Toxic elements in the environment are a threat to public health and safety. Federal and state regulations have been issued to limit and abate the presence of toxic elements in the environment. The levels of toxic elements in the U.S. population are carefully monitored. NHANES is one of the federal programs that uses urine as a proxy to assess the exposure of the US population to toxic elements. Because sample collection is non-invasive and the test results reflect recent exposure (within two days), urine is the preferred matrix for clinical diagnostics and biomonitoring of human exposures to toxic environmental chemicals. Typically, urine samples are frozen after collection while awaiting analysis. In the past, NIST produced freeze dried SRMs for quality assurance of urine measurements, but no matter how closely freeze-dried materials resemble frozen clinical test materials, the differences between them are significant when used for quality assurance. The frozen urine matrices of the new SRMs provide better commutability with the specimens of the intended clinical measurements.