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Total reflection X-ray fluorescence spectrometry: matrix removal procedures for trace analysis of high-purity silicon, quartz and sulphuric acid
Spectrochimico Acta Vol. 448, No. 5, Pnntcd in Great Briton.
pp:533-541,
1989 0
05848547/%9 s3.oo+.oo 1989 Pergamon Press plc.
Total reflection X-ray fluorescence spectrometry: matrix removal procedures for trace analysis of high-purity silicon, quartz and sulphuric acid ULRICH REUS Rich. Seifert & Co., Rontgenwerk, 2070 Ahrensburg, Federal Republic of Germany (Received 15 August 1988; in revisedfirm 30 November 1988) Almtraet-Chemical separation methods enabling trace analyses in high-purity Si, SiO, and H,SO, are presented. After removal of the matrices, the trace elements are determined by means of total reflection energy-dispersive Xray fluorescence spectrometry (TXRF). Results are presented, and a comparison is made between samples with and without matrix separation. Detection limits are found to be 2-3 orders of magnitude lower for the first case, chemical blanks being the limiting factor for the sensitivity of the combined procedure. The accuracy of the methods is proved by the analysis of a reference material (SiO,) and by the determination of recoveries (H,SO,), respectively. Possibilities for further improving the methods are outlined.
1. INTR~D~JcT~~N ALTHOUGH total reflection energy-dispersive X-ray fluorescence spectrometry (TXRF) Cl-33 has been successfully applied to a variety of materials [4,5], its restriction to thin film-like samples forms a serious drawback in the application of the method for trace analysis in nonvolatile materials. The maximum sample mass on the support is of the order of 10 pg [6] and absolute detection limits are in the pg range. Accordingly, the detection limits of trace elements in material to be analyzed are calculated to be about 1 fig/g. This restriction is valid irrespective of the kind of material concerned, e.g. aerosols t-73, soils [a], or biological materials [9]. Hence, for a more sensitrve analysis it is necessary to remove the matrix by either physical or chemical procedures, leaving as much as possible of the trace elements of interest. Such separation techniques are described in this work for samples of Si, SiO, and HzS04, all of them being important as high-purity materials in numerous fields such as those dealing with semiconductors or pharmaceutical products.
2. EXPERIMENTAL 2.1. Matrix separation procedure for Si and SiO, The same procedure is used for both types of material. Matrix removal is effected by volatilization of Si as SiF, [lo]: 2Si+4HNOJ+18HF+3H2SiF,+4NO+8H,0 SiOl + 6 HF = H2SiFs + 2 H,O H,SiF, + SiF, + 2 HF.
(1) (2) (3)
After dissolution of the material according to Eqn (1) or (2), SiF.+can be evaporated [Eqn (311when the volume of the solution is reduced. Since Eqn (2) describes an equilibrium, an excess of HF is needed to prevent hydrolysis. This excess has to be removed completely before sample preparation, since sample supports are made of quartz. If the material to be analyzed contains TiO, (ZrO,) or A1,OJ, dissolution may be incomplete. In this case a pressure digestion step should be performed prior to the evaporation of Si. Prescription. Reagents used: HF (40%, Suprapur, E. Merck, Darmstadt); HNOJ (65%, subboiled, Suprapur, E. Merck, Darmstadt). 100 mg Si or SiO, are dissolved in 1 ml HF +0.2 ml HNO, in a PTFE vessel and spiked with Rb standard solution (e.g. 100 ng Rb correspond to 1 pg Rb/g sample material). The vessel is closed and heated to 180” C for 3 h (this step may be omitted if the concentrations of TiO, and A&O, are low). After cooling, the vessel is flushed with Nz gas and the 533
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ULRICHREUS
solution evaporated to dryness at loo” C. After addition of 0.4 ml HF, evaporation is repeated. The residue is taken up in 1 ml dil. HN03 (10%) and the solution is warmed for a few minutes. Part of the solution (up to 100 ~1)is transferred onto a quartz glass sample support with a siliconized surface (see Ref. [6]) and dried. The time consumption is 40-60 min for the whole procedure (not including pressure digestion), of which most of the time is used for the evaporation steps. 2.2. Removal of H&IO, Two separate cases are considered here: (i) the determination of trace elements in (concentrated) H,SO,, and (ii) the analysis of solutions containing H,SO, as part of the solvent. The principle in both cases is to destroy H,S04 through the reaction with hydroiodic acid [ 111, i.e. H,SO,+
