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Gas chromatography (GC) and mass spectrometry (MS)
Gas Chromography GC analysis separates all of the components in a sample and provides a representative spectral output. The technician injects the sample into the injection port of the GC device. The GC instrument vaporizes the sample and then separates and analyzes the various components. Each component ideally produces a specific spectral peak that may be recorded on a paper chart or electronically. The time elapsed between injection and elution is called the "retention time." The retention time can help to differentiate between some compounds. The size of the peaks is proportional to the quantity of the corresponding substances in the specimen analyzed. The peak is measured from the baseline to the tip of the peak.
Imagine a pile of different types of balls resting at the bottom of an inclined, paved driveway. This pile includes ball bearings, marbles, ping pong balls, golf balls, wiffle balls, handballs, tennis balls, hockey pucks, baseballs, soccer balls, volley balls, basketballs, footballs, and bowling balls. Attempt to move this motley collection of balls up the driveway with a normal leafblower. Some of the pile will quickly move to the top of the driveway, some balls will migrate at varying speeds, and some balls may take an eternity to reach the end of the driveway.
The difference in the time that each type of ball takes to travel to the top depends upon the characteristics of each ball. Obviously, the lighter balls travel more quickly. Also, some balls may take longer due to their shape, like the hockey puck or the football. The different balls interact with each other as the air from the leaf blower acts on the pile. This interaction may hinder or accelerate the ball's travel as the balls strike each other. The surface characteristics of the ball may be important, as in the examples of the tennis ball and golf ball.
GC analysis depends on similar phenomena to separate chemical substances. A mixture of chemicals present in a specimen can be separated in the GC column. Some chemical and physical characteristics of the molecules cause them to travel through the column at different speeds. If the molecule has low mass it may travel more swiftly. Also, the molecule's shape may affect the time needed to exit the column. How the different substances relate to each other may cause the time needed to travel the column to increase or decrease. Interactions between the sample's molecule and the column surface may cause the molecule to be retained inside the column for a different amount of time than similar molecules that interact with the column differently.
Description of Process
The equipment used for gas chromatography generally consists of an injection port at one end of a metal column packed with substrate material and a detector at the other end of the column. A carrier gas propels the sample down the column. The technician uses flow meters and pressure gauges to maintain a constant gas flow. A gas that does not react with the sample or column is essential for reliable results. For this reason, carrier gases are usually argon, helium, hydrogen, nitrogen, or hydrogen. Many analysts use helium because it does not react. Hydrogen usually is a good carrier gas but it may react and convert the sample into another substance. The ultimate choice for a carrier gas may depend on the type of detector used.
To ensure proper separation, the sample must enter the column in a discreet, compact packet. Normally the sample is injected into the injection port with a hypodermic needle and syringe capable of measuring the specimen amount. The needle is stuck into a replaceable neoprene or silicone rubber septum that covers the injection port. The injection port is maintained at a temperature at which the sample vaporizes immediately. Ideally, the sample spreads evenly along the cross section of the column, forming a plug.
The column is a metal tube, often packed with a sand-like material to promote maximum separation. Columns are commonly obtained pre-packed by vendors. As the sample moves through the column, the different molecular characteristics determine how each substance in the sample interacts with the column surface and packing. The column allows the various substances to partition themselves.
Substances that do not like to stick to the column or packing move through the column rapidly. Substances that do not like to stick to the column or packing are impeded but eventually elute from the column. Ideally, the various components in the sample separate before eluting from the column end.
The GC instrument uses a detector to measure the different compounds as they emerge from the column. Among the available detectors are the argon ionization detector, flame ionization detector, flame emission detector, cross section detector, thermal conductivity detector, and the electron capture detector. Choosing the proper detector depends upon the use. Some considerations are that the flame detectors destroy the sample, the thermal conductivity detector is universally sensitive, and the argon ionization detector requires argon as a carrier gas. The spectral output is usually stored electronically and displayed on a monitor. The technician can produce a hard copy record.
The argon ionization detector does not detect water, carbon tetrachloride, nitrogen, oxygen, carbon dioxide, carbon monoxide, ethane, or compounds containing fluorine. The flame ionization detector does not respond to water, nitrogen, oxygen, carbon dioxide, carbon monoxide, helium, or argon. If a specimen contains water, a flame ionization detector should be used. The electron capture detector can not detect simple hydrocarbons but does detect compounds containing halides, nitrogen, or phosphorus.
Retention Time
The amount of time that a compound is retained in the GC column is known as the retention time. The technician should measure retention time from the sample injection until the compound elutes from the column. The retention time can aid in differentiating between some compounds. However, retention time is not a reliable factor to determine the identity of a compound. If two samples do not have equal retention times, those samples are not the same substance. However, identical retention times for two samples only indicate a possibility that the samples are the same substance. Potentially thousands of chemicals may have the same retention time, peak shape, and detector response.
