The urethane functional group is found almost exclusively in polymer form, known as 

. These ubiquitous polymers make urethanes an economically important functional group. Urethanes are made using diisocyanates. How to identify diisocyanates, and distinguishing among different urethane types using infrared spectroscopy, are explained in this installment of the column. 

		
		The urethane functional group is found almost exclusively in polymer form, known as 

. These ubiquitous polymers are used as wood finishes and make up polyurethane foam, also known as foam rubber. The structural framework of a urethane is shown in Figure 1.

	Note that this functional group contains a carbonyl bond with an oxygen atom and nitrogen atom attached to it. The latter is why urethanes are included in this series on organic nitrogen compounds. On the left-hand side of Figure 1, the urethane looks like an ester because of the O-C=O linkage (1), but on the right-hand side of Figure 1, the N-C=O linkage looks like an amide (2,3). As we will see, the infrared spectra of urethanes combine features of both esters and amides.

 if one carbon and two hydrogens are attached to the nitrogen atom, as 

 if two carbon atoms and one hydrogen atom are attached to the nitrogen, and as 

 if three carbon atoms are attached to the nitrogen. Figure 1 shows the structure of a secondary urethane. Note that it contains two C-N bonds and one N-H bond.

	Polyurethanes are made by reacting alcohols with a functional group called a 

. The structural framework of a diisocyanate is seen in Figure 2.

	Note that a diisocyanate contains two isocyanate -N=C=O linkages, hence the “di” in the name. A molecule with one -N=C=O unit would simply be called an 

. When urethanes and alcohols are polymerized to make polyurethanes, there can be isocyanate end groups that occasionally appear in the spectra
	of polyurethanes.

	The infrared spectrum of a diisocyanate is shown in Figure 3.

	Note that the spectrum is dominated by the peak labeled A at 2277 cm

 (assume all peak positions are in wavenumbers going forward, even if not labeled as such). This peak is due to the asymmetric stretch of the -N=C=O group, which normally falls from 2280 to 2240. This peak is the perfect example of a group wavenumber, because it is intense and shows up in a part of the spectrum where very few other functional groups absorb (5). Throughout the over 30 installments of this column, you may have noticed that most of the time we focus our attention from 3500 to 2800, and from 2000 to 400. That is because very few functional groups absorb between 2800 and 2000. The only other infrared peak we have seen that absorbs near diisocyanates is the C≡C stretch of disubstituted alkynes, which fall from 2260 to 2190 (6). These two functional groups are easily distinguished, because the diisocyanate -N=C=O asymmetric stretching peak is broader and stronger than a typical C≡C stretch. This is because of the high polarity of the -N=C=O group compared to that of an alkyne triple bond (7). The strength and unusual position of the -N=C=O stretching peak are why it sometimes appears in polyurethane spectra, even though it is in low concentration.

	Based on the structural framework of urethanes shown in Figure 1, we can predict to a point what urethane spectra will look like. Primary and secondary urethanes contain N-H bonds, and so have N-H stretches that are medium in width and intensity, and fall in the same wavenumber range as the N-H stretching peaks of amides and amines (2–4). The C=O bond in urethanes means there should be a strong C=O stretching peak (8) from around 1700, and the C-O bond means there should be a strong peak between 1300 and 1000 (9).

	The infrared spectra of primary urethanes behave as expected. Their asymmetric and symmetric NH

 stretches are found from 3450 to 3400 and 3240 to 3200, respectively. Primary urethanes have two NH stretching peaks, as do primary amines and amides (2–4), because, in all these cases, the primary nitrogen contains two N-H bonds, hence two N-H stretching peaks (3). The carbonyl stretch of primary urethanes falls at 1740 to 1730, and there is an NH

 scissors (in-plane bending) peak from 1630 to 1610.

	Unfortunately, the C=O stretching peak of primary urethanes falls in the same region as other carbonyl-containing functional groups, such as ketones, aldehydes, esters, and carboxylic acids (8,9). However, these functional groups do not contain nitrogen, hence there are no N-H stretching peaks, making distinguishing primary urethanes from these compounds easy. But primary amides have two NH

stretches, and a C=O stretch (3), as do primary urethanes. Fortunately, the primary urethane C=O stretch is at 1740 to 1730, whereas, for primary amides, it falls from 1680 to 1630 (10). This difference in C=O stretching peak position means primary amides and urethanes can be distinguished from each other. The diagnostic group wavenumber pattern, then, for primary urethanes is two NH stretching peaks, a C=O peak around 1730, and an NH

	The infrared spectrum of a secondary urethane, a “polyurethane/polyether foam,” better known as foam rubber, is seen in Figure 4.

	Because this polymer is a secondary urethane, as seen in the figure, it has a single N-H bond, and, hence, a secondary nitrogen. The spectral signature of a secondary nitrogen, as we saw for secondary amides and amines (2–4), is a single N-H stretching peak. In Figure 4, this peak is labeled A at 3326. In general, for secondary urethanes, this peak falls from 3340 to 3250. Note that the NH stretching peak in Figure 4 is of medium intensity and height, consistent with many of the other NH stretches we have seen (11).

	The carbonyl stretch in Figure 4 labeled B is split, with peaks at 1720 and 1705. In the past, a double carbonyl stretch like this indicated the presence of two separate carbonyl bonds in a functional group, such as for acid anhydrides (12) and imides (13). However, this is clearly not the case here; urethanes do not contain two carbonyl bonds. The cause of this split carbonyl peak is that some urethane C=O groups engage in hydrogen bonding, whereas others don’t. The 1720 peak is from non-hydrogen bonded, or “unassociated,” carbonyls, whereas the 1705 peak is from the hydrogen bonded, or “associated,” carbonyl groups (10). We have already studied the effects of hydrogen bonding on peaks, such as for alcohols (9). The tendency for peaks upon hydrogen bonding is to broaden and move to lower wavenumbers, as we have seen here.

