Handheld Spectroscopy: Bringing the Spectrometer to the Sample

Over the past 20 years, advances in consumer electronics (eg, all components of smartphones), micromachining (MEMS), and optical telecommunications have combined to make spectrometers portable. These instruments now range in size from a cordless drill to smaller than a deck of playing cards. These small and rugged spectrometers are now commonly used outside of the traditional lab to give answers in the field – at the point of need [1]. This has transformed operations within companies and, in some cases, entire companies.

The main handheld spectroscopy techniques can be divided into three classes: elemental analysis (X-ray fluorescence and laser-induced breakdown spectroscopy); optical molecular analysis (UV-visible, near infrared, middle infrared and Raman); and mass spectrometry (high pressure mass spectrometry and gas chromatography-mass spectrometry). Each of these classes has its own specific abilities and applications, and choosing the right technique for the problem at hand is obviously important.

Elemental analysis – metals

Portable X-ray fluorescence (XRF) is a well-established technique and has been used for at least twenty years throughout the life cycle of metals activity: from mineral exploration, mining, refining, confirmation of raw materials, in-process inspection, and finally scrap sorting.

It is obviously important to choose the right technique for the problem to be solved.

Other applications include the detection of heavy metals (eg lead) in old house paint and imported to ys, cash for gold trading, environmental surveys and cultural heritage studies. Handheld XRF has some limitations for elements lighter than silicon and the new handheld laser-induced breakdown spectroscopy (LIBS) technique is particularly useful in these cases, for tasks such as sorting aluminum alloys and lithium applications.

Optical molecular analysis

Rapid identification of chemical substances, often “white powders”, is essential in a number of different scenarios, from confirmation of incoming materials in pharmaceutical manufacturing [2], detection of counterfeit pharmaceuticals in the field, identification of suspicious materials by police, customers and border agents, and airport security personnel. The value of instant identification is remarkably evident for a hazardous material spill.

Here, a technician wearing protective gear must quickly characterize the spilled material – is it flammable, toxic, corrosive, volatile, explosive, etc. ? This cannot be achieved by sending a sample to a lab; it must be done on the spot with a reliable result and confidence in the actions necessary to remedy the spill.

Near-infrared (NIR), mid-infrared, and Raman spectrometers are all used in these areas, but each has its strengths and weaknesses. [2]. In short, Raman is often preferred due to its “point and shoot” ability, whereas mid-infrared generally requires the material to be manipulated. This tends to outweigh the problem of fluorescence in Raman spectra, and instrument manufacturers have implemented a variety of schemes to mitigate these effects.

Both techniques yield very specific spectroscopic information. NIR spectroscopy has been used for many years for the quantitative analysis of natural products (cereals, fruits, etc.) and is well suited to very heterogeneous materials.

Mass spectrometry techniques

It should be noted that optical techniques are limited to major component (percentage) analyzes in condensed phases. For analyzes of traces and complex mixtures, mass spectrometry (MS) techniques are required. MS has generally required the use of high vacuum, which has prevented miniaturization of the instrument into a portable format.

However, a variety of mass spectrometry – ion mobility spectrometry (IMS) has been used in airport security applications since the late 1980s. If your baggage has been “swabbed” and the swab has been transferred to a small office instrument, it was then an IMS. These instruments are also available as hand-held devices, usually resembling a “dustbuster” type hand-held vacuum cleaner.

Briefcase-sized gas chromatography-mass spectrometers (GC-MS) have been commercially available since about 1996. They are widely used by hazardous materials technicians, the military, and scientists in the environment to investigate and characterize suspicious areas.

Curiously, size and cost reductions now allow miniature spectrometers, spectral sensors and other photonic components to be integrated into some consumer products.

These instruments typically use narrow bore GC columns and small ion trap or quadrupole spectrometers, and these, in turn, allow the use of miniature pumps that allow portable operation even when the MS itself requires high vacuum operation. Because these instruments use a separation front and mass spectrometric detection, they are excellent for analyzing complex mixtures and for detecting trace amounts of components in these samples.

A newcomer is high pressure mass spectrometry (HPMS), using an array of micro ion traps, which only require a modest vacuum (0.1 atmospheres), allowing for a smaller pump and a portable format. HPMS is fast (seconds), reliable for programmed target chemicals, and provides a presumptive method of identification.

The commercially available instrument has “Hunter” modes, which optimize the analytical settings for specific types of targets, e.g., drug hunter mode, chemical warfare agent hunter mode, chemical warfare agent hunter mode. explosives.

The future

The current generation of commercial handheld spectrometers are smaller, lighter, yet more powerful than their predecessors of just five years ago, and their capabilities for qualitative and quantitative analyzes have increased dramatically.

This field has experienced rapid development in a circular and self-sustaining way: the desire to carry out analyzes in the field has led to the development of portable instruments, while their availability has led to the development of new applications. As a result, more and more applications will move from the lab to the charging dock and into the field, delivering real-world answers when you need them.

Handheld optical spectrometers continue to get smaller, some the size of a finger, with lower cost availability. In addition, a variety of multi-spectral sensors are available, with, for example, sixteen distinct spectral bandwidths in the visible and near infrared.

The molecular absorption bands in these regions are often very broad, so these sensors can be used for certain material identification applications. Curiously, size and cost reductions now allow miniature spectrometers, spectral sensors and other photonic components to be integrated into some consumer products. [3].

Over the next few years, we will see the growth of “smart” products containing these optical devices in household appliances (refrigerators, washing machines, dryers, ovens, vacuum cleaners), personal care (toothbrushes, hair care , cosmetics) and fitness (sports watches, connected watches).

Finally, it should be noted that the pinnacle of transportable (if not portable) spectrometers is found in the latest generation of Mars rovers, which combine both Raman and LIBS spectrometers. [4].

References:

  1. Richard A. Crocombe, “Portable Spectroscopy”, Appl. Spectrosc., 72(12), 1701-1751 (2018).
  2. Richard Crocombe, “Pharmaceutical analysis with portable spectrometers”, European Pharmaceutical Review, 27(1), 8-13 (February 2022).
  3. Richard A. Crocombe “Handheld spectroscopy in 2019: smaller, cheaper and in consumer products?” proc. SPIE 10983, New Generation Spectroscopic Technologies XII, 109830J (May 13, 2019).
  4. https://spectroscopyonmars.com
  5. Portable Spectroscopy and Spectrometry (Volume 1, Technologies and Instrumentation; Volume 2, Applications), Richard A. Crocombe, Pauline E. Leary and Brooke W. Kammrath (eds), John Wiley, Chichester UK and Hoboken NJ, USA, April 2021.

Dr. Richard Crocombe is Principal at Crocombe Spectroscopic Consulting, spectroscopy consulting.com

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