UV/VIS Spectroscopy in Microplates
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UV/VIS Spectroscopy in Microplates
Authored by: Greiner Bio-One
Originally published as an application note by Greiner Bio-One. Republished with permission.
Contents |
Introduction to UV/VIS spectroscopy
Transparent microplates (CELLSTAR®, MICROLON®) or microplates with a clear film bottom (μClear®, UV-Star®) are frequently used in ultraviolet-visible spectroscopy (UV/VIS). UV/VIS is a subclass of spectroscopy which uses visible light and adjacent near ultraviolet (UV) ranges for the determination of the concentrations and the characterisation of dissolved substances.
Most dissolved molecules absorb light in the visible or UV range at specific wavelengths. The absorption of light (Fig. 1) is a physical process where the amount of absorbed light depends on the concentration of the substance c, the thickness of the liquid layer d and a specific absorption coefficient α at a defined wavelength λ.
Different terms define the process of light absorbance. For a better overview the most frequently used terms are listed and defined in the following section.
Definition of different terms in UV/VIS spectroscopy
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| Figure 1: Principle of UV/VIS spectroscopy | |
Transmission and transmittance are often used as synonyms. In spectroscopy the physical phenomenon of light transmission through a sample can be described by the fraction of light at a specific wavelength λ that passes through the sample. If I0 is the intensity of the incident light and I1 the intensity of the transmitted light, then the transmittance T can be defined as:
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In parallel, the transmittance can be described by the light absorbance Aλ or optical density ODλ:
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The higher the optical density of the sample, the higher the absorption and the lower is the transmission of light. Often the transmission is given as a percentage (Fig. 2a). Although the absorbance or optical density does not have true units, it is customarily reported in absorbance units (AU) (Fig. 2b). The absorbance or optical density depends on the wavelength λ and the pathlength d (Fig. 1). In order to be able to compare measurements the optical density is often calibrated to a defined pathlength, typically 1 cm. Optical densities above 1.0 should be avoided in a sensitive measurement as even large differences in the optical density readouts represent only minor modifications in the transmission (Table 1). It is recommended to dilute samples with a high optical density in order to achieve results below 1.0.
| Optical density | Transmission |
| 1.5 | 3 % |
| 1.7 | 2 % |
| 2.0 | 1 % |
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Optical background of microplates in transmission measurements
As evidenced in figures 2a and 2b, the high transmission of standard polystyrene microplates drops off sharply at approximately 400 nm, whereas μClear® plates extend detection capabilities down to 340 nm. UV-Star® Microplates demonstrate the highest transmission within the lower UV range down to 230 nm. Conversely, optical density increases proportionally in accordance with transmittance wavelength values indicated for the corresponding microplate type.
Lambert-Beer’s Law
The absorption of light in a sample solution is dependent on the concentration c of the dissolved molecule, the specific molar extinction coefficient α at a defined wavelength λ and the pathlength d. With known extinction coefficient and pathlength, the concentration of a sample can be determined by using the Lambert-Beer’s equation (3). As well, Lambert-Beer’s law can be used to determine the molar extinction coefficient (4).
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An example for the determination of the molar extinction coefficient of adenosine 5’-tri-phosphate (ATP) is given in example 1.
Example 1: Determination of the molar extinction coefficient of ATP
ATP was solved in water at pH 7 and measured at 260 nm[1]. The OD260 of different ATP concentrations was determined and calibration curves for UV-Star® Microplates and cuvettes were generated (Fig. 3). The absorbance (A260) of a 30 μM/l ATP solution (c) was 0.407. The pathlength d of the sample was 1 cm. Based on the data the molar extinction coefficient was calculated using equation (4).
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UV/VIS of nucleic acids and proteins
The optical density of biomolecules such as DNA, RNA and proteins is influenced by pH, temperature and salt concentration[2]. The absorbance of a solution decreases with increasing ionic strength, e.g. DNA has approximately 23 % lower absorbance when dissolved in buffered saline rather than water[3]. Similarly, exact molar extinction coefficients are dependent upon the nucleotide sequence for nucleic acids or the amino acid type and amino acid sequence in peptides or proteins. Therefore molar extinction coefficients αλ are not commonly applied in molecular biology to determine biomolecular concentrations. Instead scientists prefer to use empirically determined standard coefficients sc (Table 2; Equation 5). Because these standard coefficients are derived from optical density measurements at 1 cm, they are only valid if the pathlength used for the sample measurement is also 1 cm. For instance, if the optical density of a double stranded DNA sample with 50 μg DNA/ml water is equal to one absorbance unit at 1 cm pathlength, then the standard coefficient of double stranded DNA is 50 μg/ml, also at 1 cm pathlength.
Concentration of sample = OD • sc • f (5)
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Example 2 shows the calculation of the concentration of an aqueous double stranded DNA sample applying the standard coefficient and the appropriate equation (5).
