A method of using spectroscopic envelopes for determining components in a sample may include selecting spectroscopic envelopes and passing input light through a sample comprising at least one absorbing component is provided. The method includes measuring throughput light with a photo-detector and determining the concentration of the at least one absorbing component in the sample using the measured throughput, wherein at least one of the plurality of spectroscopic envelopes overlaps at least one absorption band of the at least one absorbing component in the sample. An apparatus for determining components in a sample including an input light source having a spectrum and a sample container having a fixed optical path-length is also provided. The apparatus may include a plurality of pre-selected spectroscopic envelopes to select spectral portions of the throughput light from the sample; and at least one photo-detector to measure the throughput light selected by the spectroscopic envelopes.