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Background - In a laboratory setting, I use typical elemental analysis (EA) to generate about 50 µg of sulfur in the form of sulfur dioxide (SO2). The EA uses a helium carrier. As an example, a sample of silver sulfide wrapped in tin falls into the reactor set to 1000 °C at the same time as an oxygen pulse, hopefully leading to flash combustion and creation of SO2. The SO2 is carried through a desiccant (Magnesium perchlorate) followed by a quartz chip reactor set to 800 °C, and a GC column to separate out impurities by polarity. The helium flow rate through the EA is typically 80 mL / min. The pressure of helium at the top of the EA is set to approximately 135 kPa. The end detector is a stable isotope ratio mass spectrometer where we measure mass / charge (m/z) 64 and 66 (the hot quartz chip reactor exchanges oxygen atoms so as to force all SO2 to have the same oxygen isotope composition so that our only variability is from sulfur isotopes 34 and 32. The above scenario is routine and can be found in Fry B, Silva SR, Kendall C, Anderson RK. (2002) Oxygen isotope corrections for online δ34S analysis. Rapid Communications in Mass Spectrometry 16, 854-858. doi: 10.1002/rcm.651.

Observation - We were experimenting with analyzing smaller quantities of sulfur by tightening up the SO2 peak using a stainless steal loop dipped in liquid nitrogen for a standard amount of time followed by warming it to room temperature. The tubing is 1/16” OD and allowed to completely cool to liquid nitrogen temperatures before the sample is dropped into the reactor. I observed SO2 at the detector downstream of the loop still residing in liquid nitrogen at helium carrier flow rates above 30 mL / min. I had to reduce the flow rate to about 10 mL / min to completely trap all SO2 in the liquid nitrogen immersed tubing. Based on the phase diagram of SO2, even at this higher helium pressure, it seems like I am still well within the solid phase space.

Question - Are these observations expected and if so please help me understand why?

UPDATE - To add information that addresses the existing answers and comments, I have indeed tried larger diameter tubing, longer tubing, and tubing that loops into the liquid nitrogen back out and back in again which should handle the snow flake example. Also, this approach of freezing out of a helium stream works perfectly at high flow rates with N2O and CO2. My question to this Chemistry Stack then, is what about SO2 prevents easy freezing as I can accomplish with CO2 or N2O?

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  • $\begingroup$ When you say you dip the stainless steel loop into liquid nitrogen for a specified amount of time, are you confident that the loop is thoroughly cooled before the gas stream is introduced? $\endgroup$
    – J. Ari
    Commented Apr 8, 2021 at 0:29
  • $\begingroup$ Yes. I updated the post. I am absolutely certain the tubing is at liquid nitrogen temperatures before the sample is dropped into the reactor. $\endgroup$
    – ajschauer
    Commented Apr 8, 2021 at 0:56
  • $\begingroup$ In addition to the residence time point in Poutnik's answer, I would add that as the SO2 freezes, you are losing heat transfer area, which isn't much to begin with in 1/16" OD stainless steel tube. I would make the coil longer or use some bigger diameter tubing. $\endgroup$
    – J. Ari
    Commented Apr 8, 2021 at 13:56

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Phase diagrams are about thermodynamics, while you have to consider kinetics as well.

The stream may have too high linear speed at 30+ mL/min to reach equilibrium while still in the loop.

Another option is dragging particles of/containing SO2 by the gas stream by mechanical way, not allowing it to settle. Like wind taking snowflakes, not allowing them to land and stay.

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