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Anal. Chem. 1987, 59, 2798-2802
Determination of Silicon Compounds by Gradient Liquid Chromatographic Separation with Direct Current Plasma Atomic Emission Spectrometric Detection Wilton
R. Biggs,* John C. Fetzer, and Rick J. Brown
Chevron Research Company, Richmond, California 94802-0627
The separation and detection of various sHlcon-based compounds are reported. The components of the sample are separated by nonaqueous revermbphase gradlent liquid chromatography. The eMlng rlllcon speclee are detected wlth a dlrect current plasma atomic emlsdon spectrometer. Poly(dhwthylSll0xane)ollgomm aml trimethyldyl derlvatlves of sodknr sUlcate sdutlom are wed to demonstrate the technlque. A detection lbnH of 0.25 ng of Sl/s Is observed wlth the 251.6-nm emisdon line. Typkal preclslon Is In the 1-5 % relative standard devlatlon range.
The composition of aqueous sodium silicate solutions over a wide range of pH and silica concentrations has attracted the attention of many workers. Of particular interest has been the separation and detection of the various polysilicates produced through polymerization of orthosilicate. Lentz (I) first described end-blocking of silicates by trimethylsilylation, applied the technique to sodium silicate solutions, and reported the existence of silicate anions and various condensation products in varying equilibrium concentrations. Gas chromatography with flame ionization detection (GC/FID) was used for the separation/detection in this study, as well as later studies describing improvements in the derivatization procedure (2,3),new components (41, and changes in degree of oligomerization occurring with solution pH or concentration (5, 6). Since FID response varies with structure (7), the carbon/ silicon ratio must be known to quantify GC results. This posed experimental difficulties for unidentified (but potentially important) peaks. In addition, the rapidly decreasing volatility of the derivatized silicates allowed analysis of only the smaller (eight Si atoms or less) oligomers. This limitation may be particularly important, since more recent %i NMR experiments (8,9) demonstrate the existence of a t least 18 species in sodium silicate solutions. Lastly, newer derivatization techniques (IO) employing trimethylgermyl derivatives (designed to eliminate several problems encountered with trimethylsilylation) exacerbate the volatility problem because of increased molecular weight. Previous work (11) has established the feasibility of the combination of a high-performance liquid-chromatographic separation and element-specific detection for a single silicon compound. Expanding on this general theme could circumvent many of the analytical limitations discussed above. Gradient elution could reach larger oligomeric structures, since liquid chromatography (LC) requires analyte solubility in place of the GC need for volatility. Specific element detection could improve and simplify detection capabilities by responding only to silica with a constant response fador regardless of chemical structure. While LC-based separation steps have considerable potential, particularly for heat-sensitive or volatility-limited analytes, gradient elution must often be incorporated into the separation step for maximum utility. For emission-based detection techniques, gradient elution schemes can signifi0003-2700/87/0359-2798$01.50/0
Table 1. Typical Instrumental Parameters injection volume
Chromatographic
5 p L of ethyl acetate solution containing ca. 50 pg of Si
external standard hexamethyldisiloxane column Zorbax ODs, 6.2 mm X 8 cm, 3 pm mobile phase A = 70/30 V% acetonitrile/acetone B = ethyl acetate gradient 0% B to 70% B in 60 min by curve 5 0% B to 45% B in 60 min by curve 6 temperature column, ambient spray chamber, 45 OC 1 mL/min flow rate Soectrometric monochromator echelle grating groove spacing emission wavelengths entrance slit width height exit slit width height photomultiplier tube voltage photomultiplier tube amplifier gain amplifier time constant nebulizer gas flow rate
0.34 m echelle 63O,26” blaze angle 79 grooves/mm 251.6 nm 288.2 nm 100 pm 300 pm
100 pm 300 pm 1000 v
Hamamatsu Corp. R376 lo6 V/A 0.1 s 6.5 L/min
cantly complicate detection. Changes in background emission, nebulization efficiency, and excitation efficiency are expected when introducing a solvent gradient to the detector. While not universally applicable, the potential rewards for even selected successful application make a study of the gradient elution approach worthwhile. This paper reports the successful separation of the trimethylsilyl derivatives of a sodium silicate solution and other silicon-containing materials by nonaqueous reversed-phase gradient-elution LC. Detection of the separation is accomplished on-line with a direct-current plasma (DCP) emission spectrometer.
