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Anal. Chem. 1985, 57, 2979-2984
Standardization of Alumina and Silica Adsorbents Used for Chemical Class Separations of Polycyclic Aromatic Compounds Douglas W. Later*' and Bary W. Wilson Battelle, Pacific Northwest Laboratories, Richland, Washington 99352
Milton L. Lee Department of Chemistry, Brigham Young University, Provo, Utah 84602
The reproduclblllty of separating polycycllc aromatlc compounds (PAC) by adsorptlon chromatography Is known to be a function of the adsorbent's actlvlty, i.e., water content. I n thls work, a multlple-point R " calibration curve (equivalent retention volume vs. adsorbent actlvlty) was constructed by uslng a model solute/solvent system (naphthalene/hexane) for two commonly used adsorbents. The optimum actlvlties for neutral alumlna and slllclc acld, two adsorbents used in a prevlously reported separatlon method deslgned to provide chemical class separatlon of cornpiex, coalderlved mixtures, were determlned by uslng the R " callbratlon-curve method. The optimum activity ranges for these adsorbents, as used In thls separatlon method, were 1.0-1.5% H20 (w/w) for alumina and 5-8% H20 (w/w) for siliclc acld. Once the R " calibration curves were constructed for these two adsorbents, they were used to standardlre and routlnely monltor their actlvltles. Finally, thls standardlzatlon technlque was used to determine the required oven temperature at which alumina (150 "C) and silicic acld (50 "C) could be stored so that they reached and maintained their determlned activity levels.
Classical adsorption chromatography on gravity-flow open columns with alumina- and silica-based oxides has been used extensively in the separation of polycyclic aromatic compounds (PAC) that are present in complex mixtures. Recently, methods have been developed by us for the chemical class fractionation of PAC in alternate fuels, air particulate matter, fish tissue, and sediments (1-3) using neutral and basic alumina and silicic acid adsorbents. Since these methods have been reported in the literature, several inquiries and observations from other laboratories have been brought to our attention regarding the reproducibility and reliability of these class fractionation procedures. Therefore, studies were undertaken to optimize the class fractionation of PAC by standardizing the activity of the alumina and silica adsorbents used for the separations. The results are presented in this communication to assist others in obtaining similar reproducible separations. Although the separation of many PAC can be readily achieved on alumina and silica by adsorption chromatography, this technique has two distinct disadvantages. First, the highly adsorptive nature of these materials can result in solute losses due to catalytic degradation or irreversible adsorption and peak tailing, which results in less efficient separations and overlap of components. Second, the separation characteristics of adsorbents can be altered by varying their activity or water content (4-7); uncontrolled variations of adsorbent water Current address: Lee Scientific, Inc., 379 N. University, 104, Provo, UT 84601.
content can result in poor separation reproducibility. Activation of adsorbents (thermal dehydration of surface adsorbed water) is universally performed by calcination (baking) of the material. For example, alumina is completely activated at 400 O C in 8-24 h. Adsorbent activation increases selectivity, peak tailing, and adsorbent-catalyzed reactions and decreases efficiency and linear capacity ( 5 ) . Deactivation procedures generally require the addition of known amounts of water to weighed samples of activated adsorbent (8-12) or the exposure of activated adsorbents to humidity-controlled atmospheres (13). Equilibrating adsorbent activity levels has also been achieved by conditioning adsorbent stationary phases with mobile phases that contain known water levels (14). Water-deactivated adsorbents provide more efficient column beds, reduce adsorbent-catalyzed solute reactions, and reduce solute band tailing, but also reduce selectivity (5). The extent of adsorbent activation or deactivation therefore becomes a balance between efficiency, selectivity, and solute degradation and is principally determined by the required separation. Additionally, freshly dehydrated adsorbents are initially very active, but, their activity can be decreased by adsorption of H 2 0 when exposed to humid or damp environments (15,16). Hence, extra precautions must be taken to maintain the desired level of activity (water content) of prepared alumin and silica adsorbents. In order to achieve a higher degree of reproducibility and to ensure more reliable chemical class fractionation of PAC in complex matrices, a protocol was developed and is described here for standardizing adsorbent activity. Standardization procedures were initially developed for neutral alumina but have also been applied to silicic acid. Both adsorbents have been used extensively in chemical class separation methods reported by this group (1-3).
