GAS CHROMATOGRAPY AND MASS SPECTROMETERY .SUMMARIZING FROM A PHYSIC BOOK.MOHAMED ANWER RIFKY

1. Gas Chromatography and Mass Spectrometry MOHAMED ANWER RIFKY
Are two versatile techniques>>identify or measure the concentrations of anaesthetic agents or other gases present in a mixture. they are
more commonly employed as research techniques in the laboratory.
GAS CHROMATOGRAPHY: Chromatography literally means to write in colour. If a mixture of dyes is poured into a column packed with
calcium carbonate the pigments separate out as coloured bands at different levels, and the components may eventually be collected from the
outlet. Chromatography (separate a mixture into its components).1- calcium carbonate (column) >>Stationary phase which retains the
individual dyes for different periods, hence separating the mixture. The dye mixture passing through the column forms a mobile phase. 2-Gas
Chromatograph :a-The column >> material such as silica-alumina in the form of very small particles which is then coated with silicone oil. b-
The gas mixture to be analysed is injected into the inlet of the column into the mobile phase>>carrier gas (N2). c-components pass through
the column at a speed which depends on their differential solubility between the two phases. d-the column is maintained at a preset
temperature vin a thermostatically controlled oven.e-the column is usually a long coiled glass tube (added length of the column gives better
separation of the components) .
DETECTOR: The outlet of the column >> detector and a recorder. 1-The flame ionization detector: hydrogen
and air are added to the gas to produce a flame at the end of the column. The constituents of a flame are ionized
particles. If a polarizing voltage is applied across the flame by two electrodes, a current is produced>>depends
on the charged particles.As organic vapour is present in the carrier gas>>ionized in the flame>>increases the
current>>amount of organic vapour present.recording appears in the form of a peak as each component of the
sample. 2-The katharometer: Measures changes in thermal conductivity of the gas by the change in the electrical
resistance of a heated wire placed in the gas flow. More suitable for the analysis of inorganic gases such as
nitrous oxide or oxygen. 3- Halogenated compounds can be detected with greater sensitivity by an electron
capture detector(In this detector a polarizing voltage is applied across an ionization chamber in which electrons,
released from a radioactive cathode, constitute a flow of current to the anode. Halogen Gas compounds capture
these electrons and reduce the current flow. N.B it is necessary to have preliminary knowledge of the types of
substances present.
ANALYSIS OF THE RECORDER TRACING: As substances progress at characteristic rates through the
separation column>>time between the injection of a sample and its appearance at the detector >>identify the
component(retention time)>>in four minutes (as an example)>>mixture of substances may be present with
several peaks(characteristic retention time).calibration >>confirmation of the presence of the substances and
the comparison of the peak heights
INTRODUCTION OF A SAMPLE INTO THE GAS CHROMATOGRAPH: The sample is through a rubber septum. If the port is
heated>>liquids are vaporized and the volatile agent analysed>>blood samples can also,be analysed directly,as samples may be
introduced by a special gas sampling valve.
CLINICAL USE :Gas chromatography >>measuring volatile anaesthetic agents,and for trace levels in the theatre atmosphere.It is
also of use in the measurement of barbiturates,phenothiazines, benzodiazepines, steroids and catecholamines, Gas
chromatography has several advantages: 1-It allows the identification and measurement of very low concentrations of drugs, 2-
The technique is versatile (liquids and gas samples can be analysed). However, its disadvantages: 1-continuous analysis is
impossible an, 2- details of the sample constituents must be available.

2. THE MASS SPECTROMETER: 1-The sample is drawn through a tube into the sample chamber of a mass
spectrometer by a pump. 2-A molecular leak permits a few molecules of the sample to diffuse into an ionization
chamber >>bombarded by a beam of electrons passing from the hot cathode to the anode.3-charged ions which
are then accelerated out of the chamber in a narrow beam by means of the acceleration and focusing plates.4-
ions >> a strong magnetic field>> charged ions are deflected in an arc depending on their mass; the lighter ions
being deflected most. 5-only one beam of which passes through a small slit to be picked up by a detector. 6-
The signal is amplified and displayed. By varying the voltage on the acceleration and focusing plates >> the
position and speed of the beam may be altered >>streams of ions of different masses detected.7-Another
method :Quadrupole mass spectrometer: a-Four electrically charged rods in place of the magnetic field.b-The
potentials on the rods are varied so that the ions oscillate between them as they travel>>only ions of a specific
mass travel the length of the rods without being removed from the stream.8- Many molecules fragment form
during the ionization process (carbon dioxide >> carbon monoxide and oxygen ions),(CO2& N2O both have the
same mass number of 44)>>can help in the identification .9- Masss pectrometers with a rapid response time of
less than 100 ms have been used for the breath by breath analysis of expired air.Bulky and less convenient for
use in areas outside the laboratory. Anaesthetists often use alternative techniques in such areas.It can analyse
very small samples and also used in isotope studies
OTHER TECHNIQUES OF VAPOUR ANALYSIS: When an electric potential is applied across a crystal of quartz it
contracts slightly( piezoelectric effect).The crystal to which a thin oily coating has been applied>> Anaesthetic
agents dissolve in the oily coating and alter the resonant frequency of the crystal proportional to the partial
pressure of the vapour ( measured electronically). Lastly,measurement of the velocity of ultrasound in a gas
mixture is being used. N.B Henry 's law of solubility>>anaesth.in oil.
RAMAN EFFECT: 1-Electomagnetic radiation react with a molecule>>a-scattered by the molecule with an E.=
that of the incident r. but in different direction. b-Transfer of E. completely as infrared r. by CO2. c-partial transfer
of E.>>Raman effect>>vibration of the molecules>>specific bond by absorption of a particular
wavelength>>change of wavelenght. Wavelenght is proportional to E. of radiation>>the decrease of wavelength is
characteristic of the molecule type.2-It's a weak effect>>need intense light as laser. and detectors are not on the
line with radiation to identify low-intensity radiation.3-The gas monitor: a-helium-neon laser between two mirrors.
b- eight detectors (filter that allows the specific wave length& a photodetector>>convert incident rays to electrical
signal for measuring and processing). d-filters for O2,N2,CO2,N2O,the C-H bond,and three volatile anaesth. c-
drying gas by H2O removal. d-C-H bond for volatile anaesth.(a stronger signal than others),there is usually one
anaesth.at a time>>makes it more accurate. e-pump to remove degradation products from the heat.