hi and welcome to our ATR basics video
ATR is the standard technique in
infrared spectroscopy it is used for raw
material verification quality control
and of course scientific research but
how a spectra in ATR collected
first you need a suitable infrared light
source a detector and an ATR crystal
made out of six selenide Diamond
germanium silicon or other materials it
should be noted that in general the ATR
crystal material must have a higher
refractive index than the sample
investigated the sample is then placed
on the ATR crystal it is important that
the crystal and the sample have good
contact during the measurement the IR
light passes to Crystal is reflected
internally and partially absorbed by the
sample on top because of the lights
wave-like properties it will slightly
penetrate into the optically less dense
material this so-called evanescent field
interacts with the sample since the
intensity of the evanescent field decays
exponentially the light only interacts
with the first few microns of a sample
the internally reflected light now
carries the absorption information of
the sample and is collected by a
detector so far so good but what are the
benefits of using ATR compared to
transmission measurements ATR is above
all a universal sampling technique
spectral quality is almost independent
from size shape and material there is no
time consuming sampling necessary to
acquire good spectra you can analyze
solids liquids and pastes without
dilution or extra steps cleaning an ATR
accessory is particularly easy there is
an excellent sample to sample reproduce
ability and minimal operator induced
variations still sounds pretty good but
ATR has certain characteristics that
need to be discussed when talking about
the comparison of ATR and transmission
spectra of course ATR shows the same
basic band patterns as a transmission
spectrum however there are some
differences that have to
be addressed if you want to compare ATR
and transmission spectrum in
transmission all wavelength have to
travel the same distance through the
sample material the penetration depth of
ATR however is a function of wavelength
and refractive index which means that
different wavelength have different path
lengths as a result the band intensities
at different wavelength differ between
transmission and ATR additional band
shifts may Oh cure for broader signals
due to the change in penetration depth
across an absorption band both effects
are easily corrected by modern I ask
software's allowing a direct comparison
the resulting corrected spectrum is
almost identical to transmission
spectrum in practice only a very small
amount of sample is introduced to the
ATR crystal for solid samples pressure
is applied to ensure optimal contact
between sample and ATR as said before
ATR also analyzes many raw materials
like powders and pellets larger work
pieces and finished products can also be
easily tested and verified by ATR liquid
materials instantly have optimal contact
and do not require pressure to be
applied you will find that there is an
astounding amount of special ATR
accessories based on sample type or ATR
crystal this high-pressure ATR for
example is used to investigate very hard
materials or larger samples pressure is
applied with hand clutch and it features
a sturdy diamond ATR crystal
speaking of ATR crystals let's talk
about the most common ATR crystal
materials and start with diamond this
list shows the most important parameters
of ATR crystal materials diamonds stand
out with being the hardest and most
resilient when it comes to physical or
chemical stress additionally diamonds
give access to a very broad spectral
range from far infrared to UV this in
short it is a universal material for any
application on the other hand it is
quite expensive but saves money in the
long run because it's so robust and
durable it is also not
suitable for samples with an unusually
high refractive index like black rubbers
other ATR materials than diamond are
mechanically less durable and will only
withstand lower pressures germanium and
zinc selenide for example are not made
to endure high pressures and will
eventually break if the hand clutch is
combined with a wider tip the apply
pressure is reduced to suitable levels
again here is a list with the most
important properties of germanium it
stands out with a very high refractive
index resulting in a very small
penetration depth
therefore it is ideal for strongly
absorbing materials or very thin layers
on a sample surface because of other
properties that we'll discuss in another
video it is very suitable for high
resolution micro FTIR on the other side
germanium properties lead to weaker
absorbance bands for standard samples it
can be easily dented and scratched by
harder samples and has a very narrow
spectral range when compared to diamond
or zinc selenide zinc selenide is a very
widespread ATR crystal material and
easily distinguishable by its bright
yellow color a quick look at our
parameter list tells us that it offers a
much broader spectral range than
germanium while possessing a refractive
index similar to diamond it is therefore
a quite flexible material although the
most common argument for zinc selenide
are its affordable price and cost
efficiency there are certain drawbacks
however zinc selenide is a very soft
material and therefore quite prone to
scratches or other physical damages it
is also only usable in a very narrow
range of pH values since it will produce
toxic compounds like dye hydrogen
selenide in acidic environments both
materials zinc selenide and germanium
are also used for multi reflection
accessories these accessories are easily
recognizable by the long rectangular ATR
crystals but what is a multi reflection
accessory as said before it has a longer
trapezoid ATR crystal plate
in these ATR crystals the light bounces
not once but multiple times the total
number of bounces depend on the lights
incident angle as well as the length of
the plate but the question remains why
should we use such an accessory using
multi bounds ATR will significantly
increase the sensitivity for weakly
absorbing and lowly concentrated
substances as a rule of thumb the
doubling of the number of bounces will
lead to an absorbance band increase by
factor two therefore it is used for
quantifications lowly concentrated
compounds and liquids and pastes
fortunately sampling is still quite easy
just fill the liquid into the sample
tray pastes are added and then spread
with a spatula best made of silicone or
plastic to protect the ATR crystal
okay let's summarize the biggest
benefits you always record high-quality
spectra
ATR requires virtually no sampling it is
very easy to use and you only need a
very small amount of sample but you
should keep in mind that ATR only
investigates the first few microns of a
sample and homogeneity is very important
also there are special crystal materials
for certain applications and to compare
ATR spectra with transmission they
should be treated before hand I want to
close with mentioning some of the more
exciting applications of ATR like
forensic investigations of illegal drugs
and other substances the reaction
monitoring of UV curing polymers or
catalysts and the differentiation of
cannabinoids in cannabis buds and
products although may be less exciting
but the main application for sure is the
routine analysis of raw materials and
quality control of finished products
that's it about ATR if you still have
questions visit our website or contact
us see you in the next video