Quantitative analysis of underivatized amino acids in the sub- to several-nanomolar range by ion-pair HPLC using a corona-charged aerosol detector (HPLC–CAD)

Application of Normal-Phase Silica Column in Hydrophilic Interaction Liquid Chromatography Mode for Simultaneous Determination of Underivatized Amino Acids from Human Serum Samples via Liquid Chromatography-Tandem Mass Spectrometry

In neonatal screening, amino acids have a significant diagnostic role. Determination of their values may identify abnormal conditions. Early diagnosis and continuous monitoring of amino acid disorders results in a better disease outcome. An easy and simple LC-MS/MS method was developed for the quantitation of underivatized amino acids. Amino acids were separated using a normal-phase HPLC column having a totally porous silica stationary phase and using classical reversed-phase eluents. Mass spectrometry in multiple reaction monitoring mode was used for the analysis, providing high selectivity and sensitivity. A standard addition calibration model was applied for quantitation using only one isotope-labeled internal standard for all amino acids. Five calibration points were used for quantitation, and the method was successfully validated. The slopes of the calibration curves of the individual amino acids in parallel measurements were found to be similar. Since the measured slopes were reproducible, one serum sample could represent every series of serum samples of a given day. The method was tested on human serum samples and adequate results were obtained. This new method can be easily applied in clinical laboratories.

Universal response method for the quantitative analysis of photodegradation impurities in lomefloxacin hydrochloride ear drops by liquid chromatography coupled with charged aerosol detector

In terms of risk assessment especially for the impurities with different ultraviolet responses, quantitative analysis without the availability of corresponding reference substances currently poses a challenge. In this study, a universal response method was established for the quantitative analysis of photodegradable impurities in lomefloxacin hydrochloride ear drops by high performance liquid chromatography-charged aerosol detector (HPLC-CAD) for the first time. The chromatographic conditions and CAD parameters were optimized for a good separation and sensitivity. The uniform response of developed method was validated by impurity reference substances with different ultraviolet responses. In the gradient compensation HPLC-CAD method validation, good linearities were obtained with coefficient of determination (R2) all greater than 0.999 for lomefloxacin and impurity reference substances. The average recoveries of the impurities were 98.63%- 102.18% by UV and 97.92%- 102.57% by CAD, respectively. RSDs all were less than 2.5% for intra-day and inter-day precision by UV and CAD, with good precision and accuracy. The correction factor experimental results showed that the developed method provided a uniform response to the impurities with differences chromophores in lomefloxacin. The effects of packaging materials and excipients on the photodegradation were also investigated using the developed method. The results of correlation analysis showed that the packaging materials with low light transmittance and the organic excipients (glycerol and ethanol) could significantly improve the stability of lomefloxacin hydrochloride ear drops. The developed HPLC-CAD quantification method was a reliable and universal response method for quantitative analysis of impurities in the lomefloxacin. This study also revealed the key factors affecting the photodegradation of lomefloxacin hydrochloride ear drops, which guided enterprises to improve drug prescription and packaging materials and ensure the public medication safety.

Application of a porous graphitic carbon column to carbon and nitrogen isotope analysis of underivatized individual amino acids using high-performance liquid chromatography coupled with elemental analyzer/isotope ratio mass spectrometry

RATIONALE

Isolation of underivatized amino acids (AAs) using high-performance liquid chromatography (HPLC) is becoming a popular method for carbon (δ13 C) and nitrogen isotope (δ15 N) analyses of AAs because of the high analytical precision and for performing dual-isotope analysis. However, some AAs in natural samples, especially small, hydrophilic AAs, are not suitably separated using reversed-phase columns (e.g., C18) and ion-exchange columns (e.g., Primesep A).

METHODS

We developed a new method for HPLC using a porous graphitic carbon column for the separation of nine hydrophilic AAs. After purification, δ13 C and δ15 N values of AAs were determined using elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). We demonstrated the application of this method by determining δ13 C and δ15 N values of individual hydrophilic AAs in a biological sample, the muscle of blue mackerel (Scomber australasicus).

