Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 185

determined from static headspace analysis using GC, the VOCs concentration in
thesampleis:

1
K þb

C 0 ¼ C g ð7 : 21Þ

Eq. 7.20 also indicates that a compound with a higher K value (hence more volatile)
will result in high vapor phase concentration to be detected, and a low headspace to
sample volume ratio (b) will result in higher concentrations of volatile analytes in
the gas phase and therefore better sensitivity. This also implies that compound with a
very low volatility cannot be analyzed by static headspace analysis.

7.6.3 Azeotropic and Vacuum Distillation

What if a volatile compound is not purgeable? For these compounds, the purge-and-
trap method would not be applicable. The azeotropic distillation is designed for
nonpurgeable, water soluble, and volatile organic compounds. These are mostly
small molecules, including alcohol, aldehyde, and ketone as specified in EPA
Method 5031 (Table 7.3).
An azeotrope is a liquid mixture of two or more substances which behaves like a
single substance, in that it boils at a constant temperature and the vapors released
have a constant composition. Azeotropic distillation is non-conventional distillation
technique, which uses the ability of selected organic compounds to form binary
azeotropes with water to facilitate the separation of the compounds from a complex
matrix. The azeotropic macrodistillation system typically consists of a Vigreux
column and a modified Nielson-Kryger apparatus (Fig. 7.15). The polar VOCs distill
into the distillate chamber for 1 h, and are retained there. The condensate overflows
back into the pot and contacts the rising steam. The VOCs are stripped by the steam
and are recycled back into the distillate chamber.
Another distillation apparatus called vacuum distillation (EPA 5032 for details)
can be used to separate organic compounds that have a boiling point below 180 C
and are insoluble or slightly soluble in water (such as BTEX compounds). In vacuum
distillation, sample chamber pressure is reduced using a vacuum pump and remains
at approximately 10 torr (vapor pressure of water) as water is removed from the
sample. The vapor is passed over a condenser coil chilled to a temperature of 10 C

Table 7.3 Volatile and non-purgeable compounds
Acetone

Acetonitrile Acrylonitrile

Allyl alcohol 1-Butanol t-Butyl alcohol

Crotonaldehyde 1,4-Dioxane Ethanol

Ethyl acetate Ethylene oxide Isobutyl alcohol

Methanol Methyl ethyl ketone Methyl isobutyl ketone

n-Nitroso-di-n-butylamine Paraldehyde 2-Pentanone

2-Picoline 1-Propanol 2-Propanol

Propionitrile Pyridine o-Toluidine