Instant Notes: Analytical Chemistry

Alternative methods of sample injection are summarized below and in Table

1 , with schematic diagrams in Figures 2and 3.

● Split injection is used with capillary columns to prevent overloading the

stationary phase with sample. The split-point is a centrally positioned

hollow needle that allows a small part of the injected sample (2% or less) to

reach the column and vents the remainder to the atmosphere via a control

valve (Fig. 2). Sensitivity is less than with splitless injection because of the

140 Section D – Separation techniques

Injection Septum

purge

valve

Carrier

inlet

Dump valve

Control

valve

Adsorption tube

Capillary column

Fig. 2. Schematic diagram of a split/splitless injection

port. Reproduced from A. Braithwaite & F.J. Smith,

Chromatographic Methods, 5th edn, 1996, first published

by Blackie Academic & Professional.

Table 1. Gas chromatography sample injection systems

Type of Injection Method of injection Advantages or disadvantages
column system

Capillary Split Carrier-gas split 1 : 10 to 1 : 500 to Overloading of column avoided, but
reduce the amount of sample sensitivity reduced and may result in
entering the column by 90% or more discrimination between solutes with
different boiling points
Splitless Whole of injected sample condensed Increased sensitivity, but limited to low
on cooled top of column, then levels of solutes in sample, and may
released by heating broaden bands
On-column Sample condensed in cooled zone Increased sensitivity, minimal thermal
at top of column, then volatilized by degradation of solutes, no discrimination
programmed heating effects

Packed Flash-vaporization Sample injected into zone heated to Rapid volatilization of sample, but
20–50∞C above column temperature thermal degradation of some solutes
may occur
On-column Sample injected onto top of Increased sensitivity, minimal thermal
packed bed degradation of solutes, no discrimination
effects