Detector, characteristics minimum detectability

The FID can detect all organic compounds containing C and H, with the exception of formic acid and methane. It is a mass-sensitive detector. The minimum detectable mass is about 0.01-0.1 ng and the FID has a large dynamic range of 10. Other detector characteristics can be found in Table 2.4. 

Principles and Characteristics As mentioned already (Section 3.5.2) solid-phase microextraction involves the use of a micro-fibre which is exposed to the analyte(s) for a prespecified time. GC-MS is an ideal detector after SPME extraction/injection for both qualitative and quantitative analysis. For SPME-GC analysis, the fibre is forced into the chromatography capillary injector, where the entire extraction is desorbed. A high linear flow-rate of the carrier gas along the fibre is essential to ensure complete desorption of the analytes. Because no solvent is injected, and the analytes are rapidly desorbed on to the column, minimum detection limits are improved and resolution is maintained. Online coupling of conventional fibre-based SPME coupled with GC is now becoming routine. Automated SPME takes the sample directly from bottle to gas chromatograph. Split/splitless, on-column and PTV injection are compatible with SPME. SPME can also be used very effectively for sample introduction to fast GC systems, provided that a dedicated injector is used for this purpose [69,70],... 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

Interfacing the TEA to both a gas and a HPLC has been shown to be selective to nitro-based explosives (NG, PETN, EGDN, 2,4-DNT, TNT, RDX and HMX) determined in real world samples, such as pieces of explosives, post-blast debris, post-blast air samples, hand swabs and human blood, at picogram level sensitivity [14], The minimum detectable amount for most explosives reported was 4-5 pg injected into column. A pyrolyser temperature of 550°C for HPLC-TEA and 900°C for GC/TEA was selected. As the authors pointed out, GC uses differences in vapour pressure and solubility in the liquid phase of the column to separate compounds, whereas in HPLC polarity, physical size and shape characteristics determine the chromatographic selectivity. So, the authors reported that the use of parallel HPLC-TEA and GC-TEA techniques provides a novel self-confirmatory capability, and because of the selectivity of the technique, there was no need for sample clean-up before analysis. The detector proved to be linear over six orders of magnitude. In the determination of explosives dissolved in acetone and diluted in methanol to obtain a 10-ppm (weight/volume) solution, the authors reported that no extraneous peaks were observed even when the samples were not previously cleaned up. Neither were they observed in the analysis of post-blast debris. Controlled experiments with handswabs spiked with known amounts of explosives indicated a lower detection limit of about 10 pg injected into column. 

The level of noise restricts the minimum signal that can be detected and attributed to an analyte, so it is important to keep it to a minimum. A detector characteristic that is often more meaningful than the noise is the ratio of the signal-to-noise, SIN. In most chromatographic work it is... 

The pressure sensitivity of a detector is extremely important as it is one of the detector parameters that determines both the long term noise and the drift. As it influences long term noise, it will also have a direct impact on detector sensitivity or minimum detectable concentration together with those other characteristics that depend on detector sensitivity. Certain detectors are more sensitive to changes in pressure than others. The katherometer detector, which is used frequently for the detection of permanent gases in GC, can be very pressure sensitive as can the LC refractive index detector. Careful design can minimize the effect of pressure but all bulk property detectors will tend to be pressure sensitive. 

There are several important characteristics of a good detector sensitivity, dynamic range, stability, and for specific ones selectivity. Sensitivity should be in fact characterized by two parameters the ratio of the detector response to the amount of sample (sensitivity slope) and the minimum detectable level of a given compound (commonly measured for a signal to noise ratio of 3). The dynamic range is the range... 

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]

All neutrons that fall on a detector will be detected, more or less efficiently. Ideally, detectors would only be exposed to neutrons with the correct characteristics since other neutrons simply contribute to the instrument background they must be reduced to a minimum. 

There is no unique set of characteristics which can be used to describe the ideal infrared detector. The ideal detector has nearly as many definitions as there are applications. For hot box detection on railroad-caraxles, reliability and simple maintenance are measures of perfection. For satellite operations compactness, light weight, low power and high sensitivity may be the important yardsticks. In the discussion which follows, an ideal detector is defined as one whose minimum detectible power is determined by the statistical fluctuation in... 

