Anal. Chem. 1983, 5 5 , 1001-1004
:‘11“”
Lo
CONCLUSION
la
ACETATE
li
:-I _
m
I
’HEPTANE
1001
b
m
The optimum portion of an interferogram used as a vector for Gram-Schmidt orthogonalization reconstructions is heavily dependent upon the identity of the mixture componc?nts (particularly their spectral peak widths) as well as GC/FT’-IR instrument stability. When substances having similar IR absorbance spectra are detected (i.e., isomers), little difference in relative chromatogram peak height is observed when different vector displacements are used (4). However, when GC eluents have different absorbing properties, relative peak heights are dependent upon the interferogram vector sampling. Therefore, if one intends to maximize Gram-Schmidt reconstruction sensitivity for a given substance, an optimum interferogram sampling must be determined. A universally applicable optimum Gram-Schmidt vector displacement simply does not exist. However, a best “compromise” value based upon the considerations delineated here might well exist. From this and previous results (4) it does not appear that the 60-point displaced choice routinely used in commerical software is that hest compromise. In a collaborative research effort, we are currently investigating interferometer stability of various commerical GC/FT-IR instruments and its effect on optimum Gram-Schmidt interferogram vector sampling. The results of this study will be published at a later time.
LITERATURE CITED (1) de I
g=----J 4000
--’3200
2400
1600
800
W AVENUMBERS
Figure 5. (a) Gas-phase infrared spectrurn of ethyl acetate. (b) Gas-phase infrared spectrum of heptane. with increasing vector displacement clearly suggests a significant instrumental noise contribution since, in principle, S I N should decrease as data increasingly further from the centerburst are sampled.
Haseth, J. A.; Isenhour, T.
L. Anal. Chem. 1977, 4 9 , 1977-1981.
(2) Sparks, D. T.; Lam, R. 8.; Isenhour, T. L. Anal. Chem. 1982. 5 4 , 1922-1926. (3) Small, G. W.; Rasmussen, G. T.; Isenhour, T. L. Appl. Spectrosc. I W g , 33, 444-449. (4) White, R. l..; Giss, G. N.; Brissey, G. M.; Wilkins, C. L. Anal. Chem. I981, 53, 1778-1782.
RECEIVED for review January 6, 1983. Accepted March 17, 1983. The support of the National Science Foundation under Grant CHE-82-08073 is gratefully acknowledged.
Nonsuppressor Ion Chromatography of Inorganic and Organic Anions with Potassium Hydroxide as Eluent Tetsuo Okadia and ‘Tooru Kuwamoto” Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606, Japan
The elution behaviors of lnorganlc anlans and weak organic acids were investlgatced witH nonsuppressed Ion chromatography. As an eluent, a potasslum hydroxlde solution was remarkably effective for the separatlon and determlnatlon of F-, CI-, Br-, NO,-, and NOs-. For SO?- It was necessary to use a higher concentration of potassium hydroxide solution. The detection Uimlts of F-, CI-, Br-, NO,-, and NO,- were 1.5 ppb, 2.5 ppb, 15 pplb, 15 ppb, and 15 ppb, respectively. Moreover, it was found that the potasslum hydroxide eluent was also appilcable to the separatlon and determination of weak organlc trclds, such as derivatlves of phenol or benzolc acid, carboxylic acids, and dicarboxylic acids.
The separation and determination of anions, tedious and intricate tasks in the past, have been easily carried out by using ion chromatography. For example, in several reports, the
determination of CN- in air ( I ) , the blow-down water from a boiler (2),aqueous solution after the collection of a trace level ion with concentrator columns (3) and alkali metal or alkaline-earth metal (4), transition-metal ions (5, 6), and amines (4, 7) have been described. Ion chromatography as developed by Small et al. (4) usually uses a separation column or membrane-exchanger or cation-exchangercolumn to remove most of the background conductance of the eluent with a carbonate buffer solution as an eluent. Gjerde et al. described a means to determine common inorganic anions with nonsuppressor type ion chromatography by using a phthalate or a benzoate solution as the low conductivity eluent (8-10). They also investigated the separation of alkali and alkalineearth metal ions with nitric acid as the eluent ( 1 1 ) . Similarly, the authors considered the disadvantages oE the use of suppressor type ion chromatography. In it, the solute ions are partially lost by the suppressor column owing to adsorption, and organic acids having a low dissociation con-
0003-;!700/83/0355-1001$01.50/00 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
stant, less than are not detected with the bicarbonate buffer solution as an eluent (12). In this contribution the suitability of potassium hydroxide eluent for the determination of weak organic acids and inorganic anions using the nonsuppressed mode of ion chromatography was investigated. Finally, it was found that potassium hydroxide solution as the eluent is remarkably effective for the analysis of organic acids and inorganic anions.
