Impurity concentration in intense neutral beams extracted from both a 75 keV/6 A/10 s duoPIGatron ion source and a magnetic multipole line cusp ion source has been measured by magnetic deflection mass analyzer. After passage of 90% equilibrium neutralizer cell, the ion beam contains 1%–2% light impurities (such as C+, O+, H2O) and 0.02%–0.15% heavy impurities (such as Cu+, W+, Ag+). Taking into account the neutralization efficiency, the neutral beam is estimated to contain 3%–6% light impurities and 0.04%–0.3% heavy impurities. The impurities related to oxygen decrease with time during a 10 s pulse, while those related to carbon, copper, and tungsten increase slightly with time. The origin of these impurities is discussed and the source operation mode to reduce the impurity level is proposed for neutral beam injection.