Holmium oxide filter

On some detectors, a holmium oxide filter or special holmium oxide cell is integrated into the detector these can then be moved into the optical path during calibration mode. The detector itself or associated control software may then be able to check for wavelength accuracy. Note If the filter or cell is made of glass, and not quartz, 241 nm may not be used. 

Figure 10.3. (c) Absorption spectrum of holmium oxide filter. 

There are many ways to check the wavelength accuracy of a UV-Vis detector. For detectors with built-in wavelength verification, the deuterium line at 656 nm or the absorption bands at 360, 418, 453, and 536 nm in a holmium oxide filter... 

Noise and drift are measured in static (dry detector cell) and in dynamic mode at different wavelengths, e.g., 200, 254, and 390 nm. The change in the absorbance as a function of flow rate at the same wavelengths reflects flow sensitivity. Noise is expressed in AU/cm, drift in AU/hr, and flow sensitivity in ALf min/mL. Some equipment units can automatically perform calibration for accuracy. For example, some HPLC-UV/Visible detectors include holmium oxide filters for measurement and calibration of the wavelength accuracy. 

If the detector has a built-in holmium oxide filter the wavelength accuracy is tested with this filter in accordance to the manufacturer s test procedure. 

The dual-beam variable wavelength I JV/Vis absorbance detector remains the primary detector for routine analysis. Sensitivity and linearity performance have improved significantly in recent years. Noise specifications of 0.25 x 1(L5 AU and linearity of upper limit exceeding 2 AU are achievable with newer models. Typical spectral bandwidth ranges from 5 to 8nm. The lifetime of the deuterium lamp has improved to 1,000-2,000 hours. Most detectors have features such as self-aligned sources and flow cells, leak sensors, and built-in holmium oxide filters for easy wavelength accuracy verification. Many have dual- or multiple-wavelength and ratio plot output capabilities. 

For detectors with built-in holmium oxide filter we recommend in principle to use this filter. However, the determination of the UV absorption maxima of holmium oxide requires a good optical resolution (better than 2nm). Many HPLC detectors are built with a fixed slit of 8-10 nm bandwidth which is inadequate for this task. In case of doubt it is best to ask the manufacturer or to use anthracene. 

The shutter generally carries the holmium oxide filter (used for the calibration of the photodiode array) and the opaque surface used for the measurement of the dark current. Other optional filters can be included in order to afford fast built-in verification procedures for the spectrometer working parameters (e.g., neutral density glass filter for verification and calibration of absorbance scale in the visible domain). 

Wavelength accuracy is usually checked with the holmium oxide filter (absorption maxima at 361, 453.7, and 536.7nm, respectively). Such a filter is brought in the optical path of the instrument by means of the shutter. 

Holmium oxide and Easy to use Glass filter absorbs strongly... 

The wavelength reading of a spectrophotometer can be checked using solutions of known absorbance maxima and rniiiima. Potassium dichromate at pH 2.9 has maximum absorbances at 257 and 350 nm, and minima at 235 and 313 nm. A holmium oxide glass filter absorbs sharply at 279.2,222.8,385.8,446.0,536.4, and 637.5 nm. 

Works a holmium oxide glass filter (No. CS 3-138) which exhibits several sharp absorption bands in the region 240-2000 m/i. Those near 241, 280, 288 and 360 m/i are particularly suitable for wavelength calibration. The Bureau of Standards will certify such a filter or one can quickly calibrate it himself using a mercury arc. 

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