xPath question

Hello List,

I have a variable:
<xsl:variable name="HygHeadDT">
  <Header>
    <item>BMAT</item>
    <item>HBUE</item>
    <item>NURR</item>                                         
    <item>TAB1</item>
    <item>TAB2</item>
    <item>TAB3</item>
    <item>HPRO</item>
    <item>HZUS</item>
    <item>HANL</item>
    <item>BEST</item>
    <item>HDES</item>
    <item>HBAD</item>      
  </Header>
</xsl:variable>

At some point of XML processing, I want to check, if the element "id" a
different value as one of the strings of any $HygHeadDT/Header/item

<xsl:apply-templates select="//CHAPTER[id != $HygHeadDT/Header/item]"
mode="MibeHyg"/>

But this query only seems only to work, if "id" = "BMAT".

What are I m doing wrong?

thanks !!!