IR Microscopy using ATR Objectives

Why should I use an ATR objective for IR Microscopy?

For more than two decades the use of measurement accessories that use attenuated total reflectance (ATR) has significantly reduced the amount of sample preparation necessary for routine FTIR measurement. Before ATR was widely available samples could only be measured using transmission or reflection, which involves time consuming dilution, pellet making or diffuse reflectance measurement. Using ATR and with good crystal contact, virtually any sample (solid, powder liquid or film etc.) can be measured without pretreatment.

Schematic of a microscope ATR Objective
Schematic of a microscope ATR Objective

IR microscopy has similar problems with sample preparation as routine macro measurement, but these can also be remedied in the same way by using an ATR objective in place of a transmission or reflection  cassegrain.  However in microscopy the additional element of sample observation creates a second and important consideration – how to identify the correct measurement location? In transmission or reflection the upper cassegrain is typically used to observe the sample prior to measurement, until recently it has been impossible to ‘look though’ an ATR to observe the sample, this means a the upper cassegrain or a refractive objective has to be used for observation and then exchanged with the ATR for measurement; with spatial resolution to single microns, synchronization between the observed and measured images can be challenging.

Direct observation through a ClearView ATR

ClearView ATR objectives

The unique ClearView ATR developed by JASCO has overcome this limitation and it is now possible to observe the sample directly though the ATR, which is then moved into direct contact for measurement at exactly the same position. This method guarantees the observed position is exactly matched with the measured position that generates the chemical map. The visual image can also be precisely overlaid with the chemical image to create a composite.

Chemical image overlaid with the visual image using an ATR Objective
Chemical image overlaid with the visual image using an ATR Objective

Several different crystal materials used for the prism in the ClearView ATR allowing analysis of different samples different spatial resolutions, penetration depth and wavenumber range.

MaterialRefractive IndexMagnificationWavenumber RangeArea
Diamond2.4x35.27,000 to 700 cm-1180μm x 180μm
Zinc Sulfide (ZnS)2.2x35.27,000 to 700 cm-1180μm x 180μm
Germanium (Ge) Standard G454x645,000 to 650 cm-170μm x 70μm
Germanium (Ge) MG4x165,000 to 650 cm-1400μm x 400μm
Germanium (Ge) WG4x45,000 to 650 cm-11,600μm x 1,600μm

IQ Mapping

Another disadvantages of ATR measurement is the requirement for intimate sample contact; relatively strong contact is required for the evanescent field propagated from the crystal to interact with the molecules at the sample surface.  If the sample is easily deformed or likely to move, mapping can be difficult. In transmission ore reflection measurement the sample is not in direct contact with the objective, so that the stage can easily be moved to map an area, but with ATR, the objective has to be moved away from the sample to change measurement positions, this can cause a change in the shape of the material.

JASCO developed IQ Mapping to move the measurement aperture around the field of view of the ATR objective.

Instead of moving the stage IQ Mapping can be used with an ATR objective the measurement position is changed by moving the aperture around the crystal while still in contact with the sample (as shown with the red grid and yellow square in the above figure).

Typically this gives a typical measurement area of 180μm x 180μm, with the WG version of the ATR, the measurement area can be up to 1600μm x 1600μm, and with the knife-edge aperture being adjustable to define the size and shape of the measurement point.

Cross section analysis of paper pack using an ATR