A method comprising: characterizing a frequency registration deviation of a spectroscopic analysis system , the characterizing comprising recording a field calibration spectrum indicative of one or more changes of a light source of the spectroscopic analysis system relative to a calibrated state of the spectroscopic analysis system , wherein the light source is a frequency scannable laser with a laser frequency output to change over a desired frequency range, wherein the recording of the field calibration spectrum comprises use of a wavelength/frequency calibrator ; deriving a conversion function comprising at least one mathematical operation for reducing the frequency registration deviation relative to the calibrated state of the spectroscopic analysis system , and wherein the deriving of the conversion function comprises fitting a mathematical expression to model a relative frequency difference between spectral features occurring in the field calibration spectrum and known spectral markers provided by the wavelength/frequency calibrator ; converting a field spectrum recorded with the spectroscopic analysis system for a field sample comprising a sample fluid, the converting comprising applying the conversion function to the field spectrum to produce converted field spectral data comprising spectral data registered to corrected frequency response of the spectroscopic analysis system , and wherein the field spectrum recorded with the spectroscopic analysis system is converted to a known, well-characterized frequency scale such that absorbance features are correctly registered to a same frequency as in the calibration spectrum; and calculating one or more of a pressure of the field sample, a temperature of the field sample, and a concentration for one or more analytes in the field sample using the converted field spectral data. characterizing a frequency registration deviation of a spectroscopic analysis system , the characterizing comprising recording a field calibration spectrum indicative of one or more changes of a light source of the spectroscopic analysis system relative to a calibrated state of the spectroscopic analysis system , wherein the light source is a frequency scannable laser with a laser frequency output to change over a desired frequency range, wherein the recording of the field calibration spectrum comprises use of a wavelength/frequency calibrator ; deriving a conversion function comprising at least one mathematical operation for reducing the frequency registration deviation relative to the calibrated state of the spectroscopic analysis system , and wherein the deriving of the conversion function comprises fitting a mathematical expression to model a relative frequency difference between spectral features occurring in the field calibration spectrum and known spectral markers provided by the wavelength/frequency calibrator ; converting a field spectrum recorded with the spectroscopic analysis system for a field sample comprising a sample fluid, the converting comprising applying the conversion function to the field spectrum to produce converted field spectral data comprising spectral data registered to corrected frequency response of the spectroscopic analysis system , and wherein the field spectrum recorded with the spectroscopic analysis system is converted to a known, well-characterized frequency scale such that absorbance features are correctly registered to a same frequency as in the calibration spectrum; and calculating one or more of a pressure of the field sample, a temperature of the field sample, and a concentration for one or more analytes in the field sample using the converted field spectral data. <EOS>