GC-Calc FAQ
What is GC-Calc?
GC-Calc is a Windows executable program that allows you to calculate optimum conditions for GC and GC/MS operation. Given a particular column length, diameter, temperature, carrier gas, etc., GC-Calc will calculate the optimum inlet pressure, linear velocity, and flow rate so you can obtain maximum chromatographic efficiency for your separation. The program includes a handy plotting feature so you can see effects of experimental conditions on plate height. The calculations are accurate for open tubular gas chromatography with normal GC operation and atmospheric outlet, or GC/MS with the column outlet at vacuum. The calculations are not valid for packed column GC. Hopefully, you are not still using packed column GC!
Why do I need GC-Calc?
If you are using standard columns (e.g., 0.25 mm) with standard lengths (e.g, 30 m), you can probably just follow the manufacturers guidelines for optimum inlet pressure and linear velocity. However, let's say you want to use a shorter column, or you want to explore the use of hydrogen carrier gas. If you follow the column manufacturer's guidelines without taking into account the column length, the carrier gas properties, or whether the column outlet is at vacuum as in GC/MS, you could end up being way off from the optimum settings. Most GC column manufactures quote optimum settings for atmospheric outlet, not vacuum outlet. GC-Calc will help you calculate optimum settings for virtually any set of experimental conditions. Consider the following table which shows calculated optimum conditions using a variety of column lengths with either atmospheric or vacuum outlet (helium carrier gas, 100C, k = 10). The calculated optimum ranges indicate optimum - optimum practical inlet pressures and linear velocities.
|
Column length (m) |
Optimum Inlet Pressure (psig) |
Optimum Linear Velocity (cm/s) |
Outlet |
| 30 |
12 - 20 |
40 - 52 |
vac |
| 30 |
15 - 26 |
30 - 48 |
atm |
| 10 |
0.8 - 6 |
69 - 90 |
vac |
| 10 |
7 - 13 |
39 - 77 |
atm |
| 3 |
-6 to -3.5* |
125 - 165 |
vac |
| 3 |
2.2 - 6.4 |
44 - 125 |
atm |
*These pressures indicate vacuum inlet conditions are optimal
Where can I get GC-Calc?
You can download GC-Calc from our website. Click here to download. The file is a 2.4 MB zip file, so you'll need a utility for unzipping the download. Please review and accept the license agreement before downloading and installing.
What are the system requirements for running GC-Calc?
GC-Calc should run on any reasonably modern Pentium class Microsoft Windows (95/98/Me/NT/2000) PC.
What is the theoretical basis of GC-Calc?
The basis of GC-Calc goes back to the famous Golay equation which describes the relationship of chromatographic efficiency (plate height, H) and the average carrier gas velocity (v).
H = B/v + Cv
The B term relates to longitudinal diffusion of the solute zone, while the C term relates to the mass transfer of the solute in the gas phase. For open tubular columns employing thin films we can ignore the contribution of mass transfer of the solute in the liquid phase. Guiochon et al. later showed that a third term (Dv2) could be added to the Golay equation which accounts for extracolumn (i.e., instrumentation-induced) band broadening.
Interesting things happen when you connect a GC column outlet to a vacuum as in a mass spectrometer. The effects can be quite dramatic, particularly with short and/or wide bore open tubular columns. The effects are mainly due to the increase in the diffusion coefficient of the carrier gas at the lower average column pressures. The GC-Calc program is based on the work found in the following manuscript:
Theoretical and Practical Aspects of Short Open Tubular Columns at Subambient Pressures in Gas Chromatography/Mass Spectrometry (1.4 MB PDF file), by Mark E. Hail and Richard A. Yost, Anal. Chem., 1989, 61, 2402.
GC-Calc is the only publicly available software tool that we know of that can calculate the effects of the vacuum outlet as in GC/MS. Particularly, if you are interested in using shorter columns (and you should be), we think that you will find this to be a useful tool.
Has GC-Calc been validated with experimental results?
The theoretical basis if GC-Calc has been exhaustively tested with experimental data on real GC/MS instrumentation as described in the manuscript referenced above.
