Handling, Preparation and Storage of Standards

Calculations

The concentration units for chemical standard solutions used for ICP applications are typically expressed in µg/mL (micrograms per milliliter) or ng/mL (nanograms per milliliter). For example, a 1000 µg/mL solution of Ca⁺² contains 1000 micrograms of Ca⁺² per each mL of solution and a 1 µg/mL solution of Ca⁺² contains 1000 ng of Ca⁺² per milliliter of solution. To convert between metric concentration units the following conversions apply:

Table 3.1: Mass portion of concentration unit where g = gram

Table 3.2: Volume portion of concentration unit where L = liter

The difference between ppm and µg/mL is often confused. A common mistake is to refer to the concentration units in ppm as a short cut (parts per million) when we really mean µg/mL. One ppm is in reality equal to 1 µg/g. In similar fashion ppb (parts per billion) is often equated with ng/mL. One ppb is in reality equal to 1 ng/g. To convert between ppm or ppb to µg/mL or ng/mL the density of the solution must be known. The equation for conversion between wt./wt. and wt./vol. units is:

(µg/g) (density in g/mL) = µg/mL

and/or

(ng/g) (density in g/mL) = ng/mL

Therefore, if we have a solution that is 1000 µg/mL Ca⁺² and know or measure the density to be 1.033 g/mL then the ppm Ca⁺² = (1000 µg/mL) / (1.033 g/mL) = 968 µg/g = 968 ppm.

When making dilutions the following equation is useful:

For example, to determine how much of a 1000 µg/mL solution of Ca⁺² required to prepare 250 mL of a 0.3 µg/mL solution of Ca⁺² we would use the above equation as follows:

(mL_A)(1000 µg/mL) = (250 mL)(0.3 µg/mL)

(mL_A) = [(250 mL)(0.3 µg/mL)]/ (1000 µg/mL)

(mL_A) = 0.075 mL = 75 µL

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