How to Achieve Higher Injection Volumes

The standard configuration of the Multisampler can inject a maximum volume of 20 μL (G7167B and G7137A) or 100 μL (G7167A and G5668A) with the standard loop capillaries. To increase the injection volume, a Multidraw Kit can be used, where extra volume is created by installing an extension capillary between the needle seat and the injection valve. Four different kits are available: the Large Volume Injection Kit (G4216-68711, providing 80 μL of extra volume) and the Multidraw Kit (G7167-68711, providing either 400  or 1400 μL of extra volume) for use with G7167A and G7167B; the Bio-inert Multidraw kit (G5667-68711, providing either 250  or 1000 μL of extra volume) for use with G5668A; and the Bio LC Multidraw kit (G7137-68711, providing either 400  or 1400 μL of extra volume) for use with G7137A.

For higher injection volumes, larger analytical heads and sample loops can also be installed: 100 μL Analytical Head with 40  or 100 μL loops (for G7167B), and 900 μL Analytical Head with 500  or 900 μL loops (for G7167A and G7167B).

The 900 μL analytical head can also be combined with the 1400 μL Multidraw Extension Seat Capillary Kit for achieving the maximum volume of 1800 μL. In this configuration, the pressure limit is 400 bar.

Whenever a method is scaled down from a larger column to a smaller column it is important that the method translation allows for reducing the injection volume in proportion to the volume of the column to maintain the performance of the method. This keeps the volume of the injection at the same percentage volume with respect to the column. This is particularly important if the injection solvent is stronger (more eluotropic) than the starting mobile phase and any increase will affect the separation particularly for early running peaks (low retention factor). Sometimes, it is the cause of peak distortion and the general rule is to keep the injection solvent the same or weaker than the starting gradient composition. This has a bearing on whether, or by how much, the injection volume can be increased. The user should check for signs of increased dispersion (wider or more skewed peaks and reduced peak resolution) when trying to increase the injection size. If an injection is made in a weak solvent, the volume can probably be increased further because the effect will be to concentrate the analyte on the head of the column at the start of the gradient. Conversely if the injection is in a stronger solvent than the starting mobile phase, then increased injection volume will spread the band of analyte down the column ahead of the gradient resulting in peak dispersion and loss of resolution.

Perhaps the main consideration in determining injection volume is the diameter of the column as this has a big impact on peak dispersion. Peak heights can be higher on a narrow column than with a larger injection on a wider column because there is less peak dispersion. With 2.1 mm i.d. columns typical injection volumes might range up to 5 to 10 µL but it is very dependent on the chemistry of the analyte and mobile phase as discussed earlier. In a gradient separation, injection volumes of about 5 % of the column volume might be achieved while maintaining good resolution and peak dispersion. One way to achieve larger injections is to use a trapping column selected by a switching valve to capture and concentrate the injection before switching it, i.e. injecting it, onto an analytical column, see Sample Enrichment. The valve can be conveniently located in the Multicolumn Thermostat.

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