Minimum detectable quantity,

Sensitivity by itself is not sufficient to completely evaluate an LCEC system for analytical purposes. The minimum detectable quantity (detection limit) is of more practical importance. The detection limit takes into consideration the amount of baseline noise as well as the response to the analyte. The detection limit is then defined as the quantity of analyte which gives a signal-to-noise ratio of three (a S/N of 3 is the generally accepted criterion although other values have been used). For a complete description of an LCEC application, both the sensitivity and detection limit, along with the S/N criteria used, should be provided. 

In Japan, bifenox is the only registered diphenyl ether herbicide. The tolerance and/or maximum residue limits (MRLs) are established at 0.1 mg kg for cereals such as rice grain, barley and wheat, and 0.05mgkg for potatoes (Ministry of Health, Labour and Welfare, Japan). Ibe California Department of Food and Agriculture (CDFA) established the minimum detectable quantity of diphenyl ether herbicides at 0.1 mgkg for bifenox, nitrofen and oxyfluorfen. ... 

Although, as indicated by the above studies, the detector response mechanism is poorly understood, the working limits of commercially available detectors are fairly well characterized and the detector is not particularly difficult to use. The minimum detectable- quantity for nitrogen is about 10 g N/s and about 5 x... 

In TLC the detection process is static (sepaurations achieved in space rather than time) and free from time constraints, or from interference by the mobile phase, which is removed between the development and detection process. Freedom from time constraints permits the utilization of any variety of techniques to enhance detection sensitivity, which if the methods are nondestructive, nay be applied sequentially. Thus, the detection process in TLC is nore flexible and variable than for HPLC. For optical detection the minimum detectable quantities are similar for both technlqpies with, perhaps, a slight advantage for HPLC. Direct comparisons are difficult because of the differences in detection variables and how these are optimized. Detection in TLC, however, is generally limited to optical detection without the equivalent of refractive... 

Boronic Ester Relative Retention Minimum Detectable Quantity (pg of pinacol) Optimum Detector Temperature ( C)... 

While most preliminary SFC-plasma coupled techniques employed microwave-induced plasmas (MIPs), the use of ICP-MS is now increasing [469]. An advantage of microcolumn SFC-ICP hyphenation is the significantly reduced flow-rates of microcolumns compared with those of conventional columns. Both pSFC-ICP-AES [470,471] and cSFC-ICP-AES [472] were described. In the case of elemental detector selectivity (e.g. AES) complete chromatographic resolution is not required. The detector possesses linearity over several orders of concentrative magnitude. Minimum detectable quantities for nonmetals range from sub to low ng mL"1. 

Figure 1-1 shows an example of the detection limits that can be reached with EC detection. The chromatogram shows the separation and detection of the catecholamines noradrenaline (nor), adrenaline (adr), dihydroxybenzylamine (dhba), and dopamine (dopa), using a glassy carbon electrode at a working potential of +0.6 V. The minimum detectable quantity is less than 3 Pg-... 

Detection limit (defined as two times the noise level) as minimum detectable quantity is given by ... 

It is seen from table (1) that by increasing the minimum detectable quantity by an order, from 10-7 g/mi to 10 8 g/ml, the maximum (k ) value is also increased by an order but the peak capacity is only increased by about 0%. Furthermore, this increase in peak capacity is achieved at an increase in retention time from twelve hours to about five days. It follows that, attainment of increased peak capacity by increased detector sensitivity, is extremely costly in time and higher peak capacities are best achieved by the use of columns of intrinsic high efficiency. ... 

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. 

Retinol and its esters exhibit similar UV absorption spectra within a broad wavelength range and have practically equal molar absorptivities when dissolved in a given solvent. The e value of crystalline all-frans-retinol in 2-propanol at the Amax of 325 nm is 52,300 (120), which corresponds to an A m of approximately 1830. The on-column minimum detectable quantity of vitamin A using UV absorption is approximately 2 ng (121). 

