@staticmethod
def filter_to_samp_observed(df, samp):
    """
        Filter to observed values in specific sample.

        :param df: DataFrame with all intensities and annotations. Missing values should be 0.
        :param samp: The specific sample of interest
        :return: DataFrame that only has rows whether the quantified value for the sample of interest is
        nonzero.
        """
    filt = df.loc[df[samp] != 0]
    return filt
