Below 0.01 Grams: Where the Balance Gives Out
The formula says "Indole — 0.02". Your balance has a readability of 0.01 grams; seeing 0.02 on the display is easy, but the true value of that number could be anywhere between 0.015 and 0.025. In other words, the uncertainty of the dose is 25 per cent. The same uncertainty on a 20 gram raw material in the body of the formula is 0.05 per cent; nobody notices. At a trace dose, though, it is large enough to change the character of the scent.
Trace raw materials are substances used below 0.1 per cent in the formula but making a distinct contribution to the scent: indole, skatole, cis-3-hexenol, sulphur and pyrazine-type molecules, damascones, some aldehydes. Their small ratios do not mean their importance is small; most of the time it is the reverse.
This article explains how these doses are weighed reliably. The essence of the answer is a single sentence: trace materials are not weighed neat; they are weighed as dilutions.
Knowing the Safe Range of Your Balance
Every balance has two values: readability (the last digit on the display) and the lower limit of reliable weighing. The second is far above the first.
| Balance readability | Practical safe lower limit | Suitable work |
|---|---|---|
| 0.1 g | 10 g and above | Production batches, solvents, alcohol |
| 0.01 g | 1 g and above | Formula body, 10% dilutions |
| 0.001 g (1 mg) | 0.1 g and above | 1% dilutions, small trials |
| 0.0001 g (analytical) | 0.01 g and above | Laboratory; rarely needed in a perfume workshop |
Practical rule: the amount weighed should be at least 100 times the readability. On a 0.01 gram balance do not go below 1 gram; on a 1 milligram balance do not go below 0.1 grams. This rule keeps the weighing error below 1 per cent.
Other things that affect the balance: air currents (window, air conditioning, fume hood), bench vibration, static electricity (in plastic containers) and temperature differences (a bottle just out of the refrigerator). In trace weighing, each of these moves the last digit.
Weighing with Dilutions: Calculation and Examples
The way to bring a trace dose into the safe range is to dilute the substance beforehand. The dilution ratio must be high enough to carry the target dose above the balance's safe lower limit.
| Target pure dose | Balance (readability) | Dilution to use | Amount weighed |
|---|---|---|---|
| 0.05 g | 0.01 g | 10% | 0.5 g (borderline; 5 g of 1% is safer) |
| 0.02 g | 0.01 g | 1% | 2.0 g |
| 0.005 g | 0.01 g | 0.1% | 5.0 g |
| 0.005 g | 0.001 g | 1% | 0.5 g |
| 0.001 g | 0.001 g | 0.1% | 1.0 g |
Very low-ratio dilutions are prepared in two stages: first 10%, then 1% from the 10%, and if necessary 0.1% from the 1%. Each stage is weighed within its own safe range; weighing 0.01 grams of pure substance in one step and dropping it into 10 grams of solvent carries the error into the stock.
The dilution's solvent enters the formula. 5 grams of a 0.1% dilution carries 4.995 grams of DPG into the formula. In a small trial this ratio is acceptable; in a large formula, total the solvent coming from dilutions on a separate line and check the overall solvent ratio.
Trace Doses When Scaling Up
The indole you added to a 10 gram trial as 2 grams of a 1% dilution means 20 grams of pure indole in a 10 kilogram production batch. Dropping the dilution and switching to the neat substance in production looks right; but there are two traps.
The first trap is rounding. The dose you wrote as 0.02 grams in the trial may really have been 0.018 or 0.023; scaled up 1000 times, that difference comes to 5 grams. The solution is to have weighed the trial correctly with a dilution; then the scale-up is correct too.
The second trap is the solvent. In the trial, the DPG from the dilution was part of the formula. When you switch to neat indole in production, that DPG disappears and the formula changes by a few per cent. Either you use the same dilution in production too, or you add the missing solvent on a separate line.
The safest route in production is to prepare the trace materials as a separate "premix": all the trace components of the formula are brought together into a single dilution and added to the main batch in a single weighing. This both reduces the number of weighings and guarantees the same ratio in every batch.
Frequently Asked Questions
The most common questions about trace weighing.
Can I weigh trace doses with my 0.01 gram balance?
Do I need to buy a milligram balance?
Can't I add it by counting drops from a pipette?
Should I add trace materials at the start of the formula or at the end?
Does the solvent from the dilution upset the formula?
How often should I calibrate the balance?
Does static electricity really affect weighing?
Should I add trace materials neat in production?
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