
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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