ple of the reaction mixture was then injected directly into the liquid chromatograph. On the other hand, pure derivatives of individual acids were made and isolated by crystallization. Synthetic mixtures were then made of these crystalline solids. A comparison of chromatograms of a real vs. a synthetic mixture of derivatives showed no significant deviations. This indicates that the derivatization reaction is quantitative with no significant interacid interference. This is especially important for the analysis of naturally occurring fatty acid mixtures. The a,p- dibromoacetophenone derivatives of fatty acids, catalyzed by crown ethers, show excellent promise in analytical biochemistry. T h e ease of preparation, using a range of solvents and quantitative yields, makes this derivatization technique useful in a broad variety of situations dealing with fatty acids and other organic acids. Work is now being performed to evaluate the method for dibasic acids and other biologically important compounds containing acidic functionality. LITERATURE CITED (1) J. J. Myher, L. Maras, and A. Kaksis, Anal. Biochem., 62, 188 (1974). (2) D. H. McMahon and E. P. Crowell, J. Am. Oil Chem. Soc., 51, 522 (1974). (3) J. M. Timko, R . C. Helgeson, M. Necomb, G. W. Gokel, and D. J. Dram, J. Am. Chem. Soc., 96, 7100 (1974). (4) G. A. E. Arvidson, J. Chromatogr., 103, 201 (1975). (5) C. R. Scholfield, J. Am. Oil Chem. Soc., 52, 36 (1975). (6) F. A. Fitzpatrick and S. Siggia. Anal. Chem., 45, 2310 (1973). (7) F. A. Fitzpatrick, S. Siggia, and J. Dingman, Sr., Anal. Chem., 44, 2211 ( 1972). (8) R . A. Henry, J. A. Schmit. and J. F. Diekman, J. Chromatogr. Sci., 9, 513 (1971). (9) L. J. Papa and L. P. Turner, J. Chromatogr. Sci., 10, 747 (1972). (10) M. A . Carey and A. F. Persinger, J. Chromatogr. Sci., 10, 537 (1972). (11) P. J. Porcaro and P. Shubiak, Anal. Chem., 44, 1865 (1972). (12) R . W. Frei and J. F. Lawrence, J. Chromatogr., 83, 321 (1973).
(13) R. W. Frei, J. F. Lawrence, J. Hope, and R . M. Cassidy. J. Chromatogr. Sci., 12, 40 (1974). (14) R. P. Michael, R. W. Bonsaii, and P. Warner, Science (London), 186, 1217 (1974). (15) N. Nicoiaides, Science(London), 186, 19 (1974). (16) N. D. Cheronis. J. B. Entrikins, and E. M. Hodnett, "Semimicro Qualitative Organic Analysis", Interscience. New York, N.Y., 1965. (17) I. R . Politzer, G. W. Griffin, B. J. Dowty. and J. L. Laseter, Anal. Lett., 6, 539 (1973). (18) Regis Lab Notes, No. 16, August 1974. (19) J. P. Hendrickson and C. Kandall, Tetrahedron Lett., 343 (1970). (20)M. J. Cooper and M. W. Anders, Anal. Chem., 46, 1849 (1974). (21) C. J. Pederson, J. Am. Chem. Soc., 89, 7017 (1967). (22) C. J. Pederson, J. Am. Chem. Soc., 92, 391 (1973). (23) C. J. Pederson, Fed. Proc., Fed. Am. SOC.Exp. Bioi., 27, 1305 (1968). (24) C. J. Pederson and H. K. Frensdorff, Angew. Chem., lnt. Ed. Engi., 11, 16 (1972). (25) J. J. Christenson, J. 0. Hill, and R. M. izatt, Science (London). 174, 459 (1971). (26) G. Gokel and H. D. Durst, Synthesis, in press (1975). (27) D. J. Sam and H. E. Simmons, J. Am. Chem. Soc., 94, 402 (1972); 96, 2252 (1974). (28) C. L. Liotta and H. P. Harris, J. Am. Chem. Soc., 96, 2256 (1974). (29) H. D. Durst, J. W. Zubrick, and G. R. Kieczykowski, Tetrahedron Leff., 1777 (1974). (30) H. D. Durst, J. W. Zubrick. and B. I. Dunbar, Tetrahedron Left., 71 (1975). (31) D. T. Sepp, K. V. Scherer, and W. P. Weber, Tetrahedron Leff., 2983 (1971). (32) D. Landin, F. Montanari. and F. M. Pirisi, Chem. Commun., 879 (1974). (33) C. W. Bowers and C. L. Liotta, J. Org. Chem., 39, 3416 (1974). (34) M. Makoza and M. Ludwikow, Angew. Chem., 86, 744 (1974). (35) H. D. Durst, TetrahedronLett., 2421 (1974). (36) H. D. Durst, M. M. Mark, D. Dehm, and R . Banden, Tetrahedron Lett., in press (1975). (37) C. L. Liotta, H. P. Harris, M. McDermott, T. Gonzalez, and K. Smith, Tetrahedron Lett., 2417 (1974). (38) E. Grushka and E. J. Kikta, Jr., Anal. Chem., 46, 1370 (1974). (39) G. W. Gokel, D. J. Cram, C. L. Liotta, H. P. Harris, and F. L. Cook, J. Org. Chem., 39, 2445 (1974). (40) R . N. Green, Tetrahedron Lett., 1793 (1972). (41) L. C. Craig, lnd. Eng. Chem., Anal. Ed., 12, 775 (1940). (42) F. Mikes, V. Schurig, and E. Gil-Av, J. Chromatogr., 83, 91 (1973).
