Metal determination in cosmetics by ICP

It is well known that heavy metals are found naturally in the environment in rocks, soil and water and therefore exist in the manufacture of pigments and other raw materials in all industries including the cosmetics industry. Some of these metals have been used as cosmetic ingredients in the past. In some cases, measures have been implemented to reduce the amount of heavy metals to which users are exposed, including prohibiting their use in cosmetics. Lead,arsenic, cadmium, mercury, antimony and chromium are the main heavy metal ingredients prohibited in cosmetics in most countries. Cosmetic manufactures have the responsibility to ensure their products do not contain any of these metals.

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Introduction

It is well known that heavy metals are found naturally in the environment in rocks, soil and water, and therefore exist in the manufacture of pigments and other raw materials in all industries including the cosmetics industry. Some of these metals have been used as cosmetic ingredients in the past. In some cases, measures have been implemented to reduce the amount of heavy metals to which users are exposed, including prohibiting their use in cosmetics. Lead, arsenic, cadmium, mercury, antimony and chromium are the main heavy metal ingredients prohibited in cosmetics in most countries. Cosmetic manufacturers have the responsibility to ensure their products do not contain any of these metals.

Metal determination in samples is often done by ICP or ICPMS and it requires the samples to be in dissolved form. Commonly, acid digestions are the method of choice but seeing as many samples contain an important amount of silica, alumina and magnesium, some aggressive acids, such as HF must be used to obtain full dissolution. Furthermore, the quickest microwave methods can take up to 3 hours.

Knowing the risks associated with the use of HF, many laboratories look for alternative methods for obtaining full dissolution of their samples while optimizing the uptime and productivity.

The proposed methods describe alternative ways to determine the concentration of elements of interest as well as confirm the absence of heavy metals contained in cosmetics using borate fusions and peroxide fusions for ICP-OES determination.

Method

1. Sample Preparation

1.1 - Ashing
Due to their high organic and water content, each cosmetic sample and standard was submitted to an ashing procedure.
• Porcelain crucibles
• 550 °C until constant weight
• LOI is calculated for final analysis

1.2 - Borate fusion Method
Claisse® TheOx-DS® 6 position electric Fluxer
• Pt-Au (95 % / 5%) crucibles
• 34.83 / 64.67 / 0,5 (Li2B4O7 / LiBO2 / LiBr) flux
• 10 minutes of heating at 1050 °C
• 4 minutes of dissolution in 10 % HNO3 v / v

1.3 - Peroxide fusion Method
Claisse® TheOx-DS® 6 position electric Fluxer
• zirconium crucibles
• sodium peroxide flux
• 7 minutes of heating at 700 °C
• 4 minutes of cooling
• Dissolution in 20 % HNO3 / HCl v / v

2. ICP-OES

2.1 – PerkinElmer® Optima 7300DV

Table 1: Optima 7300 DV operating parameters
Plasma flow rate (Ar)16.0 L/min
Auxiliary gas flow rate0.4 L/min
Nebulizer flow rate0.8 L/min
Sample flow rate1.0 mL/min
Rinse2.5 mL/min (2 min < 10 ppm;
add. 90 sec for > 10ppm)
Shear gas pressure100 PSI
RF Power1500 W

2.2 – Samples
Table 2: Reference Materials and Samples Used to Validate the Developed Method with their calculated Losses on ignition (LOI )
SampleSupplierLOI (%)
DC60132 (Talc CRM)NCS10
SDC-1 (Mica CRM)USGS2
Sample 1 (647G-08-8008)Cosmetic Industry63
Sample 2 (6LX5-11-8001)Cosmetic Industry88
Sample 3 (6MPY-01-8001)Cosmetic Industry24
Sample 4 (6MNY-18-8001)Cosmetic Industry78
Sample 5 (9995-12-2921)Cosmetic Industry55
2.3 – Analytical Method
Table 3: Analytes of interest with selected wavelengths, detection limits and viewing modes for the peroxide fusion method and for the borate fusion method
Peroxide Fusion MethodBorate Fusion Method
ElementWavelength 
(nm)
Viewing modeMDL (mg/L)MDL (mg/L)
Al394.401Axial0.20.2
Ba413.065Axial0.1
0.1
Ca315.887Axial30.1
Cd214.440Axial0.0030.002
Co230.786Axial0.0020.01
Cr267.716Axial0.0020.03
Fe238.204Radial0.80.8
K766.490Axial30.5
Mg279.077Axial0.2
0.3
Mn257.610
Axial0.30.3
Mo204.597Axial0.050.05
Na589.592AxialND*0.4
Ni231.604Axial0.0010.008
P213.617Axial0.2
0.2
Pb220.353Axial0.07
0.03
Si251.611
Axial33
Sr460.733Axial0.040.005
Ti334.940Radial0.50.5
Zn206.200Radial0.30.3
*Not determined