EXPERIMENTAL SECTION Standard Solution. The stock solutions of 1000 ppm P,C1-, Br-, NO2-, NO3-, and SO,-2 and 0.5 M solutions of organic acids were prepared by dissolving pure KF, KC1, KBr, KN02, KNOB, K2S04,potassium salts of organic acids, and free organic acids, respectively. Working standard solutions were obtained by diluting the stock solutions with distilled water. Eluents. Potassium biphthalate solution (1mM) was prepared by dissolving the guaranteed reagent in distilled water and adjusted to pH 6.0 with 0.1 M potassium hydroxide. The potassium hydroxide solution (1-6 mM) was prepared by dissolving the guaranteed reagent in distilled water each time. It was stocked in an eluent bottle equipped with a soda lime tube. Appartus. A nonsuppressor type Toyo Soda Model HLC-601 ion chromatograph was used. The columns were precolumn 4PW1 (particle size 10-12 pm, 4 mm i.d. X 50 mm, Toyo Soda) and the separation column TSK-GEL62OSA (4 mm i.d.x 50 mm, Toyo Soda) packed with porous polymer anion exchange resin (particle size 9 f 1 bm, capacity 0.1-0.3 mequiv/g). The volume of the sample port was 100 pL. The columns and conductivity detector were maintained at 30 "C. The flow rate of the eluent was kept at 1 mL/min under a pressure of 30-32 kg/cm2. RESULTS AND DISCUSSION Principle. The specific conductivity (A) of anions is given by
I
F-
Figure 1. Ion chromatograms of inorganic anions: sample, 5 ppm each of anions; (A) eluent, 1 mM potassium biphthalate solution adjusted to pH 6.0; (E)eluent, 2 mM potassium hydroxide solution; resin, TSK-GEL IC-620SA; flow rate, 1 mL/min. F-
CI
-
where C, is the concentration of the solute anion, C, is the concentration of the eluent anion, and A, and A, are the ion equivalent conductivity of the solute anion and eluent anion, respectively. The difference in the conductivity between the solute band (A") and the background (A') is given by
Time, min
When potassium hydroxide solution is used as the eluent, the ion equivalent conductivity of the eluent anion (A,) becomes greater than that of the solute anion (Q. Therefore, the signal of conductivity of the solute anion is shown as the negative peak which is proportional to the concentration. Determination of Inorganic Anions. In order to compare the effect of eluents, the chromatograms of 5 ppm of F-, C1-, Br-, NO2-,NO3-, and SO>- were investigated by using both eluents of 1 mM potassium biphthalate solution adjusted to p H 6.0 and 2 mM potassium hydroxide solution, as shown in Figure 1. The upper elution curve is the chromatogram of when various inorganic anions (F,C1-, Br-, NO;, NO3-, SO-): using 1mM potassium biphthalate solution. The lower curve is the chromatogram of inorganic anions when 2 mM potassium hydroxide solution is used. The first large negative peak was that of water. The absolute value of the conductivity difference of the solute anion and eluent anion was greater with the potassium hydroxide solution than that which resulted from the use of potassium biphthalate solution. Therefore, as shown in Figure 1,the sensitivity of samples was enhanced several times by the use of potassium hydroxide solution compared to that from using potassium biphthalate solution. Moreover, it was found that the peak of the fluoride ion, which was not detectable due to overlapping with the
