@Test
public void testGetTotalMassNumber_IMolecularFormula() throws Exception {

    IMolecularFormula formula = new MolecularFormula();
    formula.addIsotope(builder.newInstance(IIsotope.class, "C"));
    formula.addIsotope(builder.newInstance(IIsotope.class, "O"));
    double totalExactMass = MolecularFormulaManipulator.getTotalMassNumber(formula);
    Assert.assertEquals(28, totalExactMass, 0.000001);
}