idraman mini innovative sampling technology overcomes limitations in handheld raman | 量子荧光|微型光纤光谱仪-ag贵宾会

idraman mini innovative sampling technology overcomes limitations in handheld raman

the development of handheld and portable raman systems has brought a once relatively unknown analytical technique into the mainstream. once used primarily as a research tool, raman spectroscopy is now a routine analytical technique used for applications as diverse as qa/qc and threat detection. in this application note, we introduce the ’s innovative sampling technique based on raster orbital scanning (ros) with examples of how this technique overcomes the limitations associated with other handheld and portable raman systems.

challenges in handheld raman sampling
most portable and handheld raman systems are based on a closely related dispersive design.  the advantages of this design include low power requirements, small size and enough resolution to identify a wide range of compounds. the drawback to this design is that it only measures an extremely small fixed sample volume. while the interrogation of a small sample volume is sufficient for the analysis of bulk and pure substances, most handheld applications involve difficult mixtures or heterogeneous samples and will require the high spectral resolution and large laser interrogation area of the idraman mini for accurate identification.

another issue with the current sampling approach is the tight laser focus needed to achieve high laser power for the collection of low intensity raman signals. this tight focus increases the laser power density at the sample, which creates intense heat buildup. this intense heat can damage or even ignite samples that are flammable or explosive, limiting the applicability of the instrument to non-photosensitive and non-combustible samples.

innovative raman sampling solution
the shown in figure 1 overcomes the limitations of other portable and handheld raman systems with a novel sampling technique based on ros. with conventional handheld designs, a large interrogation area (large laser spot) requires a large aperture in the spectrometer to efficiently collect the laser light. with a larger laser spot, the high spectral resolution needed for spectral matching cannot be achieved. this trade-off between aperture size and resolution is related to the etendue of the spectrometer. etendue determines a spectrometer’s sensitivity (light collection), resolution and interrogation area. the relationship among these parameters is illustrated in the diagram shown in figure 2.

the advantage of using the ros sampling technique for a sample with a dilute target material is illustrated in figure 3. in a conventional spectrometer, a tightly focused beam produces high resolution but may miss the target material completely. the increase in sampling area achieved using a large beam would require a larger aperture and consequently a loss of resolution. the idraman mini ros sampling system overcomes the trade-off between aperture size and resolution by quickly rastering a tightly focused beam over a large area. this enables the sampling of all target materials for qualification and/or mixture analysis while maintaining high resolution.

idraman mini for non-homogeneous samples
the large sample interrogation area measured using ros is important when trying to identify challenging or non-homogeneous samples. a good example of this is the analysis of ammonium nitrate porous prill (an) shown in figure 4. ammonium nitrate prill is used in the manufacturing of the blasting agent ammonium nitrate/fuel oil (anfo), a common explosive used in mining and quarries. it is also a common explosive used in the improvised explosive devices (ied) used against u.s. and nato troops throughout the world.

in an effort to reduce the number of ieds, officials have established many different laws limiting the availability of starting materials like an, which is also used as a fertilizer. in the case of an, restricting the sale of an-based fertilizers is possible due to the availability of urea as an effective replacement fertilizer. unfortunately, to avoid an restrictions, unscrupulous manufacturers sometimes label an as urea in a guise to distribute the easily converted chemical to clandestine labs. the chemicals look the same and are sold in the same prill form.

the handheld system with ros can easily distinguish between these two chemicals as shown in figure 4. the an prills are large porous beads easily identified using raman; however, the large particle size and porous nature of the prill beads make an difficult to analyze with most portable raman systems available today, due to the systems’ tightly focused sampling requirements. with a tight focus and small sample interrogation area, accurate identification is only accomplished if the focus is perfectly aligned on a single an bead. this precise focus is not always a viable or safe option in situations where external threats are imminent. with ros, the large sample interrogation area allows for the accurate identification of an without the need to focus on a single prill bead.

an example of a large sampling area achieved with ros is shown by measuring oddly shaped samples like the an prills. several an measurements were made with an in the form of prill contained in a vial. the sample was measured 10 times with the raster on and 10 times with the raster off. as shown in figure 4, for the average of the 10 spectra, ros sampling resulted in a higher intensity response. the reduced signal when the raster was off illustrates the statistical chance of actually obtaining a quality raman spectrum from the irregularly shaped prill samples.  this data illustrates how the ros interrogates more of the sample during each scan providing confident identification of the sample.

ros is so effective that the laser power used to interrogate the sample can be greatly reduced. ros creates a situation where the instantaneous laser power is high but with the fast movement of the raster the overall average power on the sample is reduced. the high instantaneous power allows the collection of quality raman spectra while the fast movement of the raster reduces the potential of damaging or destroying the sample.

damage that can result from standard sampling without ros is shown in figure 5 for an.  ammonium nitrate has a strong raman band at 1040 cm-1 when using lower laser power. but when the laser power exceeds 100 mw the sample begins to degrade and the 1040 cm-1 peak shifts toward the red. figure 5 demonstrates the shift of the 1040 cm-1 peak to 1050 cm-1 as the an begins to decompose when exposed to high laser power. this shift could cause the misidentification of an with another common nitrate such as calcium nitrate.

idraman mini for explosive or flammable samples
another advantage of the ros sampling technique is the analysis of explosive or flammable samples like black powder. black powder is made from three simple components: sulfur, charcoal and potassium nitrate (saltpeter), with the sulfur and charcoal acting as fuels and the saltpeter working as an oxidizer.

in 2002, pacific northwest national laboratory published a raman safety report strongly recommending against the use of raman for the analysis of black powder due to the potential for the laser to ignite the sample. while this recommendation may apply to handheld raman systems using a tight focus with high laser power density at the sample, the reduction of average laser power on the sample when using the idraman mini with ros permits the safe analysis of black powder. figure 6 illustrates the raw raman spectrum of goex 4f black powder using the idraman mini handheld identification system.

conclusion
limitations in the sampling techniques used with handheld and portable raman systems restrict their applicability in the identification of non-uniform and explosive or flammable samples.  these conventional sampling techniques use small sample interrogation areas and high laser power that could lead to misidentification of non-homogeneous samples or sample ignition.

the innovative ros sampling approach in the overcomes both of these limitations, giving the user a large sample interrogation area and lower laser powers for accurate identification using a handheld raman system. as shown in the analysis of ammonium nitrate prills, the with ros technology delivers threat identification with the utmost confidence. ros technology accomplishes this by increasing the interrogation spot size to provide definite and correct identification of unknown samples. in addition, ros reduces the average laser power at the sample, increasing the variety of samples that can be analyzed with the without the risk of possible deflagration or sample damage.

references

this entry was posted on thursday, june 6th, 2013 at 5:40 pm and is filed under . you can follow any responses to this entry through the feed. both comments and pings are currently closed.

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