Apodization refers to the mathematical processing technique by which the FID is multiplied pointwise by some appropriate function in order to improve the instrumental line shape. The term apodize actually derives from its Greek meaning “removing the feet”. The feet being referred to are actually the side-lobes found in the FT spectrum resulting from zero-filling a truncated FID (this phenomenon is also known as leakage).
Probably the most widely used apodization function in NMR, especially in 13C spectroscopy, is the Exponential function although other functions such as Hanning are also very popular.
In this short post, I want to introduce a new apodization function, the so-called Stanning function which gives superior results compared to Exponential and Hanning apodization functions.
The name Stanning is a play on words which combines Hanning (which forms the basis of this function) with Stan, the inventor of this apodization function to whom all credit should be given.
The performance of this apodization function is illustrated with a 19F NMR spectrum whose FID is shown in Figure 1.
This FID consisted of ca 59K acquired data points which are then extended by zero filling to a final size of 128K. As the FID has not fully decayed to zero during acquisition, resulting FT spectrum will show the expected truncation artefacts, as shown in Figure 2.
Multiplication of the FID by an exponential function, in this case with a line broadening value of 1.0 Hz results in the following spectrum where the wiggles have been significantly reduced but not in a totally satisfactory way (see Figure 3).
Application of the new Stanning function yields the result depicted in Figure 4. As it can be seen, the truncation artifacts have been further reduced whilst the resolution of the spectrum is slightly better compared to the exponential function.
The mathematical formulation of Stanning as well as some additional illustrative examples will be covered in a future blog post.