A spectrophotometer measures the transmittance or the absorbance, or both, of a particular wavelength of light after it has passed through a liquid sample. Before the transmittance or absorbance of the sample is measured, a cuvette filled only with solvent, called the blank, is placed in the spectrophotometer. Select the following step that is NOT necessary to complete before a cuvette is placed into the spectrophotometer a. Inspect the cuvette for any scratches or cracks. If present, replace the cuvette. b. Write, in ink, either sample or blank on the side of the cuvette to keep track of them c. Make sure the cuvette is oriented in the spectrophotometer in the same way each time it is used. d. Make sure the cuvette is filled only 3/4 full to prevent spilling of the sample.

The step that is NOT necessary to complete before a cuvette is placed into the spectrophotometer is option B (Write, in ink, either sample or blank on the side of the cuvette to keep track of them)

Explanation: spectrophotometer is an instrument used to measure the light intensity absorbed after being passed through a solution. Before the absorbance of the sample solution, a solvent solution called blank is used for the calibration of the machine and this blank solvent is placed in a cuvette. The procedure usually comes first before the main sample is processed. Therefore there is no need to

Write, in ink, either sample or blank on the side of the cuvette to keep track of them. This is so since sample and blank is not absorbed at the same time by the machine.

The correct answer is:

Write in ink, either sample or blank on the side of the cuvette to keep track of them (b)

Explanation:

Measurement of absorbance or transmittance is a colorimetric measurement, where the amount of light absorbed by the particles in a solution, or the amount of light allowed to pass through the solution are respectively measured, and a spectrophotometer is used to measure either the absorbance or the transmittance.

Now let me explain why the selected answer is not a necessary step in measuring with the spectrophotometer.

Cuvettes are glass or plastic transparent vessels sealed at one end used to transfer the sample or blank to be measured into the spectrophotometer. It is not necessary to write in ink (either blank or sample) on the cuvette because the same cuvette is used for transferring both the sample and the blank, this is to maintain uniformity in all the measurements, because using different cuvettes can cause biases as a result of differences in the various cuvettes, so it is completely unneccesary to label cuvettes as sample or blank cuvettes because one cuvette is used for both the sample and the blank. The blank is measured first and the spectrophotometer is zeroed, the cuvette is rinsed, and the sample is transferred to the same cuvette for measurement.

Now let me explain why the other choices are necessary steps in using a cuvette for measurement.

a. Inspect the cuvette for any scratches or cracks. If present, replace the cuvette; if there are cracks on the cuvette, the liquid (sample or blank) will leak out of the cuvette, which will make measurement wrong, because an empty cuvette might end up being measured after leakage, and if there are scratches on the sides of the cuvette, there is a tendency that the cuvette will scatter light when placed in the light path that will negatively affect absorbance or transmittance.

c. Make sure the cuvette is oriented in the spectrophotometer in the same way each time it is used; the major aim of the colorimetric measuremnt is aimed measuring only how the solutes of interest in the sample affect the absorbance or transmittance or light, hence any other cause of difference is reduced as much as possible because they can interfere with the light path. Using the same orientation for every measurement helps to maintain uniformity in the measurement. This is especially important in cuvette designs that have rough sides for handing and smooth sides for contact with the light path, if these sides are swapped for each subsequent measurement, the resultant readings will be unreliable.

d. Make sure the cuvette is filled only 3/4 full to prevent spilling of the sample; if the cuvettes are filled to greater than 3/4 of its size, there is a tendency of spillage into the measuring space of the spectrophotometer, this is not only detrimental to the health of the machine, but upon subsequent measurement, the spilled liquid can rub on the sides of the cuvette which will negatively affect reading.