2 HI = 2 H,O +I, + SOZ.
(4)
Under careful control of the reaction conditions the process will yield volatile products only. However, it is an equilibrium process which proceeds from left to right with concentrated H,SO, only, preferably. at elevated temperature and with the reaction products being removed from the system. Stoichiometry requires 5 ml HI (57%, D = 1.7 g/ml) per ml HzS04; in fact about 7 ml are needed, which must be added in small portions. To facilitate evaporation of the reaction products, the solution should have as large a surface as possible. The temperature should not exceed 130” C to prevent secondary processes leading, among other things, to elemental sulphur or HIO,. Prescription fir concentrated H2S04 [case (i)]. Reagents used: HI (57%, subboiled, Suprapur, E. Merck, Darmstadt); HNO, (65%, subboiled, Suprapur, E. Merck, Darmstadt). 0.5 ml H,SO, (96%) is put into a quartz vessel with a flat bottom (5 cm diameter). The temperature at the bottom is held at 130°C and the vessel is flushed with N,. Standard solution is added containing 50 ng Rb. 3.5 ml of HI are added in portions of 0.5 ml. After each addition, the solution will turn dark; the next portion of HI is added only when the solution has become colourless again. The completion of the reaction is easily recognized from the absence of I,-vapour (violet), but at least one more portion of HI should be added. The dry residue is then dissolved in 1 ml warm HNO, (lo%), and up to 50 ~1 are transferred to the siliconized sample support. Remainders of I, are removed by evaporating the acid solution in a vacuum. Prescription for diluted H,SO, [case (ii)]. Reagents used: as above. This sample preparation procedure is best performed on the quartz sample support, the surface of whichin this case must not be siliconized. Volumes quoted here are for a 2% H$O.+ solution; they have to be changed according to the actual concentration. 100 ~1of solution spiked with internal standard are evaporated on the surface of the support leaving 2 ~1 of H,SO, (maximum: 5 ~1).The evaporation is performed with the help of a small PTFE cylinder pressed onto the surface of the support and preventing the liquid from spreading too much (This &vice is a standard accessory of the EXTRA II TXRF system). The support is now heated to 130” C and 20 ~1 of HI are added in portions of about 3 ~1, with a waiting time of 1 min between the additions. Finally, 20 ~1 of HNO, (10%) are applied onto the dry sample spot and evaporated in a vacuum to remove remainders of I1. 2.3. Parameters for TXRF measurements The measurements of all samples were performed using standard settings for both molybdenum and tungsten target X-ray tubes (50 kV, l&38 mA). Typical measuring times were 1000 s for MO excitation and 3000 s for W-excitation. Rb was used as internal standard in concentrations varying between 20 ng/ml and 100 pg/g, respectively. Three sample plates were made from each solution resulting from a separation process. Data collection and evaluation were carried out on a Link QX 200 X-ray Analyzer equipped with a 80 mm’ Si (Li) detector with a resolution of 158 eV at 5.9 keV.
3. 3.1.
Si and
RESULTS
SiOz samples
The performance of the matrix separation procedure described in Section 2.1 has been tested by the analysis of several high-purity Si and SiO, materials. Some typical examples are discussed here. Figure 1 shows a spectrum obtained from a Si sample after matrix removal. The material contains 0.4 mg/g Ca and 0.021 mg/g Fe (no uncertainties are given by the manufacturer).
The analysis yielded 0.46 + 0.01 mg/g for Ca and 0.032 + 0.002 mg/g for Fe. Blank values
Matrix removal procedures
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6
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16 16 XI Range (k&l
Fig. 1. TXRF spectrum obtained from a Si sample after matrix removal. Concentrations of elements found are given in the text (100 fig/g Rb as internal standard).
were subtracted. The internal standard concentration of Rb was 100 &g. Other elements found are (in pa/g): Cr 2.5 + 0.5, Mn 1.3 kO.4, Ni 4.0 + 0.3, and Cu 1.4 + 0.2. Ge was also detected but its chemical yield is unknown. Detection limits vary from 0.2 ,ug/g for Sr to 10 lug/g for S but certainly improvement is possible (see below). A certified SiO, material (BCS-CRM 3 13/l, high purity silica) has been analyzed to ensure the accuracy of the method. Numerical results of this analysis, together with certified values and results of bjank experiments are listed in Table 1. Results of TXRF are in excellent agreement with certified values, and in addition to the latter about 20 more elements could be determined. A typical spectrum obtained from a sample of BCS 3 13/l after matrix removal is shown in Fig. 2. For comparison, a spectrum obtained from the same material without matrix removal, i.e. from an aqueous dispersion of SiOz, is shown in Fig. 3. The Ti and Fe signals in this spectrum correspond to 95 and 83 pg,/g, respectively (see Table 1). Except for Ca (41 + 7 pg/g), no other elements could be determined from the unseparated material. Typical values for the limit of detection vary from 4 ,ug/g (Sr) to 100 pg/g (S). After matrix removal, detection limits were found to be more than two orders of magnitude lower, e.g. 0.03 pg/g for Sr and 0.6 pg/g for S. Numerical values of detection limits for other elements can be derived from the data in Table 1 taking CL = 3 erg, where a, is the standard deviation of the blank value or the upper limit given in Table 1. The remaining Si content of the samples after matrix removal has been determined to be < 10 pg/g (two independent measurements), which yields an enrichment factor for trace elements of about 5 x 10”. Significant improvement of the method is possible by using subboiled hydrofluoric acid, since the commercial material of highest quality (Suprapur, Merck) still contains about 20 ng/ml Fe and small quantities of other elements (Ca, Ni, Pb). A PTFE subboiling device will be used in the future for that purpose. Except for Ge, chemical losses have not been observed and are not expected, since volatile fluorides (e.g. of As and Se) are not formed in aqueous systems.
