Luminescent sensors have seen tremendous growth in applications for measurement of chemical analytes such as oxygen, carbon dioxide and pH. Many of these luminescent sensors are well approximated by a single or multi-exponential model. This is particularly useful, because with one or two excitation (stimulus) frequencies, the time constant(s) of the system (luminescence lifetime decay) can be estimated.
For example, oxygen concentrations in samples such as water, are typically measured with devices that employ a luminescent probe molecule embedded in a sensing matrix. These devices measure light emitted from the luminescent probe molecule. The luminescent light is generated after excitation light has been directed to the sensing matrix containing the luminescent probe. There is a phase shift between the excitation light and the luminescent light that is measured by the device, that changes with oxygen concentration. The phase shift is then used in an empirically derived calibration, or theoretical model that relates phase shift of the luminescent probe to oxygen concentration in the sensing matrix.
Alternately, the luminescent lifetime may be calculated from the measured phase shift, and the oxygen concentration in the sensing matrix is related to the lifetime using an empirically derived or theoretical model. The oxygen concentration within the sensing matrix is generally proportional to the oxygen concentration of the sample (water), and is typically related by Henry's law. Other analytes, for example, CO2, pH, glucose, in samples are evaluated similarly, as the sensing matrix and luminescent probe are tailored for sensitivity to these particular analytes. The luminescent lifetime may be calculated from the measured phase shift, and the analyte concentration in the sensing matrix is related to the lifetime using an empirically derived or theoretical model.
Contemporary apparatus, that perform the above described functions, operate by measuring phase shift of the luminescent probe. These apparatus include a digital signal processor in communication with a synthesizer that generates a sinusoidally modulated electrical signal, and light emitting diodes (LEDs) driven by the sinusoidally modulated electrical signal. There is normally an optical filter, that improves the spectral purity of the LED, a sensing matrix that contains the luminescent probe, an emission filter that only passes the emission of the luminescent probe, and, a photodiode or similar device for converting the emitted luminescence, from the luminescent probe, into an electrical signal. These apparatus also have analog to digital converters that convert the electrical signal into a digital representation.
When the LEDs direct excitation light to the luminescent probe, the photodiode detects the emitted luminescence, and an analog to digital converter converts the emitted luminescence into a digital signal. The systems then determine the phase shift, between the sinusoidally modulated excitation light and the emitted luminescence. The digital signal processor that generates the sinusoidally modulated excitation signal can also be used to compute the phase difference between the excitation signal and the emitted luminescence. Based on the phase shift, the requisite measurement, quantity, or the like, may be determined.
U.S. Pat. No. 4,716,363 to Dukes, discloses an exemplary contemporary apparatus, that measures luminescence lifetime. The luminescence lifetime is determined by a comparison, performed by an analog processing system that implements a phase-locked servo loop. This servo loop varies the frequency of the modulated excitation light to maintain a constant phase shift between the excitation and emission of the luminescent probe. The analyte concentration is related to the resultant frequency, or the lifetime is calculated from the resultant frequency and phase. The analyte concentration is related to the calculated lifetime.
This apparatus exhibits drawbacks, in that the servo loop takes a long time to settle and it is not suited to measurements where the excitation light is turned on for short durations. Short duration measurements are desirable because the consumption of electric power is reduced and photodegradation of the sensing matrix is minimized. Moreover, the components of the apparatus must operate over a wide range of frequencies, which means that they are expensive and consume large amounts of power.
Another exemplary contemporary apparatus is disclosed in U.S. Pat. No. 6,157,037 to Danielson. This apparatus uses phase comparison, as a digital signal processor, that generates sinusoidally modulated excitation light, and implements a servo loop that varies both the modulation frequency and the phase shift through the luminescent sample. The analyte concentration is related to the resultant phase or frequency, or, calculated lifetime.
This apparatus exhibits drawbacks, in that, like the apparatus of U.S. Pat. No. 4,716,363, the servo loop takes a long time to settle. This long settling time increases the amount of time that the luminescent probe is exposed to excitation light, which can cause the luminescent probe to degrade. Additionally, the implementation of the servo loop in a digital signal processor would be computationally intensive, and requires a device with a high CPU clock frequency, and typically a hardware multiplier. Moreover, the use of sinusoidally modulated excitation is computationally intensive and requires multiple additional apparatus components, in addition to a digital processor. Also, the components necessary to perform these functions are expensive, and consume large amounts of power.
Another exemplary contemporary apparatus is disclosed on U.S. Pat. No. 6,664,111 to Bentsen. One disclosed apparatus uses sinusoidally, amplitude modulated light from an oscillator, or from a Direct Digital Synthesis (DDS) device, to excite an oxygen sensitive luminescent probe. The implementation uses a Discrete-Time Fourier Transform algorithm. The apparatus then acquires data representing the emitted luminescence, and subsequently applies a Discrete-Time Fourier Transform or Fast Fourier Transform (FFT) vector analysis method on the data to calculate the phase shift of the luminescent probe. Alternately, the phase shift is calculated by a least squares algorithm on the data. Both the Discrete-Time Fourier Transform method and least squares algorithm were implemented on software running on a personal computer.
This apparatus exhibits drawbacks in that both algorithms are sensitive to frequency errors and offsets. These frequency errors can cause errors in the measured phase and amplitude. Depending on the Fourier implementation, such as the Fast Fourier Transform, this method could also have large memory requirements. Additionally, the use of sinusoidally modulated excitation is computationally intensive and requires multiple additional system components, in addition to a digital processor. Also, the Discrete-Time Fourier Transform and least squares algorithm are computationally intensive, requiring complex and expensive components, large amounts of memory, and these components and memory consume large amounts of power.