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Introduction
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_______________________ Finnigan LTQ Hardware Manual _______________________
The LTQ MS detector consists of an atmospheric pressure ionization (API)
source, ion optics, mass analyzer, and ion detection system. The ion optics,
mass analyzer, ion detection system, and part of the API source are enclosed
in a vacuum manifold. Ionization of the sample takes place in the API source.
The specific method used to ionize the sample is referred to as the ionization
technique. The ions produced in the API source are transmitted by the ion
optics into the mass analyzer, where they are trapped in stable orbits by a
time-varying electric field. The polarity of the potentials applied to the API
source and ion optics determines whether positively charged ions or
negatively charged ions are transmitted to the mass analyzer. You can
configure the LTQ to analyze positively or negatively charged ions (called the
positive or negative ion polarity mode).
The lenses in the API source and ion optics act as a gate to start and stop the
transmission of ions from the API source to the mass analyzer. The function
of these lenses is controlled by an automatic gain control (AGC) that sets
them to transmit the optimum number of ions to the mass analyzer.
The mass-to-charge ratios of the ions produced in the API source are
measured by the mass analyzer. Selected ions are ejected from the mass
analyzer and reach the ion detection system where they produce a signal. The
signal produced is then amplified by the detection system electronics.
The ion detection system signal is analyzed by the LTQ data system. The data
system serves as the user interface to the MS detector, autosampler, LC, and
syringe pump. Refer to the online Help for more information on the LTQ data
processing and instrument control software.
Each sequence of loading the mass analyzer with ions followed by mass
analysis of the ions is called a scan. The LTQ uses several different scan
modes and different scan types to load, fragment, and eject ions from the mass
analyzer. The ability to vary the scan mode and scan type, as well as the
ionization and ion polarity modes, affords the user great flexibility in the
instrumentation for solving complex analytical problems.
This chapter provides an overview of the LTQ. Specific topics covered are as
follows:
•
Ion polarity modes
•
Ionization techniques
•
Scan modes
•
Scan types
•
Data types
•
Experiment types