536
ULRICHREUS Table 1. Trace element concentrations found in SiO, reference material (BCS-CRM 313/l) and blank samples after matrix removal with HF. Certified values are given for comparison Concentration @g/g)
Element Al P S K Ca Ti V Cr Mn Fe Ni cu Zn Ga Ge As Rb Sr Y Zr Nb MO Sn Ba Ce Hf W Pb Th
High purity SiO, BCS-CRM 313/l* 170 f 15 7.5 f 2.5 20&2 49.0 + 2.0 48.2 + 0.8 95.3 + 3.0 0.2 f 0.1 0.70 + 0.09 1.03*0.10 83.4 &-2.5 0.43 5 0.06 0.58 & 0.04 0.34 f 0.04 0.02 + 0.01
I
0.15 +0.02 0.18 * 0.027 1.32 f 0.07 0.61 f 0.83 16.Ok1.5 0.3 1 f 0.07 0.07 f 0.03 0.10 f 0.05 7.9 f 1.0 1.6 f 0.4 0.44 f 0.04 0.08 f 0.04 0.31 * 0.04 0.26 + 0.06
Blank’ < 5.0 < 0.3 0.8 f 0.2 0.52 k 0.05 0.9 + 0.3 0.052 f 0.015 < 0.01 0.040 f 0.010 0.020 + 0.006 0.35 f 0.05 0.072 f 0.005 0.030 + 0.005 0.035 f 0.005 < 0.002 < 0.002 < 0.007 < 0.005 < 0.005 < 0.006 < 0.02 < 0.01 < 0.01 5 0.06 < 0.08 < 0.03 < 0.005 < 0.005 0.019 + 0.007 < 0.010
Certified values BCS-CRM 313/l 190*21 24’ 42&17 43* 7 100*20 1’ 1.00* 0.2 84+ 7 15’ -
*Mean value and std dev. (n = 5).
+Mean value and std dev. (n = 3). *Value not certified. rGe peak observed but chemical yield unknown. nFrom unstandardized sample.
In brief, evaporation with hydrofluoric acid is satisfactorily to remove the matrix from samples of Si and SiO,, thus enabling trace analysis with TXRF in the rig/g concentration range.
3.2. Analysis of concentrated H,SO, Although the reaction according to Eqn (4) fulfils all requirements upon a matrix separation method, the technique as described in Section 2.2 turned out to be unsatisfactory for the analysis of trace elements in cont. H,SO,. Table 2 lists the results obtained from a sample of HzS04 (Selectipur, Merck) together with the corresponding blank values; the latter prevented obtaining any definite results on the sample. There is too much contamination produced by the chemicals used and/or the various steps of the preparation procedure. The hydroiodic acid (Suprapur, Merck) has been subboiled prior to use; however, in view of the large amounts needed, its purity was not sufficient. Besides, the method suffers from its time consumption. Although it seems possible to overcome these difficulties by optimization, the procedure as a whole is not suited for routine analysis.
Matrix removal procedures hwlts 6
537
(x10’)
1
0 0
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6
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16 16 20 F!angaAheJw
C
Rb : 6-
b
5
4
6t
3
Cu
In
2
Cr
As Pb
Hi h h
1
t
V j’,,,,
Pb Et
lr
Hf&
:I:...’
0 5
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7
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9
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12 13 14 Range CkeVl
Fig. 2.(a) TXRF spectrum of SiOz reference sample (BCS-CRM 313/l) obtained after matrix removal. 20 &g Rb is used as internal standard. (b) Detail from (a).
Alternatively, a procedure is currently developed which is based on the reaction of H,SO, with gaseous HI. The sample is in a flat quartz vessel inside a quartz tube which is heated and flushed with a stream of N, loaded with HI gas. Reaction will take place at the surface of the H,S04, and the reaction products will be condensed in a cooling trap. By this technique, contamination of the sample will be avoided, and the process is expected to be faster than the one used before.
ULRICHREUS
538
0
2
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6
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10
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16 16 20 Range tkeV1
Fig. 3. TXRF spectrum of SiO, reference sample (BCS-CRM 313/l) prepared from aqueous dispersion without matrix separation.
Table 2. Trace concentrations found in samples of high-purity cont. H,SO,, and in blank samples after removal of the matrix with HI Concentration (ng/ml) Element K Ca Ti Cr Mn Fe Ni cu Zn As Sr Zr Ba Pb
H,SO, cont.* 1780+40 2940 + 50 < 13 15&3 10*3 735 & 13 17+2 14&-2 57+2 <3 6kl < 12 < 24 6*2
Blank* 850 + 30 360+30 < 24 24k4 12k3 125 + 8 16k2 12*1 2251 <3 3+1 <6 < 35 5+2
*Mean value and std dev. (n = 2).
3.3. Analysis of solutions containing H,S04 As pointed out in Section 2.2, the objective here is to remove the H,SO, present as part of the solvent in order to determine the trace content of the solution. The sample analyzed for testing the procedure is a solution in 2% (0.2 M) H,SO, containing about 100 ng/ml of each of the following elements: Ca, Ti, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Cd, Sn, Pb, and U. Additionally, Rb (200 ng/ml) was added as internal standard. For a direct measurement, 5 ~1 of this solution were evaporated, leaving 0.1~1 (about 200 pg) of HzS04 on the support. The spectrum obtained from this sample (Fig. 4) hardly reveals any significant signals of the
Matrix removal procedures
0
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8
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539
16 18 20 Rangy CkeW
Fig. 4. TXRF spectrum of sample in diluted H,SO, (2%) measured directly, Le. 5 ~1 of solution evaporated to 0.1~1 (about 200 pg) of H,SO,. 200 ng/ml Rb as internal standard.