EXPERIMENTAL SECTION Apparatus. The apparatus used in these experimentshas been described previously (12). Briefly, two Waters Model M6000A HPLC pumps driven by a controller (Waters Model 660) supply the solvent gradient to the column under study, and the eluent is directed to a ceramic cross-flownebulizer of the Beckman Model SpectraSpan IIIB atomic emission spectrometer fitted with a three-electrode, DCP source. The phototube output is fed to a variable gain current-to-voltageamplifier connected to a Nelson Analytical chromatographic data system. Designs for spray-chamber interfaces specifically for HPLC/DCP are available (13);but, except for plugging the drain hole in the base of the spray chamber, the standard nebulizer/ spray chamber was used in this study. However, the spray chamber was wrapped with heating tape and kept at 45 “C, preventing “icing”of the chamber brought on by JouleThompson cooling and evaporation. In turn, this allowed complete nebulization to be maintained with the solvent systems used. 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
Table 11. Reported Detection Limits technique (wavelength,nm)
instrumental limit of detection
GC/DCP (288.2) GC/ICP (251.6) GC/MIP (251.6)
0.1 ng/s (14) 0.8 ng/s (15) 1.8 pg/s (16)
Instrument Operation. Table I lists the most commonly employed analysis conditions. The Si emission signal was optimized each day by aspirating a 50 ppm Si standard (Spex) into the plasma and monitoring the amplifier output with a digital voltmeter. Reagents. All LC solvents used were from Burdick and Jackson (Muskegon, MI). Hexamethyldisiloxane (99.9%),hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethyltetrasiloxane, and trimethylsiloxy-terminated poly(dimethylsiloxane)sof various viscosities were from Petrarch Systems, Inc. (Bristol, PA). Decamethylcyclopentasiloxane and dodecamethylpentasiloxane were from ICN Pharmaceuticals, Inc. (Plainview, NY). Octamethyltrisiloxane was from Columbia Organic Chemicals Co. (Camden, SC). Standard Reference Material 1066a (octaphenylcyclotetrasiloxane)was obtained from the National Bureau of Standards.
PROCEDURES Preparation of Derivatives of Sodium Silicate Solution. The trimethylsilylation procedure used was that described by Lentz (1). Analysis of Silicon Content of Samples and Standards. The silicon content of the various samples was determined by DCP-AES (atomic emission spectroscopy) in the conventional mode. The calibration curve was prepared by using NBS SRM 1066a (octaphenylcyclotetrasiloxane) as the silicon standard. Data Manipulation. Absolute recovery of Si was calculated by determining the ratio of the integrated area of the peaks of interest to the external standard peak area. For those runs where "drift" was evident (by comparison between external standards injected before and after the run),it was assumed to have occurred linearly during the run and the area of the peaks of interest were adjusted on a time-weighted basis prior to any calculations. This adjustment was performed as follows: corrected peak area = A ,
rsl -:(A)1
where A, = raw area of peak at retention time t , SI = area of initial standard, A = area difference between initial and final standards, and RF = run fraction. The run fraction is the retention time difference between the initial standard and the peak of interest divided by the retention time difference of the initial and final standards.
