Page 27 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GJB Revised Pages
Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

566 Analytical Chemistry

5. Coulometry and Electrogravimetry 1. Ion Sources
These techniques are dependent on an electrolysis that The common features of all ion sources are that they in-
attempts to oxidize or reduce completely all of an elec- corporate techniques for producing ions and giving them
troactive analyte species in a volume of sample solution. kinetic energy (acceleration used for introduction into the
These methods have the distinct advantage of requiring spectrometer). The electron-impact source is the most
no calibration since they are dependent on total electron widely used (Fig. 18). In this device, molecules in the
transfer, which can be quantitatively related to the total gas phase, obtained from the inlet system, are subjected
mass of the analyte. Coulometry refers to the integration to a stream of accelerated electrons (usually at a poten-
of electric current with time as electrolysis proceeds ei- tial of 70 V), and the resulting collisions between the
ther at a constant current or at a constant voltage. The particles cause molecular ionization and fragmentation.
latter parameters are chosen to optimize analysis time and These species are then injected into the spectrometer by
limit interferences. For constant current, the number of the accelerating slits. Another source is based on chemi-
coulombs of electrons consumed or produced, Q, is re- cal ionization. Here, the analyte molecule is ionized by an
lated to the current I and time t by ion–molecule reaction, not directly by electrons as men-
tioned above. A reagent gas is ionized by electrons, for
Q = It. example, methane,
+
−
−
CH 4 + e → CH + 2e ,
For variable current, the relationship becomes 4
with further reactions:

Q = Idt. + +
CH + CH 4 → CH + CH 3
4 5
+
+
CH + CH 4 → C 2 H + H 2 .
3
5
Typicalelectrochemicalcellarrangementsemployathree-
electrode potentiostat with a platinum working electrode. These particles can react with the sample molecule (MH)
It is possible in many cases to arrange the experiment in a number of ways:
so that the electrolysis deposits the analyte quantitatively
on the working electrode. The mass change of the elec-
+
CH + MH → CH 4 + MH +
trode can be related to the number of coulombs of the 5 2
+
charge associated with the deposition to determine equiv- CH + MH → CH 4 + M +
3
alent weight. These techniques have only moderate sensi-
+
CH + MH → CH 4 + MH +
tivity but provide accuracy and precision to relative values 4
of tenths of a percent. The advantages of this technique are that less energy
is transferred to the analyte molecule, causing less frag-
mentation, and the fragmentation can be controlled by the
E. Mass Spectrometry choice of reagent gas.
The field ionization technique uses the effect produced
In mass spectrometry, molecules are both ionized, posi- 7 8 −1
on molecules by a high electric field (10 –10 Vcm ).
tively or negatively depending on the imposed conditions,
In an appropriate electrode design, the electric field is
and fragmented. The particles are sorted out into groups
high enough to rip electrons from the molecule of interest.
according to their mass-to-charge ratio. A plot of the count
of the ions against the mass of the different types of ions
is called a mass spectrum. This record is valuable analyti-
callyinthatitischaracteristicofeachchemicalcompound.
The instrument required for this purpose can be divided
into several main components: (1) a device for introducing
the sample or inlet system, (2) an ion source, (3) the spec-
trometer for sorting out ions or ion analyzer, (4) a detector
of ions, (5) a recording system, (6) a vacuum system, and
(7) a complete control system including computer for data
handling and so on. There are a wide variety of such instru-
ments, which can be classified according to the methods
of ionization and/or procedures for ion analysis. Here, we FIGURE 18 Typical electron-impact source for mass spec-
shall concentrate on certain areas. trometry.