Autosamplers related

Fully automated analysis is also an option wherein the samples are placed in an autosampler and predefined HPLC-NMR experiments are performed. This is covered in detail elsewhere in this volume, but in summary, the software allows automatic detection of UV peaks in the chromatogram based on predetermined time-windows or peak intensities. The successful detection of each UV peak triggers the system to stop the flow at an appropriate time to isolate the peak in the NMR flow probe. Then data relating to the peak (intensity, retention time, etc.) are transferred to the NMR host computer and used to define the parameters for the automatically acquired NMR spectrum. This automatic NMR operation includes field homogeneity optimisation, setting and optimisation of all NMR acquisition parameters, and the predefinition of the resultant signal-to-noise ratio required in the spectrum. The measurement of 2-dimensional (2D) NMR spectra can also be performed. 

Sometimes, the root-cause might be difficult to identify due to timeline or unavailability of incurred samples (e.g., not enough volume or no authorization from the client). For example, during the initial sample analysis, variable and randomly low IS responses were observed for incurred samples only (Fig. 19). Despite similar pattern as in the case of autosampler malfunctioning, the root cause was unlikely related to autosampler issue because no CS or QC samples were affected in this case. In addition, since all the CS and QC were accepted, there should be no... 

Problems with the sample introduction may arise both in manual injection and during autosampling. Table 7 summarizes the problems related to sample introduction. 

In the case of autosamplers, although they are considered as time saving devices, their function is associated with some common problems. For example, needle depth adjustment is very critical when there is not enough sample available a needle blockage may occur from a septum. Another common problem related to the autosampler is carryover that causes the appearance of a peak in a blank injection following injection of a high concentration of sample. Sample stabihty is a figure of merit that certainly has to be evaluated to avoid imprecision and lack of sensitivity. 

In view of the excellent performance of a sampling loop as the volume-selecting element, loop-based introduction dominated during the fast development of flow injection analysis in the 1980s. Time-based introduction became popular again after the inception of sequential injection analysis and related techniques and the development of multicommutation. Nowadays, both time-based and loop-based introduction are exploited and use of an autosampler is generally advisable. 

The autosampler functions as a separate IEEE-488 device, distinct from the source system. It also Incorporates its own internal microcomputer and associated memory. It allows random access of any sample In the tray at any time during the analysis. Using the autosampler for sample introduction allows the analyst to perform other analysis-related tasks while an analysis is taking piace. Including the system controller, both monochromators, the RF system and the autosampler, the instrument assembly contains five separate microprocessors. 

Because of the different and low stability of the isoindoles obtained from the reaction of AAs with OPA/MCE reagent, alternative precolumn derivat-izations reagents, such as 3-mercaptopropionic acid (MPA) and several N-alkyl-L/o-cysteines, were proposed. The OPA/MPA and OPA/N-acetyl-L-cysteine (NAC) reagents provide more stable isoindoles compared to those formed with the OPA/MCE, and the optical resolution of enantiomeric AAs with the OPA/NAC, as well as with further N-alkyl-L/o-cysteine reagents, have opened a new area in separation of AA enantiomers. Due to robotic autosamplers, which provide excellent reproducibility even for moderately quantitative interactions, most AA analyses are performed with the OPA derivative. The unexplainable contradictions of this most popular process - relating to the particularly low stability of the OPA derivatives of six very important AAs (glycine, y-aminobutyric acid (GABA), jS-alanine,... 

A separate discussion relates to the automated off-line precolumn derivatization of the analytes, achievable by means of an autosampler. Derivatization brings a selective character to the injection process. Sample, reagents, and buffers can be transferred from vials to the loop or to an empty vial. Operations such as mixing of the reaction media, repetitive dilutions, and even sample concentration are possible. 

Therefore, verify that the fiber is stiU attached to the rod and that the coating has not stripped oft If there is no obvious problem with the fiber or the septum, then a liquid injection with a standard solution containing the analytes under investigation should be made to determine if the problem is related to the GC/MS. Fiber breakage and/or a bent rod may occur when the injector insert is plugged with septum particles. If the analysis is performed automatically, then the problem can be related to the alignment of the autosampler. 

Sample volume This is directly related to the sensitivity. As the sample volume increases also the extracted amoimt increases to a certain degree. Generally, typical 2 mL GC autosampler vials are used for direct SPME. 

Different kinds of autosamplers exist. Autosamplers can be classified in relation to sample capacity (auto-injectors vs. autosamplers, where auto-injectors can work a small number of samples), to robotic technologies (XYZ robot vs. rotating robot - the most common), or to analysis ... 

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