Instrument band-spreading

The instrument band-spreading (ITec) can be measured by replacing the column with a zero dead-volume restrictor, injecting a very small volume of sample, and measuring the resulting peak width. Table 17-5 provides band-... 

Figure 23-19 Band spreading from multiple flow paths. The smaller the stationary phase particles, the less serious this problem is. This process is absent in an open tubular column. [Adapted from H m. McNair and E. J. Bonelli, Basic Gas Chromatography (Palo Alto. CA Varlan Instrument Division. 1968).]...

On the other hand, the capability of sample preconcentration for instruments such as AAS, ICP-AES, ICP-MS, and so forth was studied [3]. After metal ions were enriched, they were eluted almost simultaneously by inorganic acid at low pH, because of their diffusion in the column is at a disadvantage for improvement of the detection limits. It has been demonstrated that metal ions such as Ca, Cd, Mg, Mn, Pb, and Zn were enriched with a good recovery at a concentration of 10 ppb each in 500 mL of the sample solution. However, the final enriched sample volume eluted from the CCC column was as large as several milliliters, due to longitudinal diffusion of the sample band in the retained stationary phase [1,3]. Additional band spreading occurred in the flow tube when the concentrated solution was eluted with an acid solution for subsequent analysis. 

This means that the contribution of a 30-cm section of 0.2S inni-Ld. tubing contributes a standard deviation of about 8.5/iL to the extracolumn band spreading. One can see that it pays to reduce the length of connection tubing. A length of only 30 on is actu y quite good for a typical HPLC instrument. 

The problem is to convert the output chromatogram, F(V to the associated number density distribution N D X via eqn (10.5), given information on the signal characteristics C(y), and the instrumental or band-spreading behaviour... 

Brief reflection on the sampling theorem (Chapter 1, Section IV.C) with the aid of the Fourier transform directory (Chapter 1, Fig. 2) leads to the conclusion that the Rayleigh distance is precisely two times the Nyquist interval. We may therefore easily specify the sample density required to recover all the information in a spectrum obtained from a band-limiting instrument with a sine-squared spread function evenly spaced samples must be selected so that four data points would cover the interval between the first zeros on either side of the spread function s central maximum. In practice, it is often advantageous to place samples somewhat closer together. 

Spectroscopic instruments in the UV and visible regions are usually equipped with one or more devices to restrict the radiation being measured to a narrow band that is absorbed or emitted by the analyte. Such devices greatly enhance both the selectivity and the sensitivity of an instrument. In addition, for absorption measurements—as we saw in Section 24C-2—narrow bands of radiation greatly diminish the chance of Beer s law deviations due to polychromatic radiation. Many instruments use a monochromator or filter to isolate the desired wavelength band so that only the band of interest is detected and measured. Others use a spectrograph to spread out, or disperse, the wavelengths so that they can be detected with a multichannel detector. 

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