Select the chemical that is used in most HPLC procedures to decrease solvent polarity.

In high-performance liquid chromatography (HPLC), the use of solvents with appropriate polarity is crucial for the separation of compounds. Acetonitrile is commonly used because it has a relatively low polarity compared to other solvents like water or methanol, making it effective in decreasing the overall solvent polarity in a mobile phase. This allows for better interactions between the analytes and the stationary phase, thus improving the separation efficiency for a variety of compounds.

Acetonitrile's low viscosity and high elution strength further enhance its suitability for HPLC applications. It also has a low UV absorption, which is advantageous for detection when using UV-based detectors. Its properties make it a versatile solvent, commonly employed in both reverse phase and normal phase HPLC methods.

In contrast, other solvents like hexane and nonane are typically too nonpolar for most HPLC applications, especially those aimed at separating polar compounds. Chloroform, while also a nonpolar solvent, does not offer the same advantages in terms of compatibility with UV detection or solvent viscosity as acetonitrile does. Therefore, acetonitrile stands out as the solvent of choice for decreasing polarity in many HPLC procedures.