“If you require certainty and stability in direct gas analysis, then SIFT-MS is your only option.”


Vaughan Langford Syft Technologies Director - Applications & Marketing

Dr. Vaughan Langford

BSc(Hons), PhD

Principal Scientist / Consultant

Achieving consistent reagent ion energies is the most critical factor in controlling analyte ionization, which in turn provides reproducible product ion formation and reliable, stable quantitation.  Consistent production of low-energy reagent ions is a unique characteristic of the Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) technique and it enables Syft to provide a built-in compound library for identification and quantitation with its instruments.

In SIFT-MS, we use a carrier gas because it enables the chemical ionization (CI) process to be controlled very effectively when compared to electron impact ionization and other forms of CI mass spectrometry. The carrier gas used in SIFT-MS plays two critical roles in controlling ionization:

  1. It cools the reagent ions prior to introduction of sample. This ensures that reagent ion energies are low and uniform, providing predictable, precise, and ultra-soft CI.
  2. It transports the reagent and product ions along the flow tube to the detection region without addition of excess energy. No electric field gradient is used to accelerate ions toward the detection region as occurs in drift tubes.  In fact SIFT-MS is the only technique that avoids electric fields in the sample ionization zone.

A good example of the ultra-soft energy of SIFT-MS ionization is illustrated by the association reaction of NO+ with ketones.  In very soft ionization conditions, the NO+ reagent ion will only “associate” with small ketones to form the ion product at m/z 30 higher than the parent ion.

Ketone + NO+ + “third body” → Ketone.NO+ + “third body”

In contrast, the drift tube environment is much more energetic.  Even at the low electric field gradients, electron transfer is observed:

Ketone + NO+ → [Ketone]+ + NO

Under truly soft ionization conditions (i.e. those used in SIFT-MS), the electron transfer reaction does not occur for ketones with fewer than six carbon atoms because their ionization energy is higher than that of NO+.

In fact, in a drift tube the product distribution is extremely dependent on the applied electric field gradient.  For ketones, the impact of increasing field gradient is two-fold:

  1. Ketone association products disappear rapidly (for acetone and butanone, over 50-fold with a change from 67 to 134 Td).
  2. The electron transfer products dramatically increase as a proportion of the total product signal.

This energy dependence makes maintaining stable ionization extremely difficult.  The flow-on effects of variable ionization in drift tubes are the major challenges in achieving and maintaining stable quantitation and providing any sort of in-built library data.  The extreme contrast between SIFT-MS and drift tube techniques is illustrated for the reaction of NO+ with acetone.

SIFT-MS soft ionisation

Drift tube ionisation

The very practical benefits of the ultra-soft ionization provided only by SIFT-MS are:

  1. Direct, real-time air analysis
  2. Maximum selectivity through minimized spectral clutter
  3. Stable quantitation
  4. A built-in library applicable across all instruments.

If you require certainty and stability in direct gas analysis, then SIFT-MS is your only option.

Written by Vaughan Langford, Director of Applications and Marketing

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