The total length of your capillary column in meters.
The internal diameter (ID) of the column in millimeters.
Retention time of an unretained compound (e.g., Methane or Air) ($t_M$).
Average Linear Velocity ($\bar{u}$):– cm/sec
Calculated Flow Rate ($F$):– mL/min
Column Dead Volume ($V_M$):– mL
Understanding GC Flow Rate and Linear Velocity
In Gas Chromatography (GC), the flow of the carrier gas (mobile phase) is a critical parameter that directly influences the separation efficiency, retention times, and peak shapes. While modern GC systems often allow you to set flow rates digitally, verifying these values or calculating them based on the dead time of an unretained peak remains a fundamental skill for method development and troubleshooting.
Why Use Linear Velocity ($\bar{u}$) instead of Flow Rate?
Volumetric flow rate (mL/min) changes with pressure and temperature, making it inconsistent as the gas moves through the column. Average Linear Velocity (cm/sec), however, describes the speed at which the carrier gas travels through the column. It is the preferred metric for optimizing efficiency because the Van Deemter equation—which relates efficiency to flow—is defined in terms of linear velocity.
How to Calculate GC Flow Rate
The most accurate way to determine the average linear velocity is by injecting a non-retained compound (like methane or air) and measuring its retention time, known as the Dead Time ($t_M$) or Hold-up Time.
Linear Velocity Formula:
$\bar{u} = \frac{L}{t_M}$
Where:
$\bar{u}$ = Average Linear Velocity (cm/sec)
$L$ = Column Length (cm)
$t_M$ = Dead Time (seconds)
Once the velocity is known, the volumetric flow rate can be approximated using the column's cross-sectional area:
Where:
$F$ = Flow Rate (mL/min)
$r$ = Radius of the column (cm)
$L$ = Column Length (cm)
$t_M$ = Dead Time (min)
Input Parameters Explained
Column Length: Usually 15m, 30m, or 60m. Note that trimming the column for maintenance changes this value and affects flow calculations.
Internal Diameter (ID): Common sizes are 0.18mm, 0.25mm, 0.32mm, and 0.53mm. Narrower columns generally require higher pressures to maintain flow.
Dead Time ($t_M$): The time it takes for a molecule that does not interact with the stationary phase to travel from the inlet to the detector.
Optimal Linear Velocities
Different carrier gases have different optimal velocities for maximum efficiency (HETP):
Nitrogen: ~12 cm/sec (very slow, steep loss of efficiency at high speeds)
Helium: ~20-40 cm/sec (standard carrier gas)
Hydrogen: ~40-60 cm/sec (fastest separations, flat Van Deemter curve)
function calculateGCFlow() {
// Get input values
var lengthM = parseFloat(document.getElementById('gc_col_length').value);
var diameterMm = parseFloat(document.getElementById('gc_col_diameter').value);
var deadTimeMin = parseFloat(document.getElementById('gc_dead_time').value);
// Validation
if (isNaN(lengthM) || isNaN(diameterMm) || isNaN(deadTimeMin) || lengthM <= 0 || diameterMm <= 0 || deadTimeMin <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// Conversions
// Length: m to cm
var lengthCm = lengthM * 100;
// Time: min to sec
var deadTimeSec = deadTimeMin * 60;
// Diameter: mm to cm (Radius calculation)
var radiusMm = diameterMm / 2;
var radiusCm = radiusMm / 10;
// — CALCULATIONS —
// 1. Average Linear Velocity (u bar)
// u = L (cm) / tM (sec)
var velocity = lengthCm / deadTimeSec;
// 2. Column Dead Volume (Vm)
// Volume = pi * r^2 * L
// Result in cm^3 (which is mL)
var columnArea = Math.PI * Math.pow(radiusCm, 2);
var deadVolume = columnArea * lengthCm;
// 3. Flow Rate (F)
// Flow = Dead Volume (mL) / Dead Time (min)
var flowRate = deadVolume / deadTimeMin;
// Display Results
document.getElementById('res_velocity').innerHTML = velocity.toFixed(2) + " cm/sec";
document.getElementById('res_flow_rate').innerHTML = flowRate.toFixed(2) + " mL/min";
document.getElementById('res_dead_vol').innerHTML = deadVolume.toFixed(3) + " mL";
// Show result box
document.getElementById('gc_results').style.display = 'block';
}