XI. USING A SPECTROPHOTOMETER
A spectrophotometer is primarily used to identify substances and determine their concentration. "Spectro" refers to the "spectrum" of wavelengths that make up both visible light and electromagnetic radiation above or below this range. The visible range is 0.0004mm (= 400 nanometers = 4,000 Angstroms) - violet - to 0.00076mm (= 760 nanometers = 7,600 Angstroms) - red. Immediately above this range is the "infrared" region of longer wavelengths, while below it is the shorter wavelength "ultraviolet" region. Ordinary light, such as light from the sun, consists of a mixture of wavelengths that is perceived by the human eye and described as "white light". However, when the component wavelengths are spread out in increasing order, the resulting "spectrum" is perceived as an orderly succession of red, orange, yellow, green, blue, indigo, and violet.
The "photo" in "spectrophotometer" refers to light (or to "photons" of light energy), and "meter" means a device capable of quantitative measurement. Putting that all together, a "spectrophotometer" is a device capable of making quantitative measurements of light intensity at selected wavelengths. Its components include a light source or sources, devices (sometimes incorporating filters) for generating beams of light of specific wavelengths from that source, special holding devices for tubes or "cuvettes" containing samples through which light beams are passed, and detectors that can measure the intensity of light that has passed through a sample.
Some spectrophotometers used in bioresearch are designed for analysis of both ultraviolet (UV) and visible (VIS) regions of the spectrum, while others are VIS range only. The former are more useful because many biomolecules of interest, such as nucleic acids and proteins, absorb wavelengths in the UV range. They are correspondingly more expensive instruments, however. You will be using a Tech Facility VIS-only spectrophotometer. However, if you use a spectrophotometer in a research laboratory, it is likely that it will be UV-VIS.
Identification of compounds: When matter interacts with energy, energy is frequently absorbed. Different types of chemical compounds have distinctive atomic and molecular structure, and will absorb energy at characteristic energy levels and wavelengths. The absorbed energy can result in changes in the energy level of electrons in the atoms comprising a molecule, as well as in vibration and/or rotation of those atoms. In some cases, absorbed light energy is re-emitted as light of lower energy and longer wavelength. This phenomenon is known as "fluorescence". An unknown substance can often be identified by comparing the amount of light of different wavelengths that it absorbs, i.e., its absorption spectrum, with the absorption spectra of known substances. Such an analysis is performed using a spectrophotometer to measure the amount of light absorbed over a range of wavelengths of incident light.
A solution looks colored to the human eye because molecules are absorbing wavelengths corresponding to the other colors. It appears as a particular color because it is NOT absorbing the wavelengths of that color; rather, they are transmitted to your eye. For example, a compound appearing blue in solution is transmitting a range of 435-480 nm wavelengths to the eye, and is typically absorbing at 580-595 (yellow). A compound appearing red is typically transmitting in a range of 610-750 nm, and absorbing in the 595-610 nm range (bluish-green).
Concentration determination: The concentration of an unknown sample can also be determined using a spectrophotometer. When a light beam is passed through a sample, part of it is absorbed, and part is transmitted through the sample. To quantify this effect, the transmittance (T) is defined as the intensity of the transmitted light (It) divided by the original (or incident) intensity of the light beam (Io). In abbreviated notation, T = It/Io.
The absorbance (A) is defined as the log10 of the reciprocal of the transmittance, i.e., A = log (1/T). This is a measure of how much light is absorbed by the sample, rather than transmitted through it. Knowing one (A or T) it is easy to calculate the other.
There is a relationship between concentration and absorbance; the higher the concentration of a substance in a solution, the greater the amount of light it will absorb, assuming that the wavelength is an absorbable one for the substance. You will determine the relationship between concentration and absorbance in Exercise #5, below. Since a spectrophotometer measures absorbance (or transmittance, from which absorbance can be calculated), the spectrophotometer can be used to determine an unknown concentration of a substance by comparison with a "Standard Curve", i.e., a graph showing the absorbance produced by various known concentrations of that same substance.
