Single reaction chamber (SRC) microwave digestion offers multiple benefits over traditional dry ashing for the sample preparation of heavy crude oils for trace metals analysis. These include increased sample throughput, reduced risk of contamination, more complete digestion and retention of volatile analytes.

	The digestion of heavy crude oil samples prior to trace metal analysis by ICP-OES has traditionally been performed by dry ashing in a muffle furnace. The oil is burned off over a burner prior to ashing in a muffle furnace. This process takes several hours and requires continual operator attention. Loss of sample and cross contamination can occur due to foaming and splashing. Additionally, volatile analytes are lost and cannot be measured. While closed vessel microwave digestion offers several advantages over dry ashing, the max sample weight of ~0.2 g is considered too small for resentative analysis. Single reaction chamber (SRC) microwave digestion, however, can digest higher sample weights, enabling microwave digestion to be applied to the sample preparation of heavy oils.


	Figure 1: UltraWAVE digestion program with 5 × 0.75 g heavy oil: ramp to 250 °C over 30 min and hold for 10 min. The line T1 shows the actual digestion temperature achieved. The applied microwave power is automatically controlled by the system so the actual digestion temperature precisely follows the programmed temperature profile. The outer temperature of the reaction chamber is also monitored (T2). Note that pressure (P) reached almost 100 bar during the run. This is due to the high weight and high organic content of the samples, which generates NO

. Pressure was released automatically at 55 min.

	Milestone's new UltraWAVE SRC benchtop digestion system can digest up to five heavy oil samples at a time, max 300 °C and 199 bar pressure. Its high temperature and pressure capability enables the complete digestion of heavy oils, which makes sample introduction to the ICP-OES easier and more routine. The Ultra-WAVE can digest 0.75 g of heavy oil (up to five samples simultaneously)-more than 3x higher sample weight than closed vessel digestion. Samples are simply digested in disposable glass vials: unlike dry ashing, there are no crucibles to clean. The system is fully automated: the sample is weighed into a vial, 10 mL HNO

 added and loaded into the UltraWAVE. Its walk away operation means labor costs are significantly reduced over dry ashing. The reaction chamber is pre-pressurized with N

, which eliminates cross contamination of loss of volatiles.


	Figure 2: Sample rack being loaded and samples after digestion.

	Sample digestion requires about one hour, including cool down time with the Milestone UltraWAVE. The digestion of heavy oils in the amounts described were sufficient for the application with increased sample throughput and reduced labor costs compared to dry ashing. Risk of contamination and loss of volatile elements is eliminated and the superior digestion quality allows for easier analysis by ICP-OES.


	25 Controls Drive, Shelton, CT 06484
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Articles

Atomic Spectroscopy

SRC Microwave Digestion

Glass Expansion has partnered with Inorganic Ventures, the well-known certified reference material (CRM) manufacturer, to investigate a simple, low cost approach to improving ICP productivity. The benefits of Glass Expansion's redesigned Niagara CM Rapid Rinse accessory were evaluated.

	Most ICP systems incorporate an autosampler. With these systems there is a significant delay between the time when the autosampler probe enters the sample and the time when the sample reaches the nebulizer. There is similar delay between the time when the probe enters the rinse and the time when the rinse reaches the nebulizer. If these delays could be eliminated, the analysis time per sample would be reduced.

	This is exactly what the Niagara CM Rapid Rinse (referred to as Niagara henceforth) achieves, by rinsing the nebulizer and spray chamber the instant the sample measurement is completed and continuing to rinse until the next sample has reached the nebulizer. Thus the rinse is carried out during the time it takes for the sample and the rinse solutions to flow from the autosampler probe to the nebulizer. Typical time saved with the Niagara is around 30% and since the valve switches immediately after the read is complete, you also minimize the amount of sample introduced to the ICP. This feature drastically reduces consumables costs and frequency of cleaning, particularly for challenging sample matrices.

	Since the original Niagara was introduced in January 2005 (2), Glass Expansion's line of enhanced productivity accessories have been continuously evolving to keep up with the demands of ICP analyses. Most recently, in 2013 the Control Module (CM) was introduced adding the suffix CM to denote this (3).

	We now introduce a new software version, called NRRV2, which includes a redesigned Method Wizard featuring an Uptake Delay Calculator, Rinse Tubing Calculator, and Time in Sample (TIS) Calculator. The addition of these calculators greatly simplifies method development.

