Convert a Bitstring into an integer value
If you only need the string for printing you can store value in an integer and then use the Serial.print(number,BIN) function to format the output as a binary value.  Appending integers to strings is a potentially costly operation both in performance and memory usage.

int Number = 0;
int tmp;

for (int i=9;i&gt;=0;i--) {
  tmp = analogRead (A0);
  Number += (tmp % 2) &lt;&lt; i;
  delay(50);
}
Serial.print(Number, BIN);