Please use the qNMR-determined moles of reagent per vial (given on each Specification Sheet) when making accurate concentration stock solutions.  When delivering solvents to vials to make stock solutions by pipet, you should be sure that your pipetting device is accurately calibrated, and you should not assume that calibration will be the same for water versus organic solvents such as methanol.  Also, note that polypropylene push-cap tubes such as Eppendorf tubes will lose solvent including water over long-term storage even when frozen.  Studies showing the solvent loss from these tubes have been published.  It is recommended to store stock solutions in glass vials with screw caps, and the screw cap needs to have a Teflon disc that forms a tight seal to the vial.  Published studies have shown minimal loss of solvent from these types of vials.  Avoid screw caps with non-Teflon caps (i.e. paper, rubber, etc.).  You also need to remember that volatile organic solvents such as methanol may be lost each time you open a vial at room temperature, so you should leave the vial open for a minimum amount of time (typically < 30 sec) and try to avoid opening the same vial more than 10-20 times.  It is also best to warm vials to ambient temperature after you remove them from the freezer to minimize the condensation of water from the atmosphere into the solvent in the vial.