There is a well established and growing need for the measurement of analytes, e.g. albumin or troponin I, which are markers for medical conditions, in samples taken from either humans or animals. At the present time the vast majority of tests carried out to measure biological compounds in a sample are carried out using laboratory based equipment. Such tests require transport of the sample to be tested to the laboratory from the place where the sample was taken from. This can cause delays in obtaining the results of the analysis, which may be disadvantageous.
Analytical systems have been developed, and commercialised that can be used at sites remote from the laboratory, thereby obviating the need for transportation of the sample and the consequential delay. Such systems normally contain a cartridge or test strip into which a small sample of a test fluid is applied, e.g. for in vitro diagnostic assays the sample may be blood, plasma, serum, urine or amniotic fluid. The cartridge also contains reagents that will selectively bind to or react with the analyte to form a target material or substance of interest such as a compound or complex or reaction mixture. In order to determine the concentration of the analyte, the cartridge is placed in a “reader” which uses optical or other means to quantify the compound or complex and thereby determine the concentration of analyte. Laboratory based analytical systems often use fluorescent measurement of dye molecules to determine the concentration of the analyte. Cartridge-reader systems can also use fluorescent measurement but have a disadvantage in that in order to achieve the desired test performance they have to use expensive, bulky optical components.
Traditionally, an optical configuration such as that shown in FIG. 1 is used for fluorescence detection in which the source 101 and detector 105 are orthogonal so that source light transmitted through the sample 103 is not directly incident upon the detector. Such devices may additionally require a short pass filter 107 and long pass filter 109. These optical probing arrangements are bulky and cumbersome to engineer into a low cost design.
Conventional fluorescent configurations of the type depicted in FIG. 1 utilise a (typically) 90° change in direction of light path between the incident source and emitted light measurement. Such arrangements are efficient but do not easily permit the light source and detector to be fabricated as a single assembly.
Alternative arrangements for optically probing a target material or substance of interest have been disclosed using in-line detection systems, that is where the light source, detection zone (containing the target material), optical detector and other required optical elements share a common optical path. Such systems have been considered for microfluidic devices.
An example of a known in-line detection system is shown in FIG. 2 including a light source 202, a first linear polariser 206 having a first polarisation direction 250, a target material 210, a second linear polariser 218 (having a second polarisation direction 270 orthogonal the first polarisation direction 250) and an optical detector 224. Polarised light 208 incident upon the target material 210 may give rise to the emission of unpolarised fluorescent light 212 some of which passes through the second polariser 218 to be detected by the detector 224. Stray polarised light from the source is extinguished by the second polariser owing to the orthogonal polarisation. In practice, it is found that a small proportion of incident light is transmitted through the polarisers and is detected by the detector. This leakage light can be of a magnitude such that at very low levels of emitted fluorescent light, as in the case of the analysis of very low concentration samples, it represents a significant interference and decreases the signal to noise ratio, leading to a loss of sensitivity of the test.
The present disclosure relates to an improved device for performing an assay. In particular, the present disclosure relates to a compact, low cost design which addresses problems related to orthogonal detection and in-line detection.