	The Uptake Delay Time is the time required for the sample to reach the valve and pass through to waste. Once the Uptake Delay Time has finished, the Niagara actuator will switch from Rinse to the Home position, directing the sample to the nebulizer (Figure 1a). The Uptake Delay Time is calculated based on the ID and length of the autosampler probe, peristaltic pump tubing ID and length, and RPM of the peristaltic pump. The Rinse Tubing Calculator calculates the optimum peristaltic pump tubing to be used in order to achieve the same flow rate that is used during analysis while using a fast pump during the uptake and post-acquisition rinsing steps of the analysis, thereby maximizing stability. TIS is the time required for the autosampler probe to remain in the sample tube during the acquisition (Figure 1b). The TIS is based the total read time, RPM of the peristaltic pump and dimensions of the uptake tubing referenced previously.

	It is well known that adding a series of bubbles can help "scrub" the uptake line. However, air bubbles introduced into the plasma can lead to instability and require longer stabilization delay times. With this latest Niagara update, we can easily add a series of air bubbles with a post wash command without the worry of introducing any air or extra argon into the plasma (Figure 1c). Once the ICP method rinse begins the air bubbles and any excess sample are directed to waste while rinse solution is directed to the nebulizer (Figure 1d).


	Figure 1: a: Niagara CM Rapid Rinse–after uptake delay; b: Niagara CM Rapid Rinse–during acquisition; c: Niagara CM Rapid Rinse–after TIS; d: Niagara CM Rapid Rinse–during ICP method rinse (post-acquisition).

	The speed and performance of Inorganic Venture's single element CRM ICP-OES certification analysis was compared with and without the use of the Niagara. For these certification experiments a Spectro Arcos EOP (Axial) ICP-OES instrument was used in combination with two Glass Expansion sample introduction systems. For samples without HF, the SeaSpray DC nebulizer was used with the Twister spray chamber. The HF-resistant package consisted of the DuraMist DC nebulizer, Tracey PTFE spray chamber and fully ceramic D-Torch (3).

	The Niagara (Figure 2), consists of an electronically controlled switching valve, Control Module and tubing kit. For maximum chemical resistance the valve is made entirely of Teflon and PEEK. The method timings for the Niagara were calculated using the newly developed Method Wizard (4).

	The Niagara allowed Inorganic Ventures to reduce their certification analysis time by 71 s per sample, roughly a 35% improvement. This allows Inorganic Ventures to run an addition 10 samples per hour with the Niagara. The performance of the ICP is unaffected by the Niagara, as an average RSD of less than 0.5% was achieved with both sample introduction systems (4).

	In addition to reducing analysis time, the Niagara also improved washout. Thallium (Tl) is known to be a "sticky" element that can necessitate long wash times. For this reason a 1000 ppm Tl solution was chosen to compare washout with and without the Niagara. Table I contains values in ppm Tl for three consecutive 2% HNO

 blank solutions that were run after a 1000 ppm Tl sample. With only a 10 s rinse, the Niagara achieved a better washout compared to a 30 s rinse without the Niagara, saving a considerable amount of time (31%). Adding bubbles reduced carryover even further, giving better than 4 orders of magnitude washout with a 46% savings in time compared to the standard setup.


	Table I: Washout experiments after analyzing 1000 ppm Tl

	The Niagara CM Rapid Rinse provides a simple, low cost approach to improving ICP productivity. Typical time saved with the Niagara is approximately 30%, with little to no changes to the current ICP method settings and no degradation in performance. The new NRRV2 software with Method Wizard facilitates easy optimization and self-installation. A unique post wash adding a series of bubbles significantly improves washout and reduces rinse time. Also noteworthy, the initial financial investment of the Niagara is 60% to 70% less than other commercially available switching valve systems.

	(1) Glass Expansion October 2013 Newsletter, "How to Achieve High Accuracy with Difficult Samples."

	(2) Glass Expansion October 2005 Newsletter, "Increasing the Productivity of ICP Analyses."

	(3) Glass Expansion February 2013 Newsletter, "Latest Developments in Enhanced Productivity Products."

	(4) Glass Expansion June 2016 Newsletter, "A Simple, Low Cost Approach to Improving ICP Productivity."