RECEIVEDfor review March 17, 1975. Accepted June 11, 1975.
Novel Ion Exchange Chromatographic Method Using Conductimetric Detection Hamish Small Central Research, The Dow Chemical Company, Midland, MI 48640
Timothy S. Stevens Michigan Division Analytical Laboratories, The Dow Chemical Company, Midland, MI, 48640
William C. Bauman Inorganic Process Research, Texas Division, The Dow Chemical Company, Freeport, TX 7754 1
Ion exchange resins have a well known ability to provide excellent separation of ions, but the automated analysis of the eluted species is often frustrated by the presence of the background electrolyte used for elution. By using a novel combination of resins, we have succeeded in neutralizing or suppressing this background without significantly affecting the species being analyzed which in turn permits the use of a conductivity cell as a universal and very sensltive monitor of all ionic species either cationic or anionic. Using this technique, automated analytical schemes have been devised for Li', Na+, Kf, Rbf, Cs+, NH4+, Ca2+, Mg2+, F-, CI-, Br-, I-, NO3-, NOz-, Sod2-, S032-, Pod3- and many amines, quaternary ammonium compounds, and organic
acids. Elution time can take as little as 1.0 miniion and is typically 3 min/lon. Ions have been determined in a diversity of backgrounds, e.g., waste streams, various local surface waters, blood serum, urine, and fruit juices.
The demand for the determination of ionic species in a variety of aqueous environments is increasing rapidly and, as a result, there is an expanding need for automated or semiautomated analysis of chemical plant streams, environmentally important waters such as waste streams, rivers, and lakes, and fluids of biological interest such as blood, urine, etc. There are many examples where there is a continual need for routine analysis of common species such
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
1801
R + OH-
Figure 1.
System for cation analysis by conductimetric chromatog-
raphy as Li+, Na+, K+, NH4+, Ca2+, Mg2+, F-, C1-, Br-, I-, S042-, NOn-, Nos-, Po43-, etc. Ion exchange resins have a well known ability to provide excellent separations of ionic species and there are a number of instances where ion exchange chromatography has been successfully applied ( I ). In recent years, however, liquid’ chromatography has moved in the direction of high speed separations and continuous effluent monitoring by detector-analyzer systems which can yield almost instant readout of analytical data. Consequently, in light of this present day practice, the usefulness of a chromatographic separation is often measured by the extent to which it can be coupled to such continuous detectors. If the ions eluting from an ion exchange column have some property which distinguishes them from the background electrolyte, e.g., absorption in the UV or visible range, then the solution to the analytical problem is fairly straightforward and many modern high speed chromatographic procedures have been developed using ion exchange resins in conjunction with spectrophotometric detectors. However, the analysis of certain ionic species is frustrated by the presence of the background electrolyte in that available detectors are not able to detect the species of interest against this background and i t has been stated (2) that this limitation in detectors is one of the main factors retarding a more widespread penetration of ion exchange into modern chromatographic methodology. The detector problem is exemplified by the case of conductimetric detection which has often been proposed and occasionally applied with very limited success in ion exchange chromatography. I t would be desirable to employ some form of conductimetric detection as a means of monitoring ionic species in a column effluent since conductivity is a universal property of ionic species in solution and since conductance shows a simple dependence on species concentration. However, the conductivity from the species of interest is generally “swamped out” by that from the much more abundant eluting electrolyte. We have solved this detection problem by using a combination of resins which strips out or neutralizes the ions of the background electrolyte leaving only the species of interest as the major conducting species in the effluent. This has enabled us to successfully apply a conductivity cell and meter as the detector system. T h e Principle of the Method. The technique employs the following train of columns, etc. (A) An eluant reservoir, (B) a pump, (C) a sample injection device, (D) the separating column wherein the species are resolved by convention1802
al elution chromatography followed by the eluant stripper column (referred to henceforth simply as the stripper column or stripper) wherein, as the name implies, the eluant coming from the separating column is stripped or neutralized. Thus, only the species of interest leave the bottom of the stripper in a background of de-ionized water where they are monitored by the conductivity cell/meter/recorder (integrator) combination. Some general principles for choosing useful eluant-separation-stripper column combinations will be outlined later but an understanding of the technique may best be gained a t this stage by giving specific details of how the method is applied. Consider, for example, the analysis of a sample containing Li+, Na+, and K+. The eluant, in this case dilute HC1, is pumped to the two columns in tandem (Figure 1) which contain a cation exchanger in the separating column and a strong base resin, OH- form, in the stripper column. If a sample containing Li+, Na+, and K+ is injected a t the head of the first column, the ions will be resolved in the separating bed and will exit a t various times from the bottom of this column in a background of HCl eluant. On entering the stripper column two important reactions take place: HCl is removed by the strong base resin
