Evaluation of Sample Pretreatment Procedures for the Determination of Phthalate Esters in Child Care Products and Children’s Toys by Gas Chromatography / Mass Spectrometry (GCMS)

Richard Whitney, PhD, Nicole Lock, Jiarui “Jerry” Wang, Zhuangzhi “Max” Wang, PhD, Clifford M. Taylor, Laura Chambers, Shimadzu Scientific Instruments, Columbia, MD, USA, 800-477-1227, www.ssi.shimadzu.com

The Consumer Product Safety Improvement Act of 2008 (CPSIA-USA) requires testing of child care products and toys in the US for specific phthalate esters by GCMS. The CPSC test method specifies solvent extraction and subsequent GCMS analysis for specific phthalate esters. In this study, analysis by pyrolysis GCMS was investigated as a technique to simplify sample preparation and improve analytical precision. The analyses were conducted on a Shimadzu GCMS-QP2010 Ultra equipped with a Frontier PY-2020is pyrolyzer, shown in the figure below.

The CPSC test method also specifies GCMS analysis in the SIM mode to monitor for low-intensity ions that are unique to specific phthalate esters. Full-scan mass spectra are extremely valuable in making unequivocal qualitative identification of specific phthalate isomers, however. Operation of the mass spectrometer in the FASST (Scan/SIM) mode allows concurrent acquisition of full-scan and SIM mass spectral data, without sacrificing sensitivity of SIM analysis.

Significant interferences, particularly terephthalate esters, are frequently present in real-world samples. An alternate chromatographic column (RXI-17Sil MS) provides excellent chromatographic resolution of bis(2-ethylhexyl) terephthalate, a common interference, from the regulated phthalate esters.

Acceptable method performance is achieved for calibration and control samples, but difficulties have been encountered in obtaining reproducible results for real-world samples using the sample extraction procedure in the CPSC method. Using the sample extraction procedure, analyses of replicate aliquots (50mg of polymer) of a sample known to contain target phthalate esters were conducted to evaluate method reproducibility. Results indicate that instrument precision is good; variability in results was previously assumed to be related to inhomogeneity of the polymer.

Introduction (1)

Pyrolysis at relatively low temperature (“thermal extraction”) was investigated as a sample introduction technique; pyrolysis offers the advantage of eliminating the time-consuming sample pretreatment procedure. It was anticipated that smaller (1mg) aliquots may show even more disparity in results than those observed with liquid extraction because sample homogeneity may be accentuated. However, improved precision was observed with replicate samples analyzed using pyrolysis relative to that obtained from replicate extractions. To further evaluate the effect of polymer homogeneity on the analytical precision, the sample was homogenized by cryogenic milling. Replicate analyses of the homogenized sample showed further improved precision relative to pyrolysis of the intact sample.

Introduction (2)

Frontier PY-2020is Shimadzu GCMS-QP2010 Ultra

Analytical Conditions

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GC Conditions

MS Conditions

The GC is operated in the splitless mode, using a RXI-5MS 30M x 0.25mm x 0.25µ capillary column (Restek), as specified by the CPSC method. Analysis using a RXI-17Sil MS 30M x 0.25mm x 0.25µ capillary column (Restek) is conducted using the same chromatographic conditions.

The MS is operated in the EI Scan/SIM (“FASST”) mode, to provide optimum sensitivity while still providing full-scan mass spectra for unequivocal qualitative identification.

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1 - Benzyl Benzoate (IS)2 - Dibutyl Phthalate3 - Butyl Benzyl Phthalate4 - Diethylhexyl Phthalate5 - Di-n-octyl Phthalate6 - Diisononyl Phthalate7 - Diisodecyl Phthalate

Diisononyl phthalate and diisodecyl phthalate appear as multiple chromatographic peaks

RXI-5MS Column 30M x 0.25mm x 0.25µ (Restek Corp)

Chromatography of Phthalate Esters

Chromatographic Interferences

Interference from non-regulated phthalate esters and related compounds masks trace detection of the regulated phthalate esters.

Noteworthy is interference of bis(2-ethylhexyl) terephthalate (a commonly-used, non-regulated plasticizer) with detection of di-n-octyl phthalate; these compounds coelute on the RXI-5MS column and have key mass spectral fragment ions in common (m/z 149, 167, 279).

di-n-octyl phthalate bis(2-ethylhexyl) terephthalate

Example of chromatography of equal concentrations of di-n-octyl phthalate and bis(2-ethylhexyl) terephthalate.

