Which compound is commonly used as an unretained marker to determine the dead time tM in LC on a C18 column?

Which compound is commonly used as an unretained marker to determine the dead time tM in LC on a C18 column?

• A. Uracil
• B. Toluene
• C. Benzene
• D. Acetone

Answer: (A)

In reversed‑phase liquid chromatography on a C18 column, the dead time tM is determined using a compound that does not interact with the hydrophobic stationary phase—an unretained marker that travels with the mobile phase. Uracil is highly polar and does not partition into the C18 phase, so it elutes with essentially no retention and defines the mobile-phase travel time through the system. That makes its observed retention time effectively equal to tM, which is exactly what you need to calculate retention factors for other analytes. The other candidates tend to interact with the C18 surface to some degree. Benzene and toluene are hydrophobic and will be retained longer than the dead time, giving longer retention times than tM. Acetone is more polar, but its retention can vary with the mobile phase composition and conditions, so it’s less reliable as a strict unretained marker than uracil. Thus uracil best serves as the unretained marker to determine dead time on a C18 column.

In reversed‑phase liquid chromatography on a C18 column, the dead time tM is determined using a compound that does not interact with the hydrophobic stationary phase—an unretained marker that travels with the mobile phase. Uracil is highly polar and does not partition into the C18 phase, so it elutes with essentially no retention and defines the mobile-phase travel time through the system. That makes its observed retention time effectively equal to tM, which is exactly what you need to calculate retention factors for other analytes.

The other candidates tend to interact with the C18 surface to some degree. Benzene and toluene are hydrophobic and will be retained longer than the dead time, giving longer retention times than tM. Acetone is more polar, but its retention can vary with the mobile phase composition and conditions, so it’s less reliable as a strict unretained marker than uracil. Thus uracil best serves as the unretained marker to determine dead time on a C18 column.