Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 183

7.6 SAMPLE PREPARATION FOR VOC, AIR
AND STACK GAS EMISSION

The extraction procedures discussed in Section 7.3 do not apply to VOCs (SPME
is an exception since it can be directly used to sample VOCs in headspace). A list
of EPA sample preparation methods for VOCs was provided in Chapter 5
(Table 5.4) and the details are described below. As can be seen from the following
discussions, methods for VOCs can be divided into two groups depending on the
types of VOCs, that is, purgeable VOCs and nonpurgeable VOCs (water soluble
VOCs). The later is of minor importance in routine analysis, and will be briefly
introduced.

7.6.1 Dynamic Headspace Extraction
(Purge-and-Trap)

In the purge-and-trap (P&T), also called dynamic headspace extraction,
separation is carried out by purging the sample, sometimes done while heating,
with an inert gas such as He or N 2 and trapping the volatile materials in a sorbent
column (Tenax, silica, charcoal). The trap is designed for rapid heating so that it
can be desorbed directly into a GC column (Fig. 7.14a). The purge-and-trap is
applicable for compounds with boiling points below 200 C and are insoluble or
slightly soluble in water. The purge efficiency can be improved for water soluble
analytes, for example, ketones and alcohols, when purging at an elevated
temperature of 80 C as compared to 20-40 C. Problems are often encountered
with the low purging efficiency of methyl t-butylether(MTBE)andrelatedfuel
oxygenated compounds such as ethyl t-butyl ether (ETBE), t-amyl methyl ether
(TAME), diisopropyl ether (DIPE), t-amyl ethyl ether (TAEE), t-amyl alcohol
(TAA), and t-butyl alcohol (TBA). These compounds are classified as volatiles

(a) (b)

Purge mode Desorb mode

To Detector

Helium Helium

Six-port
valve

GC column Trap

}
}

Gas

Sample

GC oven

VOCs

Liquid

Purging tube

Desorb
gas

Vent

Sample

Figure 7.14 Sample preparation for VOCs: (a) Dynamic headspace extraction (purge-and-trap), and
(b) Static headspace extraction (SHE)