instrument that includes the facility for both linear and reflectron modes of ion
collection. The reflectron improves resolution and mass accuracy and also allows
structure and sequence information (in the case of peptides) to be obtained by PSD
analysis.Example 3PEPTIDE MASS DETERMINATION (II)
Question Consider the following mass spectrometric data obtained for a peptide metabolite.
(i) The MALDI spectrum showed two signals atm/z¼1609 and 805.
(ii) There were two significant signals in positive ion trap MS mass spectrum at
m/z¼805 and 827, the latter signal being enhanced on addition of sodium
chloride.
(iii) Signals atm/z¼161.8, 202.0, 269.0 and 403.0 were observed when the
sample was introduced into the mass spectrometer via an electrospray ionisation
source.Use these data to give an account of the ionisation methods used. Discuss the
significance of the data and deduce a relative molecular mass for the metabolite.
Use the amino acid residue mass values in Table 9.2.Answer (i) Signals in the MALDI spectrum were observed atm/z¼1609 and 805. These
data could represent the following possibilities:
(a)m/z¼ 1609 (MþH)þ
whenm/z¼ 805 (Mþ2H)^2 þ
andm/z¼ 403 (Mþ4H)^4 þ, givingM¼1608Da
(b)m/z¼ 1609 (2MþH)þ
whenm/z¼ 805 (MþH)þ
andm/z¼ 403 (Mþ2H)^2 þ, givingM¼804Da
(ii) The distinction between the above options can be made by considering the
ion trap data. This mode of ionisation gave peaks atm/z¼805 and 827, the
latter being enhanced on the addition of sodium chloride. This evidence
suggests:
m/z¼ 805 (MþH)þ
m/z¼ 827 (MþNa)þ
givingM¼804 Da and supports option (b) from the MALDI data.
(iii) The multiply charged ions observed in the electrospray ionisation method allow
an averageMto be calculated. Using the standard formula:m 1 1 m 2 m 1 n 2 m 2 1 M(Da) z
268.0 134 2.0 402.0 804 2
201.0 67 3.0 268.0 804 3
160.8 40.2 4.0 201.0 804 4The molecular mass is clearly 804 Da, confirming the above conclusions.373 9.3 Mass analysers