Lasers Med Sci (2009) 24:917–924 DOI 10.1007/s10103-009-0659-2

ORIGINAL ARTICLE

Investigation of common Indian edible salts suitable for kidney disease by laser induced breakdown spectroscopy V. K. Singh & N. K. Rai & S. Pandhija & A. K. Rai & P. K. Rai

Received: 30 November 2008 / Accepted: 10 February 2009 / Published online: 11 March 2009 # Springer-Verlag London Limited 2009

Abstract Salt is an essential and important dietary mineral for maintaining life. Currently, the issue of the potential benefit or damage from salt intake in chronic kidney disease patients is controversial. The attempt of this article is to bring into focus the potential role of elements particularly sodium, Na, and potassium, K, which are the main constituents of dietary salts, in kidney patients by using laser-induced breakdown spectroscopy (LIBS). LIBS spectra of different salt samples have been recorded in the spectral region 200–500 nm with spectral resolution 0.1 nm and in the spectral region 200–900 nm with spectral resolution 0.75 nm. Quantitative elemental study was carried out to determine the constituents of different types of common Indian edible salts by using the calibration-free LIBS method. Our experimental results demonstrate that Saindha salt (commonly known as rock salt) is more beneficial than other edible salts for patients suffering from V. K. Singh : N. K. Rai : S. Pandhija : A. K. Rai (*) Laser Spectroscopy Research Laboratory, Department of Physics, University of Allahabad, Allahabad 211002, India e-mail: [email protected] V. K. Singh e-mail: [email protected] P. K. Rai Department of Nephrology, Opal Hospital, N. 10/60, Kakarmatta, DLW Road, Varanasi, India 221010 Present address: V. K. Singh School of Applied Physics and Mathematics, Shri Mata Vaisho Devi University, Kakryal, Reasi 182320, India

chronic kidney disease. The results of the quantitative elemental analysis of the salts obtained from LIBS measurements are also compared to atomic absorption spectroscopy (AAS). Keywords LIBS . Saindha salt . Sodium . Potassium . Calcium . Magnesium

Introduction Chronic kidney disease (CKD) is an important and widespread clinical problem that has multiple etiologies [1, 2]. The control of blood pressure, glucose, and cholesterol are important strategies to slow the progression of chronic kidney disease (CKD). Dietary salt intake may also be one of these factors. Salt intake is a controversial topic in health care today. Dietary salt intake has been debated for decades as having a potential deleterious influence on human health [3]. Few controversies in medicine have such a long history as that of whether salt is identifiably dangerous or not. Excessive dietary salt is common, as it is both a preservative and an important component facilitating taste in processed foods. Salt ingestion has been linked to the development of left ventricular hypertrophy, stroke, and kidney disease via hemodynamic and non-hemodynamic mechanisms [4, 5]. Could dietary salt be a candidate dietary exposure that needs to be considered as a risk factor for progression of CKD? Given the ubiquitous nature of salt consumption and clinical observations that have increased, dietary salt is a concern, particularly in patients whose blood pressure is salt sensitive [6–9], and CKD patients [9]. It is possible that salt may be a modifiable risk factor for progression of kidney disease.

