Sample preparation sensors

Recognition element Sample preparation Sensor SPR SPR optical platform ... 

Fig. 8.28 External view of the MIMOS II sensor head without contact plate assembly (left) MIMOS II sensor head mounted on the robotic arm (IDD) of the Mars Exploration Rover. The IDD also carries the a-Particle-X-ray Spectrometer APXS, also from Mainz, Germany, for elemental analysis, the Microscope Imager MI for high resolution microscopic pictures ( 30 pm per pixel), and the RAT for sample preparation (brushing grinding drilling (< 1 cm depth)). Picture taken at Kennedy-Space-Center KSC, Florida, USA...

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

Kinetics evaluation software generates the values of ka (rates of complex formation) and kd (rates of complex dissociation) by fitting the data to interaction models. In a sensorgram, if binding occurs as sample passes over a prepared sensor surface, the response increases and is registered upon equilibrium, a constant signal is reached. The signal decreases when the sample is replaced with buffer, since the bound molecules dissociate. 

Future generations will incorporate advances in both hardware and software to provide systems that are even more efficient, flexible, and intelHgent. TTiey will have new sensor technology to improve specificity, increase sensitivity, and lower detection Hmits. Some progress has been made in this respect by a number of workers using microwave sample preparation techniques on-Hne, for example. 

Preparation of an environmental sample for delivery to the sensor and the sample cleanup afterwards are often the rate-limiting steps in the detection of biological agents, as well. Even for biodetection, sample preparation is a chemistry and materials science issue, currently accomplished using membranes and surface-active chemistries, binders, and ligands. Biological sample preparation remains an embryonic field. 

Dew-point measurement is a primary method based on fundamental thermodynamics principles and as such does not require calibration. However, the instrument performance needs to be verified using salt standards and distilled water before sampling (see Support Protocol). To obtain accurate and reproducible water activity results with a dew-point instrument, temperature, sensor cleanliness, and sample preparation must be considered. Equipment should be used and maintained in accordance with the manufacturer s instruction manual and with good laboratory practice. If there are any concerns, the manufacturer of the instrument should be consulted. Guidelines common to dew-point instruments for proper water activity determinations are described in this protocol. The manufacturer s instructions should be referred to for specifics. 

To obtain accurate and reproducible water activity results with a dew-point instrument, the temperature, sensor cleanliness, and sample preparation must be considered. Temperature influences water activity measurements in two ways one is its effect on the water activity in the sample and the second is the effect on the dew-point measurement. 

The processes of sample preparation should not be too onerous or slow otherwise there is little incentive to use sensors. Faced with an awkward or slow sample preparation phase for sensor use, the analyst might as well as consider whether instrumental analyses with their often higher sensitivity and specificity (and which, in any case, are regarded as the gold standard for industrial measurements), coupled with on-line sample preparation procedures, might be a more efficient and effective route to chemical information. 

Above all, it is worth remembering that if insufficient attention is paid to sample preparation then the validity of an analysis will be seriously undermined [20] no matter how sophisticated the measurement instrument is, whether it be a mass spectrometer or sensor. Comments on, and reviews of modem methods of sample preparation such as those of Jinno [21] and Pawliszyn [22,23] are essential reading whether one is a developer of sensors or an instrumental analyst. 

Consideration of sample preparation also leads to the question of whether for some applications at least sensors would be more effectively constructed as a collection of modules each with a different function (sample preparation, transduction, detection, data collection) on a platform, large, small or miniaturised, rather than the old ideal of a small, self-contained device with intimately appressed layers which could return a rapid answer from a bucket of slime or some other spectacularly heterogeneous matrix. It is accepted that those with a taxonomic bent... 

Analytes in meat are not readily available to the surface of sensors. If the advantages of sensors are to be exploited, something has to be done to expose the analytes. Sample preparation will be the step that determines the performance of the sensor, and the feasibility of using it in an industrial setting. 

Turning to assays using sensors that require more than a simple cut in the sample, Bergann et al. [37] paid considerable attention to sample preparation in attempts to measure lactate in meat with a sensor (not thick film) based on reaction of hydrogen peroxide with a platinum electrode or on the reaction of ferrocene carboxylate with the active site of lactate oxidase. Sample preparation entailed extraction into buffer following grinding. In some cases, ground samples were left to allow the lactate to diffuse into the buffer solution. This was quite effective but slow (up to 90 min). Ultimately, the quality of the assay was dependent on the method of sample preparation. 

The author of this review was asked if electrochemical thick-film sensors could be used in field determinations of the level of organo-phosphate residues in sheep wool. Wool must be the epitome of matrices where sample preparation is absolutely essential before any progress with analysis using electrochemical biosensors is conceivable. Even with meat, one can imagine that analysis by a sensor of the fluid exuded into the space made by an incision might be possible indeed this has been attempted as noted above [36]. The question as to whether wool could be analysed using a biosensor brought the problems of sensors and difficult samples into sharp and inescapable focus. 

As stated by Manz, the pTAS was envisioned as a new concept for chemical sensing, needed since sensors at that time were not providing the best results in terms of selectivity and lifetime. Initially, the main reason for miniaturisation was to enhance the analytical performance of the device rather than to reduce its size. However, it was also recognised that a small scale presented the advantage of a smaller consumption of sample and reagents. Moreover, the total chemical analysis system scheme could provide an integration of several laboratory procedures such as sample preparation, filtration, preconcentration,... 

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