EXPERIMENTAL SECTION Activation/Deactivation. Neutral aluminum oxide (Brockman Activity I, 80-200 mesh, Fisher No. A950) was purchased from Fisher Scientific Company and used exclusively for this work. First, 10-g quantities were precisely weighed into acid-cleaned, heat-conditioned (400 "C, 8 h) 100-mL weighing beakers and placed in a large muffle oven held at 400 "C. Activation was achieved by calcination of the alumina samples for periods of 8-12 h. Next, the beakers were securely sealed with aluminum foil while still in the oven, placed in a sealed desiccator, and allowed to cool under a dry nitrogen atmosphere. Deactivation was achieved by evenly injecting a weighed amount of millipore grade distilled water onto the activated alumina and homogenizing the alumina/H,O on a mechanical shaker for 8-12 h. To ensure that no adverse changes occurred in standardized alumina adsorbents, the samples were sealed at all times and stored under nitrogen in a desiccator until utilized. Deactivation of silicic acid (Mallinckrodt No. 2847 100-mesh powder) was carried out by the same procedures as used for the neutral alumina except that the oven temperature used for calcination was 185 "C. Activation was again achieved by addition
0003-2700/85/0357-2979$01.50/0 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Glass Solvent Resevoii
I
VV
I
I
!I
Glass Column (llmmi d )
TIME ~rnin~
A
\ L
/J
is0
C, 1
3
2
5I h
4
-
7
0
Figure 2. Representative adsorption column chromatogram of naphthalene on neutral Al2O3 with a hexane eluent. Alumina Adsorbent
1
Sintered Glass Frit
Neutral Alum~na
Flow Regulator
F
l
R
e
c
o
r
d
e
r
f+
Teflon Tubing
20 ml Hexane
50 mi Benzene
7 0 ml
50 tnl
Chloroform Ethanol
Methanol
Silicic Acid
Figure 1. Schematic diagram of the system used to measure the retention volume of naphthalene on alumina of varying activities.
of a given weight of water to a known weight of silicic acid adsorbent. Prepared silicic acid samples were handled and stored like the standardized alumina samples. In addition to calculating the weight percent water of the adsorbents (i.e', x% H20-A1203 or x% H20-Si02) from known H20/adsorbent weight measurements, gravimetric analysis of the actual standardized samples was also performed. This was accomplished by weighing 1-g aliquots of standardized adsorbent samples into 10-mL acid-cleaned, heat-conditioned weighing bottles with ground glass covers, dehydrating the samples a t 400 OC for 12-24 h, and reweighing the calcined samples. The weight loss during baking was used to calculate the x% HzO. Again, precautions were taken to exclude the possible adsorption of atmospheric moisture by the thermally dehydrated adsorbent. Retention Volume Measurements. The equivalent retention volume of a solute on a chromatographic system is defined by eq 1,where Ro is the equivalent retention volume (mL/g), R, is
the observed retention volume (mL), V, is void volume of the column (mL), R is the adjusted retention volume (mL), and W is the adsorbent weight (g). Ro values were determined for naphthalene using precise adsorbent standards of various activity and were used to construct a calibration curve that was used to standardize, monitor, and measure the x% HzO-adsorbent of other batches of adsorbent used for chemical class fractionations. A schematic diagram of the system used to measure the retention volume of naphthalene on various x% H2O-AlZO3 is shown in Figure 1. The column system (11mm i.d. X 600 mm, Ace Glass, Inc.) was initially packed with 8 g of A1203and rinsed with several milliliters of hexane. Next, 50 r L of a naphthalene/hexane solution (approximately 20 mg/mL) was adsorbed into l g of the same A1203then placed on top of the 8-g column. The naphthalene was eluted from the column with UV grade hexane (Burdick & Jackson). An Altex Model 330 analytical optical unit (UV adsorption, 254-nm filter) and recorder were used to monitor the elution of naphthalene from the column. A variable-flow stopcock made of Teflon controlled the flow at approximately 4.0 mL/min, and exact flow rates were monitored throughout the chromatographic run. Chromatograms similar to that in Figure 2 and flow rate data provided the information necessary to calculate the equivalent retention volume for naphthalene on Alz03
5 0 ml
30 mi
50 mi
Hexane Benzene
Benzene
Benzene Ethyl ether
L
s,
s2
s,
Flgure 3. Chemical class separation on neutral alumina (see text for definition of acronyms).
