Continuous introduction of sample

Automatic continuous-flow methods Involving the continuous introduction of sample into the system are implemented by means of two different configurations (a) open, in which the flow is wasted after passing through the measuring cell, and (b) closed, in which the flowing solution is returned to the vessel to be recirculated once it has passed through the detector. 

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Elemental speciation aims at the separation, identification and quantification of the chemical forms in which the (trace) elements of interest are present. This can be accomplished by coupling a chromatographic (e.g. HPLC or GC) or electrophoretic separation technique on-line to an ICP-MS instrument. In this context, ETV is of little use as it is a discrete sample introduction system that does not permit the continuous introduction of sample into the ICR... 

Derivatisation/SPME of polar analytes in the sample matrix is the simplest way to improve an analyte s partition coefficient and enhance SPME performance [539]. With the continued introduction of new SPME fibres, the... 

The introduction of samples via nebulizers requires that they are either pneumatically or peristaltically pumped into the nebulizer for aerosol formation. This restricts the range of viscosities that can be easily handled by the nebulizer. For example highly saline or oil samples may well have to be diluted by an order of magnitude or greater. This dilution can be carried out either in a batch mode or continuously. Batch systems are quite complex in design but the rate of analysis is high. It is often the case that where dilution is required, in addition, a fast rate of analysis is also desirable. Some batch systems have been introduced commercially, notably to monitor wear metals in the oil industry. 

In static method a known amount of contaminant is introduced into a fixed volume of air in devices such as teflon bags, gas sampling bulbs and gas cylinders, etc. Dynamic methods involve continuous introduction of contaminant (at a controlled rate) into a stream of air. Static methods are generally much simpler to construct and use, however, these suffer from a number of problems. Dynamic methods, while more elaborate and relatively more expensive, offer greater flexibility in concentration range, sample volume and are also less affected by adsorption losses. 

With the on-line approach, the sample is continuously delivered to the vacuum in real time. Off-line sample introduction entails the running of microfluidic processes on the chip with later MS analysis. Therefore, the introduction of samples into the ion source generally requires breaking the ion source vacuum. 

As far as separation techniques are concerned, they can be implemented on automatic continuous analysers or robot stations as ancillary modules (dia— lysers, ion exchangers, liquid-liquid extractors). As stated in Chapter 12, chromatographic processes —particularly column (HPLC and GC), but occasionally also planar chromatographic purposes— are commonly the subject of automation. A conventional chromatograph furnished with a system for sequential introduction of samples —which can even be partially treated in a continuous fashion before of after column separation (derivatlration and post-column techniques)— markedly resembles continuous flow analysers. Gas and liquid chromatographs are often used as separative-determinative modules in robot-stalons. 

In describing some of the systems available for the sequential introduction of samples into analysers and instruments, a distinction will be made according to whether they are used with continuous or batch configurations. The introduction of liquid samples into robotic analysers shares some of the features of the operation performed with batch analysers and is described in detail in Chapter 9. 

SFA lends itself to continuous operation, and most methods can be run with a continuous flow of sample, rather than coimected to sample changen More than 100 papers specifically concerning online applications of SFA have been published. Modified laboratory systems have been less successful than those specially designed for continuous use, largely due to limited pump tube life and difficulties associated with calibration and sample introduction. In many online applications a response time of 10-15 min is adequate, and so the benefits of gas segmentation in limiting dispersion are questionable. As a result, many of the most successful commercial systems operate without air bubbles. The division between some of these systems and FIA systems is not sharply defined. 

In the 1980s, attempts were made to enable continuous introduction of liquid samples (especially aqueous buffer solutions) to ion sources of mass spectrometers. An early con-tiuous flow interface was based on the fast atom bombardment (FAB) ion source [70]. However, it was the ESI interface that greatly facilitated temporal profiling of dynamic... 

For biochemical applications, the dimensions of the introduction capillary were decreased to a few pm i.d. capillaries, mostly a glass or fused silica. This approach is called nanoelectrospray as it allows the continuous introduction of between 5 and 100 nLmin 1 of a liquid into the API source. Nano-electrospray is especially useful for applications where the sample amount is limited. In this way, it is possible to perform a series of MS experiments with a minute amount of sample, e.g. 30 min of various MS experiments with only 1 pL of sample. 

The trends begun with the general introduction of FTIR technology will undoubtedly continue. It is safe to say that the quality of the data being produced far exceeds our ability to analyze it. In fact, for many current applications, the principle limitations are not with the equipment, but rather with the quality of the samples. Thus, the shift from qualitative to quantitative work will proceed, reaching high levels of sophistication to address the optical and matrix interference problems discussed above. 

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