Pro EZGC Chromatogram Modeler Help

Table of Contents

• What is Pro EZGC?
• What’s New in the Pro EZGC Modeler?
• The Pro EZGC Interface
  • The Compounds Tab
  • The Conditions Tab
  • The Refine Oven Program Button
  • The Model View
  • Compound Structure View and Mass Spectrum
  • The My EZGC Tab
  • Printing Your Model

What is Pro EZGC?

The Pro EZGC chromatogram modeler is an advanced tool for virtually modeling gas chromatography (GC) separations. This free, online simulator allows users to simply input a compound list and instantly receive separation conditions that can be implemented directly in the lab. Results can also be further refined to meet specific analytical needs. No experimental input is needed because EZGC models are based on robust algorithms and experimental data already generated by Restek’s chemists.

What’s New in the Pro EZGC Modeler?

The latest version of Pro EZGC brings detector selection that allows you to model for both GC and GC-MS detection. With GC detection, Pro EZGC behaves as it has in the past, separating targeted compounds by retention time. With GC-MS detection, compound targeting is focused only on those compounds in isobaric groups.

This new version also has a refined Conditions tab interface, to enhance usability.

The Pro EZGC Interface

The Compounds Tab

Your work in the Pro EZGC modeler starts in the Compounds tab.

Search by Name or CAS #

Enter or copy and paste compounds into the Search by Name or CAS # list field, one per line. CAS numbers can be used as well. Note that the modeler does not currently support language variations on compound names, so use the English spelling when possible.

New to Pro EZGC is being able to choose GC or GC-MS as your detection method. GC detection is the default and attempts to separate all of your compounds. With GC-MS, the detector looks for isobaric groups in your compound list and attempts to separate only those compounds.

When you have your compounds entered and detector selected, click the Solve button. The modeler will attempt to find results on multiple phases. If more than one solution is provided, select your phase of choice from the drop-down menu.

The modeler will provide results on multiple phases when possible.

To clear your existing selections, click the Clear link beside the Solve button.

If there has been a misspelling of a compound name, the modeler will provide an alternate spelling if available. To use the alternate spelling, click its link, and it will alter the misspelled name in your list. Hit the Solve button to rerun your search.

The modeler will provide spelling suggestions when available.

Search by Phase

Alternatively, you can click the Search by Phase>> link to get more targeted results (click Search by Name or CAS #>> to return).

You can quickly toggle to Search by Phase from the header bar.

From the Search by Phase view, first choose your phase from the Phase drop-down menu, and then choose your compound class from the Library drop-down menu. A list will appear showing all the compounds available for testing in that compound class.

Next, choose your detection mode, GC or GC-MS. GC detection targets all selected compounds while GC-MS only targets—and only allows targets—of compounds appearing in isobaric groups.

To select a compound for modeling, click on its entry in the list. (Clicking the entry again deselects the compound.) When you have your selections made, click the Solve button.

If you need to maintain a minimum resolution between specific compounds, click on the checkbox in their rows. If no compounds are specifically targeted, the modeler will try to resolve all selected compounds.

 

Click the checkbox in a compound's row to target it for resolution.

Note that run times can vary significantly when you target specific compounds for resolution, and the resolution of other compounds may be sacrificed to gain resolution of your targeted compounds. Because of this, if you are targeting a subset of your full list, be sure to target all the compounds that you wish to resolve.

The Filter Compounds field can be used to quickly filter the list for a compound by name or CAS number. Once you are done, click the “X” in the Filter Compounds field to clear your search and return to the full list. Filtering in this way will not remove any of your existing selections.

You can search for a compound by name or CAS # in the Filter Compounds field.

Searching by synonym is also supported, so you can quickly find compounds in our libraries.

Compounds in the library can be found by their common names and synonyms.

To clear existing compound selections, click the Clear link in the Compounds Selected row of the legend below the compound list or choose a new phase or compound class from the Phase and Library drop-down menu. All targeted compounds can be cleared via the Clear link in the Isobars/Compounds to Resolve row of the legend.

When the detector is set to GC-MS, isobaric compounds are preselected to resolve and the modeler will try to separate any isobars in the selected compound list. A blue checkbox in a compound’s row indicates that the compound is isobaric with another compound in the selected list, and it is targeted to be resolved.