of varying HzOcontent. Isooctane (20 ML)injected onto the Alz03 column resulted in a small negative peak in the chromatogram (see Figure 2), which was used to determine the void volume, V,, of the column (Le., the void volume being the retention volume of an unretained solute). Similar procedures and apparatus were used with silicic acid adsorbent samples. Chemical Class Fractionation. In order to better determine the optimum elution parameters for chemical class fractionation of PAC on Alz03by the scheme proposed earlier ( I ) , a standard mixture of seven compounds including the following chemical classes was devised for testing the x% HzO-A1203 standards: aliphatic hydrocarbons ( ~ Z - Cpolycyclic ~~), aromatic hydrocarbons (PAH: phenanthrene, anthanthrene), nitrogen-containing polycyclic aromatic compounds (N-PAC: 2-methylindole, benzo[h]quinoline, 3-aminofluoranthene),and hydroxy polycyclic aromatic hydrocarbons (HPAH: 2-naphthol). All standard compounds used were commercially available from Aldrich (Milwaukee, WI). Approximately 250 pL of the standard solution (solutes ranged in concentration from 0.15 to 0.30 fig/pL) was fractionated according to the previously described procedure ( I ) shown in Figure 3. Each fraction was concentrated to approximately 2 mL on a rotary evaporator; further evaporation to the original sample volume was done under a dry nitrogen stream. Caution was taken to avoid solute loss by never taking the same completely to dryness. After the volumes of the fractions were brought to 250 pL, qualitative and quantitative recovery data were obtained by capillary gas chromatographic analysis. Chemical class fractionation of the various N-PAC subclasses by silicic acid adsorption chromatography as reported earlier ( I ) is also shown in Figure 3. The purpose of this separation scheme was to separate the secondary polycyclic aromatic nitrogen heterocycles (PO-PANH;e.g., indole and carbazole) from the amino-PAH (APAH) and tertiary polycyclic aromatic nitrogen heterocycles (3O-PANH). The critical step in this particular separation scheme is the selection of the fraction cut points for the APAH. Therefore, a standard solution of 1-aminonaphthalene and 6-aminochrysene (Aldrich Chemical Co.) was used t o determine the effect of percent water content of the silicic acid (x% H20-SiOZ) on the fractionation procedure. Again sample prep-
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
2981
Table I. Calibration Plot R" and % HzO-A1zO~Data
approx % Hz0-Al2O3 0.0 0.5
1.0 1.5 2.0 2.5
3.0 4.0 5.0
R", mL/g
gravimetric" % H20-A1203
64.4 f 8.3 35.7 f 1.8 25.4 f 6.7 9.6 f 1.6 5.3 f 1.6 2.7 f 0.7 2.0 f 0.7 1.6 f 0.4 0.9 f 0.3
0.2 f 0.1 0.6 f 0.1 1.1 f 0.4 2.0 f 0.1 2.2 f 0.1 2.7 f 0.0
3.1 f 0.1 3.8 f 0.2 4.7 f 0.1
calcdb % H20-A1203
no. of determinations
av OF calcd/grav 0.1 f 0.1 0.7 f 0.0 1.1f 0.2 1.8 f 0.1 2.1 f O . 1 2.6 f 0.0 3.1 f 0.1 3.9 f 0.1 5.0 f 0.0
0.0 f 0.0 0.8 f 0.2
1.0 f 0.1 1.6 f 0.1 2.0 f 0.1 2.5 f 0.0 3.1 f 0.1 4.0 f 0.1 5.3 f 0.1
Values used to plot gravimetric analysis data in Figure 4. *Values used to plot weight calculated data in Figure 4. Values used to plot "best fit" data in Figure 4. The gravimetric and calculated values for each determination were averaged; the average and standard deviation of the averages are reported.
