def concat_columns(df, sep='|'):
    """Concatenate all columns of a dataframe into a pd.Series
    """
    for i in range(df.shape[1]):
        vec = df.iloc[:, (i)].astype(str)
        if i == 0:
            column = vec
        else:
            column = column.str.cat(vec, sep=sep)
    return column
