According to one web page:
Formal union hospital worker classifications using the term "matron" referred to the woman who had the responsibility of supervising the ward nurses in a general hospital.
On 28 February 1862, she withdrew, with consent. [sources: 1, 2]
1 index to compiled service records of volunteer Union soldiers who served in organizations from the state of Pennsylvania (E Banker)
2 compiled service record (E Banker)
3 'Civil war nurses series: interesting facts about northern nurses', in Emerging Civil War, accessed 24 May 2014