General Information
The word that best describes the mass spectrometry of inorganic and Organometallic molecules is "difficult".
Many species are intractable to mass spectrometry
for one of a number of reasons.
Probably the most important issues are: oxidation state, molecular weight and polarity, the charge state of the desired species and any species that are considered to be equilibrium products.
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1. Oxidation State: Unlike organic mass spectrometry, many organometallic compounds contain atoms that can change their oxidation state with relative ease. Oxidations or reductions that require a few eV readily occur under conditions which require the application of several thousand volts (for example FAB-MS or electrospray MS). In such cases, the appearance of species which apparently contain atoms with different oxidation states to those expected should be of no surprise.
2.
Molecular Weight / Polarity: Just as with organic molecules,
the issue of volatility is important in selecting an ionization process.
Neutral organometallics tend to be relatively
volatile compared with organics of identical molecular weight, because
volatility is somewhat dependent upon the number of atoms present in a
molecule and organometallics tend to contain a few heavy atoms. For example,
Group III cube complexes of which Ga4S4(tBu)4
is an example, are volatile up to mass 1,200 or so.
Volatile organometallics often work quite well by Electron Impact. Chemical ionization typically does not work as well, since the metal atoms have a propensity to react with reagent gas molecules (ammonia).
For involatile materials, FAB and electrospray
are reasonable candidates, but this will depend upon the reactivity of
the species with the matrices and solvents employed for these ionization
techniques.
3. Charge State: Many organometallic species are prepared as salts and many of these are highly-charged ions (z > 2). Many of these species are hard, or even impossible, to analyze because they will not form stable ions in the gas phase. It is well known that electrospray ionization has a propensity to show multiply-charged ions, with perhaps as many as 100 charges on a large protein. However, the issue of charge density is most important. If an ion of mass 1400 is supposed to be a 4- ion, it will not be observed in any MS method, because the energy required to strip away all the cations and leave the species as a bare anion is just too high.
However, note that highly-charged ions may fly as cation / anion clusters
with a smaller overall net charge. It is therefore sometimes possible to
obtain useful information from such compounds, but careful interpretation
is recommended!
4. Equilibrium Species: Many organometallic species are present as equilibrium products with their starting materials. They may well be identified by other spectroscopic means, such as NMR, and such samples are then presented for MS. There are two separate issues to consider. First, does the equilibrium favor the products or reactants? If the equilibrium lies 90% towards the reagents then the likelihood of obtaining a good - quality mass spectrum of the products is low. Second, generation of equilibrium products often has the solvent species implicitly involved in their formation. Once these species are taken to the mass spectrometer, the solvent will be removed and the sample will enter the gas phase in a desolvated state. This process disrupts the equilibrium which established the products in the first place. In many cases, this will drive the products back to the starting condition.
We recommend you confer with your friendly
local mass spectrometrist before preparing any organometallic
or inorganic samples for mass spectrometry.
AT
10/4/99