Environmental samples like waste waters are generally known to contain calcium, magnesium, sodium and potassium as major constituents. It is important to assess the amount of interference at analyte wavelengths arising from the matrix as well as other analytes to select suitable analysis lines. This paper reports the measurement of partial sensitivities of 49 prominent lines of 17 impurities of interest in waste water (Ag, Al, As, B, Ba, Be, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V, Zn) using inductively coupled plasma atomic emission spectroscopy. Spectral scans for 300 pm wide windows centred (± 150 pm) around analyte wavelengths, were obtained under compromise operating conditions. Using the criterion of “true detection limit”, Q-values for line interference [QIj(λa)] and Q-values for wing background interference [QWj(Δλa)] due to Ca, K, Mg and Na have been worked out. The Q-values give a quantitative esiimate of the extent of spectral interference and thus permit a rational selection of analysis lines. It is demonstrated that by varying the relative concentration of each matrix component independently, the true detection limit of an analyte line changes and line selection in a multi-component matrix is more complex than in a single element matrix. Therefore, prior quantitative knowledge of the matrix elements is inevitable for selection of a suitable line for trace element analysis. Trace element analysis in synthetically prepared samples indicates the presence of non-spectroscopic matrix effects.