Page 308 - Tandem Techniques

Page 292

carousel was controlled automatically and there were only three positions where specific sampling
activities took place. In the first position the eluent was deposited on the potassium chloride directly
from the column, until the powder was wetted and the halide was saturated with mobile phase. The cup
was then moved to the second position, where a stream of air was drawn through the potassium chloride
to remove the solvent. After drying, the cup the was moved to the third position where infrared light
was directed through the halide, and the spectrum was taken. The use of the carousel containing
potassium chloride powder, together with the evaporation of the solvent and thus concentrating the
solute, certainly increased the sensitivity of the LC/IR combination. However, the intermittent nature of
the sample collection made the system unsuitable for modern LC columns, where many peaks can be
eluted in a few seconds.

Initially, off line sample collection of chromatographic eluents for subsequent IR examination was
employed more as an expedient than as a chosen method. Sample collection provided an opportunity to
remove interfering solvents from the sample before the IR spectra were obtained. In fact, the main
advantage of the original carousel interface of Kuehl and Griffiths [3] was to eliminate the mobile
phase. The carousel, and similar types of interface, act basically as a chromatographic 'memory',
collecting all the solutes that are eluted as a 'physical' chromatogram, consisting of localized masses of
solutes, deposited on the transport medium. The concept of a chromatographic memory was first
introduced by Karmen [4], who used a wire transport detector to collect samples of eluent onto the wire
surface, which was then stored on a reel. Subsequently, the wire was unwound from the reel, and passed
continuously through the flame of a flame ionization detector, producing a record of the separation. The
most effective LC/IR interfaces are based on this principle, but the time interval between collection and
measurement varies somewhat between different interfaces.

The Development of LC/IR Transport Interfaces

Jino and Fujimoto [5,6] adopted a similar approach to that of Kuehl and Griffiths, but employed a
potassium bromide plate as the transport system.