VS 2012 Tts ssml

I have developed a Visual Basic.net application that uses TTS (Text To Speech).

Can I please have some help to use the SSML Markup Language.

Here is my code:

TestSpeechSpeechSynthesizerObject.SpeakSsml("This sounds normal <pitch middle = '-10'/> but the pitch drops half way through")

I get the following error: System.FormatException was unhandled, Message=XML content not valid.

Can someone please give me an example of a SSML statement that works, and from that I am pretty sure I will get the hang of it.

Thanks