Comparison of percent P in phytic acid measured by
Murphy Riley and MS.
Phytic acid is a six-carbon
ring with a phosphate bound to each carbon. It ser
ves as a good example of organic phosphate.
Mass spectrometry measured a higher percentage of p
hosphate in phytic acid than Murphy Riley. On aver
age,
MS measured 23% P in phytic acid, whereas Murphy Ri
ley only measured 13%. These results are expected
as
electrospray ionization (ESI) is a much “harder” te
chnique than Murphy Riley and is thus liable to bre
ak apart larger
molecules. Although ESI is often considered a “sof
t” MS technique, the analyte is propelled quickly t
hrough a part
of the spectrometer that is held at atmospheric pre
ssure. The collisions of phytic acid ions with the
gas molecules
likely remove phosphate ions from the carbon ring.
These phosphate ions are thus detected as inorgani
c phosphate.
It is expected that the carbon ring will be strippe
d of some phosphate ions during the molybdate reduc
tion
reaction in the Murphy Riley method; however, this
should not produce as much inorganic P as in the MS
procedure, as observed. Also, natural degradation
of phytic acid to produce inorganic P is expected,
even in the
reagent bottle. Thus the phytic acid solution may
consist of a certain percent inorganic P prior to a
nalysis. The
percent P measured by Murphy Riley may be represent
ative of this “natural” degradation percentage.
These results support the hypothesis that mass spec
trometry measures P not detected by the Murphy Rile
y.