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Salt is a dietary mineral that is essential for human life and is composed primarily of sodium chloride, NaCl. Salt for human consumption is produced in different forms. It is a crystalline solid, white, pale pink, or light grey in color, normally obtained from sea water or rock deposits. Edible rock salts may be slightly grayish in color due to their mineral content. Salt is mainly obtained from two sources: rock salt and brine [10]. Rock salt is simply crystallized salt, also known as halite. It is the result of the evaporation of ancient oceans millions of years ago. Brine is water containing a high concentration of salt. The most obvious source of brine is the ocean, but it can also be obtained from salty lakes such as the Dead Sea and from underground pools of salt water [10]. Natural brines always contain other substances like magnesium chloride, magnesium sulfate, nitrates, calcium sulfate, potassium chloride, magnesium bromide, and calcium carbonate, etc., dissolved along with salt [10]. Rock salt may be quite pure, or it may contain various amounts of these substances along with rocky impurities such as quartz (crystallized form of silicon dioxide, SiO2) [10]. For table salt, however, additives are usually mixed into it. Most table salt is iodized in order to provide the trace element iodine to the diet, which helps to prevent goiter, a disease of the thyroid gland. To supply iodine, a small amount of potassium iodide is added. Table salt also contains small amounts of various chemicals including magnesium carbonate, calcium silicate, calcium phosphate, magnesium silicate, and calcium carbonate to keep the salt from absorbing water and caking [10]. There are several types of salts that are easily available in the marketplace for daily use and they contain different proportions of sodium (Na) and potassium (K). The Na and K contents in the salts are directly related to high blood pressure and kidney diseases, respectively. The levels of Na and K in these salts can help to make the choice of low sodium and low potassium salts, which are the requirement to manage these diseases. So it is very important to know the Na and K contents in these salts. Laser-induced breakdown spectroscopy (LIBS) is an analytical technique based on recording the atomic spectrum of plasma plume generated by a laser beam focused at the sample surface [11, 12]. Any kind of material (solid, liquid, or gas) without any sample preparation, and preferably without dissolution, can be studied by this technique. In the present paper, we have analyzed four types of common Indian edible salts. The concentrations of Na, K, and other constituents present in them have been determined by calibration-free laser-induced breakdown spectroscopy (CF-LIBS) [13]. The calibration-free LIBS method has recently been developed as an alternative approach to the quantitative analysis performed by the calibration curves method [14] and this method seems to be

Lasers Med Sci (2009) 24:917–924

quite suitable for the quantitative analysis of salt samples where certified reference material (CRM) samples having varying concentration of Na are not available on the market. The CF-LIBS algorithm has been discussed in detail in literature [15–18]. The results of CF-LIBS have been compared with the results obtained from atomic absorption spectroscopy (AAS).

Materials and methods Laser-induced breakdown spectroscopy (LIBS) Over the past decades, LIBS has been used as an elegant method for the analysis of materials of any kind and in any phase [19–23]. A basic LIBS system consists of a highpower pulsed laser, collection optics, and a spectrometer. When the laser is focused on the sample surface, it ablates a very small amount of material, which instantaneously superheats, generating a plasma plume that dissociates the ablated material into excited ionic and atomic species. Initially, the plasma emits a continuum radiation, but as the plasma expands and cools (0.1 to 10 µs), the characteristic atomic emission lines of the elements can be observed. These spectral signatures can be recorded and rapidly processed for analysis of the material. Salt samples BRAND A (Lona salt): Brand A is low sodium, Na, medicated salt. It also contains potassium chloride, KCl, [24]. BRAND B (Saindha salt): Brand B is Saindha salt, the salt commonly known as rock salt [10]. BRAND C (Tata salt): This is a refined iodinated salt widely used as a table salt and easily available in the Indian markets. BRAND D (Ordinary salt): This is an ordinary type of salt, which is not refined and can be obtained easily in the Indian markets. Salt samples were crushed into a fine powder and were used to form pellets. The salt samples were placed into a 13-mm diameter Evacuable KBr Die Set (Kimaya Engineers, India) and pressed using 10 t of force for 2 min by hydraulic machine to form the pellets and these pellets were finally used to record the LIBS spectra of standard samples. Experimental setup The experimental arrangement of the LIBS system is schematically shown in Fig. 1. The frequency-doubled neodymium: yttrium-aluminum-garnet (Nd:YAG) laser (Continuum Surelite III-10) having a repetition rate of

Lasers Med Sci (2009) 24:917–924

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Fig. 1 Schematic diagram of laser-induced breakdown spectroscopy (LIBS) experimental setup for the analysis of salt samples

Results and discussion We have recorded the LIBS spectra of the four types of salts described earlier. Three sets of LIBS spectra are recorded and to reduce relative standard deviation (RSD), 50 laser shots are accumulated in each set. The elements detected in these salts are Ca, Mg, Na, K, Al, Si, Sr, Ti, Zn, Fe, S, C, H, N, and O [Figs. 2 and 3]. The appearance of the atomic lines of Ca, Mg, Na, K, S, C, H, N, and O etc.,

in the LIBS spectra [Figs. 2 and 3] of different types salt samples are mainly due to the presence of compounds magnesium carbonate, calcium silicate, calcium phosphate, magnesium silicate, calcium carbonate, and nitrates, etc. in the salt samples [10]. Qualitative analysis of salts Typical LIBS spectra of Saindha salt (Brand B) and Lona salt (Brand A) are shown in Figs. 2 and 3, which clearly shows the presence of all elements mentioned above. The presence of the atomic lines of Al and Si as an impurity in the LIBS spectra (Fig. 3) of Brand A salts (low Na medicated salt) is clearly noticed. The atomic lines of Na

