Benefits

• Solvent consumption reduced by 50%
• Sensitivity increased up to x2
• Uses standard analytical HPLC equipment
• Significant cost-saving
• Performance comparable to standard 4.6 mm
 

Initial interest in the use of analytical columns of narrower bore than the industry standard 4.6 mm arose for scientific reasons. The ability to couple HPLC with techniques capable of providing characterisation data of the solute molecules necessitated the use of lower solvent volumes. Increased assay sensitivity and the need for less sample were added features.
The limited general availability of low dispersion volume equipment and the greater difficulties of manufacturing narrow bore HPLC columns have restricted developments. However the cost and environmental issues associated with the purchase and disposal of solvents is becoming increasingly important.
Consequently the utility of the intermediate 3.2 mm i.d. column is now being strongly recommended by a number of organisations. The choice represents a compromise between using standard 4.6 mm i.d. columns and microbore columns of 2.1 mm i.d. or less.
The 3.2 mm i.d. columns offer a 50% saving in solvent consumption for the same linear dynamic flow without necessarily requiring a change to lower dispersion volume injectors and flow cells. For satisfactory use of 2.1 mm or lower i.d. columns such a change is essential. When a 3.2 mm i.d. column is used with a Rheodyne valve injector model 7125 or 7725 and an 8µl flow cell some loss in performance is observed (Table 1). The loss decreases with increase in retention time.
In practice chromatograms obtained from 3.2 and 4.6 mm i.d. columns are very similar in appearance (Figure 1 below ). 

Guard cartridge columns
Our  universal guard cartridge system is recommended for use with medium bore columns.

Equipment

A Rheodyne 7125 or 7725 valve injector and an 8µl flow cell can be used with 3.2 mm i.d. columns containing 5µm particles. For critical separations a Rheodyne 8125 valve injector and a micro flow detector cell (<5µl) are recommended.
The loss in efficiency of biphenyl and phenanthrene peaks at various retention times in going from 4.6 to 3.2 mm i.d. columns using the above standard equipment is shown below (Table 1). 

Table 1

Optimum flow

A flow rate of 0.48 ml/min through a 3.2 mm i.d. column gives the same relative flow rate as 1 ml/min through a 4.6 mm i.d. column. The optimum flow rate for the medium bore column will be < 0.4 ml/min. 
 

Sample size

Existing methods can be readily adapted for use with 3.2 mm columns at the appropriate flow rate. However as an effective increase in detector sensitivity will be observed, care should be taken not to exceed detector linearity. It may be necessary to dilute samples to lower concentration levels.

Figure 1. Comparison between 4.6 mm and 3.2 mm i.d. columns
A. 4.6 mm i.d. B. 3.2 mm i.d.