Introducing the MSQ
The Source–An Introduction to API Techniques ________________________________________________
Spectral Characteristics
Polar compounds of low molecular weight (<1000 amu) typically form
singly charged ions by the loss or gain of a proton. Basic compounds (for
example, amines) can form a protonated molecule [M+H]
+
, which can be
analyzed in positive ion mode to give a peak at m/z M+1. Acidic
compounds (for example, sulphonic acids) can form a deprotonated
molecule [M-H]
-
, which can be analyzed in negative ion mode to give a
peak at m/z M-1. As electrospray is a very soft ionization technique, there is
usually little or no fragmentation and the spectrum contains only the
protonated or deprotonated molecule.
Some compounds are susceptible to adduct formation if ionization takes
place in the presence of contamination or additives such as ammonium or
sodium ions. The spectra will show other ions in addition to, or instead of,
the quasi-molecular ion. Common adducts are ammonium ions NH
4
+
[M+18]
+
, sodium ions Na
+
[M+23]
+,
and potassium ions K
+
[M+39]
+
.
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
m/z
0
100
%
322
241
100
80 99
103
145
141
187
181
244
282
261
279
344
363
[M+H]
+
[M+Na]
+
Figure 1-11. Electrospray spectrum showing a sodium adduct
The singly charged ions arising from samples of relatively low molecular
masses can be interpreted directly, as they represent the protonated or
deprotonated molecule. Electrospray, however, can produce multiply
charged ions for analytes that contain multiple basic or acidic sites, such as
proteins and peptides. As an MS detector measures mass-to-charge ratio
(m/z), these ions appear at a m/z value given by the mass of their protonated
molecule divided by the number of charges:
z
m
n
nH
M
n
=
+
+
Where, M = actual mass, n = number of charges, and H = mass of a proton.
Electrospray allows molecules with molecular weights greater than the mass
range of the MS detector to be analyzed. This is a unique feature of
electrospray.
1-12
__________________________ MSQ Hardware Manual ____________________________
