Classification of Edible Salt Products by
Tandem LA-ICP-MS and LIBS Analysis
LIBS and LA-ICP-MS have been
studied and developed extensively for sample classification and discrimination technology
for forensic, geological, gemological, and environmental applications. Recent advances in LA instrumentation now
makes it possible to obtain LIBS and LA-ICP-MS data simultaneously, allowing
analysts to obtain more complete chemical fingerprinting information about a sample. The blog piece below highlights the Tandem
LA-ICP-MS and LIBS research work of Dr. Yonghoon Lee at Mokpo National University
to apply Tandem LA-ICP-MS and LIBS analysis for discrimination of salts from
different geographical origins. More detailed technical discussion of this research was
published in Spectrochim. Acta Part B 118 (2016) 102-111.
By Dr. Yonghoon Lee, Department of Chemistry, Mokpo
National University, Republic of Korea
An average person is thought to consume a few to 10 g
of salt per day. A wide range of edible salt products from different countries
is available in local food markets. However, the price of salt products may vary
significantly from cheap table salt to more expensive mineral-rich sea salts
and pink/black rock salts. The large difference in salt price often encourages illegal
circulation of fake salt products. Therefore, it is desirable to establish reliable
fingerprints of salt products and measurement techniques to test these
products.
Chemical
composition can be utilized as the unique fingerprint for different types of salts.
Salt is a mixture of various ionic compounds in the sodium chloride (NaCl)
matrix. The concentration of magnesium (Mg), calcium (Ca), and potassium (K),
ranging from several hundred ppm to a few %, depends on the production method
and surrounding environment of saltpans or salt mines. These are the main
metallic impurities in salts.
Commercially available salt products
Laser
Induced Breakdown Spectroscopy (LIBS) is a rapid and versatile elemental
analysis technique. The optical emission from the laser induced plasma conveys
rich information about the chemical composition of samples. LIBS is
particularly effective for the analysis of light metallic elements such as Na,
K, Mg, and Ca. Thus, LIBS spectra can be chemical fingerprints of salts,
indicating their geographical origins and production methods. Rich chemical
information in the LIBS spectra can be effectively extracted and utilized for
developing classification models involving PCA analysis.
LIBS
spectra of salts from Jeung-Do (South Korea), Brazil, and Poland
Accuracy in salt classification can be improved further by combining orthogonal
chemical information with respect to LIBS data. One of the elemental analyses
that can be performed simultaneously with LIBS is Laser Ablation Inductively Coupled
Plasma Mass Spectrometry (LA-ICP-MS). LIBS and LA-ICP-MS share the same solid sampling
and sample excitation source, laser ablation.
Laser
ablation chamber for tandem LA-ICP-MS and LIBS analysis
From the laser induced
plasma, LIBS detects photons and LA-ICP-MS samples particles for ICP-MS. LIBS
analyzes light metallic elements (K, Mg, Ca, etc.) contained in salts with
relatively high concentrations (ppm to %). LA-ICP-MS can be applied for other
elements in the periodic table such as non-metallic elements and heavy metals
present especially at lower concentration.
The
two different data from LIBS (optical emission spectra) and LA-ICP-MS (mass
spectra) have been found to be independent from each other. LIBS spectra
provide chemical information mainly about K, Mg, and Ca whose concentration varies
by different production methods and interaction of rock salts with underground
fresh water. The heavy metal LA-ICP-MS mass spectra may represent the composition
contribution from the environment surrounding saltpans or salt mines. Also,
iodized salt products are easily discriminated by the 127I mass
peak.
LA-ICP-MS
spectra of salts from Haenam (South Korea), Hokkaido (Japan), Chile, Brazil,
India, and Poland
The ideal data set for developing a classification model is prepared by
combining LIBS and LA-ICP-MS spectra with optimized weighting factors.
PC
score plots based LIBS, LA-ICP-MS, and their fused spectra
The evaluation of the PLS-DA model performance based on LIBS, LA-ICP-MS,
and their fused data indicates that LIBS and LA-ICP-MS provide chemical
information complimentary to each other. Thus, the model based on the combined
data from LIBS and LA-ICP-MS led to improved classification accuracy. R&D collaboration
among Mokpo National University, Lawrence Berkeley National Laboratory, and
Applied Spectra revealed that tandem LA-ICP-MS and LIBS analysis is effective
in capturing more complete chemical fingerprints of salts for accurate
classification (Y. Lee, et al., Multivariate Classification of Edible Salts:
Simultaneous Laser-Induced Breakdown Spectroscopy and Laser-Ablation
Inductively Coupled Plasma Mass Spectrometry Analysis, Spectrochim. Acta Part B
118 (2016) 102-111).
Confusion
matrices of PLS-DA models based on LIBS, LA-ICP-MS, and Tandem LA-ICP-MS &
LIBS data
In recent years, LIBS, in combination with multivariate analysis, has
proven itself as a reliable material classification technology. Classification
of salts is one representative example of the expanding applications of LIBS.
In the future, LIBS could be hyphenated with other spectroscopic techniques,
such as LIBS-LA-ICP-MS, LIBS-Raman, LIBS-LIF, LIBS-IR, etc., for developing highly
accurate material screening analytical methods.
