System and method for determining an in vivo concentration of a small molecule substance of interest in a noninvasive manner

A method of determining an in vivo concentration of a small molecule substance of interest in a subject, wherein the small molecule substance of interest has a molecular size that is capable of passively diffusing through the skin of the subject, the method comprising:

• placing a collector for the small molecule substance of interest in contact with an area of the skin of the subject;
• maintaining the collector in contact with the skin area for a predetermined time that is sufficient to collect at least some of the small molecule substance of interest that has passively diffused through the skin;
• measuring a concentration of the small molecule substance of interest collected by the collector;
• and
• extrapolating an in vivo concentration of the small molecule substance of interest based on the measured concentration of the small molecule substance of interest that passively diffused through the skin and that was collected by the collector.