Voyager DP-263-S Betriebsanweisung Seite 540

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Appendix H Continuous Extraction Mode
H-12 PerSeptive Biosystems
H
H.2.2 Determining Laser Threshold
This section describes:
•Overview
Adjusting the laser intensity
Verifying threshold setting
H.2.2.1 Overview
Definition
Laser threshold is the minimum laser intensity required to
produce a steady detectable signal. Laser intensities above
threshold generate a dense plume of desorbed neutrals
which cause energy loss during acceleration. The observed
impact of higher laser intensity is a decrease in resolution
and mass accuracy, and a possible increase in
fragmentation.
Factors affecting
threshold
Laser threshold is affected by sample preparation. The most
significant factors affecting threshold are:
Matrix—Absorption coefficient of matrix affects the
laser intensity needed to ionize sample.
Molecular weight of sample—Higher masses
require a higher laser intensity to generate an
acceptable signal.
Sample characteristics—Different classes of
compounds in the same matrix may require different
laser intensities. For example, carbohydrates in DHB
require a higher laser intensity than peptides.
Sample contaminants—Buffers, salts, and
detergents in sample can substantially increase the
laser intensity needed.
Sample exposure—Fresh sample surfaces ionize
better than surfaces that have been exposed to
irradiation. The significance of sample exposure
varies widely between matrices and changes across
the sample surface.
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