Figure 2. Separation of inorganic anions: sample, 50 ppb each of anions; eluent, 2 mM potassium hydroxide solution; resin, TSK-GEL IC-620SA; flow rate, 1 mL/min. water peak in the use of potassium biphthalate sblution, was determined with high sensitivity, using potassium hydroxide solution. For Sod2-,which showed a broad peak and long retention time with a low concentration of potassium hydroxide, an eluent of higher concentration potassium hydroxide solution is necessary. The calibration curves of Fand C1- were linear in the concentration range 0.2-20 ppm and the linear range of other anions (Br-, NO2-, NO3-) was 0.2-50 ppm. The positive peak was observed on the injection of 40 ppm of sample solution but did not influence the elution peaks of F- and C1-. Peak heights of anions were measured instead of the peak area because it is difficult to integrate the peak areas of the low concentration solutes. Figure 2 shows the chramatogram of inorganic anions of 0.05 ppm. The detection limits of F,C1-, Br-, NO;, and NO, were 0.0015 ppm, 0.0025 ppm, 0.015 ppm, 0.015 ppm, and 0.015 ppm, respectively (the detection limit was defined as the concentration corresponding to twice the value of the noise of the base line). The credibility of the detection limit stated for F and C1- is lessened by close proximity of the water peak.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
'1003
Table I. Dissociation Constant and Retention Time of Organic Acids" pK,
benzoic acid 4.21 o-toluic acid 3.91 m-toluic acid 4.73 4.36 p-toluic acid o-chlorobenzoic acid 2.94 m-chlorobenzoic acid 3.82 p-chlorobenzoic acid 3.99 p-aminobenzoic acid 4.85 o-nitrobenzoic acid 2.17 3.45 m-nitrobenzoic acid 3.44 p-nitrobenzoic acid o-anisic acid 4.09 4.49 p-anisic acid salicylic acid 3.00 benzenesulfonic acid 0.07 p-toluenesulfonic acid p-chlorobenzenesulfonic acid phenol 10.00 o-cresol 10.29 m-cresol 10.09 p-cresol 10.26 3,5-dimethylphenol p-ethylphenol formic acid 3.76 acetic acid 4.76 glycolic acid 3.83 oxalic acid 1.27 4.27 succinic acid 4.21 5.64 1.92 maleic acid 6.23 fumalic acid, 3.02 4.38 tartaric acid 3.04 4.37 a Sample, 2.5 eluted.
X
retention time, min 2mM 4mM 6mM 11.33 8.60 17.38 17.71 10.49 27.44 28.60 8.85 8.85 26.83 28.20 6.48 15.20
8.23 6.24 11.77 12.07 7.55 17.88 18.03 6.44 6.57 17.48 17.90 5.25 10.44 b 25.9 8.60 12.28 17.96 11.99 29.48 18.0
5.76 4.46 8.01 8.37 5.34 9.75 0.10 4.63 4.81 9.60 9.64 4.18 7.18 22.0 5.97 8.14 16.34
16.50 19.47 18.53 20.03 28.0 37.3 4.10 3.68 3.60
9.66 13.67 12.60 13.80 16.18 22.30 2.79 2.59 2.54
10.80
b
b
13.13
b
b
11.03
b
b
11.53
b
b
16.17
b
b
11.10
M for each organic acid.
5.98 9.46 8.78 9.5'1
Time , min.
Figure 3. Separation of benzoic acld and its derivatives: sample, 1.O X M each of acids; eluent, 4 mM potassium hydroxide solution; resin, TSK-GEL IC-620SA; flow rate, 1 mL/min.
15.40 2.23 2.27 2.09
~~
Not
Elution Behavior of Organic Acids. On the use of potassium biphthalate eluent, the elution peaks of organic acids were not observed in one direction, and the sensitivity was low because of the similarity of ion equivalent conductivity between solute anions and biphthalate ion. Therefore, the method using a potassium hydroxide solution was applied to the analysis of organic acids. The dissociation constants and retention times of organic acids tested are shown in Table I. In the experiment, the eluents of 2 mM, 4 mM, and 6 mM potassium hydroxide E,olutionwere used, and the concentration of injected sample was 2.5 X The relationship between dissociation constants and the retention times of the organic acids, except for phenol and its derivatives which were dissociated into anions in the alkaline solution, was independent of the retention times. But the retention times of all organic acids were decreased with increasing concentration of the eluent. Dicarboxylic acids eluted as broader peaks because of high affiiity with the resin. Therefore, it was found that the use of higher concentration potassium hydroxide eluent is necessary in order to obtain the quantitative pealks. On the other hand, ortho-substituted benzoic acids eluted faster than meta- or para-substituted benzoic acids. Because of this, we speculated that the dissociation constants of phenol and its derivatives may be correlated to retention time, but this was not investigated further. The chromatograms of benzoic acids and phenols are shown in Figures 3 and 4,respectively. The concentration of the !sample was 1.0 x