-
70
aration procedures were the same as for the alumina experiments.
Gravimetric Analysis
RESULTS AND DISCUSSION Snyder has suggested that adsorbent activity varies slightly even after extensive thermal dehydration of surface-adsorbed water (8). Variations from 0.04% to 0.30% HzO-A1203were gravimetrically observed for several samples studied in this work even after 12-24 h of calcination a t 400 "C. These observations indicate that it is difficult to reproducibly and completely remove 100% of the surface-adsorbed water from the alumina adsorbents. It logically follows that calculating the x% HzO-A1203of deactivated alumina by the weight of H 2 0 added back into the material can result in an error in the absolute activity of the absorbent unless the exact H 2 0 content of the fully activated material can be measured or determined. Such factors contribute to the poor reproducibility often observed in identical chromatographic separations using different batches of alumina. Additionally, improper storage conditions or exposure to atmospheric moisture can also contribute to fluctuations in adsorbent activity. The chromatographic characteristics of an adsorbent are a better quantitative means to accurately and precisely determine the extent of activation or deactivation. Brockman (17, 18) outlined a more subjective and qualitative method for determining the activity of prepared alumina adsorbents by their ability to separate certain dyes. A more quantitative method was suggested by Snyder (8)who used the equivalent retention volume, R", for a standard solute-eluent combination to show that the retention of a solute is a function of the percent water added to calcined alumina. This method has been extended in this study t o the construction of calibration c w e s and to the determination of optimum adsorbent activity ranges for the neutral alumina and silicic acid used for chemical class fractionation of complex PAC mixtures. Neutral Alumina. Approximately 20 standard samples of x% H20-Alz03, with x varying ffrom 0 to lo%, were prepared according to the protocol outlined in the experimental section. The equivalent retention volume of a naphthalene-hexane solute-eluent combination was determined a t a constant column loading (1 X = solute weight/alumina weight) for each prepared A120, standard. Table I presents a summary of the R" and % H20-A1203 data obtained from these experiments. Figure 4 shows the curves obtained by plotting the R" results for the calculated and gravimetrically determined x% H20-A1203standards. As can be seen from Figure 4, and as implied earlier, variations, although slight, are noticeable when using weight calculations or gravimetric methods to determine the percentage of H 2 0 adsorbed on Alz03The differences are most apparent around the knee of the curve, while excellent agreement of data points is observed for percent water greater than about 2.5%'. A "best fit" curve was calculated from averages of the two methods
60
b
Weight Calculated
A-
"Best Fit"
- - -A
50
-m g
40
k 30
20
10
0
i 0
I
1
I
1
1
2
3
4
0'7 H 2 0
Figure 4. Plot of R
vs.
x%
i 5
A1203
H,O-AI,O,
for a naphthalene-hexane
solute-eluent system. Table 11. Values for Alumina Stored in Uncontrolled Environments
A1203conditionsa 1. A1203 opened and analyzed as received
from distributor 2. Al,03 opened and used 10 days, then analyzed; uncontrolled storage conditions; Batelle Pacific Northwest Laboratory, Richland, WA 3. Alz03used 4-6 mo under uncontrolled storage conditions at Brigham Young University, Provo, UT 4. Al,03 used 4-6 mo under uncontrolled storage conditions at Battelle Pacific Northwest Laboratory, Richland, WA
R", mL/g % HzOb 44.6
0.47
42.7
0.52
9.7
1.88
0.9
4.80
OAll A1203was originally classified as Brockman Activity I % H,O was extrapolated from R" data using the calibration curve in Figure 4. (0-1 % H20-A1203alumina, as specified by supplier.