Table 3. Recoveries determined for a sample of diluted H,SO, (2%) and blank values after removal of the matrix with HI Concentration (rug/ml) Element P s K Ca Ti V Cr Mn Fe co Ni CU Zn As Se Br Sr Zr MO Cd Sn Ba Pb U
Input*
109.7 f 2.0 102.5 + 2.5 106.5 -+ 1.5 94.5 + 1.0 101.8 f 1.3 104.0 + 1.5 102.8 f 2.0 98.7 f 2.0 103.4 f 2.0 107.0 _+3.5 -
105&4 110&5 115.0_+4.5 !18.0_+4.0
Found*
Blank*
< 20.0 103-lo4 206 _+12’ 133f6 86.2 + 8.0 < 0.5 98.8 f 1.5 100.2 &-2.0 4.9 f 1.0 103.1 + 1.5 103.8 + 1.5 102.5 f 2.0 106.4 f 2.5 1.5 f 1.0 < 0.5 0.4kO.l < 0.2 < 0.3 < 0.3 102.3 + 2.0 < 4.2 < 1.5 122.6 f 4.5 96.0 & 3.0
< 3.8 < 3.5 2.6 k 0.5 5.8 + 1.0 < 0.3 < 0.1 < 0.15 < 0.1 0.70 + 0.10 < 0.15 0.35 + 0.08 < 0.1 1.95 k 0.20 0.10 f 0.05 < 0.005 < 0.08 < 0.05 < 0.10 < 0.10 < 0.5 < 0.8 < 0.3 < 0.1 < 0.2
*Mean value and std dev. (n = 3). ‘Mainly from Cr input as K,CrO,.
Yield (%)
116k7 84+9 93 f 3 106k4 102k3 100+3 1OOa3 106k4 < 2.5 < 0.5
98 f 5 <5 107+7 81+6
ULRICHReus
540
elements listed above. In Fig. 5b a spectrum of the same sample is shown after removal of the HzS04, as described in Section 2.2. For comparison, a spectrum obtained from the original solution but without H,SO, is shown in Fig. 5a. The results are summarized in Table 3 along with the corresponding blank values and the chemical yields calculated from the data.
C
a
Pb
0
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6
6
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16 16 20 F.!ange (keW
0
2
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c
Fig. 5(a). TXRF spectrum of original solution used for determination of chemical yields for the H,SO, removal procedure. This spe.ctrumhas been obtained from a matrix free solution (without H,SO,). (b) TXRF spectrum from same solution as in (a) after addition and subsequent removal of H,SO,. Figure Sa and b have been normalized to the same height of the Rb peak to enable direct comparison.
Matrix removal procedures
541
As could be expected from their chemical properties, As, Se, and Sn are quantitatively volatilized by the HI. The same is probably true for Sb, Te and Hg, whose yields have not yet been determined. Minor losses are observed for Ti and U. No significant chemical loss is found (or to be expected) for other elements. For the example given here, the improvement in detection limits by the matrix removal procedure is more than a factor of 100. Detection limits can be derived from the blank values listed in Table 3 in the same manner as mentioned in Section 3.1. However, the amount of chemicals needed-and consequently the blank values-will increase with the concentration of H,SO, in the solution. Therefore, the detection limits derived from the example given here are not generally valid, but even for unfavourable cases values between 1 ng/ml for Sr and 200 ng/ml for P should be reached. The problem of the blank values, which arises from the chemicals, may be avoided by performing the matrix removal with gaseous HI as discussed above. Technical details of such a procedure will be worked out in the future. 4. CONCLUSIONS The objective of this study has been the development of sample preparation methods for trace analysis in some nonvolatile matrices. For Si and SiOz samples, such a procedure has been proved to be successful, and TXRF detection limits in an range between 10 an 200 rig/g have been reached with the perspective that further improvement is possible. Also, trace analysis in solution containing diluted H,SO, is now possible with similar sensitivity. The trace analysis of concentrated H$O, by TXRF is not yet satisfactory, but a way has been outlined to overcome the difficulties. Regardless of this restriction, we dispose now of sample preparation methods for trace analysis at the rig/g level in the above materials which hitherto could be determined with TXRF only at the pg/g level. This achievement may widen the scope of application of TXRF. Acknowledgements-The author gratefully acknowledges the support of the TXRF group at GKSS Research Center (Geesthacht, F.R.G.).
REFERENCES [l] [Z] [3] [4] [S]
[6] [7] [8]
[9]
Y. Yoneda and T. Horiuchi, Rev. Sci. Insrr. 42, 1069 (1971). H. Aiginger and P. Wobrauschek, Nucl. Instr. Methods 114, 157 (1974). J. Knoth and H. Schwenke, Fresenias Z. Anal. Chem. 291,200 (1978). A. Prange, J. Knoth, R.-P. StiiBel, H. Bdddeker and K. Kramer, Anal. Chim. Acta 195,275 (1987). W. Michaelis and A. Prange (Eds), Totalrejexions-Riintgenfhtoreszenzanalyse (1. Workshop). GKSS 86/E/61, Geesthacht (1986). K. Freitag, ZnstrumentelleMultielement-Analyse (Ed. B. Sansoni) p. 257. VCH Verlagsgesellschaft, Weinheim (1985). P. Ketelsen and A. Knochel, Fresenius Z. Anal. Chem. 317, 333 (1984). W. Gerwinski and D. Goetz, Books of Abstracts of 24th CON Spectrosc. Inc., Garmisch-Partenkirchen, Vol. 4, p. 724 (1985). A. V. Bohlen, R. Klockenkampcr, H. Otto, G. Tolg and B. Wiecken, Inc. Arch. Occup. Environ. Health 59,403 (1987).
[lo] R. Bock, Aufschlupmethoden der anorganischen und organischen Chemie. Verlag Chemie, Weinheim (1972). [ll] G. Jander and E. Blasius, Lehrbuch der analytischen und priiparatiuen anorganischen Chemie. S. Hirael, Stuttgart (1967).
pp:533-541,
1989 0
05848547/%9 s3.oo+.oo 1989 Pergamon Press plc.
