Determination of Signal to Noise Ratio-Signal Averaging.

Determination of Signal to Noise Ratio:

You may work in small groups on the lab. Put all your names on the lab report.

EFT 60 directions-

Put the 1-99 sample into the instrument, neat ethyl benzene. Tune the instrument if necessary. Set up to do the C-13 spectra. Obtain the spectra with NS =16, 4, 1, 64, 256. Use a2 to process the data. Pick the methylene C-13 peak and measure its height from the middle of its base line this will be your signal level. Now measure the height of the noise level near about 50 ppm to the left of the peak. Measure the distance between top of the noise to the bottom and divide this by 2.5. This statisically gives the average noise level. Calculate the S/N ratio. [Alternatively use the sn command to have the computer calculate the s/n ratio and write it on the spectrum.]

Graph the resulting plot of S/N (signal to noise) vs Square root (NS) and you should obtain a straight line.

On a cover page to these spectra summarize your results (briefly).

Homework questions.

1. When a decoupled C-13 spectrum is run on a 500 MHertz instrument; the sample is irradiated continuously with ______ radiation and pulsed with _______ radiation.

a) 500 MHz , 500 MHz b) 125 MHz,125 MHz

c)500 MHz, 125 MHz d) 125 MHz, 500 MHz 

2. The signal to noise ratio is 100:1 after 16 scans.  What would the signal to noise ratio be after just one  pulse?

a) 10:1 b) 4:1 c) 12.5:1 d) 25:1 e) 6.4:1

3.  All  other  things being equal,  by what  factor  would  you multiply  NS (the number of scans) in order to get the same signal to noise ratio on  a sample that is 1/100 as concentrated  as the original.

a) .01 b) .10 c) 10 d) 100 e) 10000