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Figure 1. The SplitPea™.
This horizontal reflection accessory(HATR) is ideal for analyzing optically thick, hard materials. Samples, which are difficult to analyze by conventional spectroscopic methods, such as fibers, paints, and microliters of liquids or pastes, are easily examined with the SplitPea™.
The SplitPea™ can be configured for ATR or specular reflectance, simply by changing the sample holder. In its ATR mode, the SplitPea™ features a removable crystal/sample holder for easy sample insertion and crystal cleaning. For the specular reflection and pre-alignment, the SplitPea™ features a removable sample holder and reference mirror. The SplitPea™ is enclosed in a purgable box for rapid sample exchange without interrupting the purge of the FT-IR system.
An illustration of the SplitPea™ is shown in Figure 1. Two mirrors, M1 and M2, direct the infrared beam to an ellipsoidal mirror, M3, which focuses the light onto the sample. The radiation reflected from the sample is collected by a second ellipsoid, M4. Mirrors M5 and M6 direct radiation reflected from M4 to the detector of the FT-IR spectrometer. This configuration provides a six times linear reduction of the source image on the sampling surface.
For ATR, the SplitPea™ comes with two silicon ATR crystals. Silicon is an excellent material for the ATR crystal of the SplitPea™ since it is extremely inert, allowing the analysis of even highly corrosive materials. In addition, it is very hard. This permits the application of high clamping pressures to ensure good contact between the ATR crystal and the sample. Generally, lower pressures are required to achieve the good contact needed in ATR than are needed to flatten the sample for transmission studies. Thus fewer chemical and physical changes will be induced in the sample with the SplitPea™ than with a high pressure diamond anvil cell.
Figure 2. Focusing Effects of a Hemisphere.
The Si ATR crystal is a 3-mm diameter hemisphere. This further focuses the incident radiation onto the sample (see Figure 2), providing an additional 3.4 times linear reduction of the source image. For such a short pathlength, Si is virtually transparent in the far and mid-infrared. The Si ATR crystal is beveled on the edge of its flat surface to provide a sampling area slightly larger than the 150-200 µm diameter hot spot on the crystal. This makes it easier to position small samples and maximize the clamping pressure. Because of the small size of this island, the pressure plate applies localized pressure to the sample improving contact between the sample and the ATR crystal and allowing high contact pressures to be achieved.
For applications versatility, ZnSe, Ge, and diamond mounted ATR crystals are also available. This selection offers a variety of sampling depths of volumes.
The SplitPea™ is available with a ViewThruPress™. This configuration is recommended for analyzing samples such as fibers, spots on transparent substrates, and powders. The ViewThruPress™ provides a 50X magnification of the sampling area and features precision adjustments for aligning the viewer over the active sampling area of the ATR crystal. Its unique design permits the sample to be viewed as it is compressed against the ATR crystal. The ViewThruPress™ also features an independent adjustment for focusing on the image.
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For liquid sampling, a Flow-Through Liquid Cell is available for use with the SplitPea™. This cell is o-ring sealed and features two luer fittings.
For analysis of powder with minimal spillage, a Powder Retainer is included with this ATR accessory. Both the flow-through liquid cell and powder retainer conveniently clamp onto the ATR holder.
In addition, Heatable Sampling Plates are available for sampling at temperatures up to 200°C with the diamond or Si ATR crystals and to 100°C with a Ge ATR crystal. These sampling plates feature a gasket-sealed ATR crystal, K-type thermocouple, and 24V heater. These are designed for use with the Harrick Temperature Controller.
Representative FT-IR spectra recorded with the SplitPea™ are shown in Figures 3 through 6.
Figure 3. Internal Reflectance of a Human Hair.
Figure 4. Internal Reflectance of a 20mm Poly(ethylene terephthalate) Fiber. ZnSe IRE.
Figure 5. Internal Reflectance of a Paint Chip.
Figure 6. External Reflectance of a Lottery Ticket.
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