Quotation marks within quotation marks

I'm sure someone has had this problem before. I want to check if an 
element with a particular attribute is available. I'm using the 
element-available() function which takes a string argument.
Immediately you see my problem.. I can't have quotation marks within 
quotation marks.
Any solutions?

<xsl:choose>
<xsl:when 
test="element-available('req:parameter[(_at_)name='additional_group']')">
-----------------------------------^--------------------^----------------^-^
 <xsl:value-of select="req:parameter[(_at_)name='additional_group']" />,
</xsl:when>
<xsl:otherwise>
 <xsl:text>NULL,</xsl:text>
</xsl:otherwise>
</xsl:choose>


Thanks,
-Daniel


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