LIBS Spectra of Saindha salt

Mg

2000

Intensity (arbitrary units)

10 Hz, a pulse width of 4 ns [full width at half maximum (FWHM)], and maximum laser energy of 425 mJ at 532 nm was used in the present experiment. The beam diameter was 9 mm, and the beam divergence was 0.6 mrad. A fusedsilica lens of focal length 15 cm was used to focus the laser beam directly on the surface of the salt samples kept on a rotating stage. The light emitted by the laser-generated plasma was collected and fed into the entrance slit of a grating spectrometer equipped with a charge-coupled device (CCD), using an optical fiber bundle. The dispersed light from the spectrometer (Ocean Optics, LIBS 2000+) was analyzed by using OOI LIBS 2000+ software (working in Windows 2000 Professional). The pellets of salt samples were placed on a rotating stage so that every laser pulse was incident at a fresh location on the sample surface and thus crater formation was avoided. The LIBS spectra of different types of salt samples were recorded using a 1.5-µs gate delay with a spectral resolution 0.1 nm in the spectral range of 200–500 nm and a spectral resolution of 0.75 nm in the spectral range of 200–900 nm. The averaged spectra for 50 laser shots were recorded to enhance the signal-to-noise ratio.

Ca

1500

Ca Na

1000

Ca

Sr

N Sr Mg

Mg

500

C Fe

Ca

Zn 0 200

250

300

350

400

450

500

Wavelength (nm)

Fig. 2 LIBS spectra of Saindha salt in the spectral region 200– 500 nm

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Lasers Med Sci (2009) 24:917–924 LIBS Spectra of Lona salt

Intensity (arbitrary units)

decreasing order. Thus, the concentration of K in Brand A (Lona salt), Brand C (Tata salt), Brand D (Ordinary salt), and Brand B (Saindha salt) are of decreasing order. We have also verified the above statement by performing the quantitative elemental analysis of each salt by using CFLIBS technique.

Mg

2000

1500

K

Mg

Ca Ca

1000

Na

Sr Ca

Quantitative analysis of salts

Ti

Si 500

Al

C Zn

0 200

250

300

350

400

450

500

Wavelength (nm)

Fig. 3 LIBS spectra of Lona salt in the spectral region 200–500 nm showing the atomic lines of Al and Si

at 330.2 nm [4p (2P3/2) → 3s (2S1/2)] and the atomic lines of K at 766.4 nm [4p (2P3/2) → 4s (2S1/2)] and 769.9 nm [4p (2P1/2) → 4s (2S1/2)] were selected to compare their intensity in corresponding LIBS spectra of different salts. The intensity of Na line at 330.2 nm in the LIBS spectra (Fig. 4) of Brand C (Tata salt), Brand D (Ordinary salt), Brand B (Saindha salt), and Brand A (Lona salt) are of decreasing order. The intensity of the atomic line of an element is directly proportional to its concentration in the sample, thus line intensity represents the content level of the element in the sample. This clearly reveals that the concentration of Na in Brand C (Tata salt), Brand D (Ordinary salt), Brand B (Saindha salt), and Brand A (Lona salt) are of decreasing order. The intensity of K line at 769.9 nm [4p (2P1/2) → 4s (2S1/2)] in the LIBS spectra (Fig. 5) of Brand A (Lona salt), Brand C (Tata salt), Brand D (Ordinary salt), and Brand B (Saindha salt) are of

In the present paper, we have applied the CF-LIBS method for quantitative analysis. In the CF-LIBS method there is no need to draw calibration curves to quantify the constituents present in the samples. The detail about CF-LIBS is given elsewhere [13–15, 18, 25]. Here we will verify the basic assumption like existence of stoichiometric ablation and local thermodynamic equilibrium (LTE). If in the laser-induced plasma, the rapidly heated material has the same composition as the target, then it leads to the stoichiometric ablation. In the present study, laser ablation of the salt samples were achieved using a 35-mJ Nd:YAG laser pulse at 532 nm with a 4-ns pulse duration and a focused spot diameter of 0.002 cm leading to a power density of 2.4×103 GW/cm2, which is greater than 1010 W/cm2 and satisfies the condition of stoichiometric ablation [25]. Optically thin plasma can be easily verified from the intensity ratio of two interference-free emission lines from a species having the same upper energy level. For optically thin plasma, this intensity ratio should be the same as the ratio of the corresponding transition probabilities. The intensity ratio Ca I (373.6 nm)/Ca I (370.6 nm) is 2.11 and their transition probability ratio is 1.93. Similarly, the intensity ratio Mg I (383.8 nm)/Mg I (383.2 nm) is 1.46 and their transition probability ratio is 1.32. The values of K

