This paper describes
electrospray sampling efficiency measurements
obtained on a triple quadrupole mass spectrometer equipped with a
large atmosphere to vacuum sampling aperture and modified ion optics
designed to confine the ions traveling in the intense expanding gas
beam and prevent scattering losses in the entrance optics of the mass
analyzer. Sampling efficiency, defined as the ratio of the number
of ions captured in the first vacuum stage of the entrance optics
to the number of analyte molecules entering the ion source, is a measure
of sensitivity that takes into account both ionization efficiency
at atmospheric pressure, the efficiency of transporting the ions from
atmosphere to vacuum, and the efficiency of confining them in the
subsequent gas expansion before mass analysis. Sampling efficiency
measurements were conducted under high-performance liquid chromatography
sample introduction conditions using columns and flow rates spanning
the nanoflow (300 nL/min), microflow (3–60 μL/min), and
milliflow (100–500 μL/min) ranges. The results show a
convergence in the sampling efficiencies across this range, narrowing
the sensitivity gap between the nanoflow and higher flow rate ranges
largely because nanoflow sampling efficiency has been shown to be
close to 100% for more than a decade, leaving little room for improvement.
Under situations where sample volumes are not limiting, lower concentration
detection limits can now be achieved with the higher flow rate systems
versus nanoflow as a direct consequence of the higher sample loading
capacity of the columns and the reduction in the difference in their
ion sampling efficiencies.