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A Simple, Low-Cost Approach to Improving ICP Productivity

	With the continued globalization of trade, it has become increasingly important in recent years to ensure the safety of the food supply, as there have been numerous instances of metals contamination. This has led to increased demand for better analytical techniques and lower detection limits for elements of interest. Food samples require some form of sample preparation prior to final analysis and in most cases a complete digestion of solid samples is required. As spectroscopic instrumentation has improved resulting in lowered detection limits, CEM has made advances in microwave sample preparation technology to simplify the technique and ensure complete digestions. The latest of these innovations is iWave™.

	iWave utilizes Light Emitting Technology™ (LET) that renders the vessel transparent. This allows for a temperature measurement of every sample in real time. For the last 25 years, the fiber optic probe has been considered the gold standard for temperature measurement in microwave acid digestion. Now, iWave has the accuracy of a fiber optic probe without the time consuming set up and considerable consumable cost.

	In order to highlight the benefits of this new technology, several types of food samples were prepared in a single run using standard IR and MARS 6™ with iWave. These samples were then prepared for elemental analysis.

	The samples were digested using a CEM MARS 6, with iWave contactless in-situ temperature sensing, in 75-mL MARSXpress™ vessels, on the 40-position turntable.

	Four different retail products (American cheese slices, ground sausage, granola bars, and milk powder) were digested in the MARS 6 with standard IR and iWave. Samples of 0.5 g were digested with 10 mL of HNO

, ramped to 210 °C for 20 min and held at that temperature for 15 min.


	Figure 1: iWave's contactless temperature measurements track perfectly with the fiber optic probe.

	All of the digested samples were clear and particle free upon dilution. The samples digested using iWave resulted in a more complete digestion of all the samples in this mixed food study. Figure 2 highlights the increased precision of the power over time. This ability to fine adjust the power control allows MARS 6 with iWave to analyze a greater variety of sample types in a single run.


	Figure 2: Using iWave, the power response is much tighter than the standard IR sensor, resulting in greater control of the reaction.

	Based on this mixed food study, the benefits of using microwave acid digestion with iWave technology include:

		The ability to run more diverse types of mixed foods, such as high fat sausage and high carbohydrate granola, in the same batch.

		Additional control over the sample reaction, generating a better digestion that delivers the optimum analytical results.

		A new straightforward standard for temperature measurement in microwave acid digestion.


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Preparation of Mixed Food Samples Using Microwave Acid Digestion with Novel iWave Contactless In-Situ Temperature Measuring Technology

	ATR is the most widely used FT-IR sampling technique, and is frequently used for quantitative, qualitative, and kinetic studies. New high-throughput accessories are expanding the scope of experiments by providing improved signal-to-noise performance. This application note demonstrates the potential of using a new high-throughput single-reflection diamond ATR accessory to study reaction kinetics by examining the heating of an aqueous sugar slurry.

	Infrared spectra were collected on an FT-IR spectrometer equipped with the Harrick DiaMaxATR™ single-reflection diamond ATR accessory with its Temperature Controlled Liquid Cell (Figure 1), Temperature Controller, and TempLink Software. The system was purged to remove water vapor and CO

 resolution and signal-averaged over 32 scans. A background spectrum was collected at ambient temperature. Confectioner's sugar was mixed with a small amount of water to generate a slurry and injected into the liquid cell. A sample spectrum was collected at ambient temperature. Then the cell was heated to 100 °C and additional sample spectra were collected at 2 min intervals.


	Figure 1: The DiaMaxATR™ with its Temperature Controlled Liquid Cell and associated Temperature Controller.

	Figure 2 shows the resulting spectra. Initially, there is a broad band centered around 3276 cm

, both of which are attributable primarily to water with a smaller component from the O-H stretches in confectioner's sugar, which is mainly sucrose. There are also bands in the 900–1150 cm

 region from the C-O and C-C stretches and bands in the 1400–1200 cm

 region due to the O-C-H, C-C-H, and C-O-H bends in the sugars, as expected.