-
HC1 + Resin OHResin C1- + H,O (1) The alkali metal chlorides are converted to their hydroxides M’C1- + Resin OHM’OH- + Resin C1(2) which pass unretarded through the stripper column and into the conductivity cell where they are monitored and quantified in a background of de-ionized water by either measuring the height of, or area under, the conductivity peak. An analogous scheme for anion analysis can be envisaged where sodium hydroxide is the eluant, an anion exchanger is in the separating column, and a strong acid cation exchanger in the H + form is used as stripper. An important way in which this method differs from most, if not all chromatographic methods, is in the need t o regenerate the stripper. Too frequent a need for this step would clearly be a drawback, so an important feature of this technique is the means which have been devised to render this regeneration step as unobtrusive as possible. The problem of stripper regeneration can be expressed in the form of a question, namely, how many samples can be analyzed by the system before stripper regeneration becomes necessary? The answer can in turn be approximately represented in the form of a simple equation, viz.,
-
VBCB ( 3) VACAK Xiy* where N = number of samples injected during the stripper’s lifetime (assuming maximum utilization of the available time), V, = volume of separating bed, VB = volume of stripper bed, CA = specific capacity of the separating bed, CB = specific capacity of the stripper bed, K p * * = selectivity coefficient (relative to the eluting ion y’) of the ion x * , which, in a series to be analyzed, has the greatest affinity for the separating resin. T o obtain a high chromatographic efficiency it is necessary to keep VB/VA as low as possible; otherwise the resolution obtained in the separation bed will be offset by mixing in the relatively massive void volume of the stripper bed. A value of VBlV.4 close to unity is excellent but a value of 10 or less would be acceptable. In order, therefore, that N be as large as possible it is necessary that the quantity c B / C A K p X *be kept, within certain limits, as large as possible. This can be achieved by 1) Maintaining CB as large as possible, i.e., by using conventional resins of a high degree of
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
N =
crosslinking. 2 ) Maintaining CA as small as possible. This has been achieved by using specially prepared resins of very low capacity. However, a lower limit on the capacity of the separating resin is set by the need to avoid overloading of this column by the sample injected. 3) Maintaining K p X i as small as possible. The options in the choice of eluting ion y* are limited by the further requirement that this ion be amenable to removal or neutralization in a stripping reaction. Nor should the affinity of the eluting ion y* for the separating resin be too high (Ky*x*too small) or the species of analytical interest will elute too rapidly with lack of resolution. Closely tied to point 3 is the requirement that the ions of interest not undergo any neutralization or removal reactions in the stripper column. Selecting eluting ions or resins to satisfy one of these criteria is a simple matter but the choice of those which fit all three is limited. Nevertheless a number of schemes have been proposed and are summarized in Table I. H a r d w a r e . The pump used was Milton Roy minipump with a maximum pumping speed of 160 or 460 ml/hr. The columns were obtained from Chromatronix Inc., (Now Laboratory Data Control, a division of Milton Roy Company) Berkeley, CA-the sizes used will be identified in the individual examples quoted later. A Chromatronix conductivity cell was used as detector in conjunction with a conductivity meter designed and built in the Physical Research Laboratory of The Dow Chemical Company. More recently we have used commercially available conductivity meters. The output from the conductivity meter, which is proportional to the conductivity of the sample in the cell was expressed on a strip chart recorder to provide chromatograms. Samples were injected to the columns by means of a Chromatronix sample injection valve and a pressure gauge T-ed off ahead of this valve served as a depulser. Resins a n d Eluants. The discussion of the methods for preparing the special resins will be taken u p at appropriate places later in the paper. Eluant solutions were prepared from deionized water and reagent grade chemcials. CATION ANALYSIS T h e S e p a r a t i n g Resin. A resin of very low cation exchange capacity was prepared by surface sulfonation of a styrene divinylbenzene (S/DVB) copolymer (2% DVB) by a method previously described by one of the authors ( 3 ) .It involves briefly heating (minutes) a quantity of S/DVB copolymer (2% DVB) with an excess of hot (-100 "C) concentrated sulfuric acid, which leads to formation of a thin surface shell of sulfonic acid groups. The capacity of a typical separating resin is around 0.02 mequiv/gram of starting copolymer. Apart from being a
v) W
z
2 v) W
a a 0
Iu W k W 0
Figure 2. Separation of alkali metal ions
I
No*
5
IO
20 MIN.
I5
Figure 3. Analysis of Na+, NH4+, and K+ in human urine
resin of low capacity, the pellicular nature of the sulfonated material would be expected to have favorable mass transfer characteristics due to the proximity of all of the active sites to the eluant resin interface ( 4 ) .The dimensions of the separating bed will be given in the individual examples. The S t r i p p e r Resin. The stripper resin was Dowex 1 X 8 (OH-) 200-400 mesh. The dimensions of the stripper bed will be given in the individual examples. T h e Eluant. The eluant in all the examples to be described was aqueous HC1, in the range of 0.01 to O.O2N, except for some of the amine separations where pyridine hydrochloride and aniline hydrochloride solutions were used.