RESULTS

Chromatographically, the baseline separation of hydrophilic AAs was achieved in both the standard mixture and the biological sample. We confirmed that δ13 C and δ15 N values of AA standards remained unchanged during the whole experimental procedure. The δ13 C values of AAs in mackerel muscle are also in good agreement with the values obtained using another verified method for δ13 C analysis.

CONCLUSIONS

The good separation performance of hydrophilic AAs and the reliability of δ13 C and δ15 N analyses of individual AAs using the porous graphite column offer a significant advantage over conventional settings. We suggest that, in the future, the HPLC × EA/IRMS method can be used for reliable δ13 C and δ15 N analyses of AAs in natural samples.

Single-Standard Quantification Strategy for Lignin Dimers by Supercritical Fluid Chromatography with Charged Aerosol Detection

The increased interest in utilizing lignin as a feedstock to produce various aromatic compounds requires advanced chemical analysis methods to provide qualitative and quantitative characterization of lignin samples along different technology streamlines. However, due to the lack of commercially available chemical standards, routine quantification of industrially relevant lignin oligomers in complex lignin samples remains a challenge. This study presents a novel method for universal quantification of lignin dimers based on supercritical fluid chromatography with charged aerosol detection (CAD). A series of lignin-derived dimeric compounds that have been reported from reductive catalytic fractionation (RCF) were synthesized and used as standards. The applicability of using linear regression instead of quadratic calibration curves was evaluated over a concentration range of 15-125 mg/L, demonstrating that the former calibration method is as appropriate as the latter. The response factors of lignin dimeric compounds were compared to assess the uniformity of the CAD signal, revealing that the CAD response for the tested lignin dimers did not differ substantially. It was also found that the response factors were not dependent on the number of methoxy groups or linkage motifs, ultimately enabling the use of only one calibrant for these compounds. The importance of chromatographic peak resolution in CAD was stressed, and the use of a digital peak sharpening technique was adopted and applied to address this challenge. The developed method was verified and used for the quantification of lignin dimers in an oil obtained by a RCF of birch sawdust.

Performance of ion pairing chromatography and hydrophilic interaction liquid chromatography coupled to charged aerosol detection for the analysis of underivatized amino acids

The charged aerosol detector (CAD) is frequently employed in liquid chromatography for the analysis of small polar and ionizable compounds such as amino acids and amino sugars, which provide a weak chromophore only. Separation of these compounds is achieved by means of ion pair chromatography (IPC), and, more recently, hydrophilic interaction chromatography (HILIC) techniques. However, as the CAD's response is highly dependent on the mobile phase composition, the substantial differences in the mobile phase composition of IPC and HILIC have a distinct impact on the detector's performance. This study was aimed at systematically comparing the performance of IPC and HILIC when coupled to the CAD. Therefore, the separation techniques characterized by their specific mobile phase compositions were evaluated for their influence on the CAD response and the signal-to-noise ratio (S/N) of the amino acids L-alanine, L-leucine, and L-phenylalanine applying the response surface methodology (RSM). The RSM results derived from flow injection analysis (FIA) indicated that the CAD response and thus the obtainable S/N are significantly higher in HILIC compared to IPC where the S/N decreased with the chain length of the applied ion-pairing reagent. In addition, an IPC and a HILIC method, respectively, were developed for the impurity profiling of the branched-chain amino acids (BCAAs) L-leucine, L-isoleucine, and L-valine. The beneficial effects of the HILIC conditions on the S/N observed under FIA conditions were partly offset by moderate column bleed effects when using an amide functionalized column, which facilitates the separation in the HILIC method. Satisfactory LOQs (3-10 ng on column) were obtained with both methods; however, the HILIC method was found to be slightly superior in terms of sensitivity and separation efficiency.