1100 300

Na

Black line-Lona Salt Red line-Saindha salt K Blue line-Tata Salt Green line-Ordinary Salt

1000 900 800

Intensity (Counts)

Intensity (Counts)

250

Black line-Lona Salt Red line-Saindha salt Blue line-Tata Salt Green line-Ordinary Salt

200

150

100

700 600 500 400 300 200 100

50

0 310

0 750 320

330

340

350

760

770

780

Wavelength (nm)

Wavelength (nm)

Fig. 4 A part of the LIBS spectra of Tata salt, Ordinary salt, Saindha salt, and Lona salt according to their decreasing intensity, respectively

Fig. 5 A part of the LIBS spectra of Lona salt, Tata salt, Ordinary salt, and Saindha salt according to their decreasing intensity, respectively

Lasers Med Sci (2009) 24:917–924

transition probability have been obtained from NIST [26]. The consistency between the intensity ratio of atomic lines of Ca and Mg and their corresponding transition probability is observed, which clearly demonstrates the existence of optically thin plasma. To verify the existence of LTE, the knowledge of the electron density in the laser-induced plasma is essential, which may be obtained by measuring the broadening of the spectral lines [27]. The Lorentzian line shape function fitted to the experimental data points of the emission intensities for Ca I at 422.6 nm line for salt samples lead to the observed line-width which has been corrected by subtracting the instrumental width. The electron density was calculated by using the relation:
   $lFWHM Ne
ð1Þ  1016 cm3 2w where the value of w, the electron impact parameter, is obtained from H. R. Griem [28] and the value of Ne is found to be 1.05×1018 cm−3. We have calculated the lower limit for the electron density for which the plasma will be in LTE by using the McWhirter criterion [27]   Ne cm3  1:6  1012 ð$E½eVÞ3 ðT½KÞ1=2 ð2Þ where T is the plasma temperature and ΔE is the largest energy transition. The calculated average value of plasma temperature is T=11,000 K. By substituting the value of T=11,000 K and ΔE=2.93 eV obtained from NIST [26] in the above equation, Ne comes out to be 4.2×1015 cm–3, which is three orders of magnitude less than our experimentally calculated electron density. This clearly demonstrates that the condition of LTE is fulfilled for the laser-induced plasma in our experiments. In LIBS experiments for optically thin plasma a Boltzmann population distribution can be assumed and the spectral intensity “I” corresponding to the transition between levels Ek and Ei of the atomic species α with concentration Cα, at temperature T can be expressed as

ki  Ia Ek Ca F þ ln ln ð3Þ ¼ U a ðT a Þ KB T gk Aki where, KB is the Boltzmann constant, Uα (T) is the partition function, Aki the transition probability, gk is the statistical weight for the upper level, Ek is the excited level energy, T is the temperature, and F is a constant depending on experimental conditions. In order to evaluate the plasma temperature, considering Eq. (3) and drawing a plot between  ki  Ia ln gk Aki and Ek, the slope of the plot is related to the plasma temperature T, while the intercept is proportional to the logarithm of the species concentration times the experimental factor “F”. In LTE all the plots will have the same slope but different intercept. In order to get rid of the unknown

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experimental factor F, one can use the normalization relation on the species concentration Cα of the sample X Ca ¼ 1 ð4Þ a