	The peak labeled C in Figure 4 at 1531 is the in-plane N-H bend of a secondary urethane. In general, this peak falls from 1540 to 1520. Secondary amides also have a peak from the same vibration in the same place (3). However, secondary amide C=O stretches fall from 1680 to 1630, whereas, for secondary urethanes, this peak falls from 1740 to 1705, higher than in secondary amides, allowing us to distinguish between secondary urethanes and amides.

	The diagnostic group wavenumber pattern then for secondary urethanes is a single N-H stretch, a carbonyl stretch from 1740 to 1705, and an N-H in-plane bend from 1540 to 1520. Primary and secondary urethanes can be distinguished, because the former has two NH stretching peaks, whereas the latter has one.

	A further examination of Figure 1 shows that urethanes have a C-O bond, and we have learned in general that C-O stretching peaks are intense and fall from 1300 to 1000 (9). The peak labeled D at 1222 in Figure 4 is a urethane C-O stretch.  The C-O stretch for all urethanes falls from 1250 to 1200 (10). There is a large peak at ~1050 in Figure 4 that is not labeled. It is from the C-O stretch of the ether groups (14)in this particular polyurethane/polyether foam.

	Tertiary urethanes contain no NH bonds, hence they have no NH stretching or bending peaks. Their only useful group wavenumber is their C=O stretch from 1690 to 1680 (10), which also overlaps with amides. Thus, tertiary urethanes, like tertiary amines and amides (2–4) are a functional group that is hard to distinguish by infrared
	spectroscopy.

	A summary of the group wavenumbers discussed in this column is seen in Table I.

	Urethanes are often made into polyurethanes by reaction with diisocyanates. Diisocyanates have a strong and uniquely positioned -N=C=O asymmetric stretching peak from 2280 to 2240. Like amides and amines, urethanes can be classified as primary, secondary, and tertiary, based on the number of C-N and N-H bonds. Primary urethanes exhibit two NH stretches, a C=O stretch, and an NH

 scissoring vibration. Secondary urethanes have one NH stretch, a C=O stretch, and an NH in plane bend. Tertiary urethanes are difficult to discern by infrared spectroscopy.

Infrared and Raman Characteristic Group Frequencies 

(Wiley, New York, New York, 3rd Ed., 2001).

Solution to the Last Infrared Spectral Interpretation Challenge

	The spectrum from the last interpretation challenge is seen in Figure i.

	As we work our way from left to right across this spectrum, the first peak we encounter is at 3043. We have learned that peaks between 3100 and 3000 are typically the C-H stretch of unsaturated carbons (15). However, this peak by itself cannot help us distinguish between the two most common unsaturated carbon types, alkenes or benzene rings. Alkenes have a C=C stretching peak from 1680 to 1630 (15), but, given that there are no peaks in this region in this spectrum, we can rule out an alkene. Benzene rings have ring mode peaks that are sharp, and appear from 1630 to 1400 (16). The sharp peak at 1500 can be assigned as a ring mode, indicating we have a benzene ring present.

	The most notable feature in Figure i is the pair of peaks at 1777 and 1726. Given their position and intensity, they are C=O stretching vibrations (8). The fact that there are two carbonyl stretching peaks indicates there may be a functional group present with two C=O bonds in it. Acid anhydrides are a possibility here, because we have seen that their spectra possess a double carbonyl stretching peak (12). However, the acid anhydride functional group also contains a C-O bond, and, hence, should have a strong C-O stretching peak between 1060 and 1035. There are no strong peaks in this region, meaning this is not an acid anhydride.

	We saw last time (13) that cyclic imides have a double carbonyl stretch with peaks at 1790 to 1735 and 1750 to 1680. The spectrum in Figure i has peaks at 1777 and 1726, consistent with there being a cyclic imide present.

	Now, some imides have an N-H bond, and, hence, an NH stretching peak at 3200±50 (13). The absence of an NH stretch here means there is not an NH bond, but, more than likely, a C-N bond in the imide functional group. Note that the caption in Figure i says the sample is a fiber. This means the sample is a polymer, and we probably have a polyimide where the repeat units connect through the nitrogen in the imide functional group, explaining the lack of an NH stretch.

	I am afraid this is as far as we can push this analysis. What we know is that we have a cyclic aromatic polyimide. The most common material fitting this description has the trade name Kapton, or the chemical name poly(4,4’-oxydiphenylene-pyromellitimide). Given this name, you can see why we will simply call this material Kapton. The chemical structure of Kapton is seen in Figure ii.

	Note there is also an aromatic ether in this structure, hence the C-O stretch in Figure i at 1241 (14).

is the founder and CEO of Big Sur Scientific, a maker of portable mid-infrared cannabis analyzers. He has over 30 years experience as an industrial infrared spectroscopist, has published numerous peer reviewed papers, and has written three books on spectroscopy. As a trainer, he has helped thousands of people around the world improve their infrared analyses. In addition to writing for 

 Dr. Smith writes a regular column for its sister publication

and sits on its editorial board. He earned his PhD in physical chemistry from Dartmouth College. He can be reached at: SpectroscopyEdit@MMHGroup.com â

Articles

IR Spectral Interpretation Workshop

Here we delve into the interpretation of another organic nitrogen compound, known as polyurethane, a ubiquitous polymer used for wood finishes and foam rubber.

Organic Nitrogen Compounds IX: Urethanes and Diisocyanates

Spectroscopy May 2020

Click the title above to open the Spectroscopy May regular issue, Volume 35, Issue 5, in an interactive PDF format.

Vol 35 No 5 Spectroscopy May 2020 Regular Issue PDF

The most widely used commercial techniques for solid sample analysis include laser induced breakdown spectroscopy (LIBS), arc/spark optical emission spectrometry (OES), X-ray fluorescence (XRF), and laser ablation coupled with inductively coupled plasma−mass spectrometry (LA-ICP-MS). Each technique has known limitations, including matrix suppression, diffusion and transport effects, spectral overlaps, and varying degrees of calibration challenges. Also, many laboratories determining trace impurities in solid materials are unable to reach the required limits of quantitation by direct analysis, so they must digest the samples and use a technique like ICP-MS. Recent advances in laser ablation laser ionization (LALI) have reduced many drawbacks that plague other techniques, simplifying solid sample analysis. This column examines the LALI technique coupled with time-of-flight (TOF) mass spectrometry and compares application figures of merit with other, more traditional approaches of analyzing solid materials.

