Optimizing Splitless Injections: Splitless Purge Valve Time

In my previous blog, I discussed optimizing inlet temperature for splitless injections.  Today I would like to discuss another critical parameter for splitless injections: splitless purge valve time.  The key feature of a splitless injection is that all the carrier gas flow is directed to the column and the splitless valve is closed during injection.  This allows for maximum recovery of the injected sample, making splitless injections ideal for trace level analyses.

The split valve remains closed during and after sample injection for a predetermined amount of time to allow for full volatilization and transfer of the analytes to the column.  While this time is essential for achieving the best recoveries of trace analytes, keep in mind you are also injecting a very large amount of solvent in the case of liquid injections.  Because of this, at some point, you must open the split vent to rid the inlet of excess solvent vapor.  Without doing this, the large solvent peak can potentially interfere with your compounds of interest.

So just as in selecting an ideal inlet temperature, selecting an ideal splitless hold time can also involve compromise.  You must select a hold time that is long enough to ensure complete vaporization and transfer of your analytes to the column, but not so long as to introduce excess solvent into your column, which can result in excessive tailing of the solvent peak, leading to an elevated baseline and potential interference with analytes.  The figure below provides an example showing peak areas over a wide molecular weight range of hydrocarbons vs splitless hold time.  Note that the liner is single taper with wool and the carrier gas flow is 1.5 mL/min.  As a general rule, you should allow the liner to be swept 1.5-2 full volumes with carrier gas, prior to opening the split vent.  Notice that after a certain amount of time, gains start to become insignificant.  Eventually C8 is lost in the solvent and cannot be detected.

Figure 1: Experiment to test response of early, middle and late eluting hydrocarbons, as well as solvent peak area, vs splitless valve time. The “ideal” split time is between 60 and 75 seconds, which correlates to a 1.5 to 2x sweep of the inlet liner. The solvent peak eventually interferes with C8, completely masking it. Column flow was set at 1.5 mL/min and liner was a single taper with wool.

To make your life easy, Restek has a free tool that calculates this ideal range for you: The EZGC Method Translator and Flow Calculator.  Check out the short webinar below to see how to use it:

Here are a few key points from the video to remember when filling in the EZGC Flow Calculator to calculate splitless valve time:

  1. For “Temperature”, under the Column section, enter your initial oven temperature.
  2. In the “Control Parameters” section, only enter one of the values, based upon your control mode. For instance, if you are using constant flow, fill in your column flow, whereas if you are using constant pressure, only fill in pressure.  The other values will be automatically calculated.  Be sure to select the correct outlet pressure.  For an MS detector select “vacuum” and for detectors such as FID or TCD, select “atmospheric”.
  3. In the “Inlet” section, you must enter inlet temperature and liner volume. An exact liner volume is not necessary, as having something relatively close should result in an acceptable recommendation.  The simplest way to obtain an estimate is to assume that your liner is a cylinder and apply the following formula: V = π r2 h , where V is volume, r is the radius (1/2 of the internal diameter), and h is the height (length) of the liner.  Some common liner configuration volumes are listed in Figure 2.   Note that this table is in µL, whereas the Flow Calculator uses mL.  Simply place a decimal point before the µL value to convert to mL (i.e. 900 µL = 0.900 mL).  Use the values listed under “Physical” volume.
  4. Note that after entering a value into a field, you must click outside of the field or into another field in order for the calculation to update.

Figure 2: Liner volumes for some common configurations. Use the value under “Physical” for calculating splitless valve time.

You now have an understanding of why optimizing splitless purge valve time is important.  Using Restek’s Flow Calculator makes this an easy task.  If you ever want to verify that you are in fact using the best hold time for your analysis, you can easily set up an experiment like the one shown in Figure 1, where you plot peak areas vs hold time.

For the next installment of this blog series I am going to discuss initial oven temperature.  Hope you enjoy!

 

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