Helium plasma source time-of-flight mass spectrometry: off-cone sampling for elemental analysis

Area of Science:

• Analytical Chemistry
• Atomic Spectroscopy
• Mass Spectrometry

Background:

• Elemental analysis using plasma sources often suffers from spectral interference.
• Existing methods can be limited by background noise and require robust cooling systems.

Purpose of the Study:

• To explore an atmospheric pressure helium microwave-induced plasma (MIP) ion source coupled with time-of-flight mass spectrometry (TOFMS) for elemental analysis.
• To develop an "off-cone" ion sampling technique to improve spectral quality and reduce background signals.

Main Methods:

• Utilized an atmospheric pressure helium microwave-induced plasma (MIP) ion source.
• Coupled the MIP source with an orthogonal acceleration time-of-flight mass spectrometer (TOFMS).
• Investigated the effect of sampling distance on ion signals to optimize the "off-cone" sampling mode.

Main Results:

• The "off-cone" sampling mode dramatically suppressed background signals without sacrificing analyte signal intensity.
• Eliminated spectral interference from entrainment air and working gas, enabling sensitive determination of isotopes like 40Ca, 52Cr, 55Mn, and 56Fe.
• Achieved a mass resolution of 1100 (fwhm) and detection limits in the tens of picograms per milliliter level.

Conclusions:

• The "off-cone" sampling technique offers a method for "clean" background spectra in MIP-TOFMS.
• This approach improves elemental analysis sensitivity and reduces demands on cooling systems, extending sampler plate lifetime.
• The developed system provides good mass resolution and sensitive detection limits for elemental analysis.