Equations (3) and (4) are used for the determination of the concentration of all the species present in a sample. The estimated experimental error for plasma temperature is ±17%, which is approximately equal to 2,000 K and this may be attributed to the difference in the excitation and ionization potentials between the elements. The energies of upper levels, statistical weights, and transition probabilities used for the experimental plots for each element have been obtained from NIST [26] and Griem [28]. After getting the best estimate for the plasma temperature, we have determined the concentrations of all the species by estimating the intercept bα and are tabulated in Tables 1 and 2. To check the validity of the present analysis, we estimated the concentration of Na & K in few salts using atomic absorption spectroscopy (AAS) and the results are compared in Table 1. It is clearly seen from Table 1 that the results obtained from CF-LIBS are in good agreement with the result obtained from AAS. It is clear from Table 2 that the concentrations of Ca and Mg are higher in Saindha salt in comparison to the other salts. In the Saindha salt, the concentration of Na is higher than in the Lona salt but it is lower than that of Tata salt and Ordinary salt (Table 1) and the concentration of K is less in Saindha salt in comparison to the other salts (Table 1). We have detected the presence of lighter elements like carbon, C, hydrogen, H, nitrogen, N, and oxygen, O, in these salts and have also quantified these elements which are not possible by AAS and ICP. Their concentrations are tabulated in Table 2. The concentrations of other elements like aluminium (Al), silica (Si), strontium (Sr), sulfer (S), iron (Fe), zinc (Zn), and titanium (Ti) are estimated and are tabulated in Table 2. The presence of a reasonable amount of Na in the human body helps with nerve activity, muscle contraction, and fluid balance. Thus, everyone needs some sodium, but since sodium is found naturally in foods, most people consume more sodium than they need. When kidneys fail to work properly, the extra sodium is not removed from the body. With extra sodium in the body one will feel thirsty and drink more fluids. Drinking a lot Table 1 Concentration of elements detected in salt samples by CFLIBS and AAS Sample

Brand Brand Brand Brand

A (Lona salt) B (Saindha salt) C (Tata salt) D (Ordinary salt)

Na (%)

K (%)

CF-LIBS

AAS

CF-LIBS

AAS

22.43 37.75 67.8 51.57

19.80 35.75 – –

37.94 5.39 9.7 8.92

26.90 7.9 – –

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Lasers Med Sci (2009) 24:917–924

Table 2 Concentrations of the elements detected in salt samples by CF-LIBS Sample

Ca (%)

Mg (%)

C (%)

H (%)

N (%)

O (%)

Al (%)

Si (%)

Sr (%)

Ti (%)

S (µg/g)

Fe (µg/g)

Zn (µg/g)

Brand A (Lona salt) Brand B (Saindha salt) Brand C (Tata salt) Brand D (Ordinary salt)

6.88

4.81

0.26

0.56

2.03

1.11

9.15

13.74

0.62

0.18

2

2,500

<1

16.69

9.39

0.75

4.06

16.13

6.79

N.D.

N.D.

1.4

0.13

9

5,700

66

8.9

6.6

0.15

0.96

3.18

1.3

N.D.

N.D.

1.1

0.02

142

5,100

44

13.4

5.6

0.14

2.35

7.9

3.7

N.D.

N.D.

3.38

0.04

58

20,000

22

*N.D. : Not Detected *µg/g=10–4 %

of fluids is dangerous for people with CKD. Too much fluid can cause high blood pressure, difficulties in breathing, and swelling of hands, feet and legs. Thus, the best way to control thirst is to limit sodium. This means that one needs to select the proper use of table salt containing low sodium, thus, Saindha salt is suitable for this purpose. Similarly, potassium, like sodium, helps with nerve activity and muscle contraction. When kidneys are not working properly, potassium levels will build up in the blood, which may be dangerous. Too much potassium can cause muscle weakness. Since our heart is a muscle, high potassium levels could cause it to beat abnormally or to even stop. There are no warning signs of having high potassium levels in the blood. So, like Na, excess levels of K must also be restricted in our dietary habits. It is clear from Table 1 that the concentration of K is considerably lower in Saindha salt in comparison to other salts and the concentration is very high in Lona salt which is very dangerous for CKD patients. So, these types of salts having more K contents are to be restricted for the CKD patients. Therefore, Saindha is better than the Lona salt for kidney patients. Potassium can also be controlled by the restriction of food items rich in potassium. Fats and sugars are the only potassium-free foods. Milk, bananas, oranges, potatoes, tomatoes, and dried beans are high in potassium. Salt substitutes and sodium-free baking powder are also very high in potassium. The amount of potassium our body can handle depends on our level of kidney function. Thus in summary, a diet often low in sodium as well as potassium should be used in people with abnormal kidney function. Sodium may be restricted to improve blood pressure control and to avoid fluid accumulation. Potassium is restricted if it is not excreted effectively and levels in the blood are high. The controlled amounts of each nutrient are based on the blood level of potassium, sodium and all other protein and urea. So, excess sodium and potassium must be restricted for the CKD kidney patients. Making a good decision about salts is one of the best ways to control sodium