	Figure 2: ATR spectra of sugar slurry over time at 100 °C, including plot of peak area versus time at 1519 cm

	Upon heating, the bands attributable to the O-H stretch and H-O-H bend both change shape, as the water evaporates and the primary contribution comes from the bound hydroxyl groups. The bands in the 900–1200 cm

 region also change, as the sucrose breaks down into glucose and fructose which in turn react to become small aromatic compounds and a brown-colored polymer.

 is attributed to aromatic compounds and grows as a function of time. The rate of formation of the aromatic compounds can be determined by plotting the peak area over time. Fitting the curve gives a linear relationship, where the peak area = 0.0026 x (time in minutes) with an R

 of 0.9981. Given the complexity of the reaction, it is somewhat surprising that the data could be fit easily with a linear curve but this may be due to the time scale and temperature used for the measurements.

	As seen herein, high-throughput, single-reflection ATR is valuable for examining thermally induced changes in materials. In the case of the heating an aqueous sugar slurry, it can be used to extract information about the kinetics of the reaction. Further analysis is possible to obtain more rigorous identification of the heat-induced structural changes.


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Molecular Spectroscopy

A Heated ATR Single Reflection Diamond ATR Study of an Aqueous Sugar Slurry

	The uniform blending of active pharmaceutical ingredients (API) and excipients is critical to product quality. The homogeneity of ingredients within a batch or tablet must often be rapidly determined with minimal sample modification.

	In this experiment, laser-induced breakdown spectroscopy (LIBS) analysis was used to rapidly monitor batch powder and tablet homogeneity. The distribution of pharmaceutical ingredients as a function of mixing time was monitored using the percent relative standard deviation (%RSD) of LIBS spectra. In addition, drug distribution within a non-uniformly blended tablet was assessed using LIBS spatial mapping.

	Magnesium stearate, which acts as a lubricant, and croscarmellose sodium, a disintegrant, were used as pharmaceutical additives. Chlorpheniramine maleate, a pharmaceutical with antihistamine activities, was used as the API.

	The API and excipients were mixed using a rotary blender, and analyzed using LIBS.

	In Figure 1, samples (Table I) were mixed in a low shear blender at 38 rpm. After a specific number of rotations, five samples were collected from different locations of the batch and analyzed with LIBS.

	The results from Figure 1 show that the percent relative standard deviation (% RSD) of spectral signal is a useful indicator of sample uniformity. Prior to about 80 rotations, it can be seen that there is still some non-uniformity in the blend, whereas more than 80 rotations showed %RSDs which stabilized-indicating uniformity.


	Table I: Pharmaceutical batch composition

	Several factors impact blend uniformity, including particle size and shape, electrostatic interactions, moisture content and particle surface characteristics. Uniformity analyses should be conducted routinely on blends to ensure quality-and with LIBS, this is rapidly and easily accomplished.

	In Figure 2, Chlorpheniramine maleate (Cl) was analyzed throughout two non-uniformly blended tablets by monitoring 43 separate locations. A surface plot of chlorine intensity collected after 13 pulses is displayed in the XY plane to show the degree of ingredient variation within a tablet.


	Figure 1: LIBS analysis of pharmaceutical blend uniformity.

	The results in Figure 2 demonstrate the ease with which final tablets can be monitored with LIBS to show API spatial uniformity.


	Figure 2: LIBS analysis of drug distribution in non-uniformly blended tablets.

	LIBS was shown to readily determine the minimum rotations or blend time required to produce a homogenous batch, and hence can improve production efficiency for pharmaceutical blending. Monitoring % RSD of LIBS spectra can also be used for quality assurance and blend control, as a RSD value below 5% indicates homogenous blends. Analysis of drug distribution on a tablet could also be determined using LIBS, which is useful for end-of-line product quality assurance checks.

	(1) Y. Mouget, M. Tourigny, P. Gosselin, and S. Bechard, "Three Dimensional Representation of Tablet Content Uniformity by LIBS," presented at Pittcon, 2002.

	(2) Y. Mouget, M. Tourigny, P. Gosselin, and S. Bechard, Eastern Analytical Symposium, (2001).


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Pharmaceutical Ingredient Uniformity Determination Using LIBS

The Savillex PFA cyclonic spray chamber was used with a Thermo ElementXR HR-ICP-MS for the measurement of trace element/calcium ratios in foraminiferal calcite. The signal stability and washout (particularly for boron which is a "sticky" element) were studied. The spray chamber was found to have excellent sensitivity, good short and long term stability, and fast washout (60 s for B), making it well suited to the application.

