Next, calculate the ratio of the length of the column at hand to the nominal column length, for which the method conditions
were developed. Third, multiply by the desired average carrier gas linear velocity for the method to obtain the linear velocity
goal for the specific column being installed, as shown in equation 5:
Finally, as part of verifying the setup, establish the operating or initial column temperature and then adjust the inlet pressure
as required to produce the calculated velocity.
For GC systems with electronic pressure control (EPC), the EPC system will calculate and set the correct pressure for the
desired velocity if the measured column length is entered first. However, slight variations in the column diameter from the
nominal diameter can produce a slight error in this step. The operator can approximate the apparent column diameter at this
point by comparing the measured carrier gas linear velocity to the desired value as entered into the EPC system. Because,
according to equation 2, the ratio of the velocities is equal to the square of the ratio of the diameters, a corrected diameter
can be calculated in this manner:
For the example given above, if the desired velocity was 32.64 cm/s for the 24 m × 250 μm column, but the observed velocity
was 34.9 cm/s, then the corrected column inner diameter would be:
Upon entering this corrected inner diameter into the EPC column configuration, the electronic pressure controller will adjust
the pressure downward sufficiently to produce the desired average carrier-gas linear velocity. Due to the variations in inlet
pressures from instrument-to-instrument, however, this procedure should be repeated whenever a column is set up.
Temperature and Pressure Calibration
With a better understanding of the effects of column variability on retention times, we are in a position to examine the requirements
and effects of temperature and pressure calibration in laboratories that utilize multiple gas chromatographs.
What to expect: Given that small changes in temperature or pressure can shift retention times significantly, what should gas chromatographers
expect from their instrumentation? How much pressure and temperature variation is normal between gas chromatographs that are
operating within the manufacturer's specifications? I read through a number of brochures, specification sheets, operator's
manuals, and service manuals. I found that pressure and temperature tolerances vary somewhat by manufacturer and for some
items, there was little or no information. Most GC systems produced in the past 10-15 years include provisions for oven temperature,
carrier pressure, and flow calibration via the instruments' keyboard-display and firmware. Thus, gas chromatographers can
bring these variables under some degree of control.