	Laser ablation laser ionization, coupled with time-of-flight (TOF) mass spectrometry (LALI-TOF-MS), is a brand new development that offers virtually the entire periodic table of the elements simultaneously at MS detection limits. LALI-TOF-MS has applicability to all trace element applications that require direct analysis of solid samples. LALI avoids sample digestion procedures and bypasses limitations such as matrix suppression effects, polyatomic spectral interferences, and signal instability that are exhibited in ICP-MS and other solution techniques. 

	At the heart of LALI-TOF-MS are two lasers; a sample chamber with ion source that is already under vacuum, and a time-TOF mass spectrometer. The dual-laser technology extracts and subsequently ionizes material in two discrete steps; first, ablation of the sample material, and then, ionization of the ablated material. The generated ions are then sampled into a TOF mass spectrometer for separation, identification, and quantification.

	Historically, LALI has been referred to by numerous acronyms in the literature, including 

laser desorption laser post-ionization time-of-flight mass spectrometry

 (LD-LPI-TOF-MS) (6). Most of the research regarding LALI was purely academic and performed between the late 1980s and early 1990s. At the time, cost and electronic limitations prohibited commercialization of LALI technology and other techniques took center stage. However, recent advances in computing technology and miniaturized, high-powered, solid-state lasers make LALI much more commercially viable today. Let’s take a closer look at its fundamental principles.

	The first step of LALI uses a focused laser beam to ablate (or desorb) material from a solid sample surface. It utilizes a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser with an adjustable laser wavelength. Depending on whether the application is characterizing atoms or molecules, the laser can be set to the fourth harmonic (266 nm) or fifth harmonic (213 nm) for ablation, or the fundamental 1064 nm wavelength for desorption. In a process exactly analogous to laser ionization mass spectrometry (LIMS) and laser induced breakdown spectroscopy (LIBS), the laser generates an initial set of ion or electron pairs from a temporal plasma, along with a neutral particle cloud that migrates in a direction that is normal to the sample surface.

	A short delay of <1 µs allows plasma extinction and the dispersion of plasma-generated ions before a second Nd:YAG laser at 266 nm is triggered for ionization. As shown in Figure 1, the ionization laser is aligned parallel to the sample surface, and its beam is focused inside the neutral particle cloud. The focused beam from the ionization laser has an energy density >10

, allowing for ionization of neutral particles via multiphoton ionization (MPI). MPI differs from resonant enhanced multiphoton ionization (REMPI), in that with MPI the laser is not tuned to a specific elemental or molecular frequency for ionization.

	By ionizing elements across a wide range of ionization energies, MPI serves as a highly efficient ion source and replaces the Ar plasma of ICP instrumentation. Once ionized, an ion funnel collects and focuses the ions in a low pressure (0.2–0.3 mbar) environment. After exiting the ion funnel, a quadrupole ion deflector (QID) turns the ions 90o and directs the ion beam through an Einzel lens stack and a quadrupole to further improve the beam shape. After the transfer quadrupole, the technology is equipped with a notch quadrupole filter. For applications that require high sensitivity, the notch filter increases dynamic range by selectively reducing the signal of up to four different ion masses (typically the most abundant matrix elements). Ions are then transferred to the reflectron TOF mass spectrometer that completes the mass analysis.

	To realize the real-world benefits of this technology, let’s examine the following application figures of merit compared to other solid sampling approaches: All data shown have been generated on the MassBox LALI-TOF-MS instrument
	(Exum Instruments).

	Detection limits for LALI-TOF-MS are on average approximately 100–500x lower than those of either XRF or LIBS. Typical detection limits for the majority of elements by LALI are in the region of 0.01 µg/g (10 ppb) for solids, compared to approximately 1–10 µg/g (1–10 ppm) for XRF and 10–50 ppm for LIBS. The inherent limitation of both XRF and LIBS is that they rely on the generation of X-ray- and plasma-induced excitation, and, as a result, use the measurement of photons for quantification, limiting their practical detection capability, as shown in Figure 2.

	LA-ICP-MS offers similar detection capability to LALI (and is slightly better for some elements) but suffers from two fundamental problems, thus limiting its suitability as a truly practical tool for a wide range of solid samples. The first major drawback of LA-ICP-MS is 

, or the preferential sampling of analyte elements based on their physical size. As a result, the abundances of the ions detected after separation are not entirely representative of the composition of the original sample. Besides the ablation process itself, the transport of the aerosol particles from the ablation chamber at atmospheric pressure into the ICP are impacted by differences in gravitational settling effects between smaller and larger particles, together with vaporization, atomization, and ionization differences in the plasma (less efficient for larger particles). In addition, transportation of ions through the interface cones, into the ion optics, and into the mass separation device are also important contributors to these fractionation effects. As a result, LA-ICP-MS often struggles to ensure that the analyte ions being measured are truly representative of the analyte elements in the bulk material being sampled, meaning that matrix matching of standards with samples is absolutely critical (7). Furthermore, LA-ICP-MS requires two separate systems from different vendors; this can be challenging to the end user to create a fluid workflow. This leads to high costs of instrument acquisition, high consumable costs, and a non-trivial amount of time from analysis to quantification. Figure 2 shows color-contoured heat maps of detection limits for LALI-TOF-MS (top) compared to LA-ICP-MS, XRF, and LIBS (bottom). The darkest colors represent the lowest limits of detection and clearly show that LALI offers the best detection capability across the periodic table.

