Lesson Seventeen

Lesson Seventeen

Purpose: To see how sensitive your measurements can be.
(How much of a substance must be present for you to get a
positive result?)
Materials: filtered water, salt, glass cup measure, 13 new
glass bottles that hold at least ¼ cup, 14 new plastic teaspoons,
Your skin tissue sample, paper towel.
Method: Some of the best measurement systems available
today are immunological (such as an ELISA assay) and can deTHE
tect as little as 100 fg/ml (femtograms per milliliter). A milliliter
is about as big as a pea, and a femtogram is
1/1,000,000,000,000,000th (10-15) of a gram!
1. Rinse the glass cup measure with filtered water and put one
half teaspoon of table salt in it. Fill to one cup, stirring with
a plastic spoon. What concentration is this? A teaspoon is
about 5 grams, a cup is about 230 ml (milliliters), therefore
the starting concentration is about 2½ (2.5) gm per 230 ml,
or .01 gm/ml (we will discuss the amount of error later).
2. Label one clean plastic spoon “water” and use it to put nine
spoonfuls of filtered water in a clean glass bottle. Use
another plastic spoon to transfer one spoonful of the .01
gm/ml salt solution in the cup measure to the glass bottle,
stir, then discard the spoon. The glass bottle now has a 1 in
10 dilution, and its concentration is one tenth the original,
or .001 gm/ml.