Go to the spectrophotometer and examine it for the following components, reviewing their functions:
1. TRANS/ABS button Selects between Transmittance and
Absorbance modes (measurements).
2. 100%T/0A button Sets the instrument to 100%
Transmittance or zero absorbance.
3. FUNCTION button Directs the output (reading) to
RS-232C (computer) or Printer interface.
4. DISPLAY panel Displays %T or ABS value
5. WAVELENGTH dial Sets the desired wavelength.
6. WAVELENGTH display Displays the wavelength.
7. SAMPLE COMPARTMENT Holds samples in round "cuvettes" or
small test tubes.
8. SAMPLE TUBES Contain samples for analysis (these are in
drawer or box near spectrophotometer).
OPERATION - GENERAL STEPS
Review the following information on operation of the spectrophotometer before proceeding to the next exercise. Do not actually carry out any of these steps at this time.
1. The spectrophotometer should already be plugged in. If not, plug one end the power cord into a wall outlet, the other end into rear of spectrophotometer.
2. Turn on the spectrophotometer and allow it to warm up for at least twenty minutes to stabilize the electronic components.
3. Prepare a "blank" tube. The blank is used as a control. Therefore it should contain exactly the same medium or media in the same amounts as used to prepare your samples, except that it contains none of the compound or substance under study. For example, if your sample is prepared in buffer, and you need 1ml of this sample to measure absorbance then the blank small test tube should be filled with the 1ml same buffer. If your sample is in deionized water, the blank should also be filled with deionized water.
4. Open the sample compartment and place the blank into it.
5. Turn the wavelength dial to the desired wavelength.
6. Set the filter selector located in the sample compartment to the appropriate position. Follow directions on inner panel of the sample compartment. For example, if you have set the wavelength to 560 nm, set the filter selector to position 3. Close the sample compartment. Remember there are different filter settings for different wavelength ranges. Don't forget to switch the setting when appropriate!
7. Select transmittance or absorbance mode by pressing the TRANS/ABS button. When spectrophotometer first is turned on it is set to transmittance mode (note that on DISPLAY you see red light only under TRANS, this means that transmittance of the sample is being measured). When you press TRANS/ABS button once red light goes on under ABS sign on display, and this means that now spectrophotometer is working in absorbance mode. If you press TRANS/ABS button again spectrophotometer switches to transmittance mode.
8. Press the 100% T/0A button. The display should show 100.0 for TRANS mode, or 0.000 for ABS mode.
9. Carefully open the sample compartment and remove the test tube, placing it in test tube rack nearby.
10. Fill a clean small test tube with the solution to be measured. Place the small test tube into the compartment and close it. The display shows the transmittance or absorbance immediately. Record the value(s) and remove the test tubes.
11. Repeat steps 9 and 10 for more samples to be tested at the same wavelength. For different wavelengths, repeat steps 4 through 10.
12. Remove sample tube and turn off the power after the experiment is completed. Small test tubes are disposable. Dispose of them properly into the container "Sharps Only". Do you know why test tubes which are not broken are put into the container "Sharps Only"? To review rules on disposing see module II, page 5.
13. Be sure to clean up and return all items to their original locations before leaving!
These exercises will help you to learn the basics involved in using a spectrophotometer. Read and follow the instructions carefully.
EXERCISE #2: MEASURING THE ABSORBANCE OF A COLORED COMPOUND
In this exercise, you will determine the absorption (A) and transmittance (T) of phenol-red solution in phosphate buffer, pH 7.4. Phenol-red is commonly used as a pH indicator in cell culture media, because it changes color when the pH of the medium changes, warning the investigator that the culture conditions may be less than optimal. It does not appear to interfere with growth or other functions of living cells.
1. Obtain the following:
(a) 2 empty clean test tubes from the "small test tubes" container near the spectrophotometer,
(b) one frozen tube of "stock solution" of phenol red solution from plastic container "in Tech Facility freezer. Thaw the frozen tube by letting it stand in a small beaker of warm water. Label this tube with your initials.
