Comparison of different linear calibration approaches for LC–MS bioanalysis

Straightforward quantification of endogenous steroids with liquid chromatography-tandem mass spectrometry: Comparing calibration approaches

Different calibration strategies are used in liquid chromatography hyphenated to mass spectrometry (LC-MS) bioanalysis. Currently, the surrogate matrix and surrogate analyte represent the most widely used approaches to compensate for the lack of analyte-free matrices in endogenous compounds quantification. In this context, there is a growing interest in rationalizing and simplifying quantitative analysis using a one-point concentration level of stable isotope-labeled (SIL) standards as surrogate calibrants. Accordingly, an internal calibration (IC) can be applied when the instrument response is translated into analyte concentration via the analyte-to-SIL ratio performed directly in the study sample. Since SILs are generally used as internal standards to normalize variability between authentic study sample matrix and surrogate matrix used for the calibration, IC can be calculated even if the calibration protocol was achieved for an external calibration (EC). In this study, a complete dataset of a published and fully validated method to quantify an extended steroid profile in serum was recomputed by adapting the role of SIL internal standards as surrogate calibrants. Using the validation samples, the quantitative performances for IC were comparable with the original method, showing acceptable trueness (79%-115%) and precision (0.8%-11.8%) for the 21 detected steroids. The IC methodology was then applied to human serum samples (n = 51) from healthy women and women diagnosed with mild hyperandrogenism, showing high agreement (R² > 0.98) with the concentrations obtained using the conventional quantification based on EC. For IC, Passing-Bablok regression showed proportional biases between -15.0% and 11.3% for all quantified steroids, with an average difference of -5.8% compared to EC. These results highlight the reliability and the advantages of implementing IC in clinical laboratories routine to simplify quantification in LC-MS bioanalysis, especially when a large panel of analytes is monitored.

From fundamentals in calibration to modern methodologies: A tutorial for small molecules quantification in liquid chromatography-mass spectrometry bioanalysis

Over the last two decades, liquid chromatography coupled to mass-spectrometry (LC‒MS) has become the gold standard to perform qualitative and quantitative analyses of small molecules. When quantitative analysis is developed, an analyst usually refers to international guidelines for analytical method validation. In this context, the design of calibration curves plays a key role in providing accurate results. During recent years and along with instrumental advances, strategies to build calibration curves have dramatically evolved, introducing innovative approaches to improve quantitative precision and throughput. For example, when a labeled standard is available to be spiked directly into the study sample, the concentration of the unlabeled analog can be easily determined using the isotopic pattern deconvolution or the internal calibration approach, eliminating the need for multipoint calibration curves. This tutorial aims to synthetize the advances in LC‒MS quantitative analysis for small molecules in complex matrices, going from fundamental aspects in calibration to modern methodologies and applications. Different work schemes for calibration depending on the sample characteristics (analyte and matrix nature) are distinguished and discussed. Finally, this tutorial outlines the importance of having international guidelines for analytical method validation that agree with the advances in calibration strategies and analytical instrumentation.

Isotopic Distribution Calibration for Mass Spectrometry

Mass spectrometry (MS) is widely used in science and industry. It allows accurate, specific, sensitive, and reproducible detection and quantification of a huge range of analytes. Across MS applications, quantification by MS has grown most dramatically, with >50 million experiments/year in the USA alone. However, quantification performance varies between instruments, compounds, different samples, and within- and across runs, necessitating normalization with analyte-similar internal standards (IS) and use of IS-corrected multipoint external calibration curves for each analyte, a complicated and resource-intensive approach, which is particularly ill-suited for multi-analyte measurements. We have developed an internal calibration method that utilizes the natural isotope distribution of an IS for a given analyte to provide internal multipoint calibration. Multiple isotope distribution calibrators for different targets in the same sample facilitate multiplex quantification, while the emerging random-access automated MS platforms should also greatly benefit from this approach. Finally, isotope distribution calibration allows mathematical correction for suboptimal experimental conditions. This might also enable quantification of hitherto difficult, or impossible to quantify, targets, if the distribution is adjusted in silico to mimic the analyte. The approach works well for high resolution, accurate mass MS for analytes with at least a modest-sized isotopic envelope. As shown herein, the approach can also be applied to lower molecular weight analytes, but the reduction in calibration points does reduce quantification performance.

Some unnecessary or inadequate common practices in regulated LC–MS bioanalysis

The global bioanalytical community increasingly craves scientifically sound practices and guidance where the rationale is given for each requirement. To this end, it is critical to first evaluate all the existing practices and requirements based on scientific findings and critical thinking. Here we are challenging several important common practices in regulated LC–MS bioanalysis, from the requirement of at least six different calibration concentrations, no extrapolation, use of blank and zero standard in each batch, selection of quality controls, to the way matrix effect and dilution integrity are being validated. Both the reasons why these common practices are unnecessary or inadequate and the potential solutions are presented.

Development and validation of a UHPLC-MS/MS procedure for quantification of the Pseudomonas Quinolone Signal in bacterial culture after acetylation for characterization of new quorum sensing inhibitors

The appearance of antibiotic resistance requires novel therapeutic strategies. One approach is to selectively attenuate bacterial pathogenicity by interfering with bacterial cell-to-cell communication known as quorum sensing. The PQS quorum sensing system of Pseudomonas aeruginosa employs as signal molecule the Pseudomonas Quinolone Signal (PQS; 2-heptyl-3-hydroxy-4-(1H)-quinolone), a key contributor to virulence and biofilm formation. Thus, interference with PQS production is considered as promising approach for the development of novel anti-infectives. Therefore, in this study, we developed and validated an ultra-high performance liquid chromatographic-tandem mass spectrometric approach for reliable quantification of PQS in P. aeruginosa cultures for activity determination of new quorum sensing inhibitors. The poor chromatographic properties of PQS reported by others could be overcome by fast microwave-assisted acetylation. The validation procedure including matrix effects, recovery, process efficiency, selectivity, carry-over, accuracy and precision, stability of the processed sample, and limit of quantification demonstrated that the method fulfilled all requirements of common validation guidelines. Its applicability was successfully proven in routine testing. In addition, two-point calibration was shown to be applicable for fast and reliable PQS quantification saving time and resources. In summary, the described method provides a powerful tool for the discovery of new quorum sensing inhibitors as potential anti-infectives and illustrated the usefulness of chemical derivatization, acetylation, in liquid chromatography-mass spectrometry analysis.

Sample reduction strategies in discovery bioanalysis

It is a constant challenge to provide timely bioanalytical support for the evaluation of drug-like properties and PK/PD profiles for the ever-increasing numbers of new chemical entities in a cost-effective manner. While technological advancement in various aspects of LC–MS/MS analysis has significantly improved bioanalytical efficiency, a number of simple sample reduction strategies can be employed to reduce the number of samples requiring analysis, and as a result increase the bioanalytical productivity without deploying additional instruments. In this review, advantages and precautions of common sample reduction strategies, such as sample pooling and cassette dosing, are discussed. In addition, other approaches such as reducing calibration standards and eliminating over-the-curve sample reanalysis will also be discussed. Taken together, these approaches can significantly increase the capacity and throughput of discovery bioanalysis without adding instruments, and are viable means to enhance the overall productivity of the bioanalytical laboratory.