Noise wavelength detector

In the single wavelength detector, 254 nm and 280 nm are strong lines from the mercury source and hence, are selected in most comnercial instruments. Of the three types of U.V. detectors, this one gives the lowest noise level (down to 0.00002 O.D.), but, of course, some flexibility is lost in not being able to work at other wavelengths. Nevertheless, when one purchases an instrument, this is the first detector selected. 

ASTM has issued a recommended standard practice for testing fixed wavelength detectors in LC it is ASTM 685-79. In contrast to normal practice, they have defined noise in absorbance units (AU) per unit cell length. Otherwise the specifications follow the discussions presented in Chapter 7. 

A consequence of the high sensitivities demanded by LC down to 0.0005 AUFS deflection is the need to use greater band widths with variable wavelength detectors ( 5nm being typical), otherwise the resulting noise would render the detector unusable. However, as deviations from Beer-Lambert s Law can arise due to the use of non-monochromatic radiation, it is important to check the linearity of response of the sample components... 

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 ),... 

This value is approximately 2-fold less than the typical noise of 1.5x10 au observed for a state of art variable wavelength detector, suggesting that a significant non-optlcal noise source or sources are present. 

Again, for the variable wavelength detector with Ig of 25x10 amps, one calculates a diode dark current rms noise fraction F of 8x10 and a dark current absorbance noise contribution N, (peak to peak) of 2.1x10 au. This is not a significant limit to current absorbance detectors. However It Is a significant limit to the application of photodiodes to fluorescence detection, for which Ig Is 1000-fold lower than In absorbance detection. 

Table II. Noise Source Contributions - Variable Wavelength Detector (T=0.5 sec)...

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). 

Application of Equation (1) predicts a system noise N of 7x10 Although the particular optical design described has not been reduced to practice, the theoretical estimate of 7x10 au peak to peak noise Is within a factor of 2 of the typical noise (l-2xl0 au) observed on commercial state of art fixed wavelength detectors at 254 nm. 

Reducing the non-shot Category III noise sources of the fixed wavelength detector will require attention to Improved thermomechan-Ical detector design to reduce noise due to thermal variations Imposed on the mobile phase/flow cell and on the sample and reference photodiodes, and continued reduction In HPLC reciprocating pump flow pulsations. 

All data of relevance (instrument types, serial numbers, column data, chromatographic parameters, auxiliary instruments, etc.) need to be listed in the documentation. Likewise, all chromatograms and other reports (detector noise, wavelength registration) are part of the file. The calculations must be traceable. All deviating results must be documented as well. The documentation is dated and signed by hand by the testing person and the customer (e.g. the laboratory head). 

The prototype instrument was based upon a spinning circular variable wavelength filter (CVF) and utilized a low-noise InAs detector. A CVE is a time-dependent wavelength selector that passes a narrow spectral band. The spectral region of the band depends on the angular position of the filter. 

---