Sample Consumption and Detection Limit

Note In MALDI-MS, the combination of the actual analyte and the procedure of sample preparation represent the tme limiting factors for sample consumption and detection limit. 

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In recent years, rapid advancements in photonic technologies have significantly enhanced the photonic bio/chemical sensor performance, especially in the areas of (1) interaction between the light and analyte, (2) device miniaturization and multiplexing, and (3) fluidic design and integration. This has led to drastic improvements in sensor sensitivity, enhanced detection limit, advanced fluidic handling capability, lower sample consumption, faster detection time, and lower overall detection cost per measurement. 

Compared with LC/MS/MS methods, nanoelectrospray/MS/MS methods offer additional benefits such as no sample carry-over and low sample and solvent consumptions. The major concerns surrounding columnless analysis of biological samples are matrix ion suppression and direct interference from endogenous components or metabolites of the dosed compound. Therefore, an extensive sample clean-up process must be in place to ensure the accuracy and precision of the assay. A nine-fold gain in terms of sample throughput was achieved with a nanoelectrospray/MS/ MS method that produced accuracy, precision, and detection limits comparable to those of a traditional LC/MS/MS method.14... 

The ideal analytical technique to be used in the challenging task of reconstructing past changes and recent variations in the concentration of trace substances in polar snow and ice should present several important features. Of these, extremely low detection limits, multi-element capability, low sample consumption and the possibility to avoid, as far as possible, any preconcentration step which could be a source of contamination are the most appreciated. Nevertheless, there is currently no technique with all the special features listed above several instrumental methods have been used in the past for trace element determination in polar snow and ice (see Table 3.4). 

CE is viewed as a potentially important technique for the determination of pesticide residues in environmental and food matrices. This application needs simultaneous separation of multicomponent mixtures with low limit of detection. The application of CE for the determination of pesticide residues has been aided recently by the development of improved methods of sample enrichment and detection. These methods can overcome the sensitivity limitations presented by the small sample volumes that are normally analyzed. It is somewhat perverse that this limitation should also be one of the technique s advantages in terms of reagent consumption. 

Recently, Houk s laboratory [18,19] described a direct injection nebulizer for use in ICP-MS consisting of a stainless steel tube with an inner diameter of 250 p.m held in a ceramic support tube that is inserted directly into the quartz injector tube of the ICP torch. A positive displacement gas pump is used to achieve a sample flow rate of 120 p,L/min. The low flow rates decrease sample consumption and the essentially 100% transfer efficiency improves detection limits by an order of magnitude. However, solvent loading presents a difficulty and the plasma content of solvent-derived polyatomics such as metal oxides was increased up to threefold [18]. 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

Application to solid polymer/additive formulations is restricted, for obvious reasons. SS-ETV-ICP-MS (cup-in-tube) has been used for the simultaneous determination of four elements (Co, Mn, P and Ti) with very different furnace characteristics in mg-size PET samples [413]. The results were compared to ICP-AES (after sample dissolution) and XRF. Table 8.66 shows the very good agreement between the various analytical approaches. The advantage of directly introducing the solid sample in an ETV device is also clearly shown by the fact that the detection limit is even better than that reported for ICP-HRMS. The technique also enables speciation of Sb in PET, and the determination of various sulfur species in aramide fibres. ETV offers some advantages over the well-established specific sulfur analysers very low sample consumption the possibility of using an aqueous standard for calibration and the flexibility to carry out the determination of other analytes. The method cannot be considered as very economic. 

Water samples (drinking water, rain, sea, river or waste water and others) have been characterized by ICP-MS with multi-element capability in respect to metal impurities (such as Ag, Al, As, Ba, Be, Ca, Cd, Cr, Co, Cu, Fe, Hg, K, Na, Sb, Se, Mg, Mn, Mo, Ni, Pb, Tl, Th, U, V and Zn) in many laboratories in routine mode with detection limits at the low ng I 1 range using ICP-QMS, and below by means of ICP-SFMS. Drinking water samples are controlled in respect of the European legislation (Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption). For quality control of analytical data, certified standard reference materials e.g. drinking water standard (40CFR 141.51), river water reference material SLRS-4 or CASS-2 certified reference sea-water material and others are employed. 

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