SUMMER 2023

154
THE DISTRIBUTOR’S LINK
ROB LaPOINTE FASTENER SCIENCE SPECTROSCOPY – THE ELEMENTAL CODE BEHIND THE CHEMISTRY OF METAL from page 130
Now that we understand a bit about how electron
transitions in the atom produce light, we can understand
how to determine the chemistry of a material based on
the light it produces when its electrons transition from high
energies to lower energies. If we sample a small piece of
the metal of which a fastener is made, we can put that
metal in a state of high energy.
Most often, this is the plasma form of the material.
Plasma is formed when a material is given so much energy
that its electrons are ripped away from the nuclei in a swarm
of charged particles. In this plasma, electrons will make
transitions with atomic nuclei and produce light related to
that nucleus. The color of this light can be detected by the
instrument and compared to the known spectrum of the
element to find what is present in the sample. The colors
detected will decide the presence of the element and the
brightness of the spectrum will determine the concentration
of the element.
There are several different instrument configurations
that have been developed to perform the task of energizing
material so that it emits light and then detects the color
of that light and its brightness. It is best to think of the
instruments as having a frontend which is the excitation
part of the instrument and a backend, which is the
detection part of the instrument. Excitation can happen
in a few different ways, but all are similar in that they
provide an energetic environment for the electrons to have
transition opportunities. Some examples of the frontend
energy sources are electrical arc or spark, plasma, x-ray,
and laser. Each of these has its advantage for detecting
particular elements depending on the level of energy
produced. Among the more popular instruments available
is the Themo Fisher ARL series, which uses an electric
spark to energize the material and a LECO GDS, which uses
plasma to excite the sample. X-ray Fluorescence (XRF) and
Laser Induced Breakdown Spectroscopy (LIBS) are popular
for handheld and portable instruments.
On the backend of the instrument is the detector. The
detector’s job is to see the light produced and determine
two things. The color (wavelength) of the light and its
brightness. The tricky thing about this job is the light is very
faint so the detector must be sensitive. Photo multiplier
tubes (PMTs) have long been used to detect faint light and
multiply its output signal into an electronic pulse that can
be counted and stored. PMT based instruments must be
large to accommodate the array of PMTs to be placed in the
spectrum’s light path. Although PMTs are very sensitive,
their large size prevents placing them in the dispersed
spectrum to detect all wavelengths produced by a material.
Imagine placing your eye in the path of each color being
dispersed by a water droplet (Figure3) and you have a
sense of how individual PMTs are arrayed in one of these
machines. The fact that the detector takes up space and
that not all of the detector is sensitive to light means that
you’ll be missing some photons. These instruments must
be set up to detect common colors emitted by the material
you will hope to analyze.
FIGURE 9. A LINEAR CHARGE COUPLED DEVICE
FOR DETECTING LIGHT.
A more popular detector in new instruments is the
charged coupled device (CCD) or other such solid-state
detector that can be built as a large strip and arrayed
with other strips to detect the full length of the spectrum
produced (see Figure 9). This allows the instrument to
have a smaller size and more continuous view of the
spectrum produced by any sample of material.
Once the sample has been excited, the electrons have
transitioned from high energy to low energy, the light
they’ve emitted has been detected and the data stored
as wavelength and brightness, a computer can assist by
processing the data into meaningful results.
CONTINUED ON PAGE 166