Detector minimum detectable level

Detector Minimum Detectable Level (MDL). The sample level, usually given in weight units, at which the signal-to-noise (S/N) ratio is 2. 

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. 

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). 

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. 

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... 

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... 

Plots of phase angle difference in the interferometer arms vs. time are related to heat-production vs. time, and this in turn is related to the concentration of the species responsible for heat production. Typical instrument output for the urea/urease system is shown in Figure 3. Calibration curves can be constructed as shown in Figure The system is quite stable, and reasonably sensitive. Minimum detectable levels of urea are 5 mM, compared to the 0.1-5 mM limits for traditional detectors. Over extended time periods (7 days) the relative standard deviation at 5 mM concentrations is better than 5 /.. The optimum FIA conditions were around 1.0 ml/min flow rate, with a sample loop of 0.1-0.25 ml. 

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. 

Depending on the type of detection device, different evaluation techniques are required to provide appropriate characterization. If the item has different modes of operations, each of than needs to be addressed separately. Instruments that have alarm features are tested for minimum detectable level (MDL) using the alarm time within 2 min as a olterion. Those that do not have alarm features would also use the criterion for obtaining the maximum response signal at a certain time within the 2-min exposure. A more specific evaluation procedure for each type of detector may be required. 

A detector s limit of detection (LOD) is the lowest concentration level that it can identify with a certain degree of confidence. There are many definitions of LOD, such as the concentration at which the response signal generated is three times the instrument noise level. Here LOD is referred to as the minimum detection level (MDL) of concentration that will consistently cause the detector to alarm. It is affected by background noise and blank signals. The LOD of a detector may vary widely for different chemicals. Environmental and operational conditions could drastically affect the LOD. Manufacturers normally provide LOD information obtained under optimum conditions. 

The limit of detection (LOD) or detection limit of a method is the lowest analyte concentration that the detector wiU produce a response detectable above the background, or noise level, of the system. The minimum detectable level (MDL) is the concentration level at the LOD and generally defined as three times the noise level (baseline) of the detector. LOD and MDL are the two quantifiable values that can measure the sensitivity of the method. Sensitivity is the smallest difference in the response of the detector (signal) that can be detected for the method. LOD is the smallest amount that is clearly distinguishable from the background or blank. 

To unambiguously identify the presence of a peak and, in addition, be able to give some proximate estimation of its size for quantitative purposes, the peak height needs to be at least 5 times the noise level. The detector sensitivity, or the minimum detectable concentration, (Xd), is defined as that concentration of solute that will give a signal equivalent to twice the noise level and, consequently, the concentration of solute at the limiting (k ) value must be 2.5Xd. 

Detector Sensitivity or the Minimum Detectable Concentration has been defined as the minimum concentration of an eluted solute that can be differentiated unambiguously from the noise. The ratio of the signal to the noise for a peak that is considered decisively identifiable has been arbitrarily chosen to be two. This ratio originated from electronic theory and has been transposed to LC. Nevertheless, the ratio is realistic and any peak having a signal to noise ratio of less than two is seriously obscured by the noise. Thus, the minimum detectable concentration is that concentration that provides a signal equivalent to twice the noise level. Unfortunately, the concentration that will provide a signal equivalent to twice the noise level will usually depend on the physical properties of the solute used for measurement. Consequently, the detector sensitivity, or minimum detectable concentration, must be quoted in conjunction with the solute that is used for measurement. 

Where Q, is the minimum detectable amount, R the detector noise level and s the detector sensitivity [135,146,151,152]. For a concentration sensitive detector the minimum detectable concentration is the product of Q, and the volumetric gas flow rate through the detector. The minimum detectable amount or concentration is proportional to the retention time, and therefore, directly proportional to the column radius for large values of n. it follows, then, that very small quantities can be detected on narrow-bore columns. 

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). 

The sensitivity of the detector (Xd) (or minimum detectable concentration) is defined as that concentration of solute that will provide a signal equivalent to twice the noise level. Now the concentration of solute at the peak maximum is approximately twice the average concentration of the solute in the peak, volume. Thus, the minimum detectable mass will be that mass (m) that, when dissolved in a volume of mobile phase equivalent to the peak volume, will produce a concentration of Xp/2. 

Liquid chromatography with electrochemical detection (LCEC) is in widespread use for the trace determination of easily oxidizable and reducible organic compounds. Detection limits at the 0.1-pmol level have been achieved for a number of oxidizable compounds. Due to problems with dissolved oxygen and electrode stability, the practical limit of detection for easily reducible substances is currently about 10-fold less favorable. As with all detectors, such statements of the minimum detectable quantity must be considered only with the proverbial grain of salt. Detector performance varies widely with the analyte and the chromatographic conditions. For example, the use of 100- m-diameter flow systems can bring attomole detection limits within reach, but today this is not a practical reality. 

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