Nick Morris examines new directions in spectroscopy
In many ways the spectrometry field is a mature market - the principles involved (measuring the specific frequencies of light absorbed or emitted by a sample to determine its physical or chemical make up) have been known for almost a century. However, until recently the practice of spectrometry was largely lab-based. The equipment used was delicate and bulky, meaning any substance to be sampled had to be taken to the lab. This process could be both costly and time consuming.
However, recent advances in technology, such as inexpensive, robust CCD detectors and MEMS chips, have meant that spectrometers can now be housed in small, sturdy packages. The spectrometer is becoming a 'black box' technology, rather than a specialised piece of lab equipment. The latest advances in spectrometer technology are allowing scientists to take the spectrometer out into the field. The lab is being taken to the sample.
An Ocean Optics spectrometer is being used to monitor sulphur dioxide levels close to a volcano.
Jorg Rademaker, of Ocean Optics, says: 'You don't always need a large complicated spectrometer for every application - very often smaller, hand-held devices will give a much broader range of possibilities. This is becoming even more evident as the smaller devices acquire more of the functions of larger, specialist equipment. We're finding that our customers are using our spectrometers in increasingly interesting ways. Many scientists use our spectrometers in the field for research in the life sciences, but there are many other areas where they are used, from the deep sea to examining the corona of the sun. One very interesting example recently came from the Montserrat Volcanic Observatory, where a spectrometer operating in the ultra violet region of the spectrum is measuring the sulphur dioxide content of air near a volcano and, from the data gathered, can predict when the volcano may erupt. We're also finding a lot of interest being shown in the possibilities of laser induced breakdown spectroscopy.'
Spectrometers can also be used to monitor the rate at which fruit ripens.
According to Benno Oderkerk, technical director of Avantes, one of the most significant recent advances has been towards spectrometers working as completely autonomous tools. For example, Avantes has recently released a spectrometer that incorporates a Bluetooth unit for wireless transmission of data. This setup is particularly suitable for harsh environments where you would not want to take a separate PC or laptop computer. In such an application, the spectrometer can be set up in situ and monitored remotely via its wireless connection. Environments with moist atmospheres, such as greenhouses, can be particularly harmful to conventional electronic equipment. In a greenhouse, a spectrometer can be set up to monitor the rate at which fruit ripens - the colour of a fruit, such as a tomato, will often provide a very accurate indicator of ripeness. The remote spectrometer can record the rate of ripening of the fruit, and even alert the farmer when the fruit is ready to be plucked, packaged, and shipped to market.
For Oderkerk, this represents an example of the commoditisation of the spectrometer: 'Users don't necessarily want to see a full spectrum every time they use a spectrometer; they just want to know if the result is right. This means we don't have to build full functionality into every spectrometer. In many ways we're expecting to see the spectrometer follow the route taken by other sensors, whereby it will become a high volume, low cost component. We'll soon see spectrometers appearing in completely new areas. For example, you could have a small spectrometer in your clothes to warn you if you were in danger of becoming sunburnt.'
A scematic of the Polychromix MEMS system
A prime example of new technologies expanding the use of spectrometers comes from Polychromix, based in Massachusetts. Polychromix uses an innovative MEMS array as the key component to its range of spectrometers. The MEMS chip acts as a tuneable, programmable micro-diffraction grating. Activating different areas of the MEMS chip in a specified order means that different wavelengths of light are reflected and absorbed, corresponding to the set-up of the chip. This allows the spectrometer to isolate certain wavelengths of light for analysis in sequence. Hence a full spectrograph, plotting the intensity of certain wavelengths, can be taken using a single detector module. The system also provides a very good signal to noise ratio, as half the wavelengths of light being measured are always passing through the system. This also means that noise induced within the optics of the spectrometer itself can be suppressed easily, further improving the final spectrum produced.
Because the Polychromix system only uses a single detector unit and MEMS chip instead of a series of gratings in front of a large CCD detector, the system is comparatively small and robust, making it ideal for small autonomous spectrometers.
Polychromix has recently developed the Phazir, a small, handheld device that can be used for many applications where the user needs to be able to quickly identify a material. The device holds a database of model spectra in its internal memory. The spectrum read from a sample is compared with these model spectra in order to identify the sample. The Phazir is used in the burgeoning recycling industry, where it is very important to be able to classify substances correctly so they can be dealt with appropriately. The Phazir can also be used to identify samples at crime scenes, drastically reducing the time taken by forensic scientists, which, in turn, will give police a better chance of catching criminals.
Centice develops optical sensors for customers in the molecular spectroscopy market. Mike Fuller, senior director of product management at Centice, points out that there has been a major sea-change in the field in recent months: 'There has been a paradigm shift in the way molecular spectroscopy is carried out. Our new multimodal multiplex spectroscopy (MMS) method allows for a much higher throughput than traditional slit and fibre-coupled spectrometers.'
Conventional spectrometers use a slit at the entrance to the spectrometer, but there is an inherent trade-off between resolution and light throughput. While spectral resolution increases as slit width decreases, a narrow input slit greatly limits photon throughput and consequently, the measurement sensitivity of the instrument. Fuller adds: 'MMS uses a 1mm wide aperture, while conventional methods use an aperture of 100µm. The larger aperture gives better resolution and accuracy, which makes the system ideal for applications that need a high throughput, such as medical diagnostics. There is also less degradation of the sample being examined, as the laser used to illuminate the sample is spread over a wider area.' The MMS system is currently offered within Newport's new Oriel Matrix spectrometer, which operates in the ultraviolet-visual wave bands.
Despite the growth in low-end spectrometry, driven by the need for small, fast working, and above all inexpensive instruments for simple applications, there is still a strong, growing market at the high-end of the field. According to Antoinette O'Grady, chief scientist at Princeton Scientific Instruments Acton, there is a general separation between these different areas: 'There is still a broad division between the high-end and low-end of the spectroscopy market. Some companies have tried to break into the middle ground, but without much success; the spectrometers produced to fill this niche have neither the accuracy of high-end equipment, nor the low cost per unit of the lower-end instruments.'
According to O'Grady, much of the growth at the high end of the field is being driven by the booming genetics and life sciences market: 'Raman spectroscopy, for physics and chemistry in particular, is our leading market. In a lab you're not so concerned about the portability of your spectrometer - your main concern is the accuracy of the spectrum it produces. We've seen a great increase recently in overlap or crossover research areas - research fields where scientists are moving beyond the confines of the boundaries of traditional areas of research. This is epitomised by research in areas such as biophysics, where methods traditionally associated with the physical sciences are being applied to samples from the life sciences. This would include scientists using Raman spectroscopy to examine gene sequences and proteins. These are the research areas that grab the headlines; this is where the money is.
'Fifteen years ago many researchers would build their own bespoke spectrometers and lab instruments - such activities could easily occupy the first year of PhD research! However, nowadays, with the pressures to publish papers to ensure continued funding, and other constraints, scientists no longer have the time to spend building their own equipment. Buying complete systems from companies such as Princeton Instruments Acton means they can spend more time on the science.'
The availability of small, inexpensive, accurate components is rapidly expanding the range of applications for which spectrometers can be used. There will always be a need for large lab-based equipment for the scientific community, but small, handheld devices are now becoming commonplace sights in everyday uses.