DISCUSSION Silicon Detection Limits. The most commonly used emission lines for Si are 251.6 and 288.2 nm. With hexa-
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methylcyclotrisiloxane, the 251.6-nm line showed an instrumental limit of detection of 0.25 ng of Si/s, while the 288.2-nm line gave a value of 0.1 ng of Si/s. No previous studies of LC/DCP for analysis of silicon compounds have been reported, but several GC separations coupled to various plasma detectors have been reported. The observed detection limits are listed in Table 11. The LC/DCP technique instrumental limit of detection reported here is roughly equivalent to comparable GC techniques, primarily because total-consumption nebulization conditions are reached with the mobile phase used. While the 288.2-nm line showed the lower detection limit, small base-line shifts were observed with some of the mobile phases studied. These could potentially complicate any analysis. Since the 251.6-nm line demonstrated suitable sensitivity and produced no base-line shifts with the mobile phases studied, it was selected for a majority of the later studies. Silicon Response Curve. Both the 251.6- and 288.2-nm Si emission lines were evaluated as to concentration/response linearity under both isocratic and gradient conditions and background shift during a typical gradient analysis. Under isocratic conditions, analysis using either line produced chromatographic peak areas that followed the injected mass of silicon (as hexamethylcyclotrisiloxane)linearly between 0.02 and 0.967 pg. The results of linear regression analysis were as follows: at 251.6 nm
Q = (4.869
X
10-6)A
+ (1.58 X
r= 0.99998
+ (8.1 X
r = 0.99998
and a t 288.2 nm
Q = (1.028 X 10-6)A
where Q and A are sample amounts (pg) and peak area (mV s), respectively. Development of Separation Conditions. A mixture of hexamethyldisiloxane (l-Si2), octamethyltrisiloxane (Mi3), decamethyltetrasiloxane (l-Si4),dodecamethylpentasiloxane (l-Si5),hexamethylcyclotrisiloxane (c-Si3),octamethylcyclotetrasiloxane (c-Si4),and decamethylcyclopentasiloxane(c-Si5) was used to evaluate mobile phases and columns. Table I11 lists the results of the study. Initial studies used styrene-divinylbenzene rigid, porous polymer-based column packings for the separation to eliminate emission background or base-line shifts, which might occur with silica-based columns (due to degradation of the packing). However, the expected efficiency limitations ( I 7) associated with this type of packing quickly eliminated any application. Remaining experiments used silica-based packings. Thorough flushing of the C18 silica column with a strong solvent (ethyl acetate) prior to any separation studies successfully removed (apparently) degraded silicon-containing material and produced a stable base line (no drift) with low (2-5 mV), but
Table 111. HPLC Column Comparison column
mobile phase (gradient)
broad peaks, no resolution of cyclic from linear compounds (linear) 90/10 MeOH/H20 or CH3CN in 8 rnin same as 1 (linear and concave) 90/10 MeOH/H20 to MeOH in 8 rnin base-line resolution of c-Si, and l-Si2, resolution ( R = 0.95) of other Si,-l cyclics (linear) from Si, linear siloxanes 95/5 MeOH/H20 t o MeOH in 8 rnin performance equivalent to 3 (linear) 90/10 MeOH/H20 to MeOH in 8 rnin performance equivalent to 3 (linear) 92/8 CH3CN/H20to 97/3 in 8 rnin loss of resolution between cyclic and linear siloxanes (linear) performance equivalent to 3 65/35 THF/H20 to 68/32 in 8 rnin (linear)
1. Hamilton PRP-1, 10 pm, 4.6 mm X 15 cm 90/10 MeOH/H,O to MeOH in 8 min 2. Polymer Labs, 5 pm, 4.6 mm
X 15 cm
3. Rainin Microsorb CIS Shandon CIS, 5 pm, 4.6 mm
X
15 cm
5. Zorbax ODs, 3 pm, 6.2 mm
X
8 cm
4.
results
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
I
a
I
I
3
6
9
1
I
I
12
15
18
I 21
Time, Minutes
Flgure 1. Separation of selected linear (I) and cyclic (c) methylsiloxanes. See text for names and compounds. Separation conditions: mobile-phase gradient, 90 % MeOH/ 10 % (v/v) H,O to MeOH in Wmln (linear); column, Zorbax ODs,3 Km, 6.2 mm X 8 cm; detection line, 251.6 nm.
