalytic decomposition of the sample is point or edge at a high potential will have a high