Autosampler sampling accuracy

Sample Data Table for evaluating autosampler sampling accuracy... 

FIGURE 9 Gravimetric data table showing the evaluation of autosampler sampling accuracy. 

As shown in Figure 6.21, excellent linearity was obtained, as represented by the high coefficient of correlation obtained for the least square linear regression. Similar results were obtained for the evaluation of autosampler accuracy when other analytes (propyl paraben and rhodamine 110 chloride) were employed in the determinations. Liu et al.9 conducted similar evaluations for the samples employed in the evaluation of the drug release rate profile of OROS with similar results to those discussed above. 

Online IS introduction allows loading of samples in the biological matrix without preparation. ISs were introduced online in the quantitation of propranolol and diclofenac in plasma (Alnouti et al. 2006). Plasma samples were loaded into the autosampler without pretreatment. Both the plasma sample (10 /iL) and IS (5 //I. from an IS microreservoir) were aspirated into an injection needle sequentially and injected into the sample loop. After the switching of an injection valve, the mixed solution in the sample loop was loaded into a cartridge containing washing solution for online SPE. The accuracy and precision of the online IS method were comparable (85 to 119% and 2 to 12%, respectively) to values obtained offline (86 to 106% and 2 to 16%, respectively). 

In bioanalysis, extracted samples are usually stored in either autosampler vials or wells in a plate (such as 96-well plate) sealed with pierceable caps or covers. During injection, the autosampler needle has to pierce the caps or covers to load samples. The debris may completely or partially block the autosampler needle, which would result in no sample or variably low sample volumes injected. Accordingly, no or randomly low IS responses are observed. As most autosamplers have a built-in needle flushing mechanism, the debris in the needle might be flushed out later partially or completely. Therefore, the injected volume can be back to normal at a later time without an operator s intervention. Apparently, when a needle will be blocked and when the blocked needle will be cleared by flushing, as well as how it will be blocked (completely or partially) are difficult to predict. Hence, there would be no clear pattern for this type of IS variations. However, the affected injections normally have lowered IS responses (Fig. 9). Despite lowered IS responses, the accuracy of quantitation can usually be maintained except for situations where no or very low amount of samples are injected, resulting in responses outside the limit of linear range or unacceptable S/N. 

This chapter provides an overview of modern HPLC equipment, including the operating principles and trends of pumps, injectors, detectors, data systems, and specialized applications systems. System dwell volume and instrumental bandwidth are discussed, with their impacts on shorter and smaller diameter column applications. The most important performance characteristics are flow precision and compositional accuracy for the pump, sampling precision and carryover for the autosampler, and sensitivity for the detector. Manufacturers and selection criteria for HPLC equipment are reviewed. 

Sample preparation is often one of the rate-limiting factors of sample throughput for many methods. Furthermore, it is a critical factor for determining the accuracy and precision of analytical results. The final aim of any sample preparation scheme must be to isolate and purify the compound of interest and present it in a form that is compatible with the analytical instrument. The task facing analytical chemists is to select and use the many diverse systems available, such as robots, sample processors, and autosamplers, to achieve that goal in a timely and cost-effective manner. 

Analyte stability in processed extracts for re-injection is assessed by analysis of stored QC sample extracts (minimum of low, mid and high QC samples, N > 3) and comparison of these results to nominal concentration. The mean accuracy value for a set of stability QC extract samples should be within 15% of the nominal concentration values. The stability of the analyte in reconstituted extracts should be determined by analyzing a set of QC sample extracts that have been stored at room temperature (or under other conditions consistent with storage at the autosampler). Extract stabihty is measured by at least one of the following four procedures ... 

SPE can be semi- or fully-automated to increase sample throughput and to improve both precision and accuracy. The degree of automation ranges from the parallel off-line processing of batches of up to about 10 samples using a vacuum manifold to provide suction, to on-line autosamplers, xyz liquid handlers and robotic workstations. 

---