Page 60 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology en001d42 April 28, 2001 15:9

784 Atomic Spectrometry

ground correction, could be mistaken for fluorescence.
When an ICP is used as the atom cell, the scattering inter-
ference is not observed. The ICP is so efficient at breaking
down droplets and particles that very few scatter signals
have been detected in an HCL–ICP instrument.

VI. DETECTION OF ELEMENTAL IONS

The laser-enhanced ionization (LEI) technique or elemen-
tal mass spectrometry may be used to directly measure
elemental ions. In LEI, a laser beam is used to excite the
analyte atoms in either a flame or electrothermal atomizer
cell. These atom cells are selected for LEI experiments be-
cause of their inherently low background ion population
(in contrast to plasma sources, which are rich in ions). The
laser-excited atoms undergo collisional processes that will
FIGURE 14 Laser atomic fluorescence in flame atomizers. The
ultimately lead to ionization of the analyte atoms. The ions
photomultiplier tube (PMT) is equipped with a band-pass filter.
are then collected and measured against a background cur-
rent. Although extremely sensitive, the LEI technique is
for AFS measurements because of their inherently large limited to detection of one analyte at a time in a flame or
radiant intensity. The general diagram for a flame atomic ETV atom cell. There are currently no commercial instru-
fluorescence instrument with laser excitation is shown in ments available for LEI analysis.
Fig. 14. Another approach for detection of elemental ions is to
use ICP sources (which are very rich in ion population)
in tandem with mass spectrometric detection. Under typ-
2. Detection System
ical operating conditions, about half of the elements in
Generally, monochromators are not used in AFS measure- the periodic table are singly ionized with an efficiency of
ments. For hollow cathode excitation, each HCL has a 90% or greater. The schematic diagram for an ICP–MS
paired dedicated photomultiplier tube detector. In front of instrument is shown in Fig. 15.
each photomultiplier tube is a filter, which allows a range Unlike the upright configuration in ICP–AES, in ICP–
of wavelengths to pass through it including the atomic- MS the plasma torch is placed on its side along the axis
fluorescence wavelength of the element excited by the of the mass spectrometric ion collection optics. A water-
HCL. The filter provides discrimination against the back- cooled cone is placed into the tip of the plasma to sample
ground emission from the atom cell. A high-resolution a portion of the ions generated by the ICP. The extracted
monochromator is not used for AFS, because the resolu- ions pass through another orifice, the skimmer, through
tion is provided by the specificity of the light source for the a differentially pumped region of the mass spectrometer.
element of interest. Although a low-resolution monochro- Differential vacuum pumping allows for the transfer of
mator can be, and often is, used for AFS, filters pass ions from plasma at atmospheric pressure into the high
more total light onto the photomultiplier tube and provide vacuum environment of the mass analyzer. The ions are
adequate resolution (in the range 2–10 nm) to discrim- guided through a series of lenses for the optimal signal-
inate against much of the background from the plasma. to-noise ratio at all mass ranges.
The light sources used for AFS measurement are always
modulated to allow discrimination against background
TABLE III Examples of Isobaric Interferences
emission.
Observed in ICP–MS
Analyte Mass Interfering ion
3. Spectral Interferences
Si 28 12 16 + 14 N +
C O ,
Because of the selectivity of the excitation source, 2
K 39 38 ArH +
the spectral interferences are almost completely absent.
Ca 40 40 Ar +
Nonetheless,somespectralinterferencehasbeenreported.
35
As 75 40 Ar Cl +
This interference is caused by scatter of the incident source
Se 80 40 Ar +
radiation off large droplets in the flame and, without back- 2