Thermo Trace 1310 Inlet Temperature Profile vs Agilent 7890 for Split/Splitless Injectors

Several years ago, my colleague Scott Grossman wrote an excellent article entitled “It’s a Matter of Degrees, but Do Degrees Really Matter?"  He measured the temperature profile across various Agilent inlets, demonstrating different gradients in temperature exist across inlets, depending on the type of inlet and even among the same inlet type.  One consistent finding is the top and bottom of the inlet are always cooler than the middle. The middle is closest to the actual temperature setpoint.  Depending on the inlet, some had more significant drops at the top or bottom vs others and this may affect chromatography.

I wanted to repeat this experiment on a Thermo Trace 1310 split/splitless inlet to see how it compares to an Agilent 7890 split/splitless inlet.  Please note that this was not to prove that one is better or worse, but rather to understand differences. This information will help when making decisions about inlet temperature setpoints, ultimately affecting things like vaporization potential, compound degradation, septa temperature, etc.  This is especially important in a lab operating both instruments, running high molecular weight compounds.  The fix in this case could be as simple as running one instrument at a slightly higher inlet temperature, since a difference in temperature at the bottom of the inlet will affect performance.

To obtain this data, I took a thermocouple probe and inserted it through the septum until it reached the bottom of the inlet.  After recording this temperature, I would mark and pull up the thermocouple probe in 5 mm increments, recording the temperature at each location, until I reached the top of the inlet.  I plotted the inlet temperature gradient from bottom to top for each inlet (see below).  For these experiments, I used an inlet temperature setpoint of 250 ⁰C, as this is a relatively common setting.  I also checked the Thermo inlet profile at 300 and found the same relative percent error at each inlet location compared to the 250-degree experiments.

Here’s what I found:

Comparison of inlet temperature profiles of Thermo Trace 1310 split/splitless inlet with Agilent 7890 split/splitless inlet.

What you may immediately notice is that the top of the Thermo Trace 1310 inlet is significantly cooler than Agilent’s 7890 inlet.  There is a drastic drop in temperature over the top 20 mm.  This should not affect your compound volatilization, since the sample is injected towards the center, but it will affect the temperature of the septum.  For the Thermo Trace 1300/1310 inlet, I recommend Thermolite Plus septa, which are softer than BTO septa.  BTO septa are ideal for high temperatures and will be less pliable at lower temperatures and also more susceptible to coring.  Because of the lower temperature of the septum, bleed should not be a major issue, either.

You may also notice that the bottom of the Thermo inlet is slightly cooler than the Agilent inlet.  This could affect the vaporization of analytes.  If you are using both instruments in your lab, operate the Thermo Trace 1300/1310 at a higher inlet temperature compared to the Agilent to get comparable results.  This would be most evident with response of compounds with high boiling points.

As you can see, different instruments may have different temperature gradients across their inlets.  This does not make one instrument superior to another, but can affect performance and lead to perceived differences between the instruments, if operated under the same conditions.  Also keep in mind that this data was acquired on just one inlet from each manufacturer.  It’s possible for variation to exist between inlets from the same manufacturer, as Scott Grossman demonstrated in his work.