Friday, January 24, 2014

NMR Thermometry Part 2

Yesterday I wrote a part 1 of a blog post regarding a Nature paper from October 2013.  You might want to check out that post before reading this one.

These two posts review/critique the paper "Thermal maps of gases in heterogeneous reactions" by Jarenwattananon et al.

My post yesterday was an attempt to review the publication.  I tried to minimize my comments and let the authors speak for themselves.  Today I plan to air my grievances and concerns about this paper.  

Before I begin I want to note that this paper got tons of press.  For example ..,229E6,E1MX8G,7GOMF,1

With so much attention, I'm sure my puny critiques will not be too rough for Jarenwattananon and co-workers.  I will divide them into three categories: accuracy, precision and sources of error.

#1) Accuracy.

In the body of the paper the authors state "The disagreement between NMR-derived temperatures and fibre-optic sensor measurements was at most 4%."  In the abstract they say "measurement error [is] less than four per cent of the absolute temperature."  I feel that it a bit misleading use agreement with external temperature sensors as a metric of accuracy.

Drill down into the paper and really ask yourself "what is the limit of accuracy in the temperature measurement?"  It has to be how accurately you can measure line widths of the analyte.  For example, Fig 1c shows data and Lorentzian fit for propylene in temperature calibration experiment.

Look carefully and tell me, what is the smallest line width difference you think that you can assess on such a spectrum?  Could you distinguish two spectra with line widths that differ by 1 Hz, 5 Hz or 10 Hz?    By inverting the linear equation (I'll use calibration curve for PtNP) , it is possible to calculate how much the temperature changes if line width changes by 1, 5 or 10 Hz.

T =  1 Hz / 0.16 Hz K^-1 = 6.25 K
T =  5 Hz / 0.16 Hz K^-1 = 31.25 K
T = 10 Hz / 0.16 Hz K^-1 = 62.5 K

4% of 400 K is 16 K, so the authors seem to acknowledge an accuracy of line width measurement of about 3 Hz.  I question if they can really measure the line width so accurately.  If the error is closer to +/- 25 K, then their false color thermal maps, particularly on the microreactor in Fig. 4b, could be quite misleading.

#2) Precision

In the caption for Fig. 1 and 3, the authors state that "temperature calibrations were performed for over 30 different systems." and "thermal maps were generated for more than 15 systems."  How much do the results vary from system-to-system?  Are these variations an indication of the variation in each reactor or the precision of the measurement.  If you repeat the calibration or mapping experiment several times how much variation would you see?

#3) Sources of error

I think of this experiment as almost an anti-DOSY.  In this experiment, the NMR signal in the presence of a gradient is sharpened not attenuated by molecular motion.  I surmise that this experiment works because gas molecules at elevated temperatures diffuse fast enough average gradients.  The problem as I see it is that this experiment only works if everyone agrees that the only factor that impacts the line widths is molecular motion.  You could fool yourself and think you see a cool spot if regions of the sample have any reason other than the gradient to have broad lines.  Some examples that spring to mind with heterogeneous catalysts is local concentration of paramagnetic substances. 

A related issue that troubles me is that the slope of the calibration curve (deltaF vs. T) is different for the two catalysts (PtNP and Pd-MOF).  I don't understand why.  It seems like if gradient-induced line width is narrowed by motional averaging (which is related to T) the specific catalysts should not matter.  Why is deltaF equal to -87 Hz at 400 K and G equals 0.05 G cm^-1 for PtNP and -79 Hz for Pd-MOF.  Either the material impacts the line width (which is clearly what the authors think) or these values represent errors in line width measurement.


In the end, I really love this paper.  It really made me think about stuff I don't get to think about too much line mean free path and the Maxwell distribution.  I get persnickety about a few issues, but you shouldn't let me distract you from the unprecedented view into reactor energetics this new NMR thermometry technique offers.    

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