Total reflection X-ray fluorescence spectrometry: matrix removal procedures for trace analysis of high-purity silicon, quartz and sulphuric acid ULRICH REUS Rich. Seifert & Co., Rontgenwerk, 2070 Ahrensburg, Federal Republic of Germany (Received 15 August 1988; in revisedfirm 30 November 1988) Almtraet-Chemical separation methods enabling trace analyses in high-purity Si, SiO, and H,SO, are presented. After removal of the matrices, the trace elements are determined by means of total reflection energy-dispersive Xray fluorescence spectrometry (TXRF). Results are presented, and a comparison is made between samples with and without matrix separation. Detection limits are found to be 2-3 orders of magnitude lower for the first case, chemical blanks being the limiting factor for the sensitivity of the combined procedure. The accuracy of the methods is proved by the analysis of a reference material (SiO,) and by the determination of recoveries (H,SO,), respectively. Possibilities for further improving the methods are outlined.
1. INTR~D~JcT~~N ALTHOUGH total reflection energy-dispersive X-ray fluorescence spectrometry (TXRF) Cl-33 has been successfully applied to a variety of materials [4,5], its restriction to thin film-like samples forms a serious drawback in the application of the method for trace analysis in nonvolatile materials. The maximum sample mass on the support is of the order of 10 pg [6] and absolute detection limits are in the pg range. Accordingly, the detection limits of trace elements in material to be analyzed are calculated to be about 1 fig/g. This restriction is valid irrespective of the kind of material concerned, e.g. aerosols t-73, soils [a], or biological materials [9]. Hence, for a more sensitrve analysis it is necessary to remove the matrix by either physical or chemical procedures, leaving as much as possible of the trace elements of interest. Such separation techniques are described in this work for samples of Si, SiO, and HzS04, all of them being important as high-purity materials in numerous fields such as those dealing with semiconductors or pharmaceutical products.
2. EXPERIMENTAL 2.1. Matrix separation procedure for Si and SiO, The same procedure is used for both types of material. Matrix removal is effected by volatilization of Si as SiF, [lo]: 2Si+4HNOJ+18HF+3H2SiF,+4NO+8H,0 SiOl + 6 HF = H2SiFs + 2 H,O H,SiF, + SiF, + 2 HF.
(1) (2) (3)
After dissolution of the material according to Eqn (1) or (2), SiF.+can be evaporated [Eqn (311when the volume of the solution is reduced. Since Eqn (2) describes an equilibrium, an excess of HF is needed to prevent hydrolysis. This excess has to be removed completely before sample preparation, since sample supports are made of quartz. If the material to be analyzed contains TiO, (ZrO,) or A1,OJ, dissolution may be incomplete. In this case a pressure digestion step should be performed prior to the evaporation of Si. Prescription. Reagents used: HF (40%, Suprapur, E. Merck, Darmstadt); HNOJ (65%, subboiled, Suprapur, E. Merck, Darmstadt). 100 mg Si or SiO, are dissolved in 1 ml HF +0.2 ml HNO, in a PTFE vessel and spiked with Rb standard solution (e.g. 100 ng Rb correspond to 1 pg Rb/g sample material). The vessel is closed and heated to 180” C for 3 h (this step may be omitted if the concentrations of TiO, and A&O, are low). After cooling, the vessel is flushed with Nz gas and the 533
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solution evaporated to dryness at loo” C. After addition of 0.4 ml HF, evaporation is repeated. The residue is taken up in 1 ml dil. HN03 (10%) and the solution is warmed for a few minutes. Part of the solution (up to 100 ~1)is transferred onto a quartz glass sample support with a siliconized surface (see Ref. [6]) and dried. The time consumption is 40-60 min for the whole procedure (not including pressure digestion), of which most of the time is used for the evaporation steps. 2.2. Removal of H&IO, Two separate cases are considered here: (i) the determination of trace elements in (concentrated) H,SO,, and (ii) the analysis of solutions containing H,SO, as part of the solvent. The principle in both cases is to destroy H,S04 through the reaction with hydroiodic acid [ 111, i.e. H,SO,+
