NIR

The near infrared (NIR) spectrum spans wavelengths from 770 to 3000 nm or 13000 cm-1 to 3300 cm-1 Between visible and mid-IR Broad overtone and combination bands C=O 3300-3600 cm-1 OH ~ 7100 cm-1 Mostly for quantitative rather than qualitative analysis

Advantages Most organics absorb in the NIR, but none strongly. Good S/N - wide slits Instrumentation is simple, relatively inexpensive Can get UV-vis-NIR or FTIR-FTNIR Often used with computerized analyses – where calibration is done using many samples of compositions covering the range of possible unknowns (training set). Predict multiple parameters from one spectrum

Often used in applied work Agriculture, food science, pharmaceuticals Moisture content of coal Cross-linking in chemically modified starch Identification of analgesics Recognized as an effective region to apply process spectroscopy concepts even though absorption "fingerprints" are less distinctive than in the Mid IR which is the region of choice for lab spectroscopy. 

The NIR region permits the use of probes having longer optical pathlengths which are less prone to fouling (absorptivity is low) . Less fouling means less maintenance and more reliable data. NIR radiation can be transmitted long distances (up to 400 m one way) through pliable fibers. Mid-IR radiation can be transmitted only through special non-glass fibers that are brittle and not suitable for distances greater than a few feet.

Multiple chemical parameters can be predicted on the basis of a single spectrum through computer-assisted modeling techniques. Remote spectroscopy may be deployed into even the most hazardous environments. Only the probe is exposed to the chemicals, not a technician gathering samples.

A common application: Determination of protein, moisture, starch, oil, lipids and cellulose in grains and oilseeds Wheat sales are based on guaranteed protein content. Almost 90 % of it is analyzed by NIR. Finely ground solid is irradiated and diffuse reflectance occurs. The spectrum of this scattered light depends on the composition of the sample. Use water peak at 1940 nm (5155 cm-1) Starch and protein – overlapping peaks so use two wavelengths

Octane Number The % by volume of iso-octane in a mixture of iso-octane and n-heptane that has the same knocking characteristics as the motor fuel under test.(87, 90, 91, etc) The higher the number, the slower the fuel burns—and the less likely your engine will knock. Refineries control this parameter during production.

The NIR spectra of samples of known octane rating are measured. A model is developed. A fibre optic cable is used so instrument can located at a distance from the actual sample. Results are available within seconds.

Characterizing Lipids

Both white and brown adipose tissue in rats show absorbance in three regions (1670 - 1720 nm, 1720 - 1800 nm, 1800 - 1850 nm) characteristic of lipid. The ratio of the absorbances in these three regions differ between white and brown adipose tissue. The near-IR spectrum of brown adipose tissue shows a broad band at 1820 nm that corresponds to saturated fatty acids. This band (1820 nm) was greatly diminished in dissected white adipose tissue.

Application Highlight - Needle-free blood and tissue measurements Light in the near infrared region has slightly longer wavelengths than red light. It is important for medicine because those wavelengths, for the most part, actually pass through skin and to some extent bone, allowing you to get chemical information about tissues and blood.

Patients may now encounter NIR spectroscopy at the doctor’s office. The pulse oximeter, used for measuring oxygen saturation, employs a small clip placed on the finger or ear to measure the amount of oxygen carried by the blood, along with pulse rate.

Other methods are being developed: Tissue pH and oxygenation - ‘new’ medical parameters They help identify when a person is in shock or in need of treatment. non-invasive allows physicians to take measurements continuously, once a second if desired

The prototype device currently uses two optical fibers, one shining the light into the patient and the other carrying the reflected light back to a device that analyzes the data. It needs to be even smaller for use on the international space station.

Hematocrit (red blood cell concentration) Glucose – lucrative market