Environmental Engineering Reference
In-Depth Information
Spectrometer
Radio
frequency
generator
Len
Argon
ICP Torch
Detector
Nebulizer
Pump
Sample
Data processor
To waste
Figure 9.7 Schematic diagram of inductively coupled plasma-optical emission spectrometry
(ICP-OES)
oscillating current in the ions and electrons of the support gas (Ar). The energy
transferred to a stream of argon through an induction coil produces temperatures
up to 10,000 K. The high temperature of the plasma atomizes the sample and
promotes atomic and ionic transitions, which are observable at UV and visible
wavelength. The excited atoms and ions emit their characteristic radiation, which
are collected by a device that sorts the radiation by wavelength. The intensity of the
emission for the analyte is detected and turned into electronic signals that are output
as concentration information.
There are two ways of viewing the light emitted from the ICP. In the classical
radial ICP-OES configuration, the light across the plasma is viewed radially,
resulting in the highest upper linear ranges. By viewing the light emitted by the
sample looking down the center of torch or axially, the continuum background from
the ICP itself is reduced and the sample path is maximized. The axial ICP-OES
provides better detection limits—by as much as a factor of 10, than those obtained
by radial ICP-OES.
9.2.4 Atomic X-ray Fluorescence
There are three main types of X-ray fluorescence (XRF) instruments in use today:
wavelength dispersive, energy dispersive, and nondispersive. In wavelength dis-
persive XRF, the fluorescence radiation is separated according to wavelength by
diffraction on an analyzer crystal. This is typically a very expensive type of XRF not
in common uses. The XRF with current environmental applications is the energy
dispersive. As shown in Figure 9.8, the energy dispersive XRF consists of a poly-
chromatic source (either an X-ray tube or a radioactive material), a sample holder, a
semiconductor detector, and the various electronic components required for energy
discrimination. Note that the emitted photons released from the sample are observed
from the sample at a 90 angle to the incident X-ray beam so that the incident light
will not interfere the detector. In the detector, each photon strikes a silicon wafer that
 
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