Protein Quality in Infant Formulas Marketed in Brazil: Assessments on Biodigestibility, Essential Amino Acid Content and Proteins of Biological Importance

Infant formulas, designed to provide similar nutritional composition and performance to human milk, are recommended when breastfeeding is not enough to provide for the nutritional needs of children under 12 months of age. In this context, the present study aimed to assess the protein quality and essential amino acid content of both starting (phase 1) and follow-up (phase 2) formulas from different manufacturers. The chemical amino acid score and protein digestibility corrected by the amino acid score were calculated. The determined protein contents in most formulas were above the maximum limit recommended by FAO and WHO guidelines and at odds with the protein contents declared in the label. All infant formulas contained lactoferrin (0.06 to 0.44 g·100 g⁻¹) and α-lactalbumin (0.02 to 1.34 g·100 g⁻¹) below recommended concentrations, whereas ĸ-casein (8.28 to 12.91 g·100 g⁻¹), α-casein (0.70 to 2.28 g·100 g⁻¹) and β-lactoglobulin (1.32 to 4.19 g·100 g⁻¹) were detected above recommended concentrations. Essential amino acid quantification indicated that threonine, leucine and phenylalanine were the most abundant amino acids found in the investigated infant formulas. In conclusion, infant formulas are still unconforming to nutritional breast milk quality and must be improved in order to follow current global health authority guidelines.

Determination of Free Amino Acids in Banlangen Granule and its Fractions by Solid Phase Extraction Combined with Ion-pair Hig-h Perfor mance Liquid Chromatography using a Corona-charged Aerosol Detector (SPE-HPLC-CAD)

Background:

Banlangen granules are broad-spectrum effective antiviral drugs, and have a large clinical demand in China. Free amino acid is one of the main antiviral active ingredients of Banlangen granules. The pre-processing of samples by the existing pre-column derivatization reversed- HPLC method is complicated. Therefore, the determination of free amino acids (AAs) by underivatized ion-pair HPLC-CAD is advantageous for simplifying the preparation process and improving sensitivity.

Objective:

To better optimize AAs analysis methods, here a sensitive SPE-HPLC-CAD method with a better resolution was established for the determination of underivatized AAs in Banlangen Granule for the first time.

Method:

The analytes were separated only by HPLC using a Hypercarb column with gradient elution of solvent A (20 mM nonafluorovaleric Acid in water) and solvent B (0.3% trifluoroacetic acid in acetonitrile-0.3% trifluoroacetic acid in water (1:9, v/v)) at a flow rate of 0.15 mL/min. N2 gas pressure and evaporation temperature of CAD were held at a constant 58.6 psi and 60 ℃, respectively.

Results:

This method was linear over the respective concentration range of six amino acids. The precision, accuracy, stability and recovery were satisfactory in all samples examined. And the method was successfully applied to determination of free amino acids in Banlangen granules and its fractions. The total contents of six amino acids in 28 batches of Banlangen Granule were between 1.36 mg/g-11.62 mg/g.

Conclusion:

The proposed method could be a simple, accurate and sensitive alternative approach for the determination of free AAs in Banlangen Granule.

Quantitative analysis of impurities in leucomycin bulk drugs and tablets: A high performance liquid chromatography-charged aerosol detection method and its conversion to ultraviolet detection method

Toxic impurities were found in leucomycin and its preparation, however the content determination of impurities was challengeable due to the lacking of their reference standards. In this study, we developed high-performance liquid chromatography method coupled with charged aerosol detection (CAD) for the quantification of related substance of leucomycin (kitasamycin) bulk drugs and tablets, however, the CAD was not yet popular. In order to carry out quantitation work conveniently in the laboratory without CAD instruments, a high-performance liquid chromatography method coupled with ultraviolet (UV) detection was developed with the assistant of the HPLC-CAD results. The relative response of impurities on CAD chromatogram was used for guiding the establishment of HPLC-UV method, which could achieve the quantitation task in the absence of impurity reference standards. The developed HPLC-UV method was validated according to the ICH guideline and showed good precision, reproducibility and linearity with determination coefficient higher than 0.9999. The limit of detection and quantitation were 0.3 and 0.5 μg mL^-1, respectively. The recoveries were 92.9 %-101.5 % at the spiked concentration levels of 0.1 %, 0.8 %, 1.0 and 1.2 % with relative standard deviations (RSDs, n = 3) lower than 2.0 %. Finally, the developed HPLC-CAD and -UV methods were compared by the determination of impurities in several batches of leucomycin bulk drugs and tablets. The results demonstrated that the developed HPLC-UV method was simple and reliable. This study developed methods to quantify the related substance in leucomycin and tablets, and discussed a strategy of the conversion of HPLC-CAD method to HPLC-UV method. The developed methods could be considered for implementation into pharmacopeial monographs in the future.

