Accounting for Atmospheric Pressure When Using EZGC Method Translator and Flow Calculator and the ProEZGC Chromatogram Modeler

Modern GC’s are equipped with advanced pneumatics controls, which allow for accurate control of flow rates and pressures.  In order to accurately control or calculate flow rates, atmospheric pressure must be measured by the GC, since this determines the outlet pressure of non-vacuum detectors.

Restek’s free tool, the EZGC Method Translator and Flow Calculator, has many beneficial uses to chromatographers, including translating methods, calculating column length, calculating splitless hold time, calculating average linear velocity, etc.  In order to make accurate calculations, outlet pressure must be entered. Setting an accurate outlet pressure is also important when using the ProEZGC Chromatogram Modeler, as this will lead to the best accuracy of retention time predictions.

Both the EZGC Method Translator and Flow Calculator and the ProEZGC Chromatogram Modeler allow you to select your outlet pressure as either vacuum or atmospheric (Atm), based on the detector (Figure 1).  For a mass spectrometer, you should select vacuum outlet pressure whereas, for a detector such as FID, select atmospheric outlet pressure.  Note, that when you do this, a default value of 14.70 psi (equivalent to 101.33 kPa, 1.01 bar, or 1 atm, depending on your chosen units) is auto-filled.  This equates to standard pressure at sea level.

Figure 1: Outlet pressure is found under the “Control Parameters” section. If you select “Atm” for an atmospheric pressure outlet, a default value of 14.7 psi will be automatically entered. You can manually change this value to account for elevation above sea level. Also note that pressure units can be changed using the drop-down menu located next to “Inlet Pressure”.

Increases in elevation lead to a decrease in atmospheric pressure.  For example, Denver, Colorado is at an elevation of 5,280 feet.  Here the atmospheric pressure is 12 psi, compared to the 14.70 psi found at sea level.  While this may not seem significant, it is enough of a difference to affect the accuracy of flow calculations and retention time predictions.  If you were to enter in the same inlet pressure as a lab at sea level, your average linear velocity would be higher since the pressure at the outlet of the column is lower.

Figure 2 illustrates the difference outlet pressure can have on flow and therefore analyte retention.  In the left chromatogram, the outlet pressure is set at sea level (14.70 psi) and inlet pressure is fixed at 15 psi.  In the right chromatogram, the outlet pressure is entered for an elevation of 10,000 feet (10.1 psi), say if you were in Breckenridge, Colorado, also with a fixed inlet pressure of 15 psi.  While both figures have the same inlet pressure and temperature program, the difference in outlet pressure leads to a difference in average linear velocity of carrier gas, causing the retention time of deltamethrin to vary by 45 seconds between the two chromatograms.

Figure 2: ProEZGC Chromatogram Modeler chromatograms demonstrating the difference that outlet pressure can have on retention time. The model on the left illustrates the retention time of deltramethrin at sea level, when inlet pressure is set to 15 psi. The model on the right illustrates the retention of deltramethrin at 10,000 feet, with the same conditions, except for outlet pressure, which is 10.1 psi, due to the elevation difference. Deltramethrin has a retention time difference of 45 seconds between these two models.

If your lab is at a higher elevation, you should manually enter an accurate value for outlet pressure when performing calculations in both the EZGC Flow Calculator, as well as the ProEZGC Chromatogram Modeler, rather than use the default “standard pressure” value.  Table 1 gives some example values for atmospheric pressure at various elevations.

Table 1: Atmospheric pressure vs altitude.

Keep in mind that modern GC’s should measure and account for atmospheric pressure.  Some GC’s will even display the atmospheric pressure, which you can then use for accurate calculations with the EZGC software.