	Elemental ratio determinations in foraminiferal calcite require the measurement of trace levels of Na, Mg, Sr, and ultra-trace levels of Li, B, Ba, Cd, Mn, U in a calcium rich matrix and ratioed to calcium. Samples are run in an 0.1 M HNO

/0.3 M HF matrix to prevent boron carryover between samples, so an inert sample introduction system is required. PTFE and PFA inert spray chambers generally have inferior performance to quartz spray chambers. We evaluated the recently introduced Savillex PFA cyclonic spray chamber to see if advancements in technology have improved the performance of inert sample introduction systems for ICP-MS. A Thermo ElementXR HR-ICP-MS was fitted with a Savillex inert kit comprising of an inert torch with platinum injector, PFA cyclonic spray chamber (no surface treatment) with baffle, and a C100 PFA concentric nebulizer.


	Figure 1: Savillex PFA cyclonic spray chamber.

	An acid blank and three standards (standards 0, 4, and 8) containing 10 ppm Ca and varying ppt–ppb levels of trace elements were run. After the initial data acquisition for all standards, the blank, standard 0, and standard 8 were measured 17 times each in sequence, to check for reproducibility and drift. The spray chamber was found to have good short-term stability with RSDs of 0.5–1%, and good long-term reproducibility RSDs of 1.5–2.9% over 4 h. No drift correction was applied. Washout of B from the high standard (0.74 ppb) to blank levels took <60 s, demonstrating the good washout characteristics of the spray chamber.


	Figure 2: Boron signal over 5 h-acid blank and standard 0 (2 ppt B in 10 ppm Ca), showing excellent sensitivity and washout (the higher standards in between have been excluded for clarity). For full details visit www.savillex.com/ICP.


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Performance Characteristics of a PFA Cyclonic Spray Chamber with HR ICP-MS

The measurement of Cr, Hg, Pb, Br, and Cd in polyethylene (PE) is demonstrated for RoHS rapid screening by XRF.

	The Restriction on Hazardous Substances initiative (RoHS) limits the allowable amounts of the toxic elements chromium, mercury, lead, bromine, and cadmium in plastics and consumer goods. energy dispersive X-ray fluorescence (EDXRF) is an accepted analysis technique for the rapid screening by XRF and quantification of the hazardous element according to RoHS norms. To meet the industry needs, Rigaku offers the NEX DE VS analyzer with automatic collimators and camera for sample positioning and sample image, giving QA/QC technicians the means for fast and simple screening and analysis of materials that must conform to RoHS and similar directives.

Rigaku NEX DE VS - Small Spot energy dispersive X-ray fluorescence analyzer

10, 3, and 1 mm spot size, automatic switching

High resolution for sample positioning and automatic capture of sample image

	Empirical calibration was performed using the thin certified standards available from Sumika Chemical Services, Ltd. and thick certified standards from ASI Standards. An automatic correction was used to compensate for variations in sample thickness. In this way separate methods or user input is not required for the measurement of samples having different thicknesses.

	Ten repeat measurements of ASI Standards QC check sample were measured to validate the calibration.


	Detection limits (LLD – lower limit of detection)

	Ten repeat measurements of ERM-680k were measured to validate sensitivity at low levels.

	The flexible Rigaku software allows users to design RoHS report field headers to suite specific reporting needs. Headers to report Sample Name and Analyzed By are automatically set by default and the user simply enters up to six additional report memo headers as desired.

	RoHS report is automatically generated that includes the user defined fields, sample image, and element judgment.

	The NEX DE VS with automatic collimators and camera provides excellent sensitivity and reliable precision for RoHS rapid screening by XRF down to 1 mm spot size. Simple Point Analysis Screen allows operators to easily enter report information as designed using customizable report headers and automatically generates an RoHS report complete with sample image, analysis results and Pass/Unsure/Fail judgment.


	9825 Spectrum Drive, Bldg. 4, Suite 475 Austin, TX 78717
	tel. (+1) 512-225-1796, 

RoHS Polyethylene (PE) Rapid Screening by XRF

The ultimate instrumentation for trace gas analysis would be one that can simultaneously provide a sensitive, selective, and fast, multi-gas measurement with wide dynamic range all in a compact and robust system. One of the few such promising technologies that can deliver these requirements is laser based photoacoustic spectroscopy (PAS). In this application note, the PAS technique is presented using the Cobolt Odin™ tunable mid-IR laser.

