Integrating Microlenses and Slits in Portable HPLC

Scientists from Northwestern Polytechnical University in Xi’an, China, tested a new approach for integrating micro-lenses and slits into portable liquid chromatography (LC) systems. Their findings were published in the Journal of Chromatography A (1).

For this study, a miniaturized microchip-based absorbance detector was designed and fabricated for portable high-performance liquid chromatography (HPLC) to test glycated hemoglobin (HbA_1c) levels. Portable chromatography systems have become popular in recent years; according to Yonglin Mai and Kurt Debruille of the University of Tasmania, “Field-portable and long-term ‘deployable’ chromatography systems can provide for robust at-site analysis (monitoring), allow for rapid on‑site decision making, and reduce issues arising from sample degradation and/or contamination during transportation back to the laboratory, or during long‑term sample storage” (2).

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The scientists in the study employed micro-milling techniques were in the fabrication of microchannel structures integrated onto the chip. The flow cell on chip was designed in Z-shaped to extend the optical path length. This miniaturized microchip-based absorbance detector, and the commercial chromatographic separation device were assembled for the analysis of HbA1c level. The linearity, sensitivity and stray light level of the detector were determined, along with the analytical accuracy and chromatographic reproducibility.

The microchip integrating a Z-shaped cell, two collimating micro-lenses and two ink-filled optical slits was small, providing an effective way to extend the optical path length and reduce the stray light of the microchip-based absorbance detector. Therefore, this detector indicated a low stray light level (0.011%). And the noise level (2.5 × 10⁻⁴ AU) of the detector was diminished by the use of software averaging. Moreover, the application within commercial HPLC to detect HbA1c level of hemoglobin samples showed the detector delivered a low LOD (0.5 μg/mL). In particular, the sensitivity is enhanced by 3.4 times when the optical path length was increased from 0.5 mm to 2 mm. Overall, these findings show that microchip-based absorbance detector can be used as a potential choice for the future development of general-purpose miniaturized absorbance detectors for portable and compact HPLC.

Recently, there has been a growing desire to miniaturize analytical instruments. This has been fueled by an increasing need of on-site analysis and the overarching goals of reducing instrument size, minimizing reagent usage, and lowering operational costs. However, if there is not a prior separation step in miniaturized analytical instrumentation, direct analysis of complex samples can be severely limited. To solve this challenge, high-performance liquid chromatography (HPLC) has been used, since it is a versatile and powerful analytical technique that provides nearly universal separation and detection for nonvolatile analytes. Further, HPLC can be used to analyze complex mixtures within a relatively short time, demonstrating exceptionally high detection resolution in the process. However, commercial HPLC systems, with high cost, bulky size, and a large amount of solvents consumption, are restricted to use in the laboratory. This has led to an urgent demand for portable and miniaturized HPLC devices or key system components for portable analysis to be developed.

Of the various detectors that can be used in HPLC systems, UV–visible (UV–vis) absorbance detection is considered the most popular for HPLC miniaturization because it is easier to implement, maintain, integrate, and operate. However, despite efforts to shrink other components, like light sources and receivers, without altering the structure of flow cell, achieving true miniaturization of overall detector remains challenging. Optimizing flow cell design is crucial for enhancing the performance and portability of portable HPLC systems. Microchip-based flow cells have become a revolutionary tool in the miniaturization of UV–vis absorbance, showing the advantages of small size, low-cost, and real-time monitoring. However, the depth of a microchannel on the chip is usually used as the optical path of an absorbance detector, result in a short optical path length (only on the order of tens or hundreds of micrometers); this has resulted in shortcomings regarding sensitivity.

References

(1) Ren, S.; Zhang, X.; Zhang, R.; et al. A Microchip Based Z-Cell Absorbance Detector Integrating Micro-Lenses and Slits for Portable Liquid Chromatography. J. Chromatogr. A 2024, 1730, 465099. DOI: 10.1016/j.chroma.2024.465099

(2) Mai, Y.; Debruille, K.; Edwards, S.; et al. Portable and Field‑Deployable Liquid Chromatography for Environmental Studies. MJH Life Sciences 2023. https://www.chromatographyonline.com/view/portable-field-deployable-liquid-chromatography-environmental-studies (accessed 2024-7-30)