21 Gas chromatography-mass spectrometry

Dr. Varinder Kaur

Objectives: To study the basics of gas chromatography and know the following questions.

  1. Which type of analytes is suitable for GC-MS?
  2. What are the main components of GC-MS?
  3. What is the fundamental basis in GC-MS?

1. Description

The combination of Gas chromatography with mass spectrometry as detection system is termed as Gas chromatography-Mass spectrometry (GC-MS). It is used for the separation and identification of components of a sample including drugs, environmental toxins, explosives, plant extracts, material samples like soil samples, etc. It allows analysis and detection of a substance in sub-nano levels. It is also applied by forensic experts because it can be applied to get the specific analysis to know the presence of a particular subtstance.

GC-MS is used to analyze complex organic and biochemical mixtures. The gas chromatography part works on the principle of separation of components on the basis of the volatility and partitioning of components between stationary phase and inert carries gas. The separated components are detected by the MS part according their mass-to-charge ratio (m/z) and displayed in the form of chromatogram.

The mass spectrometer is highly sensitive, selective and highly reliable with maximum accuracy as compared to conventional detectors such Flame ionization detector (FID) or electron capture detector (ECD). The MS detector is easy to handle with a variety of samples and moreover, small ‘bench-top’ GC-MS instruments are available in the market for routine applications.

2. Ionization methods

The mass spectrometers detect ionized sample molecules so the sample must be ionized for detection.

The ionized forms of a neutral molecule can be produced by the following ways.

2.1 Electron ionization

It is most important ionization method used with GC-MS. In this method, a tungsten wire is used as filament and a potential difference is set up between the filament as ions chamber. When this filament is heated, it produces electrons, which are accelerated into a chamber due to potential difference. The electrons interact with the molecule and ionize to produce a positively charged ion by the removal of an electron. The molecular ion undergoes further fragmentation due to its less stability. The fragments are formed either by the removal of a neutral molecule or a free radical from the molecular ion. The formed species further undergo fragmentation to form new fragments.

The peaks for the fragmentation are obtained on the basis of nature of compound, electron energy and ion source temperature. The main limitation of this method is the loss of molecular ion peak in the spectrum due to un-controlled fragmentation of the species in the ionization chamber. Therefore, the important information about the molecule is lost.

2.2Chemical Ionization (CI)

The chemical ionization involves low energy ionization of the molecule. This method involves ionization of a gas molecule which interacts with the analyte molecule to produce various species. The modes for the ionization of a neutral species via chemical ionization method are given below.

2.3 Positive chemical ionization (PCI)

In PCI, reagent gases like methane, isobutane or ammonia are filled in the ion source at a pressure of about 1 mbar. Therefore, the gaseous ions formed in the ionization chamber interact with the neutral gas molecules to produce various species. The formation of ionic species in case of methane as reagent gas is given below. Further these fragments react with the analyte neutral molecule to form new species. The possible interactions between gaseous methane ions and analyte are given below.

Therefore, when methane is used as reagent gas, mass fragmentation peaks pertaining to (M+H)+, (M+CH5)+ and (M+C2H5)+ species are observed in the mass spectra. However, in both the methane and isobutane, proton transfer is more facile as compared to electrophilic addition. In contrast, for ammonia as a reagent gas, addition reaction is more prevalent and therefore, peaks for (M+NH4)+ are observed in the mass spectra.

2.4 Negative ion chemical ionization (NCI)

This mode produces negatively charged species for detection in MS. There are two main methods to produce the negatively charged ions as given below.

In the proton transfer method, the proton is transferred to the reagent gas anion if its proton affinity is higher. In this case, the molecular ion is observed at (M-H)- m/e ratio. In the electron capture method, the electron produced by the reagent gas is captured by the neutral analyte molecule under the pressure conditions to produce a radical anion.

3. Analyzers used in MS

The ions formed in the ionization chamber are accelerated directly to the mass analyzer. The role of analyzer is to separate the ions according to their m/e ratio. The analyzers used in the MS are of several types.

1.  Quadrupole (Q)

2.  Ion trap (IT)

3.  Time of flight (TOF)

4.  Fourier transform resonance ion cyclotron (FT-ICR)

1.   Quadrupole mass spectrometers: It utilizes four parallel electrode in which oppositely charged electrodes are connected together. In the applied voltage conditions, ions with particular m/e ratio pass through the quadrupole and others are thrown out from the original path. The ions passed through the quadrupole filter are monitored. The quadrupole filter can be modulated to the desired mass by varying the amplitude of radio frequency voltage. Secondly, a selective mass can be determined by adjusting DC and voltage ratio. Moreover, complete spectrum can be scanned by controlling the DCand RF voltage.

2. Ion trap mass spectrometer

The basic principle of ion trap is same as quadrupole mass filter. However, it consists of a ring like (doughnut shaped) electrode and two caps making the quadrupole field three dimensional. The RF potential is applied within the circular ring and AC/DC in cap like electrodes. The ions are transported from the ionization chamber to the trap and are separated by varying the RF voltage. The ions are accelerated from the trap in ascending order of their mass and detected.

3. Time of flight

In case of time of flight mass spectrometers all the ions leave the accelerating field with same kinetic energies but with different velocities depending upon their masses. The ions are accelerated by a potential difference between A and B and pass into flight tube. They are separated according to their m/e values and collected at certain point. The time of flight is given by t = k√m/e. k is a constant depend upon length of flight time. The main benefits for time of flight mass analysers include fastest speed, high ion transmission, highest practical mass range of all mass analysers etc.

4. Fourier transform resonance ion cyclotron

The ions are formed in an ionization chamber which is further subjected to a constant magnetic field and an electrostatic field. Source of an ion formed in the ionization chamber is accelerated in one way then in other way forming a circular path by magnetic field and the ions spiral outwards to the collector. For the good sensitivity the ions should kept resonating for times in the range of milliseconds to seconds. This method offers a good sensitivity and high resolving power. The working equation for ICR can be quickly derived by equating the centrifugal force and Lorentz force experience by an ion in a magnetic field.

mv2/r = ev/B

Solving the angular frequency which is equal to v/r

ὼ = v/r = eB/m

A group of ions having same mass to charge ratio will have the same cyclotron frequency, but they will be moving independently and out of phase at roughly thermal energies. The main benefits of cyclotron analyser are the highest recorded mass resolution of all mass spectrometers. It has non destructive ion detection, ion measurement and a stable mass calibration in superconducting magnet systems.

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