soap envelope using C# problem

I'm trying to build a soap envelope using C#; I'm getting the error: "Specified value has invalid HTTP Header characters. Parameter name: name" when adding the header.
When I use the Immediate Window to examine the output of BuildSoapHeader(), I still see the escape sequence "\" literal and I wonder if that's the issue or if it's something else.

Please help!

    WebRequest webRequest = WebRequest.Create(this._fiserveURI);
    HttpWebRequest httpRequest = (HttpWebRequest)webRequest;
    httpRequest.Method = "POST";
    httpRequest.ContentType = "text/xml; charset=utf-8";
    httpRequest.Headers.Add(this.BuildSoapHeader());
    ...

        private string BuildSoapHeader()
        {
            StringBuilder retValue = new StringBuilder("<SOAP-ENV:Envelope xmlns:SOAP-ENV=\"http://schemas.xmlsoap.org/soap/envelope/\" ");
            retValue.Append("xmlns:SOAP-ENC=\"http://schemas.xmlsoap.org/soap/encoding/\" ");
            retValue.Append("xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" ");
            retValue.Append("xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\"> ");
            retValue.Append("<SOAP-ENV:Header>");
            retValue.Append("<m:PI00WEBSOperationRequest_header xmlns:m=\"http://www.FiservLSP.RequestHeader.com\">");
            retValue.Append("<m:LSPHeader>");
            retValue.Append("<m:Service>");
            retValue.Append("<m:DateTime>" + DateTime.Now.ToString() + "</m:DateTime>");
            retValue.Append("<m:uuid>" + this._fiserveUUID +"</m:uuid>");
            retValue.Append("</m:Service>");
            retValue.Append("<m:Security>");
            retValue.Append("<m:AuthenticationMaterial>");
            retValue.Append("<m:PrincipalPWD>" + this._fiservePrincipalPWD + "</m:PrincipalPWD>");
            retValue.Append("<m:PrincipalID>"+ this._fiservePrincipalID +"</m:PrincipalID>");
            retValue.Append("</m:AuthenticationMaterial>");
            retValue.Append("</m:Security>");
            retValue.Append("<m:Client>");
            retValue.Append("<m:VendorID>" + this._fiserveVendorID + "</m:VendorID>");
            retValue.Append("<m:AppID>" + this._fiserveAppID + "</m:AppID>");
            retValue.Append("<m:OrgID>" + this._fiserveOrgID + "</m:OrgID>");
            retValue.Append("<m:SessionID>" + this._fiserveSessionID + "</m:SessionID>");
            retValue.Append("</m:Client>");
            retValue.Append("<m:DataSource>");
            retValue.Append("<m:URI>" + this._fiserveURI + "</m:URI>");
            retValue.Append("</m:DataSource>");
            retValue.Append("</m:LSPHeader>");
            retValue.Append("</m:PI00WEBSOperationRequest_header>");
            retValue.Append("</SOAP-ENV:Header>");
            //retValue.Append("<SOAP-ENV:Body></SOAP-ENV:Body>");
            retValue.Append("</SOAP-ENV:Envelope>");
            return retValue.ToString();
        }