It is fortuitous that indigenous concentrations of vitamin E in the principal food sources are on the order of mg rather than yug/100 g, for the tocopherols and tocotrienols exhibit relatively low intensities of UV absorption. Individual vitamers are characterized by a slightly different absorption maximum within the wavelength range of 292-298 nm in ethanol. Published A] m values for the tocopherols at their Amax in ethanol are a-T 70-73.7 at 292 nm, (3-T 86-87 at 297 nm, y-T 90-93 at 298 nm, and <5-T 91.2 at 298 nm (126). The different absorption characteristics among the vitamers necessitates the running of individual standards for accurate quantitation of each vitamer. The absorption intensity of a-tocopheryl acetate is lower still with an A j m value of only 40-44 at the Amax of 285.5 nm (44). Reported minimum detectable quantities of a-, / -, y-, and (5-tocopherols at 295 nm are, respectively, 50 ng, 70 ng, 90 ng, and 130 ng (127). 

Method. The residue containing up to 1 pg of estrogen is dissolved in 0.9 ml of acetone. 0.1 ml of 0.01% DNS-C1 in acetone and 0.01 ml of 0.1 N sodium hydroxide are then added. The solutions are mixed and the reaction is kept at 50 °C for 30 min, cooled, diluted with 20 ml of benzene and extracted once with 5 ml of 0.1 N sodium hydroxide and twice with 5 ml of water before filtering through anhydrous sodium sulfate. The dry extract is evaporated to dryness under nitrogen. The residue is dissolved in a small volume of benzene for application of the sample to the TLC plates. The compounds are separated on plates of silica gel with chloroform-benzene—ethanol (18 2 1). The chromatogram is dried and observed under UV light. The minimum detectable quantities are 5—10 ng per spot. 

Detector sensitivity is best explained in terms of signal to noise ratio, which is the minimum detectable quantity with a signal to noise ratio of two (Willard, 1988). Detector sensitivity is linked to the method detection limit, a concept that we routinely use in environmental project work. (The definitions of detection limits in environmental pollutant analysis are discussed in Chapter 4.5.1.) The MDLs, however, while being related to detector sensitivity, greatly depend on the analytical method, sample matrix, and the analyte itself. In this chapter, we will address detector sensitivity in relative terms by comparing sensitivities of various chromatography detectors. 

The following table provides guidance in selection of electrochemical techniques by providing the relative sensitivities of various methods.1 The limit of detection of lead, defined as the minimum detectable quantity (on a mole basis), is used as the basis of comparison. 

The selectivity of electrochemical detection is further illustrated by Figure 3C (same amounts injected as per Figure 3A) which was obtained at a significantly lower potential (+ 0.500 volts), where only the three hydroquinones can be detected. When the amount of material injected was reduced by a factor of ten, the chromatogram in Figure 3D was obtained. While only 0.4 ng of hydroquinone is detected, the signal-to-noise ratio remains excellent and the amount injected could be further reduced by a factor of ten. In some ideal cases, minimum detectable quantities (S/N = 2) of 1 pg (typically 5 femtomoles) have been achieved with standard solutions. 

BTT and BTN, butane-1,2,4-triol-trinitrate DNT, dinitrotoluene EGDN, ethylene glycol dinitrate HPLC, high-performance liquid chromatography LOD, limits of detection MDQ, minimum detectable quantities NB, nitrobenzene NG, nitroglycerine NN, nitronaphthalene NT, nitrotoluene PETN, pentaerythritol tetranitrate RDX, cyclotrimethylene trinitramine SFE, supercritical fluid extraction SGC, solvating gas chromatography TDM, thermal desorption modulator TNB, trinitrobenzene and TNT, trinitrotoluene. 

Detectivity. The smallest concentration plotted in Figure 7.7 is the minimum detectable quantity, MDQ, or the detectivity. This limit was defined earlier as twice the noise MDQ is a signal 5 equal to 2N. Since the detector noise is usually specified in the same units as the signal, the detectivity can carry these same units. For example, if the signal is in... 

Under these conditions, warfarin elutes in approximately eight minutes. The minimum detectable quantity of material is approximately 5 ng. Warfarin can be extracted from rodent baits or from field wipe samples using a mixture of dioxane plus 15% water and 1% Nai Oy (27). The extract is injected directly into the HPLC system. Figure 7 is comprised of chromatograms of (a) warfarin standard and (b) an extract of commercial rat bait. 

Under these conditions, the minimum detectable quantity of dialifor Is approximately 15 ng. The compound elutes in approximately 9 minutes. Efficient analyses of wipe samples taken on Whatman 41 filter paper has been achieved. Quantitative recovery with minimum interference is obtained by extraction of the filters with ethyl acetate. 

