Today we’re going to talk about FT-IR, or infrared microscopy,
a chemical analysis technique that combines FT-IR spectroscopy with a light microscope.
In this video we’re going to cover all the basics of FT-IR microscopy
from how the FT-IR microscope works to what it’s capable of.
Before we dive into what FT-IR microscopy is,
we need to have a basic understanding of FT-IR spectroscopy first.
FT-IR spectroscopy uses infrared light to identify chemical species.
We shine the IR light on our sample,
and the IR light interacts with the molecules or compounds in that sample.
This interaction is detected by an FT-IR spectrometer.
And then, that information is used to create an FT-IR spectrum.
Every chemical species interacts with IR light in a unique way,
so each FT-IR spectrum that results is unique.
That makes the FT-IR spectrum kind of like a fingerprint for each chemical species.
Just like fingerprints can be a powerful tool for identifying people,
the FT-IR spectra are a powerful tool for identifying molecules and compounds.
Now, on a more practical level, there’s three basic ways we can do these FT-IR measurements:
transmission, reflection, and ATR.
These are called measurement techniques and each one is useful for different kinds of samples.
For transmission, the IR light shines straight through the sample before being detected.
This technique is useful for very thin samples like tissues or films.
With reflection, the IR light bounces off the surface of the sample prior to detection,
making this technique the go-to for analyzing reflective bulk samples, like a silicon wafer,
or analyzing samples that are on top of reflective surfaces, like thin films on metals.
ATR uses a crystal, which is usually made of germanium.
The crystal touches the sample, and the IR light passes through
the crystal and enters the first couple micrometers of the sample.
This technique is the most versatile of the three and can be used to analyze a
wide range of samples, from electronics to cloth, all without sample preparation.
So that’s a short overview of FT-IR spectroscopy.
If you want to know more,
check out our basics series here on YouTube where we go into more detail.
Now, FT-IR spectroscopy lets us analyze a wide variety of materials,
making this technique applicable to virtually any industry or field of study.
But if FT-IR spectroscopy can analyze all these different kinds of samples
and materials, why do we need FT-IR microscopy?
Well, say we wanted to analyze the craters on this
piece of coated metal to see what might have caused this defect.
Each crater is quite small, maybe only a few millimeters across,
and to figure out what’s going on here we need to selectively analyze one of the craters.
When we make FT-IR measurements on the spectrometer, we don’t
have the precision to choose where on a sample we want to analyze so it’s
not a good technique for analyzing this craters on this piece of metal.
But with FT-IR microscopy, we can select the exact spot we want to analyze.
FT-IR microscopy can still analyze that same broad range of sample types,
but now we have a lot of precision with our measurements.
FT-IR microscopes have so much precision that they can measure structures down to
the micrometer scale, which is around the size of bacteria.
A really common use of FT-IR microscopy is failure analysis,
like analyzing this piece of wire that has a defect on the plastic sleeve.
We’ll use the FT-IR microscope to find the root cause of this defect.
Let’s see what goes on inside the microscope while we do that.
To do this analysis, we’ll need two light sources.
An infrared light source to do the chemical analysis, and a visible
light source so we can see and visually examine our sample under the microscope.
Now, microscope optics are traditionally made of glass,
but many frequencies of IR light can’t travel through glass like visible light can.
So what do we use instead of glass?
Well, one solution is to use an IR transparent material, like germanium,
for all the microscope components so the IR light can pass through.
While this approach seems pretty straightforward, there’s a couple complications.
First of all, not all the IR light actually goes through these IR transparent materials.
So as the IR light travels through all the components of the microscope,
we lose some light here and there.
By the time the IR light reaches the IR detector, we’ve lost quite
a bit of the IR light which can make it more difficult to create a high-quality spectrum.
Secondly, some of these IR transparent materials aren’t transparent to visible
light, so we’d end up with the opposite problem we had with glass.
The IR light could travel through the microscope to do the chemical
analysis but we wouldn’t be able to visually examine our sample.
Finally, components made out of IR transparent materials tend to be very expensive.
For all these reasons, the most common approach is to use a series of mirrors
made out of a metal with high reflectivity for IR light, like aluminum or gold.
By using mirrors, we can easily direct both the IR light and visible light around the microscope.
We can even make the microscope objective
out of these metal mirrors to focus the light on the sample.
These special objectives are called Cassegrain objectives.
So mirrors aside, how do we analyze the defect in the piece of wire?
First, we’ll need to do a little sample preparation.
We usually don’t need to do any sample prep for FT-IR microscopy, but in this case we want to
analyze underneath the plastic sleeve, so we’ll hold the wire in place and cut the plastic away.
Now we can see the defect but this area is pretty small,
about 2 millimeters long and one and half millimeters wide.
So to get a better look, we’ll place this under the microscope.
First we want to see our sample, so we’ll direct visible light with
the mirrors through the objective to the sample.
The visible light travels back through the objective to either an eyepiece or a camera.
Most IR microscopes just have a camera,
because cameras let us view the sample on a computer screen and take pictures.