(see Figure 4) and used as an R" calibration curve to routinely check the activity of batches of alumina used for chemical class fractionation.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
The importance of constructing and using an Ro calibration curve is illustrated in Table 11. Variations of x % HzO-Alz03 are detected in opened and used alumina when stored under uncontrolled conditions a t different locations. These variations may arise from differences in humidity levels in the two different geographical locations. Regardless of the source, large variations in water content of alumina used for accurate separations must be eliminated if good reproducibility is to be expected. Nominally, 1% HzO-Alz03 has been used for chemical class separations done in the past (1-3). But, as can be seen from Figure 4,the R” value for 1%water-alumina falls on a steep region of the R” calibration curve. Thus, small changes in x% H@-A1203 at this activity can adversely alter the R” value and, more importantly, the chromatographic properties of the system (Le., selectivity, efficiency, and solute degradation). To determine the x% HzO-Alz03 that gave the best chemical class fractionation, a series of class separations were performed on standardized x% HzO-Alz03 samples, where x varied from 0 to 3%. A standard mixture consisting of the following selected compounds was devised to test for the associated typical difficulties encountered in this particular chemical class separation: (a) n-pentacosane, CZ5: aliphatic overlap of long-chain hydrocarbons, greater than Czo,that may elute into the PAH fraction; (b) anthanthrene: overlap of high molecular weight PAH with more than 5 rings into the N-PAC fraction; (c) 3-methylindole and benzo[h]quinoline: nitrogen “breakthrough” of the N-PAC that elute early into the PAH fraction because of their less adsorptive nature due to structural shielding of the unshared electron pair on the nitrogen heteroatom; (d) 3-aminofluoranthene: catalytic degradation and/or irreversible adsorption of the amino-PAH on alumina; and (e) 2-naphthol: hydroxy-PAH “breakthrough” into the nitrogen-rich fraction. Phenanthrene was included in the standard as a control because of its quantitative, reproducible recovery in the PAH fraction (Az, Figure 3) regardless of the alumina activity. All quantitative information was obtained by capillary gas chromatographic analysis of each alumina fraction. A plot of the relative percentage of each solute that elutes from the Alz03column into the PAH fraction (A2,see Figure 3) as a function of adsorbent activity is shown in Figure 5. Notice that 100% of the phenanthrene that elutes from the column is recovered in fraction Az regardless of activity, while none of the 2-naphthol and 3-aminofluoranthene eluted in the PAH fraction, but were totally recovered in fractions A4 and AB,respectively. Although not shown in Figure 5 , no detectable 2-naphthol eluted into the nitrogen PAC fraction, A,, over the x% HzO-Alz03 range investigated (0-3%). The percentage of the other four compounds eluting in fraction Az was a function of the activity of the alumina. Premature elution, or nitrogen “breakthrough”, of the methylindole and benzoquinoline starts to occur at about 1.7% HzO-Alz03 and precludes the use of higher percent wateralumina for the effective separation of the PAH and N-PAC chemical classes. But, aliphatic hydrocarbon and PAH recovery in the Az fraction is severely reduced and overlap into the N-PAC fraction, A,, is greater than 50% if the activity is less than 1%HzO-Alz03 (see Figure 5), due to the higher adsorptive nature of the more active alumina. The overall recovery of 3-aminofluoranthene served as an indicator of adsorbent-catalyzed degradation and fluctuated as a function of adsorbent activity. The absolute recovery of 3-aminofluoranthene in the A, fraction dropped from 55% at 1.7% HzO-Alz03to 23% at 2.1% H2O-Al2O3. This suggests that as the water content of the alumina is increased, the recovery of 3-aminofluoranthene is decreased due to moisture-adsorbent-catalyzed degradation. In summary, effective
0
05
10
1 5
x%
20
2 5
30
H20-AI203
Figure 5. Plot of percent recovery in fraction A, (see Figure 3) for selected PAC vs. alumina activity (x?h H,0-A1,03): (0)pentacosane, (0)anthanthrene, ( 0 )phenanthrene, (A)benzo[h]quinoline, (A)3methyllndole, (0)3-aminofluoranthene, and (+) 2-naphthol.
