HOW TO TEST YOURSELF
has a 1 in 10 dilution, and its concentration is one tenth
the original, or .001 gm/ml.- 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”. - Repeat with remaining bottles. Bottle #13 would therefore
be labeled “.000000000000001 gm/ml.” This is 10-15
gm/ml, or 1 femtogram/ml. - 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 bot-
tle #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). Con-
tinue 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 in-
expensive 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 6.02x10^23
molecules/M divided by 20 drops/ml). Water in bottle #12
would therefore have 10 times as many molecules in one drop,