Performance Features of an Extended Range Beamsplitter for Mid- and Near-IR Spectroscopy

Significance of the topic

The ability to measure both mid-infrared (MIR) and near-infrared (NIR) spectra with minimal instrument reconfiguration addresses a common laboratory need for versatility, throughput and operational simplicity. Many routine analyses in raw-material identification, moisture determination and FT‑Raman spectroscopy require information across both regions. A single beamsplitter that extends useful spectral coverage reduces downtime, potential alignment errors, and the need for multiple instruments while enabling complementary structural and quantitative analyses on the same sample without physical sample transfer.

Objectives and study overview

This technical note evaluates the performance of the Thermo Scientific XT-KBr beamsplitter as an extended-range optical element capable of covering roughly 11,000 to 375 cm-1, enabling both MIR and NIR measurements (including FT‑Raman) without manual beamsplitter changes. Comparative performance data versus conventional KBr and CaF2 beamsplitters are presented to characterize throughput and signal-to-noise trade-offs across the combined spectral range. Practical sampling modes including FT‑IR attenuated total reflection (ATR), FT‑Raman and NIR fiber‑optic reflectance are demonstrated to show applicability to common laboratory workflows.

Methodology and experimental conditions

Measurements were taken on Thermo Scientific Nicolet series FT‑IR instruments (examples given with the Nicolet 6700). Key experimental points:

• Single‑beam (non‑ratio) spectra were collected at 4 cm-1 resolution for beamsplitter throughput comparisons using a white light source and a DTGS detector, unless otherwise noted.
• MIR comparisons used the Ever‑Glo ETC MIR source; NIR measurements used a quartz‑halogen NIR source.
• FT‑Raman spectra were acquired using the Nicolet NXR FT‑Raman module with an InGaAs room‑temperature detector; comparison spectra for Raman regions were taken in the NIR shifted region corresponding to Raman shifts of 3600–100 cm-1.
• Example application data: powdered laundry detergent measured in ATR‑FTIR and in FT‑Raman (sample in an NMR tube) using the same XT‑KBr beamsplitter; an analgesic tablet measured by NIR fiber optic in ~20 s at 16 cm-1 resolution.
• Relative signal‑to‑noise ratios (SNR) were extracted from 100% lines and single‑beam traces to quantify performance differences between beamsplitters at selected wavenumbers.

Instrumentation used

• Thermo Scientific Nicolet 6700 (examples note that Nicolet 8700 also supports multiple installed sources/detectors)
• XT‑KBr beamsplitter (extended range)
• Standard KBr and CaF2 beamsplitters (comparisons)
• Ever‑Glo ETC MIR source and quartz‑halogen NIR source
• DTGS detector for broad comparisons
• Nicolet NXR FT‑Raman module with InGaAs room‑temperature detector
• Smart Performer ATR accessory with ZnSe crystal
• Nicolet SabIR NIR fiber‑optic sampling accessory; Smart NIR UpDRIFT and Smart NIR Integrating Sphere

Main results and discussion

• Spectral coverage: The XT‑KBr beamsplitter provides optical throughput from approximately 11,000 to 375 cm-1, creating a single optical window that spans typical NIR and MIR laboratory needs.
• NIR throughput vs KBr: Compared to a standard KBr beamsplitter (with a reflective germanium coating that strongly absorbs near 7400 cm-1), the XT‑KBr shows substantially greater energy beyond ~2000 cm-1 and remains transparent to ~11,000 cm-1.
• NIR throughput vs CaF2: A dedicated NIR CaF2 beamsplitter provides higher NIR energy above ~5000 cm-1; quantitative SNR comparisons indicate CaF2 yields superior SNR in the far‑NIR region (for example, approximately 1.6× better SNR for a cyclohexane Raman measurement in the 9298–5668 cm-1 range).
• MIR performance: The XT‑KBr outperforms standard KBr above ~2600 cm-1 (greater throughput), while below ~2600 cm-1 KBr delivers higher throughput. At 2600 cm-1 SNR is approximately identical for both beamsplitters; at 7000 cm-1 XT‑KBr outperforms KBr by ~4.6× SNR, whereas at 450 cm-1 KBr outperforms XT‑KBr by roughly 2× SNR.
• FT‑Raman applicability: Although CaF2 provides higher NIR SNR for Raman‑shifted spectra, the XT‑KBr produces high‑quality FT‑Raman spectra for the majority of routine samples, supporting efficient FT‑Raman work without an optical changeover.
• Sampling workflows: Examples demonstrate rapid switching between FT‑IR and FT‑Raman by software control (OMNIC software) within seconds, and remote/fast NIR reflectance sampling with fiber optics (tablet spectrum collected in ~20 s at 16 cm-1 resolution).

Benefits and practical applications

• Operational simplification: Using a single extended‑range beamsplitter eliminates manual beamsplitter swaps for many use cases, reducing instrument downtime and alignment effort.
• Versatility: Supports ATR‑FTIR, FT‑Raman and NIR reflectance/fiber‑optic sampling on the same optical bench, enabling complementary structural and compositional analyses without sample transfer.
• Time savings and throughput: Faster turnarounds for mixed workflows (MIR + NIR) and for labs that need to alternate between FT‑IR and FT‑Raman frequently.
• Acceptable compromises: While dedicated beamsplitters or detectors may yield optimal SNR in narrow regions (e.g., CaF2 in far‑NIR, KBr in low‑wavenumber MIR), XT‑KBr provides balanced performance suitable for routine analytical tasks across a very wide spectral range.

Future trends and potential applications

• Coating and materials advances: Development of improved broadband coatings and novel substrate materials could further extend single‑element throughput and reduce SNR trade‑offs.
• Detector evolution: Wider use of extended InGaAs and cooled detector technologies will improve NIR sensitivity, shifting the balance in favor of single‑element workflows for demanding quantitative NIR or Raman tasks.
• Integrated multi‑source/detector platforms: Software‑controlled multi‑source/detector architectures (already present in high‑end Nicolet series instruments) will continue to simplify automated switching and combined analyses.
• Fiber optics and remote sampling: Expanded adoption of fiber‑optic NIR probes and integrating spheres will enable faster process or at‑line sampling with minimal sample handling.
• Data analytics: Coupling extended‑range spectral acquisition with advanced chemometrics and machine learning will increase the practical value of broad spectral datasets for identification, quantitation and quality control.

Conclusions

The XT‑KBr beamsplitter provides a practical single‑element solution for many laboratories that require both MIR and NIR spectral information, covering approximately 11,000 to 375 cm-1. It offers significant advantages in workflow efficiency and flexibility by reducing the need for manual optical changes and supporting rapid software‑controlled switching between FT‑IR and FT‑Raman modes. Performance trade‑offs are present: dedicated beamsplitters (CaF2 for far‑NIR, KBr for the deepest MIR low‑wavenumber region) still deliver superior SNR in their optimized ranges. Nevertheless, for most routine MIR/NIR and FT‑Raman applications the XT‑KBr delivers sufficient spectral quality and SNR to justify the simplification and time savings it provides.

Reference

Technical Note 51432: Performance Features of an Extended Range Beamsplitter for Mid‑ and Near‑IR Spectroscopy, Michael Bradley, Ph.D., Thermo Fisher Scientific (2008).