(c) (from refrigerator) Phosphate buffer pH 7.4 with your initials which you made in module IX exercise 6.
Place all tubes in a test tube rack near the spectrophotometer.
2. Prepare 1 ml of a 1/100 dilution of the stock solution of phenol-red, using phosphate buffer (pH 7.4) as a diluent:
(a) Into one test tube pipette 0.99 ml (= 990 ul) buffer, using a "P-1000" Pipetman pipettor;
(b) Add 10ul of stock phenol red solution, using a "P-20" Pipetman pipettor.
(c) Label the tube "1/100 phenol red, pH 7.4"
and put your initials on it. Do you know where the volumes 990ul and 10 ul came from? To review dilutions see module VII; to review how to use a Pipetman see module V, exercise 4.
3. Follow the instructions on how to operate spectrophotometer.
4. Use 1 ml of phosphate buffer (pH 7.4) in the "blank" tube. Do you know why buffer and not deionized water or something else is used in this case? If not, go back and re-read the "OPERATION" instructions, step 3.
6. Set the wavelength at 600 nm. This wavelength is known to be absorbed by phenol-red.
7. Replace blank with new tube containing 1 ml 1/100 dilution of phenol-red stock solution.
8. Measure the A and T of your sample and record your data in your Tech Facility notebook.
EXERCISE #3: DETERMINING AN ABSORPTION SPECTRUM.
In this exercise, you will use the spectrophotometer to:
(a) determine the absorption spectrum of 1/100 dilution of phenol-red stock solution; (in Phosphate Buffer pH 7.4)
(b) draw a curve (graph) showing the absorption spectrum;
(c) use the curve to predict the absorption of phenol-red at a particular wavelength.
1. Obtain the following:
(a) 2 empty clean test tubes from the "small test tubes" container near the spectrophotometer,
(b) one frozen tube of 1/100 dilution of phenol red (in Phosphate Buffer pH 7.4) solution with your initials from Tech Facility freezer.
(c) Phosphate buffer pH 7.4 with your initials. Thaw the frozen tube by letting it stand in a small beaker of warm water. Place all tubes in a test tube rack near the spectrophotometer.
2. Follow the instructions on how to operate the spectrophotometer.
3. Use 1 ml of phosphate buffer (pH 7.4) in the "blank" tube.
4. Set the wavelength to 460-nm
5. Replace blank with new tube containing 1 ml 1/100 dilution of phenol-red dye solution and measure its absorption at 460-nm.
6. Re-insert the blank into the sample compartment. Increase the wavelength by 10 nm (to 470-nm). Re-zero it by pressing 100% T/0A button.
7. Repeat steps 5 and 6 for every 10-nm up to and including 620-nm. Don't forget to set the filter selector to the right position for the particular wavelength used (see instructions on how to operate spectrophotometer, step 6).
8. Record your data in your Tech Facility notebook.
9. Return Phosphate Buffer to the refrigerator, dispose 1/100
dilution of phenol-red solution pH 7.4 by pouring it directly into the drain and clean the sink by rinsing it with tap water. Dispose test tube to proper container and make sure that your working area is cleaned up.
10. Using a piece of graph paper, draw a graph of the Absorption Spectrum. Label the Y axis Absorbance, and the X axis Wavelength. Plot your data.
11. Use your graph to answer the following questions:
a) What absorbance values would be expected for your sample at wavelengths 475, 505, and 605-nm?
b) At what wavelengths would the absorbance of the sample be 0.62, 1.25, 1.43, 1.78?
EXERCISE #4: CHANGING THE ABSORPTION SPECTRUM BY CHANGING THE PH OF THE PHENOL-RED INDICATOR DYE SOLUTION.
As noted above, phenol-red is a pH indicator; it is red at pH 7 but turns orange-yellow at lower pH. In this exercise, you will determine how such a change in pH affects the absorption spectrum by (a) measuring the absorption spectrum of phenol-red in phosphate buffer pH 6.2 (which you prepare yourself); (b) preparing a graph showing the absorption spectrum at pH 6.2; and (c) comparing the absorption spectra of phenol-red at pH 7.4 and pH 6.2.
