Ultraviolet absorbance detector based on a high output power 235 nm surface mounted device-type light-emitting diode

Highlights

• High output power deep-UV 235 nm wide-angle SMD-type LED was characterised.

• A simple method for improving optical design of detector was presented.

• Limit of detection was boosted by 2.2-fold with 9-fold enhanced light throughput.

• The 235 nm UV detector was used in a portable capillary-scale LC.

Abstract

A new miniaturised capillary flow-through deep-UV absorbance detector has been developed using a microscale surface mounted device- type light-emitting diode (LED) (Crystal IS OPTAN 3535-series), emitting at 235 nm and with a half-height band width of 12 nm, and a high-sensitivity Z-shaped flow-cell. Compared with a previously reported TO-39 ball lens LEDs emitting at 235 nm, the new generation LED produced a 20-fold higher optical output and delivered up to 35 times increase in external quantum efficiency (EQE). The Z-cell was based on a reflective rectangular optical path with cross-sectional dimensions of 100 × 100 µm and a physical optical pathlength of 1.2 mm. Inclusion of UV transparent fused-silica ball lenses, between the SMD and the Z-cell, improved light transmission by a factor of 9 and improved the detector signal-to-noise ratio by a factor of 2.2, at the same input current. The detector was housed within an Al-housing fitted with a cooling fan and demonstrated excellent linearity with stray light down to 0.06%, and an effective pathlength of 1.1 mm (92% of nominal pathlength). The resultant detector was fitted successfully into a briefcase-sized portable capillary HPLC system, and practically demonstrated with the detection of a mixture of 13 test compounds at the sub-mg L⁻¹ level in <5 min using gradient elution.

Introduction

Ultraviolet (UV) light-emitting diodes (LEDs) have seen a great number of applications within the field of analytical chemistry, one of which is their utilisation as compact and energy efficient light sources for absorbance detection. UV-LEDs act as quasi-monochromatic light sources with a typical half-height bandwidth of 10-50 nm and thus typically do not require the addition of band-pass filters in the optical configuration [1]. Moreover, an advantageous feature of UV-LEDs over conventional light sources is that they are suitable for instant operation, with on/off response times typically down to microseconds, which eliminates the long warm-up period for detectors using conventional UV lamps. Based on these advantageous features, inclusion of UV-LEDs as light sources in miniaturised analytical instrumentation, such as absorbance detectors [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], gas sensors [20], [21], and point-of-care diagnostics [22], [23], has been increasing over recent years, in-line with regular reductions in the UV-LED wavelengths available.

In analytical chemistry, there is significant demand to move closer towards an emission wavelength of 200 nm, where such LEDs applied in absorbance detection could be used for the majority of organic solutes [1]. However, currently, the commercially available lowest wavelength UV LED is at 235 nm [24]. In 2016, our group reported the first analytical detector based on such a 235 nm LED [9]. The report was followed by multiple similar studies of the 235 nm LED detector by our and other groups [11], [15], [17]. However, in those reports the 235 nm LEDe used was a Crystal IS Optan ball lens (abbreviated as OP-BL) LED, which has now been discontinued. This leaves the Optan 235 nm surface mounted device (SMD) LED (abbreviated as OP-SMD) as the only commercially available sub-240 nm SMD-type UV-LED, which to the best of our knowledge has not been used in analytical chemistry, primarily because of its unfocussed emission. As compared to the OP-BL LED, which has a viewing angle of 15°, the OP-SMD LED has a very wide viewing angle of 120°. Hence, the direct use of the OP-SMD LED in an analytical detector would result in the loss of most of the produced light, significantly hampering the detector efficiency. Moreover, the smaller footprint of the OP-SMD LED compared to the OP-BL LED hinders the dissipation of excess heat, further lowering the performance of an analytical detector [16]. However, the successful use of the OP-SMD LED in a UV absorbance detector would pave the way towards the development of more compact and multi-wavelength LED-based detectors due to its miniaturised and planar configuration and higher optical output power.

Hence, herein, a new deep-UV LED based absorbance detector has been developed using the OP-SMD LED and a capillary Z-shaped flow-cell. Commercially available UV fused-silica lenses are used to aid focusing of the light between the OP-SMD and the Z-cell. Different optical alignments are simulated to demonstrate the illumination pattern at the entrance of Z-cell by commercial light simulation software, namely TracePro (Lambda Research Corp., Littleton, MA, USA), and to select the configuration with the highest transmission of light. Light transmission improvements by different collimation setups are compared and monitored by a previously reported radiometric output measurement (ROM) method [25]. A passive heat sink was used to dissipate the heat. The assembled detector with the best light collimation results is then demonstrated in combination with capillary HPLC as a robust, miniature and low-cost UV absorbance detection option.