Exclude category from loop not working

I have this code in my index.php file. I have a different template for a static home page, this is the blog page. I’m trying to exclude all posts with the category “new” which is tag_id “13”

<?php query_posts($query_string . '&cat=-13'); ?>
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

<a href="https://wordpress.stackexchange.com/questions/82745/<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title(); ?>"><p class="lead"><?php the_title(); ?></p></a>

<p><?php the_excerpt(); ?> <a href="<?php the_permalink()?>">read in full</a></p>

<p class="muted"><small>Written by: <?php the_author_posts_link(); ?><br>               
            <?php the_time('F jS, Y') ?></small></p><hr>

<?php endwhile; ?>

Any ideas why this isnt working?

Don’t use query_posts(). Use pre_get_posts instead:

function wpse82745_filter_pre_get_posts( $query ) {
    // Only modify the main loop,
    // and only in the blog posts index
    if ( is_home() && $query->is_main_query() ) {
        $query->set( 'category__not_in', array( '13' ) );
    }
}
add_action( 'pre_get_post', 'wpse82745_filter_pre_get_posts' );

This callback will exclude category 13 from the main loop in the blog posts index.