How to accurately weigh 1 mg powder substance?

Hello dear, you can prepared a solution with a concentration higher than those that you are needing and then from this solution you can measured X volumen that it contained Y mg required for obtaining a wanted solution for example: I want to prepare   2 mL of a solution whose concentration will be 2 mg/ml (solution A) from a solution whose concentration is 10 mg/mL(solution B)

V required of solution B for preparing solution A= Va *Ca/Cb= 2 mL * 2 mg/mL/ 10 mg/mL= 0.4 mL

I measured 0.4 mL (micropipete) and the complete with water until a total volumen  of 2 mL.

@Edwin A. Murillo, Thank you for your answer. Perhaps I did not clearly state my question. Since the standard was in the powder form with less than 10 mg, i can not get concentrated solution first and then dilute to designed concentration. Whatever, thank you for your response.

Definetely special tools: you need special high precision scales in a temperature and humidity controlled room. A standard lab scale varies too much with environmental conditions. Or you have reliable absorbance coefficients for the compound, then you dissolve it in a known volume (precisely known volume, preferable using a well calibrated volumetric flask, and calculate the concentration from the absorbance.

After one year exploring and practice, I finally found a way to weight the trace amount of organic compounds with weight around 1 mg and here the organic means the organic solvent soluble compounds. This should also work for the water soluble compounds.

1. Make sure the sample is dry enough. Usually you purchase standards from supplier and it should be dry. In case of heavy moisture effect and vapor impact, you could dissolve in methanol or acetone and then use free drying.

2. after that, cut a plastic cap from usual 2 ml tube, weight and tare the weight of cap. You should have a precise balance such as the Mettler Toledo balance with accuracy to 0.001 mg.

3. Then, use 10 ul methanol or acetone to dissolve the dry powder, dissolve the organic compound and aliquot 10 ul to the cap. Wait until the reading is stable. (I have tried the methanol and it completely vaporized in 1 hour. Methanol lost over 95% of weight in the first 10 mins and the rest part takes a bit time to vaporize.)

4. Based on the weight, you could transfer the volume of liquid to the 2 ml tube and mix well with the solvent. You could dilute further to a designed concentration as working standards.

Notes:

1.the balance should be placed in a marble table to minimize the mechanical vibration impact from other instrument. The environment around the balance should be quite without frequent pedestrians working around. For some balance, it has small cage to lock the chamber and this design would help stabilize the reading.

2. I just try the relatively volatile organic solvent. For water, I think it should work but take longer time. 

Hi Chaozhi,

I am very unfamiliar with chemistry of solutions, so please bear with me if what I am saying is illogical.

Firstly, my powders are not organic. They are things like LiF, KF, etc... Will this technique still work?

If you are using volatile solvent such as methanol, how does the balance eventually stabilize? Wouldn't it continually lose mass due to evaporation? Or does it slow enough, as you somewhat suggested, to get a good enough reading?

Please correct me if I am wrong, but the procedure to measure 1 mg of powder could be:

1) tare the balance with the plastic cap. 2) measure out 10 ul of solvent (methanol, acetone) and dissolve 1 mg of powder into it 3) you can then let the solvent evaporate and you then know you have 1 mg remaining. My question is, how do you know you are adding 1 mg of powder, if the solvent is rapidly evaporating and changing the mass from its original 10 g value?

I do not know whether my previously descripted method work on inorganic salt or not, but you could try by your self. For exaple, you could dissolve 1.000 g KF in 100 mL in volumetric flask and mix well and this would be 10,000 ppm (10 g/L or 1% [m/v]). Then you could transfer 100 uL (or 10 uL for fast time) the well mixed solution to the cap and record the reading from the accurate balance till the stable reading achieved. If you get stable reading, it should be 1 mg (0.1 ug for 10 uL). If not, that means the method I described is not applicable for inorganic in organic solvent.

Since I just tried the organic in solvent, i am not sure whether this method work on inorgaic compoounds or not. Some inorgnaic compounds are well known to be hygroscopic to form hydrate in which water content is very hard to remove. So similar case likely to happen with small organic solvent like methanol regard their strucutre.

Chaozhi Pan

Thank you very much for your reply. You clarified things very well for me.

I never thought about the inability to dry the materials in my lab, but you are right about the hygroscopic issue. My salts are extremely hygroscopic so I don't think this method would be very effective in the long run by trying to dry the materials afterwards.

Thanks again for your swift reply!