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

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

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
^Element	^Wavelength	^{Spectral correction scheme}	^Element	^Wavelength	^{Spectral correction scheme}
Al	396,153	iec	Ni	221,648	iec
As	188,979	iec	Ni	232,003	iec
As	193,696	iec	P	177,434	msf
As	228,812	iec	P	178,221	msf
Be	234,861	iec	P	213,618	iec
Be	313,107	iec	P	214,914	msf
Ca	317,93	bkg	S	180,669	iec
Ce	413,764	msf	S	181,975	iec
Ce	456,236	msf	Sb	217,582	msf
Co	238,892	iec	Si	212,412	iec
Cu	327,393	msf	Si	251,611	iec
Fe	238,204	iec	Sn	189,927	iec
Fe	239,562	iec	Sn	235,488	iec
K	766,49	bkg	Sn	242,169	iec
La	384,902	iec	Sn	283,998	iec
La	408,672	iec	Ti	368,519	iec
Mg	279,553	msf	V	290,88	iec
Mg	280,271	iec	V	292,402	iec
Mo	202,031	bkg	Zn	206,197	iec
Mo	203,845	msf	Zn	334,501	msf
Na	588,995	iec	Zr	339,199	msf
Na	589,592	iec	Zr	343,82	msf

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
^Element	^{Emission line}	^{LOQ as % of oxide form}	^{Certified value %}	^{Average results}^{(%) n=8}	^RSD %	^Recovery %
Al	396,15	0,0013	38,7	37,6	1%	97%
Ca	317,93	0,0007	0,22	0,20	1%	92%
Fe	239,569	0,0049	1,6	1,5	1%	92%
K	766,49	0,0006	1,33	1,19	1%	89%
Mg	279,552	0,0023	0,52	0,53	1%	101%
P	177,435	0,0012	0,12	0,11	3%	92%
Si	251,611	0,0007	54,9	54,1	3%	99%
Ti	368,522	0,0006	2,0	1,9	1%	97%

Table 3. Recoveries and RSD obtained for SRM NIST 78a Burnt refractory
^E^lement	^{Emission line}	^{LOQ as % of xide form}	^{Certified value %}	^{Average results (%) n=8}	^RSD %	^Recovery %
Al	396,15	0,0013	71,7	70,9	1%	99%
Ca	317,93	0,0007	0,11	0,12	2%	107%
Fe	239,569	0,0049	1,20	1,15	2%	96%
K	766,49	0,0006	1,22	1,09	2%	89%
Mg	279,552	0,0023	0,7	0,7	3%	93%
Si	212,412	0,0007	19,4	17,0	3%	88%
Ti	368,522	0,0006	3,2	3,1	2%	97%

Table 4. Recoveries and RSD obtained for CRM GBW 7106 Sandstone
^Element	^{Emission line}	^LOD		^{Certified value}		^{Analytical result n=8}		^RSD	^Recovery
^Element	^{Emission line}	^{ppm in sample}	^{% in sample as oxide}	^ppm	^{% as oxide}	^ppm	^{% as oxide}	^%	^%
Al	396,15	2	0,0004	---	3,52	--	3,40	2%	97%
Ca	317,93	2	0,0002	---	0,30	---	0,25	4%	85%
Fe	238,201	11	0,0016	---	3,22	---	3,10	1%	96%
Fe	239,569	11	0,0016	---	3,22	---	3,11	1%	97%
V	766,49	2	0,0002	---	0,65	---	0,62	3%	95%
P	177,435	17	0,0039	970	---	977	---	4%	101%
Si	212,412	11	0,0024	---	90,60		90,45	1%	100%
Ti	368,522	1	0,0002	1580	---	1529	---	2%	97%
V	292,401	2	0,0003	33	---	29	---	3%	89%
Zr	339,199	2	0,0003	214	--	194	---	3%	91%

Table 5. Recoveries and RSD obtained for NIST 8850 Zeolite Y
^E^lement	^{Emission line}	^LOD		^{Certified value}		^{Analytical result n=8}		^RSD	^Recovery
^E^lement	^{Emission line}	^ppm	^{% in sample as element}	^ppm	^{% as element}	^ppm	^{% as element}	^%	^%
Al	396,15	2	0,0002	---	8,49	88176	8,82	4%	104%
Fe	238,201	10	0,0010	174,3	---	156	---	5%	89%
Na	589,587	10	0,0010	---	7,225	78114	7,81	1%	108%
Si	212,412	10	0,0010	---	22,52	236682	23,67	2%	105%

Table 6. Recoveries and RSD obtained for NIST 8851 Zeolite A
^E^lement	^{Emission line}	^LOD		^{Certified value}		^{Analytical result n=8}		^RSD	^Recovery
^E^lement	^{Emission line}	^ppm	^{% in sample as element}	^ppm	^{% as element}	^ppm	^{% as element}	^%	^%
Al	396,15	2	0,0002	---	14,766	---	15,128	3%	102%
Na	588,993	3	0,0003	---	12,732	---	12,142	1%	95%
Si	212,412	10	0,0010	---	15,27	---	14,483	2%	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.