1. SPECTROPHOTOMETRY IN BIOTECHNOLOGY1

2. LIGHT IS A TYPE OF ELECTROMAGNETIC RADIATION
Imagine electromagnetic radiation like waves on a pond
But instead of water, electromagnetic radiation is energy moving through space
Distance from one crest to the next is the wavelength

3. WAVELENGTH AND COLOR
Different wavelengths of light correspond to different colors
All colors blended together is called white light
The absence of all light is black
Light of slightly shorter wavelengths is ultraviolet
Eyes do not perceive UV light

5. WAVELENGTH OF VISIBLE LIGHT AND COLOR
COLOR PERCEIVED
Violet
Blue
Greenish Blue
Bluish Green
Green
Yellowish Green
Yellow
Orange
Red

6. INTERACTION OF LIGHT WITH MATERIALS IN SOLUTION
When light shines on a solution, it may pass through – be transmitted OR
Some or all of the light energy may be absorbed

7. THE ABSORPTION OF LIGHT AND COLOR OF SOLUTIONS
WAVELENGTH OF LIGHT ABSORBED
COLOR OF LIGHT ABSORBED
COLOR OF SOLUTION
Violet
Yellow
Blue
Orange
Greenish Blue
Red-Orange
Bluish Green
Orange-Red
Green
Red
Yellowish Green
Violet-Red

8. BIOLOGICAL SOLUTIONS Usually appear clear to our eyes – have no color
DNA, RNA, most proteins do not absorb any visible light
But they do absorb UV light, so UV spectrophotometers are useful to biologists
Example, can use a detector that measures absorbance at 280 nm, or 254 nm to detect proteins

9. SPECTROPHOTOMETERS
Are instruments that measure the interaction of light with materials in solution

11. THE BLANK
Spectrophotometers compare the light transmitted through a sample to the light transmitted through a blank.
The blank is treated just like the sample
The blank contains everything except the analyte (the material of interest)
Contains solvent
Contains whatever reagents are added to the sample

13. WHEN OPERATING A SPEC Blank is inserted into the spectrophotometer
Instrument is set to 100% transmittance or zero absorbance

14. PROPER SELECTION, USE, AND CARE OF CUVETTES
Cuvettes are made from plastic, glass, or quartz.
Use quartz cuvettes for UV work.
Glass, plastic or quartz are acceptable visible work.
There are inexpensive plastic cuvettes that may be suitable for some UV work.

16. Use them properly and carefully
Cuvettes are expensive and fragile (except for “disposable” plastic ones).
Use them properly and carefully
a. Do not scratch cuvettes; do not store them in wire racks or clean with brushes or abrasives
b. Do not allow samples to sit in a cuvette for a long period of time
c. Wash cuvettes immediately after use

17. Disposable cuvettes are often recommended for colorimetric protein assays, since dyes used for proteins tend to stain cuvettes and are difficult to remove.
Matched cuvettes are manufactured to absorb light identically so that one of the pair can be used for the sample and the other for the blank.
Do not touch the base of a cuvette or the sides through which light is directed.
Make sure the cuvette is properly aligned in the spectrophotometer.
Be certain to only use clean cuvettes.

20. EXAMPLES Some examples of qualitative spectrophotometry
The absorbance spectra of various common solvents. Note that some solvents absorb light at the same wavelengths as DNA, RNA, and proteins
Hemoglobin bound to oxygen versus carbon monoxide
Native versus denatured bovine serum albumin (a protein commonly used in the lab)

22. OVERVIEW OF QUANTITIVE SPECTROPHOTOMETRY
Measure the absorbance of standards containing known concentrations of the analyte
Plot a standard curve with absorbance on the X axis and analyte concentration on the Y axis
Measure the absorbance of the unknown(s)
Determine the concentration of material of interest in the unknowns based on the standard curve

24. LINEAR RANGE
If there is too much or too little analyte, spectrophotometer cannot read the absorbance accurately

25. COLORIMETRIC ASSAYS
Quantitative assays of materials that do not intrinsically absorb visible light
Combine the sample with reagents that make the analyte colored
The amount of color is proportional to the amount of analyte present

26. BRADFORD PROTEIN ASSAY
A quantitative colorimetric assay
Used to determine the concentration, or amount, of protein in a sample

27. Running a Protein Assay
Prepare standards with known protein concentrations
Add Bradford Reagent to the samples and to standards
Read absorbances
Create a standard curve
Determine the concentration of protein in the samples based on the standard curve

28. MORE ABOUT THE CALIBRATION LINE ON A STANDARD CURVE
Three things determine the absorbance of a sample:
The concentration of analyte in the sample
The path length through the cuvette
The intrinsic ability of the analyte to absorb light at the wavelength of interest

29. BEER-LAMBERT LAW A =  B C Where:
A = absorbance at a particular wavelength
 = E = absorptivity constant – intrinsic ability of analyte to absorb light at a particular wavelength
B = path length through cuvette
C = concentration of analyte

30. UV METHODS
These UV methods for estimating concentration and purity of DNA, RNA, and proteins are very commonly used, are very quick, and easy to perform
However, they values obtained are not very accurate – they are rough estimates

31. CALIBRATION OF A SPECTROPHOTOMETER
Brings the readings of the spectrophotometer into accordance with nationally accepted values
Part of routine quality control/maintenance

32. CALIBRATION Two parts:
Wavelength accuracy, the agreement between the wavelength selected by the operator and the actual wavelength of light that shines on sample
2. Photometric accuracy, or absorbance scale accuracy, the extent to which a measured absorbance or transmittance value agrees with an accepted reference value

33. Wavelength accuracy is determined using certified standard reference materials (SRMs) available from NIST or traceable to NIST
An absorbance spectrum for the reference material is prepared
The absorbance peaks for reference standards are known, so the wavelengths of the peaks generated by the instrument can be checked

34. Manufacturers specify the wavelength accuracy of a given instrument
For example, a high performance instrument may be specified to have a wavelength accuracy with a tolerance of nm
A less expensive instrument may be specified to have a wavelength accuracy of + 3 nm

35. PHOTOMETRIC ACCURACY Assures that:
If the absorbance of a given sample is measured in two spectrophotometers at the same wavelength and under identical conditions
then the readings will be the same
and the readings will correspond to nationally accepted values

36. Photometric accuracy is difficult to achieve due to different instrument designs and optics
Usually photometric accuracy is not critical if the same instrument is used consistently and if its readings are linear and reproducible
Photometric accuracy is required where values from different labs and instruments are compared
Required if rely on published absorptivity constants
Likely required in a GMP-compliant facility