Example 2:
An aqueous sample of genomic DNA gave an optical density of 0.8 AU. The sample was diluted 1:25 to achieve an OD below 1.0. The measurement was done in a cuvette with a pathlength of 1 cm. With the standard coefficient of double stranded DNA (50 μg/ml) the concentration can be calculated using equation (5).
| Concentration of sample = OD • sc • f (5) = 0.8 • 50 μg/ml • 25 = 1000 μg/ml |
| Biomolecule | λ | Pathlength | Standard coefficient |
| Double stranded DNA | 260 nm | 1 cm | 50 μg/ml |
| Single stranded DNA | 260 nm | 1 cm | 33 μg/ml |
| Single stranded RNA | 260 nm | 1 cm | 40 μg/ml |
| Oligonucleotides | 260 nm | 1 cm | 20 μg/ml |
The use of standard coefficients leads to approximations rather than precise determinations of biomolecule concentration within a sample (Table 4, Table 6). However standard coefficients have proven to be easy to use with sufficient accuracy for daily laboratory work. To avoid large divergences assay conditions should be considered when determining concentration of nucleic acids or proteins on the basis of standard coefficients.
Microplates for the UV/VIS determination of nucleic acids at 260 nm and proteins at 280 nm
Standard microplates are only partially suitable for transmission measurements in the UV range (Fig. 2a and 2b). Polystyrene absorbs UV light especially in the short-wavelength range below 400 nm. μClear® Microplates with a thin polystyrene based film display much lower background values and can be used down to 340 nm. The adaptation of the patented μClear® process technology to a new, innovative UV-transparent material enabled the production of microplates that extend the transmission range down to 230 nm.
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For the determination of nucleic acid and protein concentrations at 260 nm without background interference, UV-Star® Microplates are an alternative to expensive and fragile quartz glass plates or cuvettes (Fig. 4). UV-Star® Microplates are DMSO resistant, can be stored at -20 °C without issue, and are available in 96 well, 96 well half area, and 384 well formats.
UV/VIS spectroscopy in microplates
The pathlength of a sample in a transmission measurement in a standard cuvette is normally 1 cm, as almost all cuvettes have a standardised pathlength of 1 cm. This is not the case for measuring optical density within microplates. Here the pathlength is determined by the amount and corresponding height of liquid sample filled in each well (Fig. 5 and 6). For a direct comparison of an OD measurement made in a microplate well with an OD measurement made in a cuvette, the results of the microplate well must be recalculated to a pathlength of 1 cm. The recalculation can then be carried out using equation 6:
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Adaptation to 1 cm pathlength
Several options are available to achieve the pathlength correction to 1 cm.
Mathematical approach
The simplest method to recalculate the absorbance in a microplate well to a pathlength of 1 cm is by using a mathematical approach. Depending on the geometry of the specific microplate well (i.e. customarily cylindrical shaped in the case of round well tops; rectangular in the case of square well tops), the adequate formula must be used (Fig. 7a). The dimensions of the wells of most Greiner Bio-One microplates are listed in our catalogue or can be downloaded from our website (www.gbo.com/bioscience). The calculation of the liquid height using the mathematical approach is shown in example 3 and example 4. Due to a slight tapering in the well geometry, the mean diameter of a filled well can only be estimated, and the meniscus of the liquid in the well cannot be taken into account either. However the mathematical approach is simple, without need for any special devices, with final results comparable to measurements performed with a cuvette (Table 4).
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Example 3: The volume of 100 μl is equivalent to a liquid height of 5.8 mm in a well of a half area microplate.
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Example 4: The volume of 100 μl is equivalent to a liquid height of 8.2 mm in a standard 384 well microplate.
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Calculation of the concentration of an aqueous DNA sample: Measurements taken in a half area microplate well with a sample volume of 100 μl were compared with those from a quartz glass cuvette (1 cm pathlength, 1000 μl sample volume). The pathlength of the half area microplate well was calculated using equation (7) with the dimensions given in figure 7b. The final DNA concentration was calculated using the standard coefficient of DNA (50 μg/ml). The results are listed in table 4.
| Half Area Microplate | |
| OD | 0.43 |
| Pathlength 100 μL | 0.6 cm |
| OD corrected to 1 cm (OD1cm) | 0.72 |
| Calculated DNA concentration | 36 μg/ml |
| Quartz Glass Cuvette | |
| OD 1 cm | 0.68 |
| Calculated DNA concentration | 34 μg/ml |
Absorption of water in the near infrared (NIR) as reference for a pathlength correction
Water is essentially transparent from 200 to 900 nm (Fig. 8) but in the near infrared (NIR), it has a distinctive absorption peak near 977 nm[4]. This specific absorbance band, in a spectral region where most biological molecules have little or no absorbance, can be used to measure the pathlength of light through an aqueous sample[3][5][6]. Since the absorbance (i.e. optical density) of water Areference for the pathlength of 1 cm dreference can be determined in a measurement in a standard cuvette or in a measurement in a reference well of the used microplate, filled with water equivalent to 1 cm pathlength, it can be used as a reference.
