Researchers typically use micrograms per milliliter (µg/mL) standards to calibrate gas chromatography-mass spectrometry (GC-MS) instruments for the analysis of polycyclic aromatic hydrocarbons (PAHs) in air samples. However, the measured PAH levels in ambient air are commonly expressed in nanograms per cubic meter (ng/m³). How can we convert these calibration standards from micrograms per milliliter to determine PAH concentrations in nanograms per milliliter (ng/m³) for a GC-MS response?
1 µg/mL is equal to 1 billion (109) ng/m3. So, to convert µg/mL to ng/m3, you have to multiply the number by 109.
This is because 1 µg has 103 ng, therefore 1 µg/mL is equal to 103 ng/mL. Furthermore, 1 m3 has 1 million (106) mL (this is because mL=cm3). So 103 ng/mL is equal to 103 ng/10-6 m3 = 109 ng/m3 which is the same as 1 µg/mL.
Sampling air for the analysis of PAHs is commonly done by pumping air through a sampling device and collecting the PAHs on a sorbent media. Before the analysis by GC-MS the PAHs are desorbed from the sorbent media by a solvent, resulting in a solution contaning the analytes. This solution is analysed by GC-MS.
As an example: 100 dm³ air is sampled and the PAHs collected on a sorbent. For the desorbtion 10mL solvent are used. Now we obtained a 10mL solution containing the PAHs from 100dm³ air (under the assumption of 100% retention of the PAHs on the sorbent as well as 100% desorbtion efficiency by the solvent = no losses of the analytes during these processes). Let’s say our GC-MS analysis shows for naphthalene a concentration of 0,05µg/mL in the solution. This equals to 0,5µg naphthalene in the 10mL solution, which represents the amount sampled from 100dm³ of air. So the concentration of naphthalene is 0,5µg/100dm³ air. This equals to 5µg/m³ or 5000ng/m³. Hope this helps.
Noel W Davies, of course, I agree that Peter Volansky's answer is correct. However, you get to the same answer if you multiply the initial concentration in µg/mL by 109 and then divide it with the concentration factor during the sample preparation, which is 104 in Peter's example (100 dm3 of air to 10 mL solvent).
What I've written is just a way to convert these units for a concentration in either air or solution, but you have to include the concentration factor in your calculation if there was any concentration (which is almost always in air analysis).
Of course, one can work with the mass of the analyte like Peter did in his example and therefore exclude the dilution/concentration factors.
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