1488
Anal. Chem. 1981, 53, 1488-1492
Hollow Fiber Ion-Exchange Suppressor for Ion Chromatography Timothy S. Stevens” and James C. Davis Dow Chemical USA, Michigan Division Analytical Laboratories, 574 Building, Midland, Michigan 48640
Hamish Small
Dow Chemical USA, Central Research Laboratories, 17 12 Building, Midland, Michigan 48640
A suppressor for Ion chromatography based on a countercurrent regenerated shell and tube device incorporating sulfonated polyethylene lon-exchange hollow fibers is descrlbed. Compared to the conventional lon-exchange resin bed packed suppressor column, the hollow fiber suppressor allows continuous operailon wlthout varying Interference from base line dips, ion-exclusion effects, or chemlcal reactlons. Conventional suppressor columns were found to have less band spreadlng and thls resulted in slightly poorer resolution of early eluting ions with the use of the hollow flber suppressor.
Ion chromatography is widely used for the determination of organic and inorganic ions in wastewaters, process streams, rainwaters, air samples, and food products (1). It is a unique analytical system using two ion-exchange columns in series followed by a flow-through electrical conductivity detector. The first column separates the ions in the injected sample while the second column suppresses the conductance of the electrolyte in the eluant while enhancing the conductance of the separated anions since they are detected as the acid moiety (2). The second column, called the “stripper” or the “suppressor”, complicates the use of ion chromatography relative to the practice of conventional liquid chromatography. These complications include the need to periodically replace or regenerate the suppressor column, varying elution time of weak acid anions or weak base cations due to ion-exclusion effects (3) in the unexhausted portion of the suppressor column (2), apparent reaction of some ions such as nitrite with the unexhausted portion of the suppressor column resulting in varying response depending on the percent exhaustion of the suppressor column (4),and interference from the varying elution time of a characteristic base line upset (5). Finally, there is some band spreading in the suppressor column. The ion-exchange resin packed suppressor column is not the only alternative for suppressing the conductivity of the eluant electrolyte. We have discovered that ion-exchange hollow fibers can be used in a suppressor device that eliminates most of the problems of the conventional suppressor column.
EXPERIMENTAL SECTION The ion chromatograph used was a preproduction prototype of the Model 10 unit available from Dionex Corp., Sunnyvale,CA. The eluant used was 0.003 M NaHC03and 0.0024 M Na2C03at a flow rate of 64 mL/h and a pressure of about 300 psig at room temperature. The analytical column was 2.8 X 100 mm fiied with a proprietary resin resulting in equivalent separations as the 2.8 X 500 mm Dionex column but at half the eluent flow rate. The injection loop volume was 50 pL. When suppressor columns were used they were either 2.8 X 300 mm or 9 X 110 mm filled with Dowex 50 WX16 ion-exchange resin, 200-400 mesh size, and were regenerated off-line with 1 N HzSO4. When the hollow fiber suppressor was used, the regeneration pump was used to continuously supply 0.02 N H2S04, at a flow rate of 64 mL/h. A “seven ion standard” was prepared from reagent grade salts
containing 3.3 ppm F-,4 ppm C1-, 20 ppm NOz-, 54 ppm PO4“, 10 ppm Br-, 34 ppm NO,, and 50 ppm S042-. The low-density polyethylene hollow fibers used were extruded from Dow no. 4005 resin to a size of about 300 pM i.d. and about 380 pM 0.d. and were sulfonated using 10% chlorosulfonic acid, 90’ methylene chloride, v/v, at about 42 “C (the reflux temperature of the mixture) for 30 min. The ion exchange capacity of the fibers was 1mequiv/g. A bundle of eight hollow fibers 6 ft in length was used. The hollow fiber suppressor was constructed using a coiled section of 2 mm id., 1/8 in. o.d., 316 stainless steel tubing as shown in Figure 1. The high-pressure liquid Chromatography(HPLC) end fittings were used without frits and the l/ls in. bore of the fitting on the inside end was chamfered with a twist drill to about 2.5 mm so as not to block the flow of fibers positioned on the side of the bore of the sealing tube. The hollow fibers were sealed in the sealing tube with Dow Corning room temperature vulcanizing silicone rubber bathtub caulk.