2 HI = 2 H,O +I, + SOZ.
(4)
Under careful control of the reaction conditions the process will yield volatile products only. However, it is an equilibrium process which proceeds from left to right with concentrated H,SO, only, preferably. at elevated temperature and with the reaction products being removed from the system. Stoichiometry requires 5 ml HI (57%, D = 1.7 g/ml) per ml HzS04; in fact about 7 ml are needed, which must be added in small portions. To facilitate evaporation of the reaction products, the solution should have as large a surface as possible. The temperature should not exceed 130” C to prevent secondary processes leading, among other things, to elemental sulphur or HIO,. Prescription fir concentrated H2S04 [case (i)]. Reagents used: HI (57%, subboiled, Suprapur, E. Merck, Darmstadt); HNO, (65%, subboiled, Suprapur, E. Merck, Darmstadt). 0.5 ml H,SO, (96%) is put into a quartz vessel with a flat bottom (5 cm diameter). The temperature at the bottom is held at 130°C and the vessel is flushed with N,. Standard solution is added containing 50 ng Rb. 3.5 ml of HI are added in portions of 0.5 ml. After each addition, the solution will turn dark; the next portion of HI is added only when the solution has become colourless again. The completion of the reaction is easily recognized from the absence of I,-vapour (violet), but at least one more portion of HI should be added. The dry residue is then dissolved in 1 ml warm HNO, (lo%), and up to 50 ~1 are transferred to the siliconized sample support. Remainders of I, are removed by evaporating the acid solution in a vacuum. Prescription for diluted H,SO, [case (ii)]. Reagents used: as above. This sample preparation procedure is best performed on the quartz sample support, the surface of whichin this case must not be siliconized. Volumes quoted here are for a 2% H$O.+ solution; they have to be changed according to the actual concentration. 100 ~1of solution spiked with internal standard are evaporated on the surface of the support leaving 2 ~1 of H,SO, (maximum: 5 ~1).The evaporation is performed with the help of a small PTFE cylinder pressed onto the surface of the support and preventing the liquid from spreading too much (This &vice is a standard accessory of the EXTRA II TXRF system). The support is now heated to 130” C and 20 ~1 of HI are added in portions of about 3 ~1, with a waiting time of 1 min between the additions. Finally, 20 ~1 of HNO, (10%) are applied onto the dry sample spot and evaporated in a vacuum to remove remainders of I1. 2.3. Parameters for TXRF measurements The measurements of all samples were performed using standard settings for both molybdenum and tungsten target X-ray tubes (50 kV, l&38 mA). Typical measuring times were 1000 s for MO excitation and 3000 s for W-excitation. Rb was used as internal standard in concentrations varying between 20 ng/ml and 100 pg/g, respectively. Three sample plates were made from each solution resulting from a separation process. Data collection and evaluation were carried out on a Link QX 200 X-ray Analyzer equipped with a 80 mm’ Si (Li) detector with a resolution of 158 eV at 5.9 keV.
3. 3.1.
Si and
RESULTS
SiOz samples
The performance of the matrix separation procedure described in Section 2.1 has been tested by the analysis of several high-purity Si and SiO, materials. Some typical examples are discussed here. Figure 1 shows a spectrum obtained from a Si sample after matrix removal. The material contains 0.4 mg/g Ca and 0.021 mg/g Fe (no uncertainties are given by the manufacturer).
The analysis yielded 0.46 + 0.01 mg/g for Ca and 0.032 + 0.002 mg/g for Fe. Blank values
Matrix removal procedures
0
2
4
6
6
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14
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16 16 XI Range (k&l
Fig. 1. TXRF spectrum obtained from a Si sample after matrix removal. Concentrations of elements found are given in the text (100 fig/g Rb as internal standard).
were subtracted. The internal standard concentration of Rb was 100 &g. Other elements found are (in pa/g): Cr 2.5 + 0.5, Mn 1.3 kO.4, Ni 4.0 + 0.3, and Cu 1.4 + 0.2. Ge was also detected but its chemical yield is unknown. Detection limits vary from 0.2 ,ug/g for Sr to 10 lug/g for S but certainly improvement is possible (see below). A certified SiO, material (BCS-CRM 3 13/l, high purity silica) has been analyzed to ensure the accuracy of the method. Numerical results of this analysis, together with certified values and results of bjank experiments are listed in Table 1. Results of TXRF are in excellent agreement with certified values, and in addition to the latter about 20 more elements could be determined. A typical spectrum obtained from a sample of BCS 3 13/l after matrix removal is shown in Fig. 2. For comparison, a spectrum obtained from the same material without matrix removal, i.e. from an aqueous dispersion of SiOz, is shown in Fig. 3. The Ti and Fe signals in this spectrum correspond to 95 and 83 pg,/g, respectively (see Table 1). Except for Ca (41 + 7 pg/g), no other elements could be determined from the unseparated material. Typical values for the limit of detection vary from 4 ,ug/g (Sr) to 100 pg/g (S). After matrix removal, detection limits were found to be more than two orders of magnitude lower, e.g. 0.03 pg/g for Sr and 0.6 pg/g for S. Numerical values of detection limits for other elements can be derived from the data in Table 1 taking CL = 3 erg, where a, is the standard deviation of the blank value or the upper limit given in Table 1. The remaining Si content of the samples after matrix removal has been determined to be < 10 pg/g (two independent measurements), which yields an enrichment factor for trace elements of about 5 x 10”. Significant improvement of the method is possible by using subboiled hydrofluoric acid, since the commercial material of highest quality (Suprapur, Merck) still contains about 20 ng/ml Fe and small quantities of other elements (Ca, Ni, Pb). A PTFE subboiling device will be used in the future for that purpose. Except for Ge, chemical losses have not been observed and are not expected, since volatile fluorides (e.g. of As and Se) are not formed in aqueous systems.
