Page 383 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN0011A-541 July 25, 2001 17:27

486 Organic Chemistry, Compound Detection

evaporated, and separation of charges present in the nom- F. GC–MS–MS
inally neutral solution allows positive- and negative-ion
Full-scan and selected ion monitoring (SIM) represent at
mass spectra to be recorded. The performance of LC/MS
least 95% of the MS analysis performed today. Full-scan
for β-hydroethyltheophylline is 10 pg (selected ion mon-
analysis has always offered three benefits: (1) selectiv-
itoring) or 1 ng (full spectrum) with respect to its de-
ity based on m/z detection, (2) universal detection across
tection limits. The method involving direct liquid intro-
a wide operating range and (3) library searches (spec-
duction and CI gives comparable data; for example, 50
tral comparison) for confirmation of identity. Since the
ng of vitamin B 12 gives a negative-ion spectrum of high
source of the data for SIM is the same as for full scan,
quality.
SIM offers no solutions to interference problems. The
Fourier transform mass spectrometry (FTMS) illus-
primary advantage of SIM has always been its improved
trates the speed with which instrumental developments
signal-to-noise ratio (S/N) versus full scan and its inter-
are transforming mass spectrometry. The high resolution
pretation simplicity (ion ratios).
of FTMS is probably its most important aspect. Impressive
When MS–MS is operated in a selected reaction moni-
performance data have been reported, such as a resolution toring (SRM) mode, the first step is identical to SIM. As
6
of 1.4 × 10 for m/z = 166 from tetrachloroethane and a result, isolation of the parent (precursor) ion offers no
8
10 for m/z = 18 from water. FTMS instruments are capa-
better data than SIM, assuming equivalent spectral reso-
ble of performing MS–MS experiments. Unlike a conven-
lution. If these were interferences in the full scan and SIM
tional MS–MS experiment, in which the different stages
data, these would also be observed as increased intensity
of analysis are separated in space, the separation here is
of the parent ion for MS–MS.
achieved in time. This allows the extension of the exper-
The second step, collision-induced dissociation (CID),
iment to three (MS–MS–MS) or more stages. Because
does offer a solution to this problem. The amount of frag-
FTMS instruments require very low pressures for opti-
mentation in CID varies with the collision energy, but un-
mum performance, interfacing with chromatography is
der all conditions, the secondary ions (daughters or prod-
a problem, although a GC–FTMS has been reported by
uct ones) have a direct, unique relationship to the parent
Wilkins in 1982.
ion.
One aspect of LC–MS which is rapidly being developed
Quantitation based on the MS–MS daughter ion yields
is the HPLC–MS and has great promise for the analysis
accurate results, even in the presence of an interference.
of many compounds. The primary obstacle to easy cou-
Qualitative decisions are possible in MS–MS based on
pling of HPLC to MS arises from the fact that the flow for
library searches against user libraries or ion ratioing tech-
conventional HPLC columns is approximately an order
niques similar to SIM. MS–MS is actually a more power-
of magnitude greater than can be accommodated by the
ful tool for structural elucidation of unknowns that do not
commonly used ion sources and pumping systems. The re-
search well against standard libraries.
quirement for a transfer of a maximal quantity of sample
Detailed descriptions of the tandem-in-time principles
and a minimal amount of solvent into the ion source of the
of ion-trap MS–MS have been well documented. The crit-
mass spectrometer has necessitated the development of
ical elements of ion-trap MS–MS and the resultant advan-
different interfaces. The need for introduction of smaller
tages can be summarized as:
amounts of HPLC effluents into a mass spectrometer has
stimulated the development of narrow bore columns. The
application of HPLC–MS with a direct liquid introduction Steps of the MS–MS process are executed as a timed
has been applied to the identification of marine sterol per- sequence in a single analyzer, rather than across the
oxides by Djerassi and Sugnaux in 1982. An ultrasphere path of multiple quadruple analyzer.
ODS column of i.d. 5 µm and methanol–water (99:1, v/v) Low cost of only one analyzer; high sensitivity;
solvent system were used. changes from MS to MS–MS or MS–MS–MS can be
At present it is difficult to predict if the HPLC–MS fully automated; no special expensive collision gases
technique with still many difficulties will become a rou- such as argon; good fragmentation efficiency, even for
tine tool for detection and identification of HPLC solutes. stable ions, which extends the range of applications.
It is clear, however, that this technique remains the only MS–MS offers clear, simple, error-free results even
coupled MS method by providing unambiguous identifi- with complex samples.
cation of solutes in HPLC effluents at room temperature
or even lower than room temperature. Many natural prod- When one realizes that the low picogram and even fem-
ucts that have been identified by GC–MS techniques can togramdetectionlimits(amountsoncolumn)arecommon,
be verified by HPLC–MS. MS–MS becomes even more appealing.