Page 262 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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The main advantages of ESI are molecular weight information, good
sensitivity, and suitability for large bio- or synthetic polymers as well as for
polar and even ionic compounds.

6.2.2 Principle of Operation of APCI

The APCI source is based on the original API source developed by Horning’s
18
group in the mid-1970s. Its commercial viability was only realized as an
adjunct option to ESI when it became commercially available in 1989. The
technique is very sensitive and simple to operate. It operates just as well at
high mobile phase flow rates (>1 ml/min and 4.6 mm ID columns) as it
does with lower flows and smaller ID columns. Thus, the full advantage of
the robustness of large ID HPLC columns and ease of transfer of already
established HPLC methods based on standard chromatography, is realized
with APCI.
Ionization takes place at atmospheric pressure and the ions are extracted
into the mass spectrometer by exactly the same set of skimmers used for
electrospray (Figure 6.2). In APCI there is no voltage applied to the capillary.
The liquid elutes from the capillary probe, which is surrounded by a coaxial
flow of N nebulizing gas into a heated region. The combination of nebulizer
2
gas and heat forms an aerosol which begins to evaporate rapidly. At the end
of the APCI probe is a high-voltage (2.5 to 3 kV) metal needle to produce a
corona discharge, causing solvent molecules eluting into the source to be
2
ionized. Sample molecules which elute and pass through this region of
solvent ions can be ionized by gas-phase ion molecule reactions. 3
Chemical ionization of sample molecules is very efficient at atmospheric
pressure due to the high collision frequency. Proton transfer, forming [M +
+
H] ions, occurs in the positive-ion mode, and either electron transfer or
–
proton transfer, forming [M – H] ions, occurs in the negative-ion mode.
The moderating influence of the solvent clusters on the reagent ions and the
influence of the high gas pressure reduce fragmentation during ionization
and results in primarily [M + H] , [M – H] , and/or adduct ions. As in ESI,
+
–
ions are transported into the high vacuum system of the mass spectrometer
by use of a nozzle-skimmer arrangement. The probe can be situated axially
or orthogonally (as in Figure 6.1) to the sampling orifice.
The main advantages of APCI are molecular weight information on
volatile molecules, typical flow rates up to 2 ml/min, and good reproducibility
for quantitation.

6.2.3 Comparison between ESI and APCI

Flows in the range of 0.2 to 2 ml/min can be used with APCI. This permits
direct coupling of 2.1 mm and 4.6 mm ID HPLC columns to the APCI

© 2004 by CRC Press LLC