Chemistry Reference
In-Depth Information
Autosampler
PC
SL1
P4
SV1
Sample
Detection unit
Conv
Ref
SL2
RC
HC
P1
R1
D
W
Condenser
SV2
R2
P2
P7
P5
P6
W
P3
W
R3
R4
Distillation unit
Figure 8.6
Schematic diagram of an automated microdistillation/flow-injection system. Conv, converter; D,
detector; HC, heating coil; P1-P7, pump; RC, reaction coil; Ref, condenser; R1-R4, reservoir; SL1, SL2, sample
loop for distillation and detection, respectively; SV1, SV2, six-way valves; T, thermostated bath; W, waste.
double circuit: one for the carrier gas (generally helium) and the other for the auxiliary gas, which is used to
pressurize the vial. The two circuits are connected by a six-way valve which operates automatically as
required in each step of the head-space procedure (basically, conditioning, pressurization, sample-loop filling,
injection and purging). Automating head-space procedures increases their reproducibility, and reduces
operator involvement and risks. Modern head-space units use relatively small volumes: typically less than
5 ml for samples and 10 ml or less for vials.
Pawliszyn
et al
. [27, 28] combined head-space separation and hollow-fibre membrane extraction in a
single module to develop the simplest, least problematic choice by virtue of its avoiding the need for a
sampling pump and a flow-meter. In addition, head-space methodology avoids plugging of the extraction
system by placing the membrane above the sample without physical contact. The head-space membrane
module can be made rugged and suitable for long-term field monitoring applications. MESI systems can be
operated in a batch or multiplex mode. Unlike conventional single-injection or batch GC, a multiplex approach
introduces a random series of sample concentration pulses through the inlet of the GC column, which
responds to each concentration pulse independently. Component peaks from different injection pulses overlap
to form a multiplex chromatogram at the GC detector. The multiplex technique adds the response obtained
from a multiplicity of small samples to give a response reflective of a single large sample, thus increasing the
sensitivity of the analysis. However, the most salient feature of the multiplex technique as regards
environmental friendliness is the sorbent interface, which can be much simpler than in batch MESI.
