Detection limit flame ionization

Midha and Charette carried out quantitative determinations of quinidine in plasma and whole blood. Cinchonidine was added to the plasma sample to be analyzed as an internal standard. The alkaloids were extracted with benzene at pH 12.0. The residue from the extract was mixed with 25 yl of trimethylanilinium hydroxide in methanol, and aliquots (1-2 ul) were injected into the gas chromatograph in which the injection port was held at 350°C. The methyl derivatives of quinidine and the internal standard gave well separated symmetrical peaks. Detection by flame ionization gave a linear response over the range 0.2-12.0 vg quinidine/ml plasma. The limit of detectability was 0.05 ug/ml and the method was adequate for following blood profiles of 200 mg quinidine sulphate doses in humans. The recovery of quinidine... 

If quantity is a limiting factor in the analysis, as it is often the case, one may increase the sensitivity of detection by selecting from the mass spectrum of the fatty acid derivative one or two diagnostic ions of reasonable abundance and focus the mass spectrometer on those, a technique called selected ion monitoring (SIM). (For instance, one may choose the ion of m/z 175 from the spectrum of methylarachidonate shown in Fig. 3.2.) Alternatively, detection by flame ionization (FID) may be routinely used when quantities of the analyte are not limiting. 

Samples to be examined by inductively coupled plasma and mass spectrometry (ICP/MS) are commonly in the form of a solution that is transported into the plasma flame. The thermal mass of the flame is small, and ingress of excessive quantities of extraneous matter, such as solvent, would cool the flame and might even extinguish it. Even cooling the flame reduces its ionization efficiency, with concomitant effects on the accuracy and detection limits of the ICP/MS method. Consequently, it is necessary to remove as much solvent as possible which can be done by evaporation off-line or done on-line by spraying the solution as an aerosol into the plasma flame. 

Flame emission spectrometry is used extensively for the determination of trace metals in solution and in particular the alkali and alkaline earth metals. The most notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples for clinical diagnosis. Simple filter instruments generally provide adequate resolution for this type of analysis. The same elements, together with B, Fe, Cu and Mn, are important constituents of soils and fertilizers and the technique is therefore also useful for the analysis of agricultural materials. Although many other trace metals can be determined in a variety of matrices, there has been a preference for the use of atomic absorption spectrometry because variations in flame temperature are much less critical and spectral interference is negligible. Detection limits for flame emission techniques are comparable to those for atomic absorption, i.e. from < 0.01 to 10 ppm (Table 8.6). Flame emission spectrometry complements atomic absorption spectrometry because it operates most effectively for elements which are easily ionized, whilst atomic absorption methods demand a minimum of ionization (Table 8.7). 

PDMS = polydimethylsiloxane. PA = polyacrylate. CW = Carbowax. DVB = divinylbenzene. FID = flame ionization detection. NPD = nitrogen-phosphorus detection. TSD = thermionic-specific detection. LOQ = limit of quantitation. LOD = limit of detection. TCA = trichloroacetic acid. PICI-MS = positive ion chemical mass spectrometry. SIM = selected ion monitoring. 

Detectors range from the universal, but less sensitive, to the very sensitive but limited to a particular class of compounds. The thermal conductivity detector (TCD) is the least sensitive but responds to all classes of compounds. Another common detector is the flame ionization detector (FID), which is very sensitive but can only detect organic compounds. Another common and very sensitive detector is called electron capture. This detector is particularly sensitive to halogenated compounds, which can be particularly important when analyzing pollutants such as dichlorodiphenyltrichloroethane (DDT) and polychlorobiphenyl (PCB) compounds. Chapter 13 provides more specific information about chromatographic methods applied to soil analysis. 

Elimination of wet chemical sample preparation enables a complete analysis to be performed and data to be quickly analyzed. The detection limits are in the low part-per-million range using mass spectrometric detection. Alternatively, detection of compounds can be achieved by all common gas chromatography detectors (flame ionization detector, electron capture detector and flame photometric detector), and detection limits are determined by the method of detection employed. 

Arakawa et al. [96] have pointed out that methyltin compounds may be extracted from complex matrices and analysed by conventional gas chromatography. However, the procedure is lengthy, involving multiple steps where speciation may be altered and vessel adsorption effects may be large. Detection limits achievable with a flame ionization detector are 10-100pg [97],... 

Flame ionization detectors are capable of detecting virtually all organic compounds and show a lower limit of detection of approximately 1 X 10-9 mol. They also show good linearity of response and the fact that they do not respond to oxides of carbon or nitrogen or to water makes them particularly convenient for aqueous samples. They have the disadvantage, however, that samples are destroyed unless a stream-splitting device is incorporated. 

Gas chromatography (GC) equipped with a flame ionization detector has been employed for measuring the concentration of 1,2-dibromoethane in the tissues of two workers following exposure (Letz et al. 1984). A detection limit of 0.5 ig of 1,2-dibromoethane per gram of tissue was achieved. In the same report, Letz et al. (1984) detected ppm (mg/L) levels of bromide ion (a metabolite of... 

Methods for Determining Biomarkers of Exposure and Effect. As noted in Section 6.1, methods are available for the qualitative and quantitative measurement of 2-hexanone after it is separated from its sample matrix (Anderson and Harland 1980 Fedtke and Bolt 1986 Nomeir and Abdou-Donia 1985 White et al. 1979). High-resolution gas chromatography for 2-hexanone analysis has been developed to the point that the instrumental capability to separate volatile analytes by HRGC is, for the most part, no longer the limiting factor in their analysis. Flame ionization detection has enabled detection at very low levels and MS has assured specificity in measurement. 

The amount of cresol in the concentrated extract can then be determined by high performance liquid chromatography (HPLC) (DeRosa et al. 1987 Yoshikawa et al. 1986) or gas chromatography (GC) coupled to either a flame ionization detector (FID) or a mass spectrometer detection system (Angerer 1985 Needham et al. 1984). Separation of the cresol isomers by gas chromatography is readily accomplished, and the use of an appropriate internal standard allows the determination of their concentrations. Although exact detection limits were not given for the above GC methods, a concentration of 10 ppm appears to be readily determined. 

---