Analysing Plastics for Evidence of Toxic Metals by Laser Ablation-ICP-MS in Collision Mode

Environment-related concerns in the past decade have caused governments around the world to formulate regulatory guidelines to curb the spread of certain toxic substances from processed materials into the environment. Toxic substances such as arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg) and lead (Pb) are closely monitored.

Regulations have been set up by the US and the EU to safeguard people from toxic substance pathways migrating into the human body. Health officials are focusing on two specific pathways. They are:

  • The migration of trace elements from consumer plastics, such as product packaging, computer hardware and peripherals into the surrounding ecosystem when they are deposited or incinerated into landfills
  • The migration of toxic elements from plastic parenteral nutrition products such as catheters, tubing, IV bags, and product containers into the human body which are in direct contact with the blood stream

Conventionally mineral acids are used to digest polymeric materials to dissolve them. However these acid digestion processes can be time consuming and complex with the risk of elemental losses or contamination.

In a micro-destructive method laser ablation solid sampling helps greatly decrease the time taken to prepare a sample and to attain a high sample throughput. Here, the data provided illustrates an innovative laser ablation solid sampling technique, which holds real promise as a feasible method capable of precise multi-element quantitative evaluation.

Experiment

For this experiment a NWR213 laser ablation system was coupled to a NexION® 300D quadrupole ICP-MS system provided with a reaction/collision cell. The system was operated in both standard mode, with no collision cell gas, and in collision mode, with He collision cell gas, to understand the pros and cons of the analytical capabilities of both modes. The surface of the samples was scanned to provide a bulk analysis. Table 1 lists the main operating parameters.

Table 1. Instrumental Parameters of LA-ICP-MS

Laser Ablation System NWR213
     Sample Cell 100 mm Two-Volume Cell
     Fluence 9 J/cm2
     Spot Size 100 μm
     Scan Speed 200 μm/sec
     Scan Time 60 secs
     He Sample Cell Gas Flow Rate 0.9 L/min
     Ar Mixing Gas Flow Rate 0.8 L/min
ICP-MS NexION 300D
     RF Power 1350 W
     He Collision Cell Gas Flow Rate (Standard mode) 0 mL/min
     He Collision Cell Gas Flow Rate (Collision mode) 5 mL/min
Isotopes Monitored 13C, 52Cr, 75As, 79Br, 111Cd, 202Hg, 208Pb

For performing calibration, two polyethylene standards - IMEP-10 and BCR681, were used. Three polyethylene samples, whose compositions were known at the time of manufacture, were tested as “unknown” to measure instrument performance. White represented an "uncontaminated" sample, yellow samples represented a sample containing high levels of Cr and Pb and green represented a high Br content. Inter- and intra-experimental accuracy was evaluated by running two fully automated analytical tests. Each test comprised six replicate analyses of each sample.

Results

Table 2 displays the complete results of the tests. Both the standard and collision modes displayed a good amount of accuracy. The application of 13C to normalise the elemental signals could be the reason for this. However it also highlights the stability of the NWR213 laser ablation system and the NexION 300D ICP-MS.

Table 2. Determined concentrations of toxic elements in polyethylene by LA-ICP-MS both with and without Collision mode

White A White B Yellow A Yellow B Green A Green B
As Standard Mode
%RSD
ND
-
ND
-
ND
-
ND
-
ND
-
ND
-
μg/g
%
Collision Mode
%RSD
ND
-
ND
-
ND
-
ND
-
ND
-
ND
-
μg/g
%
Reference Value 0 0 0 μg/g
%
Br Standard Mode
%RSD
1.31
88.0%
7.23
35.0%
8.46
65.5%
8.91
42.7%
94.3
7.88%
406
9.67%
μg/g
%
Collision Mode
%RSD
ND
-
ND
-
ND
-
ND
-
435
2.95%
443
1.83%
μg/g
%
Reference Value 0 0 450 μg/g
%
Cd Standard Mode
%RSD
ND
-
ND
-
ND
-
ND
-
ND
-
ND
-
μg/g
%
Collision Mode
%RSD
ND
-
ND
-
ND
-
ND
-
ND
-
ND
-
μg/g
%
Reference Value 0 0 0 μg/g
%
Cr Standard Mode
%RSD
ND
-
ND
-
2542
2.92%
2490
7.31%
ND
-
ND
-
μg/g
%
Collision Mode
%RSD
ND
-
ND
-
2487
3.82%
2487
1.86%
ND
-
ND
-
μg/g
%
Reference Value 0 2480 0 μg/g
%
Hg Standard Mode
%RSD
ND
-
ND
-
ND
-
ND
-
ND
-
ND
-
μg/g
%
Collision Mode
%RSD
ND
-
ND
-
ND
-
ND
-
ND
-
ND
-
μg/g
%
Reference Value 0 0 0 μg/g
%
Pb Standard Mode
%RSD
ND
-
ND
-
7648
4.40%
7459
3.51%
ND
-
ND
-
μg/g
%
Collision Mode
%RSD
ND
-
ND
-
8919
4.60%
8935
6.78%
ND
-
ND
-
μg/g
%
Reference Value 0 8900 0 μg/g
%

The key observations are listed below:

  • In the collision mode Br results were more repeatable when compared to the standard mode
  • No false positives for Br in collision mode as no Br was noticed in the white or yellow samples
  • Accurate results were obtained for both Cr and Pb in the yellow sample using the collision mode.

Conclusions

It was possible to establish toxic elements in polyethylene because of the use of He as a collision gas in the universal cell of the NexION 300D ICP-MS, which enhanced the analytical capability of the LA-ICP-MS to identify the toxic elements. This technique can be extensively applied to the study of various materials. Setting up the method is simple and fast due to the ability of measuring all of the elements in the collision mode.

This information has been sourced, reviewed and adapted from materials provided by PerkinElmer.

For more information on this source, please visit PerkinElmer.

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