Sample carry-over

Carry-over in continuous segmented-flow analysers arises essentially from the undesirable mixing of successive samples in three parts of the system, 

In theory, the steepness of the rising and falling portions determines the extent of overlap between transient signals. The steeper the signals are (theoretical situation), the lower is the probability of overlap. A measure of carry-over is given by the so-called degree of Interaction 

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Historical instrumental problems (plugging with larger samples, carry-over effects)... 

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... 

In air segmented continuous flow analysis, air bubbles help to reduce the interaction between adjacent samples (carry-over)... 

Sample Aspiration aitd Dispensing. Precisely metered amounts of the sample have to be aspirated rapidly and without allowing intersample contamination, known as sample carry-over. Generally, pipetting is accomplished by motor-driven syringes connected through a fluid line to a thin aspiration probe (see Fig. I)... 

In most analyzers, sample carry-over is minimized by one or more of the following techniques sensing the liquid level in the sample container, and limiting the probe immersion in the liquid to a few millimeters or rinsing the aspiration probe inside and out after each immersion. 

Digitally controlled autosamplers that incorporate a loop injector are available commercially. These sophisticated devices are extremely precise and can be programmed for continuous and automated operation. In addition, the sample loop is flushed automatically with the mobile phase between samples to prevent sample carry over. The ability to inject multiple aliquots from a single sample vial or to add reagents from designated vials to derivatize the analyte just before injection are additional features of many autosamplers. 

If the peak becomes smaller with every blind injection there is a high probability of sample carry over. Measures as explained above under Sample Carry Over . If the peak appears with more or less constant size it may be an injection peak (system peak). Measures can be difficult or impossible. The mobile phase must not be detector-active, i.e. the UV cutoff wavelength must be taken into consideration. Perhaps the peak disappears when a mobile phase from another batch or another manufacturer is used, especially in low UV and with acetonitrile or trifluoroacetic acid. Injected air can produce a peak. ... 

Fig. 5.6 Influence of the flow-rate on sample carry-over in a continuous segmented configuration. Carry-over decreases from 1 to 3.

Built-in rinse cycles serve to prevent sample carry-over. A typical instrument process sequence is ... 

Headspace techniques are often the method of choice since there is virtually no sample preparation involved. One simply places the food sample in a closed vessel, allows the headspace to equilibrate and then samples the headspace with a gas-tight syringe or an automated sampling system. Problems with sample carry over in the syringe and reproducibility favor automated systems for headspace sampling. The primary limitation of headspace sampling is a lack of sensitivity. One may not isolate sufficient quantities of indicator compoxmds to permit accurate and precise quantification. However, the simplicity, reproducibility and speed of this method make it exceptionally desirable for quality control purposes. 

Baseline Compensation Analysis—A baseline compensation analysis, or baseline blank, is performed exactly like an analysis except no injection is made. A blank analysis must be performed at least once per day. The blank analysis is necessary due to the usual occurrence of chromatographic baseline instability and is subtracted from sample analyses to remove any nonsample slice area fijom the chromatographic data. The blank analysis is typically performed prior to sample analyses, but may be useftil if determined between samples or at Ae end of a sample sequence to provide additional data regarding instrument operation or residual sample carry-over from previous sample analyses. Attention must be given to all factors that influence ba%line stability, such as column bleed, septum bleed, detector temperature control, constancy of carrier gas flow, leaks, instrument drift, etc. Periodic baseline blank analyses should be made, following the analysis sequence protocol, to give an indication of baseline stability. 

Although glass and silicon microtechnologies ensure high-precision structures, these materials are fragile and expensive for disposable devices, and the fabrication methods are complex, costly, and time-consuming, requiring the use of cleanroom facilities for the fabrication process (Faustino et al., 2015 Ziaie, 2004 Patel et al., 2008 Dresselhaus et al., 2010). Additionally, silicon is optically opaque and a semiconductor, which makes it inappropriate for some types of separation and detection mechanisms (with the risk of sample carry-over and crosscontamination) (Duffy et al., 1998). These limitations have led to an increase in research into low-cost alternative materials, such as polymers (Patel et al., 2008 Whitesides et al., 2001). 

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