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The absorbance of water Areference can be set in relation to the absorbance of the sample Asample leading to the actual pathlength of the sample dsample.
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As the absorption of water is temperature dependent, sample measurements may be subject to error if they are not made at exactly the same temperature as the reference. To avoid these errors, reference measurements can be conducted at or near 1000 nm (e.g. 997 nm). At this wavelength, the isosbestic point of water, the temperature has a minor influence on the absorbance of water[3][5]. Nevertheless, 977 nm is close enough to the isosbestic point to get decent reduction of the temperature dependence.
For a more precise measurement the background of the aqueous sample should be considered by deducting the absorption at 900 nm. So equation (9) may be modified to:
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In parallel to the absorption measurement at 977/997 nm and 900 nm the absorption (Aλ sample) of the aqueous sample should be measured at the wavelength λ whereas λ is defined as the specific wavelength necessary for the determination of the concentration of the substance (i.e. for DNA the wavelength of detection is 260 nm).
With the determined pathlength of the sample (dsample), the absorption of the sample (A λ sample) can be corrected to 1 cm (Aλ sample 1cm) as follows:
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Example 5: In example 5 the calculation for the concentration of double stranded DNA using the pathlength correction is explained. The absorption of an aqueous DNA sample of 100 μl in a well of a half area microplate at 977 nm was 0.15 AU, the background at 900 nm was 0.04 AU. The absorption of the reference (water) was 0.24 AU at 977 nm and the background at 900 nm was 0.06 AU. The absorption of the sample at 260 nm was 0.47 AU, and the background 0.07 AU.
| (A977 - A900)sample = 0.15 - 0.04 = 0.11 (A977 - A900)reference = 0.24 - 0.06 = 0.18 Aλ sample = 0.47 - 0.07 = 0.40 |
Using equation (10) the pathlength of the sample can be determined:
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Using equation (11) the optical density of the sample can be corrected to 1 cm:
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Using the standard coefficient and equation (5) the final DNA concentration of the sample can be calculated:
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Microplate readers with pathlength correction
In 1994 Molecular Devices Inc. (Molecular Devices Inc., Sunnyvale, CA, USA; www.moleculardevices.com) introduced a new microplate reader equipped with a monochromator, rather than bandpass filters, for flexible wavelength selection[5]. Another monochromator based reader, the PowerWave™, was introduced by BioTek Instruments Inc. (BioTek Instruments Inc., Winooski, VT, USA; www.biotek.com)[6]. The innovative technology of both readers facilitated readouts below 340 nm and supported the high-throughput measurement of nucleic acids and proteins in microplates, in tandem with the increasing importance of genomic and proteomic research projects.
Both manufacturers supply software programs to enable a pathlength correction to 1 cm. The basis of the pathlength correction in both devices is a reference measurement of the absorbance of water in the near infrared (NIR) as described in the previous section. The usage of a device with an integrated software program is certainly the most popular and most convenient method to determine the absorbance of a sample at 1 cm. More information about the reader technology of Molecular Devices Inc. and BioTek Instruments Inc. can be found on their websites and in application notes[5][6].
Standard curve on the basis of calibrated standards
Finally, the most classical method to determine the concentration of a sample in UV/VIS spectroscopy is the implementation of references to establish a standard curve. Calibrated standards with known concentrations are used to create a straight line (Fig. 9). If the sample volume remains constant for all measurements, the pathlength has not to be considered for this type of calibrated standard determination. In addition, determinations made from both cuvettes and microplates will give the same results when the same standards are used (Fig. 9, Table 5). The advantage of this method is obvious: No special devices, optical filters, monochromators or complicated and time consuming calculations are necessary. The method is simple and precise.
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| Cuvette | Half Area 170 μl | Half Area 80 μl | |
| Optical density [y] | 0.668 | 0.669 | 0.342 |
| Linear regression | x = (y + 0.0015) / 0.0201 | x = (y - 0.0277) / 0.0198 | x =(y - 0.0038) / 0.0102 |
| DNA concentration [x] | 33 μl/ml | 32 μg/ml | 33 μg/ml |
Conclusion
The various strategies described in this application note to measure and calculate unknown DNA concentrations of aqueous samples all yielded results of sufficient accuracy for daily laboratory work, with only negligible differences between results obtained with microplates vs. cuvettes (Table 6).
| DNA concentration | |
| Mathematical approach, Half Area Microplate | 36 μg/ml |
| Pathlength correction, Half Area Microplate | 33 μg/ml |
| Calibration standards, 1 cm pathlength, Half Area Microplate | 32 μg/ml |
| Calibration standards, quartz glass cuvette | 33 μg/ml |
Compared to measurements with cuvettes, microplates allow the measurement of many samples in parallel with sample volumes down to 30 μl. The samples can be easily retrieved and even stored within the microplates. Therefore disposable microplates offer an ideal alternative to expensive and fragile quartz glass cuvettes or quartz glass microplates.
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