Instrumental quantification limit

Another factor of interest in defining fhe instrumental limitations is the instrumental quantification limit (IQL), which may be defined as the smallest amount of an analyte that can be reliably quantified by the instrument. 

Extending this principal to equation (10), the instrumental quantification limit (IQL, Cq) may be calculated using the equation... 

The approach described in Lee Aizawa (2003) consists of two steps for the determination of the LOD and LOQ. These firstly involve determination of the Instrumental Detection Limit (IDL) and Instrumental Quantification Limit (IQL), and using these values to estimate the Method Detection Limit (MDL) and Method Quantification Limit (MQL), following calculation of the LOD and LOQ for the extraction/analysis method. 

Historically, EELS is one of the oldest spectroscopic techniques based ancillary to the transmission electron microscope. In the early 1940s the principle of atomic level excitation for light element detection capability was demonstrated by using EELS to measure C, N, and O. Unfortunately, at that time the instruments were limited by detection capabilities (film) and extremely poor vacuum levels, which caused severe contamination of the specimens. Twenty-five years later the experimental technique was revived with the advent of modern instrumentation. The basis for quantification and its development as an analytical tool followed in the mid 1970s. Recent reviews can be found in the works by Joy, Maher and Silcox " Colliex and the excellent books by Raether and Egerton. ... 

Quantification Limit of determination Instrument for detection and quantification... 

The intensity of the current produced by analyte ions is relevant in quantification. Limits of detection are improved when fragmentation is reduced or eliminated and the ion current, attributable to the analyte, is present as a single species. For instance, using Cl often improves both detection and quantification limits when compared to El, although the controlled fragmentation used in selected reaction monitoring can also improve detection limits. Fragmentation as it applies to specific quantification techniques for small molecules is discussed in connection with the quadrupole family of instruments (Sections 3.3.3.1 and 3.3.5). Quantification for biopolymers, particularly proteins, is presented in Section 3.5.1.9. 

A two-step approach was used for the determination of method detection and quantification limits for the sulfur analytes, as described in Lee Aizawa (2003). The two step approach takes into consideration several factors that affect the analyte signal, including instrumental noise, variability in instrumental sensitivity, and variability in method efficiency, matrix effects and interference, and is simple to follow. Other methods, such as the Hubaux-Vos approach for the calculation of the detection limit can also be used, as reported in Fedrizzi et al. (2007). However, this later approach is complicated, time consuming and does not take either the variabiUty in method efficiency or the matrix effects into consideration (Lee Aizawa, 2003). A brief discussion on how to conduct the method vahdation using the two steps approach is mentioned in this section. 

The detection and quantification limits determined here (cl and cq) do not take the matrix interferences into account, because RMSE was determined from calibration standards. The value cq is used in the next step to spike the blank to compute the LOD and LOQ of the method, which incorporates instrumental variations. Consequently, both matrix/analytes and the extraction/analysis are taken into account for the determination of LOD and LOQ. 

In some cases, data and analyte recovery or concentration may be reported down to instrumental detection limit values (left-censored data) provided that the necessary explanation accompanies the results, and indicates changes in sample size, final aliquots prior to analysis, or method quantification including the use of internal standards or surrogates. 

Atmospheric pressure chemical ionization (APCI) is the ionization source that provides lower chemical noise and, subsequently, lower quantification limit than electrospray ionization (ESI) which is more robust. The use of mass spectrometric methods can be expected to increase, particularly as they become easier to use and the costs of instrumentation continue to fall. Despite the enormous progress in analytical technologies, methods based on HPLC with fluorescence detection are the most used today for aflatoxins instrumental analysis, because of the large diffusion of this configuration in routine laboratories. 

Encinar et al. developed a method for the accurate determination of sele-noamino acids in human serum by species-specific isotope dilution analysis. A human serum was enzymatically digested, and then the selenoamino acid and carboxymethylated selenocysteine were separated and quantified by HPLC-ICP-MS. Quantification of selenomethionine was carried out by isotope dilution using a synthetic Se-labeled counterpart. The selenomethionine in samples was also measured by using selenomethionine as an internal standard. The instrumental detection limit was down to 75 fg Se and the precision was better than 5% RSD. ... 

Although SFE and SFC share several common features, including the use of a superaitical fluid as the solvent and similar instrumentation, their goals are quite distinct. While SFE is used mainly for the sample preparation step (extraction), SFC is employed to isolate (chr-omatography) individual compounds present in complex samples (11 -15). Both techniques can be used in two different approaches off-line, in which the analytes and the solvent are either vented after analysis (SFC) or collected (SFE), or on-line coupled with a second technique, thus providing a multidimensional approach. Off-line methods are slow and susceptible to solute losses and contamination the on-line coupled system makes possible a deaease in the detection limits, with an improvement in quantification, while the use of valves for automation results in faster and more reproducible analyses (16). The off-line... 

In addition to the aforementioned methods, TLC in combination with other instrumental techniques have also been used for quantification of inorganic species. For example, two-dimensional TLC coupled with HPLC has been utilized for the separation and quantification of REEs in nuclear fuel fission products using silaiuzed silica gel as layer material [60]. In another interesting method, REEs in geological samples have been determined by ICP-AAS after their preconcentration by TLC on Fixion plates [32]. TLC in combination with neutron activation has been used to determine REE in rock samples on Eixion 50 x 8 layers with the sensitivity limit of 0.5 to 10 pg/g for 10- to 30-mg samples [41]. A combination of TLC and A AS has been utilized for the isolation and determination of zinc in forensic samples [27]. 

One of the limitations of the portable field survey instruments in the measurement of americium is that their quantitative accuracy depends on how well the lateral and vertical distribution of americium in the soil compares with the calibration parameters used. These methods can provide a rapid assessment of americium levels on or below surfaces in a particular environment however, laboratory-based analyses of samples procured from these environmental surfaces must be performed in order to ensure accurate quantification of americium (and other radionuclides). This is due, in part, to the strong self absorption of the 59.5 keV gamma-ray by environmental media, such as soil. Consequently, the uncertainty in the depth distribution of americium and the density of the environmental media may contribute to a >30% error in the field survey measurements. Currently, refinements in calibration strategies are being developed to improve both the precision and accuracy (10%) of gamma-ray spectroscopy measurements of americium within contaminated soils (Fong and Alvarez 1997). 

The quantification capability is normally limited by the detector and/or the ion source. The MCP that is often utilized in TOF instruments cannot fully handle the ion currents that are produced in MALDI and are often saturated to some extent. With other ion sources, such as SIMS, the detection system is less strained so the detector is less limiting. Instead the ion source will limit the quality in quantification. Magnetic sectors and also qudmpoles are more often utilized when quantification is important. 

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