Product Survey: Gel documentation systems
by Harald Zähringer, Labtimes 05/2017
CCD camera-based gel imagers offer a whole bunch of illumination options and are about to enter the last territories ruled by laser scanners, such as the analysis of 2D differential gels.
DIY gel doc system made by HKUST University’s iGEM team. Photo: iGEM HKUST
Capturing gels on pictures with gel documentation (gel doc) systems is routine in many life science labs. In the old days (not entirely sure they were always the good old days) of biochemistry and molecular biology, gel documentation was mostly done with the legendary GelCam. Simple enough, the GelCam was basically a Polaroid camera mounted on a hood that served as a small portable darkroom. The hood was imposed over the gel lying on the glass plate of a transilluminator desk and, after pulling the trigger of the GelCam, an instant black and white photo was pulled out of the camera, showing the protein or DNA bands on the gel.
The cool thing about the GelCam was its simplicity and also its honesty: the gel picture was glued into the lab book without any further image “manipulation”. But the Polaroids only allowed a qualitative estimation of the protein or DNA bands on the gel.
Aficionados of vintage cameras and Polaroid photos may still get hold of used GelCams on ebay or lab equipment auctions on the internet. But it’s way easier to buy a digital version of the GelCam with a modern, scientific grade, digital camera instead of the old analogue Polaroid camera. The simplest models don’t even have a fixed scientific-camera on top of the hood. Instead, popular smartphones may be mounted above the platform to capture the gel images.
More sophisticated digital offsprings of the GelCam are already equipped, for example, with 12-bit CCD cameras, providing a native resolution of about one or two megapixel and an image depth (gradation) of 4,096 (212) shades of grey. They usually come with exchangeable emission filters to document, for example, ethidium bromide or SYBR green-stained gels. Included entry-level imaging software packages, running on an external PC, allow visualisation and documentation of gels as well as basic quantifications of stained protein or nucleic acid bands.
If you need more illumination options, higher resolution, more grey shades and powerful band quantification software, you should take a closer look at bench top systems. Their basic set-up is pretty much the same for all models, though the design may slightly vary between different manufacturers: the gels are placed on a drawer that glides out of the built-in darkroom after opening a small door. A transilluminator with two or three different UV tubes is integrated in the drawer to illuminate, for example, ethidium bromide or SYBR gold-stained gels. The UV table may be adapted with conversion screens for transmission of white or blue light to detect Coomassie-stained gels or safe fluorescent dyes, respectively. Additional multi-coloured LEDs or epi white lights and blue LEDs installed above the drawer, at the side of the darkroom, enable fluorescence imaging and overhead white and blue light imaging.
Higher priced, stand-alone instruments, with a built-in computer system and touchscreen-controlled imaging software, usually provide even more illumination features, such as accessory infrared laser diodes. They may be configured with different cooled 16-bit CCD cameras, providing up to 65,536 grey levels and nine or even more megapixel resolution.
Gel doc systems with high-resolution CCD cameras already rival laser scanner-based systems in the analysis of 2D differential gels (2D-DIGE). In 2D-DIGE experiments, two protein probes (e.g. disease and control) as well as an internal standard are labelled with different fluorescence dyes and separated through isoelectric focussing and PAGE. Since all probes are loaded on a single gel, variations between two different gel runs, are eliminated. Due to the internal standard and the sensitive fluorescence label, 2D-DIGE indicates even slight variations in the expression of differently labelled proteins.
The analysis of 2D-DIGE gels usually requires a very expensive laser scanner system that reads the gel point-by-point and converts the emitted light signals into an accurate gel image. The scanning process is pretty slow and takes, for example, about half an hour for a 21 x 27 centimetre-sized gel. And you should also have ample storage capacities on your PC, since the gel data set requires about 12 MB storage space.
But recent progress in LED illumination and CCD camera resolution enables CCD-based gel doc systems to compete with laser-based systems in analysis of 2D-DIGE gels. Ralf Rabus’ group at the University of Oldenburg, Germany, compared the performance of GE’s Typhoon 9400 laser scanner, which is the benchmark instrument in 2D-DIGE imaging, with a new CCD-based 2D-DIGE imager developed by the German company Intas (Proteomics 16, 1975-79). The team prepared protein extracts from various environmental bacteria strains, separated the proteins on 2D-DIGE gels and digitalised replicate gels with the Typhoon and the 16-bit CCD imager. The latter utilises fluorescence dye-specific LED arrays, arranged on both sides of the darkroom at an optimised angle to evenly illuminate the complete 2D-DIGE gel area.
It is no surprise that the CCD imager operated at a much faster speed (three minutes to capture a 21 x 27 cm gel) than the Typhoon and required less storage space (3.8 MB per image). But there was also no significant difference in the number of spots that both systems detected. The camera-based system identified only slightly less spots than the laser scanner and “recovered between 86 to 90% of the protein spots detected in images acquired by the laser scanner”. The average ratio numbers, which are the true biological relevant numbers indicating statistically significant changes in protein levels of probes and controls, were, however, very similar for both systems.
It seems that CCD gel doc systems are just about to conquer one of the last strongholds of laser-based systems.
First published in Labtimes 05/2017. We give no guarantee and assume no liability for article and PDF-download.
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