Oils and such on a weight can also make a difference in the mg range. Sorry, I've been engrained with the bs from my chem and physics classes.
Haha we chem students are so predictable. I too felt compelled to chime in and give my two cents concerning the subtleties of taking mass measurements. :D
I'm sure you, like me, have been drilled near to death with this stuff Cloudy, but I'll just throw it out there in case anyone else happens to be interested:
When measuring the amount of mass present in very small quantities of matter, one must be exceedingly careful if a reasonable degree of accuracy is desired. Even the heat from your hands (transferred to the weighing bottle through handling) can skew readings in the milligram range. This is particularly significant when using a balance with a enclosed balance pan (especially analytical balances, which are extremely sensitive).
To preface,
The so-called Ideal Gas Law:
PV = nRT
P is pressure in atmospheres (atm).
V is volume in liters (L).
n is the amount of gas present, expressed in number of moles (mol).
R is the so-called 'Gas Constant' which is approximately equal to 0.0821 (L * atm) / (mol * Kelvin)
T is temperature in kelvins.
Since pressure and temperature vary proportionally ( P = nRT / V; T is in the numerator ), the increase in temperature causes the air-pressure to increase within the balance enclosure. This increase in pressure applies force to the sample by the surrounding atmosphere, which further depresses the balance pan and causes the instrument to register an erroneously high mass reading.
Weighing a sample in an uncovered weighing bottle can also result in an erroneously high mass reading. The ambient air contained within the weighing bottle will diffuse into the balance enclosure. Since the amount of air is increasing, this can be thought of as increasing the variable 'n' in the ideal gas equation. Since n is also proportional to P ( P = nRT / V; n is in the numerator), the addition of extra air from an uncovered weighing bottle will drive up your mass reading as well.
And as if there wasn't already enough crap to potentially mess with your data...
Most common materials, especially plastics and to a certain extent pyrex glass, can retain a fairly high amount of electrostatic charge ("static electricity" basically-- like when you take yer socks out of the dryer).
Most balances, even the majority of the non-analytical balances we use to weigh drugs, don't actually measure the mass of an object: instead, they calculate the object's mass through an indirect method.
They use an electrical current to induce a magnetic field under the balance pan and then measure the magnitude of perturbation caused in the field by the object depressing the pan. Since the strength of the magnetic field is proportional to the strength of the current inducing it, the amount of force (in newtons) necessary to cause such a magnitude of perturbation can be derived quite easily by a small computer chip.
So Newtonian mechanics tells us that force is equal to mass times acceleration (
F = m*a), and the sample depresses the balance pan only with acceleration due to gravity. So acceleration, a, is equal to g = 9.80665 m/s². So finally mass, m, is equal to F in newtons divided by g (m=F/g). Since F= xNewtons = xkg/m·s², and g = 9.8m/s², when you divde F by g, the s² drops out and you're left with essentially a weight measurement in kilograms per square meter. Since the size of the balance pan in m² is a known quantity, the balance can then display a mass measurement in grams or milligrams.
So anyways, electrostatic charges present in the analyte material can sometimes gain electromotive force and flow as current (like when you pull off a wool sweater in the dark and see sparks). This current will cause additional perturbation in the induced magnetic field, in turn skewing the force reading-- which then causes the device to return an erroneous mass reading.
So yeah, there are definitely plenty of weighs to screw up weighing stuff. :D
That's why I always suggest that people should employ an extra safety net such as liquid measurement when dealing with exceptionally potent compounds, you should never fully trust your instrumentation.