Generate Multi parameter URL in C# WPF

Hi,
i am trying to build URL string lik:

var Tag =
                    string.Format(@"<img src=""{0}?camp_id={1}&ukey_id={2}"" alt="" width=""0"" height=""0"" 
						   style=""width: 0px; height: 0px; border:0px;"" />",
                                           baseURL,campaignId,ukeyid);

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the proble is that when i am dibuggin it the string looks good:
http://www.mydomain.com/somepage.aspx?camp_id=23&ukey_id=3332

but when i am embedding the string in a HTML page it turned into:
 http://www.mydomain.com/somepage.aspx?camp_id=23&ukey_id=3332

the '&' become &

how do I encoding this string to work as a link?
thanks.