Instrumental detection limits definition

For flame atomic absorption spectrophotometry, the detection limit Is defined as the concentration that produces absorption equivalent to twice the magnitude of the background fluctuation. No mention is made of the blank or blank correction. This definition implies an instrument detection limit rather than a detection limit of a complete analytical procedure. Finally, no mention Is made of the need to determine the variability of responses. 

Out of these efforts have come a multitude of terras such as Limit of Detection, Method Detection Limit, Instrument Detection Limit, Limit of Quantitation, Criterion of Detection and a multitude of symbols such as the less than sign, RD, TR (for trace), U, M, J, T and w, and K. EPA is now proposing LTL (less than lower limit of detection) and LTC (less than criteria of detection) in computer standards. Some of the conventions come with rigid definitions for prescribed use and others come with vague definitions and allow for "analytical judgment" and flexibility. 

Limit of detection The method you choose must be able to detect the analyte at a concentration relevant to the problem. If the Co level of interest to the Bulging Drums was between 1 and 10 parts per trillion, would flame atomic absorption spectroscopy be the best method to use As you consider methods and published detection limits (LOD), remember that the LOD definition is the analyte concentration producing a signal that is three times the noise level of the blank, i.e., a S/N of 3. For real-world analysis, you will need to be at a level well above the LOD. Keep in mind that the LOD for the overall analytical method is often very different than the LOD for the instrumental analysis. 

This group of elements contains a large volume of information on analytical laboratory method requirements and procedures. Therefore, the laboratory that will conduct the analysis should provide this information to the project team for the incorporation into the QAPP. For example, the selection of analytical laboratory methods and QC requirements definitely needs input from the analytical laboratory, particularly if low detection limits or non-routine analyses are concerned. Analytical instrument calibration and maintenance requirements should also be developed as a cooperative effort with the analytical laboratory or by the individuals who are well-versed in laboratory practices and procedures. 

Various approaches have been used to define detection limit for the multivariate situation [24], The first definition was developed by Lorber [19]. This multivariate definition is of limited use because it requires concentration knowledge of all analytes and interferences present in calibration samples or spectra of all pure components in the calibration samples. However, the work does introduce the important concept of net analyte signal (NAS) vector for multivariate systems. The NAS representation has been extended to the more usual multivariate situations described in this chapter [25-27], where the NAS is related to the regression vector b in Equation 5.11. Mathematically, b = NAS/ NAS and NAS = 1/ b. Thus, the norm of the NAS vector is the same as the effective sensitivity discussed in Section 5.4.9.1 A simple form of the concentration multivariate limit of detection (LOD) can be expressed as LOD = 3 MINI, where e denotes the vector of instrumental noise values for the m wavelengths. The many proposed practical approaches to multivariate detection limits are succinctly described in the literature [24],... 

We should examine the precision of measurements made at this limit before accepting this definition of detection limit. The repeated measurement of the instrumental response from a sample containing analyte at the detection limit will lead to the analyte being reported as below the detection limit for 50% of the analyses. The relative standard deviation, RSD, of such measurements is given by... 

Column dimensions—length (L) and column inner diameter (dc or i.d.)— control column performance (N, speed, sensitivity, sample capacity) and its operating characteristics (flow rate, back pressure). Designations of various column types based on column inner diameters and their associated characteristics are shown in Table 3.1. Note that void volume, sample capacity, and operating flow rate are proportional to (dc)2, while detection limit, or sensitivity, is inversely proportional to (dc)2. Note also that prep columns (>10mm i.d.), microbore (micro columns (<0.5 mm i.d.) will require specialized HPLC instruments (see Chapter 4). There is a definitive trend toward the increased use of shorter and smaller inner diameter analytical columns due to their higher sensitivity performance and lower solvent usage.9"11 This trend will be explored later. 

The threshold of sensitivity varies according to the instrument and the element being considered. Numerous comparative tables on detection limit values exist which are continually being updated as a result of the progress made in instrumentation. The sensitivity threshold corresponds to the minimum concentration of element in solution that will yield an analytical signal for which the amplitude is equal to three times the standard deviation calculated for an analytical blank. This classic definition leads to rather optimistic values and very variable with respect to the element. The detection limit of the instrument represents the concentration of an element, which allows detection with a confidence of 95 per cent (cf. Chapter 22). 

Eor each method, DLs are taken from several (2-10) different references, typically interpreted as the number of references consulted, from which data were extracted, but occasionally as a combination of the reference consulted and those listed therein some duplication of information is to be expected. Values are different due to different instruments, conditions, definitions (2 s, 3 s, etc.) which were not rationalized to a common basis but accepted as is. Estimation of DLs is just that, an estimation from measurements with an RSD in the vicinity of 50%, negating, in this author s opinion, the need for very firm decisions on a uniform measurement basis. Eor each element, a best (lowest) DL was selected and a median value was calculated from the range of values tabulated median values (with some rounding) were selected for listing in this table as a representation of typical detection limits. 

