Acetylsalicylic acid (Aspirin) belongs to a class of medications known as non-steroidal anti-inflammatory drugs (NSAIDs) and has been among the most popular medications for treating mild to moderate pain. It also reduces inflammation, clotting of blood, fever, redness, swelling and discomfort caused by medical disorders such as headaches, infections and arthritis. Methimazole is used to treat hyperthyroidism, a condition that occurs when the thyroid gland produces too much thyroid hormone.

Mass spectrometry has played a significant role in the characterization and analysis of small molecules such as Methimazole and Aspirin. The present study was undertaken to assess the structures and mechanisms of formation of the principal fragment ions in the ESI mass spectra of these two drugs. The CID mass spectra of Methimazole and Aspirin and their corresponding deuterated analogs have been studied in both the positive and negative ion modes.

Decomposition mechanisms are proposed for the principal fragment ions using H/D exchange and the combination of multiple-stage CID at low collision energy with high-resolution mass measurements. The mass spectra of Methimazole and Aspirin can serve as useful models for structural determination of chemically or biologically modified Methimazole/Aspirin or related compounds.

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