EZGC Method Translator and Flow Calculator Glossary
The EZGC Method Translator is a tool built for gas chromatography (GC) method development. Generally, the goal of Method Translation is to allow alteration of GC column format, carrier gas, flow, etc., while keeping peak elution order—NOT retention times—the same. (Note that Method Translation assumes that the GC stationary phase type remains the same between Original and Translation methods.)
Some of the most practical uses for Restek's EZGC Method Translator are listed below:
- Increasing speed of analysis through decreasing column length and/or decreasing inner diameter and/or switching to a faster carrier gas (e.g., going from helium to hydrogen).
- Updating the oven temperature program through Translation after column trimming for maintenance so peak elution orders do not change.
- Improving Original methods in separation and/or speed of analysis by solving for Efficiency or Speed in Translation.
- Translating methods from GC-FID (or other atmospheric outlet detector) to GC-MS (vacuum outlet) or vice versa.
Basic Navigation in the EZGC Method Translator and Flow Calculator
"White" cells are user-entry cells. "Blue" cells are locked cells that contain calculated values. In the Method Translator, the Translation's Control Parameters can be unlocked by selecting the Custom translation method in the Results section.
Highlighting numerical values using the mouse allows easy user entry of new values. A double mouse click in any user-entry cell highlights the value automatically for user entry. Hitting the Tab key while in a cell updates the cell with the user entered value and moves to the next cell for additional user entry, if necessary.
In the Control Parameters section for both the Method Translator and Flow Calculator, a double mouse click in the Outlet Flow, Average Velocity, Holdup Time, or Inlet Pressure cell will make that cell the "set point" around which the other control parameters are calculated. Column dimensions (and Temperature, in the Flow Calculator) can then be changed, and the set point value will remain fixed. A blue arrow denotes the "set point" cell.
Original / Translation
Original GC method parameters can be entered in the far left column. The EZGC Method Translator will generate a method in the Translation column that is based on the selection of Efficiency, Speed, Translate, or Custom in Results.
Helium, Hydrogen, Nitrogen
The drop-down menu allows selection of Carrier Gas, including helium, hydrogen, and nitrogen, the most commonly used carrier gases in gas chromatography. In general, hydrogen is the fastest carrier gas with no efficiency loss, followed by helium, and then nitrogen, which for optimum separation capability is quite slow. Helium is the most widely used gas for gas chromatography–mass spectrometry (GC-MS) applications.
GC column length in meters. Most new GC columns are listed with their nominal length on the box (e.g., 30 m). However, like Restek, most vendors usually provide extra column (from about 0.5 m to even 2 m or more). Accurate column length can be determined by counting the number of column loops on the cage and multiplying that number by pi, then multiplying by the column diameter on the cage (e.g., the column diameter for a Restek® GC column on a 7-inch cage is 7.08 inches, or 0.1798 m).
Inner diameter (ID) of the GC column fused silica tubing without stationary phase in mm. Restek provides this number on the original GC column box and the column tag.
Stationary phase film thickness of the GC column in µm. Restek provides this number on the original GC column box and the column tag.
Phase Ratio, or ß (beta), is calculated as the inner diameter of the column divided by 4 times the film thickness in µm. In general, a larger Phase Ratio column (i.e., thinner film) is better for analyzing semivolatile compounds, and a smaller Phase Ratio column (i.e., thicker film) is better for analyzing volatile compounds and has greater sample loading capacity. The Phase Ratio is an EZGC Method Translator–calculated value from user-entered column Inner Diameter and Film Thickness values and cannot be changed by the user.
The volumetric GC column flow rate in mL/min. The Outlet Flow is calculated from the Inlet Pressure (gauge) of the selected carrier gas, the GC oven/column temperature, and the GC column Outlet Pressure (abs). The reference temperature and reference pressure used for flow calculation are 22°C and 1 atm, respectively.
The Average Velocity or, more correctly, the average linear velocity of the carrier gas is the average speed of the carrier gas through the column in cm/sec.
The Holdup Time is the time in minutes it takes an unretained GC peak to travel the length of the column. Holdup Time is sometimes referred to as "dead time" or "void time."
Inlet Pressure (gauge) is the "head pressure" or pressure on the inlet side of the GC column. The drop-down menu allows choice of psi, kPa, bar, or atm pressure units, which are then used for both Inlet Pressure and Outlet Pressure (abs).
Outlet Pressure (abs)
Outlet Pressure (abs) is the pressure on the outlet end of the GC column in psi (or other selected units, such as kPa, bar, atm). The default value is 0.00 (zero), which is the outlet pressure of a GC column installed in a mass spectrometer since it is under vacuum. For detectors that operate at atmospheric pressure (e.g., FID, ECD, TCD, NPD, etc.), it is common to use 14.70 psi, which is the atmospheric pressure at sea level. A simple click on "Atm" or "Vacuum" below the Outlet Pressure (abs) entry area allows selection of 14.70 or 0.00 psi (or corresponding values for kPa, bar, or atm).
Isothermal, Ramps, Number of Ramps
The GC oven program can be toggled between Isothermal or Ramps (up to 4) in the Method Translator. Isothermal means that the GC oven temperature will not be programmed during the analysis. Selecting Ramps and then entering the Number of Ramps and Original oven program values allows translation of GC oven temperature programs.
Temp (°C), Hold Time (min), Ramp Rate (°C/min)
Temp (°C) is the initial oven temperature (first row) and the final temperature of any ramp used for oven programming (subsequent rows). Ramp Rate (°C/min) is the rate at which the GC oven is being programmed for any ramp used. Hold Time (min) is the initial oven temperature hold time (first row) and any final temperature hold time of any ramp used for oven programming (subsequent rows).