How do I determine the capacity ratio (k)?
Capacity ratio (k) is determined from a measure of the retention time of your analyte, Tr, and the elution time of an unretained peak, To (e.g., the air peak). k can be calculated by:
k = (Tr - To) / To
If you do not know the capacity ratio for your analyte, simply use the default value.
What is a typical value for extra column peak width?
Extra-column peak width (due to the instrumentation itself) can add significantly to overall band broadening, particularly at high gas velocities. Extra-column effects typically add a constant peak width to the experimental data. This is generally instrument dependent. A value of 0.4 sec is a typical measurement obtained using a common GC/MS system.
What does it mean when I get results from GC-Calc indicating negative optimum inlet gauge pressures?
Recall that a gauge pressure of zero is actually 1 atmosphere or 760 Torr. A negative optimum inlet gauge pressure indicates subambient pressure (i.e., below 760 Torr). Subambient optimum inlet pressures are predicted as you decrease the column length in GC/MS. You get to subambient optimum inlet pressures very easily with a carrier gas like Hydrogen, which has a very high diffusion coefficient and low viscosity. For Helium carrier gas, if you use a column shorter than ~10 m, subambient inlet pressures are required to operate the column at peak efficiency. For Hydrogen, you would need a column ~15 m long to avoid subambient optimum inlet pressures. Use GC-Calc to explore these scenarios!
What do the ìOptimizeî radio buttons do?
When optimization is set to "Yes" , GC-Calc iteratively calculates plate heights over a range of inlet pressures. The optimum inlet pressure is the pressure obtained at the minimum plate height (H), or maximum number of theoretical plates (N). The program also returns the optimum linear velocity and the estimated peak width of the analyte. When optimization is set to "No", GC-Calc simply calculates plate height and number of theoretical plates using the current inlet pressure set in the Column Conditions box.
What is meant my ìoptimum practical linear gas velocityî?
The optimum practical linear gas velocity or OPGV is defined as the carrier gas velocity that yields the maximum number of theoretical plates per unit time. OPGV is determined by dividing the number of theoretical plates (N) by the retention time (Tr) at each linear velocity during the iterative calculation. The OPGV is determined at the maximum N/Tr. This is really where you should be running, since you obtain the best resolution per unit of analysis time. The chromatographic resolution lost by operating at the slightly higher OPGV is typically only ~10% or less relative to operation at the normal optimum linear velocity.
What are some operating tips reinforced by GC-Calc?
- Vacuum outlet operation is our friend! When the GC column outlet is at vacuum, the diffusion coefficient of the carrier gas increases allowing you to operate at higher linear velocities and obtain shorter analysis times without sacrificing resolution. The effect is more pronounced as the column is shortened. At some point, depending on your column i.d. and carrier gas, very short columns will require subambient inlet pressures for optimum operation. A method for operating under these subambient conditions is outlined in the manuscript referenced above. Another practical aspect of short-column operation is that solutes elute at much lower temperatures. This may be useful for compounds that are thermo-labile or are otherwise highly retained on traditional length columns. Weíve been able to analyze solutes otherwise considered ìnon-GCableî using short-columns (e.g., 3 meters) in GC/MS.
- Use a low viscosity carrier gas. You can also lower your average column pressure further by using a low viscosity carrier gas, like Hydrogen. For this reason, Hydrogen will always provide the highest optimum linear velocities and shortest analysis times relative to the other carrier gases. If you decide to use Hydrogen, make sure to exercise caution and ensure that your system is leak free.
- Use the shortest possible column for your separation! Even though the number of theoretical plates increases directly with column length, resolution only increases with the square root of column length. By using a column half as long you can cut your analysis time in half and only give up a little in resolution. A shorter column may also help you elute components that are difficult to analyze as discussed above.
What's GC-Calc look like?
Screen shot of the main window used for parameter entry:
Screen shot of the results window:
Screen shot of the plot window:
Version history:
-
Version 1.2, 4/23/01 - Enhanced plotting features and added an installer, which made the download larger. Earlier releases may not have worked on PC's without the required DLL's and OCX's. This should not be a problem now. > Download here <
 
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