536
ULRICHREUS Table 1. Trace element concentrations found in SiO, reference material (BCS-CRM 313/l) and blank samples after matrix removal with HF. Certified values are given for comparison Concentration @g/g)
Element Al P S K Ca Ti V Cr Mn Fe Ni cu Zn Ga Ge As Rb Sr Y Zr Nb MO Sn Ba Ce Hf W Pb Th
High purity SiO, BCS-CRM 313/l* 170 f 15 7.5 f 2.5 20&2 49.0 + 2.0 48.2 + 0.8 95.3 + 3.0 0.2 f 0.1 0.70 + 0.09 1.03*0.10 83.4 &-2.5 0.43 5 0.06 0.58 & 0.04 0.34 f 0.04 0.02 + 0.01
I
0.15 +0.02 0.18 * 0.027 1.32 f 0.07 0.61 f 0.83 16.Ok1.5 0.3 1 f 0.07 0.07 f 0.03 0.10 f 0.05 7.9 f 1.0 1.6 f 0.4 0.44 f 0.04 0.08 f 0.04 0.31 * 0.04 0.26 + 0.06
Blank’ < 5.0 < 0.3 0.8 f 0.2 0.52 k 0.05 0.9 + 0.3 0.052 f 0.015 < 0.01 0.040 f 0.010 0.020 + 0.006 0.35 f 0.05 0.072 f 0.005 0.030 + 0.005 0.035 f 0.005 < 0.002 < 0.002 < 0.007 < 0.005 < 0.005 < 0.006 < 0.02 < 0.01 < 0.01 5 0.06 < 0.08 < 0.03 < 0.005 < 0.005 0.019 + 0.007 < 0.010
Certified values BCS-CRM 313/l 190*21 24’ 42&17 43* 7 100*20 1’ 1.00* 0.2 84+ 7 15’ -
*Mean value and std dev. (n = 5).
+Mean value and std dev. (n = 3). *Value not certified. rGe peak observed but chemical yield unknown. nFrom unstandardized sample.
In brief, evaporation with hydrofluoric acid is satisfactorily to remove the matrix from samples of Si and SiO,, thus enabling trace analysis with TXRF in the rig/g concentration range.
3.2. Analysis of concentrated H,SO, Although the reaction according to Eqn (4) fulfils all requirements upon a matrix separation method, the technique as described in Section 2.2 turned out to be unsatisfactory for the analysis of trace elements in cont. H,SO,. Table 2 lists the results obtained from a sample of HzS04 (Selectipur, Merck) together with the corresponding blank values; the latter prevented obtaining any definite results on the sample. There is too much contamination produced by the chemicals used and/or the various steps of the preparation procedure. The hydroiodic acid (Suprapur, Merck) has been subboiled prior to use; however, in view of the large amounts needed, its purity was not sufficient. Besides, the method suffers from its time consumption. Although it seems possible to overcome these difficulties by optimization, the procedure as a whole is not suited for routine analysis.
Matrix removal procedures hwlts 6
537
(x10’)
1
0 0
2
4
6
6
10
12
14
16 16 20 F!angaAheJw
C
Rb : 6-
b
5
4
6t
3
Cu
In
2
Cr
As Pb
Hi h h
1
t
V j’,,,,
Pb Et
lr
Hf&
:I:...’
0 5
6
7
6
9
10
11
12 13 14 Range CkeVl
Fig. 2.(a) TXRF spectrum of SiOz reference sample (BCS-CRM 313/l) obtained after matrix removal. 20 &g Rb is used as internal standard. (b) Detail from (a).
Alternatively, a procedure is currently developed which is based on the reaction of H,SO, with gaseous HI. The sample is in a flat quartz vessel inside a quartz tube which is heated and flushed with a stream of N, loaded with HI gas. Reaction will take place at the surface of the H,S04, and the reaction products will be condensed in a cooling trap. By this technique, contamination of the sample will be avoided, and the process is expected to be faster than the one used before.
ULRICHREUS
538
0
2
4
6
6
10
12
14
16 16 20 Range tkeV1
Fig. 3. TXRF spectrum of SiO, reference sample (BCS-CRM 313/l) prepared from aqueous dispersion without matrix separation.
Table 2. Trace concentrations found in samples of high-purity cont. H,SO,, and in blank samples after removal of the matrix with HI Concentration (ng/ml) Element K Ca Ti Cr Mn Fe Ni cu Zn As Sr Zr Ba Pb
H,SO, cont.* 1780+40 2940 + 50 < 13 15&3 10*3 735 & 13 17+2 14&-2 57+2 <3 6kl < 12 < 24 6*2
Blank* 850 + 30 360+30 < 24 24k4 12k3 125 + 8 16k2 12*1 2251 <3 3+1 <6 < 35 5+2
*Mean value and std dev. (n = 2).
3.3. Analysis of solutions containing H,S04 As pointed out in Section 2.2, the objective here is to remove the H,SO, present as part of the solvent in order to determine the trace content of the solution. The sample analyzed for testing the procedure is a solution in 2% (0.2 M) H,SO, containing about 100 ng/ml of each of the following elements: Ca, Ti, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Cd, Sn, Pb, and U. Additionally, Rb (200 ng/ml) was added as internal standard. For a direct measurement, 5 ~1 of this solution were evaporated, leaving 0.1~1 (about 200 pg) of HzS04 on the support. The spectrum obtained from this sample (Fig. 4) hardly reveals any significant signals of the
Matrix removal procedures
0
2
4
6
8
10
12
14
539
16 18 20 Rangy CkeW
Fig. 4. TXRF spectrum of sample in diluted H,SO, (2%) measured directly, Le. 5 ~1 of solution evaporated to 0.1~1 (about 200 pg) of H,SO,. 200 ng/ml Rb as internal standard.
