Limit of identification

An alternative expression for the detection limit, which minimizes both type 1 and type 2 errors, is the limit of identification, (Sa)loi> which is defined as... 

Normal distribution curves showing the definition of detection limit and limit of identification (LOI). The probability of a type 1 error is indicated by the dark shading, and the probability of a type 2 error is indicated by light shading. 

Somsen et al. [796] have reported the use of SERR spectroscopy for the in situ selective determination and semi-quantitative analysis of structurally similar dyes separated by TLC. The limits of identification of the TLC-SERRS method (ca. 5ng applied) were sufficient for acquisition of spectra of impurities present in the certified dye standards. SERRS may also be used for in situ identification of highly fluorescent molecules on HPTLC plates. 

The use of surface-enhanced resonance Raman spectroscopy (SERRS) as an identification tool in TLC and HPLC has been investigated in detail. The chemical structures and common names of anionic dyes employed as model compounds are depicted in Fig. 3.88. RP-HPLC separations were performed in an ODS column (100 X 3 mm i.d. particla size 5 pm). The flow rate was 0.7 ml/min and dyes were detected at 500 nm. A heated nitrogen flow (200°C, 3 bar) was employed for spraying the effluent and for evaporating the solvent. Silica and alumina TLC plates were applied as deposition substrates they were moved at a speed of 2 mm/min. Solvents A and B were ammonium acetate-acetic acid buffer (pH = 4.7) containing 25 mM tributylammonium nitrate (TBAN03) and methanol, respectively. The baseline separation of anionic dyes is illustrated in Fig. 3.89. It was established that the limits of identification of the deposited dyes were 10 - 20 ng corresponding to the injected concentrations of 5 - 10 /ig/ml. It was further stated that the combined HPLC-(TLC)-SERRS technique makes possible the safe identification of anionic dyes [150],... 

Throughout this text the term sensitivity will be employed synonymously with limit of identification. The concentration limit is the greatest dilution in which the test gives positive results it is expressed as a ratio of substance to solvent or solution. For these two terms to be comparable, a standard size drop must be used in performing the test. Throughout this book, unless otherwise stated, sensitivity will be expressed in terms of a standard drop of 0 05 ml. 

Determination online-HPLC-DAD recovery above 80% and limits of identification of 20 to 40 ng/g for 1.5% organic matter for 3.5% organic matter identification limits of 30 to 50 ng/g... 

Before dealing with the apparatus required for spot test reactions, it is necessary to define clearly the various terms which are employed to express the sensitivity of a test. The limit of identification is the smallest amount recognizable, and is usually expressed in micrograms (pg), one microgram being one-thousandth part of a milligram or one-millionth part of a gram ... 

Reaction type Visible change Interference Limit of identification (M)... 

The method is suitable for the identification and quantification of mixtures of substances. Depending on the IR absorptivity of the substance and the distance run in the chromatogram, the limits of identification, the validated detection limits, and the limits of quantification lie between 15 ng and 2.5 pg. 

The term Limit of Identification, which was first suggested by the author, has found almost universal acceptance as x. When the values of x and y are known, the concentration (or dilution) of the test solution with respect to the material being sought or identified is easily stated. This leads to the Concentration Limit or better its reciprocal the Dilution Limit The following simple relation exists between the dilution limit, the volume of the test solution, and the identification limit ... 

In this text, the limits of identification and the dilution limits which can be attained by the respective spot tests are given immediately after the outline of the various procedures. A tabular summary (Chapter 6) presents the identification limits of a large number of the more important spot reactions. If the drop volume, which ordinarily is around 0.05 ml, is taken into account, the dilution limits corresponding to the identification limits can be readily computed. 

In accord with their limits of identification and the corresponding dilution limits, spot reactions can be applied directly only within certain concentration ranges of the material to be detected. When higher dilutions are presented, preliminary concentration is essential. Evaporation of considerable volumes of liquid was formerly employed almost exclusively, but this has been replaced in many instances by the use of so-called "collectors". The latter are solids which have the ability to fix on their surface traces of the sought material. The action of collectors is due to the formation of mixed crystals in some cases, or to mutual precipitation, or adsorption. Traces accumulated from highly dilute solutions by these gathering aids can then be tested directly on the collector by suitable spot reactions, or the gathered material may be put into solution in a small volume before the test is applied. 

A great improvement in the limit of identification of spot reactions, sometimes 100 fold, is obtained through the use of the akro reaction. It consists in the carr5dng out of spot tests in the tiny point (5 mm ) of a paper triangle shown in Fig. 34. 

Limits of Identification. 1 y sulfur 2.5 y KCNS y NagSgOg, 5y FeS04. A positive response was given by less than 0.5 mg of PbS BijjSa, SnSj, KgSgOg, CuS, and HSOgNHg through the formation of COS. 

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