Oxytetracycline (MW 460.43) belongs to an antibiotic tetracycline with broad-spectrum antimicrobial effect, and is one of the most commonly used pharmaceutical drugs. The main fragment ions of oxytetracycline, corresponding to peaks at m/z 443 and 426, can be found in the full-scan mass spectrum of MPTDI-TOF MS (Figure 5b), and similar result with MSn data can be obtained by MPT-LTQ MS (Supplementary Figure S-1b). The protonated oxytetracycline contains multiple alcoholic hydroxyl groups and forms the ion at m/z 443 easily by losing a water molecule, and then losing an ammonia molecule to generate the ion at m/z 426. The possible fragment pathway of oxytetracycline at the MPT ionization source is shown in Supplementary Figure S-2b. It can be proven that microwave plasma thermal desorption and ionization at atmospheric pressure technology is expected to simplify qualitative analysis.