Drift Region
Definition and meaning of Drift Region in chemistry.
Drift region refers to the field-free flight tube in a time-of-flight mass spectrometer, or the low, uniform-field tube in ion mobility spectrometry, through which ions travel and separate before reaching the detector.
In more detail
In time-of-flight mass spectrometry, ions of the same charge are given equal kinetic energy by an accelerating voltage, so lighter ions move faster through the drift region and arrive sooner, with flight time proportional to the square root of mass-to-charge ratio. In ion mobility spectrometry, ions instead drift through a neutral buffer gas under a weak, constant electric field, separating according to collision cross-section, shape, and charge rather than mass alone. The length and field uniformity of the drift region are the main levers for resolving power: a longer, more uniform region gives ions more time (or distance) to separate before detection, sharpening the resulting spectrum.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Also known as | Flight tube (TOF-MS); drift tube (IMS) |
| Governing relation | Flight time t ∝ √(m/z) in TOF-MS |
| Typical conditions | Field-free (TOF) or ~10–20 V/cm uniform field with buffer gas (IMS) |
In a linear time-of-flight instrument, ions accelerated through 20 kV enter a 1-meter field-free drift tube; a singly charged ion at m/z 100 travels faster and reaches the detector before a singly charged ion at m/z 400.
Frequently asked questions
Why must the drift region be field-free in time-of-flight mass spectrometry?
Separation must depend only on the velocity differences ions acquired during initial acceleration; any additional field inside the drift region would distort the simple relationship between flight time and mass-to-charge ratio.
What property does the drift region separate ions by in ion mobility spectrometry?
Ion mobility, which depends mainly on the ion's collision cross-section (size and shape) and charge as it migrates through a buffer gas under a weak electric field.