	Recent developments in technologies for mid-IR light sources allow cantilever-enhanced (CE) PAS instruments to now run with monochromatic and tunable wavelengths, broad spectral selectivity, and high output powers bringing sensitivity and selectivity with sub ppb levels. There are a number of laser technologies now available on the market capable of providing spectrally flexible narrow-band emission in the mid IR spectral range; quantum cascade lasers (QCLs), inter-band cascade lasers (ICLs), DFB diode lasers, and optical parametric oscillators (OPOs).


	Figure 1: Cobolt Odin™. Complete OPO system with all-integrated laser head (pump and OPO) and driver unit.

	In many aspects the OPO technology is the most ideal choice for driving CE-PAS instruments due to the broad spectral selectivity and tuning capability within the 2.8–3.6 μm range. This wavelength region is not readily reachable with QCLs and the available power levels of DFBs and ICLs are comparatively low. Advancements in OPO design and laser packaging technology mean the traditional drawbacks for OPOs of bulkiness and complexity has now been removed.

	The Cobolt Odin™ is a compact pulsed OPO with center wavelength between 2–5 μm and tunable >60 nm where the linewidth can be tailored from <1.5 nm to <0.2 nm, depending on the selectivity and dynamic range requirements. The proprietary HTCure™ manufacturing technology ensures reliability and robustness and means the laser is essentially insensitive to varying ambient conditions (for example, resistant to 60 G shocks and exposure to -20 C to +70 C cycling) bringing reliability and ease-of-use to instrumentation for in-field trace gas detection.


	Figure 3: OPO/CE-PAS measurement of EtOH, MeOH and CH4.

	Experimental demonstration of the OPO/CE-PAS capability was performed using the Cobolt Odin™ operating between 3405–3463 nm (110 mW, 10 kHz). The OPO was coupled into one of Gasera's commercially available PA201 research photoacoustic detector for laser sources to measure ethanol, methanol, and methane.

	Respective univariate detection limits obtained were (at 2x rms, with 1s CIT): for EtOH 7.7 ppb, for MeOH 11.4 ppb and for CH4 35 ppb. The total response time of the system was about 30 s, including the automatic gas exchange and the signal processing for a reduced tuning range.

	The compact design and high mid-IR output power from the Cobolt Odin™, combined with Gasera's novel cantilever enhanced photoaccoustic measurement technology, allow the OPO/CE-PAS technology to offer extreme sensitivity and selectivity for compact, reliable, industrial gas analysis and monitoring.

	(1) J. Peltola, M. Vainio, T. Hieta, J. Uotila, S. Sinisalo, M. Metsälä, et al. 

(8), 10240 (2013), doi:10.1364/OE.21.010240.


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	tel. 46 8 5 45 912 30, fax 46 8 5 45 912 31
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Tunable Mid-IR Lasers Set to Shrink Trace Gas Analysis Systems Using the PAS Technique

	Diamond is an important material choice for sampling optical elements in Fourier transform-infrared (FT-IR) spectroscopy. The multi-phonon absorption in the mid-infrared, although temperature dependent, is weak for thin optical elements and can be referenced to an empty cell background. However, the cost of diamond optics requires the sampling area to be very small. As a result, the signal-to-noise ratio (SNR) advantages of the FT-IR systems are frequently stretched to the limit and more energy throughput is usually quite helpful.

	Frequently the sampling technique of choice is attenuated total reflection (ATR) for its benefits of ease of analysis. ATR elements can be manufactured from most optical materials of relatively high refractive index. Ge, Si, ZnSe, ZnS, KRS-5, and diamond is often used for this application. For most of these materials, a broad band anti-reflection (BBAR) coating can be applied to both surfaces of a window or the entrance and exit facets of an ATR element and can significantly increase the energy throughput of the sampling accessory or system.

	Since diamond and ZnSe have similar refractive indexes a similar BBAR can be used on Diamond once the binding layers of the coating are appropriately modified.

	Spectral Systems has developed its Diamond XP-BBAR(tm) coating with spectral performance as shown in Figure 1.


	Figure 1: Transmission spectrum of uncoated Diamond window shown in blue. Transmission spectrum of Diamond XP-BBAR coating on both sides of a Diamond window shown in green.

	It is seen in Figure 1 that the energy and thus the SNR is about 30% better through most of the analysis region of a typical FT-IR instrument.

