Differential mobility detector

Differential mobility detector

Differential Mobility Detector (DMD) is a type of microelectromechanical system, radio frequency modulated IMS (MEMS RF-IMS) device.

Varian CP-4900 MicroGC, four channel, with Differential Mobility Detector (first from the right)

Due its small size, it easily fits into portable units, such as transferable gas chromatographs or drug/explosives sensors. For instance, it was incorporated by Varian in its CP-4900 DMD MicroGC, and by Thermo Fisher in its EGIS Defender system, designed to detect narcotics and explosives in transportation or other homeland security applications.


Specifications for DMD as part of Varian CP-4900 MicroGC[1]

  • Detection limit: 100 ppb for MES
  • Dynamic range: 103
  • RSD < 2% at 10 times the LDL
  • Gas requirements: 400 mL/min ‘Zero’ Air or N2.[2]

However, independent researchers[3] report that RSD for such compounds as hydrogen sulfide, methyl mercaptan, and ethyl mercaptan is from 4 to 11%.

Radioactive source in DMD[4]

Type of source Sealed source (capsule in holder)
Capsule Type Rolled Foil 6.3 mm x 1.3 mm x 5 μm
Holder Type Manifold
Isotope (radio nuclide) Ni – 63 (also written as 63Ni or Nickel 63)
Source Manufacturer
Amount of activity per source 185 MBq (equal to 5 mCi)
Physical State of Source Solid (deposited on one face of the foil)
Half life Ni – 63 100.1 years
Energy of beta for Ni – 63 (average / max) 17 keV / 67 keV
Minimum Specific Activity 555 GBq per gram
Type of decay β (beta)
Progeny (Ni – 63 decays to) Cu – 63 (stable, not radioactive)
ISO – 2919 : 1999 Testing Classification C32211
Maximum temperature of the source 180°C
ISO – 2919 : 1999 Radio Toxicity Classification Group B, subgroup B2

Detectable Substances

DMD is able to analyze the substances which can be ionized by the β particles emitted from 63Ni. In this, it is very similar to the ECD. Varian has given the following specifications:[5]

Analyzer Application Detection Limit DMD Why Markets
Sulfur odorants in Natural Gas THT 0.5 mg/m3 (100 ppb) Safety Gas transportation
TBM Environmental Gas Distribution
MES City gates
Fuel cells
Sulfur-free odorants in Natural Gas Methyl Acrylate 0.35 mg/m3 (100 ppb) Catalysts
0.45 mg/m3 (100 ppb)
Ethyl Acrylate
Anesthetic gases in air Halotane 100 ppb Safety Operating Theaters
Enflurane Anesthetic gas companies
Isoflurane Recovery rooms
Sulfur hydrocarbons in HC matrices Hydrogen Sulfide 200 ppb Safety Petro-chemical processes
Carbonyl sulfide Environmental Catalysts
Methyl Mercaptan Corrosion Fuel cells
De-sulfurisation companies
Mercaptanes C1-C4 in gas Methyl Mercaptan 100 ppb
Ethyl Mercaptan
iso-Propyl Mercaptan
n-Propyl Mercaptan
TBM, tert-Butyl Mercaptan
CFC’s & HCFC’s in air For example 100 ppb Safety Personal monitoring
CFC-11 Environmental Environmental companies
CFC-12 Emission control companies
CFC-113 CFC & HCFC Producers


  1. ^ Varian document CP501677, February 2004.
  2. ^ The authors of this article note that one 5 liter gas cylinder of Zero Air was enough for three to four days of normal work in the field.
  3. ^ in Russian: Zhiltsov I.N. and Chupin V.V., New Possibilities of Gas Chromatographic Analysis: the Differential Mobility Detector (DMD)
  4. ^ Varian document CP501378.
  5. ^ Presentation provided by Varian to the users of CP-4900 MicroGC with DMD.

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