Michael P. Balogh

Although we most often associate the suffix "-omics" with biologically important protein studies in living systems, the concept nonetheless applies to large and diverse groups of compounds in other important areas. Ryan Rodgers heads a petroleum research group in Professor Alan Marshall's group at the National High Magnetic Field laboratory at Florida State University. He offers rare insight into the need for extremes of accurate mass measurement, and its attendant data handling, to characterize hydrocarbons in petroleum production. The search for increasingly scarce petroleum reserves has impelled developers to adopt deep and ultradeep-water production techniques, producing oil in 5000–10,000 feet of water and thereafter drilling 5000–20,000 feet into the Earth's crust. Such projects entail a $1 billion, or more, decision regarding whether to produce an oil reservoir. The extreme circumstances prohibit sending a diver down to address production issues and limits the resolution of seismic instrumentation needed to discern what the reservoir looks like. Seismic data are still obtainable and important however the quality of the data is limited by both the depth of the water and depth of the ocean floor sitting on top of the reservoir. When the chemistry is not well understood, unfortunate and costly things can happen as oil is removed from a reservoir. For instance, Rodgers' group investigates the chemistry associated with asphaltene deposition in deep, undersea, petroleum-delivery lines. Asphaltene deposition is a sort of coronary artery disease for the petroleum industry. Asphaltenes aggregate on the inside of pipes as they bring oil to the surface producing backpressure, clogging valves and putting pressure of a different sort on oil production. Because it is impractical to send a diver down to remedy the problem, Rodgers' goal is to find a way for workers to anticipate the problem before it becomes catastrophic. Without sufficient knowledge of the chemistry in the reservoir before attempting to extract it to the surface the oil company risks the need to station a cleaning vessel on the surface, above its production facility, at a cost of between a half-million to one million dollars a day to clean the pipes.

So how does the industry get information on such complex mixtures? The National High Magnetic Field laboratory's high-field Fourier-transform ion cyclotron resonance (FT-ICR) building houses both 9.4-T and 14.5-T FT-ICR mass spectrometers. The instruments are fitted with various ionization sources — atmospheric-pressure photoionization (APPI), electrospray (ESI), field desorption, and low-voltage electron ionization — a variety made necessary by the analytical complexity and diversity of problems encountered in petroleum assays.

The studies focus on small molecules and the complete analysis of complex mixtures. You need only a single drop of crude oil to perform the analyses. That drop, which weighs about 30 mg, is then diluted to about 0.1 mg/ mL, so it yields about 300 mL of stock solution. Thereafter you perform an array of MS techniques on the stock-solution sample. These techniques include positive-ion ESI, negative-ion ESI, field desorption or low-voltage electron ionization, and APPI, and they yield thousands upon thousands of identified entities while consuming less than 1% of the original drop of crude oil.

Consider the design of the ion cyclotron. It ensures that the higher the field, the higher the resolution; the more ions you can pack into the cell, the higher the dynamic range. When analyzing oil we are dealing with numerous, closely related hydrocarbons. Nevertheless, the analytical approach resembles that of many other characterizations of closely related analytes. Complex mixture studies often require extended dynamic range, as in the case of drug metabolite studies. MS provides a wealth of information as well as the characteristics of the ionization method used. For instance, positive ESI is limited to ionizing analytes more basic than the solvent in which they are contained. So when you are looking at bases, you use positive ESI. But the converse is true when you are looking at acids. In that case, you use negative ESI.

These are the primary information levels derived from high-resolution MS analysis:

• Accurate mass, from which elemental compositions can be assigned.
• The compound class. For example, the presence of hydrazine carbon identifies ethyl benzene as a component of the hydrocarbon class.
• The number of rings and double-bond equivalents (DBE), or the Z-number, indicates the type of compound. For example, one ring and three double-bonds indicates a DBE of 4.
• The carbon number is simply the number of carbons in the molecule and the ratio of hydrogen but it can suggest the amount of alkyl substitution in the molecule.