RESULTS AND DISCUSSION The theory of operation of the hollow fiber suppressor used for the determination of anions with an eluant electrolyte of sodium carbonate is shown in Figure 2. The sulfonated polyethylene fiber wall allows sodium ion to permeate out and hydrogen ion to permeate in. Carbonate ions and sulfate ions do not permeate the fiber wall because of Donnan exclusion forces (6). Sample anions also do not permeate the wall while the cations do. Thus, the highly conducting sodium carbonate of the eluant is converted to feebly conducting carbonic acid and the sample anions emerge with enhanced conductivity because they elute in the acid form. The sulfuric acid regenerant is converted to sodium sulfate. Several limitations are seen with the system. If the flow rate of sodium carbonate solution through the device i s too fast, there will not be enough time for sodium ion to diffuse from the center of the fiber flow stream to the fiber wall and incomplete suppression will occur. The device reported here could suppress a maximum eluent flow rate of 80 mL/h. The number of mequiv/min of sodium ion into the device must be matched by at least as many mequiv/min of hydrogen ion flowing countercurrent around the outside of the fibers. Some excess of hydrogen ion is usually required. The device used in this work required at least a 10% excess of regenerant. Donnan exclusion forces are not always sufficient to prevent “leakage” of regenerant into the eluent stream, especially a t relatively high regenerant concentration. However, no detectable leakage of regenerant was seen with 0.02 N H2S0b Comparison with Conventional Suppressor Columns. Two suppressor columns were compared to the hollow fiber device. The first was 2.8 mm X 300 mm containing about 1.8 mL of resin. This is a small suppressor column having a lifetime, under the conditions listed, of about 6 h. The second suppressor column tested was 9 mm X 110 mm containing about 7 mL of resin. This larger column has a lifetime of about 24 h. The most common eluant used for the determination of anions in ion chromatography contains 0.003M NaHC03 and
0003-2700/81/0353-1488$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 HPLC Column End Fitting
1489
Eluent Outlet
Silicone Rubber
1 1 1 6 Tubing \
Center Tube
I
Eluent Inlet
1116Tubing
[Regenerant Outlet 1116Tubing
Regenerant Inlet
1116"Tubing
1
Schematic drawing of a hollow fiber suppressor.
Flgure 1.
r
Na>CO,
H2C%
Flgure 2. Theory of operation of
the hollow fiber suppressor
0
5
10
20
15
6
0
M,"","
Flgure 4.
haustion.
0
0
2.5
5
0
2.5
5
7.5
0
2.5
10 M#""t*
7s
m
Response variations obsewed with suppressa column ex-
5
Minutes
Chromatographicresponse to an injection of water with the use of suppressor columns and the hollow fiber suppressor. Flgure 3.
0.0024 M Na2C08in deionized water. The suppressor converts these eluant electrolytes to carbonic acid which has little conductivity because of its weak acid character. When an injection of deionized water containing no anions is made, two negative peaks are seen in the chromatogram. The first, called the water dip, is related to the plug of injected water moving though the system with eluant of slightly higher conductivity on either side of it. The second, called the carbonate dip, is larger and is the vacancy chromatogram for the carbonate not in the injected sample but present in the eluant. In other words, carbonic acid elutes through the unexhausted portion of the suppressor column a t a reduced rate because of ionexclusion effects. Figure 3 shows the water dip and the carhonate dip for the two freshly regenerated suppressor columns and the hollow fiber device. The base line for the 9 X 110 mm suppressor was unsteady because it had not "aged. Large suppressor columns require many hours of rinsing after regeneration, depending. on the hatch of resin used, to wash out leachables, and this one gave a steady base line the following day. Note that the larger suppressor's water dip came later due to increased void volume
0
5
10
15
20
Chromatogram of the seven ion standard using the small suppressor wlumn.
Flgure 5.
and that the separation between the water dip and the carbonate dip was much greater for the larger suppressor because of increased ion-exclusion effects. Finally, both dips were combined in the hollow fiber suppressor hecause of the absence of ion-exclusion effects. The carbonate dip elution time approaches the water dip elution time with the use of a suppressor column as the resin bed exhausts. This can affect the response factor of ions meluting with the variable carbonate dip. Figure 4 shows this effect on an injection of a mixture of seven anions with a freshly regenerated 9 X 110mm stripper and when this column was about half exhausted. In the first chromatogram, the carbonate dip eluted at about 6 min, coeluting with NO;. In the second chromatogram, the carbonate dip eluted at ahout
1490
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
column
x
Example iromarogram iith holiow ber suppressor
column
cetate
Acetate
S0,'Z
n
a I 5
I
I
J
10 Minutes
15
20
Figure 6. Chromatogram of the seven ion standard using the hollow fiber suppressor.
