Make very accurate solutions without weighing I am currently facing a hen-egg problem. For a LC-MS method development I need to prepare reference solutions which are used to calibrate the method. Since the method is to be used for quantification in the pg/mL range, the reference solutions should have the most accurate concentrations possible. Of course I know how to prepare exact solutions in volumetric flasks, but here substance quantities are used which can be weighed easily and also quite exactly. In my case, only a few milligrams of the substance are available to me, which is why I cannot weigh high masses. The precision balance that I have at my disposal has a lower weighing limit of 1 mg, which is about the same as the mass that I would have to weigh. However, since the lower limit is very error-prone, I do not like this method. Unfortunately, I cannot think of an alternative solution that would allow me to create an exact solution without weighing. One could of course dissolve any amount of substance in the desired solvent and then determine the concentration by LC-MS, but this is exactly where the hen-egg problem mentioned above occurs. I cannot quantify via LC-MS until I have a suitable method and an internal standard. Are there any other ideas how to get exact concentrations using small amounts of the desired substance?
Are there any other ideas how to get exact concentrations using small amounts of the desired substance?

Every mass spectrometry lab should have microbalances to prepare samples and standards. A lab which can afford an LC-MS can also afford a semimicro balance or a microbalance. Such balances are meant to weigh 0.01 mg to 0.001 mg (with extreme caution).
Two simple options:
(i) Since you are apparently not the first user of LC-MS system in your group, ask how other senior researchers prepared standards in your lab?
(ii) Another (crude) option is that you try weighing by difference. You will tare your analytical balance to zero. By an analytical balance I mean a balance which reads to four decimal places in grams, this is the most commonly used in research labs. Place your standard container on the balance and read the mass. Carefully transfer approximate amounts to your desired container in which the standard will be prepared, and note the weight of the standard container again. The change in mass corresponds to the amount transferred. Practice it with some commonly available substance to see if this works and if your balance is sensitive enough or not. Balances age with time.