Quantification by GC-MS

Setting up a dosage method includes three important steps (1) development of the GC-MS method (optimizing parameters for chromatography and mass spectrometry), (2) optimization of the steps of sample preparation (extraction, re-concentration, chemical derivation, etc.), and (3) validation. 

As previously noted, sample preparation theory is not presented in depth in this volume. Certain practical issues are discussed in the context of examples. The validation of analytical methods is a very large subject that could fill several books. Method validation is covered in Section 7.4. The discussion is not exhaustive it presents the basics of validation of a GC-MS method and should allow beginners to learn about validation and determine the quality of their analytical processes. 

From a strictly technical view, it would be unreasonable to consider setting up a dosage method in GC-MS without an autosampler. Even if the use of an internal standard makes proceeding by manual injection theoretically possible, practical experience indicates that it is extremely difficult to validate a method through manual injections. 

Analytical chemists know that two quantification methods are possible with chromatography quantification with external calibration and quantification with internal calibration. The first mode is only briefly presented in this chapter because it is not adapted to precise quantification. Priority is therefore given to quantification with internal calibration. The development of dosing methods is discussed elsewhere in this chapter. 

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Dibromoethane can be isolated from soil samples by liquid-liquid extraction and subsequent quantification by GC/MS (Sawhney et al. 1988). Low ppb levels of 1,2-dibromoethane in soil were reported using this technique. 

If sample does not contain fat or oil, the duration of the extraction should not exceed 1 hr. If it contains fats one must verify the extraction time that is necessary for an efficient aroma recovery. Depending on the matrix, one must also verify the appropriate extraction time, prior to beginning. Generally, this time does not exceed 1 hr. Solvent injection and quantification by GC/MS after the SDE is usually used to verify the minimum extraction time needed. 

Two compounds are currently of particular interest in paper and board. Diisopropyl naphthalene (DIPN) is a mixture of isomers that until recently were widely used in carbonless copy papers as ink solvents. Although it is currently being replaced it occurs as a persistent contaminant in recycled paper and board. Various studies have shown that it is able to migrate from paperboard into food. There is a draft CEN analytical method available. This method involves acetone extraction and quantification by GC-MS using diethyl naphthalene as an internal standard. There is currently no limit for DIPN but levels are being monitored to reduce concentrations in recycled paperboard. 

Although the volatility and thermal stability presented by BPA and BPF make them suitable for detection and quantification by GC-MS, derivatization procedure can... 

Chapter 7 is dedicated to quantification using GC-MS coupling. The techniques of dosage are generally often ignored in the books devoted to mass spectrometry so that quantification by GC-MS remains a puzzle for numerous chemists. Laboratory audits show that it is when attempting to perform quantitative analysis that operators have to face the main difficulties and often make mistakes. An example of the development of a quantification method is presented. 

For details of the clean-up of the pyrolysate of the DIN oven see reference 12. Identification and quantification of PBDD/F was performed by GC/MS (refs. 8-12). This was done for all brominated PBDD and PBDF from mono- through octabromo compounds using external standards which were either prepared (refs. 11-13) or purchased. There exists a total of 210 brominated compounds of PBDD/F. Since not all isomers are available a complete isomer-specific determination could not be performed. 

Plasticiser/oil in rubber is usually determined by solvent extraction (ISO 1407) and FTIR identification [57] TGA can usually provide good quantifications of plasticiser contents. Antidegradants in rubber compounds may be determined by HS-GC-MS for volatile species (e.g. BHT, IPPD), but usually solvent extraction is required, followed by GC-MS, HPLC, UV or DP-MS analysis. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out. The determination of antioxidants in rubbers by means of HPLC and TLC has been reviewed [58], The TLC technique for antidegradants in rubbers is described in ASTM D 3156 and ISO 4645.2 (1984). Direct probe EIMS was also used to analyse antioxidants (hindered phenols and aromatic amines) in rubber extracts [59]. ISO 11089 (1997) deals with the determination of /V-phenyl-/9-naphthylamine and poly-2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ) as well as other generic types of antiozonants such as IV-alkyl-AL-phenyl-p-phenylenediamines (e.g. IPPD and 6PPD) and A-aryl-AL-aryl-p-phenylenediamines (e.g. DPPD), by means of HPLC. 