and potassium. Saindha salt is liked by most of the Indians. The taste of Saindha salt is excellent and comparable with other types of salts despite having lower concentrations of Na and K. The presence of other elements like CA and Mg possibly contributes to good taste of Saindha salt. Calcium is the most abundant mineral found in the body, which is supplied from the food we eat and from calcium supplements. Calcium recommendations for people with CKD are different from others. Kidney disease causes imbalances in bone metabolism and increases the risk of a type of bone disease called renal osteodystrophy, and these imbalances can cause calcium to deposit in the blood vessels which can contribute to heart disease. Therefore, to overcome these imbalances, calcium supplementation is recommended to the patients of CKD. Dietary salt rich in calcium can also be a better way of calcium supplementation. Our experimental results (Table 2) show that the concentration of Ca is higher in Saindha salt than other common edible salts; therefore it is a better salt substitute for CKD patients. The higher percentage of Ca in Saindha salt (rock salt) is probably due to the presence of calcium sulfate, magnesium carbonate, and calcium carbonate, etc., in rock salt [10]. Magnesium is the fourth most abundant mineral in the body and is needed for more than 300 biochemical reactions. It also helps to maintain normal muscle and nerve function [29, 30]. So, there is an increased interest in the role of magnesium in preventing and managing disorders such as hypertension, cardiovascular disease, diabetes, and kidney failure. Dietary magnesium is absorbed in the small intestines and is excreted through the kidneys [30, 31]. Dietary magnesium does not pose a health risk, however, pharmacologic doses of magnesium in supplements can promote adverse effects such as diarrhea and abdominal cramps. Risk of magnesium toxicity increases with kidney failure, when the kidney loses the ability to remove excess magnesium. Very large doses of magnesium-containing laxatives and antacids also have been associated with magnesium toxicity [31]. Therefore, it is important for one to be aware of the use of any

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magnesium-containing foods and drugs. For kidney patients, the recommended salt intake is less than 3–4 gm [32] and the recommended dietary dose for magnesium for the adults of age group 19–30 years is 400 mg per day and for adults of age group 30 plus is 420 mg per day [33]. Our experimental results (Table 2) show that the concentration of Mg is higher in Saindha salt in comparison to other salts. The presence of a higher amount of Mg in Saindha salt (rock salt) than other salt may be due to the presence of magnesium sulfate, magnesium carbonate, etc., in rock salt [10]. According to our reports (Table 2), Saindha salt contains approximately 95 mg of Mg in 1 gm of Saindha salt and, consequently, patients would consume 285 mg Mg, for 3 gm of the salt each day, which is within the safety limit [33]. Thus, based on the Mg concentration, Saindha salt is not harmful for the patients suffering from CKD. Aluminium, Al, toxicity has been associated with CKD. Al is absorbed by the intestines and is rapidly transported into bone, where it disrupts mineralization and bone cell growth and activity. Its toxicities result in painful forms of renal osteodystrophy, bone disease, and osteomalacia, and other forms of the disease. The element Al is not detected in Saindha salt and Lona salt contains an appreciable amount of Al that can produce toxic effects in the patients of CKD. So, regarding this point of view also, it is better for the patients of CKD to take Saindha salt.

Conclusions In summary, we believe that there is sufficient evidence to indicate that dietary salt may be a modifiable risk factor for progression of kidney disease. The present study reveals that Saindha salt is suitable for patient suffering from kidney disease. Further clinical trials will answer the question as to whether specific salt is important in delaying progression of kidney disease. Experiments in the laboratory demonstrated that the LIBS method is suitable for the detection and quantification of the constituents present in different types of salts, which is helpful in managing CKD patients. Acknowledgments The financial assistance from the DRDO project (No. ERIP/ER/04303481/M/01/787) is gratefully acknowledged. The authors also thank Mr. Pradeep Ranjan (Sr. Manager R&D, IFFCO Phulpur Unit, Allahabad, India) for providing us the facility of AAS. We are also much grateful to Prof. S.N. Thakur, B.H.U., Varanasi, India for valuable discussions.

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