**1. Introduction**

For long-term environmental studies, such as for prospective epidemiology studies, it is often advantageous to store collected environmental samples for future retrospective analyses. Traditionally, stored environmental samples have included human tissue and fluids, animal and plant tissues, soils, sediments, and ice cores [1]. Such samples can be used to evaluate results of government policies, health of an animal population, or temporal trends in ecosystems or exposures [2]. In longitudinal studies, this also permits spreading costs of analyses over time – an important consideration as analysis for environmental contaminants can be expensive. Additionally, it provides more flexibility to analyze subsets of samples in nested case–control studies for specific health outcomes or for inclusion of new target analytes or analysis methods. For instance, concern about the presence of pesticides, pharmaceuticals and personal care products (PPCPs) has heightened over the past several years due to the presence of these chemicals in wastewater, groundwater, surface water and drinking water [3–10].

Information about sample stability in long-term studies is critical to determine, if there will be analyte loss or gain or degradation under specified storage conditions and storage period. Failure to evaluate stability could result in inaccurate results and biased exposure assessments due to partial or complete analyte decomposition, chemical transformation, or loss/gain.

The National Children's Study (NCS) was a longitudinal cohort study that aimed to follow 100,000 children from birth until 21 years of age to evaluate the health effects of environmental exposures, including chemical, physical, biological, and psychosocial factors. A variety of different types of samples were collected during the NCS pilot study including environmental samples and biospecimens. The environmental samples that could not be stored because of known instability (e.g., badge samples for air oxidants) were analyzed immediately after collection. Other samples and biospecimens were stored in the NCS repository. The plan was to store the other environmental samples (e.g., water, soil, and dust) and biospecimens (e.g., blood, urine) until children aged, analyzing only samples as requisitioned for children included in subsequent case–control studies.

As part of the NCS pilot, we considered which types of environmental samples could be stored for extended periods of time. Many factors can affect stability of chemical compounds in stored environmental samples over time, such as temperature, humidity, pH, and microbial enzymatic activity of the sample matrix, physicochemical properties of the analytes themselves and their reactions with the matrix, container materials, and other analytes present in the sample [11].