0
2.5
5
0
2.5
5 Minutes
7.5
0
2.5
5
Figure 7. Chromatographic response to an injection of acetate with the use of suppressor columns and the hollow fiber suppressor.
4 min, coeluting with C1-, resulting in a larger response for NO2- and a smaller response for C1-. Figure 5 shows the chromatogram of the same seven ions using a freshly regenerated 2.8 X 300 mm suppressor column. Here the carbonate dip eluted between P and C1- and would migrate under F- as the suppressor exhausted. Figure 6 shows the chromatogram of the same seven ions using the hollowfiber suppressor. The carbonate dip coeluted with F- but its effect is easier to cope with because it is constant since the hollow fiber suppressor does not exhaust with use. When the chromatograms in Figures 4-6 were compared, the band spreading with the hollow fiber device as most apparent in the peak width and height for F- was greater than with the suppressor columns. The band spreading of each suppressor was quantitated by removing the analytical column from the system, changing the eluent to deionized water, and injecting a sample. The resultng peak was then triangulated and the peak width at base line measured and converted to microliters. The injection volume was then subtracted from this value and the result used as an indication of the band spreading of the system without the effect of the analytical column. The band spreading of each suppressor was 130 pL, 300 p L and 525 pL, respectively, for the 2.8 X 300 mm suppressor, the 9 X 110 mm suppressor, and the hollow fiber suppressor. The increased band spreading with the use of the hollow fiber suppressor has a deleterious effect on ions eluting early in the chromatogram. The effect is not noticed for the broader peaks of later eluting ions. The determination of weak acid anions, such as acetate, using a suppressor column is complicated by ion-exclusion effects in the unexhausted portion of the column. Figure 7 shows chromatograms for the injection of a NaAc standard using the 2.8 X 300 mm suppressor, the 9 X 110 mm suppressor, and the hollow fiber device. Both suppressor columns, and especially the 9 x 110 mm one, broaden and retard the acetate peak due to ion exclusion effects. As the columns exhaust, these ion-exclusion effects decrease, resulting in sharper and taller peaks that elute sooner. The use of the hollow fiber device theoretically could have resulted in the loss of the acetate response (and there may indeed be some loss) since potonated acetate is not rejected from passage through the fiber wall. However, Figure 7 clearly demonstrates the applicability of acetate determination using a hollow fiber suppressor.
0
5
10 15 ppm chloride
20
25
Figure 8. Calibration curves for chloride. 2 8 x 300 mm suppressor column A
0
9 x 110 m m suppressor column
10
-
20 30 ppm bromide
40
50
Figure 9. Calibration curves for bromlde.
Comparisons were made between the use of the suppressor columns and the hollow fiber device to determine response linearity for chloride, bromide, and acetate (see Figure 8-10). The data in Figure 8 indicated that the use of both the 2.8 x 300 mm and hollow fiber suppressors resulted in increased response factors with increasing concentration of chloride. The linearity with the 9 x 110 mm suppressor was good. The lower response with the hollow fiber device was caused by the broader chloride peak as a result of band spreading in the hollow fiber device. Although the linearity of the 9 X 110 mm suppressor was good, the data in Figure 4 indicate that as the 9 x 110 mm column exhausts and the carbonate dip begins
ANALYTICAL CHEMISTRY, VOL. 53, NO.