Table I. Schemes for Conductimetric Chromatography Cation Analysis Separating colunin
Eluant
Resin - H' Resin - Ag' Resin - Cu"
HC1 AgNO,
Resin - anilinium
Aniline hydrochloride AgNO, - HNO,
Resin - Ag*
Cu(N0J2
-
Stripper column
Stripping reaction
1st Resin - C12nd Resin - OH-
AgC 14 Resin - OH- + HNO,
Resin - OH'
--.
-+
Anion analysis
Resin - OHResin - phenate
NaOH Na-phenate
-
Resin - OH- - HC1 Resin C1- H,O Resin C1- iAgNO, Resin NO3- + AgC1, Resin amine + Cu(NO,), --+ Resin amine. C u (NO,), Resin OH- + aniline HC1 aniline + Resin C l - + H,O Resin - C1- + AgNO, Resin NO,- +
Resin - OHResin - ClResin - amine
Resin - H+ Resin - H'
Resin H' Resin H'
+ NaOH
--+
Resin NO,-
4 H,O
-
Resin Na* + H,O + PhONa+ .--, Resin Na* + PhOH
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
1803
A
A
B
No'
Na' W
VI
z
0 a VI
W
a a
P W VI
0 I0
co
z m 0
W
a a
ew
K+
0
i
W
I0 W
0 W
-5
5
IO MIN.
IO
15
5
Figure 4. Analysis of Na+ and K+ in dog's blood serum
K+,
Separations. Alkali Metal Ions. Details of the columns, resins, eluants, etc., used in the various examples cited, are summarized in Table 11. Figure 2 shows the separation of the alkali metal ions. Linear flow rate through the separating column was increased by using microbore columns and complete separations of sodium from potassium were attainable within as little as three minutes. Figures 3-5 show examples of the application of the technique t o Na+, K+, ~~
IO
15MIN.
5
IO MIN
Figure 5. Analysis of ( A ) orange juice and ( 8 )grape juice for Na+,
and NH4+
and NH4+ determination. Figure 3 shows the chromatogram of human urine-the peak between Na+ and K+ was identified as NH4+. Figure 4 shows the elution of Na+ and K + in dog blood serum. In Figure 4A, the K+ peak is barely perceptible above the base line but shows as a substantial peak in Figure 4B as a result of increasing the detector sensitivity after the Na+ peak had passed through. This example illustrates the ability in this case t o determine a small
~
Table 11. Elution Conditions Used in the Examples Illustrated i n Various Figures
Fig.
2
3 4 5 6 7
8
Separating column
9 mm x 250 mm SSS/DVB 180-325 mesh Sp. cap 0.016 mequiv,/g Same as in Fig. 2 Same as in Fig. 2 Same as in Fig. 2 9 mm X 250 mm SSS/DVB 180-325 mesh Sp. cap 0.024 mequiv/g 2.8 mm X 300 mm SSS/DVB 180-325 mesh Sp. cap 0.024 mequiv/g
9 mm X 250 mm SSS/DVB 180-325 mesh
9 mm x 270 mm Dowex 1 X8 OH200-400 mesh Same Same Same 9 X 250 mm Dowex 1 X8 OH200-400 mesh 1s t column 9 mm x 250 mm Dowex 1 X8 Cl200-400 mesh 2nd column 9 m m X 250 mm Dowex 1 X8 OH200-400 mesh 9 mm x 250 mm Dowex 1 X8 OH-
Sp. cap 0.024 mequiv/g 9 2.8 mm x 500 mm SSSIDVB 180-325 mesh Sp. cap 0.024 mequivlg 13A 2.8 mm x 300 mm SA Dowex 2 resin SA Dowex 2 resin 13B SA Dowex 2 resin
200-400 mesh 9 mm x 300 mm Dowex 1 X8 C1200-400 mesh 2.8 mm x 300 mm Dowex 50W X8 H' 200-400 mesh 200-400 mesh
14
9 mm x 250 mm Dowex 50W X8 H' 200-400 mesh 2.8 mm X 250 mm Dowex 50W X8 H' 200-400 mesh
2.8 mm
X
300 mm
S A Dowex 2 resin
15
1804
Flow rate,
Stripping column
Same a s Fig. 14
ANALYTICAL CHEMISTRY, VOL.
47, NO. 11,
Eluant
0.OlLVHC1
ml/hr
160
Sample size rnl'
0.1
Sample composition
0.01A' in each of Li, Na, K , Rb, and C s chlorides. Urine diluted 10-fold Serum diluted 10-fold Undiluted juices 8 ppm monomethylamine 8 ppm dimethylamine 20 ppm trimethylamine 20 ppm tetraethyl ammonium bromide 60 ppm tetra-n-butyl ammonium bromide
0.01A' HC1 0.01.V HC1 0.01S HC1 0.01,V HC1
460
0.1 0.1
0.0021Y AgNO,
230
0.1
0.001.71 aniline hydrochloride 0 .OO 1J I HC 1 0.05S &NO,
4 60
0.1
92
0.1
0.015F sodium phenate 0.015F sodium phenate 0.035F NaOH 0.015F sodium phenate 0.015F sodium
60
0.01
Untreated Midland city water
60
0.01
Untreated Lake Huron water
64
0.01
64
0.01
0.01S in each of sodium mono-, di-. and t r i chloroacetate 0.01S in each of chloride, bromide, and iodide
0.1 0.1
0.0004S HNO,
SEPTEMBER
phenate
1975
14 ppm monoethylamine 20 ppm diethylamine 40 ppm triethylamine Untreated Saginaw river water
W v)
z
0
a v) w
[L
TEA
a
2 W 0
+ W
n
5
0
IO
15MIN.