There is a long history of efforts to produce such tight definitions for detection limits that no play is afforded to the human imagination in their establishment. However, up to the present, little success has been achieved. Nevertheless, atomic absorption has shown better repeatability of detection limits than most other techniques. When one laboratory using a given type of instrument achieves a certain limit, it can be expected that another laboratory with the same type of instrument can duplicate the limit within a factor of about three. 

SIMS instrument designs based around FT-ICR have been able to replicate many of the advantages displayed by such mass filters when applied in mass spectrometry. As an example, mass resolution values of 385,000 have been demonstrated via the single ion method (see Section 5.1.1.1.1) albeit using the 50% definition (Smith et al. 2011). This was reported for molecular secondary ions produced via Cgo primary ion impact. Also demonstrated was the possibility of imaging the organic ions to unprecedented sensitivity and detection limits. This was carried out by synchronizing the pulsed Cgo beam raster with the FT-ICR mass filter detection electronics, i.e. the microprobe method (see Section 5.3.2.2). 

Note that in spite of the mathematical definitions cited, detection limits are rather nebulous quantities. Because they depend on many variables, a factor of 2—3 times uncertainty in the values can be anticipated. They can vary significantly between various manufacturers instrumentation and are especially sensitive to different modes of sample introduction. They can also be modified by the optimization for the determination of specific elements. When performing multielement analyses, a compromise of optimization must be tolerated. This compromise usually results in the achievement of optimal detection limits for only a few elements, with the remainder often being a factor of 2—3 times their optimized values. Also, because detection limits are so dependent on operating parameters, it is prudent to frequently (i.e., with each batch of samples analyzed) compute detection limits to reliably report ultra-trace concentration levels. Care must be taken to not report too many significant figures when stating detection limits, so as to be consistent with the probability level selected in the computation. Typical published detection limits for various types of instrumentation are tabulated in Table 10.1. 

Capillary electrophoresis (CE) is a separation technique for ionic or ionizable compounds. CE is particularly attractive because the instrumentation is inexpensive and separations are quick and efficient. As with GC and LC, CE can be coupled to and flame photometric detection (FED) to detect alkylphosphonic acids [30-32]. Indirect UV absorbance detection with CE has also been used for the analysis of nerve agents and their degradation products [33]. In an attempt to meet the demands of portable and efficient field instruments, miniaturized analytical systems with CE microchips have also been made for the separation and detection of alkylphosphonic nerve agents [34]. The aforementioned CE procedures all provide rapid identification without extensive sample preparation. CE is most likely to be used as a guide in order to select the appropriate methods for further analysis by more definitive techniques such as GC-MS, as most of the products detected and analysed are degradation products [35]. A review depicting various CE separation techniques, lab-on-a-chip technology and detection limits has been compiled by Pumera and is shown in Table 3.1. 

In analytical and in environmental sciences there is a constant challenge to enhance instrumental performance and to detect lower and lower trace amounts. It seems also to be a challenge for statistical refinement of the definition of the limits briefly considered above. Until now the soundest approach to this field is probably that given by LUT-HARDT et al. [1987]. The reader who is not forced to follow statutory regulations (DIN, ISO, BS,...) may try his own literature search. 

A reference method is an analytical method with thoroughly documented accuracy, precision and low susceptibility to interferences. The accuracy and precision shall be demonstrated by direct comparison with the definitive method and primary reference material or, where not available, with other well-characterized and documented analytical approaches) (Boutwell, 1977). As long as accuracy and imprecision are within the limits, each technique or method is acceptable as a reference method. However, for reference methods one always looks for a method easily applicable in the laboratory. Therefore, the expensive instrumentation and the relatively low sample capacity make IDMS suitable as a definitive method rather than as a reference method. For some applications, however, IDMS is the method of choice, allowing a more specific detection than the existing methods in the laboratory. 

The most common traditional definition of the quantum/classical limit is the point at which Planck s constant h - 0. However, this is an unreasonable stipulation [33] because h is not dimensionless and its value can therefore not be varied. A possible operational condition could be formulated in terms of a dimensionless parameter of the form h/S 1, where S is the action quantity in a given situation. It could be argued that for S sufficiently large compared to h, measurement at the macroscopic level cannot detect quantum effects because of limited instrument resolution. This argument implies that the coarse-grained appearance of a classical world is simply a question of experimental accuracy and that every physical system ultimately displays quantum features and that there is no classical limit. 

Kidd and Spinney (201) ascribed the fact that only seven chemically shifted resonances are detected for the [NbCl Br6 ] system to the non-statistical formation of the mixed halogenoniobates by concluding that only the cis isomer is formed for those constituents where geometric isomerism is conceivable. This view has recently been challenged (202) and definitive proof is given for the statistical coexistence of all ten possible isomers. The failure to observe the remaining isomers is definitely due to instrumental limitations, i.e. the... 

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