Fast GC is a technique that allows you to reduce the analysis time while keeping an adequate resolution power, thus increasing your throughput. The technique can be applied to simple through to complex mixtures and provides 3-10x faster analysis compared to conventional GC.

Fast GC utilises shorter capillary GC columns having a smaller internal diameter (usually referred to as “narrow-bore” columns). Typically a 100μm ID x 5 or 10m length columns are used. A high temperature ramp rate (usually more than 15°C/min) and a fast data acquisition rate is also needed.

The theoretical separation power of a gas chromatographic capillary column is the number of the theoretical plates (N). N is calculated as

N=L/H

where L is the column length and H is the height of the theoretical plates that can be approximated to the column’s internal diameter (ID).

Reducing the internal diameter of the column whilst keeping the value of N constant means we can shorten the length of the column as well. In fact a 100μm ID x 10m column has 100,000 theoretical plates, similar to a 25m x 0.25mm ID conventional capillary column.

Column efficiency (N per m) is inversely proportional to column diameter; smaller diameter columns have higher theoretical plates per meter. However resolution is a square root function of the theoretical plate number. Doubling column efficiency theoretically increases resolution only by 1.41 times (the square root of 2), but is usually observed to be lower in practice.

Were there no downsides to reducing column diameters (and length), they would be in universal use in GC and GC-MS systems. However instrument performance is important and limits the adoption of Fast GC.

Reducing column diameter influences the following.

• efficiency: imroved
• retention: shortened
• pressure: increased
• carrier gas flow rate: reduced
• capacity: reduced