Detectors lifetime

Instrument and maintenance costs are high. Prices range from about 180,000 (U.S. dollars, 1998) for quadrupole instruments to almost 1,000,000 (U.S. dollars, 1998) for a fully capable multicollector sector-based instalment and laser ablation sampling. About 10 to 20 L/min of Ar is used by the ICP. Sampling and skimmer cones cost 800 to 3000 (U.S. dollars, 1998), depending on material. Detector lifetime may be less than 1 year. Vacuum pumps have limited lifetimes. Of course, the rapid multielement analysis capabilities, low detection limits, and isotope measurements often provide information that makes ICP-MS successful financially as well as scientifically. 

ICP-MS ionization energy of elements must be lower than that for Ar (15.8 eV). The determination of nonmetals is possible only with modem instruments and detector lifetime is limited. 

The third limitation, the counting capability of the detector, depends on the detector type used, the voltage divider design, and the requirements for IRF stability, long-term gain stability, and detector lifetime. For conventional PMTs the practical limit for TCSPC is of the order of 5 to 10 MHz. For MCP-PMTs the maximum count rate is 200 kHz to 2 MHz, depending on the MCP gain used. 

Theory. If two or more fluorophores with different emission lifetimes contribute to the same broad, unresolved emission spectrum, their separate emission spectra often can be resolved by the technique of phase-resolved fluorometry. In this method the excitation light is modulated sinusoidally, usually in the radio-frequency range, and the emission is analyzed with a phase sensitive detector. The emission appears as a sinusoidally modulated signal, shifted in phase from the excitation modulation and partially demodulated by an amount dependent on the lifetime of the fluorophore excited state (5, Chapter 4). The detector phase can be adjusted to be exactly out-of-phase with the emission from any one fluorophore, so that the contribution to the total spectrum from that fluorophore is suppressed. For a sample with two fluorophores, suppressing the emission from one fluorophore leaves a spectrum caused only by the other, which then can be directly recorded. With more than two flurophores the problem is more complicated but a number of techniques for deconvoluting the complex emission curve have been developed making use of several modulation frequencies and measurement phase angles (79). 

The simplest fluorescence measurement is that of intensity of emission, and most on-line detectors are restricted to this capability. Fluorescence, however, has been used to measure a number of molecular properties. Shifts in the fluorescence spectrum may indicate changes in the hydrophobicity of the fluorophore environment. The lifetime of a fluorescent state is often related to the mobility of the fluorophore. If a polarized light source is used, the emitted light may retain some degree of polarization. If the molecular rotation is far faster than the lifetime of the excited state, all polarization will be lost. If rotation is slow, however, some polarization may be retained. The polarization can be related to the rate of macromolecular tumbling, which, in turn, is related to the molecular size. Time-resolved and polarized fluorescence detectors require special excitation systems and highly sensitive detection systems and have not been commonly adapted for on-line use. 

Because of the underlying photophysics, fluorescence lifetimes are intrinsically short, usually on the order of a few nanoseconds. Detection systems with a high timing resolution are thus required to resolve and quantify the fluorescence decays. Developments in electronics and detector technology have resulted in sophisticated and easy to use equipment with a high time resolution. Fluorescence lifetime spectroscopy has become a popular tool in the past decades, and reliable commercial instrumentation is readily available. 

Both TCSPC and TG benefit from operation in SPC mode. SPC results in little or no noise and a high photon-economy [10]. Therefore, TCSPC and TG are ideal for high spatial and lifetime resolution imaging [24], Both techniques offer high image contrast also on dim samples. However, the dead-time of the detectors and the point scanning character limit the throughput of these systems. 

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