Fast GC columns
Fast GC - all you need to know
What is Fast GC
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.
What do you need for Fast 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
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
Column: usually columns 0.25mm/0.32mm I.D. x 25m, 30m or 50m length
Temperature rates: 1 - 15°C/min.
Injection: using standard injection techniques, is possible to inject quite large quantities (typically 1 - 2μL of a diluted solution with a split ratio of 1:20)
Carrier Gas: typical flows are not less than 0.8mL/min with head pressures of 40 - 130kPa depending on column dimensions and carrier gas type
Peak width: 2 - 5 seconds
Detector: any type of detector for GC can be used
Analysis time: 20 - 60 min
Column: usually columns 0.05mm/0.10mm I.D. x 2.5m, 5m or 10m length
Temperature rates: 15 - 60°C/min.
Injection: injection quantity has to be at least 10x less than conventional GC. Usually split ratio of 1:100 or higher are used with diluted solutions (< 100ppm).
Carrier Gas:typical flows do not exceed 0.9 - 1mL/min with higher head pressures (200-250kPa for 0.10mm I.D. columns, and 300kPa or more for 0.05mm I.D. columns) depending on column dimensions and carrier gas type
Peak width:0.5 - 2 seconds.
Detector: any type of detector for GC can be used. The acquisition frequency should be at least 50Hz
Analysis time: 1 - 10 min.