Influence of the mobile phase composition on hyphenated ultraviolet and charged aerosol detection for the impurity profiling of vigabatrin

Recently, charged aerosol detection (CAD), a universal detection technique in liquid chromatography, has been introduced into monographs of the European Pharmacopoeia (Ph. Eur.), which now employs HPLC-UV-CAD for assessing the impurities of the drug vigabatrin. The separation of vigabatrin and its impurities is facilitated by ion pair chromatography (IPC) in the compendial method using tridecafluoroheptanoic acid (TDFHA) as ion-pairing reagent. However, the subsequent detection of the impurities by UV-CAD is considerably impaired due to the substantial amount of ion-pairing reagent applied in the method generating high levels of background noise. In this study, the influence of the mobile phase composition on the background noise of the CAD was evaluated applying response surface methodology. The model's results indicated that the chain length of the ion-pairing reagent is a predominant factor for noise generation. Thus, an alternative method for the impurity analysis of vigabatrin using mixed-mode chromatography (MMC) instead of IPC was developed. The dual separation mechanism of the MMC column enabled the choice of a mobile phase better suited for the individual requirements of the UV-CAD detectors, while maintaining excellent selectivity. The MMC method does not require the addition of a post-column solution to reduce the TDFHA concentration in the mobile phase, and, therefore, needs less instrumentation. Moreover, the sample concentration could be halved due to the improved LOQs of the impurities (<50 ng on column) and the analysis time could be shortened (30 to 20 min) due to improved separation efficiency. The MMC method was validated with respect to ICH guideline Q2(R1).

Impurity profiling of Compound Amino Acid Injection (6AA) using ion-pair high performance liquid chromatography coupled with corona-charged aerosol detection and high resolution mass spectrometry

The complex industrial production process of amino acids (AAs) leads to the existence of a certain amount of impurities in Compound Amino Acid Injection (6AA). It is difficult to obtain its comprehensive and systematic impurity profile using conventional ultraviolet (UV) detectors due to lack of a suitable chromophore in the structures of AAs and their impurities. In our study, a universal ion-pair high performance liquid chromatography (HPLC) method combined with high resolution mass spectrometer (HRMS) and charged aerosol detection (CAD) was developed to identify and determine the content of impurities in Compound Amino Acid Injection (6AA), respectively. After optimizing the content of trifluoroacetic acid (TFA) and heptafluorobutyric acid (HFBA) in the mobile phase on a C18 AQ column, HPLC-CAD method was developed and nine unknown impurities were detected. These impurities were successfully identified using HPLC coupled with orbitrap mass spectrometry and confirmed with their reference substances. The CAD parameters setting was optimized to improve the sensitivity and linearity of the methods before the developed method was validated. The results of validation reflected that the limit of detection (LOD) was approximately 2 ng (corresponding to approximately 0.02 % of L-isoleucine in injection). Under the optimized power function value (PFV) of CAD, the linear range of each impurity was 1 ∼ 200 μg mL^-1 (the linear range of one of the impurities with higher content was 2 ∼ 400 μg mL^-1) with coefficients of determination (R²) greater than 0.998. The recovery rates for nine impurities were 93.37 % ∼ 110.23 %. This study made full use of the qualitative functions of HRMS and the versatility of CAD, revealing possible impurities in the 6AA injection, which could provide reference for the safety research of it.

Simultaneous determination of six main components in Bushen Huoxue prescription by HPLC-CAD

BACKGROUND

The Bushen Huoxue prescription is a traditional Chinese medicine formula treating diabetic retinopathy, which was developed by our research group. Catalpol, puerarin, salvianolic acid B, ginsenoside Rg1, ginsenoside Rb1 and ginsenoside Rd are six of main effective components, which could be partly representative of this prescription. The corona charged aerosol detector (corona CAD) is one kind of universal detectors equipped with the high performance liquid chromatography (HPLC). The CAD has many advantages for the analysis of complex mixtures, but too few applications in traditional Chinese medicine compounds.

OBJECTIVE

The aims of this study are to establish a method for the determination of six components in Bushen Huoxue prescription, and to increase the use of the CAD in traditional Chinese medicine compounds.

