Migratable element analysis of liquid glue determines how much of potentially harmful heavy metals such as lead, cadmium, mercury and chromium(VI) can be released from water-based adhesives under simulated gastric conditions. Because liquid and sticky adhesives used in toys, stationery and craft products fall under the strictest migration limits of standards such as EN 71-3, ISO 8124-3, ASTM F963 and GB 6675, accredited third-party laboratories rely on acid extraction followed by ICP-OES, ICP-MS and speciation techniques to verify safety and regulatory compliance. This article explains the scope, elemental limits, analytical workflow and practical applications of migratable element testing for liquid adhesives.
Technical Overview
Liquid glues, including polyvinyl acetate (PVA) school glues, white glues, craft adhesives, gel glues and slime components, are among the most widely handled chemical products by children. During normal use these adhesives contact skin, fingers and mouths, and accidental ingestion is a foreseeable scenario for young users. For this reason, chemical safety assessments focus not on the total amount of heavy metals present in the formulation, but on the migratable (bioavailable) fraction that can dissolve in simulated gastric fluid and be absorbed by the body.
Migratable element analysis reproduces the acidic conditions of a child's stomach in the laboratory. A measured test portion of the liquid glue is extracted with dilute hydrochloric acid at body temperature, and the resulting extract is analyzed by modern plasma spectroscopy to quantify elements such as lead, cadmium, mercury, arsenic and hexavalent chromium. The result is compared against migration limits established by international toy safety and stationery safety standards.
The key regulatory references for liquid adhesives include:
- EN 71-3 (Safety of toys – Part 3: Migration of certain elements) – covers 19 elements and assigns liquid or sticky materials to Category II, which carries the strictest limits.
- ISO 8124-3 (Safety of toys – Part 3: Migration of certain elements) – internationally harmonized with the EN approach.
- ASTM F963 (Standard Consumer Safety Specification for Toy Safety) – specifies soluble heavy metal limits for toy substrates and surface coatings.
- GB 6675.4 (Chinese national toy safety standard on migration of certain elements) and GB 21027 (General safety requirements for student supplies), which apply to glues used by school-age children in China.
- EU Toy Safety Directive 2009/48/EC and US CPSIA, which mandate chemical compliance including lead restrictions for children's products in the European and North American markets.
Because liquid adhesives are classified as Category II ("liquid or sticky") materials under EN 71-3, a larger potential ingestion volume is assumed, and the allowable migration limits are far lower than those for dry, brittle or scraped-off materials. This makes accurate, low-level elemental analysis essential for any glue intended for children's products.
Testing Samples
Water-Based White and School Glues
PVA emulsion glues, washable school glues and glue pens are the most common submissions. Typical contaminant sources include processing equipment, fillers, colorants, preservatives and recycled packaging. Sufficient homogeneous sample (typically 5–10 g per element suite) is required to allow replicate extraction and confirmatory testing.
Craft, Hobby and Slime Adhesives
Liquid craft glues used for slime kits, glitter glues and colored adhesives frequently contain pigments and effect additives. Cadmium- and lead-based pigments, barium-containing fillers and organotin-based biocides are historically documented sources of non-compliance and warrant targeted screening.
Specialty and Industrial Liquid Adhesives
Adhesives incorporated into toys during manufacture (assembly glues, sticker adhesives, adhesive coatings) are also evaluated. Where the glue cannot be physically separated, the whole component or composite material is tested, and the result is assessed against the applicable category.
Sample Intake and Handling
Laboratories homogenize the adhesive, record lot identification, and store aliquots in acid-cleaned containers to prevent cross-contamination. Samples for chromium(VI) and organotin determination are protected from light, since speciation can change during storage.
Testing Items
Depending on the destination market, the following element suites are evaluated as migration values in mg/kg of sample:
- Nineteen-element suite (EN 71-3 / ISO 8124-3): aluminium, antimony, arsenic, barium, boron, cadmium, chromium(III), chromium(VI), cobalt, copper, lead, manganese, mercury, nickel, selenium, strontium, tin, organic tin and zinc.
- Eight-element classic suite (ASTM F963, GB 6675.4, GB 21027): antimony, arsenic, barium, cadmium, total chromium, lead, mercury and selenium.
Representative Category II migration limits for liquid or sticky toy materials under EN 71-3 include:
- Arsenic: 2.3 mg/kg
- Cadmium: 1.3 mg/kg
- Chromium(VI): 0.010 mg/kg
- Lead: 2.3 mg/kg
- Mercury: 3.8 mg/kg
- Antimony: 11.3 mg/kg
- Barium: 1125 mg/kg
- Aluminium: 560 mg/kg
- Boron: 375 mg/kg
- Chromium(III): 9.4 mg/kg
Comparable soluble heavy metal limits under ASTM F963 include lead 90 mg/kg, cadmium 75 mg/kg, arsenic 25 mg/kg, mercury 60 mg/kg and selenium 500 mg/kg. Because limit values are periodically revised, manufacturers and laboratories should always verify the figures in the current published edition of each standard before issuing or acting upon a compliance report.
