GC Detectors and Mass Spectrometry

The more advanced instrumental methods of analysis, including GC, for the detection and identification of expls are presented (Ref 90) Pyrolysis of expls in tandem with GC/MS was used for the identification of contaminant expls in the environment (Ref 108). Isomer vapor impurities of TNT were characterized by GC-electron capture detector and mass spectrometry (Ref 61). Volatile impurities in TNT and Comp B were analyzed using a GC/MS the GC was equipped with electron capture and flame ionization detectors (Ref 79). The vapors evolved from mines, TNT, acetone, toluene, cyclohexanone and an organosilicon, were analyzed by GC/MS (Ref 78). Red water produced by the sellite purification of crude TNT was analyzed by GC/MS for potentially useful organic compds, 2,4-dinitrotoluene, 3- and 4-sulfonic acids (Ref 124). Various reports were surveyed to determine which methods, including GC/MS, are potential candidates for detection of traces of TNT vapors emitted from land mines factors influencing transportability of TNT vapors thru soil to soil/air interface are dis-... 

The chiral columns used for liquid chromatography may also be used for supercritical fluid chromatography (SFC). SFC offers important advantages over HPLC and GC in the separation of enantiomers. First, SFC provides a higher resolution per unit of time than does LC, because the diffusion rates in the mobile phase and linear velocities are higher. Second, SFC chromatography is carried out at temperatures well below those used in GC. LC and GC detectors, such as FID (flame iruiized detectors) and mass spectrometry (MS), may also be applied to SFC (Chamberlain et al. 1998). 

The recent development and comparative application of modern separation techniques with regard to determination of alkylphosphonic acids and lewisite derivatives have been demonstrated. This report highlights advantages and shortcomings of GC equipped with mass spectrometry detector and HPLC as well as CE with UV-Vis detector. The comparison was made from the sampling point of view and separation/detection ability. The derivatization procedure for GC of main degradation products of nerve agents to determine in water samples was applied. Direct determination of lewisite derivatives by HPLC-UV was shown. Also optimization of indirect determination of alkylphosphonic acids in CE-UV was developed. Finally, the new instrumental development and future trends will be discussed. 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

In soil analysis, HPLC is used much like GC in that soil is extracted and the extract, after suitable cleanup and concentration, is analyzed. One major difference between them is that HPLC does not require the components to be in the gaseous phase. They must, however, be soluble in an eluent that is compatible with the column and detector being used. A second difference is that both a syringe and an injector are used to move the sample into the eluent and onto the column. Detection is commonly by UV absorption, although RI, conductivity, and mass spectrometry are also commonly used. Conductivity or other electrical detection methods are used when analysis of ionic species in soil is carried out [3,78],... 

Lanina et al. 1992 Oishi 1990). These methods include gas chromatography (GC) combined with mass spectrometry (MS) and high-performance liquid chromatography (HPLC) combined with an ultraviolet detector (UV). No comparisons can be made between methods since no data were given regarding sensitivity, recovery, or precision. 

Other methods for the determination of aromatics in naphtha include a method (ASTM D5580) using a flame ionization detector and methods that use combinations of gas chromatography and Fourier transform infrared spectroscopy (GC-FTIR) (ASTM D5986) and gas chromatography and mass spectrometry (GC-MS) (ASTM D5769). 

GC, gas chromatography FID, flame ionization detector MS, mass spectrometry EQL, estimated quantitation limit (the EQL of Method 8270 for determining an individual compound is approximately 660 qg/kg (wet weight) for soil/sediment samples, 1-200 mg/kg for wastes (dependent on matrix and method of preparation), and 10 J,g/L for groundwater samples) NR, not reported... 

Carbamate pesticides can be determined using different detectors in GC or HPLC analysis. A characteristic feature of a carbamate molecule is the nitrogen atom, which can form the bases for quantitation and some carbamates also contain chlorine, sulfur, or other heteroatoms in the molecule. This allows the use of various detection techniques for their determination (139,140), such as electrical conductivity (165), alkali flame (141) photometry, and mass spectrometry (44,166). 

Organochlorine pesticides and OPPs have been determined mainly using GC, because of the stability and volatility that most of them show under chromatographic conditions and, particularly, the availability of element-selective detectors that display high selectivity for OCPs (electron-capture detector, ECD), and OPPs (flame photometric detector, FPD, and nitrogen phosphorus detector, NPD). Mass spectrometry-based detection is also more popular in GC than in HPLC (1,2,12,16). 

GC isotope ratio mass spectrometry [7] and GC using a caesium bromide thermionic detector [8] have been used to determine, respectively, carboxylic ethers in apples and tetraethyl pyrophosphate in chloroform-acetone extracts of crops in amounts down to 0.01 ppm. 

Low-pressure He-ICPs have been used as chromatographic detectors for mass spectrometry [111,112,114], Organotin compounds [112] were speciated by using a plasma operated at 100 W. A low-pressure torch must be constructed from a quartz tube of dimensions approximately 150 mm long and 6 mm outer diameter to sustain such a plasma. This torch is connected at one end to the GC interface and to the sampler cone at the other. Heated transfer lines must be used for reproducible transfer of the GC analytes. As the low-pressure system may not completely at-... 

One of the attractions of SFC is that it can use both GC- and LC-like detectors, including the almost universal flame ionization detector (FID) for nonvolatile and volatile analytes after separation on either capillary or packed columns. Selective responses could be also obtained from a number of detectors as NPD, ECD, FPD, ultraviolet, Fourier transform infrared, nuclear magnetic resonance, and mass spectrometry. 

One of the most powerful detectors for gas chromatography is the mass spectrometer. The combination of gas chromatography and mass spectrometry is known as GC/MS. As discussed in Chapter 28, a mass spectrometer measures the mass-to-charge ratio (m/z) of ions that have been produced from the sample. Most of the ions produced are singly charged (z = 1), so that mass spectrometrists often speak of measuring the mass of ions when mass-to-charge ratio is actually measured. 

There are a variety of detectors for GC systems, however, mass spectrometry (MS) is generally accepted as the best overall. GC-MS is a very powerful and popular technique and therefore has a wide range of different application areas. The major reasons for using GC-MS are ... 

We shall successively examine GC and LC detectors. Since mass spectrometry is now widely used as the detector in both modes, it deserves a special section. 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

Mass Spectrometric Detectors The combination of l.C and mass spectrometry would seem to be an ideal merger of separation and detection. Just as in OC, a mass spectrometer can greatly aid in identifying specic.s as they elute from the chromatographic column. There are major problems, however, in the coupling of these two techniques. A gas-phase sample is needed for mass s >ectromclry. and the output of the LC column is a solute dissolved in a solvent. Asa first step, the solvent must be vaporized. When vaporized, however, the I.C solvent produces a gas volume that is 10 -1000 limes greater than the carrier gas in GC. Hence, most of the solvent must also be removed. 

Separations by GC are very popular for non-polar organic compounds. Prior to the separation however extraction from the sample and preconcentration have to be performed as well as derivatization. The last step is usually done by addition of Grignard reagent (RMgX) and its excess after the process is removed by acid addition. As detectors after GC separation successfully are used ICP-AES (for N, P, S, C, Br, Cl, Hg, Zn, Pb) with detection limit in the pg range atomic fluorescence and mass spectrometry. 

---