A second pooled QC sample set was developed and is routinely integrated in profiling studies as a Long Term Reference (LTR) , allowing independent monitoring of precision within a study, providing a point of reference for relating measurements across multiple profiling studies, and allowing the general monitoring of system performance across studies and years of operation. Over time, LTR materials (human urine, plasma and serum) have also become important as a system suitability test mixtures (SSTMs), allowing rapid assessment of an LC-MS system's accuracy (retention time, m/z and signal intensity measurement of spiked in MR and IS as well as some most abundant endogenous metabolites commonly detected in biofluid samples) after method-specific setup but before acquisition of study data. Each pooled QC sample is injected regularly throughout the sample analysis sequence (see Figure , following page). 17