Figure 6. Separation of monomethyl amine (MMA), dimethyl amine (DMA), and trimethyl amine (TMA)
amount of one ion, K+, in a relatively much higher background concentration of another ion, Na+. The concentrations of the Na+ and K+ in the serum were 4050 and 165 ppm, respectively. The chromatograms of orange juice and grape juice are illustrated in Figure 5. Organic Amines and Quaternary Ammonium Compounds. Several organic amines and quaternary ammonium compounds were successfully detected and/or resolved using dilute hydrochloric acid as the eluant as ex-
0
5
IO
15 MIN.
Figure 7. Separation of tetraethyl ammonium (TEA) and tetra-n-butyl ammonium (TBA) ions
plained in Table I. In the stripper, the protonated amine is converted to the free amine and the quaternary salt to quaternary ammonium hydroxide. Table I11 lists the elution data obtained. Note that elution times vary with the amount of sample injected. The effect is minor at low to moderate concentrations but becomes significant a t higher ~~
Table 111.Elution D a t a for Aminesa Sample
ppm Concentration
Peak max. elution t i m e , min.
Peak width at half height, min
Monomethyl amine 8 6.5 0.6 Monomethyl amine 80 6.3 0.7 Monomethyl amine 800 5.2 1.2 Dimethyl amine 8 8.2 0.9 Dimethyl amine 80 7.7 1.o Dimethyl amine 800 6.3 1.4 Trimethyl amine 8 10.3 1.1 Trimethyl amine 80 9.9 1.3 Trimethyl amine 400 8.7 1.1 Tetramethyl ammonium B r 10 12.0 1.4 Tetramethyl ammonium B r 100 12.3 1.6 Tetramethyl ammonium B r 1000 9.8 1.8 Monoethyl amine 14 7.2 0.8 Monoethyl amine 140 6.8 0.8 Diethyl amine 20 10.8 1.3 Diethyl amine 200 10.9 1.3 Triethyl amine 40 15.5 1.8 Triethyl amine 400 14 .O 2.2 Tetraethyl ammonium B r 100 27.5 3 .O Tetraethyl ammonium B r 1000 22.0 4.4 ii-Butyl amine 20 14.2 1.8 12-Butyl amine 200 12.7 1.8 Cyclohexyl amine 200 23.6 3.8 Cyclohexyl amine 1000 19.5 3.8 Tri-/z- butyl amine >30 min. Tetra-tl-butyl ammonium B r >30 min. Monoethanol amine 20 5.3 0.7 Diethanol amine 20 5.5 0.8 Triethanol amine 2 00 5.9 1.2 Monoisopropanol amine 20 5.6 0.7 Diisopropanol amine 40 6.2 1.o Triisopropanol amine 400 6.6 1.7 Ammonia 1 5.1 0.5 Ammonia 10 5.1 0.5 Ammonia 100 4.9 0.7 Separating column: 9 mm X 250 mm SS/DVB; 0.024 mequiv/g; 180-325 mesh. Stripper column: 9 mm x 250 mm Dowex 1 X8 OH-, 200-400 mesh. Eluant: 0.01NHC1. Flow rate: 460 ml/hr. ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
1805
v) W
t
v) W
B In
z
MEA
W
0 &
a
w fn
K
a
0
z
e
a
W W CI
c
W V
c W
n
0
5
IO
15 MIN.
Figure 8. Separation of monoethylamine (MEA), diethylamine (DEA), and triethylamine (TEA) using aniline hydrochloride as eluant
concentrations. Peak width a t half height is listed to permit calculations of resolution and HETP. As an example, a resolution of mono-, di-, and trimethyl amine is shown in Figure 6. Some of the amines listed in Table I11 took more than 15 minutes to elute so eluants with a higher affinity for the surface sulfonated resin were used to reduce the elution time. These eluants were silver nitrate-nitric acid, a mixed eluant, and a double stripper, see Table I, and aniline hydrochloride-hydrochloric acid mixed eluant. The presence of the acid is needed to protonate the amines to cations. Figure 7 shows the resolution of tetraethylammonium bromide with the silver nitrate-nitric acid eluant and Figure 8 shows a resolution of mono-, di-, and triethyl amine using the aniline hydrochloride-hydrochloric acid eluant. Note that both systems decreased the time needed for the determinations. The detection limit of conductimetric chromatography for the lower molecular weight amines is normally 0.1-1 ppm. Diualent Metal Ions. A scheme such as the one used in the alkali metal separation has a drawback when applied to the case of Ca2+,Mg2+ determination. The problem derives from the high affinity that these divalent ions have for the separating resin. Consequently, the selectivity factor becomes dominant (see Equation 3), and the large amounts of acid required to elute these species off the resin greatly shortens the lifetime of the stripper. T o get around this problem, we therefore sought an eluting species which would have the following characteristics: 1) Have a significantly higher affinity for the sulfonated resin than H+.2) Be easily and completely removed by the stripper by a reaction which would not remove the species being determined, viz., Ca2+and Mg2+. The scheme devised employs Ag+ as the eluting ion which has roughly ten times the affinity for a sulfonic acid resin that H + does. After passing from the separating column, the Ag+ is removed by precipitating it as AgCl on the C1- form of an anion exchanger. The spent stripper is regenerated by a solution containing ammonium hydroxide (IN)and ammonium chloride (IN). A typical chromatogram of a sample which contained Ca2+and Mg2+ is shown in Figure 9-the first peak is due to the alkali metals, Na+ and K+, which were also present in the sample. Recently a p-phenylene diamine/HCl eluant with a strong base re'sin stripper has been found to be a superior system to the silver system.
