GC-MS Operation

After the optimum GC conditions and MS mode have been selected, the operating parameters of the GC-MS spectrometer are set accordingly. When the initial oven temperature attains an equilibrium state, a sample solution in the range of 2-5 p is injected into the gas chromatograph. The amount of sample injected depends on the number and the concentration of the constituents in the sample solution. The constituents are separated in the capillary column and continuously introduced into the ion source of the MS system at a pressure in the order of 10-6 to 10-7 Torr, whereupon the mass spectra corresponding to the individual components in the sample are generated. Irrespective of the MS operational mode used, the mass spectra thus produced are scanned 

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At such a rate of scanning, it is even possible to examine eluants from capillary GC (gas chromatography) columns during GC/MS operations. 

Gas Chromatography and Mass Spectrometry A Practical Guide is designed to be a valuable resource to the GC/MS user by incorporating much of the practical information necessary for successful GC/MS operation into a single source. With this purpose in mind, the authors have kept the reading material practical and as brief as possible. This guide should be immediately valuable to the novice, as well as to the experienced GC/MS user who may not have the breadth of experience covered in this book. 

GC-MS operated in electron impact (El) mode was only sporadically used for the determination of some UV filters such as 4-MBC, EHMC, and OC. Separation was achieved on a 60 m x 0.25 mm i.d. DB-5 column, with 0.25-pm film thickness. For quantification of the compounds, data acquisition was performed in selected ion monitoring (SIM) mode recording three characteristic ions per compound. GC-MS allowed the differentiation between the two isomers (cis/trans) for 4-MBC and EHMC. 

In the Introduction, we presented a brief overview of our research interests and research strategies. A more detailed account has been published, which also includes a description of our early work using SPME/GC-MS, the specifics on our GC-MS instrumentation, and a typical set of GC-MS operating conditions (Goodwin, Rasmussen, Schulte, Brown, Davis, Dill, Dowdy, Hicks, Morshedi, Mwanza and Loizi 2005). 

Figure 7.1 Total ion current (TIC) chromatogram obtained by GC-MS analysis of a resin (Pinus sylvestris). The diterpenoid resin acids were methylated (using diazomethane) to improve chromatographic performance. Peak identities 1, Methyl pimarate 2, Methyl sandaracopimarate 3, Methyl isopimarate 4, Methyl palustrate 5, Methyl dehydroabietate 6, Methyl abietate 7, Methyl neoabietate. For GC-MS operating conditions, see Heron and Pollard (1988).

Mass-spectral (MS) Analysis. Mass spectral analyses were obtained using a computerized Finnigan Model 1015 gas chromatograph-mass spectrometer (GC-MS) operated at 70 eV. Samples were introduced via direct insertion probe or by utilizing the GC-MS combination, both operated over a programmed temperature range. 

At the time of writing, a new blinded mode software was being developed. This new version is based on different GC/MS operating software and will include additional security features such as unique installation numbers and check sum. Also, the need of AMDIS security level filters is being reviewed. 

Extensive technical measures have been implemented to allow the ISP to protect confidential, commercial, or sensible military information during on-site analysis. These measures include in particular the option of blinding the GC/MS operating software and security level filters of AMDIS (Automated Mass Spectral Deconvolution and Identification System), the GC/MS raw data processing software. These measures can be applied separately or combined, offering the ISP the choice of a gradual restriction of the information revealed to the IT (for details see Chapter 2). 

If the chromatographic peak separation between 13Ci2-2,3,7,8-TCDD and 13Ci2-l,2,3,4-TCDD is not resolved with a valley of <. 25% on the DB-5 (or equivalent) column, or 2,3,7,8-TCDD is not resolved from the closest eluting isomer with a valley of 25% on the SP-2331 (or equivalent) column, then the Contractor shall adjust the GC/MS operating conditions and rerun the affected sample. This criterion applies to sample analyses. If this criterion is not met for a calibration standard, all associated samples must be rerun. 

Table 3.3 GC-MS operating conditions and parameters used for the identification of LSD [5]...

Typical GC-MS operating conditions and parameters employed for the analysis of cannabis products are shown in Table 4.1. 

