"FTIR Analysis"

In infrared (IR) spectroscopy, a sample is irradiated with infrared light and the IR radiation absorbance is plotted as a function of wavelength (frequency). 

FTIR is an acronym for Fourier Transform Infrared spectroscopy.  IR usually refers to the mid range ( 2.5 um to 200 um) infrared absorption spectrum of a material.  The “Fourier Transform” refers to the “Fast Fourier Transform” (FFT) of an interferogram produced by a Michelson interferometer and transmitted through a sample material.  The sample interferogram is transformed to its component frequencies, i.e. its IR spectrum.

Chemical compounds (particularly organic materials) absorb IR radiation by vibration of atoms in the molecule.  Specific atomic groupings (functional groups) in a molecule absorb IR radiation at distinctive wavelengths. Detection of IR absorptions at these specific wavelengths indicates the presence of particular functional groups in the irradiated compound.

Different samples of a pure organic compound will give identical IR spectra.  Hence, the IR spectrum of a material can be used as a "fingerprint" for chemical identification. 

The simplest way to use IR spectroscopy for identification is to build spectral databases of known compounds and to compare any unknown to the databases.  The method is analogous to analyzing an unknown fingerprint by comparing it to fingerprints of known subjects in various governmental databases.

Because of the large number of known (and unknown) organic compounds in the world (billions), it is quite possible that the IR spectrum of a particular unknown material does not exactly match anything in your database.  However, the IR spectrum of the unknown can tell what type of organic compound it is.  For example:  the CH stretch frequencies (around 3100 to 2800 cm-1) can tell one whether the compound is aromatic or aliphatic (or both).  Aromatic CH stretches are 3000 cm-1 or above.  Aliphatic CH stretches are around 2900 – 2800 cm-1.  If both types of absorptions are seen in the spectrum, the unknown sample has both aromatic and aliphatic parts.

This general spectral analysis method can distinguish between hydrocarbons (such as petroleum products), esters (such as fatty acid esters), amides (such as nylons and proteins) and so forth.

All the IR spectral analysis described so far, assumes that the unknown samples are pure.  This may or may not be case for any random unknown. 

Mixtures of compounds give spectra that are combinations of the individual component spectra.  These mixtures, alloys and blends may be intentional formulations.  For example:  the spectrum of an ABS (acrylonitrile-butadiene-styrene) sample is a combination of the component spectra in specific ratios.

IR spectroscopy follows Beer’s Law, which means that absorption is directly proportional to concentration.  With calibration standards, one can use IR to quantitatively analyze organic mixtures.  Also in the case of something like ABS, it can be easily determined if a particular batch was properly formulated (i.e. in the correct component proportions).

"What is this stuff?"

There is a general procedure to answer “what is this material?”,.  It is very much like playing “20 questions”.  “20 questions” is a word game where one tries to identify an object by asking the minimum number of questions and winning if the number of question is less than twenty.  The questions have to be yes or no answerable.

“ Is it bigger than a bread box?” is usually the first question.  The purpose is to establish physical scale.  In the WTS game, this corresponds to visual and microscopic visual inspection.  This step would include things like photo documentation and dimensional measurements. 

Simply looking at material samples can give one a large amount of information.  Most people can visually determine if is a material is metallic, wooden, plastic (polymeric), mineral or biological in origin. 

The next questions in the “20 questions” game are usually, “Is it vegetable?, Is it mineral? or “Is it animal?.  A more scientific question is “What is the elemental composition of the material, is it organic (carbon based chemistry) or is it inorganic (every other element chemistry in the periodic table).

There are several ways to get an elemental analysis of a sample.   One of the most convenient methods is called Energy Dispersive X ray (EDX) spectroscopy.  X Ray Flouresence (XRF) is another convenient method.  Most commercial Scanning Electron Microscopes (SEM) have EDX detectors on them, so one can examine an unknown sample microscopically and get elemental analysis for the same machine time.

Most SEM/EDX instruments can detect all the elements on the periodic table heavier than beryllium (Be) in a solid or nonvolatile fluid sample.

 A material sample without any carbon allows one to guess the identity of the material from the elements present.  For example:  an unknown sample that has only silicon and oxygen is probably silica (silicon dioxide) either as amorphous glass or crystalline quartz.  Either form can be determined with X Ray Diffraction (XRD).

A sample primarily composed of carbon is probably organic and has to be examined with Infrared spectroscopy (using a Fourier Transform Infrared (FTIR) spectrometer).

In infrared (IR) spectroscopy, a sample is irradiated with infrared light and the IR radiation absorbance is plotted as a function of wavelength.  An organic compound absorbs IR radiation by vibration of atoms in the molecule.  Atomic groupings (functional groups) in a molecule absorb IR radiation at distinctive wavelengths.   Pure organic molecules will give identical IR spectra.  Hence, the IR spectrum of a material can be used as a "fingerprint" for identification.  Mixtures of compounds give spectra that are combinations of the individual component spectra.

In 20 questions, once the general early questions are answered, details can be determined with the remaining follow up questions.  For WTS, one can do a similar inquiry.  For example:  A solid sample is examined.  It looks like a plastic (polymer).  EDX results show only carbon.  FTIR results can distinquish hydrocarbon polymers like polyethylene, polypropylene or polystyrene.  If the sample were polyethylene, other techniques such as Differential Scanning Calorimentry (DSC) can tell from melting points whether the polyethylene is Low Density Polyethylene (LDPE), Linear Low Density Polyethylene (LLDPE), High Density Polyethylene (HDPE) or Ultra  High Density Polyethylene (UHDPE).

The seemingly simple question of  “what is this material?”, carries a multitude of unspoken implications with it.  For the material analyst, all the unspoken implications and expectations of the questioner have to be understood before a satisfactory (and billable) answer can be given.

The seemingly “snarky” counter-question is “why do you want to know?”.  Basically, the question is what is the context and background of the problem.   If the questioner is conducting a failure analysis of their manufacturing process and they have isolated a particular contaminant whose identity and source will explain the problem, then the analysis is expected as a “commodity analysis”.   Most people conduct investigations at this level and expect a “testing service” to provide them with “relevant data”, not failure analysis (with concurrent higher costs).    

Materials identification could be part of a contamination issue where contamination identification will lead to contamination source identification and removal.   Unfortunately, materials analysis is not simply operating a Star Trek tricorder and  having a computer voice saying something like “the material is a petroleum grease from a leaking crankcase”. 

Background and context will be used to determine the overall process and specific techniques used to determine the identity of the sample material.  For example;  a client brings a dark blob of “oily” material in a piece of aluminum foil and asks; “what is this material?”.    The analyst then asks; “why do you want to know?”  The answer/question of  “is the sample material a petroleum grease from a leaking crankcase?” will allow the analyst to determine the overall investigation process and specific method.   Ie. take  Fourier Transform Infrared (FTIR) spectra of the sample and an exemplar sample from the leaking crankcase and determine if they are both the same aliphatic hydrocarbon lubricant.  If , for example, the EDX analysis of both of the samples were identical, with a large carbon peak and trace amounts of iron, and identical FTIR spectra, then the answer to the question “is the sample material a petroleum grease from a leaking crankcase?” is a highly probable, yes.  However, if the EDX analysis of one of the samples showed a large carbon peak and trace amounts of iron and the other sample showed only a single carbon peak, and identical FTIR spectra, then the answer to the question “is the sample material a petroleum grease from a leaking crankcase?” is a maybe.