I
,
1
I
I
1
3
5
7
9
Time, Minutes
Table IV. Response Comparisons for Selected Siloxanes mobile phase compound 1-Si, c-Si, l-Si4 c-Si, 1-Si, c-Si, NBS SRM 1066A
A"
Bb
C'
100 f 2 101 f 2 102 f 2 99 f 2 98 f 4 95f2
101 f 3 102 f 2 100 f 3 101 f 3 100 f 1 96f4
100 f 2 103 f 2 100 f 3 101 f 2 100 f 2 94f2
98 f 2
100 f 1
100 f 2
70/30 (v/v) acetonitrile/acetone. b35/15/50 (v/v/v) acetonitrile/acetone/ethvl acetate. Ethvl acetate.
observable, background emission. These preliminary experiments also showed significant dependence of selectivity with mobile phase. No resolution of linear Si,, from cyclic Sin-l is seen with acetonitrile as the major component of the mobile phase. Replacing acetonitrile (a group VIb solvent) (1419)with T H F (group 111)or MeOH (group 11) at equivalent solvent strengths produced some (R = 0.95) resolution between cyclic and linear species. Figure 1 shows a typical separation. Acetonitrile was chosen over MeOH for the bulk of the weak solvent portion of the mobile phase because several percent water would have to be added to the MeOH to achieve equivalent retention of the derivatized samples described below. When the water/methanol mobile phase was tried, it did not volatilize completely. The possibility then existed that the detector response would no longer be independent of the solvent gradient composition. Dependence of Detector Response on Type of Silicon. For purposes of these experiments, the DCP must be capable of a response that either is independent of the type of silicon compound or varies in a predictable (and repeatable) manner. Since the compounds generated by silylation of silicate solutions are not commercially available, the series of silicon compounds used in the previous section was selected to study this potential problem. Both cyclic and linear structures are represented, and more importantly, a wide range of volatility is included. The experiments were performed in the following manner: Under several isocratic conditions, each compound listed in Table IV and a hexamethyldisiloxane standard were alternately injected. The integrated responses of each injection pair were then determined and the integrated response per mass of silica calculated. The response of each compound was then expressed as a percentage of the hexamethyldisiloxane response based on equivalent silicon mass. The results of these
Flgure 2. Chromatogram of dodecylmethylcyclopentaslloxane (peak D) sample. Peak A is the hexamethyldisiloxane external standard; Peaks B, C, and E are unknown impurities. Separation conditions: Zorbax ODS column; MeOH (isocratic) mobile phase; detection line, 251.6 nm.
