INTRODUCTION

Gas chromatography- mass spectroscopy (GC-MS) is one of the so called hyphenated analytical techniques. It is a method that combines the features of gas chromatography and mass spectrometry to identify different substances within the sample. Additionally, it can trace elements in materials that were previously thought go undetected by other technologies. Gas chromatography separates the components of a mixture and mass spectroscopy characterizes each of the components individually. By combining the two techniques, an analytical chemist can both qualitatively and quantitatively evaluate a solution containing a number of chemicals.

In general, chromatography is used to separate mixtures of chemicals into individual components. Once separated, the components can be evaluated.

Gas chromatography

In all chromatography, separation occurs when the sample mixture is introduced or injected into a mobile phase. In gas chromatography (GC), the mobile phase is an inert gas such as helium. In liquid chromatography, the mobile phase is solvent.

The mobile phase carries the sample through what is referred as a stationary phase. The stationary phase is a usually chemical that can selectively attract components in a sample mixture and contained in a tube that referred as column. The columns can be glass or stainless steel of various dimensions.

The mixture of compounds in the mobile phase will interacts with the stationary phase. Each compound in the mixture interacts at a different phase. The difference in the chemical properties between different molecules in a mixture will separate the molecules as the sample travel down the length of column. Compounds that interact fastest will exit the column first and the slowest will exit the column last. Different mixtures of chemicals can be separated by changing the characteristics of the mobile phase and stationary phase. And this separation can be further refine by changing the temperature of the stationary phase by the used of oven or the pressure of the mobile phase.

As the compounds are separated, they exit from the column and enter a detector. The detector is capable of creating an electronic signal whenever the presence of compounds is detected. The greater the concentration in the sample, the bigger the signal. The signal then processed by a computer and generates a graph from the signal. The time when the injection of sample mixture is made to when elution occurs is referred to as the retention time (RT). The graph generates called a chromatogram. Each peak in chromatogram represents the signal created when the compound elutes from the column into detector.

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Figure 1: Chromatrogram generatd by a CG

By knowing the retention time (RT) for a given compound, assumptions about the identity of the compounds can be made. However, some compounds that have similar properties may have the same retention time. Therefore, further analysis is made to make true identification of the compounds in a sample containing unknown components.

Mass spectroscopy

When gas chromatography separates the components of a mixture, mass spectroscopy characterizes each of the components individually. As the individual compounds exit from the GC column, they enter the electron ionization detector. There, they are bombarded with a stream of electrons causing them to break apart into fragments. The fragments are charged ions with a certain mass. The mass of the fragments divided by the charge is called the mass to charge ratio (M/Z). The mass-to-charge ratio of a cation is equal to the mass of the cation divided by its charge.

mass−¿−charge ration= mass of cationcharge of cation

Since most fragments have a charge of +1, the M/Z usually represents the molecular weight of the fragment.

mass−¿−charge ration=mass of cation+1

¿massof cation

The mass of the molecular ion is equal to the molecular weight of the compound.  Thus, the mass-to-charge ratio of the molecular ion is equal to the molecular weight of the compound. The detector in the mass spectrometer counts the number of ions with a specific mass. This

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information is sent to a computer and a mass spectrum is created. The mass spectrum is a graph of the number of ions with different masses that travelled through the filter.

Figure 2: Mass-spectrum generated by MS

Gas chromatography-mass spectroscopy (GS-MS)

Figure 3: 3D depiction of GC-MS output

Figure 3 represents a three-dimensional graph generated when the GC is combined with the MS.

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INSTRUMENTAL COMPONENT

Figure 4: GC-MS

Carrier gas

The carrier gas used is inert gas which is helium, argon and carbon dioxide. Usually, the carrier gas is chosen based on the type of detector used. It is also contains a molecular sieve to remove impurities and water.

Sample injection port

A micro syringe is used to inject sample through a rubber septum into a flash vapouriser port at the head of the column. The temperature of the sample is usually 50°C higher than the boiling point of the least volatile component of the sample.

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The injector can be used in two ways namely split and splitless. It contains a heated chamber. The carrier gas enters the chamber and leave. The sample vaporizes to form a mixture of carrier gas, vaporized solvent and vaporized solutes. It then moves along the column.

Columns

There are two types of columns – packed and capillary. The capillary column is more efficient than the packed columns.

Column temperature

The optimum column temperature is dependent upon the boiling point of the sample.

Detectors

Different detectors give different types of selectivity. A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound.

