Standard Operating Procedure: Preparation of High-Quality KBr Pellets for FTIR Analysis

Objective

To define a standardized process for creating high-transmittance, impurity-free KBr sample discs, ensuring clear characteristic peaks and minimizing baseline interference in FTIR spectroscopy.

Material & Environmental Specifications

• KBr Purity: Use only FTIR-grade (Spectral Grade) KBr (purity $\ge$ 99.99%). Avoid analytical or chemical grades as they contain trace carbonates and nitrates that cause ghost peaks.
• Drying: KBr powder must be baked in an oven at 120°C – 150°C for at least 2 hours. Store in a desiccator or a 60°C warming cabinet.
• Humidity Control: The laboratory humidity must be < 50% RH (ideally < 40%). Perform all grinding and assembly under an infrared heating lamp to prevent rapid moisture absorption.

Equipment Preparation

• Mortar & Pestle: Use an Agate set only. Glass or porcelain tools can introduce silica impurities (causing absorption peaks at 1000 $cm^{-1}$).
• Cleaning: Wipe the mortar, pestle, and die sets with anhydrous ethanol three times. Air-dry or use a heat gun; ensure no solvent residue remains.

Step-by-Step Procedure

Phase A: Mixing and Grinding

1. Sample Ratio:
   • Strong Absorbers (Organic): Sample : KBr = 1:200 to 1:500.
   • Weak Absorbers (Inorganic): Sample : KBr = 1:50 to 1:100.
2. Grinding Technique: * Place the sample in the mortar first and grind to a fine powder.
   • Add KBr in 2-3 increments. Use a gentle circular motion until the mixture is homogeneous and the texture resembles fine flour (particle size $\le$ 2 $\mu$m).
   • Note: Over-grinding can increase the surface area and lead to excessive moisture absorption.

Phase B: Die Assembly and Pressing

1. Loading: Place the die base on a level surface. Transfer the mixture into the die sleeve using a small spatula. Gently tap or rotate the plunger to ensure the powder is distributed evenly.
2. Vacuuming (Critical): Connect the vacuum pump to the die. Draw a vacuum for 1–2 minutes. This removes trapped air, preventing the formation of micro-bubbles and cracks under pressure.
3. Applying Pressure: Place the die in the hydraulic press. Slowly increase pressure to 8–10 tons (for a standard 13mm die).
4. Pressure Maintenance: Hold the pressure for 60–90 seconds to allow the KBr crystals to fuse into a unified, transparent lattice.
5. Controlled Release: Slowly release the pressure valve. Rapid decompression causes internal stress, leading to “spider-web” fractures or “cloudy” discs.

Quality Inspection (QC)

A qualified KBr pellet should be semi-transparent to transparent (like thin ice), with a smooth surface and no visible graininess.

Observation	Probable Cause	Corrective Action
Opaque/White Pellet	Large particles or low pressure	Increase grinding time; check press gauge
Cloudy Patches	Trapped air or moisture	Ensure vacuum is working; dry KBr longer
Cracks/Fractures	Rapid decompression or uneven loading	Release pressure slower; level the powder
Large Water Peak (3400 $cm^{-1}$)	High humidity or wet sample	Operate under IR lamp; dry sample to constant weight

Post-Analysis Maintenance

• Blank Control: Always run a “Blank KBr Pellet” (no sample) as your background to subtract atmospheric $CO_2$ and residual $H_2O$ interference.
• Die Storage: KBr is highly corrosive to steel. Immediately after use, clean all die components with ethanol, dry them, and store them in a desiccator.

Summary of Key Improvements

• Standardized Terminology: Switched to industry-standard terms (e.g., “Spectral Grade,” “Hydraulic Press”).
• The Vacuum Step: Explicitly added the vacuuming phase, which is the most common reason for failed (cloudy) pellets in original workflows.
• Troubleshooting Table: Provided a quick-reference guide for identifying and fixing common prep errors.
• Pressure Parameters: Clarified the tonnages and hold times for consistent results.