def convert_g_to_mgUi(XY, Y_list):
    """
    Converts nuclides from ORIGEN simulations measured in grams to 
    concentrations measured in mg / gUi

    Parameters
    ----------
    XY : dataframe of origen sims with nuclides measured in grams
    Y_list : list of columns in DB that are not features (nuclides) 

    Returns
    -------
    XY : dataframe of origen sims with nuclides measured in mg / gUi
    
    """
    
    nucs = XY.columns[~XY.columns.isin(Y_list)].tolist()
    # [x (g) / 1e6 (gUi)] * [1000 (mg) / 1 (g)] = x / 1000
    XY[nucs] = XY[nucs].div(1000, axis=0)
    return XY