9, AUGUST 1981
Table I. Response Variations Observed during the Lifetime of the 2.8 mm X 300 mm Suppressor Column elapsed ambient time since peak height in chart units for teomp, regeneraC tion, h Fc1NO,Po;BrNO,so:77.0
28.7
10.5
34.0
9.5
26.0
42.2
75.0 76.2
29.0 31.0
10.0
33.5 34.5
9.2 9.5
25.0 26.5
42.0 43.0
77.0 78.8 79.0
32.5 32.0 31.0
17.0 22.2
35.5 35.5 35.7
10.2 10.2 10.0
27.5 27.2 27.2
42.8 43.2 43.0
79.0 79.0
32.0 32.0
27.2 30.5
36.5 36.8
11.2 11.2
29.6 29.8
44.3 44.0
32.0
8.1
81.5 83.5
31.8
11.6 11.2
30.5 30.0
45.0 44.7
0.2 0.5
72.0 73.0
31.8
11.4 11.0
30.5 30.0
44.5 44.2
22.0 22.0 22.0 22.0 22.0 22.0 22,o 22.0 22.0 22.5 22.5 22.0 22.5 22.5
0.3 0.7 0.8 1.5 2.5 3.2 4.1 5.1 5.5 6.2 6.8 7.3 7.7
22.5 22.5 22.5
31.8
12.0 18.8
33.8 36.5 35.5 36.2 Regenerated Suppressor 10.8 11.5
37.5 37.0
21.0
21.0 21.5 21.5 21.5 22.0 22.0 22.0 22.0 22.0
Ac29.8 34.0
51.0 76.0
31.0
0.8
Table 11. Response Variations Observed during the Use of the Hollow Fiber Suppressor peak height in chart units for ambient elapseld temp, “C time, Ih Fc1NO,PO,,BrNO,20.5 21.0
0.3 1.0
1.5 2.0 2.5 3.0 4.2 4.5 5.5 6.0 6.6 7.1
1491
so4,-
34.2
11.2 11.0
28.8 29.0
45.2 45.0
10.5 10.8 10.6
35.6 36.0 36.0
11.5 11.0 11.6
30.5 30.0 31.2
45.5 47.0 46.0
23.0 23.5
10.2 9.5
35.5 36.5
11.1 11.8
30.0 32.0
46.5 47.2
23.8
10.0
36.5
11.0
30.0
46.0
29.0 28.8
24.3 22.0
12.0 11.6
33.8
28.8 29.2 29.8
22.2 22.6 22.8
29.6 30.5 30.6
to coelute with chloride, the responsivenessof chloride would change significantly. The data in Figure 9 indicate that the use of any of the suppressors tested would1 result in linear response to bromide. The data in Figure 10 indicate that the responsiveness of acetate falls off a t higher concentration with the use of the 2.8 X 300 mm suppressor. This is expected because of the weak acid nature of acetic acid. the linear responsiveness with the use of the hollow fiber suppressor a t the higher concentrations of acetate was fortunate but cannot be readily explained. The hollow fiber suppressor calibration curve for acetate did not pass through the origin of the plot because acetate coeluted with the carbonate dip. The final comparison made was to inject the seven-anion standard and the acetate standard periodically during the lifetime of the 2.8 X 300 mm “small” suppressor column and for a comparable length of time using the hollow fiber suppressor to observe response variations with time (see Tables I and 11). The data in Table I indicate a general instrumental upward drift in the response of C1-, PO4%,Br-, NO”, and SO?not related to exhaustion of the suppressor column as evidenced by the response continuity for these ions when the column was regenerated. Acetate and P response, measured by peak height, increased with suppressor column exhaustion. However, the acetate arid F- responses remained constant when response was measured by peak area using a planimeter. In contrast, both the peak height and peak area response of NOz- varied by a factor of about 3 with exhaustion of the suppressor column. Nitlrite forms nitrous acid in the unex-
Ac 42.8
43.8 44.2 43.5
‘r 2.8 x 300 mm Suppressor column Hollow fiber suppressor
0
20
40
60
80
100
Ppm Acetate (as sodium acetate)
Figure 10. Calibration curves for acetate.
hausted portion of the suppressor column and nitrous acid reacts with the resin affecting response depending on suppressor column exhaustion. The data in Table I1 indicate that the use of the hollow fiber device results in stable response from all of the ions tested. Thus, the significant advantage of the hollow fiber suppressor with regard to calibration is that
1402
Anal. Chem. 1981, 53, 1492-1497
responsivenessis not a function of suppressor exhaustion but remains reasonably constant with use. CONCLUSION The replacement of the conventional ion-exchange resin bed suppressor column with the hollow fiber suppressor allows continuousoperation of an ion chromatographwithout varying interference from base line dips, ion-exclusion effects, or chemical reactions. Future work needs to be directed at reducing the band spreading of hollow fiber suppressors, at the development of aminated membrane suppressors for cation determination using ion chromatography and at easier means of fabricating hollow fiber suppressors.
LITERATURE CITED (1) Maugh 11, T. H. Science I980 208, 164. (2) Small, H.;Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 47 1801. (3) Wheaton, R. M.; Bauman, W. C. Ind. Eng. Chem. 1953, 45, 229. (4) Koch, W. F. Anal. Chem. 1979, 57, 1571. (5) Wetzel, R. A.; Anderson, C. L.; Schleicher, H.; Crook, G. D. Anal. Chem., 1979, 57, 1571. (6) Heftman, E. “Chromatography”, 2nd ed.; Reinhold: New York, 1976; p 296.
