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 de

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.
3. Use the “water” spoon to put nine spoonfuls of filtered
water in bottle #2. Use a new spoon to transfer a spoonful
of salt solution from bottle #1 to bottle #2 and stir briefly
(never shake). Label bottle #2 “.0001 gm/ml”.
4. Repeat with remaining bottles. Bottle #13 would therefore
be labeled “.000000000000001 gm/ml.” This is 10-15
gm/ml, or 1 femtogram/ml.
5. Do the skin test with water from bottle #13 as in Lesson
Five. If you can detect this, you are one hundred times as
sensitive as an ELISA assay (and you should make a bottle
#14 and continue if you are curious how good your
sensitivity can get). If you can not, try to detect water from
bottle #12 (ten times as sensitive as ELISA). Continue until
you reach a bottle you can detect.

Calculate the error for your experiment by assuming you
could be off by as much as 10% when measuring the salt and

water adding up to 20% error in each of the 13 dilutions. This is
a total error in bottle #13 of 280%, or at most a factor of 3. So
bottle #13 could be anywhere from 0.33 to 3 femtogram/ml. If
you can detect water from bottle #13, you are definitely more
sensitive then an ELISA, in spite of your crude utensils and inexpensive
equipment! Note that the starting error of using 2.5 gm
instead of 2.3 gm only adds another 10% error.
If you want to calculate how many salt molecules you can
detect, select the concentration at the limit of your detection, and
put 2 drops on a square inch of paper towel and rub into your
skin. Assume one drop can be absorbed. If you can detect water
from bottle #13, you have detected 510,000 molecules (10-15
fg/ml divided by 58.5 gm/M multiplied by 6.02x1023
molecules/M divided by 20 drops/ml). Water in bottle #12
would therefore have 10 times as many molecules in one drop,
and so forth. Even if your error is as much as a factor of 2
(100%), you can still get a good idea of what you can measure.
Atomic absorption standards start at exact concentrations; it
is easy to make a more exact dilution series with them. When
testing for iridium chloride by this skin test method, I was able to
detect 3025 molecules!
Troubleshooting:
Always extend your set until you get a negative result (this
should happen by at least bottle #18). If you always “detect” salt,
then you shook the bottle!
Never try to reuse a bottle if you spill when pouring into it.
Get another new bottle.