Dayana Oropeza

Laser plasma spectrochemistry

An overview of laser plasma spectrochemistry is presented to demonstrate its wide range of capabilities. Laser plasmas offer the ability to perform elemental, isotopic, molecular, quantitative and qualitative sample analysis with sub-micron spatial resolution, and each feature can be measured at standoff distances. Obviously, these attributes are not all achievable at the same time, but they can be optimized for specific applications. This manuscript gives a sampling (pun intended) of the research in our group that has demonstrated each of these capabilities. Although the technology is commonly referred to as LIBS (laser-induced breakdown spectroscopy), the authors prefer to use laser plasma spectrometry to represent the underlying science.

Assessment of the precision and accuracy of thorium (232Th) and uranium (238U) measured by quadrupole based inductively coupled plasma-mass spectrometry using liquid nebulization, nanosecond and femtosecond laser ablation

The precision and accuracy of the 238U/232Th ratio were evaluated from liquid nebulization and direct solid sampling repetitive pulsed laser ablation. Nanosecond and femtosecond pulsed lasers at 266 nm wavelength were utilized for the ablation studies. The ICP-MS and sampling parameters were optimized for each procedure; flow rates, gases, laser energy and other parameters were optimized for the particular sampling approach and therefore will not be the same. The work is not a comparison per se but represents performance metrics for three optimized sampling modalities. As expected, nanosecond pulsed ablation provided the greatest inaccuracy (>30%) from the nominal 238U/232Th bulk ratio. This deviation from bulk ratio is attributed to incomplete vaporization of large particle agglomerates produced by nanosecond laser ablation. Femtosecond pulsed ablation provided inaccuracy (∼1–3%) approaching that of liquid nebulization (∼1%). In terms of temporal relative standard deviation (TRSD) and relative standard deviation (RSD), liquid nebulization provided the best precision for the 238U/232Th ratio (TRSD ∼3–5%, RSD ∼0.2–0.6%), femtosecond laser ablation (TRSD ∼5–12%, RSD ∼1%) and nanosecond laser ablation (TRSD ∼25–48%, RSD ∼9–12%). Laser ablation requires less sample to achieve these performance metrics, in some cases less than a factor of 100-times depending on the entrainment and transport efficiency.

Simultaneous 3-dimensional elemental imaging with LIBS and LA-ICP-MS

Laser Induced Breakdown Spectroscopy (LIBS) and Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) are used simultaneously for spatially resolved mapping of major and trace elements and isotopes within a Bastnasite rare earth ore sample. The combination of the two techniques provides complementary measurements for elements that are separately unattainable due to low sensitivity and/or strong interferences. Two dimensional (2D) layer-by-layer mapping, 2D cross-sectional imaging and three-dimensional (3D) volume rendering of elements and isotopes in the Bastnasite matrix are presented. These results pave the way for improved 3D elemental imaging through simultaneously acquired LIBS and LA-ICP-MS measurements.

Rapid bulk analysis using femtosecond laser ablation inductively coupled plasma time-of-flight mass spectrometry

Laser Ablation (LA) for direct solid sampling has advanced to provide accuracy and precision similar to that obtained using SN. Analytical performance metrics such as accuracy, precision, and limits of detection obtained when using femtosecond lasers are similar to those by SN. The reason for this improved performance is that the femtosecond laser produces essentially mono-disperse nanometer sized particles that are relatively easily digested in the ICP. In addition, the femtosecond laser material interaction is a photophysical process significantly reducing melting and elemental fractionation. A remaining aspect of ablation sampling is representative sampling; how much mass must be ablated to accurately analyze an inhomogeneous sample. The quantity of sample needed for bulk analysis is dependent on the inhomogeneity (or heterogeneity). Obviously, this same issue exists with sample digestion, and milligrams to grams of material are used to address this requirement. Laser ablation is beneficial in that only micrograms or less of sample are needed for the analysis. However, for bulk analysis of an inhomogeneous sample, the question is how much mass is required to adequately represent the bulk sample, and maintain the benefits of no sample digestion. The goal of this study was to demonstrate bulk analysis of a homogeneous and inhomogeneous sample by ablating large sample volumes using a high repetition rate (20 kHz) femtosecond pulsed laser. This approach is beneficial for bulk analysis of heterogeneous samples, for rapid analysis of difficult to dissolve samples, and for analyzing impurities over a large surface of homogeneous samples.

