<pre><code>df.ix[df.Nucleus == 'H','Reference Value'] = 30
df.ix[df.Nucleus == 'C','Reference Value'] = 180
df['delta'] = df['Reference Value'] - df['Isotropic Shift']
Atom Number Nucleus Isotropic Shift Reference Value delta
1 C 49.3721 180.0 130.6279
2 C 52.9650 180.0 127.0350
3 C 36.3443 180.0 143.6557
4 C 50.8163 180.0 129.1837
5 C 50.0493 180.0 129.9507
6 C 49.7985 180.0 130.2015
7 H 24.0772 30.0 5.9228
8 H 23.7986 30.0 6.2014
9 H 24.2922 30.0 5.7078
10 H 24.1632 30.0 5.8368
11 H 24.1572 30.0 5.8428
12 C 102.9401 180.0 77.0599
</code></pre>