chemical class separation on alumina is highly dependent on the activity of the alumina. Additional measures can be taken to enhance chemical class separation of PAC on alumina. For example, if a heavy-end or high boiling point material is analyzed, a 1.5-1.6% H20Alz03can be used and the volume of fraction AI can be increased (normally 20-25 mL) to elute the longer chain hydrocarbons into the proper fraction. A UV monitor can be used to ensure that no PAH (fluorescent material) elute into the aliphatic fraction. On the other hand, if a medium or low boiling point material is analyzed 0.8-1.2% HZO-Al2O3can be used because higher molecular weight aliphatics and PAC are not generally present in these samples. Additionally, the problems of nitrogen- and hydroxy-PAH “breakthrough” normally associated with lower molecular weight PAC can be minimized due to the greater selectivity of the more active alumina. Thermally dehydrating alumina at 400 “C for 8-12 h, and then homogenizing the adsorbent on a shaker for 8 h after adding a weighed amount of water is a tedious and timeconsuming process. Therefore, the possibility of standardizing the moisture content of the alumina adsorbent by simply storing it in an oven at known temperature was investigated. In theory, this process would only partially dehydrate the surface water and result in a much simplified method for preparing the adsorbent for chemical class separation. Several standardized alumina samples ranging in activity from approximately 1 to 5% H20-Alz03 were prepared. The exact activity of these standards was determined by using R” measurements and the calibration curve of Figure 4. Measurements of changes in x% Hz0-A1203 were carried out
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
2983
100.
N ul
c
G
,
,f 50 c
g 5
m
E 0
Time (hrl
Flgure 6. Plot of alumina activity (x% H,O-AI,O,) at 150 "C.
vs. activation time \
0
gravimetrically, as described in the experimental section. Alumina activity as a function of time is plotted in Figure 6 for the various standardized Alz03 samples during calcination a t an oven temperature of 150 "C. Regardless of the initial activity of the starting alumina adsorbents, a t the end of 24 h all the samples were standardized at 0.92-1.20% H20-A1203. In fact, within 2 h a t this temperature, all alumina samples were activated a t 1.0-1.5% Hz0-A1203. By experimentation with different oven temperatures, it was found that other ranges of activity could easily be achieved. For example, storage of A1203a t 100 "C for similar periods resulted in an alumina with between 1.5 and 2.0% water content. This simplified standardization procedure was adopted for preparing Alz03prior to its use for chemical class separations of PAC. No attempt was made to control the humidity level around or in the oven, and the exact temperatures required to standardize the alumina in a chosen activity range, such as 1.0-1.2% HzO-A1203,could vary in different geographic locations due to humidity level variations. Alumina adsorbent standardized by this technique can and should be routinely evaluated by the R" calibration-curve method to ensure that it remains activated within the specified range. Silicic Acid. Silicic acid has a much higher water content than alumina. The Si02adsorbent used in this study ranged from 10-15% HzO by weight as received from the manufacturer. Scott (7) explained the high water content of silicas by showing that there were three distinct layers of physically adsorbed water on the silica surface. As with chemical class fractionation on alumina, the class separation of N-PAC and the elution of AF'AH on silicic acid is dependent on the activity of the adsorbent. The silicic acid optimum activity was determined by using similar procedures as described for the standardization of AlZO3. If the adsorbent was too active (i.e,, over-dehydrated),the APAH were more strongly adsorbed and overlapped into the 3"-PANH fraction (S3, see Figure 3). Conversely, the adsorbent lost selectivity when the moisture content, x% HZ0-SiOz,was too high and the APAH eluted early into fraction SI with the 2O-PANH. These observations are graphically summarized in Figure 7, which is a plot of the recovery of a two- and four-ring APAH in fraction S2 as a function of adsorbent activity. From this plot, the optimum x% H20-Si02 for the complete elution of the APAH into fraction Sz, without overlap into fractions SI or S3, was 5-8%
1
2
3
4
6
5 Y'O
7
8
9
1011
12
13
1415
H~O-SIO~
Flgure 7. Plot of compound percent eluting in fraction S2 (see Figure 3) for I-aminonaphthalene and Gaminochrysene vs. sillcic acid activity (x% H20-SiOZ).
H20-Si02. Under these optimized conditions, a complete separation of 2-5-ring 2O-PANH from the APAH and 3"PANH was observed. This activity range is not as critically narrow as the alumina activity, and standardization of the adsorbent a t this activity was achieved by heated storage a t approximately 50 "C. As with the neutral alumina, an R" calibration curve was constructed and used for routine monitoring of the silicic acid adsorbent activity.