Femtosecond laser ablation multicollector ICPMS analysis of uranium isotopes in NIST glass

We utilized femtosecond laser ablation together with multi-collector inductively coupled plasma mass spectrometry to measure the uranium isotopic content of NIST 61x (x = 0, 2, 4, 6) glasses. The uranium content of these glasses is a linear two-component mixing between isotopically natural uranium and the isotopically depleted spike used in preparing the glasses. Laser ablation results match extremely well, generally within a few ppm, with solution analysis following sample dissolution and chemical separation. In addition to isotopic data, sample utilization efficiency measurements indicate that over 1% of ablated uranium atoms reach a mass spectrometer detector, making this technique extremely efficient. Laser sampling also allows for spatial analysis and our data indicate that rare uranium concentration inhomogeneities exist in NIST 616 glass.

A metric for evaluation of the image quality of chemical maps derived from LA-ICP-MS experiments

For laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) imaging experiments – as well as other techniques used for elemental or molecular mapping – the accordance of the measured distribution with the actual distribution is of utmost importance to guarantee reliability of the obtained images. In most experiments reported in the past, the experimental conditions have been chosen so that washout effects and signal carry-over are minimized by scanning the sample surface very slowly. Therefore, measurement times become very long and decently resolved images will require acquisition times of several hours up to more than one day. To increase the application range of LA-ICP-MS for imaging it is important to decrease the measurement times, which is best accomplished by increasing the scanning rates. However, depending on the instrumentation, this can lead to blurring and compromised image quality. In this work, we present a metric to compare the measured elemental distribution with their actual distribution based on a sample with visually distinguishable features. This approach allows quantitative determination of the image quality and enables comparison of multiple measurement conditions. This information can be used for method optimization, to get a reasonable tradeoff between image quality and measurement time.

Laser-Ablation Sampling for Accurate Analysis of Sulfur in Edible Salts:

We evaluated the performance of laser ablation analysis techniques such as laser-induced breakdown spectroscopy (LIBS), laser ablation inductively coupled optical emission spectrometry (LA-ICP-OES), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), in comparison with that of ICP-OES using aqueous solutions for the quantification of sulfur (S) in edible salts from different geographical origins. We found that the laser ablation based sampling techniques were not influenced by loss of S, which was observed in ICP-OES with aqueous solutions for a certain salt upon their dissolution in aqueous solutions, originating from the formation of volatile species and precipitates upon their dilution in water. Although detection of S using direct laser sampling with LA-ICP-MS has well-known isobaric and polyatomic interferences, LIBS and LA-ICP-OES showed good accuracy in the detection of S for all salts. LIBS also provided the ability to identify the dominant chemical form in which S is present in salts. Correlation between S and oxygen, observed in LIBS spectra, provided chemical information about the presence of S2– or SO 4 2 - , which are associated with the origin and quality of edible salts.

Analysis of Plant Leaves Using Laser Ablation Inductively Coupled Plasma Optical Emission Spectrometry: Use of Carbon to Compensate for Matrix Effects:

Direct solid sampling by laser ablation into an inductively coupled plasma synchronous vertical dual view optical emission spectroscope (LA-SVDV-ICP-OES) was used for the elemental analysis of nutrient elements Ca, B, Mn, Mg, K, and Zn and essential (non-metallic) elements P and S in plant materials. The samples were mixed with paraffin as a binder, an approach that provides better cohesion of the particles in the pellets in addition to supplying carbon to serve as an internal standard (atomic line C I 193.027 nm) as a way to compensate for matrix effects, and/or variations in the ablation process. Precision was in the range of 1–8% relative standard deviation (RSD) with limit of detection in the range of 0.4–1 mg/kg–1 and 25–640 mg/kg–1 for metallic and non-metallic elements, respectively.