Abstract: A method of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) under O₂ and NH₃ reaction modes was developed for the determination of tantalum (Ta), sulfur (S), phosphorus (P), and silver (Ag) in high-purity hafnium (Hf), and the reaction mechanisms were investigated. Ammonia precipitation effectively separated the matrix, thus eliminating spectral interferences and matrix effects from zirconium (Zr) and Hf on the determination of Ag and S. However, poor recoveries were observed for the detection of Ta and P, necessitating direct determination. Product ions scanning revealed complex reaction products between Ta⁺ and O₂/NH₃. After parameter optimization, when the O₂ flow rate was set to 45%, the mass-to-charge ratios (m/z) of the first quadrupole (Q1) analyzer and the second quadrupole (Q2) analyzer were established at m/z 181 and 263, respectively, effectively eliminating Hf matrix interference on Ta. At an O₂ flow rate of 30%, the primary ion products of P⁺ and S⁺ were ³¹P¹⁶O⁺ (m/z 47) and ³²S¹⁶O⁺ (m/z 48), respectively, allowing interference-free detection by setting Q2 to these m/z values. Under an NH₃ flow rate of 35%, Ag⁺ formed Ag(NH₃)₂⁺ (m/z 141), which was detected by setting Q1 and Q2 to m/z 107 and 141, respectively. Internal standard correction was performed using scandium (Sc), caesium (Cs), and thallium (Tl), under optimized experimental conditions, the calibration curves for Ta, S, P, and Ag determination exhibited excellent linearity (R²>0.999). The method demonstrated detection limits ranging from 0.00082 to 0.13 µg/g, with relative standard deviations (RSD, n=11) of 4.3%-9.4% and spiked recoveries of 92%-110%. This method offers low detection limits, high accuracy, and excellent precision, making it suitable for the rapid and accurate determination of trace Ta, S, P, and Ag in high-purity hafnium.