1. Obtain the following: 2 empty clean test tubes from the small test tubes container near spectrophotometer, + one frozen tube of "stock solution" of phenol-red with your initials from the freezer. Also obtain from refrigerator Phosphate Buffer solution pH 6.2 which you made previously in module IX, exercise 6.
2. Prepare a 1/100 dilution of phenol-red stock solution, following the instructions in step 2, Exercise #2. However, this time use Phosphate buffer pH 6.2 as a diluent.
3. Prepare a blank sample. What should it contain?
4. Repeat EXERCISE #3, including the absorption spectrum graph. In your Tech Facility lab notebook, describe how pH can affect the absorption spectrum of a compound such as phenol red. What wavelengths are absorbed most heavily when the compound appears red to the human eye? What wavelengths are absorbed most heavily when the compound appears yellow? On the basis of your results, try to explain why phenol red, at pH 7.4, looks red in white or incandescent room light. What wavelengths has it effectively prevented from reaching your eye?
5. Return stock solution of phenol-red to the freezer and Phosphate Buffer to refrigerator. Dispose 1/100 dilution of phenol-red solution pH 6.2 by pouring in directly into the drain, and clean the sink. Dispose small rest tubes and clean the area.
EXERCISE #5: ABSORBANCE VERSUS CONCENTRATION.
As noted above, the absorbance of a compound in solution at a given wavelength is a function of its concentration, so that the spectrophotometer can be used to determine the concentration of compounds. In this exercise you will:
(a) Measure the absorption of different dilutions of the phenol-red solution.
(b) Produce a graph, known as a Standard Curve, of Absorbance verses Concentration for phenol red.
(c) Use the Standard Curve to determine the concentration of an unknown sample of phenol red.
(d) Produce data illustrating "Beer's Law", which states the relationship between the absorbance and the concentration of a solution.
1. Obtain the following:
(a) 1 frozen tube of the original stock phenol-red dye solution (in Phosphate Buffer at pH 7.4) with your initials on it from the freezer. We will call the concentration of phenol-red in this tube "X" g/ml.
(b) 8 small test tubes from the test tube box near the spectrophotometer;
(c) from the plastic container labeled "unknown concentration of phenol-red solution" in the freezer a 1.5 ml microfuge tube labeled "1 ml phenol-red unknown", i/e/. a solution of unknown concentration.
2. Assume that the concentration of phenol-red stock solution is "X" g/ml. For this exercise you will need to make different dilutions of phenol-red using original stock solution. You will prepare 1 ml of each of the following dilutions of phenol-red stock solution : 0.8X, 0.6X, 0.4X, 0.2X and 0X (= no phenol red). This means that 0.8X tube will contain only 8/10 of the original 1X concentration of phenol-red stock solution. Example: To prepare 0.8X you will need 0.8 ml of original 1X stock solution and 0.2 ml of Phosphate buffer, pH 7.4.
Prepare an ingredient table in your lab notebook showing dilution factors, volume of stock solution used, volume of diluent used for all of the remaining dilutions (to review dilutions see the Tech Facility workbook, module VII). Use the Phosphate buffer, pH 7.4, as a blank tube (it will be your "0X" tube) and as the diluent for all tubes.
3. Set wavelength dial to 615-nm, following procedure as presented under OPERATION, above.
4. Record reading of a blank as your 0X; It should read 0.
5. Measure the absorbance of the different dilutions starting with absorbance of 0.2X dilution.
6. Insert blank tube into sample compartment. Make sure that it reads zero, and, if not, re-zero it (press 100 T/OA button).
7. Repeat steps 6-7 to measure the absorbance values for 0.4X, 0.6X, 0.8X, 1X, and for sample labeled "unknown concentration".
