Bristol University - Gas Chromatography Mass Spectrometry (GC/MS)
Comparison of gas chromatography-mass spectrometry and high-performance liquid Data were evaluated by graphical investigation of difference-plots and. This chapter illustrates the open-split interface that offers a convenient connection between the GC column and the mass spectrometer. The open-split interface. Gas chromatography mass spectrometry (GC/MS) is an instrumental technique, comprising a gas chromatograph (GC) coupled to a mass spectrometer (MS), by .
A softer ionization fragments the molecule to a lower degree than the hard ionization of EI.
Gas chromatography–mass spectrometry
One of the main benefits of using chemical ionization is that a mass fragment closely corresponding to the molecular weight of the analyte of interest is produced. In positive chemical ionization PCI the reagent gas interacts with the target molecule, most often with a proton exchange.
This produces the species in relatively high amounts. In negative chemical ionization NCI the reagent gas decreases the impact of the free electrons on the target analyte.
This decreased energy typically leaves the fragment in great supply. Analysis[ edit ] A mass spectrometer is typically utilized in one of two ways: The typical GC-MS instrument is capable of performing both functions either individually or concomitantly, depending on the setup of the particular instrument. The primary goal of instrument analysis is to quantify an amount of substance. This is done by comparing the relative concentrations among the atomic masses in the generated spectrum.
Two kinds of analysis are possible, comparative and original. Comparative analysis essentially compares the given spectrum to a spectrum library to see if its characteristics are present for some sample in the library.
Gas Chromatography Mass Spectrometry (GC/MS) Information
This is best performed by a computer because there are a myriad of visual distortions that can take place due to variations in scale. Computers can also simultaneously correlate more data such as the retention times identified by GCto more accurately relate certain data. Deep learning was shown to lead to promising results in the identification of VOCs from raw GC-MS data  Another method of analysis measures the peaks in relation to one another.
The total mass of the unknown compound is normally indicated by the parent peak. The value of this parent peak can be used to fit with a chemical formula containing the various elements which are believed to be in the compound. The isotope pattern in the spectrum, which is unique for elements that have many natural isotopes, can also be used to identify the various elements present.
Once a chemical formula has been matched to the spectrum, the molecular structure and bonding can be identified, and must be consistent with the characteristics recorded by GC-MS. Typically, this identification is done automatically by programs which come with the instrument, given a list of the elements which could be present in the sample. Conversely, selective ion monitoring SIM only monitors selected ions associated with a specific substance. This is done on the assumption that at a given retention time, a set of ions is characteristic of a certain compound.
This is a fast and efficient analysis, especially if the analyst has previous information about a sample or is only looking for a few specific substances. When the amount of information collected about the ions in a given gas chromatographic peak decreases, the sensitivity of the analysis increases. So, SIM analysis allows for a smaller quantity of a compound to be detected and measured, but the degree of certainty about the identity of that compound is reduced.
Full scan MS[ edit ] When collecting data in the full scan mode, a target range of mass fragments is determined and put into the instrument's method.
The determination of what range to use is largely dictated by what one anticipates being in the sample while being cognizant of the solvent and other possible interferences. Additionally if one is to use a large scan range then sensitivity of the instrument is decreased due to performing fewer scans per second since each scan will have to detect a wide range of mass fragments. Full scan is useful in determining unknown compounds in a sample.
It provides more information than SIM when it comes to confirming or resolving compounds in a sample. During instrument method development it may be common to first analyze test solutions in full scan mode to determine the retention time and the mass fragment fingerprint before moving to a SIM instrument method.
Selective ion monitoring[ edit ] In selective ion monitoring SIM certain ion fragments are entered into the instrument method and only those mass fragments are detected by the mass spectrometer. The advantages of SIM are that the detection limit is lower since the instrument is only looking at a small number of fragments e.
More scans can take place each second. Since only a few mass fragments of interest are being monitored, matrix interferences are typically lower.
Gas Chromatography/Mass Spectrometry (GC/MS)
To additionally confirm the likelihood of a potentially positive result, it is relatively important to be sure that the ion ratios of the various mass fragments are comparable to a known reference standard. Applications[ edit ] Environmental monitoring and cleanup[ edit ] GC-MS is becoming the tool of choice for tracking organic pollutants in the environment. The cost of GC-MS equipment has decreased significantly, and the reliability has increased at the same time, which has contributed to its increased adoption in environmental studies.
Criminal forensics[ edit ] GC-MS can analyze the particles from a human body in order to help link a criminal to a crime. Law enforcement[ edit ] GC-MS is increasingly used for detection of illegal narcotics, and may eventually supplant drug-sniffing dogs.
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It involves identifying an acid metabolite of tetrahyhydrocannabinol THCthe active ingredient in marijuana, in urine samples by employing derivatization in the sample preparation. In drug screening, GC-MS methods frequently utilize liquid-liquid extraction as a part of sample preparation, in which target compounds are extracted from blood plasma.
These systems run on a host of technologies, many of them based on GC-MS. Chemical warfare agent detection[ edit ] As part of the post-September 11 drive towards increased capability in homeland security and public health preparedness, traditional GC-MS units with transmission quadrupole mass spectrometers, as well as those with cylindrical ion trap CIT-MS and toroidal ion trap T-ITMS mass spectrometers have been modified for field portability and near real-time detection of chemical warfare agents CWA such as sarin, soman, and VX.
Chemical engineering[ edit ] GC-MS is used for the analysis of unknown organic compound mixtures. One critical use of this technology is the use of GC-MS to determine the composition of bio-oils processed from raw biomass.
GC-MS is extensively used for the analysis of these compounds which include estersfatty acidsalcoholsaldehydesterpenes etc.
It is also used to detect and measure contaminants from spoilage or adulteration which may be harmful and which is often controlled by governmental agencies, for example pesticides. Two were brought to Mars by the Viking program. GC-MS can determine compounds in urine even in minor concentration. These compounds are normally not present but appear in individuals suffering with metabolic disorders. When the resulting peak from this ion is seen in a mass spectrum, it gives the molecular weight of the compound.
Due to the large amount of energy imparted to the molecular ion it usually fragments producing further smaller ions with characteristic relative abundances that provide a 'fingerprint' for that molecular structure.
This information may be then used to identify compounds of interest and help elucidate the structure of unknown components of mixtures. CI is a less energetic way of ionising a molecule hence less fragmentation occurs with CI than with EI, hence CI yields less information about the detailed structure of the molecule, but does yield the molecular ion; sometimes the molecular ion cannot be detected using EI, hence the two methods complement one another.
Gas Chromatography Mass Spectrometry (GC-MS) Information | Thermo Fisher Scientific - BR
Once ionised a small positive is used to repel the ions out of the ionisation chamber. The next component is a mass analyser filterwhich separates the positively charged ions according to various mass related properties depending upon the analyser used.
Several types of analyser exist: The most common are quadrupoles and ion traps. After the ions are separated they enter a detector the output from which is amplified to boost the signal. The detector sends information to a computer that records all of the data produced, converts the electrical impulses into visual displays and hard copy displays.