Sulfur compounds play a major role in determining the flavor and odor characteristics of many food substances. Often sulfur compounds are present in trace levels in foods making their isolation and quantification very difficult for chromatographers. This study compares three gas chromatographic detectors the flame photometric detector, sulfur chemiluminescence detector and the atomic emission detector, for the analysis of volatile sulfur compounds in foods. The atomic emission detector showed the most linearity in its response to sulfur the upper limit of the linear dynamic range for the atomic emission detector was 6 to 8 times greater than the other two detectors. The atomic emission detector had the greatest sensitivity to the sulfur compounds with minimum detectable levels as low as 1 pg. 

Hence, all compounds having a conductivity less than the carrier gas will be detected by the TCD, which is a universal concentration-sensitive detector. The TCD is nondestructive, and may be used for preparative separations. The detector has however low sensitivity, and the minimum detectable (M D) mass is about 10 ng even using He or H2 as carrier gas. Other detector characteristics can be found in Table 2.4. The TCD is commonly used for determination of light and permanent gases in packed or PLOT columns. The TCD is well suited for portable gas chromatographs because it is easily miniaturized and does not require extra gases. 

Detector Minimum detectable amount (gs"Y Minimum detectable amount on-column Linear range Selectivity Other characteristics Main application... 

The properties of the detection system employed for a chromatographic analysis can be as critical as the properties of the column itself and it is now well understood that column performance and detector performance are interactive. The properties of the detector controls the mass sensitivity and the concentration sensitivity of the whole of the chromatographic system. Detector characteristics also determine the minimum... 

Some FDD detectors also offer an electron capture mode being selective for monitoring high electron affinity compounds such as freons, chlorinated pesticides and other halogen compounds. For such type of compounds, the minimum detectable quantity (MDQ) is at the femtogram or low picogram level. The FDD is similar in sensitivity and response characteristics to a conventional radioactive ECD, and can be operated at temperatures up to 400 °C. For operation in this mode. He and CH are introduced just upstream from the column exit. 

The sensitivity of a detector is not the minimum mass that can be detected. This would be the system mass sensitivity, which would also depend on the characteristics of the apparatus as well as the detector and, in particular, the type of column employed. During the development of a separation the peaks become broader as the retention increases. Consequently, a given mass may be detected if eluted as a narrow peak early in the chromatogram, but if eluted later, its peak height may be reduced to such an extent that it is impossible to discern it from the noise. Thus detector sensitivity quoted as the minimum mass detectable must be carefully examined and related to the chromatographic system and particularly the column with which it was used. If the data to do this are not available, then the sensitivity must be calculated from the detector response and the noise level in the manner described above. 

The sensitivity or MDC of a detector is not the same as the minimum mass that can be detected. This would be the system mass sensitivity, which will depend on the characteristics of the column and the chromatographic properties of the solute, as well as the detector specifications. In all chromatographic systems, the peak becomes broader as the retention increases. Con-... 

The difficulties inherent in off-line analysis of HPLC peaks are becoming less of an unavoidable problem because HPLC also has another advantage over other techniques. The most commonly used HPLC detectors are ideal for PAH analysis. Fluorescence excitation-emission and UV absorbance detection are both highly sensitive and very selective for PAHs. These methods detect electronic transitions in the PAH molecules. The transition energies are determined by the PAH size and shape. Therefore, isomeric species that differ in ring configuration also differ in spectral character. The locations of absorbance maximums and the intermediate minimums, as well as the relative intensities of each, form a unique pattern characteristic of a particular PAH (4). 

The ratio of the signal to the noise is a convenient characteristic of detector performance. It conveys more information about the lower limit of detection than does the noise alone. Commonly, the smallest signal that can be attributed to an analyte is one whose signal-to-noise ratio or S/N, is 2 or more. An S/N ratio of 2 is shown in Figure 7.3 it can be seen that this is certainly a minimum value for distinguishing a peak from the background noise. Sharp spikes which exceed an S/N of 2 should not be interpreted as peaks as these often arise from contamination and represent a different type of detector instability. 

---