ANION ANALYSIS The S e p a r a t i n g Resin. Anion exchangers of very low (surface) capacity have been used in the separating bed. At 1806
J
0
5
IO
I 5 MIN
Figure 9. Separation of calcium and magnesium
the inception of this work, it was known that resins of this type were available commercially, but since none was readily a t hand an alternate route to such a resin was devised. I t has been known for some time that cation and anion exchangers have a marked tendency to clump together-a manifestation of the strong electrostatic interaction between polycationic and polyanionic materials in general. This clumping property is exploited in the following manner to produce an effective anion exchanger of very low capacity. A strong base anion exchanger (Dowex 1, Dowex 2, IRA 900) is thoroughly ground in a rod mill and the larger particles are removed by sedimentation. A very dilute suspension of the fine particles is then passed through a column packed with a surface sulfonated S/DVB resin where the fine anion exchange particles are agglomerated with the surface of the SSS/DVB. Eventually the surface of the SSS/DVB becomes saturated with the fine resin particles and they break through from the column, a t which time, after a brief water rinse, the column is considered ready for use. I t has been established by electron micrographs that the dry particle size of a useful ground material is around 0.5-2 micron. Furthermore, by careful size fractionation, it is possible to prepare columns of a variety of capacities and resolving powers. For convenience in referring to these surface agglomerated resins, we will use the simple designation SA followed by the name of the anion exchange resin used, e.g., SA Dowex 2. The S t r i p p e r Resin. Dowex 50W X8 (H+) was used as stripper and was regenerated by 0.25N HzS04 when exhausted. T h e Eluant. In devising a scheme for anion analysis, an obvious choice for eluting species is the hydroxide ion since it is so conveniently neutralized by a strong acid resin in the stripper bed. On the other hand, on the basis of its selectivity in anion exchange reactions ( 5 - 7 ) ,the choice is not such a desirable one-within a large series of anions, the hydroxide ion is one of the least tightly held, Le., K ~ H - ~ is in most cases large. Furthermore, with respect to other ions, it is less tightly held on a Dowex 1 than on Dowex 2 and it is for this reason that in this work a Dowex 2 type resin has been employed in the separating column. As a result of this unfavorable selectivity factor, fairly high concentrations of OH- have to be used to elute ions of even moderate affinity, which in turn shortens the lifetime of the stripper. The low displacement potential of the hydroxide ion also leads to extensive tailing of the elution curves of the more tightly bound ions which has the undesirable
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
Table V. Elution of Anions by Sodium Phenatea
Table IV. Elution of Anions by Sodium Hydroxide/ Sodium P h e n a t e Mixturesa Eluani: Anion
0 . 0 4 5 ~CH'i0.005F
Eluant:
Pho-
0 . 0 3 5 ~@ ~ ' / 0 . 0 1 5pho' ~
6.51 6.19 Br7.47 118.0 IO,' 5.81 NO,' 7.47 NO'6.29 so,26.72 sod27.15 ~ 0 ~ 3 8.48 Formate 5.14 Acetate 8.59 C1 acetate 6.67 C1, acetate 13.6 Oxalate 7.15 Separating column: 2.8 mm X 300 mm "SA Dowex 2 Resin" Stripper column: 9 m m x 250 mm Dowex 5OW X16 ( H + )200-400 mesh. Flow rate: 60 m1,ihr.
