﻿ Volatile Organic Compounds - Chromatography Forum

## Volatile Organic Compounds

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### Volatile Organic Compounds

Hi,

Is there anybody working on volatile organic compounds in waste gas?

I am new on this area and i don't understand the calculation of the voc?

A calibration like 2,5ppm - 5 ppm - 10 ppm - 25 ppm - 50 ppm is too high for GC-MS?

### Re: Volatile Organic Compounds

Gas-phase ppm is not the same as condensed-phase ppm. Think of gas-phase ppm as µL analyte/Liter of gas or µmole analyte/mole gas. It comes from the ideal gas law so the ratios are the same. Take a gas that is 50 ppm acetaldehyde (MW = 44 µg/µmole) and there is 10 mL of that gas in the headspace above 10 mL of water in a 20 mL headspace vial. All of the AA starts in the vapor (at 21 °C and 1 atm pressure) and assume it is quantitatively transferred to the water:

mole gas in the vial = PV/RT = 1 atm x 0.01 L/0.0821/(273 + 21) = 4.14 x 10^-4 mole
50 µmole AA/mole gas x 4.14 x 10^-4 mole gas = 0.0207 µmole AA in vial
0.0207 µmole AA x 44 µg AA/µmole AA/10 mL = 0.091 µg AA/mL

This completely discounts the partitioning behavior of the analyte between the gas phase and the condensed phase. You can see that 50 ppm gaseous is quite a bit different than 50 ppm in a condensed phase like water.

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