At 280 nm, the minimum detectable quantity of pentachlorophenol is approximately 50 ng. Greater sensitivity may be achieved by monitoring at 254 nm. With samples extracted into benzene, however, 280 nm is preferred because of the large extinction coefficient of benzene at 254 nm. 

The methodology for assessing LoDs (i.e., the minimum detectable quantity in the ISO terminology) in the case of a linear regression model (LRM) has been... 

DETECTOR MINIMUM DETECTABLE QUANTITY (gsec-1) LINEAR RANGE TEMPERATURE LIMIT (°C) REMARKS... 

Figure 3. Simultaneous analysis for ascorbic acid and dehydroascorbic acid with the use of a gradient analysis HPLC method. The minimum detectable quantities were 10 ng/ injection for ascorbic acid and 500 ngl injection for dehydroascorbic acid (80) column, LiChrosorb NH2, 10 fxm mobile phase, 0.005M KH2PO4, pH 3.5 and CHsCN detection, ascorbic acid, 268 nm and dehydroascorbic acid, 228 nm. (Reproduced, with permission, from Hewlett-Packard.)...

Refractive index detection allows an extremely wide latitude in the selection of the eluent type, eluent pH and the ionic strength. In principle, refractive index detection can be substituted for conductance or UV absorption detection in many separations. However, in early work, refractive detection was found to be only moderately sensitive and was considered to be somewhat interference-prone [71]. Minimum detectable quantities for common anion such as chloride nitrate, or sulfate were reported to be in the 20 ng to 50 ng range (compared with 1 to 5 ng for direct conductance detection). 

A separation of arsenite, arsenate and monomethylarsonate (MMA) is shown in Fig. 6.23. The detection limit for arsenic was 10 pg/L and the minimum detectable quantity was 100 pg. It was also possible to separate and detect selenium(IV) and (VI) under similar conditions. 

Because the minimum detectable quantity for a TCD Is proportional to the concentration of sample molecules at the peak maximum, C, the Instantaneous concentration, C, for a Gaussian distribution in volume units Is ... 

A conventional capillary column, 0.200mm x 12.5M with a 3 of 250 has a sample capacity of about 50 ng at k 10. In comparison, a 0.480 mm x 28M column with a 8 of 250 has a sample capacity of about 540 ng at a k - 10. These very low sample capacities affect the XCD requirements very severely since the minimum detectable quantity, l.e. the detectivity, approaches the maximum sample capacity of the column as the column diameter decreases. 

Examination of the response of the 30 Ul TCD as a function of flow rate (see Figure 4) reveals apparent linear response for several different flow rates, and a decrease In the minimum detectable quantity proportional with the decrease In total flow through the detector. This strongly supports the concept that the TCD operates with greatest sensitivity and minimum distortion... 

The flame-based detector was reported to accept in excess of 20 ui/mln of 10-25Z aqueous methanol without extinction of the flame Optimum response was obtained at flow rates below 5 iii/min. Compatible solvent systems were aqueous methanol (up to 50%), acetone and ethanol (up to 40%) The minimum detectable quantity (at 5 times noise) measured for the FPD was 2 pg P. The dual-flame TSD can also be directly Interfaced with mlcrocaplllary packed columns The TSD was reported to be compatible with 75 to 100% aqueous methanol The utilization of microbore column LC-TSD for the analyls of nitrogen, phosphorous, and halogen containing compounds is particularly Important in studies of biomolecules, and drugs and their metabolites in physiological fluids ... 

The minimum detectable quantity (for Cj-Cg compounds) is in the range of 20-100 pg. Tests of this type have the advantage that no special equipment is needed in the chromatographic laboratory and they can be applied rapidly whenever any unexpected problem arises. 

DETECTOR MINIMUM detectable QUANTITY (gsec-. ) LINEAR RANGE temperaturi LIMIT ( C) REMARKS... 

Bowers, L.D. Cyclosporine analysis by high-performance liquid chromatography precision, accuracy, and minimum detectable quantity. Transplant.Proc., 1990, 22, 1150-1154 Gupta, S.K. Benet, L.Z. HPLC measurement of cyclosporine in blood plasma and urine and simultaneous measurement of its four metabolites in blood. J.Liq.Chromatogr., 1989, 12, 1451-1462 [cyclosporine A plasma urine simultaneous metabolites LOD 30 ng/mL cyclosporine D (IS) column temp 70]... 

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