The importance of sample preparation stems from three major concerns—removing interferences from the biological sample matrices, concentrating the analyte(s) of interest, and improving analytical system performance (8). An industry survey noted a marked increase in methods requiring limits of quantitation of less than 1 ppb, and the trend toward trace analyses is not diminishing (9). Optimized sample preparation techniques that provide high enrichment factors become crucial for these dilute concentrations.
The choice of sample preparation method should depend on the quality of the data required. It makes little sense to invest weeks of development time attempting to achieve pg/mL sensitivity for a screening assay. However, it may be important to invest such time to develop and validate a method for a lead drug candidate undergoing human safety assessments that are subject to FDA regulatory scrutiny. Indeed, validating analytical procedures is the process of determining a suitable method that is capable of providing useful analytical data. It is important to bear in mind that a method that is valid in one situation could be invalid in another (10, 11).In the pharmaceutical industry, the most common biological sample matrix is plasma. Moreover, it is common practice to dilute troublesome matrices like urine or cerebrospinal fluid with plasma and apply previously developed plasma extraction protocols. Drugs are most commonly isolated from plasma using one (or occasionally, a combination) of either liquid–liquid extraction, protein precipitation, or solid phase extraction. Other less common choices include column-switching (LC–LC), affinity extraction, and ultrafiltration.