We investigated the abundance ratio of ^ { 12 } C/ ^ { 13 } C in planetary nebulae by examining emission lines arising from C iii 2 s 2 p~ { } ^ { 3 } P _ { 2 , 1 , 0 } \rightarrow 2 s ^ { 2 } ~ { } ^ { 1 } S _ { 0 } .
Spectra were retrieved from the International Ultraviolet Explorer archives , and multiple spectra of the same object were coadded to achieve improved signal-to-noise .
The ^ { 13 } C hyperfine structure line at 1909.6 Å was detected in NGC 2440 .
The ^ { 12 } C/ ^ { 13 } C ratio was found to be \sim 4.4 \pm 1.2 .
In all other objects , we provide an upper limit for the flux of the 1910 Å line .
For 23 of these sources , a lower limit for the ^ { 12 } C/ ^ { 13 } C ratio was established .
The impact on our current understanding of stellar evolution is discussed .

The resulting high signal-to-noise C iii spectrum helps constrain the atomic physics of the line formation process .
Some objects have the measured 1907/1909 flux ratio outside the low-electron density theoretical limit for ^ { 12 } C. A mixture of ^ { 13 } C with ^ { 12 } C helps to close the gap somewhat .
Nevertheless , some observed 1907/1909 flux ratios still appear too high to conform to the presently predicted limits .
It is shown that this limit , as well as the 1910/1909 flux ratio , are predominantly influenced by using the standard partitioning among the collision strengths for the multiplet ^ { 1 } S _ { 0 } – ^ { 3 } P _ { J } 
according to the statistical weights .
A detailed calculation for the fine structure collision strengths between these individual levels would be valuable .