A modified mirror reaction was developed to prepare a sensitive and reproducible Ag nanofilm substrate for the surface-enhanced Raman scattering (SERS) analysis of arsenate (As(V)) and arsenite (As(III)). A good linear relationship between the SERS intensity of As(V) and As(III) and their concentrations in the range from 10 to 500 mu g-As/L was achieved using the SERS substrate. As(V) and As(III) appear to be adsorbed on the Ag nanofilm through formation of surface complexes with Ag, based on comparisons of the Raman spectra of the arsenic species in solutions, on the SERS substrate, and in silver arsenate and arsenite solids. As(V) and As(III) species on the SERS substrate and in the solids had the same Raman band positions at 780 and 721 cm(-1), respectively. The effect of eight ions in natural waters on the SERS analysis of As(V) was studied. K+, Na+, SO42-, CO32-, and NO3- in the range of 0.1-100 mg/L did not interfere with the SERS detection of As(V) for a As(V) concentration greater than 100 mu g-As/L While Cl-(50 mg/L), Mg2+ (10 mg/L), and Ca2+ (1 mg/L) were found to quench the SERS intensity of 100 mu g/L As(V). Cl- (at concentrations >10 mg/L) formed silver chloride with the adsorbed Ag+ and decreased the SERS detection limits for arsenic species. The mechanism of the Ca2+ effect on the SERS analysis of As(V) was through the formation of surface complexes with As(V) in competition with Ag. When the Ca2+ concentration increased from 0 to 100 mg/L, the amount of As(V) adsorbed in Ag nanoparticles was reduced from 38.9 to 11.0 mu g/mg-Ag. When the Ca2+ concentration increased to values higher than 1 mg/L in the As(V) solution, the As(V) peak height was decreased in the corresponding SERS spectra and the peak position shifted from 780 to 800 cm(-1). The fundamental findings obtained in this research are especially valuable for the development of sensitive and reliable SERS methods for rapid analysis of arsenic in contaminated water. (C) 2010 Elsevier Inc. All rights reserved.