Hardware Charge Deconvolution

The most effective technique to deal with complex spectra due to multiply charged ions is to achieve the full separation between signals corresponding to different charge states and to resolve their isotopic patterns. Beyond a molecular weight of about 2000 u this requires high-resolving mass analyzers. 

Example At molecular weights of some 10 u the isotopic distribution of organic ions becomes several masses wide (Fig. 11.20). The minimum resolution for its full separation is always equal to the ion s mass number (Chaps. 3.3.4 and 3.4.3). Lower resolution can only provide an envelope over the distribution. At insufficient resolution, the resulting peak may even be wider than the envelope. [98] 

Magnetic sector instruments (Chap. 4.3) were used first to demonstrate the beneficial effects of resolution on ESI spectra of biomolecules. [96,103,104] Fourier transform ion cyclotron resonance (FT-ICR, Chap. 4.6) instruments followed. [105-107] The more recently developed orthogonal acceleration of time-of-flight (oaTOF, Chap. 4.2 and 4.7) analyzers also present an effective means to resolve all or at least most peaks. [108-110] 

Example ESI on a magnetic sector instrument set to R = 20,000 allows for the flail resolution of isotopic peaks in case of medium-molecular weight proteins (Fig. 11.21). This enables the direct determination of the charge state of the ions from the spacing of the isotopic peaks, i.e., 1 1h- for the lysozyme ion due to the average spaces of Am = 0.091 u and 13h- for the myoglobin ion due to Am = 0.077 u. In this particular case, the lysozyme [M+llH] ion serves as a mass reference for the accurate mass measurement of the unknown [M+13H] ion. [103] 

Note The resolving power required to fully resolve isotopic patterns is independent of the charge state of the ion. The minimum numerical value of R is always equal to the ion s mass number. 

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