Question

1. what is the best wavelength to use to examine the smallest visible samples? Why?

2. If the resolution of an FTIR depends on the distance the mirror travels, why are relatively long moving distances such as 2 or 4 cm not used? What is the trade-off?

3. Which of the three contributing factors to band broadening in chromatography is independend of the flow rate? Explain why.

Answer 1

I will answer question 1, question 2 and 3, post them in another post cause I'm unsure in the answer for those:

Using the maximum wavelength gives us the best results. This is because at the peak absorbance, the absobance strength of light will be at the highest and rate of change in absorbance with wavelength will be the smallest. Measurements made at the peak absorbance will have the smallest error.

It really depends on what is the largest source of error. Taking the readings at the peak maximum is best at low absorbance, because it gives the best signal-to-noise ratio, which improves the precision of measurement. If the dominant source of noise is photon noise, the precision of absorbance measurement is theoretically best when the absorbance is near 1.0. So if the peak absorbance is below 1.0, then using the peak wavelength is best, but if the peak absorbance is well above 1.0, you might be better off using another wavelength where the absorbance is closer to 1. Another issue is calibration curve non-linearity, which can result in curve-fitting errors. The non-linearity caused by polychromatic light is minimized if you take readings at either a peak maximum or a minimum, because the absorbance change with wavelength is the smallest at those wavelengths. On the other hand, using the maximum increases the calibration curve non-linearity caused by stray light. Very high absorbances cause two problems: the precision of measurement is poor because the transmitted intensity is so low, and the calibration curve linearity is poor due to stray light. The effect of stray light can be reduced by taking the readings at awavelength where the absorbance is lower or by using a non-linear calibration curve fitting technique. Finally, if spectral interferences are a problem, the best measurement wavelength may be the one that minimizes the relative contribution of spectral interferences (which may or may not be the peak maximum). In any case, don't forget: whatever wavelength you use, you have to use the exact samewavelength for all the standards and samples.

Hope this helps