Full-scan mass spectra are extremely useful in qualitative identification. SIM data alone would make qualitative identification difficult in these cases.

Chromatography on RXI-17Sil MS Column

Bis(2-ethylhexyl) terephthalate (“DOTP”) is well-resolved from the regulated phthalate esters on an RXI-17Sil MS 30M x 0.25mm x 0.25µ capillary column (Restek), using the same chromatographic conditions as with the RXI-5MS column. (Elution of butyl benzyl phthalate is somewhat later than on the RXI-5MS column.)

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DOTP interference is well-resolved from the regulated phthalate esters

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1 - Benzyl Benzoate (IS)2 - Dibutyl Phthalate3 - Butyl Benzyl Phthalate4 - Diethylhexyl Phthalate5 - Di-n-octyl Phthalate6 - Diisononyl Phthalate7 - Diisodecyl Phthalate

Basic Method Performance

Fundamental method performance is satisfactory:• Good chromatography, with excellent linearity and precision for

analytical standards and QC samples• Excellent sensitivity with FASST (Scan/SIM)

Issues with method implementation:• Background contamination with phthalate esters from various

sources – method blanks are very critical.• Interferences from non-regulated phthalate esters and related

compounds – chromatographic and mass spectral interferences with detection and quantitation of regulated phthalate esters.

• Disparate analytical results have been obtained in several laboratories. The cause of this disparity was previously assumed to be variable homogeneity of solid samples.

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Sample Preparation Scheme for Replicate Analyses of Intact Vinyl Film Sample

Summary of Replicate Results for Extracted Aliquots

1 2 3 40.0

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Std Dev (concentra-tion)

Diisodecyl Phthalate Concentration (mg/Kg)

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Diisononyl Phthalate Concentration (mg/Kg)

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Std Dev (concentra-tion)

Diethylhexyl Phtha-late Concentration (mg/Kg)

Diisodecyl phthalate

Diisononyl phthalate

Diethylhexyl phthalate

Graphical representation of the results show instrument precision (Std Dev, shown by the dark blue bars in the graphic) is considerably better than precision between aliquots.

Results for dibutyl phthalate and butyl benzyl phthalate are very low (single digit precision), so results for these analytes are not shown here.

The sample preparation for these analyses was conducted simultaneously by the same analyst, so sample preparation technique was not assumed to be the source of disparity of the results

These results indicate that disparity in results of replicate sample analysis are related to sampling (sample homogeneity) or sample preparation, not poor precision and accuracy in the instrumental analysis.

Observations / Conclusions

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Solutions for CPSC Method Performance Issues

CPSC Method Performance Issue

Difficulties in qualitative identification of regulated phthalate esters with SIM data.Low level detection of some analytes difficult due to low relative abundance of quantitation ions and multiple chromatographic peaks.Monitoring of low abundance ions for quantitation gives rise to large variations in relative abundance ratiosChromatographic and spectral interferences are encountered from non-regulated phthalate esters and terephthalate esters.Disparate results are frequently obtained for analysis of replicate analyses of solid samples.

SolutionConduct analyses using Scan/SIM function (FASST) for unequivocal qualitative identification.Excellent sensitivity is achieved using Scan/SIM function (FASST), even for analytes with multiple chromatographic peaks (DINP,DIDP).Use of a convenient software feature to monitor ion abundance ratios allows easy modification of method parametersOne major interfering compound is easily resolved by using an RXI-17Sil MS chromatographic column (Restek).

Results indicate that difficulties are related to sampling/extraction rather than analysis. Sample homogenization and analysis of replicate sample aliquots is highly recommended.

Sample Introduction by Pyrolysis• Sample introduction by low-temperature pyrolysis (“thermal extraction”) was

investigated for applicability to this method (pyrolysis is not currently authorized by the CPSC Method):

• Pyrolysis has the advantage of greatly simplifying sample pretreatment – significant time savings.

• Pyrolysis has traditionally not been considered an accurate quantitative technique.

• Pyrolysis conditions were chosen to thermally desorb the phthalate esters while not decomposing the polymer itself.

• Conditions for Pyrolysis GCMS (Pyr-GCMS) are similar to those used for analysis of polymer extracts:

• Pyrolyzer temperature 325oC; interface temperature 320oC. Sample aliquots (0.4mg) were thermally desorbed for 1.0 min.

• A 60M RXI-17Sil MS column was used to optimize separation; the split injection mode was used to optimize sample transfer from the pyrolyzer to the GC; additional sample was added to maintain detection limit.