	In XRF, the sample is irradiated with a beam of high-energy X-rays. As the excited electrons in the atom fall back to a ground state, they emit secondary X-rays that are characteristic of those elements present in the sample. Its inherent weakness lies in the fact that because the generation of secondary X-rays is dependent on the analyte–matrix bond, matrix-matched standards are a critical requirement for reliable quantification. So, for applications that require multielement measurements across a variety of different matrices, this is a well-recognized limitation of the XRF technique. On the other hand, LIBS uses plasma-generated photons produced by a combination of excitation and ionization from the laser, which are significantly affected by the matrix elements. LALI, however, has more efficient ionization and significantly lower matrix effects, making quantification more straightforward. LALI’s strength over LIBS is that the source ionizes gas-phase particles within the neutral particle cloud, instead of relying on plasma generated ions during ablation. As a result, the LALI particles in the neutral cloud are significantly less variable across different matrices. Ionization of neutral cloud material also results in stoichiometric accuracy, enabling quantification of a variety of sample matrices without the need for matrix-matched standards.

	Another major advantage of the LALI design is that ionization occurs under vacuum in the sample chamber. It is a completely static system, held at high-vacuum (~10

 mbar) in the TOF, and a pressure gradient to ~10

 mbar in quadrupoles and the ion optics. The pressure in the LALI sample chamber is maintained at ~0.2–0.3 mbar with an inert helium cooling gas system. The low pressure of the ion source results in a significant improvement in sensitivity, because it greatly reduces losses associated with gas transport from atmospheric pressure to a vacuum system.

	The removal of the plasma source also has the advantage that thermal emission of contaminant ions from the cones or injector is eliminated, greatly improving the ability to determine many of the volatile elements at both low and high masses, including Na (mass 23) and Pb (masses 204, 206, 207, and 208). Additionally, without a plasma source or any carrier gas, LALI does not rely on components with dynamic fluctuations, thus leading to long-term signal stability without requiring regular instrument tuning and calibration. Figure 3 shows the long-term stability of a suite of trace elements in a NIST 610 standard reference glass over a period of 8 h, with the precision (% RSD) of multiple measurements. Note that each of the three sets of data points (morning, afternoon, and night) represents an average of 1000 scans.

	Without an ICP source, the technique does not require a carrier gas, eliminating all polyatomic molecular interferences derived from argon. In addition, because there is very little opportunity for air entrainment inside the ionization chamber, all oxygen, nitrogen, and carbon-based polyatomic and isobaric interferences are drastically reduced. Elimination of these ionic species significantly improves the detection for a whole host of elements, including Si, S, Ca, Mg, Si, Cr, and Fe, which are traditionally problematic for ICP-MS. This is exemplified in Figure 4, which show a 0–130 amu scan of a high purity nickel certified reference material (IARM-191A, Analytical Reference Materials, Intl.). The major isotopes of nickel at masses 58, 60, 61, 62, and 64 amu are shown in the center of the scan, while zoomed sections are shown in the boxes. It can be seen that spectral peaks for the different isotopes of Mg, Si, S, Fe, Cr, Cu, Ca, Mo, and Sn are clearly being detected and measured at the low ppm level. It should also be emphasized that using an ICP-MS instrument, either as a solution technique or coupled with a laser ablation system, there would be large spectral overlaps from 

Ca. The green traces, which represent theoretical isotopic distributions, closely match the experimental results across the entire mass range, indicating that no such spectral overlaps are present.

	To emphasize the purity of the elemental spectral fingerprints across the entire mass range, Figures 5a and 5b show a LALI mass spectrum of a NIST 610 standard reference glass. Figure 5a exemplifies a suite of transition elements in the spectral region 0–90 amu, free of the most common polyatomic and isobaric interferences, while Figure 5b shows some rare earth elements in the spectral region 90–220 amu, where doubly charged interferences are commonly observed. Theoretical isotopic distributions are shown in green.

–90 amu) of NIST 610 standard reference glass, with zoomed regions of specific elements highlighted; (b) LALI mass spectrum (90–220 amu) of NIST 610 standard glass, with zoomed regions of specific elements highlighted.

	LALI also provides considerable improvements in transmission efficiency of an ion beam compared to techniques that generate ions at atmospheric pressure. For example, ions generated by the plasma discharge of an ICP-MS instrument at atmospheric pressure are generally transferred in several stages before reaching the high-vacuum mass analyzer.  Each stage transition of cones and/or lenses removes a significant portion of ions. For instance, LA–ICP–MS has a very high ionization efficiency for elements with a first ionization potential (FIP) less than 8 eV, but only ~1 in every 10

 ions reach the detector (~0.01–0.001% transmission efficiency). The LALI source is already under vacuum, so it does not suffer from transmission loss going from atmospheric pressure to vacuum. Removing the atmospheric or vacuum interface greatly improves transmission efficiency, allows for higher sensitivity, and further reduces matrix effects by removing plasma-ion spatial interaction effects.

Identification of Atomic and Molecular Species 

	LALI is capable of analyzing both inorganic and organic species, which is difficult to carry out with other techniques that suffer from the problems previously discussed (8). The ability to analyze organic compounds by LALI has been studied by many groups for applications including planetary missions and crude oil analysis (6,9). The analysis of organics utilizes the infrared (IR) component of the Nd:YAG laser (1064 nm). The intense IR pulse flash heats the sample (10

 K/s) to desorb intact material from the sample surface (10). Following desorption, organic compounds are ionized via MPI, using the secondary ionization laser as previously described. The ability to analyze both organics and inorganics in the same analytical run enables mapping (or bulk characterization) of both in the same sample almost simultaneously after a quick mode switch. To enable accurate assessment and interpretation of molecular species, an optional high resolution TOF mass spectrometer with a resolving power of up to 14,000 is available.

	LALI–TOF-MS is suited to many application areas by customizing the configuration of the mass spectrometer, based on the spectral demands of the analysis. Under normal conditions for carrying elemental analysis, the resolving power (m/dm) of the TOF mass spectrometer is in the order of 700–1100 (the range for quadrupole MS is typically 400–500). However, for characterizing molecular species, the resolving power can be increased from 7000 up to 14,000. Figure 6 is a plot of resolving power (as m/dm) against mass (

) for the typical configuration used for trace element analysis.

