Limit-of-quantification

The limit of quantification, LOQ, is a figure of merit that expresses the ability of a measurement process to quantify adequately an analyte and it is defined as the lowest amount or concentration of analyte that can be determined with an acceptable level of precision and accuracy [73]. In practice, the 

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Limit of quantification (LOQ) The smallest amount of contaminant that can be... 

Specificity, Accuracy, Precision, Limit of detection, Limit of quantification, Practicability and applicability under normal laboratory conditions, Susceptibility to interference, Stability)... 

Today, when a pesticide with no detectable residues is registered for use, a Tolerance or maximum residue limit (MRL) is established at the lowest concentration level at which the method was validated. However, for risk assessment purposes it would be wrong to use this number in calculating the risk posed to humans by exposure to the pesticide from the consumption of the food product. This would be assuming that the amount of the pesticide present in all food products treated with the pesticide and for which no detectable residues were found is just less than the lowest level of method validation (LLMV). The assumption is wrong, but there is no better way of performing a risk assessment calculation unless the limit of detection (LOD) and limit of quantification (LOQ) of the method were clearly defined in a uniformly acceptable manner. 

Several terms have been used to define LOD and LOQ. Before we proceed to develop a uniform definition, it would be useful to define each of these terms. The most commonly used terms are limit of detection (LOD) and limit of quantification (LOQ). The 1975 International Union of Pure and Applied Chemistry (lUPAC) definition for LQD can be stated as, A number expressed in units of concentration (or amount) that describes the lowest concentration level (or amount) of the element that an analyst can determine to be statistically different from an analytical blank 1 This term, although appearing to be straightforward, is overly simplified. If leaves several questions unanswered, such as, what does the term statistically different mean, and what factors has the analyst considered in defining the blank Leaving these to the analyst s discretion may result in values varying between analysts to such an extent that the numbers would be meaningless for comparison purposes. 

Eortify each of these sub-samples with the analyte(s) of interest such that the concentration of the analyte(s) in the matrix equals the estimated limit of quantification (ELOQ). 

As discussed earlier, a concentration that would produce a signal of approximately 10-12 times the standard deviation of the blank (or in this case xeloq) is considered to be the limit of quantification. Therefore, if the LOQ was set at 10 times 5eloq, for 7 replicates (6 degrees of freedom) of the fortified control samples... 

Recoveries, limit of detection and limit of quantification 7.2.1 Crops... 

With fortification levels between 0.010 and 0.50mgkg, average recoveries from untreated plant matrices range from 78 to 106% with the limit of quantification (LOQ) and the corresponding limit of detection (LOD) of famoxadone being as follows ... 

Net recoveries of tebuconazole from matrices fortified at 0.01-7.0 mg kg ranged from 78 to 116%. The limit of detection (LOD) based on control interferences in matrices ranged from 0.001 to 0.01 mg kg. The limit of quantification (LOQ) based on recoveries was established at 0.01 mgkg ... 

With fortification levels between 0.01 and O.lmgkg, recoveries from untreated plant matrices ranged from 70 to 113%. The limit of detection is 0.002 mg kg and the limit of quantification is 0.01 mg kg 7... 

With a fortification level of 0.20 mg kg recoveries of buprofezin from untreated pepper, tomato, squash and eggplant samples were 76,73,77 and 92%, respectively. The limit of quantification of buprofezin is 0.01 mg kg ... 

As a more sensitive detection method, MS can be very useful in amino acid determinations. For example, S-carboxymethyl-(R) cysteine or SCMC, is a mucolytic agent used in the treatment of respiratory diseases. The development of a method utilizing high performance IEC and atmospheric pressure ionization (API) mass spectrometry to quantify SCMC in plasma has been described.66 This method is simple (no derivatization needed), rapid (inn time 16 min.), sensitive (limit of quantification 200 ng/mL in human plasma), and has an overall throughput of more than 60 analyses per day. API-MS was used successfully with IEC to determine other sulfur-containing amino acids and their cyclic compounds in human urine.67 IEC has also been used as a cleanup step for amino acids prior to their derivatization and analysis by gas chromatography (GC), either alone or in conjunction with MS.68 69... 

There are special problems in bioequivalency determinations when conventional pharmacokinetic studies are not possible. For example, when drugs are administered intranasally for direct treatment of receptors in the nasal mucosa, the concentration of drug in plasma may be below the limit of quantification. In such cases we are forced to attempt measurement of clinical response. The subjectivity and/or low precision of this type of study can be a serious problem. 

The extraction of the AOT samples consisted of taking a 50-mL subsample and extracting by separatory funnel with hexane and 10% NaCl in water. The cleaned-up and dried hexane extract was then injected onto a gas chromatograph (GC) with an ECD (electron conductivity detector). The minimum quantifiable limit (MQL), or limit of quantification, for the method was 0.002 ug/mL dislodging solution. A typical set of chromatograms can be seen in Figures 1 and 2. 

The limit of quantification (LQ, limit of determination, quantitation limit), Xiq, being the lowest content which can quantitatively be determined with a given precision (mostly expressed by the relative uncertainty of measurement)... 

Measured value at LQ (MLQ) (also minimum quantifiable value, IUPAC [1995]) Limit of quantification, LQ... 

For this reason, a number of analysts uses a further limit quantity, namely the limit of quantification, xLq, (limit of determination), from which on the analyte can be determined quantitatively with a certain given precision (Kaiser [1965, 1966] Long and Winefordner [1983] Currie [1992, 1995, 1997] IUPAC [1995] Ehrlich and Danzer [2006]). This limit is not a general one like the critical value and the detection limit which are defined on an objective basis. In contrast, the limit of quantification is a subjective measure depending on the precision, expressed by the reciprocal uncertainty xLq/AxLq = k, which is needed and set in advance. The limit of quantification can be estimated from blank measurements according to... 

Formally, an analytical result x,- can be calculated from y, by means of the corresponding calibration function. When this result (from repeated measurements) should be reported, it must be taken into account that the relative uncertainty amounts minimally 100% (see Sect. 7.5, item (1) p. 201) and, therefore, it holds that (x x)- That means, that the uncertainty interval of analytical results calculated from measured values nearby the critical value covers a range of about 0... 2x. As additional information, the limit of quantification, xLq, should be given. 

The uncertainty of the results between the detection limit and the limit of quantification decreases continuously up to the precision set in advance by the precision factor k. In reaching and exceeding the quantification limit, analytical results can be reported as usual see Sect. 8.1. 

An analytical value, Xlq, above which quantitative determinations are possible with a given minimum precision. The condition on precision must be declared in each case. For a given precision k = xlq/Axlq, the limit of quantification can be estimated by Eqs. (7.48) and (7.49). 

For factual reasons, the limit of quantification cannot be lower than the limit of detection —> The declaration of precision must always be given because it is an inherent component of LQ... 

Limit of detection Limit of quantification Confidence interval... 

PFOA observations To evaluate MPI-MCTM model results observational data of PFOA from ship cruises in the Atlantic, Indian and Pacific Oceans were taken from literature (summarised in Yamashita et al (2008)). The data was collected between 2002 and 2006 in a global ocean monitoring initiative. Samples were taken from ocean surface water. Vertical profiles were sampled in the Labrador sea, the Mid Atlantic ocean, the South Pacific ocean and the Japanese sea, where water probes were done at several depths down to 5500 m. The limit of quantification for PFOA was determined as 6 pg/L. 

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