Google Analytics Event Tracking onClick Code

You are mixing classic code with universal code. It will not work. You need to replace this:

_gaq.push(['_trackEvent', 'Button', 'Click', 'Purchase Details',, false]);

With this:

ga('send', 'event', 'Button', 'Click', 'Purchase Details');

GAJS reference for Events: https://developers.google.com/analytics/devguides/collection/analyticsjs/events#implementation