Noise optical absorbance detectors

Nonoptical Noise Sources in High-Performance Liquid Chromatographic Optical Absorbance Detectors... 

Such "non-optlcal" noise sources are significant in current HPLC optical absorbance detectors in that they limit the reduction of fixed wavelength detector noise below approximately 10 absorbance units (au) and limit the noise achievable in deuterium lamp-based photodiode array detectors to approximately 4x10 au, (The above noise values are given as peak to peak noise as seen on a recorder, which is approximately 6 times the rms noise for the case of a Gaussian noise distribution (1 ),... 

Consider the case of an optical absorbance detector. Each individual noise current n translates into an absorbance noise Nj through the logarithmic relationship ... 

From the viewpoint of detector design. It Is Instructive to classify noise sources In terms of the dependence of the term on the photodetector current 1. (Note that the absorbance signal A of an optical absorbance detector Is not dependent on 1. ) Common noise sources can thus be divided Into three categories ... 

For HPLC optical absorbance detectors, the term (2qB/l ) 1.0 and thus an x-fold Increase In will reduce the absorbance noise contribution by >73ET... 

Table V. Noise Source Contributions in 1984 State of Art Optical Absorbance Detectors - assumes T =0.5 sec. For a given detector, each noise source contribution is normalized to that of its shot noise. 

Detecting single nano-objects in a far-held laser spot requires carefully optimized setups, discussed in several reviews and books [3, 19]. Hereafter, we assume ideal conditions, perfect optical elements (detectors, sources, hlters, etc.), to concentrate on the fundamental limitations to signal-to-noise ratio in optical single-molecule studies. We consider three main detection techniques applied to individual small absorbers, emitting or not photoluminescence direct absorption, huorescence and photothermal contrast. 

As well as noise arising due to particular electrical or optical devices, noise may be caused by external sources, such as mechanical vibrations, or pick-up of external electrical signals. Detector noise tends to increase with the age of the instrumentation. Certain components have limited lifetimes and are considered as expendable. An example is the deuterium lamp of a UV absorbance detector, which typically has a rated life of 2000 h or so. After this time the lamp will probably not fail completely. 

Now, one must consider how to Improve optical absorbance detection limits, that Is - how to reduce noise. One strategy Is to Increase I through use of a fixed wavelength detector based on a discrete line lamp and an Isolation filter to select the UV line of Interest generated by the lamp (typically Zn, Cd or Hg lamps having major lines at 214 nm, 229 nm, 254 nm, respectively). 

Optical absorbance detection In HPLC Is currently limited by several sources of "non-shot" or "non-optlcal" noise. As shown In Summary Table V, a state of art variable wavelength UV absorbance detector comes within a factor of two of Its 8x10 au shot noise limit. In this case the dominant noise sources are shot noise and Johnson thermal noise. 

The relatively low throughput of reverse optic/polychroma tor designs utilized In photodiode array UV absorbance detectors and an electronic readout noise Incurred In scanning an array currently limit noise In this detector to approximately 5x10 au. 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

Two UV detectors are also available from Laboratory Data Control, the UV Monitor and the Duo Monitor. The UV Monitor (Fig.3.45) consists of an optical unit anda control unit. The optical unit contains the UV source (low-pressure mercury lamp), sample, reference cells and photodetector. The control unit is connected by cable to the optical unit and may be located at a distance of up to 25 ft. The dual quartz flow cells (path-length, 10 mm diameter, 1 mm) each have a capacity of 8 (i 1. Double-beam linear-absorbance measurements may be made at either 254 nm or 280 nm. The absorbance ranges vary from 0.01 to 0.64 optical density units full scale (ODFS). The minimum detectable absorbance (equivalent to the noise) is 0.001 optical density units (OD). The drift of the photometer is usually less than 0.002 OD/h. With this system, it is possible to monitor continuously and quantitatively the absorbance at 254 or 280 nm of one liquid stream or the differential absorbance between two streams. The absorbance readout is linear and is directly related to the concentration in accordance with Beer s law. In the 280 nm mode, the 254-nm light is converted by a phosphor into a band with a maximum at 280 nm. This light is then passed to a photodetector which is sensitized for a response at 280 nm. The Duo Monitor (Fig.3.46) is a dual-wavelength continuous-flow detector with which effluents can be monitored simultaneously at 254 nm and 280 nm. The system consists of two modules, and the principle of operation is based on a modification of the 280-nm conversion kit for the UV Monitor. Light of 254-nm wavelength from a low-pressure mercury lamp is partially converted by the phosphor into a band at 280 nm. 

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