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− +
moderately moderately
ionic polar non-polar non polar non-polar polar ionic
− − ESI + +
APCI APCI
ESI ESI

electron
ions in increasing capture increasing ions in
solution acidity basicity solution
region
Figure 2.1 Ionization–continuum diagram showing the regions of effective use-
fulness of the various interfaces for LC–MS, including atmospheric pressure
chemical ionization (APCI), electrospray ionization (ESI), and electron impact (EI)
ionization. (Reprinted with permission from Anal. Chem. 73, 5441, 2001. Copy-
right 2001 American Chemical Society.)
developing stage of the method. Unfortunately, this approach is not often
followed or not reported in the literature.
Nevertheless, some studies were specifically devoted to the usefulness of
various ion sources in toxicological analysis. Dams et al. carried out a com-
parative performance study of the three ion sources for the ion-trap–based
mass spectrometer (pneumatically assisted ESI, sonic spray, and APCI), using
morphine as a model compound analyzed under different elution condi-
tions. The influence of solvent modifier, buffer, pH, and volatile acids was
studied. The composition of the mobile phase had a serious impact on the
ionization efficiency. Strong similarities were observed between the perfor-
mance characteristics of ESI and sonic spray, whereas APCI showed com-
pletely different behavior. The authors concluded that APCI source showed
the greatest potential, due to robustness, applicability to higher flow rates,
and positive response to acids or buffers. 28 Thurman et al. assessed the
sensitivity of APCI and ESI sources used for detection of 75 pesticides
belonging to different classes. The experiments were performed in chromato-
graphic conditions optimized for particular groups of pesticides in full-scan
mode and in positive and negative ionization modes. It was stated that
neutral and basic pesticides were better detectable with APCI, while ESI gave
better signal for cationic and anionic herbicides. Generally, the optimization
of mobile phase composition and use of proper ion source was of paramount
importance. 29
Yang and Henion compared the performance of APCI and recently
introduced atmospheric pressure photoionization (APPI) for the determi-
nation of idoxifene and its major metabolite. APPI assured six to eight times
higher sensitivity than APCI. Both sources showed similar response to
matrix suppression. The authors concluded that APPI might be an addi-
tional tool in LC/MS. 30

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