Detectors limits of detection

Detectors for Gas Chromatography (continued) Detector Limit of Detection Linearity... 

Detector Limit of Detection (gS/s) Linear Dynamic Range (decades) Selectivity ... 

In hplc, detection and quantitation have been limited by availabiHty of detectors. Using a uv detector set at 254 nm, the lower limit of detection is 3.5 X 10 g/mL for a compound such as phenanthrene. A fluorescence detector can increase the detectabiHty to 8 x 10 g/mL. The same order of detectabiHty can be achieved using amperometric, electron-capture, or photoioni2ation detectors. 

Limits of detection become a problem in capillary electrophoresis because the amounts of analyte that can be loaded into a capillary are extremely small. In a 20 p.m capillary, for example, there is 0.03 P-L/cm capillary length. This is 1/100 to 1/1000 of the volume typically loaded onto polyacrylamide or agarose gels. For trace analysis, a very small number of molecules may actually exist in the capillary after loading. To detect these small amounts of components, some on-line detectors have been developed which use conductivity, laser Doppler effects, or narrowly focused lasers (qv) to detect either absorbance or duorescence (47,48). The conductivity detector claims detection limits down to lO molecules. The laser absorbance detector has been used to measure some of the components in a single human cell (see Trace AND RESIDUE ANALYSIS). 

The near-ir spectmm of ethylene oxide shows two peaks between 1600—1700 nm, which are characteristic of an epoxide. Near-ir analyzers have been used for verification of ethylene oxide ia railcars. Photoionization detectors are used for the deterrnination of ethylene oxide ia air (229—232). These analyzers are extremely sensitive (lower limits of detection are - 0.1 ppm) and can compute 8-h time-weighted averages (TWAg). 

Detectable concentration ranges are mbe-dependent and can be anywhere from one-hundredth to several thousand ppm. The limits of detection depend on the particular detector mbe. Accuracy ranges vary with each detector mbe. The pump may be handheld during operation (weighing from 8 to 11 ounces), or it may be an automatic type (weighing about 4 pounds) which collects a sample using a preset number of pump strokes. 

The System described in the previous section has been extended with a sulfur chemiluminescence detector (SCO) for the detection of Sulfur compounds (32). The separated fractions were thiols + sulfides + thiophenes (as one group), benzothio-phenes, dibenzothiophenes and benzonaphtho-thiophenes. These four groups have been subsequently injected on-line into and separated by the GC unit. Again, no overlap between these groups has been detected, as can be seen from Figure 14.20, in which the total sulfur compounds are shown and from Figure 14.21 in which the separated dibenzothiophenes fraction is presented. The lower limit of detection of this method proved to be 1 ppm (mg kg ) sulfur per compound. 

What is meant by the selectivity of a detector Define the limit of detection of a detector. 

The selectivity of a detector is often related to its limit of detection, i.e. the more selective it is, then the lower the background noise is likely to be, and consequently the lower the limit of detection. 

The term sensitivity is often used in place of the limit of detection . The sensitivity actually refers to the degree of response obtained from a detector, i.e. the increase in output signal obtained from an increasing amount or concentration of analyte reaching the detector. Care must therefore be taken when these terms are being used or when they are encountered to ensure that their meanings are unambiguous. 

There are a number of properties of a detector that determine whether they may be used for a particular analysis, with the most important being (a) the noise obtained during the analysis, (b) its limit of detection, (c) its linear range, and (d) its dynamic range. The last three are directly associated with the analyte being determined. 

Figure 2.6 Detector response curve showing (a) ideal behaviour, (b) real behaviour, (c) its linear range, (d) its dynamic range, (e) the noise level, and (f) the limit of detection at three times the noise level.

Assay sensitivity is defined here as the concentration of analyte that inhibits the observed absorbance by 50% or the IC50. The lower limit of detection (LLD) is the lowest analyte concentration that elicits a detector response significantly different from the detector response in the absence of analyte. In some cases, the LLD is defined as three standard deviations from the mean of the zero analyte control. In other cases, the LLD is defined empirically by determining the lowest concentration of analyte that can be measured with a given degree of accuracy. Readers are referred to Grotjan and Keel for a simplified explanation and to Rodbard for the complete mathematics on the determination of LLD. 

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

The limit of detection for this instrument is about 10 pg/ ml for polystyrene in 2-butanone,163 which is close to two orders of magnitude higher than that of the deflection-type DRI. Moreover, the response of the ELSD is linear over only two decades in concentration.163 The ELSD is a useful backup detector when the DRI or UV detectors are not appropriate, e.g., when the UV absorbance or RI change is a function of copolymer composition as well as concentration or in gradient elution systems where changes in solvent composition cause drift in baselines of the UV and DRI detectors. Compounds about as volatile as the solvent are poorly detected by ELSD. 

Table 4.22 Lower limits of detection (LLD) of some GC detectors...

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