The two main types of detection used with photomultipliers are photon-counting and analog.
When photons of light strike the cathode of a photomultiplier tube (PMT), they dislodge electrons. These electrons pass from the cathode into the dynode chain (a series of high-voltage steps), which amplifies and directs them, to the anode.
The pulses are then transmitted to an amplifier, and then on to a discriminator, which filters out the lower-intensity noise pulses. A second amplifier changes the voltage to a useable level.
The advantages of photon-counting are:
The signal-to-noise ratio of the data is higher. Most noise is generated after the cathode, usually in the dynode chain. These noise pulses are lower intensity and are removed by the discriminator. In the analog system, noise gets added directly to the signal, and thus cannot be removed.
The stability of a photon-counting system is better. All PMTs need a high-voltage supply. A small change in the voltage supplied will not affect the pulse count of a photon-counting system; bill will affect the analog portion because of the integration of pulse height changes.
Photon counting is unaffected by an RC time constant, which is an inherent part of analog systems’ electronics. (The RC time-constant refers to an electronic circuit made of a resistor and capacitor.) Analog systems rely on RC time-constants to reduce noise, but thereby extract a price. One problem introduced by the RC time-constant is accuracy. An RC circuit discharges only 67% of its charge after one time constant has elapsed. For example, with a time constant of one second, 33% of the signal still is stored within the circuit, reducing the accuracy. Another problem is the speed of the electronics. At any signal level, RC circuits approach their final value exponentially; thus, the faster you scan, the more skewed your data look. This effect does not happen to digital signals, such as with photon counting.
The precision, or practical range, of photon counting is greater than most analog systems. The range for photon counting is usually based on the PMT or the band pass of the first amplifier, in the range of 3–4 million counts per second. Analog systems are usually based on the range of the analog-to-digital converter (ADC).
on the other hand, the range is 0 to 3 million. This means that small changes and signals will be more clearly resolved via photon counting.
In analog mode, electrons also are generated by photons of light striking the cathode of a PMT. The resulting analog signal from the anode is the DC portion of the pulses. Bear in mind that this DC current is the sum of all pulses regardless of their source (cathode, dynode chain, leaky components, etc.). Therefore, the noise is more tightly bound to
Photon-Counting vs. Analog
With strong light intensity, such as in absorption measurements, analog methods may be adequate. For low-light techniques, such as fluorescence or spectroscopes, photon-counting enables you to extract the maximum information from your data.
Chemiluminescence is the
light emission accompanying
redox reactions, e. g.
interaction of two radicals
or reactions where peroxides
are involved. Intrinsic
of human and
animal cells and tissues is
the result of reactions of
free radicals, including
nitric oxide and radicals
of oxygen and lipids
The Cell Chemiluminescence method allows revealing presence of inflammation focuses. With acute heart pain state (cardiodynia) the Chemiluminescence method allows differentiating myocardial ischemia from myocardial infarction, as in the case of infarction the cell chemiluminescence intensity increases 100 times as much. The Chemiluminescence method allows detecting myocardial infarction focuses in the cases when they are not observed by electrocardiographical examination – for example, with posterior mycrocardial infarction.
The Chemiluminescence method allows estimating the level of disease gravity and efficiency of the therapy performed in most cases of inflammatory diseases, including rheumatoid arthritis, lung diseases (pulmonary tuberculosis, acute pneumonia), and in inflammation focuses of internal organs (destructive appendicitis, pancreatitis, cholecystitis).
Chemiluminescence of blood serum shows the level of antioxidant reserves of the organism as its lowering points out at the risk factor of stimulation of various kinds of diseases. The method allows checking-up antioxidant therapy (vitamins C and E) and medical preparation treatment efficiency.
Determination of blood serum chemiluminescence intensity allows estimating the oxidized lipoprotein rate in blood, as the heightened concentration of oxidized lipoproteins is the essential index of the risk factor of atherosclerosis and essential hypertention challenge.
Yu. A. VLADIMIROV
AS AN ANALYTICAL TOOL IN
Yu. A. VLADIMIROV
Универсальный хемилюминометр SmartLum 5773
2004 – 2009