Constant Flow, Constant Pressure, Constant Linear Velocity
The drop-down menu allows selection of Constant Flow or Constant Pressure for the GC carrier gas. Constant Flow is the maintenance of a constant Outlet Flow of carrier gas during analysis. Constant Pressure is a constant inlet pressure (head pressure) on the GC column during analysis. Constant Linear Velocity is the maintenance of a constant Linear Velocity of carrier gas during analysis.
Solve for Efficiency, Speed, Translate, Custom
A Constant Flow solution for Speed is generally based on "Speed-Optimized Flow" (SOF), a definition for carrier gas flow from [1,3]. SOF in mL/min is: Hydrogen (10 x column ID mm); Helium (8 x column ID mm); Nitrogen (2.5 x column ID). SOF, when used with the Optimal Heating Rate (OHR, °C/min as 10/holdup time min, [2,3]), generates the best peak capacity production per time, i.e., the most theoretical peaks in a chromatogram under those conditions. Note that this concept does not always yield the best separations, but it is a very good place to start for method development. It is most practical for Phase Ratios from 125 to 625.
A Constant Flow Efficiency solution is generally based on "Efficiency-Optimized Flow" (EOF), where EOF for GC carrier gas is SOF divided by the square root of 2 [1,3]. When combined with OHR, EOF generates a better peak capacity than SOF with OHR.
The Translate option uses a combination of flow (in the case of column ID differences and/or carrier gas differences) and oven temperature program adjustments to match the Translation column efficiency to that of the Original column efficiency. Note that this does not mean the Translation chromatogram will maintain the same retention times, or the same resolution of critical pairs, as the Original chromatogram.
Custom allows entry of Outlet Flow, Average Velocity, Holdup Time, and Inlet Pressure (gauge) values in the Translation column of the Method Translator. Only the Oven Program is translated when Solve for Custom is selected.
The Run Time (min) is calculated for both Original and Translation methods from the Oven Program values.
Speed is the ratio of the Original method Run Time divided by the Translation method Run Time. Depending on the Translation, speed will either be gained (a number higher than 1) or lost (a number lower than 1).
Use FC Values for Original
"FC" is "Flow Calculator." A mouse click on this button allows quick transfer of values from the EZGC® Flow Calculator to use as "Original" values for the Method Translator.
Use FC Values for Translation
"FC" is "Flow Calculator." A mouse click on this button allows quick transfer of values from the EZGC® Flow Calculator to use as "Translation" values for the Method Translator.
EZGC Flow Calculator Glossary
Note: Where terms overlap between the EZGC Method Translator and EZGC Flow Calculator, including under Carrier Gas, Column, and Control Parameters headings, terms are not defined again here. See above for definitions of these terms.
The EZGC Flow Calculator provides the user with important information on carrier gas flow parameters as column pressures and temperatures are changed.
An important feature of the Flow Calculator is providing suggested Splitless Valve Time for splitless injection based on the generally accepted rule of sweeping the inlet liner 1.5 to 2 times with carrier gas to effect quantitative sample transfer prior to activating the purge valve (sweeping to the split vent).
The temperature of the GC inlet in °C.
The volume, or "empty space," of the GC inlet liner in mL. In a simple open liner configuration, the volume can be calculated by V = πr2h, where V = volume of the GC inlet liner in mm3 (µL), π = pi (approximately 3.1416), r = radius of inlet liner in mm (inner diameter in mm / 2), and h = height of inlet liner in mm. An example volume calculation for an HP/Agilent GC straight splitless inlet liner with 4 mm ID and 78.5 mm length (height) would be 3.1416 x 22 x 78.5 = 986 mm3 (986 µL or 0.99 mL). Any volume taken up by packing material (e.g., wool) or taper is typically ignored in the Liner Volume calculation. Restek provides inner diameters and heights for its GC inlet liners, as do most vendors. Volumes for commonly available HP/Agilent GC splitless inlet liners are listed below:
1 mm ID x 78.5 mm h = 0.06 mL
2 mm ID x 78.5 mm h = 0.25 mL
4 mm ID x 78.5 mm h = 0.99 mL
5 mm ID x 78.5 mm h = 1.36 mL
The calculated GC inlet flow in mL/min based on Column Length, Inlet Pressure (gauge), Outlet Pressure (abs), and Inlet Temperature. The Inlet Flow can either be higher or lower than the Outlet Flow depending on Column Length, Inlet Pressure, and Inlet Temperature.
Splitless Valve Time
For splitless injection only, this value is a suggested range for Splitless Valve Time in minutes based on sweeping the inlet liner volume with carrier gas flow 1.5 to 2 times prior to activating the purge valve (or opening the split vent). It is important to note that this range is calculated from accurate user entry of Inlet Temperature and Liner Volume.
Use MT Original Values
"MT" is "Method Translator." A mouse click on this button allows quick transfer of "Original" values from the EZGC Method Translator to use as values for the Flow Calculator.
Use MT Translation Values
"MT" is "Method Translator." A mouse click on this button allows quick transfer of "Translation" values from the EZGC Method Translator to use as values for the Flow Calculator.
- L.M. Blumberg. Theory of fast capillary gas chromatography. Part 3: Column performance vs. gas flow rate. J. High Resolut. Chromatogr. 22 (1999) 403–413.
- L.M. Blumberg and M.S. Klee. Optimal heating rate in gas chromatography. J. Micro. Sep. 12 (2000) 508–514.
- L.M. Blumberg. Theory of Gas Chromatography, in: C.F. Poole (Ed.), Gas Chromatography,Elsevier, Amsterdam,2012, pp. 19–78.
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