Fundamental physical law informs that neutrons play no role in electron probe microanalysis, and therefore, mass-based models follow an improper route to data reduction. Castaing's mass-based first approximation inappropriately remains the starting point-see any microprobe textbook-for development of quantitative electron probe microanalysis. By incorrectly relating intensity of characteristic x rays to mass rather than to charge, such conventional models demand an isotope (mass) effect; our robust experiments detected none. (C) 2000 American Institute of Physics. [S0021-8979(00)09111-8].