Minimum detectable level

The flame-photometric detector (FPD) is selective for organic compounds containing phosphoms and sulfur, detecting chemiluminescent species formed ia a flame from these materials. The chemiluminescence is detected through a filter by a photomultipher. The photometric response is linear ia concentration for phosphoms, but it is second order ia concentration for sulfur. The minimum detectable level for phosphoms is about 10 g/s for sulfur it is about 5 x 10 g/s. 

Nuisance levels for odors are not absolute, but are related to the minimum detectable level for 50 per cent of the population. These levels have been explored by Warren Spring Laboratories, who have concluded that five times the minimum detectable level is likely to give rise to complaint. 

The minimum detectable level is commonly given in terms of the mass flow rate in grams per second. 

Air samples collected in the Sacramento Valley area of California near sites where methyl parathion was heavily used on rice were analyzed by Seiber et al. (1989). Methyl parathion concentrations ranged from 0.2 (minimum detectable level) to 25.67 ng/m depending on the location and time of sampling. Methyl paraoxon, the oxygen analog of methyl parathion, was also detected at a maximum of 3.07 ng/m. The highest concentrations of both compounds were found at sites near locations of heaviest use. 

Methyl parathion was determined in dog and human serum using a benzene extraction procedure followed by GC/FID detection (Braeckman et al. 1980, 1983 DePotter et al. 1978). An alkali flame FID (nitrogen-phosphorus) detector increased the specificity of FID for the organophosphorus pesticides. The detection limit was in the low ppb (pg/L). In a comparison of rat blood and brain tissue samples analyzed by both GC/FPD and GC/FID, Gabica et al. (1971) found that GC/FPD provided better specificity. The minimum detectable level for both techniques was 3.0 ppb, but GC/FPD was more selective. The EPA-recommended method for analysis of low levels (<0.1 ppm) of methyl parathion in tissue, blood, and urine is GC/FPD for phosphorus (EPA 1980d). Methyl parathion is not thermally stable above 120 °C (Keith and Walters 1985). 

Tissue Culture Assay. Kogure et al. (48) report a novel tissue culture assay for detecting several types of sodium channel blockers. The mouse neuroblastoma cell line ATCC CCL 131 is grown in RPMI 1640 supplemented with 13.5% fetal bovine serum and 100 pg/ml gentamycin, in an atmosphere of 5% C0 95% air at 37 C. Ninety-six well plates are seeded with 1 x 10 cells in 200 pi of medium containing 1 mM ouabain and 0.075 mM veratridine. Veratridine and ouabain cause neuroblastoma cells to round-up and die. In the presence of sodium channel blockers (e.g., TTXs or STXs), the lethal action of veratridine is obviated and cells retain normal morphology and viability. An important feature of this assay is that a positive test for sodium channel blockers results in normal cell viability. Since bacterial extracts can contain cytotoxic components, this assay offers an advantage over tests that use cell death as an endpoint. The minimum detectable level of TTX is approximately 3 nM, or approximately 1/1000 mouse unit. 

The minimum detectable level is estimated with the dinifroaniline signal-to-noise ratios (S/N). With fortification levels between 0.2 and 0.5mgkg the recovery of trifluralin from plant matrices is 70-99% with the LOD/LOQ being 0.005 mg kg according to the analytical method of the Ministry of the Environment, Japan. In multiresidue analysis by GC/NPD, the percent recoveries of pendimethalin from each crop with a fortification level of 0.25 mg kg were brown rice 70, potato 70, cabbage 80, letmce 89, carrot 84, cucumber 64, shiitake 74, apple 76, strawberry 99, and banana 99%. The LOD for each sample was 0.01 mg kg for pendimethalin. In residue analysis by GC/ECD, recoveries of the majority of dinifroaniline herbicides from fortified samples of carrot, melon, and tomato at fortification levels of 0.04—0.10 mg kg ranged from 79 to 92%. The LODs were benfluralin 0.001, pendimethalin 0.002 and trifluralin 0.001 mg kg for the GC/ECD method. ... 

