Sample preparation by borate fusion of catalysts and high silica samples for ICP-OES analysis: method development and results

The analysis and determination of metals and trace metals in high silica and alumino-silicate samples such as catalysts and Zeolites are never easy, mainly because of the dissolution process. Samples containing silica often need hazardous chemicals (HF), strong acids (HClO4) or microwave digestions.

This application note describes a new approach using lithium borate fusion as the sample preparation method as well as the optimization of ICP-OES parameters. Many elements showed a signal suppression varying from 0 to 40 % while Plasma ionization effects from the borate solution enhanced the K, Na, and Rb signals.
High levels of Si, Al, Ca and Na from the sample and Li from the matrix required a strict scheme to correct all spectral interferences. The PerkinElmer® WinLab 32 software for the Optima™ ICP-OES allowed for three spectral correction techniques and they were used as follows:
• Background correction if no wing lines nearby;
• Multi-component spectral fitting ( MSF ) when there is a partial overlapping;
• Inter-element corrections ( IEC ) when lines are completely overlapped.

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Introduction

The analysis and determination of metals and trace metals in high silica and alumino-silicate samples such as catalysts and Zeolites are never easy, mainly because of the dissolution process. Samples containing silica often need hazardous chemicals (HF), strong acids (HClO4) or microwave digestions.

This application note describes a new approach using lithium borate fusion as the sample preparation method as well as the optimization of ICP-OES parameters. Many elements showed a signal suppression varying from 0 to 40 % while Plasma ionization effects from the borate solution enhanced the K, Na, and Rb signals.

High levels of Si, Al, Ca and Na from the sample and Li from the matrix required a strict scheme to correct all spectral interferences. The PerkinElmer® WinLab 32 software for the Optima™ ICP-OES allowed for three spectral correction techniques and they were used as follows:

  • Background correction if no wing lines nearby;
  • Multi-component spectral fitting ( MSF ) when there is a partial overlapping; 
  • Inter-element corrections ( IEC ) when lines are completely overlapped.

Method

1 Fusion

1.1 Apparatus: Claisse® M4-30 fluxer
• Works on 9 psi propane gas
• Automated process and automatic pouring of sample into diluted acid solution
• 95 % Pt /5 % Au crucible, Flat bottom, 20 ml, 24 g nominal weight, part number P-0120-00
• Teflon Beaker, 150 mL

1.2 Method: Fusion program used

Fig.JPG
Figure 1. Fusion Program parameter

2 ICP-OES

2.1 Apparatus: PerkinElmer® Optima™ 7300DV
• Nebulizer: Cross-flow
• Scott spray chamber
• Alumina injector
• RF: 1500 Watts
• Argon flow rate:
• Auxiliary: 0.4 L/min
• Nebulizer: 0.8 L/min
• Plasma: 16 L/min
• Shear gas: 100 psi
• Sample flow: 1.0 mL/min
• Polypropylene tube, 50 mL
• Polypropylene tube, 15 mL
• Syringe, 10 mL
• Filter: Milipore 45 μm

2.2 Analytical Method

Table 1. Wavelength used for each element and inter-element correction type used
ElementWavelengthSpectral
correction
scheme
ElementWavelengthSpectral
correction
scheme
Al396,153iecNi221,648iec
As188,979iecNi232,003iec
As193,696iecP177,434msf
As228,812iecP178,221msf
Be234,861
iecP213,618iec
Be313,107iecP214,914msf
Ca317,93bkgS180,669iec
Ce413,764msfS181,975iec
Ce456,236msfSb217,582msf
Co238,892iec
Si212,412iec
Cu327,393msfSi251,611iec
Fe238,204iecSn189,927iec
Fe239,562iecSn235,488iec
K766,49bkgSn242,169iec
La384,902iecSn283,998iec
La408,672iecTi368,519iec
Mg279,553msfV290,88iec
Mg280,271iecV292,402iec
Mo202,031bkgZn206,197iec
Mo203,845msfZn334,501msf
Na588,995iecZr339,199msf
Na589,592iecZr343,82msf

3 Chemicals

• Nitric acid (Omnitrace)
• Ultra-pure water (conductivity <2 μS)
• 10000 ppm commercial standard solution of each element analyzed
• Borate lithium flux, 98.5 % Lithium metaborate/1.5 % Lithium bromide (LiM), pure grade, catalog number C-0610-66
• SRM NIST 76 a Burnt refractory
• SRM NIST 78 a Burnt refractory
• CRM GBW 7106 Sandstone
• NIST 8850 Zeolite Y
• NIST 8851 Zeolite Y

Results

Table 2. Recoveries and RSD obtained for SRM NIST 76a Burnt refractory
ElementEmission lineLOQ as % of 
oxide form
Certified
value %
Average results  
(%) n=8