CONCLUSIONS The importance of using standardized alumina and silica adsorbents for chemical class separation has been demonstrated. A protocol for standardizing the activity of neutral alumina and silicic acid adsorbents has been outlined, and a simple, rapid method using an open-column, gravity-flow chromatographic system and an R" calibration curve has been proposed for controlling and routinely monitoring the water content of these two adsorbents. After profiling several PAC standard compounds, it was determined that for optimum class separation, then x% Hz0-Al2O3should be standardized at 1&1.5%, but different activities of alumina should be used depending on the chemical nature of the material to be class separated. The silicic acid optimum activity was found to be 5 4 % H20-SiO2 for class fractionation of 2"-PANH, APAH, and 3"-PANH. An activity of 1.0-1.5% HzO-A1203was easily maintained by storing the adsorbent in an oven a t 150 "C. Likewise, 5 4 % Hz0-SiO2 was obtained by storing the adsorbent a t 50 "C. Registry No. A1,0,, 1344-28-1; SiO,, 7631-86-9; silicic acid, 1343-98-2.
LITERATURE CITED (1) Later, D. W.; Lee, M. L.;Bartle, K. D.;Vassilaros, D.L.;Kong, R. C . Anal. Chem. 1981, 53, 1612-1620. (2) Vassilaros, D. L.; Stoker, P. W.; Booth, G. M.; Lee, M. L. Anal. Chem. 1982, 54, 108-112. (3) Lee, M. L.; Vassilaros, D. L.; Later, D. W. I n t . J . Environ. Anal. Chem. 1982. 1 1 . 251-262. (4) Bartle, K. D i Lee, M. L ; Wlse, S. A. Chem. SOC. Rev. 1981, 10, 113-1 58.
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(5) Karger, B. L.; Snyder, L. R.; Horvath, C. I n “An Introductlon to Separation Science”; Wliey: New ‘fork, 1974. (6) Scott, R. P. W.; Kucera, P. J . Chromatogr. Sci. 1975, 13, 337-346. (7) Scott, R. P. W. J . Chromatogr. Scl. 1980, 18,297-306. (8) Synder, L. R. J . Chromatogr. 1981, 6 , 22-52. (9) Snyder, L. R. J . Chromatogr. 1982, 8 , 319-342. (IO) Snyder, L. R. J . Chromatogr. 1971, 83, 15-44. (11) Popl, M.; Dolansky, V.; Mostecky, J. J . Chromatogr. 1874, 91, 649-658. (12) Hurtublse, R. J.; Allen, T. W.; Sllver, H. F. Anal. Chim. Acta 1981, 126, 225-227. (13) Lindsey, A. J.; Pash, E.; Stanbury, J. R. Anal. Chim. Acta 1956, 15, 291-293. (14) Engelhardt, H.; Wiedeman, H. Anal. Chem. 1973, 45, 1641-1446. (15) Giles, C.H.;Easton, I. A. Adv. Chromatogr. 1988, 3 , 67.
(16) Lee, M. L.; Novotny, M. V.; Bartle, K. D. I n “Analytical Chemistry of Polycycllc Aromatic Compounds”: Academic Press: New ‘fork, 1981. (17) Brockman, H.; Schodder, H. Chem. 8 e r . 1941, 7 4 8 , 73. (18) Brockman, H. DISCUSS.Faraday Soc. 1949, 7 , 58.
RECEIVED for review February 25,1985. Resubmitted August 12,1985. Accepted August 12,1985. This work was supported by the U.S. Department of Energy, Contract No. AC0676RLO-1830, with Pacific Northwest Laboratory, and the Department of Energy, Division of Biomedical and Environmental Research, Contract No. DE-AC02-79EV10237, with Brigham Young University.
