Crime dramas have become a mainstay on network television programming. Each week viewers tune in to find out whodunnit and with which object: the rope, the candlestick, or the lead pipe? As portrayed in crime dramas, items recovered from a crime scene are sent to "The Lab," where a series of curious individuals -- magicians, perhaps -- analyze the samples and spit out an answer. This post aims to elucidate the mechanisms that analytical chemists employ to provide answers to the dastardly quandaries that cause Detective Stabler's pretty little brow to furrow.
The Big Black Box
By and large, mass spectrometers are among the most widely used instruments in analytical laboratories. These instruments are used to unequivocally provide the molecular weight of substances based on the mass-to-charge ratio of the substance in question. The molecular weight can then be used for identification if the substance is unknown. Many types of mass spectrometers exist; for the purpose of this post, the mechanics of a standard triple quadrupole mass spectrometer will be outlined.
As the name denotes, a triple quadrupole mass spectrometer consists of three quads: two mass filters, Q1 and Q3, separated by an RF-only collision cell, Q2. Each quad consists of four cylindrical rods that serve as electrodes. Rods opposite each other are held together electrically, one pair to the positive terminal of a DC power source and the other to the negative terminal. Radio frequency potentials are also applied to each pair of rods. Together, the voltages applied to the quad serve to propel charged particles (i.e., the ionized sample) along the length of the instrument.
Triple quadrupole mass spectrometers can be operated in two modes. In the first and most basic mode, MS mode, the molecular weight of the intact sample is determined. In MS/MS mode, the ionized sample is fragmented, or broken apart, in Q2 by collision with gas particles, and the molecular weights of the fragments are determined. The resulting fragments create a fingerprint, of sorts, that can also aid in identification of the sample. This feature proves particularly useful in distinguishing between substances that may have the same molecular weight, referred to as "isobaric species." To clarify, obtaining the molecular weight in MS mode is analogous to stepping on your bathroom scale to determine your body weight. However, there are undoubtedly many people with the same weight. This is where MS/MS comes in. Let us imagine that you were, say, broken apart into several unique pieces. Those pieces could be used to identify you out of the masses. (Suddenly, the whole sum-of-my-parts argument just got a lot more complex.)
The Front of the House
If the business end of the mass spectrometer can be described as an overworked, crowded kitchen, using the same restaurant analogy, the inlet can be thought of as the front door. It's what gets the sample into the instrument. Many types of inlets exist, and selecting the appropriate one is largely dependent upon the physical properties of the sample. If the sample is volatile and readily enters gas phase, gas chromatography (GC), though an analytical technique in its own right, can be used as a sample inlet. This is called GC-MS and is commonly used to analyze illicit drugs. Alternatively, if the sample is liquid and has UV absorbance, such as a blood sample, liquid chromatography (LC) may be used as the sample inlet.
Similarly, the ion source can be thought of as the reception area. To use more scientific terminology, the source is the part of the instrument that ionizes the sample and accelerates the newly charged particles into the instrument using kinetic energy. Remember, according to Newtonian physics, kinetic energy is dependent upon the mass of the substance as well as its speed; this property is utilized once inside the mass filter.
The Next Step
Now that we understand what a mass spectrometer is and how it operates, let's discuss how it can be used to answer a forensic question that might appear on a crime drama:
The blood sample of a victim, found unconscious near a popular nightspot, was sent to toxicology for analysis. LC-MS was performed on the plasma, and the resulting chromatogram and mass spectrum confirmed the presence of ketamine at a concentration of 20.0 mg/L . The plasma concentrations of norketamine were more than double than that of ketamine. GC-MS/MS analysis showed a racemic mixture of R and S enantiomers of the drug; a database search linked the ratio between the two isomers to a European supplier.
This information is invaluable to detectives attempting to solve a crime. Ketamine, though popular amongst partygoers for recreational drug use, is also commonly used as a date-rape drug. Traditionally, the drug has been used as an anesthetic between the range of 1.0 and 6.3 mg/L. The victim's plasma ketamine concentration is well outside that range, and from this it can be deduced that the cause of death was acute intoxication. Norketamine, a metabolic derivative of ketamine, is found at high concentrations when the drug is administered orally as opposed to intravenously. Using this fact, the detective can begin scouring the streets for dealers with merchandise in the tablet form over the more commercially available liquid version. Because the database match confirmed that the drug is likely European in origin, the detective can then begin narrowing down which crime syndicate might be involved.