Purpose
This Reference Material can be used to qualify the accuracy and linearity of the absorbance scale of spectrophotometers with bandwidth less than 2 nm in the far ultraviolet region of the spectrum (210 nm – 260 nm). It is accepted for this purpose by most pharmacopoeias and other regulatory bodies.
Description and Discussion
Nicotinic acid solutions with concentrations from 6 to 24 mg/l in dilute (0.1 M) hydrochloric acid, sealed in high quality far UV quartz cells. Supplied with a 0.1 M hydrochloric acid blank.
The spectrum of nicotinic acid, when prepared in acid solution, has characteristic peaks at approximately 213 nm and 261 nm. Within the concentration range 6-24 mg/l, if the absorbance scale of a narrow bandwidth (<2 nm) spectrophotometer is linear, the apparent absorbance of a series of concentrations measured at these peaks will be a linear function of concentration.
Solutions are available at higher concentrations giving absorbance values up to 2.4A. Starna do not recommend the use of Nicotinic Acid for linearity measurements above 1.0 A
Typical absorbance values at these wavelengths for the different concentrations are as follows:
| Concentration | Absorbance at Certified Wavelength | |
| 213nm | 261nm | |
| 6 mg/L | 0.24 | 0.281 |
| 12 mg/L | 0.477 | 0.556 |
| 18 mg/L | 0.73 | 0.849 |
| 24 mg/L | 0.988 | 1.149 |
Typical spectra
Note: The above values are for guidance only. Because the absorption bands are asymmetric, measured values will be spectral bandwidth dependent. The Calibration Certificate accompanying the references gives actual values measured at a bandwidth of 1.0 nm, and only these certified values should be used for instrument qualification. On request, Starna can provide certified values at other bandwidth values.
Catalogue Number
These references are available in a convenient set containing four concentrations and a hydrochloric acid blank cell:
| Catalogue Number | ||||||
| Nicotinic Acid Linearity Set 6, 12, 18 & 24 mg/l & Blank Cell | RM-06A12A18A24A | |||||
References are also available individually. Catalogue numbers and nominal certified values are given below. Any combination of references can be ordered as a set by combining the catalogue numbers as in the example above. A hydrochloric acid blank cell is supplied with each reference or set of references.
| Concentration | Absorbance at Certified Wavelength | Catalogue Number | |
| 213nm | 261nm | ||
| 6 mg/l | 0.244 | 0.233 | RM-06A |
| 12 mg/l | 0.489 | 0.466 | RM-12A |
| 18mg/l | 0.727 | 0.694 | RM-18A |
| 24mg/l | 0.968 | 0.924 | RM-24A |