We report on the semi-quantitative analysis of pulsed laser induced plasma species as well as thin film compositions of La_0.6Sr_0.4MnO₃ grown on SrTiO₃ substrates under various background pressure regimes using an ¹⁸O isotope labelled La_0.6Sr_0.4MnO₃ target. The importance of negative metal-oxygen or positive metal-oxygen ions to influence the final oxygen composition of the thin film is illustrated through the use of mass spectrometry, where the chemical reactions between the laser ablated target species with the oxygen background molecules are directly characterized. We find that the influence of metal-oxygen negative ions is not as important as the metal-oxygen positive ions to the final oxygen composition of the LSMO film, due to their low stability in high background partial pressures. Furthermore, we observe that the oxygen incorporated in La_0.6Sr_0.4MnO₃ thin films coming from the target is ∼44%, 29% and 1% at 2 × 10^-3 mbar, 1 × 10^-2 mbar and 2 × 10^-1 mbar, respectively. When growing films at 10^-1 mbar on ¹⁸O₂ exchanged substrates, almost all oxygen originates from the background and almost none from the substrate or target.