	To demonstrate the practical benefits of resolving power, a spectral scan of five barium isotopes at 138, 137, 136, 135, and 134 amu using a resolving power of 750 is exemplified in Figure 7, showing the clean separation and the isotopic purity of the Ba isotopes (theoretical distribution of the isotopes is shown in green). A comparison of experimental vs. theoretical abundances for each of the isotopes is also shown, which demonstrates very good agreement, within experimental error.

Macro and Micro Analysis and Imaging Applications

	This technology is well suited for both macro and micro analysis, together with laser imaging studies. This is exemplified in Figure 8, which shows the sample chamber opening to allow the sample tray to be loaded.

	A macro-camera then opens to take a high-precision, spatially located picture. After the sample chamber door closes the chamber begins to pump down to vacuum, and the high-resolution image from the macro-camera is loaded onto the touch screen interface. The macro-image is used to enable navigation around the samples, as shown in Figure 9. Moving back and forth on the screen physically moves the sample stage within the chamber, and aligns the desired area properly with the lasers. “Pinching” on the image zooms in and switches to a live microscopy image that allows precision when choosing an area to analyze. From the live view, spots, lines, or rasters for maps are chosen. After selecting the type and number of sampling areas, analysis and data processing are automated, and all that remains is to interpret the results. This capability is well-suited for microanalysis of small surface inclusions, depth profiling analysis for coating thickness, or two-dimensional, high-spatial-resolution, multi-elemental mapping, rapidly becoming an important research tool in the field of geological, biological and medical imaging studies (11).

	LALI-TOF-MS provides high specificity and high sensitivity quantitative analysis for both bulk and micro analysis, together with elemental and chemical mapping, without the need for complex quantification schemes or matrix-matched standard materials. The technique’s simple design enables field portability, and its easy-to-use, intuitive interface makes personnel and training requirements very straightforward. Together, these factors open a wide range of potential inorganic and organic application areas. As a result, this emerging technology is expected to have a large impact in the field of geological, metallurgical, petrochemical, agronomy, agricultural, biological, and cannabis studies.

		P. Hurtado, F. Gamez, and B. Martinez-Haya, 

		Y. Bussweiler, O. Borovinskaya, and M. Tanner, 

is principal of Scientific Solutions, a consulting company that serves the application and writing needs of the trace element user community. He has worked in the field of atomic and mass spectroscopy for more than 40 years and has written over 100 technical publications including a 15-part tutorial series on ICP-MS. He recently completed his fourth textbook entitled 

Measuring Elemental Impurities in Pharmaceuticals: A Practical Guide,

 and is currently working on his next book project which focuses on heavy metals in cannabis and hemp. He has an advanced degree in analytical chemistry from the University of Wales, UK, and is also a Fellow of the Royal Society of Chemistry (FRSC) and a Chartered Chemist (CChem).

Atomic Perspectives

Recent advances in laser ablation laser ionization TOF mass spectrometry (LALI-TOF-MS) are allowing laboratories to reach required limits of quantitation for trace impurities in solid materials by direct analysis, while avoiding common limitations of other techniques.

Advances in Trace Element Solid Sample Analysis: Laser Ablation Laser Ionization TOF Mass Spectrometry (LALI-TOF-MS)

In situ high-pressure Raman measurement of ascorbic acid has been conducted in the pressure range of ambient pressure to 14 GPa. A phase transition occurs at 3.8G Pa, and, at the pressure of 3.8 GPa, a small C=O band appears at 1767 cm

, and the intensity increased with further compression. Furthermore, as the pressure increases, the crystal symmetry of ascorbic acid becomes lower.

	Ascorbic acid, also known as vitamin C, is an acidic and strongly reducing polyol containing six carbon atoms. It is a colorless and odorless flake crystal, soluble in water, but insoluble in fat solvents (1,2). Ascorbic acid is easily oxidized to dehydrogenated vitamin C, but it is stable in an acidic environment. It is easily oxidized by oxygen, and is susceptible to temperature, pH, copper ions, iron ions, and light (3). Ascorbic acid is widely found in fresh vegetables and fruits, as well as in other foods. It is one of the important nutrients for human nutrition. If the body lacks ascorbic acid, it can cause scurvy, characterized by bleeding in the skin and gums (4,5). In addition, it is a neuromodulator, and is used by neurons in many different functions and enzymatic reactions (6–8). Due to its high reactivity to oxygen, ascorbic acid has received extensive attention related to the prevention and treatment of cancer (9–13).

	In the past two decades, high-pressure in situ Raman spectroscopy, using diamond anvil cell technology, has become one of the important means to study molecular structure and properties under high pressure (14–20). We can study the changes in material structure and electron orbital changes through Raman spectroscopy (21–26).

	Pressure is an effective thermodynamic parameter, and is considered a powerful tool for studying the detailed structure and physical and chemical properties of matter. Pressure can effectively change the lattice structure, and therefore shorten the distance between atoms-this affects the nature of chemical bonds between molecules, molecular conformation, crystal structure, and solid stability (27–30). Pressure-induced phase transition is a common phenomenon in high-pressure science. To date, a high-pressure Raman study of ascorbic acid has not been reported.

	The ascorbic acid used in this experiment was purchased from Sigma Aldrich, and was used without purification. The high-pressure cell used in the experiment is a symmetric diamond anvil cell (DAC). The DAC has two diamonds with 500 μm culet size. A T301 (thickness 250 μm) steel gasket was used as a sealing device for generating high-pressure. Ruby powder, having a diameter of about 12 μm, was used as an internal standard to calibrate and convey the pressure in the cavity. The ruby fluorescent line R1 (at a wavelength of 694.2 nm) linearly moves as the pressure increases (31). No additional pressure-transmitting medium was used in this experiment.

