Page 382 - 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

Organic Chemistry, Compound Detection 485

(FAB), secondary ion mass spectrometry (SIMS) and fis- analysis of mixtures. The two principal advantages of
sion fragment methods] which allows ionic, nonvolatile this system can be illustrated by considering a complex
compounds to be examined by MS. A further develop- coal liquid mixture. The signal due to a dioxin spike is
ment is the integration of separation and analysis tech- lost in the chemical noise from the other constituents
niques represented by tandem mass spectrometry (MS– and single-stage mass spectrometry is not capable of
MS) and the much improved liquid chromatography-mass analyzing for it. MS–MS filters against chemical noise
spectrometry (LC–MS) interfaces. The latter capability and allows a high-quality spectrum of the dioxin to be
is already proving its effect with respect to GC–MS but recorded. In addition to improving detection limits in this
without the limitation to volatile, low-mass compounds. way, tandem mass spectrometry provides alternative scan
MS–MS is a two-dimensional form of spectrometry which modes which can be employed to search the data bank for
often improves signal-to-noise ratios as well as provid- particular information. For example, chlorinated dioxins
ing entirely new capabilities, such as that of scanning a are characterized by the loss of COCI, so a scan for the
mixture for all constituents having particular structural reaction reveals all dioxins present in the mixture. The
subunits. GC–MS–MS system can decrease GC–MS detection lim-
The GC–MS combination has had considerable impact its by an order of magnitude. It has been reported that the
on biological and environmental research, providing a spe- drug, isosorbide-5-mononitrate, a coronary vasodilator, is
cific means of characterizing constituents of mixtures and metabolised to the glucuronide, which can be determined
having sufficient sensitivity and quantitative accuracy to in urine by a simple MS–MS procedure to 0.1 ng/mL.
trace constituents. The recent discovery in 1977 of the neu- The improved detection limit in the MS–MS experiment
ral excitotoxin, quinolinic acid, in the mammalian brain is the direct result of minimizing interferences. Both
at the level of nanomoles per gram was achieved by GC– the sensitivity and the speed of analysis with MS–MS
MS of the volatile hexafluoroisopropanol diester deriva- can be illustrated with the administration of the drug
tive with electron impact (EI) ionization. Quantitation was tetrahydrocannabinol in doses of 0.1 mg/kg for eight days
based on standard addition and single-ion monitoring. down to 10 −11 g/ml by using a combination of GC–MS
The GC–MS technique was applied in the discovery of with simple MS–MS to avoid extensive sample cleanup.
19-hydroxylated E prostaglandins by first protecting the Sensitivities in parts per trillion have been reported in
unstable β-ketol system by oximation. This discovery, in MS–MS studies on animal tissue. High-resolution mass
1974, implied that the previously identified prostaglandins spectrometry and MS–MS have been used to achieve
could be artifacts. absolute detection limits of less than 10 −12 g for tetra-
Challenging structural problems. including protein chlorodibenzodioxin and a GC–MS–MS combination has
structure determinations, can often be solved by a com- produced spectra with 250 fg samples (<20 parts per tril-
bination of techniques. Two mass spectrometric methods lion) at the rate of 30 samples per day. In terms of sample
were used to deduce the structure of the 112-amino-acid throughput, the determination of trichlorophenol in serum
antitumor protein macromycin derived from streptomyces at concentrations as low as 1 ppb and a rate of 90 samples
culture. Partial acid digestion gave a mixture of di- to per hour was reported. Characterization of the foodstuff
hexapeptides, which was derivatized and analyzed by GC– contamiant (and chemical warfare analog) vomitoxin at
MS with EI. 25 pg in wheat at a rate of 10 minutes per samples was also
Tandem mass spectrometry has been applied in the dis- reported.
covery and confirmation of the structure of the metabo- A different approach to the characterization of mixtures
lites of the drugs primidone, cinromide, and phenytoin in of nonvolatile compounds is LC–MS. The first practical
plasma and urine extracts; analyses were completed in less LC–MS interface was based on complete removal of the
than an hour, using concentrations of 1 to 50 µg/mL. The solvent and temporary storage of the solute during trans-
study is predicted on the speed and flexibility of MS–MS port by a moving belt or wire into the ion source. In the
scans made with a triple quadrupole instrument, and on source the sample is either thermally desorbed and ionized
the realization that metabolites often retain a large portion by electron or chemical ionization or the belt is bombarded
of the parent drug structure. by an energetic beam to create secondary ions. The large
Analysis of complex mixtures has often involved pumping capacities of chemical and atmospheric ioniza-
time-consuming procedures such as extraction, centrifu- tion sources make it possible to work at flow rates consis-
gation, and chromatography. MS–MS and LC–MS are tent with normal column operation, for example, 2 mL of
recent developments that reduce such time-consuming aqueous mobile phase per minute. The solvent itself acts
methods. By linking two mass spectrometers in tandem as the reagent gas in these experiments. An alternative,
it is possible to employ the first as a separator and the the thermospray procedure does not use any externalion-
second as an analyzer and, hence, to perform direct ization technique. An aerosol generated in the interface is