METHODS

HPLC-CAD analysis was performed on an Inertsil ODS-SP (4.6 mm × 250 mm, 5 μm) with a mobile phase consisting of 0.5 % formic acid solution(A)-acetonitrile(B) at a flow rate of 1 mL/min (0-7 min, 1 % B; 7-12 min, 1 %-12 % B; 12-22 min, 12 %-19 % B; 22-40 min, 19 %-28 % B; 40-43 min, 28 %-33 % B; 43-50 min, 33 % B; 50-65 min, 33 %-42 % B). The column temperature maintained at 30 ℃, the injection volume was 20 μL, the atomization temperature mode was LOW, the filtration constant (filter) was 3.6 and data collection rate was 10 Hz. The methodology was examined and the linearity of regression of different functions was compared. Sixteen batches of samples were prepared and their contents were determined.

RESULTS

The six compounds showed a better linearity (R² > 0.9990) in their concentration ranges when using the linear function. The average recoveries were 99.18 %-101.30 %. Although the RSD value of puerarin and ginsenoside Rg1 was slightly out of 3 % during the average recovery investigation, all the other methodological investigations of the six components were within 3 %. The precision, stability and repeatability of the method were good. In sixteen batches of Bushen Huoxue prescription samples, the contents of six components were 0.3138 %-0.6042 % for catalpol, 0.8095 %-1.2917 % for puerarin, 0.7416 %-1.1189 % for salvianolic acid B, 0.0231 %-0.0418 % for ginsenoside Rg1, 0.0702 %-0.1724 % for ginsenoside Rb1, 0.0384 %-0.1196 % for ginsenoside Rd.

CONCLUSION

In this experiment, a method for the determination of six components in Bushen Huoxue prescription based on HPLC-CAD was established with high accuracy, good repeatability and simple operation, and it can provide references for the improvement of quality standard of the Bushen Huoxue prescription. It is reasonable and accessible for the CAD application in the determination of traditional Chinese medicine compound prescriptions.

A method for stable carbon isotope measurement of underivatized individual amino acids by multi-dimensional high-performance liquid chromatography and elemental analyzer/isotope ratio mass spectrometry

RATIONALE

To achieve better precision and accuracy for δ¹³ C analysis of individual amino acids (AAs), we have developed a new analytical method based on multi-dimensional high-performance liquid chromatography (HPLC) and elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). Unlike conventional methods using gas chromatography, this approach omits pre-column chemical derivatization, thus reducing systematic errors associated with the isotopic measurement.

METHODS

The separation and isolation of individual AAs in a standard mixture containing 15 AAs and a biological sample, spear squid (Heterololigo bleekeri) were performed. AAs were isolated using an HPLC system equipped with a reversed-phase column and a mixed-mode column and collected using a fraction collector. After the chromatographic separation and further post-HPLC purification, the δ¹³ C values of AAs were measured by EA/IRMS.

RESULTS

The complete isolation of all 15 AAs in the standard mixture was achieved. The δ¹³ C values of these AAs before and after the experiment were in good agreement. Also, 15 AAs in the biological sample, H. bleekeri, were successfully measured. The δ¹³ C values of AAs in H. bleekeri varied by as much as 30‰ with glycine being most enriched in¹³ C.

CONCLUSIONS

The consistency between the δ¹³ C values of reference and processed AAs demonstrates that the experimental procedure generates accurate δ¹³ C values unaffected by fractionation effects and contamination. This method is therefore suitable for δ¹³ C analysis of biological samples with higher precision than conventional approaches. We propose this new method as a tool to measure δ¹³ C values of AAs in biological, ecological and biogeochemical studies.