	Raman spectra were measured using a Renishaw instrument via confocal Raman spectroscopy. The excitation source was a 514.5 nm Argon Particle Laser (Spectra Physics 160M). Backward Raman scattering was measured with a 50× objective lens. The laser power used was 10 mW. The accumulation time for the measurement was typically 60 s. All of the experiments were carried out at constant room temperature (~21 

	Ascorbic acid crystal belongs to the monoclinic crystal system, and its space group is P21 (32,33). Figures 1 and 2 show the crystal structure and Raman spectrum of ascorbic acid, respectively, in a normal-pressure environment. The Raman spectrum of ascorbic acid is consistent with what Singh and associates have reported (8). The strong Raman scattering lines located at 1124, 1651, 1664, and 2915  cm

 are assigned to C-O bending, C=C stretching, and C-H stretching, respectively. Peaks at 562 and 583 cm

 are attributed to be OH out-of-plane deformation and C=C ring stretching mode, respectively. Detailed Raman vibrational assignments are given in Table I.

High-Pressure Raman  Spectroscopy of Ascorbic Acid

	Figure 3 shows the high-pressure Raman spectra of ascorbic acid. It can be seen that as the pressure increases the bandwidth of most Raman bands will gradually widen. The Raman peaks shift to a higher wavenumber region. This is because, as the pressure is gradually increased, the distance between atoms decreases, and the force constant of the valence bonds increases, so that the vibration frequency increases. Figure 3 shows the Raman spectra of ascorbic acid recorded at selected pressure for the 100–3200 cm

 spectral region. Raman spectra below 100 cm

 cannot be observed, due to the notch filter in use. In addition, it is difficult to identify the Raman peaks at around 1340 cm

, due to the first-order Raman peaks of the diamond cell. Furthermore, as can be seen from Figure 3, with the increase of pressure, the vibration intensity of the Raman band at 583 is enhanced. This may be due to the compression of the C=C ring, and the change in the dihedral angle of the OH outside the ring. And at the pressure of about 3.8 GPa, a weak C=O stretching band appears at 1767 cm

. Further compression will result in a gradual increase in the intensity of this band-possibly the result of the activation of the silent mode, due to changes in molecular symmetry (23).

Frequency–Pressure Relationship of Ascorbic Acid

	In general, the Raman frequency pressure relationship can provide useful information about the crystal structure at high pressure. In addition to the disappearance of the original peaks and the appearance of new peaks, the frequency–pressure relationship is also an important indicator of the pressure-induced phase transition (34–35). As can be seen in Figure 4, with an increase of pressure, the relationship between frequency and pressure is basically linear. However, at about 3.8 GPa, the frequency–pressure slope of the Raman peaks of ascorbic acid changes significantly. The four linear equations at 562, 624, 693, and 3000 cm

 are changed from y = 3.2708x+563.19, y = 2.377x+625.15, y = 7.3727x+695.54, and y = 9 .85x+3004.54 to y = 1.84702x+565.392, y = 1.3612x+626.75, y = 3.285x+700.8, and y = 6.545x+3022.8287, respectively (

]). The sudden changes in the frequency-pressure relationship should correlate with the material phase transition. Therefore, we believe that a first-order phase transition of ascorbic acid occurs at approximately 3.8 GPa. We define the phase under pressure of 3.8 GPa as phase I, and when the pressure is higher than 3.8 GPa as phase II. The change in the molecular properties of ascorbic acid caused by phase transition remains a subject to be studied.

	Additionally, a change in the symmetry of the molecular point group in ascorbic acid appears to accompany the phase change at 3.8 GPa. New bands at 1510 and 1767 cm

 appear in phase II, indicating that the symmetry of ascorbic acid in phase II is lower than phase I. This also may be related to the fracture and rearrangement of the H-bond structure. Due to the effect of high pressure, most of the frequency–pressure Raman peaks of ascorbic acid become smaller. We also believe that the bonds in the crystal become more rigid after the phase transition.

	In this work, we used high–pressure Raman spectroscopy (~14 GPa) to study the structure and Raman spectra of ascorbic acid. By observing changes in peak positions and intensities at approximately 583, 693, and 1746 cm

, it is shown that the internal and side chains of the ascorbic acid ring are changed under high pressure. Furthermore, we analyzed the frequency–pressure relationship of ascorbic acid, and found that ascorbic acid undergoes a phase transition at 3.8 GPa, and the symmetry of the crystal structure also changes. We hope that the results of this study can provide some insights into the study of the structure of ascorbic acid under high pressure.

	This work was supported by the Jilin Provincial Research Foundation for International Scientific and Technological Cooperation Projects (Grant No.20170414026GH), and the Jilin Provincial Science Foundation (Grant No.20180101219JC).

		B.M. Tolbert, M. Downing, R.W. Carlson, et al., 

Study on the Stability of Vitamin C and the Determination of the Content of Fruits, Vegetables and Fruit Juice by Ultraviolet Spectrophotometry, 

(Marcel Dekker, New York, New York, 2002).

		C.Y. Panicker, H.T. Varghese, and D. Philip, 

 (Marcel Dekker, New York, New York, 1997).

		G. Singh, B.P. Mohanty, and G.S.S. Saini, 

		R.E. Patterson , E. White, A.R. Kristal, et al., 

		K.N. Prasad, A. Kumar, V. Kochupillai V, et al., 

		L. Beer, J.L. Brusso, R.C. Haddon, et al., 

		K.W. Chapman, G.J. Halder and P.J. Chupas, 

		F.J. Lamelas, Z.A. Dreger, and Y.M.  Gupta, 

		A. Olejniczak, K. Ostrowska, and A. Katrusiak, 

		R.O. Goncalves, P.T.C. Freire, H.N.  Bordallo H N, et al., 

		S. Meijiao, W. Kai, X. Shengnan, et al., 

, A. Katrusiak and P.F. McMillan, Eds. (Kluwer, Dodrecht, The Netherlands, 2004), pp. 367–392.

, A. Katrusiak and P.F. McMillan, Eds. (Kluwer, Dodrecht, The Netherlands, 2004), pp. 411–428.

		W.L. Liu, Z.R. Zheng, R.B. Zhu, et al., 

		V.V. Mozhaev, K. Heremans, J. Frank, et al., 

		R.J.C. Lima, A.M.R. Teixeira, P.T.C. Freire, et al., 

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		C. Mitterdorfer, J. Bernard, F. Klauser, et al., 

Peer-Reviewed Research

A Raman study of vitamin C, also known as ascorbic acid, is conducted at high pressure to determine phase changes and crystal symmetry through spectral interpretation.

