Lab shutterbugs looking for new gel-documentation systems this year could be pleasantly surprised: Manufacturers are selling better cameras at lower prices. With companies now offering a largely common set of features, competition will center on cost and improved imaging equipment. And that's a good thing, because rapid developments in imaging hardware mean that last year's dream system is likely to be this year's budget-minded purchase.
Imaging hardware ranges from simple scanner-based systems to high-end chemiluminescence and microarray readers. This buyer's guide focuses on low-cost systems designed primarily for visual imaging and fluorescent imaging of DNA gels, excluding visible-light scanner-driven systems. Key features to consider include the imaging system and software, light source, and whether the system includes a cabinet-style darkroom or portable hood.
HOOD VS. CABINET
The first step in buying a gel-documentation system is to decide where and how it will be used. If the system will replace an old film-based setup in a dedicated darkroom, a portable hood may be a good choice. Designed to fit over an existing transilluminator (which lights the gel from below), portable hoods generally provide a relatively inexpensive route into the gel-documentation arena.
For labs without a dedicated darkroom nearby, cabinet-style systems present a convenient alternative. The cabinet resembles a miniature darkroom, housing a small, light-tight box containing one or more illumination sources. Cabinets vary considerably in design, so size, compatibility with existing transilluminators, and dimensions of imaged gels all should be considered. Unlike portable hoods, cabinets support epi-illumination, in which the gel is lighted from above, as well as transillumination.
Whether illuminating from above or below, an ultraviolet light source is key, as it enables fluorescent imaging of DNA. Visible light also is useful, to capture Coomassie-stained protein gels, for instance. Some companies offer a choice of visible or ultraviolet epi- and trans-illumination sources as well as filters for specific wavelengths. A popular alternative to a dedicated visible light transilluminator is a converter plate, which is placed on top of an existing transilluminator to convert the UV light into blue or white visible light.
WHICH IMAGING SYSTEM?
Next, the researcher must decide on an imaging platform. Charge-coupled device (CCD) video systems represent a well-established technology, but increasingly they are being replaced by lower-cost digital camera-based systems for some applications. The key parameters to consider in both cases are bit-depth and resolution, which describe the underlying image sensors.
Most CCD-based systems discussed here collect eight bits of information per pixel, whereas digital cameras typically collect eight bits for each of three colors per pixel. (But don't confuse 24-bit color, which comprises three eight-bit measurements, with a 24-bit CCD.) For visual imaging and exposures typical in standard DNA gel applications, eight-bit, noncooled CCDs or digital cameras are sufficient.
For demanding applications such as chemiluminescence and faint fluorescent imaging, in which background noise from long exposures can saturate the image, higher bit depth (e.g., 10- or 12-bit) and cooling can produce better images. Such cameras are often more expensive than their 8-bit counterparts, but as cost continues to drop, the low-noise images produced by these high-quality systems become increasingly accessible.
Resolution refers to the number of pixels per captured image. Most CCD cameras produce images from 0.4 to 1.4 million pixels (megapixels, or MP), which is generally sufficient for most small-gel documentation. But large gels, multiple gels, and gels with smaller, more demanding band patterns may be difficult to visualize with a low-resolution CCD. For these applications, purchasing a digital camera, with available resolutions between 2.3 and 5.0 MP, might make more sense. The dynamic range and megapixel rating for digital cameras must be interpreted cautiously, however, because each element is recording only one color. Nonetheless, a digital camera, generally speaking, makes an excellent choice when high resolution and low cost are important.
There are many other important but less-easily quantified requirements for a good imaging system, including quality optics and electronics. As with software usability, these are best evaluated in person or at least by examining examples of actual image files for planned applications.
Though several standalone choices are available, many systems require a computer to drive the imaging hardware. Even if a computer is necessary, its purchase with the system generally is optional. A lab with reasonably up-to-date computers often will be able to use an existing machine via an available USB or FireWire connection to drive the documentation system.
In many labs, the software used to run and process gel images ranks among the most frequently used computer applications. Such programs should be flexible enough to accommodate any application the lab is likely to encounter, yet intuitive enough for the average user. At a minimum, basic functions such as image capture and data storage should be easily executed.
Because individual preferences differ widely, it is important to field-test the software of the systems under consideration. A number of systems offer free software trials, while other manufacturers are willing to provide systems for evaluation. Several available packages can meet specific documentation requirements, including GLP (good laboratory practice) audit trails.
The image files these systems produce can be stored on the computer's hard drive, floppy disk, or even on a flash memory card, but a hard copy of the data requires a printer. Digital thermal printers can produce excellent true-grayscale images, but for most applications, the more versatile high-resolution laser or dye-sublimation printers will do the job well.
ON THE UPGRADE PATH
For researchers who may in the future require different lighting schemes, applications, or even different imaging systems, upgradeability is an important consideration. With some systems, for instance, upgrading to a cooled, higher-bit-depth camera for chemiluminescence or to epi-illumination can be accomplished using an existing cabinet. Ask company representatives about the enhancements that are available for a particular system being considered.
The chart accompanying this article and the expanded online version