The methoxy-polybromodiphenyl ethers (MeO-PBDEs) were analyzed together with PBDEs by GC-MS. The GC-MS methodology is optimized for the detection and quantification of eight different congeners including the 5-MeO-tetra-BDE-47, 6-MeO-tetra-BDE-47, 4-MeO-tetra-BDE-49, 2-MeO-tetra-BDE-68, 5-MeO-penta-BDE-99, 5 -MeO-penta-BDE-100, 4 -MeO-penta-BDE-101 and 4 -MeO-penta-BDE-103. 

HBCD determinations were carried out simultaneously with mono- to hepta-BDEs. Technical HBCD consists of three diastereomers a, (3 and y-HBCD. HBCD can be determined by GC-MS, but until now, the three diastereomers have not been separated by this technique. However, as apparently the response factors of the three diastereomers do not differ very much [25], HBCD can be quantified as total HBCD by GC-MS. Quantification was carried out by external standard. 

Organotin compounds enriched from a diethylether extract of a snow sample collected from the city of Gdansk, Poland and analyzed are shown in Fig. 22 b, c [286]. Gas chromatography with atomic emission detection (GC-AED) run in the chlorine and tin channels, respectively, revealed the presence of tributyltin chloride and this was subsequently confirmed by GC-MS and GC-AED analyses of an internal standard solution (e.g., 1-chlorooctane) of that compound. Quantification was based on the response to chlorine (wavelength 479 nm) in the AED system, and a detection limit of 0.5-1 ng/1 was achieved for all the reference substances. 

As already mentioned, it is the volatile constituents that serve to identify fruit type and variety. Broadly speaking, qualitative analysis will identify the principal substances present in the volatiles fraction as representative of a particular fruit type, but it is the relative proportions of these substances that will reflect the variety. Alcohols, volatile acids, esters, carbonyl compounds, and low-boiling hydrocarbons are the principal groups represented. Analysis by GC-MS (gas chromatography coupled with mass spectroscopy) can be used to provide quantification and identification of the various constituents. 

Analytical pyrolysis with field ionization mass spectrometry (online Py-FIMS) or in combination with GC/MS (Curie point Py-GC/MS) led to a significant increased number of identified subunits (e.g., Bracewell et al., 1989 Schulten et al., 2002). In addition, the application of tetramethylammonium hydroxide (TMAH) methylation, followed by GC/MS, was successfully applied. The most abundant pyrolysis products identified are benzene, phenol and furan derivatives, aliphatic and carboxylic compounds, and indene derivatives (Schulten et al.,2002). New approaches have been used for the quantification of n-alkyl fatty acids of DOM and isolated fractions in the form of individual compounds after solvent extraction followed by derivatiza-tion with TMAH. 

The 10-undecenoic acid motif has also been attached to isosorbide in the preparation of a fatty acid-/carbohydrate-based monomer [131]. ADMET polymerization in the presence of C3 and C4 produced fully renewable unsaturated polyesters (Scheme 18). Most importantly, the transesterification of these polyesters with MeOH, and subsequent analysis by GC-MS of the products, allowed for the quantification of double-bond isomerization during ADMET in a very simple manner. This strategy was then extended to fatty acid-based ADMET polyesters synthesized in the presence of indenylidene metathesis catalysts [132]. With these studies, the knowledge on the olefin isomerization in ADMET reactions was widened, and it is now possible to almost completely suppress this undesired side reaction. 

Gas chromatography-selected ion monitoring (GC-SIM) is often used for the quantification of GAs by GC-MS, an internal standard labeled with stable isotopes (usually with deuterium) being used as the most reliable and sensitive method. A mass chromatogram reconstructed from full-scan GC-MS is also used for semiquantification. The amounts of GAs are determined by measuring the peak areas of ions characteristic for each GA and comparing these areas to those of authentic samples. When an internal standard labeled with a stable isotope is used, the ratio of the area of an ion peak characteristic for a sample and the area of the corresponding peak of the labeled internal standard is used for quantification. The analysis of GAs by GC-MS has been discussed in numerous publications and review articles.254-256... 

Table 6.3 Calibration and calcnlated data, obtained by GC-MS analysis, used for the quantification of cocaine in a drug sample...

Chlordane compounds were detected in the blubbers of Weddel seals caught near to the Syowa Station (75). Sampling, dissection and pretreatment were all conducted so as to prevent any possible contamination from chlorinated hydrocarbons, e.g., by washing the electric devices, knives, polyethylene bags and other materials with high purity acetone. Quantification was performed by GC-MS and values of 12-62 ng g were ascertained. Airborne transport can probably account for the presence of these substances in Antarctica. 

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