The Effect of High Pressure on Ascorbic Acid Structure

We present our annual review of products introduced at Pittcon or during the previous year, broken down by the following categories: accessories, atomic spectroscopy, components, imaging, mass spectrometry, mid-infrared (mid-IR) spectroscopy, near-infrared (NIR) spectroscopy, nuclear magnetic resonance (NMR), Raman spectroscopy, software, ultraviolet-visible (UV-vis) spectroscopy, and X-ray.

	This review focuses on new instrumentation, components, and accessories launched from May 2019 through April 2020, announced either directly to 

 through form submissions or by those presented as new at the recent Pittsburgh Conference (Pittcon) in Chicago, Illinois, where we spoke directly with vendors.

	This year’s Pittcon was interrupted by a larger societal concern-the sudden spread of the coronavirus. Pittcon was not canceled, but many exhibitors chose not to attend. Even so, there were still a large number of exhibitors, representing a cross section of the industry (from smaller suppliers to large instrument companies). The total attendance, reported as 9011, was, nevertheless, quite respectable, despite the challenges.

	In this review, we are interested only in products that fall under the umbrella of spectroscopy, whether they be instruments, components, accessories, standards, software, or anything else related to spectroscopic measurement.

	One of the points of interest at Pittcon is the awards for instruments  considered the most innovative. This evaluation is, of necessity, subjective, and involves a selection from among all the instruments on display at Pittcon. Starting three years ago, the selection methods and criteria were changed, and the awards are now called the Pittcon Today Awards. A selection committee, consisting of a blue-ribbon panel of experts, evaluates the entries based on ingenuity, creativity, implementation, and outcomes. Instruments are categorized by the size of the company (small, medium, and large) that makes them, based on their sales figures. This allows companies to compete with others in their same class. There are gold, silver, and bronze awards in each category, for a total of nine awards. We are pleased that this year four awards went to spectroscopy instruments. In the small company category, Waveguide won the gold for world’s first small, battery-powered, handheld NMR instrument and Aeris Technologies won bronze for a real-time mid-IR-based ultrasensitive ethylene oxide gas analyzer. In the large-company category, Bruker won gold for its Lumos 2 FT-IR microscope, and Horiba won bronze for the LabRam Soleil instrument that combines Raman, UV-vis, and NIR spectroscopy.

	The review that follows is organized alphabetically by wavelength region or type of spectroscopy-for example, mid-infrared (IR), Raman, X-ray, and so forth-except that we place the two non-instrument categories (accessories and components) at the end. Table I sorts the companies alphabetically, and lists the categories they appear in. We arrange our review to allow readers to compare instruments from different manufacturers, although this process sometimes classifies low-end handheld instruments with high-end research tools. The categories used to classify the products are:

	Our categories fluctuate from year to year, depending on trends in the submissions. The core spectroscopy headings are consistent, but the imaging category, for example, has been transient. This year, we received enough products classified as imaging to warrant that being a category in itself, rather than only including imaging versions of various technologies under the parent instrument type. Fluorescence was a popular category some time ago, but has fallen out of favor for several years. Last year it returned, but that proved to be only temporary; this year, there were again no submissions in that category, despite its analytical popularity and presence at trade shows.

 are tools used with an instrument-like sample preparation or optical devices-whereas 

 are parts used as integral parts within instruments, like lasers or detectors. The software category includes instrument control and data processing, storage, and transmission, as well as spectral databases (libraries) and other collections of specialized information, besides programs that analyze the data.

	One of our goals in writing this instrumentation review, as well as spending time at instrumentation trade shows (such as Pittcon), is to identify larger themes from the aggregation of submissions.

	Some years ago, we had seen environmental solutions trending, especially around microplastics. This year, environmental issues were muted under the public health solutions around opioids, cannabis, and COVID-19. The plenary lecture at Pittcon, and many hallway conversations, were dominated by these subjects, and vendors were touting testing and research tools. The absence of several vendors, especially from Asia and some parts of Europe (and the reticence of attendees to shake hands), put an exclamation point on this.

	Within the instrument submissions themselves, we see many tools and instruments relating to Raman spectroscopy. We discuss this further in the Raman spectroscopy section, but note here that component and accessory suppliers, as well as instrument manufacturers, have identified Raman spectroscopy as a significant market. Overall, we saw in recent years many handheld instruments and new laser sets, and also found that large instruments and software were prevalent
	this year.

	In the expanding world of software, we reported last year on the emerging uses of computers in artificial intelligence (AI) applications and integrating with the internet of things (IoT), as expansion of IoT is starting to come to fruition, albeit not as soon as we originally expected. We note this in our
	review below.

	One continuing trend was the combination of various technologies into a single instrument. Several companies, for example, are now combining Raman spectroscopy with other optical spectroscopies, such as UV-vis or infrared. These combinations are often augmented with special software to enable the instrument control computer to find commonalities or other relationships between the spectra produced by the different technologies. Some details about this are apparent in the Raman spectroscopy section of
	this review.

	Vendors have all identified the trend in many laboratories of downsizing staffing, even with a continued increase in sample loads. This means a single worker must be able to move between multiple instruments without major barriers. Easy sample transfer and shallow learning curves via workflow software are becoming common needs. With instrument vendors, this information is often buried, both in their product development and their marketing literature, under the prevalent and long-standing emphasis on hardware performance (signal-to-noise, resolution, speed, and so forth), but it becomes immediately apparent when you watch vendors demonstrate their instruments. The demonstration is increasingly a software show. This trend is discussed further in the software section of this review.

	This year, we added a new tactic for the trend predictions. We contacted subject-matter experts to discuss future trends in their technologies. We thought that people working in the same area might be seeing the same forces affecting their technology. Below, we share some of the responses we received to this inquiry. Because these comments represent opinions rather than hard facts, to preserve privacy, we do not identify the company or person responding with each comment, but we list the expected trends for the technology. For those companies that responded to this question and have multiple products, their response is listed under the technology category that their instrument containing the response belongs.

