JavaScript and TypeScript scope issue

I am trying to use typescript, Knockout and azure mobile services to get some data. The problem is when I get data from azure, it's async and I can not get the viewmodel object any more. I think it's some scope/closure issue, please help me find the solution. It is null in the call back function for Done();

Here is my TypeScript code:

class pmViewModel {
    public welcomeMsg: string;
    public totalCount: number;
    constructor () {
        this.welcomeMsg = "";
    }
}

class Manager {
    client: any;
    portalVM: pmViewModel;
    constructor () {
        this.client = new WindowsAzure.MobileServiceClient("url", "key");
        this.portalVM = new pmViewModel();
    }

    LoadData() {
        console.log("load data for management portal");
        this.portalVM.welcomeMsg = "Hello";
        this.portalVM.totalCount = 0;
        var dataTable = this.client.getTable('dataTable');

        dataTable.take(1).includeTotalCount().read().done(this.populateTotal,this.portalVM);

        ko.applyBindings(this.portalVM);
        };

        populateTotal(result, vm) {
            console.log(result);
            console.log(vm); //null here
            this.portalVM.totalCount = 100000;////this is also null
        }
}

and generated JavaScript code:

var pmViewModel = (function () {
    function pmViewModel() {
        this.welcomeMsg = "";
    }
    return pmViewModel;
})();
var Manager = (function () {
    function Manager() {
        this.client = new WindowsAzure.MobileServiceClient("url", "key");
        this.portalVM = new pmViewModel();
    }
    Manager.prototype.LoadData = function () {
        console.log("load data for management portal");
        this.portalVM.welcomeMsg = "Hello";
        this.portalVM.totalCount = 0;
        var dataTable = this.client.getTable('dataTable');
        dataTable.take(1).includeTotalCount().read().done(this.populateTotal, this.portalVM);
        ko.applyBindings(this.portalVM);
    };
    Manager.prototype.populateTotal = function (result, vm) {
        console.log(result);
        console.log(vm);
        this.portalVM.totalCount = 100000;
    };
    return Manager;
})();