ATR Measurement of an Irregular Solid Sample

It is possible to use infrared spectroscopy as an analytical measurement technique for determining the molecular constituents or chemical groups of a highly range of solid, liquid, and gaseous sample types.

Certain polymer and plastic type sample in a bead, raw or a pellet form can be termed as hard, irregular shaped samples and these cannot be ground with KBr powder to form a uniform sample mix suspended in the KBr matrix for infrared spectral analysis using a transmission technique.

However these can be analyzed as they are with the ATR spectroscopic technique for collecting a representative IR spectrum. This obviates a timely sample preparation method for a thin film production for a transmission spectral measurement in case the reflection technique for quantitative and qualitative analysis is enough.

Application

The application is the ATR measurement of an irregular shaped, hard solid sample so as to obtain a wide spectral transmission range of light frequency collection for the sample as possible.

Equipment and Method

The Quest ATR Accessory P/N GS10801-B was used along with the single reflection diamond extended range ATR crystal puck option on the black surface colored optical unit to obtain a comprehensive spectral range for the sample between 4000cm^-1 and 400cm^-1 for conducting ATR measurement of a plastic or polymer bead material.

The polymer and plastic bead samples studied include the following:

• PET (Polyethylene Terephthalate) 100% as small, hard, short rod-type fragments.
• Blue Coloured Plastic Pen Cap (Polypropylene) as small, hard, shards of material.
• Grey Plastic Construction Brick (Acrylonitrile Butadiene Styrene) as small, hardish fragments.
• PVA (Polyvinylacetate) as sottish beads.
• PolyB (Polyethylene) as hard beads.

Sample Preparation was done as follows:

• A specific example of the sample is taken and placed over the diamond ATR crystal of the Quest ATR puck assembly option
• Another example of the same sample was repeated for two further spectral collections in order to ensure result consistency
• A set load from the anvil arm assembly, either the flat or pellet anvil option was used for covering the sample when it contacted the ATR crystal for correct positioning of the sample and applying a constant and reproducible force to obtain an acceptable IR ATR spectrum.
• Then the sample was ready for spectral data collection

Spectral Data

Figures 1 to 6 show the infra red spectra collected for the samples studied

Figure 1. PET (Polyethylene Terephthalate) 100% hard, “rod” with Flat Anvil on diamond ATR puck of Quest ATR Accessory

Figure 2. PET (Polyethylene Terephthalate) 100% hard, “rod” with Pellet Anvil on diamond ATR puck of Quest ATR Accessory

Discussion

The key objective of this is to try and determine whether the flat or pellet steel option is better for a specific plastic or polymer type.

As seen in Figures 1 and 2 in the case of PET sample there is a very small difference in whether the flat or pellet anvil is used to bring the sample contact with diamond for a reliable ATR spectrum to be collected.

For the blue plastic pen cap sample the ATR spectra indicates this material to be a polypropylene form. Similar to PET samples pen cap fragments that were cut for sampling were harder than some other.

The spectrum presented as Figure 3 is an example of the spectrum collected for this sample type using the flat anvil for the load force against the diamond crystal.

Figure 3. Blue Pen Cap (Polypropylene) hard, “shard” with Flat Anvil on diamond ATR puck of Quest ATR Accessory

Figure 4 shows the grey plastic construction brick sample with consistent spectra for the examples of sample run using the pellet anvil to provide the contact force. The spectra are consistent in sensitivity and the polymer is identified as ABS.

Figure 4. Plastic Construction Brick (Acrylonitrile Butadiene Styrene) small, hardish fragment with Pellet Anvil on diamond ATR puck of Quest ATR Accessory

For the PVA (polyvinylacetate) sample shown as Figure 5, the polymer beads were quite soft and so all three sample examples gave a reliable PVA IR spectra using the pellet anvil, but a user might be able to decide that for such softish polymer bead types the flat anvil would also work just as well for consistent pressing.

The PolyB (polyethylene) sample shown as Figure 6. was in a solid bead form and similar to the PVA sample gave consistent spectra for the three sample examples chosen when the pellet anvil was used.

Figure 5. PVA (Polyvinylacetate) sottish bead with Pellet Anvil on diamond ATR puck of Quest ATR Accessory

Figure 6. PolyB (Polyethylene) hard bead with Pellet Anvil on diamond ATR puck of Quest ATR Accessory

Conclusion

While using the Quest ATR with a diamond ATR puck, a fairly broad spread of plastic and polymer types in various shapes, forms and hardness have produced good reproducible spectra. It is just a matter of discerning the sample type for its shape, size and hardness to know if the flat or pellet anvil may be better employed to allow for the force required to bring the sample type into close contact with ATR crystal, but this could be learnt with experience of use.

This information has been sourced, reviewed and adapted from materials provided by Specac.

For more information on this source, please visit Specac.