8. Dispose any left over stock solution of phenol-red you may dispose it by pouring in directly in the sink. Make sure that sink is cleaned up. Also dispose both Phosphate Buffers pH 6.2 7.4 by pouring them to the drain. Wash the plastic reagent bottles that were used for storing these buffers. Dispose centrifuge tube that was used for storing of phenol - red solution by placing it in the Bio Hazardous waste container.
9. Draw a Standard Curve on graph paper by plotting Absorbance (Y axis) versus Concentration (X axis). Note that a standard "curve" can, in fact, not be curved! It is usually a straight line!
10. Using your Standard Curve, determine the concentration of phenol red in the sample with unknown concentration.
11. What is the general relationship between concentration and absorbance for this and other compounds? This is known as Beer's Law.
EXERCISE #6: Protein assay.
In this exercise, you will use the spectrophotometer to determine the concentration of protein in standard and unknown solutions. The method employed is known as the "Bradford" protein assay, after its inventor. It is based on the ability of a particular dye to bind to protein, plus a change in the absorption spectrum of the dye that occurs as it binds.
A "dye" can be defined as a chemical compound that has two properties: (a) visible color, and (b) the ability to bind (attach to) other kinds of compounds. In the case of the Bradford assay, the dye known as Coomassie blue, under acidic conditions, changes its absorption spectrum so that it will absorb well at 595 nm after binding, but not before. The amount of bound dye, as detected by absorbance at 595 nm, is proportional to the amount of protein present. Therefore, a standard curve showing absorbance of the dye versus known protein concentrations can be used to determine the concentration of an unknown protein solution from its bound dye absorbance.
In this exercise you will:
(a) Prepare known protein samples by dilution of stock protein.
(b) Measure their absorbance at 595 nm
(c) Plot a Standard Curve
(d) Determine the concentration of unknown protein sample by using the Standard Curve.
PROCEDURE (wearing protective gloves and eye wear!):
1. Obtain the following: 6 small test tubes from small test tubes container near the spectrophotometer, frozen 1.5 ml microfuge tube containing 250 um of 2 mg/ml BSA in water (Bovine Serum Albumin, the protein used as a standard) (from plastic container labeled "Stock solution of BSA" in lab freezer), and a 50 ml centrifuge test tube containing 15 ml of small of the Coomassie Protein Assay Reagent from a plastic container labeled "Coomassie Protein Assay Reagent".
2. Prepare 0.1 ml of each of the following dilutions of the stock 2mg/ml BSA: 0, 0.5, 1.0, 1.5, 2.0 mg/ml, using deionized water as a diluent. Note that 0 mg/ml has no protein in it and this tube will serve as your blank tube. Use the Pipetman micropipettors, and follow an "ingredients table" written in advance into your Tech Facility notebook. To review dilutions see module IX.
3. Prepare an additional tube containing 0.1 ml of an unknown concentration of BSA (obtain 1.5 ml microfuge tube containing 0.1 ml of unknown concentration of BSA from plastic container labeled "Unknown concentration of BSA stock solution" from the freezer and thaw it then using plastic transfer pipet transfer it into small test tube which you should label "Bradford Unknown".
4. Set wavelength to 595 nm. All measurements will be made at this wavelength.
5. Add 3.0 ml of Protein Assay Reagent** into all sample tubes including Blank. Also add 3.0 ml of Protein Assay Reagent into the unknown tube.
6. Insert Blank into sample compartment and press 100%T/OA button. This will set the absorbance value for 0 mg/ml to zero.
the Bradford Assay Reagent contains phosphoric acid to achieve low pH; handle/pipette it with care!
7. Read and record absorbance for the 0.5 mg/ml sample.
8. Repeat steps 6-7 for rest of the samples including sample of unknown concentration.
9. Plot a Standard Curve of Absorbance (Y axis) versus Concentration (X axis).
10. Using your Standard Curve, determine the concentration of the unknown sample from its absorbance.
11. Dispose all samples by pouring them directly into drain and clean the sink. Dispose small test tubes properly. Return test tube racks and other equipment to original module location, so that it is ready for use by someone else. Make sure that the area were you worked is cleaned up and ready for use by other students.