DNA concentration and purity measurements, which use absorption spectroscopy, are the first step in analysis for many life science applications. This includes the detection of genetic disorders and pharmaceutical development of insulin, antibiotics and hormones. Quantitative analysis of DNA concentration and purity is based on UV absorption spectroscopy. Both DNA and protein absorb UV light, but have different absorption peaks at 260 nm and 280 nm respectively. The absorbances at these wavelengths determine the concentration of DNA and protein, while the ratio of the absorbances determines the purity of the DNA sample.

In research and clinical labs, nucleic acids are needed in applications such as polymerase chain reaction (PCR), and DNA sequencing for pathogen detection, genetic testing, and related studies.  The suitability of the method used to extract the nucleic acids is determined by characterizing the quantity and quality (purity and intactness) of the extracted DNA. DNA extracts must meet requirements for purity or assays may result in false negatives with serious consequences.

Leading manufacturers are seeking alternative light sources for these analysis instruments in order to reduce application costs without sacrificing measurement accuracy. Spectrometers for DNA concentration and purity measurements often rely on xenon flash lamps, which offer instant on/off for quick evaluation with high linearity of measurement over a wide concentration range. However, these lamps emit a broad spectrum and require costly components, which is why manufacturers are turning to LEDs for new design innovations.

Monochromaticity Simplifies Design

Within the narrow range of wavelengths defined by the absorbance measurement for either DNA or protein, UVC LEDs can match the measurement performance of xenon flash lamps. Figure 1 compares the spectral irradiance of a 1 mW, 260 nm UVC LED with a 15 W Xenon flash lamp.

As Figure 1 shows, unlike the broad, complex spectra of UV lamps, LEDs have simple spectra—a single peak with a narrow spectral bandwidth. In spectroscopy, if stray light—from a multiple peaks or from a single, broad peak—reaches the detector it interferes with quantification. The narrow peak of the LED allows for measurement accuracy and sensitivity without the use of filters to remove the unwanted wavelengths.

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High Light Output Improves Measurement

This high light output of the LED also allows for a lower detection limit of 0.5 ng/µl for concentration of double stranded DNA (dsDNA) and the excellent spectral quality of the LED leads to linearity of measurement over three orders of magnitude of concentration from 0.5 – 2000 ng/µl (see Figure 2).

Reducing Costs – Not Performance

The performance and monochromaticity of LEDs results in a simpler instrument design with fewer optical components than the xenon flash lamp-based version. This lowers the overall instrument cost. Additionally, power sources for UVC LEDs are less complex, and less costly. The reduction in component costs allows for a more cost effective instrument to be manufactured without sacrificing performance for DNA purity measurements. The following table compares typical optical component costs for a fixed wavelength detector for measurements at 260 nm and 280 nm. The xenon flash lamp system assumes two filters to achieve those wavelengths while the UVC LED system uses two LEDs—one at each wavelength. 

  Xenon Flash Lamp System UVC LED System
Light Source $600 $600
Power Supply & Trigger Socket* $1000 $50
Excitation Filter for 260 nm $350 $0
Excitation Filter for 280 nm $350 $0
Silicon photodiodes (UV enhanced) $100 $100
Total $2400 $750

*Systems using xenon flash lamps require a trigger socket in addition to the power supply. This is not neccesary for LED-based systems.

The component costs provide a significant difference in the initial system cost. However, the efficiency of the system is also a factor that contributes to the overall costs. In the system examples above, the power consumption for the UVC LED system is approximately 2 watts (1 watt per LED). A typical xenon flash lamp will operate at an average power of anywhere from 2-60 watts. The UVC LED system also offers a more efficient light source for fixed wavelength measurements. As a significant amount of light output from the xenon flash lamp is filtered out in the unwanted wavelengths, the LED provides more power output at the desired wavelength as seen in Figure 1 above.

Visible LEDs are well known for their long lifetimes. Although UVC LEDs are an emerging technology and the lifetime still does not match that of their visible counterparts, the lifetime does exceed that of existing lamps. The longer the life of the light source, the more measurements that can be taken without the additional cost of replacement light sources and added maintenance costs.

Although the lifetime of a xenon flash lamp can be provided in hours (as seen in Figure 3), it is often represented in the number of flashes (or measurements) with a typical life of 1E9 flashes. For an on time of 1 millisecond per measurement, the typical lifetime of a UVC LED of 8,000 hours at 20 mA would translate to 3E10 measurements before requiring replacement. That’s 30 times more measurements than the xenon flash lamp.

By developing UVC LED-based instruments, manufacturers can reduce instrument costs by over 30% for DNA purity and concentration measurements. Because these applications rely on a fixed set of wavelengths, these instruments can match and sometimes exceed the performance of systems using UV lamps. UVC LEDs enable longer instrument life, higher reliability, and increased productivity while reducing overall costs for the end user. These new devices are driving innovations in instrument design for the life sciences to address key market trends around productivity, cost reduction, and miniaturization.

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