Table V. Response Comparisons in o-Xylene % of
compound hexamethyldisiloxane octamethyltrisiloxane hexamethylcyclotrisiloxane octamethylcyclotetrasiloxane dodecamethylpentasiloxane decamethylcyclopentailoxane octaphenylcyclotetrasiloxane
hexamethyldisiloxane response 100 83 f 3 87 f 2 78 2 74 + 2 71 3 76 + 2
+ +
bp, "C 99-100 152-53 134 175-76 101 (2.7 X lo2 Pa) 330-34(13.3 Pa)
comparisons are shown in Table IV. Integrated peak areas were used for comparison purposes because peak heights varied considerably with mobile-phase composition. The relative response for dodecylmethylcyclopentasiloxane appears low when compared to the other compounds selected. I t is believed that this is due to contaminants in the sample rather than the beginning of a trend to lower response/silicon with decreasing volatility or other change in response with the type of silicon compound analyzed. Figure 2 shows a chromatogram of the dodecylmethylcyclopentasiloxane. The nature of the silicon-containing contaminants is unknown. In principle, unless the contaminants are cyclic homologues, they could be responsible for the poor recovery of silicon with dodecylmethylcyclopentasiloxanesince formula weights were used in calculating the expected silicon concentration for each compound. The relationship between detector response and complete vaporization of the column eluent can be demonstrated by comparing the response of these same compounds when oxylene is used as the mobile phase. o-Xylene was chosen to ensure that the mechanical efficiency (13) of the nebulizer/ spray chamber combination would be less than 100% at 45 "C. Table V shows the response of these compounds calculated as in Table IV and again expressed as a percentage of the hexamethyldisiloxane response. The two salient observations from the data are (1) the examined compounds all produced lower (for the same mass of silicon) responses than hexamethyldisiloxane, and (2) for the more volatile compounds, an inverse relationship exists
ANALYTICAL CHEMISTRY, VOL. 5 9 , NO. 23, DECEMBER 1, 1987
between boiling range and response. It is assumed that two factors contribute to these observations. The first factor is the volatility of the compound. In this small sample of compounds, the response (expressed as a percentage of the hexamethyldisiloxane response) trends downward as the volatility (represented by the boiling point) trends upward. Previous work (20,21)has demonstrated that the volatility of a particular chemical form of an element can lead to changes in its response during pneumatic nebulization. Second, with o-xylene the mechanical efficiency is less than 100% (incomplete nebulization), so an unknown fraction of the analyte is rejected from the sample introduction system, even with hexamethyldisiloxane. Both of these factors decrease the amount of analyte reaching the plasma, leading to decreased response in general. By total nebulization, both of these potential contributions to changing response can be eliminated. No eluent is rejected, and, if other losses of analyte can be prevented and plasma-related processes remain constant, a reasonable equivalency of element response under constant solvent composition conditions should be expected. Effects of Changing Solvent Composition. A separation performed by using a solvent gradient necessitates changing the composition of the eluent flowing to the nebulizer with time. Changing the solvent composition would be expected to produce several problems. These would include changes in the emission background (which would manifest itself in a base-line shift) and in changing element response brought on by changes in nebulization1 transport efficiency of the analyte or in the plasma characteristics. Appropriate line selection can address the spectral problems to a degree, and, in the case of silicon, the 251.6-nm line appears to be the optimum choice to limit changes in the emission background. But, in the broader sense, one is bound by the characteristics of the element of interest. This natural limitation will prevent the universal application of gradient separations with emission detection when using the approach described here, but other avenues will undoubtedly develop. Thermal gradient elution (22) is one possible alternative. Changing element response can be minimized by total (or constant fraction) transport and consumption of the analyte, assuming plasma-related processes remain constant. Totalconsumption nebulization is the most direct means of accomplishing the task and would ensure equivalent response of the analyte in the plasma regardless of time-dependent characteristics in the solvent compositions. With this requirement in mind, the choice of DCP as the plasma source becomes critical. While other plasma emission sources (inductively coupled plasma, ICP; microwave-induced plasma, MIP) represent potential options, a review (23)of the relative merits of each device support our experience that the DCP, with its ability (24,25)to tolerate high flow rates of typical organic solvents used in HPLC separations, represents the emission source best suited for the application at hand. To investigate the validity of this approach, various siloxanes were repeatedly injected as the composition of the column eluent was changing in a manner similar to that which would occur during a typical solvent gradient. The peak area of each response was then calculated, compared with the initial peak area response at start of run (to give a relative response), and the experiment repeated to obtain a measure of the expected variation in response at selected points in the gradient. The same experiment was then conducted under isocratic (constant solvent composition) conditions. Figure 3 shows a plot of the data. Limit lines have been drawn showing the mean response (rtl standard deviation) for both isocratic and gradient data sets at equivalent points. If the sensitivity of Si changed with mobile-phase composition, one would expect an upward or downward trend for the limit lines when the gradient re-
Iaocratlc
Iaocratlc
Mean t 1s
Mean 1s
Gradlent M""'_~s
-
2801
Gradient Mean 1s
-
110 108
.
lO6 104
t
:I 90
,
,
20
40
,
,
,
,
,
SO
80
100
120
140
Gradient Completed, %
Flgure 3. Relative silicon response observed under isocratic and gradient elution conditions. Separation conditions: column, Zorbax ODs; isocratic mobile phase, 70130 acetonitrilelacetone; gradient, 70130 acetonttrllelacetone to 7012 119 (vlvlv) ethyl acetatelacetonitrile/acetone; detection line, 251.6 nm.
