Biomedical Engineering Reference
In-Depth Information
The nominal mass is the integer part of the molecular mass and was
commonly used with older low-resolution spectrometers. Mass
spectrometry is a complex subject. Reference [4] gives a good overview
of the fi eld.
4.3 Metabolomics and metabonomics
In this chapter the focus will be on the application of mass spectrometry
to metabolomics and metabonomics. Both terms refer to the study of
naturally occurring metabolites in living systems. 'Metabo
l
omics'
originally referred to the study of metabolites in cellular systems, whereas
'metabo
n
omics' [5] referred to the metabolic response of entire living
systems. Subsequently, confusion has arisen by the inconsistent adoption
of either term [6]. These terms are often used interchangeably, so the
term 'metabolomics' will be used in the text that follows.
In animal and human studies the study of metabolites is often by
analysis of fl uids such as urine, saliva, blood plasma or cerebral-spinal
fl uid. In this way the sampling may be non-invasive (urine/saliva) or at
least non-lethal, and therefore allows the repeated collection of samples
from the same individual. In plant metabolomics, a portion of the plant
(or whole plant) is used. As removal of plant tissue is a disruptive event,
it is rare that any repeated sampling will come from the same part of the
plant tissue or even the same plant. Instead groups of plants are grown
under carefully controlled conditions, with the assumption that the
variation between individuals will be smaller than any treatment effects.
Extracts of the biological material are then presented to the analysis
system where a 'snapshot' of the metabolites present in the sample is
determined. This kind of global metabolite profi ling is often called
'untargeted analysis' where the hope is that many compounds may be
identifi ed either by comparison with known, previously run compounds
or that tentative identifi cation by the mass spectrum or accurate mass
may be made. This is in comparison to 'targeted analysis' where standard
dilutions of known compounds are run in advance to determine the
quantitative response of the instrument and to identify metabolites.
The consequence of untargeted analysis is that the quantitative
information is relative between treatment groups. Within any particular
sample the amounts of analytes detected may not represent their true
concentration but a comparison of a control and treated sample for a
particular peak will give a relative measure of that metabolite's
concentration between samples. Moreover, the data revealed by
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