Ease of Operation. Since the MBI can remain installed during normal GC/MS operations, the change-over to LC/MS can be accomplished within I hour. The only serious malfunction is the breaking of the Kapton belt which means a shut-down and removal. Re-installation normally takes several hours. 

A total ion monitor (TIM), though not part of the interface, is often useful for the GC MS operation. The TIM is an ion detector positioned in the analyzer tube between the ion source and the magnet and is adjustable to collect a certain percentage of the total ions formed. This detector response is registered on a dual pen strip chart recorder with that of the GG flame detector (Figure 6). There may be some difiFerences in peak ratios because of differences in detector responses to the various compounds the mass spectra are taken at the TIM peak maxima since the ions are then most concentrated. Pesticide residue analysis by GG MS... 

Mass Spectrometry. The use of supercritical CO2 as a mobile phase provides an efficient means of transferring these labile compounds to the mass spectrometer Ion source. Modifications to the Flnnl-gan 4500 are mini mal and fully compatible with normal GC-MS operation. Conversion between GC-MS and SFC-MS modes involves only an injector substitution and a column change. Figure 3 Illustrates the reconstructed ion chromatogram obtained for the separation of... 

Determination of BERs in particulate matter and dust is commonly carried out by GC/MS operating in the negative ionization with SIM (Gevao et al. 2006 Karlsson et al. 2007 Stapleton et al. 2005, 2006 Wilford et al. 2005 Tan et al. 2007), allowing obtaining LODs in the low ng g level. Nevertheless, MS in the El mode with SIM (Rudel et al. 2003 Knoth et al. 2002 Stapleton et al. 2006 Harrad et al. 2006), tandem mass spectrometry (MS/MS) (Regueiro et al. 2006), or even micro-electron capture detection (pECD) (Regueiro et al. 2007), have also... 

Most of fragrance determinations in indoor environments have been focused on synthetic musks, and have been performed on dust (Table 5). Synthetic musk fragrances have been extracted from house dust by PLE (Gevao et al. 2006 Rudel et al. 2003). Fromme et al. (2004) carried out the PLE extraction of both polycyclic and nitromusk in indoor dust with n-hexane/diethyl ether (19 1) and further determination by GC/MS operating in the El mode with SIM. Recently, Peck et al. (2007) reported the extraction of musk compounds from the indoor dust standard reference material SRM 2585 with dichloromethane at 100°C and 2000 psi. After clean-up on an alumina SPE cartridge, a GPC column and volume concentration, recoveries in the range 73-90% were obtained. 

MIMS detection limits (LOD, S/N > 3) were determined by analysis of reference solutions and were compared with (a) LODs obtained by P T/GC/MS operated as described in the previous section, and (b) LODs reported by USEPA Method 8260B (Table 16.1). MIMS LODs were (a) smaller than those obtained by P T/GC/MS for the organohalogen compounds and for benzene by about, in some cases, one order of magnitude (b) comparable to the other technique for toluene, ethylbenzene, styrene. 

Almost any commercial gas chromatograph can be interfaced to a mass spectrometer and, although its operation is similar to that when used in the standalone mode, there are certain features that are specific to successful GC-MS operation. 

GC-MS instruments are a capital expense item they are more complicated to operate than a GC, and there is a lack of skilled GC-MS operators. Few colleges train students on GC-MS systems due to both a lack of systems for teaching purposes and the lack of expertise of many college professors. 

The mass spectra were acquired on a Hewlett-Packard 5985B GC/MS operating in the electron impact (El) mode with an electron energy of 70 ev. Unless otherwise noted, the samples were introduced via a 5 % phenylmethyl silicone column into the ms source, which was maintained at 200°C. 

GC-MS is still widely used technique in environmental, forensic, and planetary (space) sciences. It is, however, limited to volatile and thermally stable compounds as they are injected to the GC via a high-temperature (250-300°C) injection port. Nonvolatile compounds can be analyzed after specific derivatiza-fion such as methylafion, silylafion, etc. however, that requires additional sample preparation time. This is not always feasible as HPLC-MS is a better technique for a large variety of nonvolatile compounds, including those of biological importance. These include drugs and their metabolites, peptides, proteins, oligosaccharides, and oligonucleotides. For more details about GC/MS operation... 

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