public function authenticate()
    {
        $object = 'login';
        $url = sprintf('/api/%s/version/%s', $object, $this->version);
        $parameters = array(
            'email'    => $this->email,
            'password' => $this->password,
            'user_key' => $this->userKey,
            'format'   => $this->format
        );

        $this->apiKey = $this->doPost($object, $url, $parameters);

        return $this->apiKey;
    }