Quantification of the limits of detection (LOD), or minimum detectable levels (MDL statistically defined in Section 13.4), is an important part of any analysis. They are used to describe the smallest concentration of each element which can be determined, and will vary from element to element, from matrix to matrix, and from day to day. Any element in a sample which has a value below, or similar to, the limits of detection should be excluded from subsequent interpretation. A generally accepted definition of detection limit is the concentration equal to a signal of twice (95% confidence level) or three times (99% confidence) the standard deviation of the signal produced by the background noise at the position of the peak. In practice, detection limits in ICP-MS are usually based on ten runs of a matrix matched blank and a standard. In this case ... 

The minimum detectable level, or detection limit, is defined as that concentration of a particular element which produces an analytical signal equal to twice the square root of the background above the background. It is a statistically defined term, and is a measure of the lower limit of detection for any element in the analytical process. (This definition corresponds to the 95% confidence interval, which is adequate for most purposes, but higher levels, such as 99% can be defined by using a multiplier of three rather than two.) It will vary from element to element, from machine to machine, and from day to day. It should be calculated explicitly for every element each time an analysis is performed. 

The procedure described by Suzuki et al. [11, 12], discussed in section 9.1.1.1 for the determination of chlorinated insecticides in soils has also been applied to hexane extracts of river sediments using high-resolution gas chromatography with glass capillary columns. Minimum detectable levels of a-BHC, fs-BHC, -BHC, P-BHC, Heptachlor, Heptachlor epoxide, Aldrin, Dieldrin, Endrin, p,p -DDE, p,p -TDE and p,p -DDT in lOOg samples of bottom sediment were 0.0005, 0.0032, 0.0014, 0.0040, 0.0012, 0.0020, 0.0014, 0.0020, 0.0056, 0.0032, 0.0080 and 0.0120mg kgr1 respectively. 

In a completely different area, the setting of minimum detectable levels, the politically expedient method may be counter productive to the expansion of knowledge. Dr. Kurtz related an example of this "We had a local manufacturer dribble amounts of kepone and mirex in the local streams from waste disposal on their site. At one point a few wells of homeowners along the stream were found to contain kepone at levels in the low parts per trillion. Later in annual state testing programs these wells were found not to contain any kepone. Was there a lowering of contamination What had probably happened was that the minimum detectable level was set above most of these early levels, and the later samples were less than this minimum level. It took on a suspicious air that the state had purposely set this level to avoid future controversy in this area."... 

The county landfill in the diagram was monitored to verify that toxic compounds were not leaching into the local water supply. Wells drilled at 21 locations were monitored over a year and pollutants were observed only at sites 8, 11. 12, and 13. Monitoring all 21 sites each month is very expensive. Suggest a strategy to use composite samples (Box 0-1) made from more than one well at a time to reduce the cost of routine monitoring. How will your scheme affect the minimum detectable level for pollutants at a particular site ... 

It is important to specify detectors independent of column parameters and of sample size. One parameter that does this is minimum detectable level, MDL. It is the "level" of sample in the detector at the maximum of the peak, when the signal-to-noise ratio is two. The term detectability is sometimes used for MDL. Variations of this definition are sometimes given which require the signal-to-noise ratio to be either one, three, or five. The parameter is also defined sometimes in terms of root-mean square (rms) noise. Peak-to-peak noise can be taken as six times rms noise. 

If the first definition of sensitivity is used, then "high sensi-vity" is better if detectable level is meant, then "low sensitivity" is better. To avoid confusion, it is better to refer to minimum detectable level or to response factor. 

What is the smallest "linear level Is it the minimum detectable level, or is it a level sufficiently higher than the noise to allow a measurement to be made with a precision equivalent to the allowable deviation from linearity Understandably, instrument manufacturers prefer the former definition, chromatographers with the responsibility of specifying the accuracy of their analyses prefer the latter. 

This corresponds to 1 ng/cm3, giving a peak twice the noise level. Better minimum detectable levels can be achieved if even higher currents are used. 

The part-per-billion sensitivity for the fixed gases makes the helium detector the most suitable for trace analysis of these gases. A minimum detectable level of 4x10 1<4g/sec and a linear... 

The minimum detectable level for phosphorus is about 2x10 12g/sec and that of sulfur is 5xl0-1 g/sec. 

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