Gas chromatography/mass spectrometry (GCMS)
is a powerful tool for organic analysis. GCMS is similar to GC, with one important
difference: the use of a mass spectrometer (MS) for detection. The MS allows
peaks eluting from the GC to be positively identified using their mass spectrum.
The mass spectrum is a fingerprint for the individual compound.
In GCMS, the mass spectra are usually generated in one of two ways. The more
common mode, called electron impact (EI) ionization, uses electrons to ionize
the compounds as they elute from the GC.
Initially, ions of the intact parent molecule (molecular ions)
are formed by collision with the electron beam. This also imparts excess energy,
and the ions fragment in characteristic and predictable ways. The individual
fragments are detected based on their mass-to-charge ratio, which is usually
equivalent to their mass for small molecules (since most of the ions generated
are singly-charged). This is a relatively harsh mode of ionization, which
yields more fragmentation, and therefore more structural information can be
extracted from the spectrum. These types of spectra are reproducible from
instrument to instrument using a fairly standard set of conditions. Because
of this, libraries of spectra are compiled, and spectra of unknown peaks can
be searched against these libraries to aid in identification. That makes EI
ideal for samples where the components are entirely unknown.
The second ionization mode is chemical ionization (CI). In CI,
a reagent gas (methane and isobutane are commonly used) is introduced into
the MS at a pressure of about one torr. This moderates the energy transfer,
and less fragmentation is induced in the analyte molecules. In fact, frequently
no fragmentation is seen at all. The only ions present in many CI spectra
are protonated molecular ions, and adducts with the ionization gas. This is
useful in determining molecular weights, but frequently gives little or no
structural information. CI can also be a more sensitive technique than EI.
This mode is often used in metabolite studies, where the structure of the
parent drug is known, but the metabolites are not.
GCMS has been used extensively in the environmental industry for many years,
but it can also be successfully applied to other disciplines. WCAS has used
GCMS for many "non-routine" analyses in the past, such as organic
alcohols in cardboard packaging, phthalate esters and cement residues in PVC
tubing, trace contaminants from ethylene oxide sterilization, residual monomers
in polymer formulations, off-gassing products from packaging materials, and
many others. GCMS is also invaluable in validation of pharmaceutical GC assay
methods, which require identification of degradant peaks from stressed samples.
It can also be of use in routine stability studies, as a mechanism to help
identify potentially toxic degradation by-products.
GCMS is also useful in cases like, "I ran this sample by GC, and I have
an unknown peak. Can you find out what it is?" In this situation, no
other instrument will do. We can perform a GCMS analysis using identical conditions
to those used for the initial GC analysis, and obtain mass spectral information
on the unknown peak. This may be an unknown contaminant at a waste site, or
an unknown impurity in a pharmaceutical product. Either way, determining its
identity can be critical to the client process, and GCMS is the best way to
accomplish this. If you have unusual organic analytical problems, GCMS may
be able to provide you the answers you are looking for. For
a quotation...
