def find_approximate_rt(peaklist, rt):
    """
    Find a peak with the closest retention time if it's within a tolerance.
    :param peaklist: list, of Peaks
    :param rt: float, retention time to match to
    :return: Peak, the peak matching the retention time within the tolerance, if any
    """
    peaklist = [peak for peak in peaklist if peak.rt]  # clean list for only those with RTs
    return next((peak for peak in peaklist if rt - .011 < peak.rt < rt + .011), None)