Analysis of Output
Less than ideal spectral peaks may indicate less than ideal analytical procedures or equipment. The technician can readily observe whether the output exhibits unsatisfactory results. Ideally, the spectral peaks should be symmetrical, narrow, separate (not overlapping), and made with smooth lines. GC evidence may be suspect if the peaks are broad, overlapping, or unevenly formed. If a poorly shaped peak contains a steep front and a long, drawn-out tail, this may indicate traces of water in the specimen.
The GC technician should inject the specimen into the septum rapidly and smoothly to attain good separation of the components in a specimen. If the technician injects the specimen too slowly, the peak may be broad or overlap. A twin peak may result from the technician hesitating during the injection. A smoothly performed injection, without abrupt changes, should result in a smoothly formed peak.
Mass Spectrometry
MS identifies substances by electrically charging the specimen molecules, accelerating them through a magnetic field, breaking the molecules into charged fragments and detecting the different charges. A spectral plot displays the mass of each fragment. A technician can use a compound's mass spectrum for qualitative identification. The technician uses these fragment masses as puzzle pieces to piece together the mass of the original molecule, the "parent mass."
The parent mass is analogous to the picture on top of a puzzle box, a guide to the end result obtained by putting together the fragment masses, or puzzle pieces. From the molecular mass and the mass of the fragments, reference data is compared to determine the identity of the specimen. Each substance's mass spectrum is unique. Providing that the interpretation of the output correctly determines the parent mass, MS identification is conclusive.
Description of Process
Today many different types of MS instruments exist, each one using a different apparatus and process for producing mass spectra. This article's description of the MS process will limit itself to a basic description of a conventional large magnet mass spectrometer. Such a MS instrument contains a sample inlet, an ionization source, a molecule accelerator, and a detector.
MS analysis requires a pure gaseous sample. The sample inlet is maintained at a high temperature, up to 400° C (752° F), to ensure that the sample stays a gas. Next the specimen enters the ionization chamber. A beam of electrons is accelerated with a high voltage. The specimen molecules are shattered into well-defined fragments upon collision with the high voltage electrons. Each fragment is charged and travels to the accelerator as an individual particle. In the acceleration chamber the charged particle's velocity increases due to the influence of an accelerating voltage. For one value of voltage only one mass accelerates sufficiently to reach the detector. The accelerating voltage varies to cover a range of masses so that all fragments reach the detector.
The charged particles travel in a curved path towards the detector. When an individual charged particle collides with the detector surface, several electrons (also charged particles) emit from the detector surface. Next, these electrons accelerate towards a second surface, generating more electrons, which bombard another surface. Each electron carries a charge. Eventually, multiple collisions with multiple surfaces generate thousands of electrons which emit from the last surface. The result is an amplification of the original charge through a cascade of electrons arriving at the collector. At this point the instrument measures the charge and records the fragment mass as the mass is proportional to the detected charge.
The MS instrument produces the output by drawing an array of peaks on a chart, the "mass spectrum." Each peak represents a value for a fragment mass. A peak's height increases with the number of fragments detected with one particular mass. As in the case of the GC detectors, a peak may differ in height with the sensitivity of the detector used.
Analysis of Output
Each substance has a characteristic mass spectrum under particular controlled conditions. A technician can identify a specimen by comparing the specimen's mass spectrum with known compounds. Quantitative analysis is possible by measuring the relative intensities of the mass spectra.
Usually a mass spectrum will display a peak for the unfragmented molecule of the specimen. This is commonly the greatest mass detected, called the "parent mass." Like the picture on a puzzle box, the parent mass is used to fit the pieces together from the other peaks in the mass spectrum. The parent mass reveals the mass of the molecule while the other peaks indicate the molecule's structure. Determining the parent peak and consequently the molecular mass of the specimen is the most difficult part of MS analysis. Identifying the parent mass is outside the scope of this article. Assuming that a technician can correctly determine the molecular mass, the technician makes an educated guess of the specimen's identity and compares the mass spectrum to reference spectra for confirmation. The mass spectra for larger molecules containing carbon are complicated and require tedious calculations that are subject to error.
GC/MS Combination
The GC device is generally a reliable analytical instrument. The GC instrument is effective in separating compounds into their various components. However, the GC instrument can not be used for reliable identification of specific substances. The MS instrument provides specific results but produces uncertain qualitative results. When an analyst uses the GC instrument to separate compounds before analysis with an MS instrument, a complementary relationship exists. The technician has access to both the retention times and mass spectral data.
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