Order by custom field value (meta)

I have a custom field called: track_number . I want to order my posts byu the track number numerically.

This is currently how I call my code. WHat do i need to add to make it order the posts by the track_Number custom field?

<?php $my_query = new WP_Query('category_name='.$post_title.'&order=asc&showposts=100');while ($my_query->have_posts()) : $my_query->the_post(); $do_not_duplicate=$post->ID;?>