To develop an MMI method using Large Volume Injection (88xx GC)
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To develop an MMI method using Large Volume Injection

This topic provides a procedure to change from a splitless injection using a split/splitless inlet to a solvent vent mode injection using a Multimode Inlet (MMI). It applies mainly to large volume injections (LVI) using a MMI, but the concepts can apply to general MMI use. This topic does not consider all items that can impact an analysis, for example the liner, solvent, analyte boiling points, or polarity. This topic also assumes knowledge of your data system—when to save a method, how to start a run, how to set up a single run, etc.

The main advantage of the MMI solvent vent mode is that you can inject slowly into the inlet, allowing large amounts of solvent to be evaporated from the liner while concentrating the analytes inside the liner. This requires an injector with variable speed injections, a "large" syringe, and knowledge of the sample and the solvent. When developing a solvent vent method, the goal is to determine the injection rate, inlet temperature, inlet flow, and inlet pressure needed to evaporate selectively the solvent but not the analytes. The development technique is first to calibrate the system for analyte recovery and then scale up the conditions to achieve the desired method improvement. The most significant parameters are:

Inlet temperature. Hold the inlet temperature at or below the solvent boiling point until after all the sample has been injected. This is important so that you do not vent the more volatile analytes along with the solvent. Solvent vent methods work best when the boiling point of the solvent is at least 100 °C lower than the first eluting analyte. This allows an unpacked liner to be used which gives the most inert inlet possible. Smaller boiling point differences can be handled but typically require some type of retentive packing in the liner (e.g. Tenax).

Injection speed. If using a MMI, use the Solvent Elimination Calculator. This calculator requires the solvent type, injection volume, and boiling point of the first eluting analyte to determine the correct injection speed. Normally, the injection speed is set so that the rate of injected liquid solvent just matches the rate of eliminated vaporized solvent. This establishes a concentrating zone inside the liner to retain the analytes of interest.

Vent time. Make sure the vent time setpoint is greater than the time the needle spends in the inlet. If the vent time is too short, you will inject more solvent than necessary. This can lead to distorted peak shapes and too much solvent reaching the detector. As you increase the injection volume, the vent time will also increase to eliminate more solvent.

Vent flow. The vent flow rate is directly proportional to the solvent vent rate. The vent flow is set to be high enough to remove the solvent in a reasonable time without removing the analytes. For high vent flow rates, the inlet backpressure will increase. This may require the method to control the vent pressure above ambient in order to maintain method precision.

Vent pressure. The vent pressure affects how easily the solvent vaporizes and how much enters the column during solvent evaporation. The lower the vent pressure, the more quickly the solvent is removed and the less solvent that enters the column. Vent flow can impact vent pressure in the following way. As the vent flow increases, the actual vent pressure minimum will also increase. For best method precision, the vent pressure should be set slightly above ambient pressure so that the instrument reliably controls it.

Rijks and coworkers developed the theory behind solvent elimination in these inlets. The following simplified equation is used to calculate the solvent elimination rate from the parameters described above.

Solvent elimination rate  =  C  x  Pvap/Tinlet  x  Po/Pi  x  F

Where:

C = solvent related constant

Pvap = vapor pressure of solvent at inlet temperature

Tinlet = inlet temperature

Po = outlet pressure

Pi = inlet vent pressure

F = inlet vent flow

If using an MMI with an MSD, another tip for method development is to scan for solvent ions. Detecting the solvent ions can be useful in determining how much residual solvent is being transferred to the column.

To develop an MMI method for large volume injection

  1. Start with an existing hot splitless method using a 1 uL injection volume. Run an analysis on the MMI with the same conditions to get a baseline chromatogram for peak areas and retention times.
  2. If the existing method was in hot splitless mode, rerun the analysis in cold splitless mode on the MMI. For cold splitless:
    • Start with the inlet temperature cold. A good initial temperature is 10 °C below the solvent boiling point. For example, if using methylene chloride (boiling point 39 °C), start with a temperature of 30–39 °C.
    • Hold this initial temperature for 0.1 min then program at 600 °C/min to the hot splitless method temperature.
    • Use Splitless mode.
    • Use the existing injection parameters (typically Fast mode)
  1. Note the analyte peak areas and retention times. This run will serve as the baseline for analyte recovery.
  2. Change the inlet mode to Solvent Vent.
  3. Click on the Solvent Elimination Calculator to open the calculator.
    1. Select the solvent type from the drop down menu. If your solvent is not listed, you can approximate it by using a solvent with a similar boiling point.
    2. Input the injection volume. For this initial test, use 5 uL. This allows you to use the same syringe and the same sample from the initial test.
    3. Enter the boiling point of the first eluting analyte, if you know it. Otherwise, leave the value at 150 °C. Click Next to see the elimination rate, suggested injection speed, and suggested vent time. You can change the control parameters in this screen and recalculate the values but for now, just use the values already present.
    4. Click Next to see the parameter changes proposed. Note that the oven parameters are modified by the calculator but the changes are not included unless the boxes are checked. This allows you to decide which parameter changes you want to use. For example, if you have developed a retention time locked method, you may not want to change the oven initial temperature or time. However, the calculator is trying to give you the best parameters for a successful analysis, so any changes not used may limit the method.
    5. Click Confirm and Copy to download these parameters to the Edit Method window. The changes selected have been made to this screen but remember that these changes have not been downloaded to the instrument yet. If you are satisfied with the method, click OK or Apply to send the changes to the instrument. If you select Cancel, the method changes from the calculator are not used and your method remains unchanged.
    6. Click OK to send the new method to the instrument. Using the same sample, run an analysis with the 5 uL injection.

In most cases, you should see five times the peak area for the analytes. If the response of all analytes is too low:

  • The dispense speed is too fast. Liquid was injected into the inlet and pushed out the vent.
  • The vent time is too long. The inlet started to heat while the vent was open.

If the response of early eluters is too low:

  • The inlet temperature is too high.
  • The Vent Flow is too high.
  • The Vent Time is too long.