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HOW DOES IT WORK?

The Gas Chromatography – Mass Spectrometer (GC-MS) instrument is made up of two components which is;

1. Gas Chromatography (GC)2. Mass Spectrometer (MS)

The GC functions as its separate the chemical mixture into pulses of pure chemicals while MS will identifies and quantifies the chemicals.

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Schematic of GC-MS

1. Gas Chromatography (GC) Injection port – a small amount of the chemical mixture is injected into the GC

and the sample is carried by inert gas through the instrument. The inert gas usually used is helium (He). Next, the inject port is heated to 300 °C for the chemicals in the sample to vaporize and become gas.

Oven – A specialized oven is built as one of the outer part of the GC. The column is heated to move the molecules through it. The average temperature of the oven is around 40 °C to 320 °C.

Column – The column which is coated with a special polymer coating on the inside and 30 meter thin tube is situated inside the oven. The mixture of the chemicals is separated based on their votality and carried through the column by helium. Lighter or smaller molecules will travel faster and quicker through the column than the heavier or larger molecules that is low in votality.

2. Mass Spectrometer (MS) Ion Source – From the GC, the chemical molecules is then entered the MS.

Here, ionization occurred as the molecules are bombarded with electrons. This result in the breakdown of the molecules into fragments and positively charged particles called ions.

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Filter – the ions travel through an electromagnetic field that will filters the ions based on their respectively masses. The filter then scans through the range of masses as the ions stream come from the ion source.

Detector – the detector counts the number of ions with a specific mass. The information collected then passed to the computer and mass spectrum is created. A graph of mass spectrum showing the number of ions with different masses that traveled through the filter.

ANALYSIS

MS is utilized in one of two ways: full scan or selected ion monitoring (SIM). The typical GC-MS instrument can perform in both ways depending on the setup of the instrument.

The objective of the instrument analysis is to quantify an amount of substance. It is done by comparison between the relative concentration and the atomic masses in the generated spectrum. There are two kind of analysis which is comparative and original. Comparative analysis is works by comparing the given spectrum to a spectrum library to see if its characteristics are present for some sample in the library. Usually, this is done by computer because it can correlate more data to more accurately relate certain data.

Other method is by measuring the peaks in relation to one another. In this method, the tallest peak is assigned 100% of the value while other values and peaks are assigned proportionally. The total mass of the unknown compound is usually the indicated by the parent peak. The isotope pattern that exists in certain compounds can be used to detect the various element presences. Once the chemical formula gas been matched to the spectrum, the molecular structure and bonding can be identified, and consistent with the characteristics recorded by the GC-MS.

A “full spectrum” analysis considers all the “peaks” within a spectrum. SIM conversely only monitoring selected ions associated with a specific substance. At given retention time, a set of ions is characteristic of a certain compound is predicted. This is more fast and efficient analysis. SIM allows for a smaller quantity of a compound to be detected and measured, but the degree of certainty about the identity of the compound is reduced.

Using full scan mode in collecting data, a target range of mass fragments is determined. An example of a typical broad range of mass fragments is between m/z 50 – 400. Then a MS should not be set to look for mass fragments too low. Large scan range resulted in less sensitivity of the instrument since it needed to detect wide range of mass fragments. Full scan is used to detect the unknown compounds in a sample. It gives more information than SIM.

In SIM certain ion fragments are inserted into the instrument method and only those mass fragments are detected by the mass spectrometer. The advantages of SIM are the detection limit is lower since the instrument is only looking at a small number of fragments during each scan.

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APPLICATION

Medication Drug detection Fire investigation Environmental analysis Explosives investigation Identification of unknown samples

REFERENCES

1. Gas Chromatography-Mass Spectroscopy Background. Retrieved 23 November, 2013 from http://www.gmu.edu/depts/SRIF/tutorial/gcd/gc-ms2.htm

2. Gas chromatography-Mass spectrometry. Retrieved 23 November, 2013 from http://www.smithsdetection.com/gc-ms.html

3. Mass-to-Charge Ratio. Retrieved 23 November, 2013 from http://science.uvu.edu/ochem/index.php/alphabetical/m-n/mass-to-charge-ratio/

4. Gas Chromatography. Retrieved 23 November, 2013 from http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm.

5. Amirav, A.; Gordin, A. Poliak, M. Alon, T. and Fialkov, A. B. (2008). "Gas Chromatography Mass Spectrometry with Supersonic Molecular Beams". Journal of Mass Spectrometry 43: 141–163.

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