Background

For over 100 years, researchers have been investigating and evaluating the impacts of the fixed-combination medications which have the potential of increased effectiveness, decreased toxicity, and decreased drug resistance; thus, it has become a standard for the treatment of many diseases [1].

The benefits of fixed-dose pharmaceutical combinations are well acknowledged, and there has been a dramatic increase in the activities of the pharmaceutical industry to supply a new drug combination, which challenges the quality control to determine their multiple analytes in several presented ratio regardless of how different drug concentrations differ from each other, as there are some combinations in where the concentration of one of the two drugs is high (major component) where the concentration of the second drug is very small (minor component). The minor component is pointed according to its ratio in the mixtures as well as its absorptivity at different wavelengths so this is leading to a challengeable spectral banding which hindered their simultaneous determination.

Main text

Mathematical ultraviolet spectrophotometric analytical methods have appeared overwhelmingly in the last few years and have replaced the other chromatographic ones to a pronounced degree [2]. This can be due to their usability, less time and solvent usage, lack of prior extraction or separation steps, and lower cost of the course [3,4,5,6]. In this review, the ultraviolet spectrophotometric methods are classified in dealing with the minor component in five different spectral manipulation pathways as follows:

Methods based on spectral signal enhancing process

These methods involve the two techniques namely area under the curve (AUC) and derivative, where AUC can be easily calculated using spectra manager program and it has the advantage of increasing the sensitivity since the value belongs to AUC is greater than the absorbance value for the same substance, even in lower concentrations, also it can be applied either in zero-order spectrum or in the derivative spectrum. However, in case of having a component with an unspecified peak in the zero-order absorption spectrum, the derivative technique is a better solution which gives a derived spectrum with identified peaks which also derivative that can be used to improve overlapped spectra resolution, but it has the disadvantage of rising signal to noise ratio and requires specifying some parameters of the derivative process [7,8,9].

Methods based on obtaining the hidden spectral information of the minor component by spectro-chemometric tools

Chemometrics and spectrophotometer are a powerful combination method for obtaining the hidden spectral information in spectral data and for improving robustness, analytical frequency, and practicality for analytical methods. However, this technique could not be applied without a special program like Matlab®. It is anticipated that the spectro-chemometric methods can provide a fast-quantitative study of the pharmaceutical dosage forms [10,11,12,13,14].

Methods based on evaluating the geometrical relationship of standard addition or blank subtraction

Where in standard addition technique, different concentrations of minor component (x) are added to the binary mixture (x.y), and then, the obtained regression equation from the geometric representation of signal versus Cx is used to determine Cxminor in the mixture. This technique has the advantage of utilizing several standard solutions which minimize the error in determining component concentration. The second case of geometric representation depends on placing different concentrations of the minor component in the reference cell and then recording mixture solution (x.y) against each reference cell, in order to obtain a linear representation of signal versus different reference cells. These methods succeeded in determining the minor compound without any interference with the major one, but they demand many experiments to construct the geometric representation of signal against (Cminor) [15,16,17,18].

Methods based on ratio spectra

These methods handle with ratio spectra by different strategies like subtraction, multiplication, factor calculation, mean centering, and derivative in order to resolve the interference between the components in the mixture, and in many cases, these methods have the ability of determining each component in zero-order, so we could get the spectral profile of each component, and that acts as a fingerprint determination. This ability gives these methods a great advantage over the other ones because they cannot only determine the compounds in the mixture, but also eliminate interference with excipients that exhibit UV absorbance and that may hinder the determination of the components of the mixture [19,20,21,22,23,24,25].

Methods based on sample enrichment by spiking or spectrum addition technique

These methods are the best choice when the concentration of the minor component is deviating from Beer’s law which happens in the situation of low concentrations where the process of determining this compound by smart spectrophotometric methods becomes impossible without utilizing these techniques which increase the concentration of the minor component to fall within the linearity of the developed method. The first technique depends on adding known concentrations of a pure minor component to the pharmaceutical preparation before proceeding the developed methods and then subtracting the added concentration before calculating the claimed concentration of the minor component by the developed methods [18]. The second method utilizing spectrum addition instead of sample spiking to increase the concentration of the minor component by adding a standard spectrum of a pure minor component to the pharmaceutical preparation to achieve its linearity and then subtracting as mentioned in the first technique. These techniques are very helpful when the concentration of minor components is very low and deviated from its linearity [22].

The reported applications of the previously ultraviolet spectrophotometry methods are listed in Table 1. Also, the advantages and disadvantages of ultraviolet spectrophotometric methods for the determination of the minor components presented in different fixed-dose pharmaceutical combinations are listed in Table 2.

Table 1 Examples for applications of ultraviolet spectrophotometric methods for the determination of the minor component presented in different fixed-dose pharmaceutical combinations
Full size table
Table 2 Advantages and disadvantages of ultraviolet spectrophotometric methods for the determination of the minor component presented in different fixed-dose pharmaceutical combinations
Full size table

Conclusion

The most notable progress in the field of ultraviolet spectroscopy was experienced in the last two decades (2000–2020). The integration of the mathematical equations contributes to the discovery of new paths in the field of research for the application of UV spectroscopic methods in the determination of fixed-dose combination drugs with minor components.

Determining this mentioned drug combinations in pharmaceutical formulations with reasonable accuracy and precision has tested the applicability of the existing methods; thus, it may be excellent alternatives to other hyphenated analytical techniques.