RSD %

Recovery %

Al396,150,001338,737,61%97%
Ca317,930,0007
0,220,20
1%92%
Fe239,5690,00491,61,51%92%
K766,49
0,0006
1,33
1,191%89%
Mg279,5520,0023
0,520,53
1%101%
P177,4350,00120,12
0,113%92%
Si251,6110,0007
54,954,13%99%
Ti368,5220,00062,01,91%97%
Table 3. Recoveries and RSD obtained for SRM NIST 78a Burnt refractory
ElementEmission lineLOQ as % of
xide form
Certified 
value %
Average results 
(%) n=8
RSD %Recovery %
Al396,150,001371,770,91%99%
Ca317,930,00070,110,122%107%
Fe239,5690,00491,201,152%96%
K766,490,00061,221,092%89%
Mg279,5520,00230,7
0,73%93%
Si212,4120,000719,417,03%88%
Ti368,5220,0006
3,23,12%97%
Table 4. Recoveries and RSD obtained for CRM GBW 7106 Sandstone
ElementEmission line
LOD
Certified valueAnalytical
result n=8
RSD
Recovery
ppm in sample% in sample as oxideppm% as oxideppm% as oxide%%
Al396,1520,0004---3,52--
3,402%97%
Ca317,9320,0002---0,30---0,25
4%85%
Fe
238,201110,0016---3,22---3,101%96%
Fe239,569110,0016---3,22---3,111%97%
V766,4920,0002---0,65---0,623%95%
P177,435170,0039970---977---4%101%
Si212,41211
0,0024---90,6090,451%100%
Ti368,5221
0,00021580---1529---2%97%
V292,40120,000333---29---3%89%
Zr339,199
20,0003214--194---3%
91%
Table 5. Recoveries and RSD obtained for NIST 8850 Zeolite Y
ElementEmission lineLODCertified valueAnalytical
result n=8
RSDRecovery
ppm% in sample as elementppm% as  elementppm% as  element%%
Al396,1520,0002---8,49881768,824%104%
Fe238,201100,0010174,3---156---5%89%
Na589,587100,0010---7,225781147,811%108%
Si212,412100,0010---22,5223668223,672%105%
Table 6. Recoveries and RSD obtained for NIST 8851 Zeolite A
ElementEmission lineLODCertified valueAnalytical
result n=8
RSDRecovery
ppm% in sample as elementppm% as elementppm% as  element%%
Al
396,1520,0002---14,766---15,1283%102%
Na588,99330,0003---12,732---12,1421%95%
Si212,412100,0010
---15,27---14,4832%95%

Discussion

High lithium matrix can cause signal suppression or signal increase for some elements.
In order to counteract this effect, matrix matching and internal standards were used.
Calibration solutions of each element were done in a LiM matrix, same as the sample.
Calibration curves were done using concentration range from 0.1 ppm to 10 ppm and 4 points were used for each element. Correlation of each curve was higher than 0.999. Samples were diluted to be within the calibration range. Dilution of sample prevented any clogging caused by a high salt matrix.

Experiments were conducted using different NIST standards, CRM (certified reference material) and SRM (standard reference material) of Zeolites, high silica samples and other composition known catalysts. Complete sample dissolutions were accomplished in rapid fashion without the use of HF. The length of the fusion program was less than 12 minutes including dissolution. A 10 % HNO3 solution in ultra-pure water was used as solvent for the Lithium metaborate- sample mixture.

Because of the interferences from the sample or the matrix, different wavelengths were used for the analysis. A minimum of two lines were selected per element, an atomic and an ionic line if possible. A line having no overlapping was selected for quantification when possible.

With the use of matrix matching and strict interference management we have been able to get recoveries of 90 to 100 % for most of the elements in each sample tried. RSD are less than 4 % on a 8 replicate test. 1 or 2 % RSD were obtained for most elements showing a good reproducibility for the complete analytical process.

Conclusion

High salt content of borate solutions can be managed in routine and research ICP-OES laboratory environments. To do so, spectral interferences from the matrix itself must be managed with proper emission line selection and a combination of background correction, multi-component spectral fitting (MSF), and inter-element spectral corrections (IEC).

Volatilization of light element can be prevented by using an appropriate fusion program.

Lithium borate fusion method for ICP and AA is faster than standard acid dissolution on a hot plate or a microwave for several sample types. Good recoveries were obtained without the use of HF or HClO4 and the dissolution was always complete even for refractories such as silicates and chromites.

Borate fusion gives very accurate and precise results for ICP-OES applications. It allows for a rapid and safe alternative to acid digestions with strong acids (HF, HClO4) for the dissolution of high silicate matrices.

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