	Since there are no suitable coating materials for the very long wavelength portion of the spectrum the coated element is not as good, but still usable, below 600 cm

. If a liquid nitrogen cooled MCT detector is used, this portion of the spectrum is usually not recorded by the spectrometer.


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Diamond Anti-Reflection Coating

 applied to Al-coated glass was measured using the 10Spec, a near-normal specular reflection accessory. The reflection spectrum served as input data for FilmStar™ software to calculate SiO

	Many applications in the optics industry require highly reflective surfaces to maintain laser intensity throughout the beam path. Aluminum is an optimal material in the visible spectrum (400–800 nm) for these applications. However, due to the softness of Al, proper maintenance of the surface requires an abrasion-resistant overcoat to allow for periodic cleaning of the optical surface.

 can protect Al while retaining a reflective surface. Previous work indicates SiO

 provides a barrier against abrasion (1,2). Based on testing outlined in MIL-C-48497, a minimum thickness of 200 nm is recommended.

	At the PIKE Technologies facility (Madison, WI), Al-coated glass substrate was pre-cleaned via ion assist with Ar gas for 10 min at a chamber pressure of approximately 1x10

 Torr. Immediately following the Ar pre-cleaning, a thin SiO

 film was deposited onto the substrate at approximately 200 °C via electron beam physical vapor deposition.

	Specular reflectance of the sample was measured between 400–800 nm using the 10Spec specular reflection accessory (PIKE Technologies, Madison, WI) in a research-grade spectrophotometer. The accessory has a 10 degree fixed angle of incidence (Figure 1). Its optics condition a focused beam into a collimated beam, creating a uniform 10 degrees striking the sample. All reflective optics allow spectral measurements from the UV to the far IR.


	Figure 1: 10Spec near-normal specular reflection accessory.

	FilmStar software (Princeton, NJ) was used to model the reflectance of SiO

 over the visible region. To maximize the optical properties of the coating, FilmStar's index-solving algorithm along with additional optimization features were used to model an ideal protective coating over Al assuming the refractive index of SiO

 coating exhibited uniform measured reflectance over the wavelength range (Figure 2). Using the visible reflection spectrum in conjunction with FilmStar's damped least squares optimization feature, the coating thickness was determined to be 220 nm (3). When compared to the FilmStar model, the reflectivity of our SiO

 coating is very similar. The modeled reflectance in FilmStar is fairly uniform over the wavelength range and greater than 83%, which is acceptable for most applications. A small discrepancy can be seen at approximately 450 nm due to a slightly different absorption, or k-value, utilized by FilmStar.


	Figure 2: Reflectivity of the FilmStar modeled SiO

 coating (blue) and the measured visible spectrum from the deposited SiO

 coating on an Al-coated glass substrate (red).

	The 10Spec specular reflection accessory is used to make near-normal measurements. Spectra obtained may be used as input and verification data for FilmStar and other optical measurement modeling software.

	(1) U. Schulz, S. Jakobs, and N. Kaiser, 

	(2) M. Mazur, D. Wojcieszak, J. Domaradzki, D. Kaczmarek, S. Song, and F. Placido, 


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Predictive Modeling of SiO2 Film Thickness on Aluminum

	In free space optics, light energy is collected from open beams and surfaces and sent on to the spectrometer. Free space optics can be used in various spectroscopy applications and are best deployed when the geometry between the light source and the spectrometer is defined and when these components are held in a rigid mechanical structure. Understanding how free space optics function within the spectrometer setup will help users to optimize experiments and obtain best results.

	Ocean Optics spectrometers are designed to project a spectrally convolved image of the input slit plane onto the detector (Figure 1). For example, if a light source comprising three monochromatic lines is projected onto the slit (aperture), then three distinct images of the slit would be seen at the detector plane. The images would be separated from each other by an amount determined by the wavelength dispersion of the system.


	Figure 1: Light enters the spectrometer at its entrance aperture.

	In free space coupling, an image of the subject is projected onto the slit. The details of the image projected onto the slit do matter. For example, if the image of a circular object is projected and greatly de-magnified onto the slit, the image of the object may be lesser in size than the slit and produce an effect similar to having a fiber smaller than the slit. In this case, the experimenter has to be careful with horizontal alignment of the image projected onto the slit. Horizontal shift will appear as a peak wavelength shift; however, the shift can never exceed the slit's original FWHM (full width at half maximum, an expression of spectrometer resolution).