I
I 12
I
I
25
38
I 51
I 04
lime, Minute8
Separation of oligomers in 10 CS trimethoxy-terminated poly(dimethylsi1oxane)(41.4 pg of Si inlected). The external standard is hexamethyidisiioxane. The gradient is 0 - 7 0 % ethyl acetate in 60 min. Flgure 4.
sponses are compared with the isocratic responses. Both data sets have a slight downward trend, but we believe that is due to drift in response common to the DCP. To limit the impact of this drift, the gradient was completed in 15 min, a much shorter time than that used in the work described later. The experiment was also repeated with different silicon compounds with equivalent results. Thus, under the conditions of the study, the solvent composition can be varied without altering the response of silicon. Although not studied in detail, it is likely that this response independence is a general phenomenon if total effluent-consumption conditions can be established and maintained during a separation. Here the DCP has a distinct advantage over other plasma sources. The very high nebulizer gas flow (6.5 L/min) relative to the ICP (-0.5 L/min) permits complete transport of volatile organics to the plasma. Further, the much higher power density (23)of the DCP relative to the ICP permits successful operation even with this increased vapor load. Precision of Gradient Analysis. The precision to be expected of a typical gradient analysis was evaluated by repetitively analyzing a sample of 10 CStrimethylsiloxy-terminated poly(dimethylsi1oxane)for oligomer distribution. A
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
Table VI. Precision Study oligomer no.
wt %
SD
RSD
Si 6 Si 7 Si 8 Si 9 Si 10
0.60 3.09 6.53 8.50 8.91 8.55 7.87 7.10 6.36 5.67 4.99 4.39 3.86 3.37 2.95 2.55 2.23 1.92 1.67 1.44 1.24 1.06 0.90 0.77 0.64 0.55
0.015 0.045 0.083
2.5 1.4 1.3 0.6 1.0
Si 11 Si 12 Si 13
Si 14 Si 15 Si 16 Si 17
Si 18 Si 19 Si 20 Si 2 1
Si 22 Si 23 Si 24 Si 25 Si 26 Si 27 Si 28
Si 29 Si 30
Si 31
0.05 0.09 0.09 0.09 0.06 0.06 0.04 0.02 0.02 0.02 0.03 0.04 0.05 0.03 0.03 0.01 0.02 0.03 0.03 0.04 0.03 0.03 0.03
1.1 1.2
0.9 0.9 0.7 0.4 0.5 0.5 0.9 1.4 2.0 1.3 1.6 0.6 1.4 2.4 2.8 4.4 3.9 4.7 5.5
typical chromatogram is shown in Figure 4. Given the length of the gradient used (1h), drift of the DCP source is inevitable. To ameliorate this circumstance, an external standard is injected before and after the peaks of interest have eluted. Normally, the difference between peak areas of the external standards is 410% (final/initial ratio for external standards for the six runs used in Table VI: 0.906,0.947, 1.009, 0,919, 0.956, and 0.904). The assumption is also made that the drift occurred linearly between injections of the external standard. The raw peak areas are then adjusted on a time-weighted basis. Table VI shows the result for such a set of analyses. Table VI lists only the first 25 oligomers. In fact, data were collected out to the 40-mer, but these larger oligomers contribute very little to the total silicon present in the sample. The precision for analysis of the major oligomers is acceptable for quantitative applications; however, values for the relative standard deviation (RSD)
