Dilute and shoot approaches

Liquid samples, other than those that are inherently liquid, can arise from the solid sample extraction techniques described above. As mentioned previously, sometimes a simple dilute-and-shoot approach can be utilized, i.e., add solvent to the sample and then inject directly into the instrument. Other times, evaporation of residual liquid can be utilized—then the sample is either directly injected, or if the sample is evaporated to dryness, a new solvent can be added. Often, however, the residual matrix causes interference and the following techniques can be employed for further sample cleanup. 

Sample preparation refers to a family of solid/liquid handling techniques to extract or to enrich analytes from sample matrices into the final analyte solution. While SP techniques are well documented, few references address the specific requirements for drug product preparations, which tend to use the simple dilute and shoot approach. More elaborate SP is often needed for complex sample matrices (e.g., lotions and creams). Many newer SP technologies such as solid-phase extraction... 

This chapter provides the novice and the experienced analyst with an overview of sample preparation techniques focusing on solid dosage forms. It describes the best practices in the dilute and shoot approach, and the tricks of the trade in grinding, mixing, sonication, dilution and filtration of drug products. Selected case studies of sample preparations for assays and impurity testing are used to illustrate the strategies, trade-offs... 

In the early years of LC-MS/MS application in clinical laboratories, chromatographic separation was looked upon as rather unnecessary with tandem mass spectrometers being understood as extremely selective measuring devices. Thus, many LC-MS/MS methods with minimal degree of chromatographic resolution and analyte retention times close to the void time of the chromatographic systems ( dilute and shoot approaches) have been described. However, from the issues discussed so far, the requirements of proper sample preparation and sufficient chromatographic separation prior to MS/MS detection have become evident. 

FIGURE 21.3 Recovery of stable-labeled standards spiked in urine. Color indicates recovery, according to the scale on the right. Despite the better recovery of many metabolites with some solid-phase extraction (SPE) cartridges, expediency often dictates a dilute-and-shoot approach (see text). OASIS SPE cartridges were obtained from Waters Corp. and used per manufacturer instructions. 

In HPLC-MS coupling, careful sample preparation is recommended. The dilute-and-shoot approach is not possible, especially when handling real-world samples such as environmental and biological samples. In these cases sample preparation techniques (see also Chaps. 12.8 and 12.9) such as liquid-liquid extraction, solid-phase extraction [119], or even affinity techniques (see also Chap. 12.11) directly coupled with LC-MS [120] can be useful. 

Direct injection of plasma or supernatant after protein precipitation on a short column with a high liquid flow rate is a common method for reducing analysis time in the pharmaceutical industry. The direct injection of a sample matrix is also known as the dilute-and-shoot (DAS) approach.62 DAS can be applied to all types of matrices and approaches and is the simplest sample preparation method with matrix dependency. Direct injection can also be approached through the extraction of eluent from PPT, SPE, and LLE onto a normal phase analytical column. The procedure is called hydrophilic interaction liquid chromatography (HILIC)70110111 and it avoids the evaporation and reconstitution steps that may cause loss of samples from heat degradation and absorption. 

Major Matrix Components Showing Potential for Dilute-and-Shoot (DAS) Approach... 

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