Results

1. Peroxide Fusion Method

Table 4.1: Accuracy and precision measurements on SDC-1 using the peroxide fusion method
ElementWavelength (nm)Average
experimental
values (%) n = 10
Certified
values
(%)
Accuracy
(%)
Precision
(%) 
Al394.4017.98.3622952
Ca315.887BQL1.0006(104)(12)
Fe238.204
4.64.42041053
K766.4902.62.7229963
Mg279.0770.991.0191961
Mn257.610BQL0.0880(90)(1)
P213.617BQL0.0698(92)(3)
Si251.6113430.75721092
Ti334.9400.610.60551003
Table 4.2: Accuracy and precision measurements on DC60132 using the peroxide fusion method
ElementWavelength
(nm)
Average
experimental
values (%) n = 10
Certified
values 
(%)
Accuracy
(%)
Precision
(%)
Al394.4014.04.0329993
Ca315.887BQL1.7081(104)(6)
Fe238.2041.7
1.8465942
K238.204BQL0.0108--
Mg279.0771717.7896962
Mn257.610BQL0.0183(82)(1)
P213.617BQL0.048(110)(7)
Si251.611
2522.30131141
Ti334.9400.320.31171034
Table 4.3: Recovery results on pre-fusion spikes (n=5) using the peroxide fusion method
ElementWavelength
(nm)
Sample
1 (%)
Sample

2 (%)


Sample
3 (%)
Sample
4 (%)
Sample
5 (%)
DC60132
(%) 
SDC-1
(%)
Ba413.06510810210710610411087
Cd214.440108103959710210098
Co230.786949197969810097
Cr267.71695110111111114115106
Mo204.5978510510110410710286
Ni231.604998699100102101
84
Pb220.353898697971009385
Sr460.733100961041051129486
Zn206.200103101112921109791

2. Borate Fusion Method

Table 5.1: Accuracy and precision measurements on SDC-1 using the borate fusion method
ElementWavelength
(nm)
Average experimental values (%) n = 10Certified values (%)Certified values (%)Precision
(%) 
Al394.4018.38.3221002
Ca315.8871.11.00061053
Fe238.204
4.64.42041033
K766.4902.62.7229942
Mg279.0771.0
1.01911012
Mn257.610BQL0.088(84)
(2)
Na589.5921.11.5208753
P213.617BQL0.0698(75)(5)
Si251.611310.06981012
Ti334.9400.60.6055983
Table 5.2: Accuracy and precision measurements on DC60132 using the borate fusion method
ElementWavelength
(nm)
Average experimental
values (%) n = 10
Certified values (%)Accuracy
(%)
Precision
(%)  
Al394.4014.3
4.03291052
Ca315.8871.9
1.70811143
Fe238.2041.81.8465963
K766.490BDL
0.0108--
Mg279.0771817.78961032
Mn257.610BDL0.0183(76)
(2)
Na589.592BDL0.0182--
P213.617BDL0.0480(92)(3)
Si251.6112422.30131082
Ti334.9400.360.31171022
Table 5.3: Recovery results on pre-fusion spikes (n=5) using the borate fusion method
ElementWavelength
(nm)
Sample
1 (%)
Sample
2 (%)
Sample
3 (%)
Sample
4 (%)
Sample
5  (%)
DC60132
(%)
SDC-1
(%)
Ba413.06599921011021009599
Cd214.4408788988711411387
Co230.786100105100105101101101
Cr267.71610811110294111108110
Mo204.59710210610410610410398
Ni231.6049810499101104100101
Pb220.353
86879185938677
Sr460.73395988696
879184
Zn460.73310094102102108
113111

Discussion

The following criteria were taken into consideration in selecting the elemental wavelengths:
(a) the freedom from spectral interferences
(b) the different sensitivities and expected concentration in the samples.

The most sensitive line was not always used in order to avoid spectral interferences and to remain in the linear range. Observed interferences were compensated for by modifying the processing parameters (e.g. adjusting the background correction points, applying multicomponent spectral fittings (MSF) or inter-elemental corrections (IEC)). To compensate for instrumental signal variations, Ge (209.419) was added to every solution as an internal standard.
Since some elements were present as majors and others were present at trace levels, it was decided that the method validations would be done following two (2) avenues. For both dissolution methods, the accuracy and precision were evaluated on the high-concentration elements whereas spike recoveries were performed on the samples and the CRMs (Tables 4.3 and 5.3) to monitor the low concentration elements. The accuracy was determined by calculating the elemental recovery of certified reference materials (CRMs). The precision was determined by preparing and measuring 10 replicates of various CRMs. The results for each CRM are presented in Tables 4.1, 4.2, 5.1 and 5.2. The accuracy, precision and recovery results obtained demonstrate that the developed methods both perform very well. 

Conclusion

Peroxide fusions and borate fusions combined with the simultaneous ICP-OES (Optima 7300 DV) have the analytical capabilities to perform the analysis of many high and low concentration elements in cosmetics following a controlled ashing procedure. Metal components were measured in a variety of cosmetic samples and reference materials, demonstrating good accuracy, precision and recovery. The dual dissolution capabilities of TheOx® allowed us to validate two dissolution methods and find that both peroxide fusions and borate fusions can be used for metal determination in cosmetic products. The results obtained show that borate fusions have a slight advantage due to the lower MDLs for the majority of the elements. Finally, both methods confirm the absence of heavy metals in the cosmetic samples.

Acknowledgments: Sylvain Roy, Lab technician | Claisse and Aaron Hineman, Product Specialist | PerkinElmer®  

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