Fast Atom Bombardment and Tandem Mass Spectrometry for Characterizing Fluoroalkanesulfonates Philip A. Lyon* 3M, Central Research Laboratories, 201 -BS-05, St. Paul, Minnesota 55144 Kenneth B. Tomer and M. L. Gross Midwest Center for Mass Spectrometry, Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588
A serles of perfluoroalkanesulfonateswere examlned by uslng fast atom bombardment (FAB) Ionization comblned wlth tandem mass spectrometry (MWMS). Both positive and negative ion FAB spectra yleld lnformatlon for determlnlng molecular weight and ldentlfylng counterlons. Abundant parent Ions are desorbed and undergo mlnlmal fragmentation. Structural informatlon Is obtained from the collision actlvated dlssoclatlon (CAD) spectra of selected parent Ions. Comparlsons of colllslonally actlvated decomposltlons are made wlth hydrocarbon analogues. The fluoroalkanesulfonates undergo at least two remote charge slte fragmentations. The more facile Is loss of C,Fzn+, followed by losses of a perfluoroalkene. A second, less abundant serles of fragments Is formed by losses of the elements of CnF2n+z,a process that may be analogous to the parallel ellmlnallons of the elements of C,H,, +2 from carboxylates and alkyl sulfates. Perfluoroalkanesulfonates containing a single hydrogen atom have also been determlned by uslng FAB MS/MS and their fragmentation pathways elucidated. The comblnatlon of FAB and MS/MS should be useful for analysis of mixtures of fluorinated surfactants.
Perfluoroalkanesulfonates have considerable commercial interest as anionic surfactants ( 1 ) . As is characteristic of most industrial surfactants, they are provided as complex mixtures. Extensive purification is normally not possible nor necessary for the compounds to satisfy the requirements of most commercial surfactant applications. Nevertheless, their properties as surfactants depend on the nature of the mixture, and characterization methods are required. But as for other surfactants, their ionic nature increases considerably the problems of separation and analysis. Perfluoroalkanesulfonates may have other uses as mass calibrants in mass spectrometry. Perfluoroalkane mixtures are well-known calibrants for positive electron ionization mass spectra. However, the requirements of mass standards have become more demanding in order to cope with the enhanced
abilities of new mass spectrometers equipped with desorption ionization and mass analyzers which can deal with molecules up to and greater than 10000 amu. The perfluorotriazines satisfy the requirement up to approximately mass 2000, and they can be used for both electron ionization and desorption ionization. To press beyond mass 2000, inorganics such as CsI have been extensively used as calibrants for fast atom bombardment (FAB) mass spectra. Recently, Heller et al. (2) and Roberts and White (3) demonstrated the ability of perfluoroalkanesulfonates to be desorbed as high mass clusters under conditions of FAB and field desorption (FD), respectively. Hydrocarbon sulfonates were reported earlier (4)to desorb as L,M,+,+ clusters, where L is the alkylsulfonate and M is a counter metal ion; so the phenomenon of clustering is not surprising. What is significant is the higher abundance high mass ions produced from the fluoroalkanesulfonates as compared to CsI. Apart from the ion clustering exhibited by the fluoroalkanesulfonates, little attention has been paid to other mass spectral features of these compounds. Previously, we have demonstrated the power of FAB combined with tandem mass spectrometry (MS/MS) for characterizing both anionic (4) and cationic (5) surfactant mixtures. In this paper we report extending the combined technique to the fluoroalkanesulfonates. The fragmentations of collisionally activated fluoroalkanesulfonates are deciphered and compared with the decomposition reactions of the hydrocarbon analogues.
EXPERIMENTAL SECTION Chemicals used in this study were obtained from 3M Commercial Chemicals Division but most are commercially available from other sources (e.g., PCR Research Chemicals, Gainesville, FL, or Fluka Chemical Corp., Hauppauge, NY). The compounds were used without further purification. The purity of these materials is discussed in the Results section of this paper. The mass spectra were obtained with a Kratos MS-50 triple analyzer mass spectrometer (6). The instrument is comprised of a high-resolutionMS-I (a standard Kratos MS-50) follwed by an electrostatic sector, MS-11. A standard Kratos FAB source equipped with an Ion Tech atom gun was used. The samples were dissolved in glycerol or triethanolaminefor analysis. A small drop
0003-2700/85/0357-2984$01.50/00 1985 American Chemical Society
