![Text Box: MULTIPLE PARALLEL MASS SPECTROMETRY TECHNIQUES
FOR LIPID AND VITAMIN D ANALYSIS
WILLIAM CRAIG BYRDWELL
USDA, ARS, FOOD COMPOSITION AND METHODS DEVELOPMENT LABORATORY, BELTSVILLE, MD
Introduction
Lipid analysis is a complex, even daunting, undertaking due to the wide range
of classes containing fatty chains, and the variety of chain lengths and degrees
of unsaturation (number of double bonds) encountered. Lipid classes range
from non-polar molecules such as triacylglycerols (TAG) and sterols to polar
molecules such as saccharolipids (liposaccharides) to zwitterionic or charged
molecules such as phospholipids (PLs). Table 8.1 lists the lipids that were
defined by “A Comprehensive Classification Scheme for Lipids” (Fahy et al.,
2005) and its update (Fahy et al., 2009). Given the wide range of polarities, a
single liquid chromatographic approach is not suitable for all classes of lipids.
Typically, non-polar molecules such as triacylglycerols are separated using
reversed-phase (RP) high performance liquid chromatography (HPLC) [often
non-aqueous reversed-phase (NARP) HPLC], while polar molecules are often
separated by normal-phase (NP) chromatography. Of course, this is merely a
generalization, since some classes may be separated by either approach. For
instance, a separation of phospholipid classes is often done by NP-HPLC,
while molecular species within a class may be separated by RP-HPLC. Similarly,
the class of secosteroids known as “vitamin D” may be separated from
other classes by NP-HPLC, while species within the class, i.e., vitamin D2 and
vitamin D3, may be separated by RP-HPLC. In some cases, the best separation
is a compromise between a separation of classes and a separation of molecular
species, such as the separation of sphingomyelin from dihydrosphingomyelin
using an amine column.
Just as the wide range of polarities of lipid classes means that no one type
of chromatography is universally applicable, it is also true that no one type of
mass spectrometry technique is universally applicable to all lipids analyzed
by LC-MS. Some molecules, such as phospholipids, which are easily ionized
or are already ionic, are very amenable to electrospray ionization (ESI) MS.](image1444.gif)
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Contact the author:
Dr. Wm. Craig Byrdwell
c/o AOCS Press P.O. Box 17190 Urbana, IL 61803-7190
Phone: 217-359-2344 E-mail: sales@aocs.com Web address: |