	Moving the image vertically from the center of the slit will cause the apparent amplitude to change. Light entering the vertical center of the slit is most efficiently coupled through the system. Light entering at the top or bottom of the slit will not be routed to the detector as efficiently.

	Also, changing the angle of input causes the apparent intensity to change due to vignetting in the spectrometer. Light entering at 0° to the normal has the highest efficiency. To minimize these effects, ensure that the horizontal and vertical offsets, as well as horizontal and vertical angles, are fixed throughout the experiment.

	All of these alignment issues are greatly reduced if the projected image overfills the slit and overfills the numerical aperture of the spectrometer. This spatial and angular overfilling allows small angle changes and small offset changes to be irrelevant. If the image projected onto the slit plane is not properly focused then the field of view is not a well-defined region and will instead have fuzzy edges.

	The SMA bulkhead has an aperture that has been sized to restrict the angle of light entering the spectrometer (Figure 2). This has been designed to optimally fill the optical elements without overfilling, which would increase stray light. The full angle defined by a ray from the slit to the aperture is roughly 13.5°. The full angle defined by a ray from the opening of the SMA to the slit is greater than the 13.5°. This means that in a free space optical setup that is properly focused on the slit, the SMA connector is not restricting any light.


	Figure 2: An aperture in the spectrometer's SMA bulkhead restricts the angle of light entering the bench.


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Free Space Coupling and Spectrometer Performance

	Transition metal dichalcogenides (TMDs) display physical characteristics that hold great promise for applications in nano- and optoelectronics. These properties are strongly influenced by thickness, edge structures, lattice point defects, and functionalization of the material. For comprehensive structural and chemical characterization of low-dimensional TMDs, more than one technique is required. Here we present an analysis of WS

 flakes using correlative confocal Raman imaging and AFM.


	Figure 1a: High resolution AFM image of CVD-grown WS

 substrate with varying numbers (1, 2, >2) of layers and a total height of up to 7 nm.

	Using a confocal Raman-AFM microscope of the new WITec alpha300 microscope series, Raman spectra and the photoluminescence of CVD-grown WS

 were analyzed. AFM unambiguously revealed the numbers of atomic layers by measuring the height of the material (Figure 1a). However, this can also be determined by the material's Raman spectra with an excitation wavelength of 532 nm. Using a RayShield coupler, several peaks could be identified and were used for analysis (Figure 1b). The low frequency Raman signal at 8.5 relative wavenumbers (cm

) assessed as being specific for single-layer (1L) structures. This band corresponded perfectly with the 1L WS

 in the AFM image and decreased with the number of layers (Figure 1c). The peak at 416 relative wavenumbers (cm

) increased incrementally with the number of layers (not shown).


	Figure 1b: Raman bands at low wavenumbers detected with RayShield technology. Excitation wavelength = 532 nm. 

	Another direct indicator of the physical and morphological features of TMDs is their photoluminescence (PL) that results from the recombination of carriers generated by photoexcitation. Its intensity and position are functions of the bandgap and thus depend on the layer count, because only the direct bandgap present in 1L WS

 allows for direct recombination of electrons and holes. Therefore PL intensity is strongest in TMD monolayers. WS

shows strong PL around 639 nm wavelength (not shown). In the microscope image the highest PL intensity correlated perfectly with the AFM and Raman data of the monolayer flake (Figure 1d).


	Figure 1c: The peak intensity map of the WS

 Raman signal at 8.5 relative wavenumbers (cm-

 in the AFM image. The signal decreases with the number of layers. 

	In the case of low-dimensional TMDs, photoluminescence can serve as an effective method for investigating the electronic band structure as well as the local chemical structure of the material. Uniform PL at 8.5 relative wavenumbers (cm

) detected with WITec RayShield technology provides clear evidence of high quality 1L WS

 crystals, whereas intensity shifts indicate structural inconsistencies and local modifications. The correlation of PL data with Raman imaging and AFM yields a comprehensive picture of the material.


	Figure 1d: The intensity of the PL at about 635 nm after excitation at 532 nm matches well to the thickness of the material. The fewer the layers the stronger is the PL. This can be explained by the transition of a direct bandgap in 1L WS

 to an indirect bandgap in multilayer material. The upper left 1L WS

Sample courtesy of Ting Yu, Nanyang Technological University, Singapore.


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Application Notebook-09-02-2016