Measuring Diffusion

• Pertinent pulse sequences are “vpge1” and “vpge_cnvct”

• Suggested literature reference is A.S. Atieri et al., JACS, 117:7566, 1995.

• Currently only installed on 6001, because vpge_cnvct requires triple axis gradients

• “vpge1” is used in most instances. “vpge_cnvct” should be used for temperatures above 30C.

• For “vpge1”, two parameters are adjusted/arrayed to optimize the experiment and collect a series of data points for calculation of the diffusion coefficient. These are:

  • d3: typical values from 50 to 100ms

  • gt2: typical values from 1 to 4ms.

• “d3” should be set with a larger value for smaller molecules.

• The two parameters are optimized so that when “gt2” is arrayed, a series of spectra produces a linear decrease in observed signal approaching a null. An array of 10-12 points is usually sufficient for the calculation. The first and last points are typically discarded.

• After collecting the data, two macros are used to process and analyse the data: “bca” and “idif”.

  • “bca” is a simple macro to adjust the baseline in the entire array of spectra.

  • “idif” integrates the peaks in the spectra and performs calculations to prepare the data to be graphed for linear regression analysis. “idif” queries the user for a file name for the data to be output in a comma separated table format that can be imported into a program such as MS Excel. The first column represents the experiment specific parameters in the equation: A(2t) = A(0) exp[-(gamma*delta*G)^2 * ( DELTA – delta/3)/Ds], where delta = PFG duration in s, G = gradient strength in Gauss/cm, and DELTA = time between PFG pulses. Further columns are the natural logarithm of A(2t)/A(0) for each peak of the spectra.

• N.B. “idif” saves the text file in the experiment directory (i.e. vnmrsys/expX), neither in the user's default directory nor in the current working directory.

• For the “vpge_cnvct” experiment, two gradients are employed to cancel out the effects of convection currents. These gradients (gt2 and gt3) MUST be arrayed simultaneously, not consecutively.