A method was established for the determination of the elemental composition in plant samples using the characteristic argon fluorescence peak in the air as a normalization standard via low power total reflection X-ray fluorescence (LP-TXRF) spectrometry using polychromatic X-ray excitation. The source of the Ar peak was demonstrated both theoretically and experimentally. According to the theoretical study, the characteristic Ar peak mainly originates from the optical path of the incident and reflected X-rays and the standing wave field formed by the incident and reflected X-rays. Additionally, the experiments confirmed the main contribution of the standing wave fields formed by the incident and reflected X-rays to the Ar characteristic peaks, and the influence of reflector type and sample deposition amount on the Ar characteristic peaks was excluded. Three different plant certified reference materials (CRMs) were employed to evaluate the effectiveness of the proposed methodology for multi-elemental quantitative analysis. The results showed that the Ar-peak normalization method resulted in comparable accuracy and precision compared with the conventional internal standard method, and a long-term stability check under the same conditions showed the good stability of the argon peak.