But the traditional microscope eyepiece is still has its place in certain applications.
Now that we can see our sample under the microscope, it’s time to do the FT-IR analysis.
The lighter region on the sample corresponds to
some contamination so we’ll select a point in this region to analyze.
Our sample needs to interact with the IR light for the FT-IR analysis, so the IR light will now
travel through the microscope to the sample in a similar way that the visible light did.
The exact way the IR light travels through the microscope here depends on
the measurement technique we use, that is transmission, reflection, or ATR.
For this sample, we’ll use ATR because it offers very high spectral quality.
Now, the IR light interacts with the sample and then we’ll collect the light and guide to
towards the detector, which will register the IR light so we can obtain our FT-IR spectrum.
But there’s one issue: we only want to analyze and
get the FT-IR spectrum of this specific point on the sample.
The objective did focus the IR light on this point,
but the IR beam is still larger than the region we wanted to analyze.
So if we detected this IR light as it is right now, we wouldn’t end up with a
clear picture of what’s happening just in our region of interest.
So what do we do?
We use an aperture. Apertures block some of this IR light from reaching the detector.
That way, we only detect the IR light from the point we want to analyze.
There’s a couple different aperture styles we can use in our microscope.
The simplest kind of aperture is called a pinhole aperture,
which is just a small hole that’s close in size to the region we’re analyzing.
There’s usually a wheel of different size apertures,
so you can select an size that’s pretty close to what you need.
The more advanced option is a knife-edge aperture.
Knife-edge apertures are made of four blades.
These blades can move to make a rectangular shape of any size, in any orientation.
That way, this style of aperture can exactly
match the size of the region on the sample we want to analyze.
So we send the IR light through an aperture to block the extra light from reaching the detector,
and now the IR light is detected and we’ll get our FT-IR spectrum.
To figure out what the contamination on the wire is, we’ll use the reference library.
The reference library contains thousands of spectra of different molecules and compounds,
and we’ll search through it until we find a match.
To make things easier, computer software can
search through the reference library for us to automatically identify our sample.
A reference library search on this spectrum
identified the contamination as a mixture of PVC and Fluorocarbon.
Now that we know that, we can figure out what caused the defect in the piece of wire.
But there’s one more part of this microscope we haven’t really discussed, the detector.
The detector collects IR light so we can get the FT-IR spectrum.
To get high-quality results, the detector has to capture enough IR light.
But often with FT-IR microscopy we’re analyzing very small areas with a very small aperture,
so a lot of IR light is blocked from reaching the detector.
That means if we want to analyze really small areas,
we’ll need a really sensitive detector to get an accurate analysis.
Luckily there’s several detector options available
for FT-IR microscopes so we can get the sensitivity we need.
Going from least sensitive to most sensitive,
we have DLaTGS detectors, TE-MCT detectors, and LN-MCT detectors.
With the most sensitive detector, the LN-MCT detector, we can analyze
samples as small as 5 micrometers, which is about the size of a human red blood cell.
However, this super sensitivity comes with tradeoff: these are high maintenance detectors
that must be continuously filled with liquid nitrogen for cooling.
TE-MCT detectors have slightly lower sensitivity.
They can measure anything larger than about 10 micrometers,
which is about the size of flour particle after milling.
However, they are continuously cooled with thermoelectric cooling,
so they’re more convenient to use.
DLaTGS detectors require no cooling at all, but they can only measure
samples larger than 50 micrometers which is about the size of the average human hair.
That’s still small, but that’s in the range of things we can already see with the naked eye.
So DLaTGS detectors are limited to applications where we’re analyzing areas
that are relatively large, in the range of what we can see without a microscope.
Now all these detectors are called single-element detectors.
Single Element Detectors make one FT-IR measurement on one point of the sample
at a time so you end up with one FT-IR spectrum.
There’s a whole other class of detectors, called imaging detectors that can measure
over a thousand points on a sample at once so you can get much more IR data.
Another difference between single element detectors and imaging detectors is
that imaging detectors don’t require an aperture, so all the IR light is
captured during a measurement, leading to faster and higher quality results.
We need the capabilities of an imaging detector to create IR images, which are
colored images that help us visualize the chemical composition of a sample.
IR images are made of thousands or even millions of FT-IR spectra.
But IR imaging and imaging detectors are a topic deserving of their own video,
so we’ll leave this discussion here.
Now we know how an FT-IR microscope works, let’s quickly touch on a few applications.
FT-IR microscopy is a useful when want to analyze small structures or samples,
or specific areas on a sample.
That makes it convenient for failure analysis and
quality control to quickly identify contaminates or defects in products.
In forensics it can analyze paint layers or small fibers that may be evidence.
And in environmental research, it can analyze soil or small microplastic particles.
An FT-IR microscopy can analyze lots of different materials, so there’s many,
many more applications of this technique.
But for now, this should give you a good idea of what FT-IR microscopy can do.
Thanks for watching this video, we’ll see you in next one.