Reduction of spectral interferences and noise effects in laser ablation molecular isotopic spectrometry with partial least square regression – a computer simulation study

Abstract

The fundamental analytical accuracies and precisions attainable by laser ablation molecular isotopic spectrometry (LAMIS), with emphasis on the impacts from spectral interferences and measurement noise, were investigated by means of computer simulation. The study focused on the analysis of a minor isotope at sub-to single-percentage abundance level. With a natural abundance about 1.1% for 13 C, the C 2 Swan band (d 3 Π ga 3 Π u) with  = +1 was selected as a representative system. The characteristics (e.g., noise amplitude and distribution, signal strength, and signal-to-background ratio) of the simulated spectra were experimentally characterized. Partial least square (PLS) regression was used to extract isotopic information from the simulated molecular spectra. In the absence of any spectral interference and with the use of a calibration set consisting of eleven isotopic standards, the theoretical accuracies and precisions with signal accumulation from 100 laser shots are about 0.002% and 0.001%, respectively, in absolute percentage abundance of 13 C. The theoretical analytical accuracies slightly degrade, but are adequate for many applications, to 0.004% and 0.008% respectively, for calibrations involving only three and two isotopic standards. It was found that PLS regression is not only immune to both source-flicker and photon-shot noise, but is also effective in differentiating the spectral patterns from the analyte against those from spectral interferences. The influences of spectral interference from single or multiple atomic emission lines were simulated, and new ways to minimize their impacts were formulated and demonstrated. It was found that the wavelength range selected for the computation of the normalization factor should not contain any spectralinterfering peak, and a properly chosen wavelength range increases the tolerance of spectral interference by at least one order of magnitude. With matrix-matched calibration standards, the precisions (expressed as RSDs of the determined 13 C isotopic abundances) degrade from ~1‰ in 2 the absence of spectral interference, to ~3‰, 10‰ and 20‰ with spectral-interfering peaks that are 10, 100 and 1000, respectively, stronger than the molecular bandhead of the analyte. The study concluded that PLS regression is a powerful and indispensable tool for extraction of isotopic information from LAMIS spectra.

Combination of atomic lines and molecular bands for uranium optical isotopic analysis in laser induced plasma spectrometry

“…Similar to our previous studies [10,22] [11,12,24]) but those for molecular UO [25][26][27] are far from complete, the methods for generating simulated spectra were accordingly different for atomic lines and molecular bands. For simulations involving only atomic emission lines, the simulated spectra were all based on theoretical computation.…”

“…Extraction of isotopic abundance information from the LIBS or LAMIS spectra was achieved with partial least square (PLS) multivariate regression [10,22]. As PLS regression is a purely empirical approach by matching the emission pattern of the sample with unknown 235 U/ 238 U ratio to a set of standard reference spectra; it is particularly useful in uranium LAMIS analysis because spectroscopic constants of many rovibronic bands for UO are not documented.…”

“…For molecular bands, as will be explained in detail in Section 3. In all cases, emission intensities, signal-to-background ratios, and measurement noise (detector-read, photon-shot and source-flicker) were all experimentally determined and incorporated into the simulation model [10,22]. For the two U3O8 pelletized samples, the measured pulse-to-pulse flicker noise was 14.8% RSD.…”

“…All spectra simulation and PLS analyses were performed with in-house written LabVIEW programs (LabVIEW 2015, National Instruments). The PLS algorithm used in this study was a PLS1 type and three principal components were utilized in all PLS calibration models [22].…”

“…Nevertheless, the LAMIS-LIBS combined approach shows its potential as a viable supplement and valuable alternative for U isotopic analysis in situation that the single U II 424.437 nm line suffers interference and cannot be utilized. Because analyte signals (LIBS or LAMIS) are highly correlated, the use of multiple spectral features as in the LAMIS-LIBS approach provides the necessary resource for the PLS algorithm to differentiate the spectral patterns from the analyte against those from spectral interferences[22] an attribute that would be difficult to implement if only a single spectral component (e.g., one atomic emission line) is used.…”