When the detector is set to GC, the default is that no compounds are preselected to resolve. In this instance, targeted resolution of compounds is performed by either manual checkbox selection, or by clicking the "Target All" checkbox.

Current phases and libraries available in the Pro EZGC modeler.

The Conditions Tab

Once you have generated your initial modeled result, you can tailor the model to further meet your needs in the Conditions tab.

Alter the conditions of your model in the Conditions tab.

Your initial view of the Conditions tab allows you to alter a limited subset of parameters. To enable the disabled fields, click the Custom radio button in the Gas Flow Parameters section. Custom unlocks all Gas Flow parameters and Column dimensions.

The Custom mode unlocks all the fields for editing.

Column Dimensions

Length (m)

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 length (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 (mm)

Inner diameter (ID) refers to the inner diameter of the GC column fused silica tubing without stationary phase. Restek provides this number on the original GC column box and the column tag.

Film Thickness (µm)

Stationary phase film thickness of the GC column is measured in µm. Restek provides this number on the original GC column box and the column tag.

Available Columns

To pick a Restek-supplied column dimension, choose an alternative from the Available Columns list. You can switch to Custom Mode in the Results section to alter the column dimensions directly.

Gas Flow Parameters

 

Carrier Gas

The Pro EZGC modeler supports the use of hydrogen, helium, nitrogen, and argon/methane as carrier gases.

In general, hydrogen is the fastest carrier gas with minimal efficiency loss across a range of flow rates. Helium is the next fastest carrier gas and is the most widely used gas for gas chromatography–mass spectrometry (GC-MS) applications. Nitrogen is the slowest carrier gas, and optimum separations often require long analysis times.

 

Control Method

The modeler supports Constant Flow, Constant Pressure, and Constant Linear Velocity control methods that can be selected from the Control Method drop-down menu. 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 mode uses pressure changes to maintain a constant linear velocity.

Column Flow (mL/min)

The volumetric GC column flow rate is in mL/min. The column 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.

Average Velocity (cm/sec)

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.

Holdup Time (min)

The holdup time is the time in minutes that it takes an unretained GC compound to travel the length of the column. Holdup time is sometimes referred to as "dead time" or "void time."

Inlet Pressure

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 from the inlet pressure drop-down menu, which are then used for both inlet pressure and outlet pressure (abs).

Efficiency, Speed, and Custom Buttons

The Efficiency and Speed radio buttons allow you to quickly alter the model, changing the flow to speed-optimized or efficiency-optimized flow. You can also choose Custom to re-enable the disabled fields in the interface.

With Custom enabled, you may alter any one of the four control parameters, Column Flow, Average Velocity, Holdup Time, or Inlet Pressure, and the other three parameters’ values will then be calculated by the modeler based upon the parameter you alter.

Note: If you want to alter column dimensions or carrier gas while maintaining the value of one of the control parameters, double click on that parameter’s field (Custom mode only). It will then be marked by a blue arrow, indicating that the other parameters will be calculated around it. If you then alter the Carrier Gas or Column Dimensions, that parameter will remain fixed.

For example, you could set the flow of your model to 2 mL/min, and then fix the new hold time by double clicking on the holdup time field. Now, any changes to your carrier gas, column dimensions, or oven program will maintain that holdup time.

Double-clicking a parameter will lock its value. (Indicated by the blue arrow.)

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).

 

Oven Program

You can change the oven program in this section and add or remove ramps. To add ramps, change the value in the Number of Ramps field to a value from 1 to 5 or use the radio button to switch to Isothermal.

When adding ramps, add your ramp details starting with the earliest ramp, entering the ramp rate, the temperature, and any hold time.

If a resolution lower than baseline is acceptable, you can change it in the Target Resolution field.

Refine Oven Program

The Refine Oven Program button is a quick way to refine your model’s oven program for your specific compounds. The button examines a wide range of alternate temperature ramps and returns a model that meets or bests the previous run time and compounds resolved if possible. If more refinements may be possible, a circular arrow appears next to the button. The button will disable when no further refinements are available. You can easily undo any changes via the Undo button (or pressing Ctrl-Z).

