Fiber Optic Sampling Tools for Absorbance Applications
Your optimal choice of a sampling accessory for absorbance depends on the application. Among the criteria to consider are the sample media (liquid or gas); the optical density of the sample; and whether the measurement will be made in situ, as in a process stream, or within a reaction chamber. As you sort through the criteria for each application, you are presented with a wide range of Ocean Optics accessories to choose from, from general-purpose labware such as cuvettes and cuvette holders to more sophisticated tools such as transmission dip probes and flow injection analysis devices.
Thanks to its simplicity, accuracy and ease of use, absorbance is the most widely used spectroscopic technique for studying liquids and gases. An absorbance spectrum can be used as a qualitative tool to identify or “fingerprint” substances, or as a quantitative tool to measure the concentration of a molecule in solution.
Absorbance measurements can take many forms. Absorbance works equally well for gases and liquids, and has been integrated into industrial applications and consumer products alike. Samples no longer need to fit into the familiar cuvette-in-a-cuvette holder setup, as flow cells, dip probes, micropipetters, folded gas cells and micro-cuvettes allow the sampling optic to be customized to the sample.
With modular spectroscopy, users have more flexibility to choose the instrumentation wavelength range and resolution best suited for their absorbance measurements, and are able to move between sampling optics quickly and easily for measurements in the lab or field. Indeed, with our extensive line of spectrometers, light sources and sampling accessories – and the flexibility to switch among them with great ease – you have many options for tackling your absorbance experiment.
Choosing Sampling Accessories for Absorbance
Here are some criteria to consider when selecting sampling accessories for your absorbance setup:
- Where will you make the measurement? More precisely, where will the sample be situated as you make your measurement? If you can isolate the sample in a container such as cuvette or test tube, your options extend to cuvette holders and similar devices. If you need to make the measurement in situ, in real time – in a process stream, for example — you’ll need a transmission dip probe or flow cell that can be immersed in the stream.
- Is the sample a liquid or a gas? Gases can pose a problem, as their greatly reduced density requires either a highly absorptive sample or extremely long pathlength. Depending on the sample, a 10 cm pathlength may be sufficient, achieved using a CUV-UV-10 or variable pathlength cuvette holder with cuvette, or simply by placing an adjustable collimating lens and cuvette holder directly into the gas to be measured.
- What is the optical density of your sample? The OD of your sample directly affects the pathlength of the device you’ll need for your measurement. The more optically dense the sample, the shorter the pathlength that is required. For example, SpecVette™ specialty cuvettes are available in pathlengths as short as 250 µm and standard cuvettes come in 1 cm and 10 cm pathlengths. On the other hand, low OD solutions require longer pathlengths, with options including longpass flow cells of up to 500 cm pathlength. For more on the relationship of absorbance with sample concentration and pathlength, see the Beer’s Law entry in the Ocean Scholar Library.
- What is the sample volume? The amount of sample you measure affects your choice of sampling accessory. We offer microvolume-level cuvettes and pipetters for applications with sample volumes as low as 2 µl.
- Does the sample require continuous flow? We offer sampling accessories such as flow injection Z-cells, capillary electrophoresis cells and flow cells that can be coupled with pumps, tubing and fittings to stream sample fluids for absorbance and other measurements.
Sampling Accessories for Absorbance at a Glance
|Cuvettes||Quartz and plastic||General-purpose lab work|
|Cuvette Holders |
|Standard and multipurpose; temperature regulated; and integrated with spectrometer (see the Flame-with-cuvette holder example) |
|General-purpose lab work; applications requiring precision control of sample temperature; users who need a compact absorbance setup |
|Small-Volume, Short-Pathlength Sampling |
|Cuvettes with sample volume as low as 5 µl or pathlengths as short as 250 µm||Analysis of proteins, nucleic acids and other small-volume samples; measurement of optically dense solutions such as inks and dyes|
|Flow Cells |
|Z-cells for flow injection analysis; low-volume sampling cells; and process-ready cells||Microfluidics handling, low concentration absorbance measurements and process control|
|Transmission Dip Probes |
|General-purpose, harsh-environment and high-pressure,/high-temperature||General-purpose lab work and measurements in chemically or physically harsh environments|
|Absorbance Standards||NIST-traceable standards for UV (200-450 nm) and visible (400-900 nm) wavelengths||Quality control and applications where verifying absorbance response is important|
Considering Absorbance Standards for Your Application
Some absorbance applications may require the use of absorbance standards, which are used to verify the accuracy and consistency of the response of an absorbance system relative to NIST-traceable standards. Often this level of verification is required as part of quality control procedures in biomedical and pharmaceutical industries. Also, neutral density filters can perform the same function, but do not account for factors such as proper preparation of stock solutions, liquid and cuvette handling, or the quality of cuvettes being used. We offer absorbance standards for UV and visible wavelengths.
Additional Resources on Absorbance Measurements
- Basic Absorbance Spectroscopy Setup
- Absorbance Spectroscopy Using the Flame and OceanView Software
- Spectroscopy for Clinical Analysis
- Spectrometer Repeatability in a Process Environment
Application Notes and Tech Tips
- Measuring DNA Absorbance with the STS-UV Microspectrometer
- Quantitative Absorbance Spectrophotometry
- Improving Absorption Measurements through Light Source Selection
- Measuring Beer Color with OceanView
Example Absorbance Setups
|Maya2000 Pro||This high-resolution spectrometer is configured for UV absorbance in the sample setup. We use Grating #H7 set for 200-300 nm, with a 5 µm slit as entrance aperture and a detector collection lens for increased sensitivity.|
|D-2000 (discontinued light source)||Deuterium source produces continuous output from ~215-400 nm|
|CUV-UV-10||Cuvette holder for 10-cm pathlength cuvettes|
|CV-Q-100||Cylindrical quartz cell with Teflon stopper; has volume of 28.2 mL|
|QP455-025-XSR-BX||Pair of extreme solarization-resistant patch cord assemblies, 455 µm diameter, 0.25 m length|
|Flame-S-UV-VIS||A modular spectrometer is preconfigured for 200-850 nm and has a 25 µm slit and order-sorting filter|
|DH-2000-BAL||Balanced deuterium tungsten halogen light source provides illumination from 215-2000 nm|
|CUV-UV||This sturdy cuvette holder accepts 1 cm pathlength cuvettes|
|CV-Q-10||A quartz cuvette is recommended for UV applications in particular|
|QP450-2-XSR||Two 450 µm extreme solarization-resistant optical fibers will transmit and receive light in this setup|
|Absorbance Standards (optional)||NIST-traceable photometric absorbance standards for 200-450 nm (STAN-ABS-UV) and 400-900 nm (STAN-ABS-VIS) ranges|