Techniques Available

Transmission IRM is recommended where possible and is predominantly used for solid samples. If you’re interested in liquid or gaseous samples, please contact the IRM team. Transmission requires that samples be very thin (between 5-10 µm), flat and highly polished where applicable. Solid samples should be microtomed to the correct thickness ranged for analysis, either freestanding or deposited onto an IR transmitting window such as CaF₂, BaF₂, ZnSe (preferably 0.5 mm in thickness) depending on the window properties and your required wavenumber coverage. Less frequently, Si or silicon nitride (Si₃N₄) may be used as substrates for transmission IR.

A micro-compression cell with diamond or CaF₂ windows, to sandwich your sample in place under the beam, is also available for use. 

Examples of samples for transmission:

• Biological tissues – microtomed and mounted onto an IR transmitting window

• Cultured cells – grown on a substrate such as CaF₂ or Si₃N₄ windows, and freeze-dried or fixed and air dried

• Polymer multilayers – microtomed and placed in a compression cell between two windows of diamond or CaF₂

ATR relies on a form of internal reflection to enable the analysis of more difficult samples with IR spectroscopy, e.g. samples that cannot be microtomed for transmission measurements or that do not adequately reflect IR for reflection. Compared to transmission and reflectance FTIR, ATR offers the advantage of enhanced spatial resolution (where a crystal with a high refractive index is used such as Ge).  Furthermore, the total internal reflectance phenomenon that ATR relies on (occurs when IR radiation travels through a high refractive index crystal a onto a low refractive index sample surface), results in an evanescent wave that penetrates the surface of the sample, rather than the bulk

The IRM beamline has a Bruker single-bounce micro-ATR accessory with a Germanium crystal tip (diameter=100 microns, refractive index=4). Traditional microscopic ATR methods often cause sample damage, contamination and re-positioning issues, where the ATR crystal is attached to the objective. However, with the macro-ATR methods developed in-house and described below, these issues are avoided by single contact ATR mapping.

Two models of the macro-ATR are available for use:

Open

Open

Reflection studies measure the specular reflectance or transflectance from the surface of a sample. For samples that can reflect IR directly (specular reflectance), it is vital that they be optically polished (to a roughness of 1 µm or less) to reduce scattering of the reflected beam and other interference features.

Transflectance studies are more commonly conducted at the beamline and describe the reflection observed when a sample is deposited as a relatively thick layer (1 - 20 µm) on a non-IR absorbing, reflecting surface e.g. a highly polished metal slide (Au or Al). Experiments on layers thinner than this typically required use of our Grazing Angle Objective (GAO) (see below in Grazing Incidence). Transflectance effectively produces a double-pass transmission spectrum as the beam passes through the sample twice upon reflection, however scattering effects from the surface can impact the data and transmission measurements are preferred where possible.

Examples of samples for reflection:

• Minerals - finely polished with alumina or diamond to a surface roughness of 1 µm or less

• Biological tissues or cultured cells mounted or grown on reflective surfaces, such as Mirr-IR microscope slides (Kevley Technologies, Ohio) or gold coated glass

The IRM beamline has a Bruker Vis/IR Grazing Angle Objective (GAO, 15×) for grazing incidence analyses. This objective is specifically designed to analyse thin layers or films <1 µm thick that are deposited onto non-IR absorbing, reflecting surfaces e.g. highly polished metal slides (Au or Al) or indium tin oxide (ITO) coated glass, for example. GAOs increase the optical path length of a sample by essentially skimming the beam across the surface using high angles of incidence. This objective uses an incident angle of ~84° and shows excellent sensitivity down to monolayer thicknesses, particularly when used with polarised light, and good spatial resolution. The Bruker design relies on transflectance from the sample surface, effectively increasing the sample path length to again assist sensitivity, and uses a polariser in the IR beam; p-polarised light is polarised perpendicular to the surface of the sample and shows enhanced IR absorption when large angles of incidence are used.

Examples of samples grazing angle experiments:

• Thin (sub micron) coating on metallic or other highly IR-reflective surfaces

• Corrosion products on metallic surfaces

Time resolved spectroscopy (TRS) using a FTIR spectrometer offers the advantage of being able to monitor rapidly changing chemical or physical properties of a system in high temporal resolution (up to 65 spectra/sec using a 160 kHz scan velocity @ 16 cm^-1). It allows for the simultaneous observation of the progression and/or decay of various chemical species during an event.

The rapid scan experiments can be initiated by an external trigger, such as a change in voltage, which can be programmed into the rapid-scan method editor in OPUS, making it ideal for the observation of kinetic chemical changed induced by an electrochemical potential event, to provide an example, More information on rapid scan FTIR spectroscopy can be found on the Bruker website.