Due to the paraantiferromagnetic transition of oxygen at 45 K, the contamination with this element may deeply affect measurements performed with high-resolution magnetometers around this temperature. We have studied this spurious effect by measuring with a commercial SQUID-based magnetometer (Quantum Design, model MPMS-XL) the magnetization of granular La1.85Sr0.15CuO4 (LSCO) superconductors by using different sample holders. Our results demonstrate the crucial role played by the O2 physisorption in the extended surface of the LSCO grains, an effect that may be deeply mitigated by encapsulating the grains into an epoxy resin (EPO-TEK 301). This procedure may be also useful for high-precision magnetization measurements at low temperatures in other granular or porous materials.