Electron capture detector linearity

GC is coupled with many detectors for the analysis of pesticides in wastewater. At the present time the most popular is GC-MS, which will be discussed in more detail later in this section. The flame ionization detector (FID) is another nonselective detector that identifies compounds containing carbon but does not give specific information on chemical structure (but is often used for quantification because of the linear response and sensitivity). Other detectors are specific and only detect certain species or groups of pesticides. They include electron capture,nitrogen-phosphorus, thermionic specific, and flame photometric detectors. The electron capture detector (ECD) is very sensitive to chlorinated organic pesticides, such as the organochlorine compounds (OCs, DDT, dieldrin, etc.). It has a long history of use in many environmental methods,... 

P33 Analyses wereper/ormed on a gas chromatograph equipped with an electron capture detector (ECD) and a gas chromatograph coupled to a mass-selective detector working in mass spectrometry-mass spectrometry (MS-MS) mode, to achieve better limits of detection and selectivity. The proposed method yields high sensitivity, good linearity, precision, and accuracy. (From Dellinger et ah, 2001)... 

With enantiomer analysis, however, a linear detector response is indispensible. Thus, for the correct determination of. say, 0.1 % of an enantiomeric impurity, linearity within a concentration range of at least three orders of magnitude is required. It is generally accepted that the flame ionization detector (FID) does fulfill this requirement, but it is recommended that the linear detector response is verified via dilution experiments31. In contrast, the linear response range of the electron capture detector is low, being only two to three orders of magnitude. 

The electron capture detector (ECD) is also a concentration-dependent detector, and like the TCD will give a higher response for a given compound at lower carrier flowrates. Carrier flow-rate must be carefully controlled. Usually a 95% argon - 5% methane mixture is used for carrier gas. Presence of oxygen or water in the carrier gas results in loss of sensitivity and a compression of the linear range. 

DISCUSSION. Figure 10.11 shows the separation of a group of gases from blood. The procedure described depends on complete equilibration between the gas and liquid in the syringe. The period to achieve equilibration depends on the gas, and can be as little as 1 min for acetone and as much as 30 min for sulfur hexafluoride. Linear calibration curves can be obtained when peak height is plotted against gas concentration for either the flame ionization or electron capture detector. The reproducibility (n = 10) for ethane gave a standard deviation of 23% of the mean concentration while it was 2.3% for halothane and 1.8% for ether. 

Following this procedure, recovery of greater than 80% was obtained from DBA-spiked urines at levels of from 0.05 ppm to 100 ppm. The electron capture detector was linear for chloro-benzilate from 0.1 yg/ml to 2 yg/ml and for DBP from 0.005 yg/ml to 0.5 yg/ml. The lowest concentrations which produced reasonably good peak shapes (relative to noise) were 0.2 yg/ml for CB and 0.02 yg/ml for DBP. Background at the retention time of DBP averaged 0.014 ppm for control rat urines. Although this is a low value, it is highly recommended that further application of this method should include an adsorption column clean-up step such as the alumina column used by Bartsch and coworkers (3). 

Measurement with an electron capture detector has been made using the pentafluoropropyl derivative in the plasma of neonates with conparable precision and linearity over the range 10 to 1000 ng(54). 

Assuming that the detector signals are proportional to the masses of the products, the yield of DFBP is proportional to the absorbed dose however, the yields of the major products are less than proportional above 10-15 kGy. Some of the minor products show yields that are more than proportional at this range, and become more prominent at larger doses. If the detector sensitivity is assumed to be constant per monomer unit, i.e. the sensitivity of the tetramers is twice that of the dimer and 4/3 times that of the trimer, then the yield of all the products can be calculated. This total yield was found to be linear with the dose for the whole range studied (up to 25 kGy). Absolute yield can be measured only for DFBP, for which a standard exists, and it is 0.0465 molecule/100 eV. Measuring the formation of total polymer by the total ion current in the GC/MS gave a total yield of 1.7 molecules/100 eV, and similar yields are obtained by the electron capture detector. 

Giachetti et al. [60] compared the performance of mass selective detector (MSD), electron capture detector (ECD) and nitrogen-phosphorus detector (NPD) of gas chromatography systems in the assay of six nonsteroidal antiinflammatory drugs in the plasma samples. As a practical test, six NSAIDs (mefenamic, flufenamic, meclofenamic and niflumic acids, diclofenac and clonixin) added to plasma samples were detected and quantified. The analyses were carried out after solvent extraction from an acidic medium and subsequent methylation. The linearity of response was tested for all the detection systems in the range of 1-25 ng/mL. Precision and accuracy were detected at 1, 5 and 10 ng/mL. The minimum quantifiable level for the six drugs was about 1 ng/mL with each of the three detection systems. 

This again emphasizes the need for an improved procedure for defining detector specifications. The linear dynamic range of the electron capture detector is again ill-defined by many manufacturers. In the DC mode the linear dynamic range is usually relatively small, perhaps two orders of magnitude, with the response index lying... 

The flame ionization detector (FID), which, for fluorinated compounds, has the advantage of a greater linearity range in spite of its lower sensitivity in comparison with the electron capture detector (BCD), has been extensively applied to organic metal chelates in particular. Comparisons of the different detectors were carried out (Table 1.2) For special analyses such as the determination of SO2 by reaction to SO2F2 with the radioactive F-isotope or by utilization of H-labelled 3-diketon-ates, radiometric measurements for detection are employed ... 

The device functions in the same way as the conventional electron-capture detector with a radioactive source. The column eluent enters just below the third electrode, any electron-capturing substance present removes some of the free electrons, and the current collected by the fourth electrode falls. The sensitivity claimed for the detector is 0.2-1.0 ng, but this is not very informative as its significance depends on the characteristics of the column used and on the k of the solute peak on which the measurements were made. The sensitivity should be given as that solute concentration that produces a signal equivalent to twice the noise. Such data allow a rational comparison between detectors. The sensitivity or minimum detectable concentration of this detector is probably similar to the conventional pulsed ECD (viz. 1 X 10 g/mL). The linear dynamic range appears to be at least three orders of magnitude for a response index of r, where 0.97 [Pg.607]

Electron capture detectors are highly sensitive and have the advantage of not altering the sample significantly (in contrast to the flame ionization detector, which consumes the sample). The linear response of the detector, however, is limited to about two orders of magnitude. 

At this point, the sample is analyzed by gas chromatography (GC), the analytical method of choice for volatile halogenated hydrocarbons. Information on the analysis of these samples by GC is presented in Section 6.2, with a discussion of the advantages and disadvantages of each method. The technique of Antoine et al. (1986) showed a 5% variance on a series of 2 ppb spiked samples, and the analysis had a linear response ranging from 0.5 to 50 ppb. Although infra-red spectrometry has less sensitivity than electron capture detectors (ECD), Hall electroconductivity detectors (HECD), and mass spectrometrlc detectors (MS), it has been used to quantify the levels of... 

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