Analyzing Essential Oil Composition And Purity

Application Need:
It is difficult to ensure the quality and purity of essential oils because they are complex
mixtures of many different chemical constituents, and can be affected by growing
and harvesting conditions, distillation technique, handling, and storage conditions.
In addition, products can pass through many hands before reaching
the consumer and some unscrupulous distillers or sellers alter essential oils by diluting, cutting, 
or extending them, while selling them as “pure.”
Solution:
Use polarimeters, refractometers, and density meters to measure an essential oil, identifying
its composition and purity.
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Overview: An essential oil is a liquid that is generally distilled (most frequently by steam or water)
from the leaves, stems, flowers, bark, roots, or other elementsof a plant. Essential oils,
contrary to the use of the word "oil is not really oily feeling at all. Most essential oils are
clear, but some oils such as patchouli, orange and lemongrass are amber or yellow in color.
There are a few techniques available for analyzing essential oil composition
and purity, but there is no technique that can absolutely guarantee an oil’s composition and purity.
There are three common quality control methods for checking essential oils, all of which involve measuring physical parameters;
polarimetry, refractometry, and density. All three are quick, non-destructive measurement methods. 
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Method 1; Measuring  Specific Rotation using Polarimetry.The specific rotation of a substance is defined
as the optical rotation of a solution containing 1 g/ml in a 100 mm polarimeter tube.
It is affected by temperature (20°)  reference temperature) a wavelength (usually the sodium D  line, 589 nm.
is used), and can be measured by a polarimeter, which uses the following formula :
where:
α is the measured optical rotation
d the light path in dm
c the concentration of g/ml.
How this is done;
Fill a clean 100 mm., polarimeter tube with the diluted
(or pure) essential oil. Place the filled sample tube in the polarimeter and record the measured value. Compare the calculated value
with the expected value.
Some examples;
Lemon     +57 degree  to +70 degree
Mint         -17 degree  to  -24 degree
Orange     +94 degree to +99 degree
Spearmint -45 degree  to -60 degree

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Method 2;
Measuring Refractive Index. 
The refractive index of an essential oil is a unique number that designates how the oil responds to and bends light. 
Essentially, it is a measurement that tests how the speed of light is altered when passing through the oil. An oil's
refractive index can be compared to that of a reliable sample.
Refractive indices of some essential oils:
Some examples;
Lemon     1.474 to 1.476
Lavender  1.459 to 1.464
Orange    1.470 to 1.474
Spearmint1.455 to 1.460
Rose       1.451 to 1.484

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Method 3:
Measuring Gravity (Specific Gravity).
Measured using a densitometer, the specific gravity of an essential oil is a unique number that measures the density of a particular oil
in comparison with the density of water.Specific gravity readings are measured at precise temperatures and pressures, as
temperature and pressure can impact the measurements. Particular oils have known ranges of specific gravity in which the oil
is considered to be unadulterated and pure.
Some examples:
Lemon      0.849 to 0.855
Lavender   0.870 to 0.888
Orange     0.842 to 0.846
Spearmint 0.917 to 0.934
Rose        0.848 to 0.861
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