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What is the purpose of deconvolution?

Mass spectrometers do not directly measure mass but instead measure m/z.

Mass (M) ≠ m/z

But

m/z = (M+zA)/z

where A is the mass adduct providing charge and z = number of charges.

So, knowing the charge of an analyte is pivotal in determining its Mass. One way is to look at the spacing between the isotopes.

z = 1 / spacing between the isotopes

A one charge state will have a 1 Da difference, 2 charge state = 0.5 Da, 3 charge state = 0.33 Da, etc… However, the resolution of the instrument becomes a limiting factor. The alternative method to determine charge of a peak is not impacted as quickly by the resolution of the instrument.

Using two adjacent peaks in the same series you can use algebra to determine their charges using a simple formula.

z1 = z2+1

z2 = (mz1-A)/(mz2-mz1)

where mz1 < mz2

Having to do this manually for each spectra is extremely time consuming so having a program like ProMass that utilizes a deconvolution algorithm (ZNova) is beneficial. In the example below you will notice that ProMass has identified 3 peaks (1056.4, 924.3, and 821.8) as being in a charge series and identified their charges. After deconvolution they correspond to the Mass of 7402.5 Da seen in the upper panel.

Another benefit of deconvolution is that it allows for the determination of a more accurate signal intensity of an analyte. In the example above you can see that the signal for this analyte is divided across >3 m/z signals. The deconvoluted mass of 7405.2 is equal to the sum of each identified signal in the charge series. This is extremely beneficial for low concentration samples as well as more ‘noisy’ spectra. This same benefit can be seen with deisotoping high-resolution MS data as it sums the intensities of all the identified isotopes of the analyte. A brief explanation of deisotoping can be found here.

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