F-
c 1-
0.005F P h O -
Anion
Elution v o l m l
6.72 6.72 9.65 V. large 5.55 9.87 7.36 8.05 10.2 16.5 ( ? ) 7.25 8.27 6.67 V. large 11.3
effect of reducing the sensitivity for detecting the species in the effluent. It was, therefore, clearly desirable to find a suitable alternative to OH- as an eluting ion and a species that seemed a likely candidate was the phenate ion. Not only did it have a more favorable selectivity coefficient on Dowex 2, 0.14 for phenate vs 1.5 for OH-, but an acid form resin as stripper would convert it to phenol which being a very weak acid, would be only feebly dissociated and contribute little to the conductivity of the effluent from the stripper. Accordingly, the phenate ion was extensively investigated as a displacing ion. T h e Phenate System. Tables IV and V summarize the results of an extensive series of elutions of a wide variety of anions wherein the concentration of OH- and PhO- and the size of the stripper was varied. In general, the order of elution of ions is in accord with previous work (5-7). However, there are a number of exceptions. 1) Using the large stripper bed (9 X 250 mm) Felutes a t the same time or later than C1- (Table IV) which is not consistent with their selectivities for anion exchange on Dowex 2 (6). 2) Likewise, formate, acetate, and monochloroacetate elute later than would be expected on the basis of anion exchange selectivities alone. 3) Phosphate elutes after sulfate under some conditions (Table IV) and before it under other conditions (Table V). The reason for some of these inconsistencies (1 and 2) becomes apparent when one makes a more detailed study of the function of the stripper bed for it is this bed that is the cause of these elution reversals. Consider Figure 10 which illustrates a stripper bed in the process of being exhausted and lists the various ionic species existing in the exhausted sodium form zone and in the unexhausted hydrogen form zone. The elution volume (VE) through this stripper bed of species containing the ion X-, the ion of analytical interest, is given by the expression T h e first term, V,, is simply the retention volume attributable to displacement of the void volume of the stripper bed. B, the retention of The second term, K ~ x N ~ Vdescribes the species X- by the sodium form of the stripper where K1 is the distribution coefficient of X- between the resin and
0 , 0 1 F PhO-
Fc1Br-
1.92 3.48 7.8
1.8 2.5 4.62
1NO,NOZIo3-
7.8 4.02 1.88
4.62 2.9 1.62
Br0,-
so,2so,'co,2-
>34.2 2 1 .o 13.5
Crod2-
11.6 5 .O >25.2
PO,,-
0.015F PhO-
1.68 2.2 346
16.3 3.52 2.6 1.68 1.94 5 -76 3.88 3.78 21.8 4.72
Formate 2.30 2.06 Acetate 2.34 2.22 1.84 Propionate 2.64 C1 acetate 2.74 2.22 C1, acetate 5.34 3.58 C1, acetate 10.1 Glycolate 1.84 Oxalate 11.6 5.76 Mal e at e 7.46 4.2 Fumarate 10.4 5.56 Succinate 3.66 Malonate 3.78 Itaconate 7.56 Benzoate 10.9 7.18 6.3 Ascorbate 2.1 15.88 Citrate 42.4 Separating column 2 8 m m X 300 m m "SA Dowex 2 Resin" Stripper column 2 8 mm x 300 mm Dowex 50" X8 (H-) 200-400 mesh
NO+ R -
Figure 10. The
c
AQUEOUS
-----
PHASE
7- - - - - I
NO+OH-
No++ONo' X-
species in the aqueous phase in the phenate stripper'
column mobile phase, X N is ~ the fraction of the bed that has been exhausted, and VB is the volume of the stripper. Due to Donnan Exclusion ( 8 ) , K1 is very small and term 2 in Equation 4 is therefore negligible. The third term, K2 (1 - X N ~ ) V Bdescribes , the retention of X- and related species by the unexhausted H+ form resin. Since the X- ion is converted to its corresponding acid at the Na+R-/H+R- boundary, the elution behavior of X type species through the H+R- zone will be very dependent on the strength of this acid. If the acid is strong, then the predominant species in this region of the bed will be X- which will be effectively excluded from H+R-, that is K P will be very small, and term 3 will be negligible. If, on the other hand, the acid is weak, a fraction of X will exist as the undissociated H X species which being uncharged, is not subject to ion exclusion forces and can enter the hydrogen form resin quite readily. In other words K2 is no longer
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
1807
SO:-
GI'
A
A
W
Lo z
a0 . Lo rr W K 0 t
0 W
W In
t
0 W
z
0
n In W
a
I
I
I
5
IO
15
I 20MIN.
L
2
8
6
4
IO MIN
a
0 IV W
I-
W
n a 0 t 0 Y
t
D W
, 2
0
5
15
IO
20MIN.
Figure 11. Elution of CI- and SO4*- by ( A ) sodium phenate, ( B )
NaOH
6
4
8
Figure 13. Chromatograms of (A) Midland Huron water
IO MIN.
City
water, ( B ) raw Lake
;I'
IO'
W
Lo z
0
a.
IO0
i W n
a
PEAK HEIGHT
a
+ U 0
ImV)
W I-
n W
10-1
5
162 IO'
102
I o4
lo3
NANOGRAMS OF C I - OR
of very small magnitude and term 3 can become so significant that it is the dominant factor controlling the elution order through the complete train of columns and is the cause of the elution reversals already noted. Not only can term 3 be of significant magnitude in certain cases but it also varies in magnitude as X N varies ~ from zero (fresh stripper) to unity (exhausted stripper). This gives rise to the undesirable result that the elution volume of the species of interest depends on the degree of exhaustion of the stripper column. Since one has no control over the value of K P or X N ~ the , effect of the third term can be reduced and the drifting of peak position minimized by keeping VB as small as possible. A parallel case exists for chromatography of cations which form weak bases and it is therefore recommended t h a t , i n t h e chromatography of species which convert t o partially dissociated forms i n t h e stripper bed, t h e volume of this bed be k e p t as small as practicable. On the basis of the arguments already presented, the elution of species which form highly dissociated derivatives in the stripper bed is sensitive only to the first term in Equation 4 and the size of the stripper bed is therefore determined solely by the considerations expressed in Equation 3. The shifting elution position of phosphate (anomaly 3 above) is for a reason quite different from those just discussed. I t is believed that the conditions in the separating
