In mass spectrometry

Mass spectrometry

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

, resolution measures of the ability to distinguish two peak

Summit (topography)

In topography, a summit is a point on a surface that is higher in elevation than all points immediately adjacent to it. Mathematically, a summit is a local maximum in elevation...

s of slightly different mass-to-charge ratio

Mass-to-charge ratio

The mass-to-charge ratio ratio is a physical quantity that is widely used in the electrodynamics of charged particles, e.g. in electron optics and ion optics. It appears in the scientific fields of lithography, electron microscopy, cathode ray tubes, accelerator physics, nuclear physics, Auger...

s ΔM, in a mass spectrum

Mass spectrum

A mass spectrum is an intensity vs. m/z plot representing a chemical analysis. Hence, the mass spectrum of a sample is a pattern representing the distribution of ions by mass in a sample. It is a histogram usually acquired using an instrument called a mass spectrometer...

.

Resolution and Resolving Power

There are two different definitions of resolution and resolving power in mass spectrometry.

IUPAC definition

The IUPAC definition for resolution in mass spectrometry is = resolution = resolving power

where a larger resolution indicates a better separation of peaks. This definition is used in a number of mass spectrometry texts. This use is also implied by the term "high-resolution mass spectrometry."

A high value for resolution corresponding to good separation of peaks is similar to the convention used with chromatography

Chromatography

Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures....

 separations, although it is important to note that the definitions are not the same. High resolution indicating better peak separation is also used in ion mobility spectrometry

IUPAC defines Δm as the resolving power.

Resolving power definition

Some mass spectrometrists use the definition that is similar to definitions used in some other fields of physics and chemistry. In this case, resolving power is defined as: = resolving power.

The minimum peak separation ΔM which allows to distinguish two ion species is then called: = resolution.

Resolution and resolving power, when defined in this way, are consistent with IUPAC recommendations for microscopy

Microscopy

Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...

, optical spectroscopy. and ion microscopy (SIMS) but not gas chromatography.
This definition also appears in some mass spectrometry texts.

Measuring peak separation

There are several ways to define the minimum peak separation ΔM in mass spectrometry, therefore it is important to report the method used to determine mass resolution when reporting its value.
The two most widely used are the peak width definition and the valley definition.

Peak width definition

In the peak width definition, the value of ΔM is the width of the peak measured at a specified fraction of the peak height, for example 0.5%, 5%, 10% or 50%. The latter is called the full width at half maximum

Full width at half maximum

Full width at half maximum is an expression of the extent of a function, given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value....

(FWHM).

Valley definition

The valley definition defines ΔM as the closest spacing of two peaks of equal intensity with the valley (lowest value of signal) between them less than a specified fraction of the peak height. Typical values are 10% or 50%. The value obtained from a 5% peak width is roughly equivalent to a 10% valley.