Table 3. Recoveries determined for a sample of diluted H,SO, (2%) and blank values after removal of the matrix with HI Concentration (rug/ml) Element P s K Ca Ti V Cr Mn Fe co Ni CU Zn As Se Br Sr Zr MO Cd Sn Ba Pb U
Input*
109.7 f 2.0 102.5 + 2.5 106.5 -+ 1.5 94.5 + 1.0 101.8 f 1.3 104.0 + 1.5 102.8 f 2.0 98.7 f 2.0 103.4 f 2.0 107.0 _+3.5 -
105&4 110&5 115.0_+4.5 !18.0_+4.0
Found*
Blank*
< 20.0 103-lo4 206 _+12’ 133f6 86.2 + 8.0 < 0.5 98.8 f 1.5 100.2 &-2.0 4.9 f 1.0 103.1 + 1.5 103.8 + 1.5 102.5 f 2.0 106.4 f 2.5 1.5 f 1.0 < 0.5 0.4kO.l < 0.2 < 0.3 < 0.3 102.3 + 2.0 < 4.2 < 1.5 122.6 f 4.5 96.0 & 3.0
< 3.8 < 3.5 2.6 k 0.5 5.8 + 1.0 < 0.3 < 0.1 < 0.15 < 0.1 0.70 + 0.10 < 0.15 0.35 + 0.08 < 0.1 1.95 k 0.20 0.10 f 0.05 < 0.005 < 0.08 < 0.05 < 0.10 < 0.10 < 0.5 < 0.8 < 0.3 < 0.1 < 0.2
*Mean value and std dev. (n = 3). ‘Mainly from Cr input as K,CrO,.
Yield (%)
116k7 84+9 93 f 3 106k4 102k3 100+3 1OOa3 106k4 < 2.5 < 0.5
98 f 5 <5 107+7 81+6
ULRICHReus
540
elements listed above. In Fig. 5b a spectrum of the same sample is shown after removal of the HzS04, as described in Section 2.2. For comparison, a spectrum obtained from the original solution but without H,SO, is shown in Fig. 5a. The results are summarized in Table 3 along with the corresponding blank values and the chemical yields calculated from the data.
C
a
Pb
0
2
4
6
6
10
12
14
16 16 20 F.!ange (keW
0
2
4
6
6
10
12
14
16 16 20 Range CkeW
c
Fig. 5(a). TXRF spectrum of original solution used for determination of chemical yields for the H,SO, removal procedure. This spe.ctrumhas been obtained from a matrix free solution (without H,SO,). (b) TXRF spectrum from same solution as in (a) after addition and subsequent removal of H,SO,. Figure Sa and b have been normalized to the same height of the Rb peak to enable direct comparison.
Matrix removal procedures
541
As could be expected from their chemical properties, As, Se, and Sn are quantitatively volatilized by the HI. The same is probably true for Sb, Te and Hg, whose yields have not yet been determined. Minor losses are observed for Ti and U. No significant chemical loss is found (or to be expected) for other elements. For the example given here, the improvement in detection limits by the matrix removal procedure is more than a factor of 100. Detection limits can be derived from the blank values listed in Table 3 in the same manner as mentioned in Section 3.1. However, the amount of chemicals needed-and consequently the blank values-will increase with the concentration of H,SO, in the solution. Therefore, the detection limits derived from the example given here are not generally valid, but even for unfavourable cases values between 1 ng/ml for Sr and 200 ng/ml for P should be reached. The problem of the blank values, which arises from the chemicals, may be avoided by performing the matrix removal with gaseous HI as discussed above. Technical details of such a procedure will be worked out in the future. 4. CONCLUSIONS The objective of this study has been the development of sample preparation methods for trace analysis in some nonvolatile matrices. For Si and SiOz samples, such a procedure has been proved to be successful, and TXRF detection limits in an range between 10 an 200 rig/g have been reached with the perspective that further improvement is possible. Also, trace analysis in solution containing diluted H,SO, is now possible with similar sensitivity. The trace analysis of concentrated H$O, by TXRF is not yet satisfactory, but a way has been outlined to overcome the difficulties. Regardless of this restriction, we dispose now of sample preparation methods for trace analysis at the rig/g level in the above materials which hitherto could be determined with TXRF only at the pg/g level. This achievement may widen the scope of application of TXRF. Acknowledgements-The author gratefully acknowledges the support of the TXRF group at GKSS Research Center (Geesthacht, F.R.G.).
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[6] [7] [8]
[9]
Y. Yoneda and T. Horiuchi, Rev. Sci. Insrr. 42, 1069 (1971). H. Aiginger and P. Wobrauschek, Nucl. Instr. Methods 114, 157 (1974). J. Knoth and H. Schwenke, Fresenias Z. Anal. Chem. 291,200 (1978). A. Prange, J. Knoth, R.-P. StiiBel, H. Bdddeker and K. Kramer, Anal. Chim. Acta 195,275 (1987). W. Michaelis and A. Prange (Eds), Totalrejexions-Riintgenfhtoreszenzanalyse (1. Workshop). GKSS 86/E/61, Geesthacht (1986). K. Freitag, ZnstrumentelleMultielement-Analyse (Ed. B. Sansoni) p. 257. VCH Verlagsgesellschaft, Weinheim (1985). P. Ketelsen and A. Knochel, Fresenius Z. Anal. Chem. 317, 333 (1984). W. Gerwinski and D. Goetz, Books of Abstracts of 24th CON Spectrosc. Inc., Garmisch-Partenkirchen, Vol. 4, p. 724 (1985). A. V. Bohlen, R. Klockenkampcr, H. Otto, G. Tolg and B. Wiecken, Inc. Arch. Occup. Environ. Health 59,403 (1987).
[lo] R. Bock, Aufschlupmethoden der anorganischen und organischen Chemie. Verlag Chemie, Weinheim (1972). [ll] G. Jander and E. Blasius, Lehrbuch der analytischen und priiparatiuen anorganischen Chemie. S. Hirael, Stuttgart (1967).