BIOELECTRONICS
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,
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.
Sensitivity of Pollutant-In-Product Testing...........................
Get some slides of Salmonellas and Shigellas and find some
milk that tests positive to at least one. Make a dilution series of
the milk up to bottle #14, being careful not to shake the bottles.
Start with 2 drops of milk in bottle #1. Use an eye dropper to