Sampling techniques drug analysis

SPE is increasingly being adopted as a useful method of sample preparation where extraction into an organic solvent would have originally been employed. The reasons for this are outlined in the Keypoints box. The technique has been employed more extensively in the clean up of biological samples prior to analysis but there are increasingly useful examples of its application to the analysis of drugs in formulations. 

HPLC as a comparative technique has a number of advantages, including the fact that it does not require the analyte to be volatile, it does not require any pre-treatment of the sample prior to analysis, such as derivatization, it can be automated and can be used quantitatively. The HPLC technique used is a reversed-phase system and employs ion-suppression. Cannabinoids are weakly acidic drugs due to the presence of phenolic moieties on the aromatic rings. 

LC/MS/MS to support the clinical development [104-108]. The conventional sample preparation procedures require labor-intensive sohd-phase extraction (SPE) sample pretreatment steps and extensive method development time for the LC/MS drug analysis. One of the popular alternatives to SPE is the resurgence of the on-line SPE or column switching techniques [109-111]. More recently, this strategy has been further developed to determine the drug candidates in human plasma by LC/MS/MS analysis [104,109,112]. 

The third experiment classified samples by discriminant analysis. In one series of blends, the CAF, BUT, and ASA concentrations varied independently between 90 and 110% of labeled claim. In another series, one of the three drugs was excluded from the mixture, while the others were varied between 90 and 110%. The Mahalanobis distance statistic was used for classification of formulations. This technique was used for samples of complete formulations (all three drugs at 100% of label strength), borderline formulations, and samples lacking one active component. 

In the numerous studies on the analysis of theophylline in biological fluids, only a few HPLC systems have been used. However, a variety of sampling techniques have been described, ranging from direct injection of plasma to solvent extraction techniques. These techniques will be discussed briefly later (see below). Whereas series of drugs has been reported not to interfere with the analysis of theophylline18,27,33,35.48,68,75,96,104,105,106,111,1127 122.126.127.128.1S6.160.183.191.193.196> some drugs interfere, e.g. aBtpic1ll1n and methi-... 

Single-enzyme systems, typically obtained from cDNA expression systems, are an effective technique for the study of individual enzyme reactions and their products. The incubation of enzyme, buffer, necessary cofactors, and drug produces extremely clean samples that are amenable to either simple cleanup procedures or direct injection into the LC/MS instrument (Chap. 6). These isolated enzymes can be stored at -80°C for extended periods of time, making them readily available whenever necessary. In addition, they are reasonably easy to work with either by hand or in a robotic environment where high-throughput incubation, sample preparation, and analysis are possible. 

With respect to sample extraction, it should be emphasized that homogenization using a probe is an intrusive technique. Compatibility is required between the sample probe and extraction vessel and the sample solvent, drug, excipients, and related substances. For this verification, it is frequently desirable to also prepare the development samples manually and to compare chromatographic profiles. Furthermore, the probe must be cleaned between extractions. A cleaning procedure should allow for the most concentrated sample solution to be prepared without carryover above a specified limit. This may be measured by running an analysis blank immediately after the sample. 

For methods to measure trace impurity, the sample can be saturated with respect to the active ingredient because the solubility of the impurity is independent of the solubility of the drug. This technique, sometimes called the swish technique, allows for the use of small diluent volume and is an excellent technique for analysis of traces impurities such as degradants or residual solvents. However, recovery experiments are critical to demonstrate that the analyte is not adsorbed or entrapped by the solid drug. 

Plasma samples for PK analysis by a one-compartment bolus IV model are typically collected at 5 to 12 time points after drug administration. The time interval between samples generally starts out small for early samples and increases for later samples. The drug concentration in each sample is then measured by some type of analytical chemistry technique. 

As well as sampling, another important aspect of sample preparation for drug analyses is the homogenisation of a subsequent extraction of the sample. This step is necessary, particularly if we have a powder that is not homogenous. A number of homogenisation techniques are available in drug analysis and these are summarised in Table 11.1. 

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