When collecting Infra Red spectral data of liquid or solid samples it is preferable to obtain a spectral range extending from 4000cm-1 to 400cm-1. Conventionally using liquid transmission cells with KBr windows allows for this transmission range of light to be collected, but there may be a compromise of the liquid cells structural integrity.
The attenuated total reflectance technique is an alternative method for spectral collection. Liquid or solid samples are bought into direct contact with an ATR crystal material and internally reflected and measurable light from the sample crystal interface results in a spectrum being collected from a specific penetration depth into the sample material itself.
A monocrystalline type Ilia diamond ATR crystal element can be used to achieve a similar spectral range of collection for the transmission range of 4000cm-1 to 400cm-1 using the ATR technique.
The Specac QuestATR accessory offers the choice of a diamond, ZnSe or germanium ATR crystal puck to be used on a dedicated optical unit with mirrored reflectance optical components only.
Pharmacopeia regulations in Japan require pharmaceutical samples to have infrared spectral data collected for them between the frequency range of 4000cm-1 to 400cm-1 (2.5µm to 25µm wavelength).
Two solid pharmaceutical samples were used as examples to measure. A white crystalline powder Theophylline (C7H8N402) is one sample. The second was an ibuprofen tablet (C13H1802) which was orange in colour. Both samples were taken and prepared as traditional KBr pellets for transmission purposes to measure over the range of 4000cm-1 to 400cm-1 and also studied for collection of an ATR spectrum over the same spectral range.
Equipment and Method
A 7mm diameter KBr pellet was formed for transmission measurement. In order to make the KBr pellet, the Specac Basic Solid Pack P/N GS01150, containing a 2 ton maximum load capability Mini-Pellet Press using a 7mm diameter pellet die assembly was used.
For ATR measurement, the Quest™ ATR Accessory P/N GS10801-B was used with the single reflection diamond extended range ATR crystal puck option on the optical unit. The spectra were collected using a Thermo Nicolet iS5 instrument set at a resolution of 4cm-1 for 16 scans at a gain of 1.
A small amount of theophyline and ibuprofen samples were added to an excess of KBr powder to make the KBr 7mm pellet. A proportion of the ground powder sample was transferred to the 7mm pellet die assembly and a solid KBr disc was pressed from application of a 1.75 ton load to the die assembly from the mini pellet press. The KBr sample pellet was then ready to be mounted in the spectrometer for spectral data collection.
Figure 1. IR Spectrum of 7mm Diameter KBr Pellet of Theophylline
Figure 2. IR Spectrum of Theophylline on Quest ATR Diamond Accessory
For preparing the sample for the ATR spectral measurement a small amount of the Theophylline and Ibuprofen powder samples were taken “as is” and placed over the diamond crystal of the Quest™ ATR puck. A set load from the anvil arm assembly of the Quest™ optical unit using the stainless steel flat anvil (P/N GS10820) option was applied to the powder sample to bring it into close and reproducible contact with the diamond crystal. The sample was then ready for spectral data collection.
Figures 1, 2 and 5 show the infrared transmission spectra collected for the Theophylline sample prepared as a 7mm KBr pellets and run on the Quest ATR Accessory.
Figures 3, 4 and 6 show the infrared transmission spectra collected for the Ibuprofen sample prepared as a 7mm KBr pellets and run on the Quest ATR Accessory.
When compared to the collected spectra there will be a difference in the absorption intensities for the sample over the spectral frequency (or wavelength) range when measured as a transmission or as an ATR technique.
Figure 3. IR Spectrum of 7mm Diameter KBr Pellet of Ibuprofen Powder
Figure 4. IR Spectrum of Ibuprofen Powder on Quest ATR Diamond Accessory
When the sample is analyzed with the ATR technique it is possible that certain weak spectral features that have typical group frequency vibrational modes in the higher frequency region of an IR spectrum may not be easily observed. If a transmission method of spectral collection could be achieved for the same sample then there may be a brighter chance of observation of these spectral features to confirm one way or another the existence of the species from an IR spectrum.
However considering overall sample handling, preparation and consistent measurement of the two example pharmaceuticals, the ATR spectra that have been collected are indicative of a qualitative and, if the sample is homogenous in nature, quantitative measurement technique over the spectral range required.
A key advantage is that the sample has been taken “as is” for quick and easy presentation for measurement by this technique, whereas to produce a KBr pellet for a transmission measurement certain steps need to be followed such as the sample/KBr mix proportion to consider, a method of grinding to use and a process of pressing into a solid disc, which all contribute to the overall qualitative and quantitative result of an IR spectrum obtained.
It is observed that for both these pharmaceutical samples, their KBr pellet transmission IR spectra, and ATR IR spectra compare very well. To emphasize the spectral features collected for both the Theophylline and Ibuprofen samples using a diamond ATR crystal on the Quest ATR accessory, these have been shown as Figures 5 and 6 respectively over the range from 650cm-1 to 400cm-1. These spectral details could not be achieved with use of a ZnSe ATR crystal for any measurement.
Figure 5. IR Spectrum of Theophylline Sample on the Quest ATR Diamond Accessory
Figure 6. IR Spectrum of Ibuprofen Powder on Quest ATR Diamond Accessory
In this instance, the solid pharmaceutical samples chosen have allowed for the formation of a KBr pellet to be analyzed accordingly, but not all solid samples can be ground to form a KBr pellet.
The ATR technique via use of the Quest ATR Accessory with a diamond crystal top plate does though allow for the measurement of a very wide range of pharmaceutical sample types, be they solid, seml-solld or liquid In nature and the spectral data can be collected for the sample over the frequency region between 4000cm-1 and 400cm-1 if this is a mandatory requirement..
This information has been sourced, reviewed and adapted from materials provided by Specac.
For more information on this source, please visit Specac.