Increased insect herbivore performance under elevated CO2 is associated with lower plant defence signalling and minimal declines in nutritional quality

Changes in insect herbivore performance under elevated atmosphere carbon dioxide concentrations e[CO₂] are often driven by changes in the nutritional and defensive chemistry of their host plants. Studies addressing how the prolific pest cotton bollworm (Helicoverpa armigera) responds to e[CO₂] show that performance usually declines, often associated with lower nutritional (e.g. nitrogen (N) concentrations) quality of host plants under e[CO₂]. We investigated the impacts of e[CO₂] on nutritional quality and anti-herbivore (jasmonate) defensive signalling in lucerne (Medicago sativa) when challenged by H. armigera. While foliar N decreased under e[CO₂], other aspects of nutritional quality (soluble protein, amino acids, foliar C:N) were largely unaffected, potentially due to increased root nodulation under e[CO₂]. In contrast, e[CO₂] greatly reduced jasmonate signalling in M. sativa following H. armigera attack; jasmonic acid concentrations were ca. 56% lower in attacked plants grown under e[CO₂]. Concurrent with this, relative growth rates of H. armigera were ca. 66% higher when feeding on e[CO₂]-grown plants. In contrast with previous reports, which we meta-analytically summarise, we provide the first evidence that H. armigera performance can increase under e[CO₂]. This may occur in plants, such as M. sativa, where e[CO₂] has limited impacts on nutritional quality yet reduces jasmonate defence signalling.

Analytical methods for amino acid determination in organisms

Amino acids are important metabolites for tissue metabolism, growth, maintenance, and repair, which are basic life necessities. Therefore, summarizing analytical methods for amino acid determination in organisms is important. In the past decades, analytical methods for amino acids have developed rapidly but have not been fully explored. Thus, this article provides reference to analytical methods for amino acids in organisms for food and human research. Present amino acid analysis methods include thin-layer chromatography, high-performance liquid chromatography, liquid chromatography-mass spectrometer, gas chromatography-mass spectrometry, capillary electrophoresis, nuclear magnetic resonance, and amino acid analyzer analysis.

Analytical strategies for the determination of amino acids: Past, present and future trends

This review describes the analytical methods that have been developed over the years to tackle the high polarity and non-chromophoric nature of amino acids (AAs). First, the historical methods are briefly presented, with a strong focus on the use of derivatization reagents to make AAs detectable with spectroscopic techniques (ultraviolet and fluorescence) and/or sufficiently retained in reversed phase liquid chromatography. Then, an overview of the current analytical strategies for achiral separation of AAs is provided, in which mass spectrometry (MS) becomes the most widely used detection mode in combination with innovative liquid chromatography or capillary electrophoresis conditions to detect AAs at very low concentration in complex matrixes. Finally, some future trends of AA analysis are provided in the last section of the review, including the use of supercritical fluid chromatography (SFC), multidimensional liquid chromatography and electrophoretic separations, hyphenation of ion exchange chromatography to mass spectrometry, and use of ion mobility spectrometry mass spectrometry (IM-MS). Various application examples will also be presented throughout the review to highlight the benefits and limitations of these different analytical approaches for AAs determination.

Revealing the inner workings of the power function algorithm in Charged Aerosol Detection: A simple and effective approach to optimizing power function value for quantitative analysis

In recent years, charged aerosol detection (CAD) has become a valuable tool for fast and efficient quantitative chromatographic analysis of drug substances with weak UV absorption. In analytical method development using CAD, the power function settings available in the instrument software are key for linearization of the signal response with respect to analyte concentration. However, the relatively poor understanding of the power function algorithm has limited a more widespread use of CAD for quantitative assays, especially in the late stage of method validation and GMP laboratories. Herein, we present an approach to understand the inner workings of the power function value (PFV), the PFV optimization algorithm, as well as a method to determine the optimum PFV based on the signals acquired at PFV = 1 (default CAD settings). The exponent and the constant in the PFV equation used for modeling follow a trend as a function of PFV. The CAD signal at any PFV was modeled based on the signal acquired at PFV = 1, the modelling was successful for two analytes at different concentration levels on two different CAD detectors of the same model. This method reveals the functionality of the PFV which substantially simplifies the workflow needed to optimize the detector signal. The accuracy between the experimental and theoretical results showed high correlation and always resulted in the same optimum PFV determined by both ways. The approach described in this investigation simplifies the selection of the optimum PFV at which the signal is more linear, the signal-to-noise is higher, and the area reproducibility is better. The power function algorithm elucidated herein enables determination of optimum PFV from minimal experimental output and excellent overall accuracy. This paper provides an approach that includes no data transformation outside the vendor software, a very important requirement to easily validate and report results in a GMP environment.