Just a description of the substrate and its location should be enough. If the sample you know to be a comparison sample comes back positive, you need to deal with that. If you want the lab to make an actual comparison, however, you will need to tell the lab analyst that # x is a comparison sample for # y.

From ASTM E1459

4. Marking Evidence Upon Original Collection
4.1 General Requirements:
4.1.1 Each item of evidence will be assigned a unique
alphanumeric designator by the investigator who collects the
evidence, or by someone designated to assign item numbers.
4.1.2 Whenever possible, sequential numbers will be used
for evidence associated with a particular incident, event, or
scene.
4.1.3 The location and condition of each item should be
documented prior to collection.
4.1.4 Each item should be properly protected in an appropriate
manner and marked or tagged with the following
information:
4.1.4.1 Item number,
4.1.4.2 Case or incident number,
4.1.4.3 Identification of person who collected item,
4.1.4.4 Date item collected, and
4.1.4.5 Brief description.
4.1.5 If possible, the evidence should be sealed in a tamperevident
container.
4.2 The following procedures are intended as an example of
a procedure that will satisfy the requirements of 4.1. They may
be adapted to the requirements of a specific incident or agency
as required.
4.2.1 At each scene, assign one individual to package, label,
and inventory evidence.
4.2.2 Give each scene a separate identification number. This
may be the same as the incident or report number, or may be a
combination of an incident or report number and a scene specific
number.
4.2.3 Give each item collected at a specific scene a unique
sequential number.
4.2.4 Separately package and mark each item with the
information called for in 4.1.4.

John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
[www.firescientist.com]