public static function getMonoisotopicMass($acid)
    {
        $value = @constant('pgb_liv\php_ms\Core\AminoAcidMono::' . $acid);

        if (! is_null($value)) {
            return $value;
        }

        if (strlen($acid) > 1) {
            throw new \InvalidArgumentException('Value must be a single amino acid. Input was ' . $acid);
        }

        throw new \InvalidArgumentException('Value must be a valid amino acid. Input was ' . $acid);
    }