concat - node set or a string

I would like to write a node set that allows me to automatically show the
elements of a pers data.
Therefore I defined 2 variables: self_quirxi and qadmin_table
...
<xsl:variable name="self_quirxi"
select="document('/qxml/quirxi.xml')/quirxi" />
...
<xsl:variable name="qadmin_table" select="." /> selects surname,
lastname,...

Here I want to give out the node set, but it is only a string!
<xsl:value-of
select="concat($self_quirxi,'/globals/pers_datas/pers_data/',$qadmin_table)" />

Where is the bug?
markus


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