*
t
15
20 MIN.
Figure 14. Separation of halide ions
SO,'-
Figure 12. Calibration plots for CI- and Sod2-
1808
I
IO
column dictate its elution position rather than those in the stripper column since phosphoric acid is a strong acid and should be subject only to term 1 in Equation 4. I t is well known from ion exchange studies that ion affinities are strongly dependent on the valence of the exchanging ions with higher valence ions being the more tightly held, all other factors being equal. In the case of the phosphate ion the species involved in exchange in the separating column (pH 10 to 11) is not a single species but rather a mixture of HP042- and Po43-. Consequently, conditions which favor the formation of the trivalent Pod3- a t the expense of the divalent HP042- will promote the retention of phosphate in the separating column and an excess of sodium hydroxide above that necessary to neutralize the phenol (Table IV) is such a condition. Separations. A number of separations and analyses of anions are illustrated in Figures 11to 15. Figure 11 shows the comparison between the hydroxide and phenate systems in the elution of C1- and S042-. Calibration plots were constructed for these two ions and are shown in Figure 12 which illustrates the close linear dependence between peak height and the amounts of ion present over a wide range of concentration. Figure 13A illustrates the chromatogram of a local municipal water. Three of the peaks were attributable to C1-, C03*- and S042-, and F- was suspected as being the origin of the first peak to elute, particularly since it did not appear in the chromatogram of water from Lake Huron (Fig-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
Table VI. Comparison of Analysis by Conductimetric Chromatography (CC) a n d Other Methods Concn, ppm
ton detemined
10
2a
3"
22 23 1.4 2.2 43 47 46 13 12 14 46 54 50 28 30
49 48
24 24 1.8 2.5 44 49
Method
N a+
cc
Na'
AA~
K+ K' Ca2+ c a2+
cc AA
c a2+
EDTA titration
Mg'" Mg'' Mg2'
cc
AA
cc
AA EDTA titration
cc
c1c1-
AgNO, titration Neutron activation
c1-
so,'-
cc
so42-
Turbidimetry
4 .O
3.7 74
74 74 21 19 20 122 117 118 44 49
47 14 12 14 57 55 56 28 28
P 45 45 2.3 2.6 52 56 53 15 13 16 91 100 99 32 32
S a m p l i n g locations o n the Saginaw R i v e r . A t o m i c absorption.
I
I
CONCLUSIONS MCA
L
1
5
IO
15
20
25 MIN.
Separation of monochloroacetate (MCA), dichloroacetate and trichloroacetate (TCA)
Figure 15. (DCA),
ure 13B) which is the source of the municipal water. Confirmation that the first peak was very probably due to Fwas obtained from chromatograms of raw Huron water spiked with known amounts of fluoride. I t is worth noting that, in these experiments, a distinct peak due to fluoride was obtained for only 3.8 nanograms of added F- which attests to the sensitivity of the technique. Figures 14 and 15 illustrate two other separations obtained with the anion analysis system. Table VI shows results of analyses of a local river water a t four different sampling locations made by the subject method and a t least one accepted analytical method. The average overall variance was 7.6%. This level of agreement was obtained with samples of river.water generally believed to contain many components, in addition to the ones determined, none of which apparently affected the results under the conditions used.
A practical ion exchange chromatographic method for anions and cations has been developed which uses a conductivity cell as detector. This is made possible by a unique combination of resins which separates the ions of interest and strips or neutralizes the eluant from the background. Analytical schemes have been devised for a large variety of organic and inorganic cations and anions which can be determined with good accuracy and precision. The method has a large dynamic range and is capable of determining small amounts of one species in a high background of other ions. The method is particularly attractive for anions in light of the special and varied techniques usually required for their analysis. With this technique, a great diversity of anionic species may be quantified by a simple universal detector, the conductivity cell. NOTE. The methods described in this publication are the subject of pending patents. LITERATURE CITED (1) W. Reiman Ill and H. F. Walton, "Ion Exchange in Analytical Chemistry", Pergamon Press, Oxford, 1970. (2) J. E. Salmon, Third IUPAC Analytical Chemistry Conference, Budapest, 1970. (3) H.Small, J. Inofg. Nucl. Chern., 18, 232 (1961). (4) J. J. Kirkland, "Modern Practice of Liquid Chromatography", Wiley Interscience, New York, 1971. (5) S. Peterson, Ann. N.Y. Acad. Sci., 57, 144 (1953). (6) R. M. Wheaton and W. C. Bauman. Ind. Eng. Chern. 43, 1088 (1951). (7) R. Kunin and R. J. Myers, "Ion Exchange Resins", Wiley, New York, p 154. (8) F. Helfferich, "Ion Exchange" McGraw Hill, New York, 1962.
RECEIVEDfor review December 5 , 1974. Accepted February 24, 1975.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
1809
