Gas Chromatography/Mass Spectrometry

Gas Chromatography/Mass Spectrometry

Gas chromatography separates mixtures of volatile and semi-volatile organic compounds into individual components using a temperature-controlled, open tubular column. The sample is flash vaporized, and the molecules are swept onto the gas chromatography column with an inert carrier gas. Separation occurs as the components partition themselves between the stationary phase on the inner wall of the column and the mobile phase (the carrier gas). The time it takes for a given molecule to traverse the entire length of the column is known as the retention time. The retention time is a function of the chemical structure of the component, the column type and the temperature profile it was subjected to during the chromatography experiment. It depends on the relative affinity of the compound for the stationary and mobile phases. The mass spectrometer then detects the components that elute from the end of the gas chromatography column. In the mass spectrometer, energetic electrons bombard the component molecules, ionizing some of them. This ionization process can also produce fragment ions which often provide structural information about the molecule. The ions are then accelerated by an electric field and enter a mass analyzer, where they are separated according to their mass-to-charge ratios. By plotting the abundance of ions as a function of mass-to-charge ratio, a mass spectrum is generated. The mass spectrum can be a unique “fingerprint”, allowing identification of unknown compounds. This “fingerprint” is compared with a database of over 107,000 unique chemical compounds. If a reasonable match is obtained, the analyst uses this information to help identify the compound. Frequently, the database is of no help. This can occur for one of two reasons:

A) The mass spectrum contains unique chemical information, that is, abundant, unusual fragment ions—yet no compound in the database is even close to matching it, or

B) The mass spectrum is not very unique at all. It matches quite well with as many as a thousand other compounds. In this case, the best that can be done is to place it within a chemical class.

Compounds often cannot be analyzed by a particular method, because they are not in a form amenable to the analytical technique. Examples of this problem are non-volatile compounds for gas chromatographic analysis and insoluble compounds for HPLC. Many compounds that are not stable under the conditions of the technique also fall into this category. Di- and tri- basic acids as well as hydroxy acids are good examples of this type of molecule. The derivatization procedure modifies the chemical structure of the compound, so that they may be analyzed by Gas Chromatography.

Dynamic Headspace Analysis
Dynamic Headspace Analysis (HSA) was developed primarily for the analysis of volatile compounds in matrices that could not be directly injected into a gas chromatograph. These matrices include polymers, cosmetics and toiletries, food and beverages, environmental samples, and biological specimens not suitable for direct injection. In dynamic HSA, the sample is placed in a closed chamber, heated to a specified temperature, and the atmosphere surrounding the sample is continuously swept with a stream of dry inert gas. The components that outgas from the sample are collected and analyzed by GC/MS. The temperature normally used for this test is 85°C, and the time-at-temperature is typically three hours. For refractory materials, temperatures as high as 400°C can be used.

Pyrolysis
Pyrolysis is a technique normally used to analyze non-volatile organic compounds such as wood, paper or polymers, by GC/MS. In pyrolysis the sample is heated rapidly to 750°C or higher in order to thermally decompose it. A high temperature such as this is sufficient to break the polymer backbone, forming smaller, more volatile fragments. By examining these fragments, it is sometimes possible to deduce the structure of the polymer chain. Some polymers un-zip during pyrolysis to yield only the original monomer. This technique is used frequently to examine materials for the presence of additives such as plasticizers, anti-oxidants, flame retardants, UV-stabilizers, or sizing treatments applied to cloth samples.

Solids Probe
Direct solids probe analysis is a volatilization technique that places the sample under vacuum near the mass spectrometer's ion source. The sample's temperature can either be raised to a preset maximum or it can be heated gradually, in a temperature programmed fashion. During this time, the molecules that volatilize from the sample continuously enter the mass spectrometer's source, and are ionized much in the same fashion as described above with GC/MS. The disadvantage of this technique is that there is no separation step. This is yet another way to analyze non-volatile samples using mass spectrometry.

Enhanced Sensitivity for Quantitation with Tandem Mass Spectrometry (MSMS)
Most of the applications of GCMS in our laboratory are involved with the identification of unknowns—usually trace-level contamination studies. However, frequently we get requests from a customer asking us to verify that a particular contaminant has been successfully removed from the sample submitted. The compound in question might be a lubricating oil, or a cleaning agent, for example. In cases such as these in which compound identification is not requested, the sensitivity of the mass spectrometer for a particular analyte can be increased greatly.

In single-stage Mass Spectrometer systems, this is accomplished using Selected Ion Monitoring, or SIM. With this technique, the most abundant (and characteristic) ion in the mass spectrum of each component to be quantitated is selected for scanning, and the instrument is programmed to scan this ion rather than the entire mass spectrum. Using this technique, the limit of detection can frequently be lowered by a factor of 10 - 50. The disadvantage of this method is that most of the mass spectral information for the component is lost. This is not much better than having a gas chromatograph with a flame ionization detector. For, in trace level work, any component that has an ion with the same mass as the one being scanned, and the same retention time will be mistaken for the component of interest. These small peaks are called chemical noise.

On the other hand, with a tandem (dual-stage) Mass Spectrometer system, the chemical noise can be reduced almost to zero, lowering the limit of detection significantly.

In Tandem Mass Spectrometry (as the name implies), a second mass spectrometry stage is added to the first. This allows the analyst to select one of the fragment ions from the first mass spectrum for further fragmentation. Now, we have a mass spectrum of an ion, whereas in single-stage mass spectrometry we have a mass spectrum of a molecule. Recalling our discussion earlier about Selected Ion Monitoring, we can now apply this same principle to tandem mass spectrometry. With this technique, we select the most abundant fragment ion from the first mass spectrum for second-stage fragmentation. This is called the parent ion. A mass spectrum is obtained. From this spectrum, we select the most abundant ion to monitor—in the same manner as SIM. This is called the daughter ion. Now, when the sample is chromatographed, only those peaks will show up at the desired retention time that contain the daughter ion originating from the parent ion that was selected from the original mass spectrum. This greatly reduces the signal background or chemical noise.


Strengths

Complex mixtures can be separated for the identification of organic components.
Quantitative information is readily obtainable.
The technique is capable of trace level determination of organic contamination; low to mid-ppb level for liquids, and low nanogram level for solids (Dynamic Headspace Analysis).
Weaknesses

The sample must either be volatile, or capable of derivatization.
If the sample itself is not volatile (as in headspace, pyrolysis or direct probe samples), then the material being analyzed (detected) must be volatile.
This is not a surface technique.


Typical Applications Identification and quantification of volatile organic compounds in mixtures, outgassing, residual solvents, liquid or gas injections


Signal Detected Molecular/characteristic fragment ions


Elements Detected Molecular ions to mass 800


Detection Limits 400 ng (full scan)
10 ng (outgassing)


Depth Resolution -


Imaging/Mapping -


Lateral Resolution/
Probe Size -