What sample preparation technique is commonly used to clean urine samples before LC-MS/MS or GC-MS analysis?

• A. Direct injection without cleanup.
• B. Solid-phase extraction (SPE) or liquid-liquid extraction (LLE) to remove impurities and concentrate analytes.
• C. Centrifugation alone.
• D. Filtration with a coffee filter.

Answer: (B)

Before LC-MS/MS or GC-MS analysis, cleaning up urine samples is crucial to reduce matrix effects and concentrate the target analytes. Urine contains salts, urea, proteins, and a variety of endogenous compounds that can suppress ionization, contaminate the source, or obscure the signal. The goal of sample preparation is to remove these impurities while retaining the analytes of interest so the instruments can detect them more accurately and sensitively.

Solid-phase extraction and liquid-liquid extraction are commonly used because they effectively separate analytes from most interfering substances. In solid-phase extraction, the sample passes through a cartridge that traps the analytes while impurities are washed away; the analytes are then eluted in a small volume, yielding a cleaner extract with higher concentration. Liquid-liquid extraction relies on partitioning compounds into an immiscible organic solvent, separating them from the aqueous matrix, and often concentrating the analytes in the organic phase. These approaches substantially improve signal quality and reduce matrix effects for both LC-MS/MS and GC-MS analyses.

Direct injection without cleanup exposes the system to significant matrix effects and potential contamination. Centrifugation alone removes particulates but leaves dissolved impurities that can still cause ion suppression. Filtration with a coffee filter would not effectively remove many dissolved impurities and can lead to inconsistent recovery.