Warning! When refining the oven program, compounds not already meeting the Target Resolution may be sacrificed to achieve better run times for the compounds that are adequately separated. If the optimization does not resolve all the targeted compounds to the Target Resolution after refinement, undo to the initial starting position, enter a lower value for your Target Resolution, and try again. To maintain the resolution of particular compounds, lower your Target Resolution to match the resolution of those compounds before refining.

To maintain the resolution of peaks 35 and 36 above, change the target resolution to 0.50 before refining.

 

The Target Resolution field is defaulted to 1.5, which is full baseline separation. Raising this value may result in longer run times and lowering this value may decrease the run times (sometimes significantly).

 

Results

To undo any change you have made in the Conditions tab, click the Undo button in the Results section or press Ctrl-Z on your keyboard. To redo a change, hit the Redo button or press Ctrl-Y on your keyboard.

The Compounds Separated value shows the number of compounds that meet or exceed your Target Resolution. (See more on Target Resolution in the Refine Oven Program section.)

The Model View

The modeled results appear in the right panel of the interface. You can expand the view (hiding the tabs) via the << arrows beside the My EZGC tab. To collapse the view and re-enable the tabs, click the >> arrows in the upper left corner of the interface.

The expanded view is great for printing your modeled result.

Zoom in on the graph by clicking and dragging across a region of the graph with the mouse (or by tapping and dragging with a finger if using a tablet). This method can be used again to zoom in tighter. To zoom out, double-click (or double tap) on the graph.

Clicking and dragging over a region of the chart will zoom that region.

 

Hovering over a peak’s number (desktop browser version only) will give you its retention time; resolution (to nearest peak); and elution temperature. For coeluting peaks, it will show those values for all coeluters.

Also, for a separation containing isobaric compounds, the resolution of those isobars is easily visualized by using the Available Isobars menu below the modeled chromatogram. The Critical Pair (see note below) for each isobar will be listed in order of resolution from least to most resolved. Any isobaric critical pair with resolution values at or above Rs = 1.5 (baseline resolved) will appear as a green trace. If the resolution between isobaric critical pairs drops below Rs = 1.5, the trace will appear either orange or red based on the degree of coelution.

Available Isobars can easily be visualized in the chart.

 

NOTE: If three or more isobaric compounds are present for an isobar, as shown in colored traces in the figure above, only the two most closely eluting compounds will be displayed in the Available Isobars drop-down list. These two are defined as the “critical pair” for that m/z ion.

The peak list shows retention time; resolution (to nearest peak); peak width; and elution temperature.

When an isobar is selected, the peak list will highlight each compound in the isobar and color each compound's row individually based on the compound's resolution.

Compound Structure View and Mass Spectrum

To view more information on a particular compound, click on the compound name. In the popup that appears, you will see details on the compound as well as its mass spectrum as available. The mass spectrum chart can be zoomed in by clicking and dragging over a region of the chart. Mousing over the end point of a bar will give you the relative abundance for that particular ion fraction.

If you are viewing information on a compound that coelutes with another compound in your model, there will be a View Coelutions button below the Mass Spectrum chart. Click it and the spectra of the coeluting compounds will be overlaid on the chart for ease of comparison.

The My EZGC tab

The My EZGC tab houses your saved models.

To save a model, do the following:

1. Give it a title by clicking on Untitled. Click here to edit. at the top of the modeled result. A title textbox field will become visible. Enter your title and click OK or hit the Enter key on your keyboard to lock in the new title.
2. Hit the Save button in the top bar of Pro EZGC.

Untitled saves are possible but not recommended. You can add a title and resave even after loading the model on a subsequent visit.

If you need to save a variation on a model, make your changes in the Conditions tab, change the model’s title as needed, then click the Save A Copy button. A new entry will appear in your My EZGC list of saved models. You can then compare your models easily by clicking on their entries in the My EZGC list.

You can resave a model loaded from the My EZGC list by clicking the Save button.

To delete a model, click the X in the upper right corner of a model’s listing and click the OK button in the confirmation alert that pops up.

Printing Your Model

You can print your model by selecting the Print button in the toolbar. For models with long compound lists, we suggest expanding the model view first before printing. In the expanded view, long peak lists are split into two columns.