Product Survey: Microplate readers

Lighting Specialists
by Harald Zähringer, Labtimes 01/2014

Figures: Gheorghe Christols

Microplate readers or simply plate readers belong to the most basic and essential instruments in life science labs. What makes them stand out against other ubiquitous lab hardware, however, is their astonishing versatility.

Microplate readers are the archetypical jack-of-all-trade instruments in life science laboratories. They are used to analyse all kinds of assays from protein and DNA/RNA quantification, ELISAs, fluorescence and luminescence assays such as FRET and BRET, to AlphaScreen and other sophisticated light-based techniques.

The many different features and options of microplate readers, however, may cause potential buyers quite a headache. A typical quest for a new microplate reader may be found on the website of the life science network “Researchgate”.

The million dollar question

One of the contributors is looking for a plate reader for standard applications, such as ELISAs, protein and DNA quantifications and fluorescence and luminescence assays in 96-well plates, so he asks the Researchgate followers the simple, albeit challenging, question “Which plate readers are good and user-friendly and which are not”? Six of the followers responded to his post – proposing six different microplate reader models offered from four different manufacturers. Hence, the poor guy still has to choose from six models and there are many, many more out there, which may also come into question.

The many different features of plate readers may confuse potential buyers; the basic components of individual plate readers, however, are very similar or even identical. First of all, every microplate reader needs some sort of a microplate carrier that accepts different kinds of microplates. Most instruments can handle 96-well and 384-well formats. High throughput readers may also accept plates with higher densities such as 1,536 or even 3,456 wells. Plate readers covering the whole range from 6 to 3,456 wells are pretty rare but you may get one if you’re willing to pay a considerable sum of money.

Laboratories cranking out numerous microplate assays per day may also focus on instruments with additional plate stackers or look for plate readers that can be integrated into an automatic workflow.

Filters or monochromators

The fundamental part of any microplate reader, however, is the inbuilt optical system, which is necessary to analyse the reactions taking place in the microplate wells. Usually, the light of a xenon flash lamp is split into the desired wavelength by either a filter or a monochromator system and is then conducted through the microplate wells to excite, for example, fluorescent molecules.

The fluorescent light emitted from the plate well passes a second filter or monochromator system before entering a photo­multiplier tube (PMT) that converts the optical signal into a current, which is finally transferred into an appropriate chart. Depending on the intended use of the microplate reader, most instruments are equipped with either filters or monochromators for wavelength selection. Both have their pros and cons.

Filter systems used in microplate readers have high light transmission rates and efficiently block the unwanted light spectrum. They are cheap and may be easily changed to switch between different wavelengths. Hence, some manufacturers almost exclusively equip their instruments with filters instead of monochromators.

Highly flexible

So, why use monochromators in microplate readers, at all? The major advantage of monochromators is their flexibility. ­Monochromators separate white light with a diffraction grating into a spectrum that is projected on a small exit slit to select a specific wavelength. The wavelength may be adjusted in tiny steps by rotating the grating in small increments.

To minimise stray light, a second monochromator is connected in series to the first one, with the exit slit of the first monochromator acting as the entrance slit for the second. Since a monochromator pair is usually implemented on the excitation and the emission side of the light path, they actually function as a quadruple monochromator.

Monochromator pairs

Microplate readers equipped with Linear Variable Filter (LVF) monochromators have only recently been introduced. They are based on two consecutive, linear variable filter slides, one acting as short pass filter, the other as long pass filter. Proper alignment of these slidable filters enables the selection of the desired wavelength when a focused light beam passes through them.

A variable dichroic mirror separates the two LVF monochromators, which are implemented in both the excitation and the emission light paths.

Hybrid microplate readers, equipped with the best of both worlds, i.e. filters and monochromators, usually offer the widest range of multimode plate reading applications.

The inbuilt filter system is unbeatable in fluorescence applications with low signal levels, such as FRET-assays, where high transmission efficiencies are essential. The additional monochromators enable, for example, spectral scanning of microplate wells and the fast selection of any wavelength that is necessary to perform or optimise a given assay.

If your group has tons of money to spend, a highly flexible hybrid multimode reader, capable of performing any given ­microplate assay, is certainly not the worst option. If you are short of money, however, you should simply go for an affordable ­microplate reader that can handle your specific assays without any bigger problems. But in any case, you should carefully check the considered instrument(s) in a real lab setting before buying.

First published in Labtimes 01/2014. We give no guarantee and assume no liability for article and PDF-download.

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