What is SIM/SCAN?
In High Resolution Gas Chromatography/Mass Spectrometry (GC/MS), organic compounds are separated from the matrix and each other by an open tubular column, which has a gas moving through it (the mobile phase) and a liquid polymer on the walls of the tube (the stationary phase). Compounds partition themselves differently between the mobile and liquid phase and hence come out of the end of the column (are eluted) at different times. Given any one particular set of conditions (type of column, length, temperature program, etc.), any one compound should be eluted from the column at the exactly the same time each analysis. Thus in GC alone, the primary method for identifying the compounds is the so-called retention time (RT). The problem with GC alone is the huge possible number of organic compounds which, given experimental error, will be eluted from the column at or close to the same time as many other compounds. The addition of the mass spectrometer for identification makes multi-component analysis not just feasible but routine.
Once they reach the mass spectrometer/analyzer, the target compounds eluted from the GC column are then subjected to an ionizing source in the mass spectrometer. In most environmental work this source is an electron beam. This beam ionizes the molecules and causes them to break apart in characteristic ways depending upon the structure of the individual molecule. These charged fragments, called ions are of differing size and hence have varying weights. It is then the role of the "analyzer" of the mass spectrometer to "weigh" and tabulate the number of fragments of each size. There are a number of techniques for performing this tabulation, which is based on mass and charge, including quadrupole, ion-trap, time-of-flight mass analyzers. In all instances the "normal" mode in which the analyzer operates is called the "SCAN" or "FULL SCAN" mode. As an example, one might start the Scan by tabulating all ions of mass/charge (m/e) = 35 atomic mass units (amu). The MS tabulates (using an electron multiplier) m/e=35 for a fraction of a second, then moves on to m/e=36, tabulates for the same time then on to m/e=37 and so forth until the end of the range that is being scanned is reached (e.g., m/e=35 to 300). The electronics then reset and the scan is begun again at m/e=35. A single scan from m/e=35 to m/e=300 typically takes about 1/2 to 1/4 of a second in a typical EPA Method TO-15 analysis. Another way of saying this is that the instruments tabulator (analyzer and electron multiplier) spends about 2 milliseconds on each m/e fragment before moving on to the next m/e fragment in each full scan of the spectral range. The final result is what is commonly referred to as the "mass spectrum" of the compound in question. It is a plot of m/e fragment "weight" along the x-axis compared to the relative abundance of that fragment as the y-axis. As with the GC column alone, given any one set of mass spectrometer conditions the "fragmentation pattern" or spectrum for any individual compounds should be identical each time it is measured.(see "Tuning") Figure 1. is the mass spectrum of benzene from the author's instrument. Of note is the relatively simple spectrum whose feature is the large fragment at m/e=78 (called the molecular ion since benzene's molecular weight equals 78 atomic mass units (amus). The next most abundant fragments are only about one-fifth as abundant as the molecular ion. Figure 2. is the mass spectrum of carbon tetrachloride (MW=153.8). Although this molecule is quite different than benzene the mass spectrum show only two ions of high abundance and all others less than about 25% of the most abundant ion. In the case of carbon tetrachloride the molecular ion is not featured in the spectrum. (see electron impact spectra)
If the mass spectrum is a constant (for constant conditions) then owing to the enormous diversity of structure of organic molucules, the mass spectrum becomes the equivalent of the human "fringerprint"; it is virtually different for every compound. In practical environmental analysis, this holds true although many compunds with similar structure have similar spectra and some other assistance in identification must be used. All in all, the mass spectrum has come close to being the "absolute proof" of compound identity and because of that GC/MS has been the most used technique in analytical chemistry for decades.
Although running your GC/MS in full scan mode is the best way to either assure oneself of the identity of target compounds, it does suffer from an inefficiency. As mentioned above a full scan of all the masses is made about three to four times a second in environmental applictions. However, most compounds can be identified using only a few ions in the spectra. In fact most targets of interest in environmental work are identified using only one to four ions at most. This occurrs because (1) mass spectra in general only contain a few high abundancy ions and (2) owing to the use of the GC retention time as an additional identifying piece of data, there is no need for use of the less abundant ions (which may suffer from lower precision as well). as well). As a result of From these facts, it is clear that the majority of the time in full scan mode is spent tabulating ions which will never play a role in the data analysis. Another way of saying that is that the mass spectrometer is not being used to analyze meaningful data most of the time.
SIM ANALYSIS
In Selected Ion Monitoring (SIM) mode, the analysts selects the m/e ratios (ions, also called masses) that he wishes to monitor. If that is, for example, a single ion as in the case of m/e=78 for benzene, the analyst can set the system to monitor and tabulate only that one m/e. The spectrometer spends all its time on that one mass and does not waste time monitoring mass=35,36... etc to the end of the range. A hundred times more ions can be tabulated, resulting (in theory) in a hundred fold increase in sensitivity. Although there are many practical considerations prior to analyzing by SIM, the above explaination is the basis for the increased sensitivy available from Selected Ion MOnitoring (SIM). An added bonus is that most GC/MS systems for years have been able to run SIM on several m/e ratios (ions) throughout the GC/MS analysis and to switch to different ones in the GC/MS run by programming. This allows one to run SIM on more than one compound. Although limited by various factors SIM runs containing twenty or more compounds are possible. Of course, the addition of more target compounds limits the boost in sensitivity acheived by the SIM mode of operation.
Mass Spectrometers have been able to perform Selected Ion Monitoring since their inception. However, only more recently have systems been able to perform both modes of operation in a single analysis. Owing to increased speeds of electronics, the concept of analysis containing both a full SCAN GC/MS analysis and a SIM method is now possible. As mentioned above, the more target compounds that are added to a SIM analysis (and hence more ions tabulated), the more one approaches the SCAN mode. In spite of that gains of from 5 to 50 fold are still possible using SIM/SCAN analysis. Figure 3 is a comparison of the ion chromatograms (from SCAN data) for benzene (m/e=78), and the three internal standards (bromochloromethane m/e=130, difluorobenzene m/e=114, and chlorobenzene-d5 m/e=117), TOP TRACE and the SIM data from the same analysis. The sample in question is a laboratory blank. Note how the internal standards are easily visible in both the full scan trace and the SIM analysis, however, benzene ( and other targets of the SIM run ) are only visible in the SIM Analysis.
Although this is a simplistic example of the power of SIM/SCAN and many factors affect the ability to analyze target compounds at low concentrations, the basic premise outlined above still holds true.
How does SIM/SCAN help our clients
The analysis of the California Human Health Screening Levels (CHHSLs) compounds is an excellent example where this technique is very powerful. Of the fifteen compounds, eight are at levels where a SCAN analysis can easily produce desired reporting limits. The remaining seven are substantially lower and the guidelines specify levels where SIM is more appropriate for meeting reporting limits. By combining both methods the entire concentration range of the CHHSL guidelines and compounds can be treated in one analysis, saving time and money.