Testing Methods
Sample Preparation and Gastric Extraction
The reference procedure follows the EN 71-3 / ISO 8124-3 protocol. A weighed test portion of the homogenized glue is mixed with hydrochloric acid solution (approximately 0.07 mol/L) in a defined mass-to-volume ratio. The mixture pH is checked and, where necessary, adjusted to within the range of 1.1 to 1.3 to replicate stomach acidity. The vessel is agitated at 37 °C ± 2 °C for one hour and then allowed to stand for one hour under temperature control. The slurry is subsequently centrifuged and filtered to obtain a clear migration solution for instrumental measurement.
Instrumental Determination
- ICP-OES (inductively coupled plasma optical emission spectrometry) is the workhorse technique for most elements, offering robust throughput and multi-element capability.
- ICP-MS (inductively coupled plasma mass spectrometry) is applied when required detection limits are very low, notably for chromium(VI) at the 0.010 mg/kg Category II level, and for arsenic, cadmium and lead at trace concentrations.
- AAS (atomic absorption spectrometry), including graphite furnace and hydride generation techniques, serves as a complementary method for confirmatory analysis.
Speciation Analysis
Total migrated chromium is split into trivalent and hexavalent species, because chromium(VI) is a carcinogen with a limit roughly three orders of magnitude lower than chromium(III). Established approaches include spectrophotometry with 1,5-diphenylcarbazide and ion chromatography coupled to ICP-MS. Organic tin compounds are determined separately, typically by solvent extraction and derivatization followed by gas chromatography–mass spectrometry (GC-MS).
Quality Control and Data Integrity
ISO/IEC 17025-accredited third-party laboratories support each batch with reagent blanks, duplicate extractions, matrix spikes, certified reference materials and interference-matched calibration. Method detection limits, measurement uncertainty and recovery rates (commonly acceptable within 80–120%) are documented so that pass/fail decisions against low Category II limits remain statistically defensible.
Applications
- Toy safety compliance: demonstrating conformity with EN 71-3 and the EU Toy Safety Directive for glue-based toys, sticker books, activity kits and slime sets placed on the European market.
- North American market access: verifying soluble heavy metal compliance under ASTM F963 and supporting CPSIA lead documentation for children's art and craft adhesives.
- Student stationery certification: meeting GB 21027 and GB 6675.4 migration requirements for school glues, glue sticks and liquid adhesives sold in China.
- Supply chain quality control: incoming inspection of resins, emulsions, pigments, driers, fillers and preservatives to intercept contamination before production.
- Formulation development: screening alternative raw materials such as lead-free driers, cadmium-free pigments and tin-free biocides during R&D.
- Root cause investigation: identifying the contaminant source when a finished glue lot fails, and validating corrective actions through retesting.
FAQ
What are migratable elements?
Migratable elements are the portion of elements in a material that can be extracted under simulated gastric conditions, meaning the fraction a child could actually absorb after accidental swallowing. This bioavailable fraction, rather than total metal content, is what toy and stationery safety standards regulate.
Why is liquid glue placed in Category II under EN 71-3?
Liquid and sticky materials adhere to fingers and are more easily ingested in larger amounts than dry or scraped-off toy components. The standard therefore assumes a higher potential intake and applies the lowest migration limits of the three material categories.
How is migration testing different from total heavy metal testing?
Total testing digests the entire sample and measures everything present, while migration testing uses a mild acid extraction that mimics stomach conditions. A glue can contain a relatively high total metal content yet pass migration testing, or vice versa, because only the soluble fraction is assessed.
Which standard should my liquid glue be tested against?
The choice depends on the target market and product classification: EN 71-3 or ISO 8124-3 for toys in Europe and internationally harmonized markets, ASTM F963 for the United States, and GB 6675.4 or GB 21027 for toys and student supplies in China. Many manufacturers commission the combined element suite to cover all major markets with a single submission.
How much sample is required and what is the typical turnaround?
Approximately 5 to 10 g of homogenized liquid glue per element suite is generally sufficient, including reserves for confirmation. Routine migration testing with ICP-OES/ICP-MS typically completes within five to seven working days; speciation of chromium(VI) or organic tin may extend the schedule slightly.
What happens if a result exceeds a limit?
The laboratory first verifies the result through repeat extraction and confirmatory analysis. If the exceedance is confirmed, manufacturers should trace the source, common culprits being pigments, driers, catalyst residues or preservatives, reformulate with cleaner raw materials, and resubmit verified lots before shipment.
Are solid glues and tape adhesives tested the same way?
The extraction chemistry is identical, but the material category differs. Solid glue sticks are typically evaluated as dry materials, and adhesives scraped from tape or labels may fall under the scraped-off category, each with its own set of migration limits.