		“[There is] a general trend towards higher resolution X-ray and neutron indirect detection cameras.”

		“[There is a] continuous trend of improving the data storage, handling, and processing allowed hyperspectral imaging to take place as it generates very heavy images. One image captured by [trade-name redacted] can require 1 Gb [of data], as it contains a lot of information of a scene.”

		“Shortwave infrared (SWIR) imaging is growing in many biological and chemical analysis applications.”

		“Rapid growth of several industrial market segments (including chiral chemicals, asymmetric synthesis, cannabinoids, essential oils, and others) have created a strong demand for analytical techniques capable of fast isomer-selective chemical analysis. The emerging [trade-name redacted] technology addresses this demand, … offer[ing] extraordinary chemical specificity combined with fast analysis and ease of use.”

		“Photonics devices [will] become much more advanced. This requires [technicians] to use optical components with extreme specifications. These specifications need to be measured and confirmed, so this leads to design of more advanced metrology and testing equipment.”

		“Close to 20 years ago, the first portable X-ray fluorescence (XRF) devices were released for easy-to-use metal and mining applications for non-scientific users. A decade ago, there was a strong thrust into the portable optical/vibrational spectroscopy market, with Raman, NIR, and FT-IR for safety, security, military, and pharmaceutical raw-material ID analysis. One company [name redacted] extends this portable theme further with the world’s first portable NMR, which addresses the gaps of XRF and optical and vibrational [spectroscopy] in the areas of biologics, vaccines, foods, and, ultimately, in medical diagnostics. The theme or trend is routine chemical screening and non-scientific operators who need to make a fast and precise decision at the point of need.”

		“The pharmaceutical industry is not making large investments at this time, as market consolidation and acquisitions are on the rise in this sector. The demand for field identification of controlled substances and threats is global, with needs specific to geographic regions. Investment in technology is tied to national budgets, and therefore impacted by geopolitical and global economic trends.”

		“With continued improvements in Raman technology, more sophisticated trace-level and low-concentration measurements are becoming possible.”

		“Automation is among the most important long-term trends in the development of scientific instruments. Using optimization and calibration routines, along with pre-set and saved experimental workflows, reduces input demands on the researcher, while allowing them to focus on the data they acquire. Errors are minimized, while operational turnover is maximized, enabling laboratories and user facilities to generate more, and more consistent, data within a given amount of time.”

		cloud-based analytical solutions-instrument management and reporting, mostly-from both customers and

		“Hyperspectral imaging is [a] growing field that is finding applications in processing, medical imaging, environmental applications, and earth sensing. [Trade-name redacted] allows incorporation of domain knowledge necessary for interpretable results required by engineers and scientists.“

		“Chemometrics is falling under a broader field of data science and machine learning. In addition to AI tools, our package is more flexible than most tools, and allows incorporation of domain knowledge necessary for interpretable results required by engineers and scientists.”

		“The increasing development of handheld or smaller sized spectrometers.”

		“NIR in general is moving away from technology experts with years of training and experience, to users of the technology that need to get results. [Company name redacted] is responding by creating intuitive products that provide top performance and capabilities in a user-friendly format.”

		“[Increasing use of the] home office.”

		“General optimism and more government money being spent in science, especially [in the fields of] astrophysics and physics. For example, more of our customers are getting funded via National Science Foundation/Department of Energy, and even NASA.”

		“[Technology] is driving toward ever-lower scatter, while demanding high efficiency.”

		“Today’s trace metals analysis laboratories are constantly being driven to achieve lower levels of detection, which, in turn, put greater demand on clean chemistry techniques and applications.”

		“Increasing automation in the laboratory has led to a demand for easy-to-operate pumping systems that do not require pressure to be set individually.”

	Some current (and even past) practices may become future trends, if and when they become more widespread and generalized. In the meanwhile, sporadic mentions of these practices appear in advertisements, in scientific literature, and even in this review. For example, some instrument manufacturers have created a capability of remotely controlling and collecting data from their own instruments (networking) for a long time. As we said last year, when that capability expands to include more than one type of instrument, then it would be fair to rename that localized networking as belonging to IoT, but not until then. Similarly, can the current capabilities of the application of chemometric analysis to instrument data, to perform quantitative and qualitative analysis, be called 

	As we have noted almost since we first started trying to detect instrument trends, the trend toward hyphenated techniques has been coming along slowly, but surely, over the years. A recent twist on that concept, broadening the idea of applying multiple technologies to an analysis, is the packaging of dissimilar, and indeed sometimes even orthogonal, technologies in a single instrument package, without the two technologies necessarily interacting (as hyphenated methods do). We first noted this trend several years ago, but, even in last year’s review, there was only one example of this combining of technologies, With hindsight, we can see the beginnings of this trend several years ago, when Raman spectroscopy was combined with an atomic force microscope (AFM) in one unit. In this review, there are multiple companies demonstrating combined technologies; interestingly, all of them include Raman spectroscopy as one of the technologies.

	Some other potential miscellaneous trends can be seen if you keep your eye tuned for them, one of them being a change in the nature of applications from being chemistry-focused to becoming more focused toward biochemical and biomedical applications.

	Both Thermo Fisher Scientific and Perkin
	Elmer introduced new inductively coupled plasma (ICP) solutions, striving for increased sensitivity and speed. In presenting these instruments, these companies focus on use in a wide range of markets (including lubrication, pharmaceuticals, and environmental markets). In contrast, PS Analytical focuses on mercury monitoring, with a continuous operations profile.

Featured Article

Our annual review of products introduced at Pittcon or during the previous year.

Spectroscopy-05-01-2020

Organic Nitrogen Compounds IX: Urethanes and Diisocyanates

Vol 35 No 5 Spectroscopy May 2020 Regular Issue PDF

Advances in Trace Element Solid Sample Analysis: Laser Ablation Laser Ionization TOF Mass Spectrometry (LALI-TOF-MS)

The Effect of High Pressure on Ascorbic Acid Structure

2020 Review of New Spectroscopic Instrumentation