RECEIVED for review March 2, 1981. Accepted May 8,1981. A United States Patent application has been filed on behalf of the authors covering the subject of this contribution.
Laser Ionization Mass Spectrometry of Nonvolatile Samples E. D. Hardln’ and M. L. Vestal” Department of Chemistty, Universlty of Houston, Houston, Texas 77004
A new laser Ionization mass spectrometer has been developed whlch employs a movlng stainless steel belt onto whlch the sample Is electrosprayed for continuous sample Introduction. Ionization Is produced by focusing the output of a tunable dye laser onto the movlng belt. The mass spectrometer system Is a conventlonal quadrupole system, wlth the exceptlon of a gated boxcar integrator to process the pulsed Ion beam. Mass spectra have been recorded for a number of nonvolatlle blomolecules Including saccharides, amlno aclds, peptldes, nucleosldes, and nucleotldes. Generally these spectra show intense catlonlzed molecular Ions, often Includlng multiple alkall addltlon, and little fragmentatlon. The major llmltatlon on the technlque at present Is the rather poor reproduclbllity of the spectra. Ion tlme-of-flight dlstrlbutlons have been measured whlch show that Ions produced by laser desorptlonAonlzation have broad kinetlc energy distrlbutlons wlth most probable kinetic energles of about 6 eV and wlth hlgh-energy tails extendlng beyond 25 eV. The time-of-flight dlstrlbutlons also show that most of the hlgh mass ions observed result from metastable decomposltlon of larger clusters formed lnltially at the surface.
In a number of areas of organic and biochemical research there is a growing need for high mass, high sensitivity mass spectrometry applicable to thermally labile molecules of low volatility (I). In particular, it is often very important to obtain easily identifiable ions characteristic of the intact molecule so that the molecular weight can be determined. In response to this need, several new techniques have been developed (2). These include field desorption (3), chemical ionization (4), plasma desorption (5), laser desorption (6),organic SIMS (7, 8), and very rapid sample heating with “in-beam’’ chemical (9-11) or electron ionization (12-14). In our own laboratory we have recently discovered a new soft ionization technique which employs thermal production of a charged macroscopic particle beam containing the sample. This technique employs very rapid heating of a liquid solution to form a particle beam
R. A. W e l c h F o u n d a t i o n Predoctoral Fellow. 0003-2700/81/0353-1492$01.25/0
and impact of this particle beam on a mildly heated surface (15).
Several of these techniques, such as field desorption and chemical ionization, both conventional and “in-beam”, have been relatively well characterized, and the apparatus is commercially available; however, with the exception of gas-phase chemical ionization, it appears that none are satisfactorily understood. Recent work employing these new ionization techniques has demonstrated that gas-phase ions characteristic of the intact molecule can be produced for previously intractable molecules. Despite the enormous advances of recent years many problems are not yet solved, and none of these new techniques has, as yet, achieved wide-spread acceptance. The present research was undertaken to explore the feasibility and practicality of laser desorption and ionization for mass spectrometric analyses of nonvolatile organic molecules adsorbed on, or contained in, solid surfaces. In our view, a practical technique should provide both molecular weight and structural information on a wide range of nonvolatile and/or thermally labile compounds; it should be compatible with conventional rapid scanning mass analyzers, both magnetic and quadrupole;and it should be suitable for combination with the techniques commonly used for separating and purifying mixtures of involatile compounds such as liquid (LC) amd thin-layer chromatography (TLC). Most of the earlier laser desorption studies have used some form of simultaneous ion detection, employing either timeof-flight analysis (16), electrooptical ion detection, or photoplate (17). Recently, the use of fast electrical scanning with laser desorption from a direct exposure probe in a chemical ionization source has been reported by Cotter (18). The use of repetitive pulsed laser desorption with magnetic scanning over a limited region of the spectrum has been reported by Heresch, Schmid, and Huber (19). The use of laser desorption using a moving belt inlet system with a tightly enclosed chemical ionization (CI) source and a quadrupole mass spectrometer has been reported by Hunt, Bone, and Shabanowitz (20). In the present work we have developed a new laser desorption mass spectrometer which uses a moving belt system to continuously supply fresh sample to a fully open laser vaporization/ionization region. The mass spectrometer is a 0 1961 American Chemical Society
