lipidomics pt.2

Question 1

what kind of ionizing method is ESI

Answer 1

ESI is a ‘soft ionizing’ method that leads to
very little fragmentation of molecules

Question 2

What is tandem ESI-MS

Answer 2

Fragmentation requires tandem ESI-MS (also
seen as MS2 and MS/MS): a method that
selectively isolates an ion of interest and
fragments it in the presence of increased
voltage)

Question 3

what is tandem ESI-MS important for

Answer 3

Important for NL and precursor ion scanning

Question 4

ESI-MS in lipidomics

Answer 4

Different classes of lipid can be detected
using either positive ion mode or negative
ion mode, depending on their charge
following ionization:

With few exceptions, most lipids require a
combination of ion mode plus scanning
mode

Question 5

lipids in negative ion mode are detected in the form

Answer 5

[M-H] -

Question 6

lipids in postive ion mode are detected in the form

Answer 6

Lipids in positive ion mode are weakly
detected in the form [M+H] + , but strongly
detected in the form [M+X] + , where X can be
Na+ , Li + , NH4+ (which is added to the solvent)

Question 7

isobaric lipid

Answer 7

a lipid under same pressure
and temperature in a mass spectrometer
that exhibits the exact same m/z value
under Q1MS conditions

Question 8

MALDI-MS

Answer 8

involves the ionization of
molecules that are mixed with a matrix
material and adhered to a surface

Question 9

process of MALDI-MS

Answer 9

Ionization is a two-step process:
– the matrix (which is usually a small molecule
with a conjugated diene that can rapidly
absorb UV) is stimulated by a UV laser,
leading to its ionization and vaporization
– The ionized matrix material in turn ionizes the
molecules in the form of [M+nH] n+ and [M-H] -

Question 10

ionization method of MALDI-MS

Answer 10

Like ESI-MS, MALDI-MS is a soft
ionization method

Question 11

what differs MALDI-MS from ESI-MS

Answer 11

Unlike ESI-MS, MALDI-MS can readily
detect molecules that exhibit a m/z of up to 30,000
– This is advantageous for assessing large
molecules such as peptides and nucleotides,
but it is a little overkill for analyzing lipids on one-dimension

Question 12

how has MALDI-MS evolved

Answer 12

MALDI-MS has profoundly evolved, such
that one-dimensional data obtained by
MALDI-MS can be collected on a two-
dimensional sample
* This is extremely useful for assessing
lipids in different regions of tissue:
– Tissue slices can be mounted on a slide,
coated with matrix material, and micron-sized regions of tissue can be assessed for lipid content

Question 13

naming lipids

Answer 13

Assigning a molecular species is based on
the acyl chains and NOT the backbone
* Example: CE 18:0
– The cholesteryl backbone is not considered in
the calculation of “18:0”
– The 18:0 represents the acyl chain associated
with the cholesteryl backbone

Question 14

naming phospholipids and acylglycerides

Answer 14

Example: PC 18:0-18:1
– The glycerol backbone is never considered in
the assignment of a phospholipid (& same for
MG, DG & TG), but ONLY the acyl chains
– This example indicates that the sn-1 acyl
chain has 18 carbons and zero unsaturation,
while the sn-2 has 18 carbons and one
unsaturation
* You CANNOT definitively say the sn-2 is from
oleate or vaccenate without extra analyses though

– Another group may examine this and call it
PC 36:1
* WHY? The group did not distinguish what is in the
sn-1 or sn-2 position
* Thus, the 36:1 could be 18:0-18:1, BUT ALSO it
could be 16:0-20:1, 20:1-16:0, 14:0-22:1, etc…

Question 15

distinguishing acyl chain requires

Answer 15

acyl fingerprinting