• A single SIM group was monitored to simplify data display.12

Polymer Sampling for PyrolysisTwo procedures were used in sampling of the vinyl sheet sample for pyrolysis GCMS analysis; 0.4mg sample aliquots were used for pyrolysis GS analysis:

1. Small circular aliquots of the sample were punched out of the sample directly.

2. The sample was cryogenically milled to a fine mesh and an aliquot was taken for analysis.

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Aliquots were punched from the vinyl sheet at various positions

The sample was cryogenically milled, resulting in a fine powder.

0.4mg sample aliquots were taken from the cryogenically milled sample. (Sample cup is also shown.)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 Conc. Ratio0.0

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Area Ratio(x0.1)

Calibration Using PYR-GCMS

• A six-point internal standard calibration was conducted from 0.25-20ppm. This calibration range corresponds to the same sample concentration of phthalate esters in the polymer as with the CPSC (solvent extraction) method.

• Linearity was R2>0.99 for all of the analytes in both the full-scan and SIM modes.

• Example calibration curves are shown below for diisononyl phthalate and diisodecyl phthalate.

140.0 0.5 1.0 1.5 2.0 2.5 3.0 Conc. Ratio

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Area Ratio(x0.1)

Diisononyl Phthalate Diisodecyl Phthalate

Precision of Analytical Standards Results using PYR-GCMS

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Compound Name%RSD Scan

%RSD SIM

Dibutyl Phthalate 7.07 5.72

Benzyl Butyl Phthalate 7.42 7.65

Diethylhexyl phthalate 5.13 5.04

Di-n-octyl phthalate 4.40 4.88

Diisononyl Phthalate 4.12 4.82

Diisodecyl Phthalate 6.67 6.13

Replicate analyses for eight runs of analytical standard using PYR-GCMS show excellent instrumental precision with sample introduction by pyrolysis (equivalent precision to liquid injection).

13.00 13.25 13.50 13.75 14.00 14.25 14.50 14.75 15.00 15.25

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Precision for analysis of 8 replicatesMass chromatograms of diisodecyl phthalate for analysis of 8 replicates

Precision of Real-world Sample Results using PYR-GCMS

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Replicate injections for eighteen runs of vinyl polymer sample show reasonable instrumental precision with sample introduction by pyrolysis. Precision is improved significantly by homogenization of the sample.

Precision for analysis of 18 replicates of vinyl polymer by PYR-GCMS

Compound Results for Intact Sample Results for Cryo-milled Sample

Mean (mg/Kg) RSD(%) Mean (mg/Kg) RSD(%)

Dibutyl Phthalate 10 34 11 11

Diethylhexyl Phthalate 44 32 311* 14

Diisononyl Phthalate 600 43 501 11

Diisodecyl Phthalate 65 35 60 15

*Disparity in results for diethylhexyl phthalate is attributed to possible contamination during sample transport or cryo-milling.

The CPSC test method has been demonstrated using Shimadzu GCMS-QP2010S and GCMS-QP2010 Ultra operated in the FASST (Scan/SIM) mode. Acceptable basic method performance was demonstrated: excellent sensitivity, linearity, and precision at low concentrations were attained.

Interferences from terephthalate esters (non-regulated compounds with similar structures) have been encountered in real-world samples. An alternate chromatographic column (RXI-17Sil MS) was employed to provide chromatographic resolution of the interferences from the target phthalate esters.

Disparate results have been observed for replicate analyses of real-world solid samples. To cast light on this issue, replicate aliquots of a vinyl film sample were prepared according to the CPSC method and analyzed six times each. Replicate analyses of each aliquot showed good statistical agreement, but disparate results were obtained for each of the sample aliquots. These results originally suggested that disparity in the results can be attributed to homogeneity of solid samples or irregularities in the sample extraction process.

Summary

Sample introduction using pyrolysis was demonstrated to be a convenient and reliable analytical procedure for determination of phthalate esters in a vinyl film sample. Excellent linearity and precision for analytical standards and QC samples were obtained using pyrolysis for sample introduction. Variable results were observed for analysis of 0.4mg aliquots punched from the vinyl film sample, but statistical agreement was better using pyrolysis on the intact sample than that from analysis of replicate extraction aliquots using the CPSC method.

The vinyl film sample was homogenized using cryogenic milling, resulting in a fine powder. It was anticipated that the homogeneous sample would give more precise analytical results. Indeed, analytical precision was improved considerably using the homogenized sample.