I
I
1
10
20
30
Time , m i n
Figure 4. Separation of phenol and its derivatives: sample, 1.5 X lo4 M for phenol, 2.0 X M for its derivatives: eluent, 2 mM potassium
hydroxide solution; resin, TSK-GEL IC-620SA; flow rate, 1 mL/min. M for benzoic acids, 1.5 X M for phenol and 2.0 X M for derivatives of phenol. As revealed from Figures 3 and Table I, ortho- and meta-substitutions and para-substitutions of toluic acids or ortho- and para-substitutions and metasubstitutions of chllorobenzoic acids were easily separated, but 0-, m-, and p-cresols were not completely separated. The shoulder peak of p-ethylphenol was identified as carbonate ion, because the peak area increased with the addition of carbonate ion in the sample solution. In conclusion, it was found that inorganic anions and weak organic acids were detected with high sensitivity by using potassium hydroxide solution without a suppressor column. From this fact, it is considered that the utility of nonsuppressed ion chromatography can be widely extended to the analysis of various ionic chemical species by properly selecting the eluent. ACKNOWLEDGMENT The authors wish to thank Toyo Soda Manufacturing Co., Ltd., for the use of their instruments. Registry No. Benzoic acid, 65-85-0; o-toluic acid, 118-90-1; m-toluic acid, 99-04-7;p-toluic acid, 99-94-5;o-chlorobenzoic acid, 118-91-2; m-chlorobenzoic acid, 535-80-8;p-chlorobenzoic acid, 74- 11-3; p-aminobenzoic acid, 150-13-0; o-nitrobenzoic acid, 552-16-9; m-nitrobenzoicacid, 121-92-6;p-nitrobenzoic acid, 6223-7; 0-anisic acid, 579-75-9;p-anisic acid, 100-09-4;dicyclic acid, 69-72-7; benzenesulfonic acid, 98-11-3; p-toluenesulfonic acid,
1004
Anal. Chem. 1983, 55, 1004-1009
104-15-4;p-chlorobenzenesulfonic acid, 98-66-8;phenol, 108-95-2; o-cresol, 95-48-7; m-cresol, 108-39-4;p-cresol, 106-44-5; 3,5-dimethylphenol, 108-68-9;p-ethylphenol, 123-07-9; formic acid, 64-18-6; acetic acid, 64-19-7; glycolic acid, 79-14-1; oxalic acid, 144-62-7;succinic acid, 110-15-6;maleic acid, 110-16-7;fumaric acid, 110-17-8; (+)-tartaric acid, 87-69-4; potassium hydroxide, 1310-58-3.
(5) Cassidy, R. M.; Elchuk, S. Anal. Chern. 1982, 5 4 , 1558-1563. (8) Nordmeyer, F. R.; Hansen, L. D.; Eatough, D. J.; Rollins, D. K.; Lamb, J. D. Anal. Chern. 1980, 52, 852-856. (7) Bouyoucos, S. A. Anal. Chern. 1977, 4 9 , 401-403. (8) Gjerde, D. T.; Fritz, J.
(9) Gjerde, D. T.; Schmuckler, G.; Fritz, J. S.J . Chromafogr. 1980, 187, 35-45. (IO) Fritz, J. S.; Gjerde, D. T.; Becker, R. M. Anal. Chem. 1980, 52, 15 19- 1522. (11) Gjerde, D. T.; Fritz, J. S. Anal. Chern. 1981, 5 3 , 2324-2327. (12) Smith, F. C., Jr.; Chang, R. C. CRC Crlt. Rev. Anal. Chern. 1980, 197-217.
LITERATURE CITED (1) Dolzlne, T. W.; Esposito, G. G.; Rinehart, D. S.Anal. Chern. 1982, 5 4 ,
470-473. (2) Stevens, T. S.;Turkelson, V. T. Anal. Chem. 1977, 4 9 , 1178-1178. (3) Wetzel, R. A.; Anderson, C. L.; Schieicher, H.; Crook, G. D. Anal. Chem. 1979, 5 1 , 1532-1535. (4) Small, H.; Stevens, T. S.;Bauman, W. C. Anal. Chern. 1975, 4 7 , 1805-1809.
S.;Schmuckler, G. J . Chromafogr. 1979, 186,
509-5 19.
RECEIVED for review
January 3, l983. Accepted
1983.
z2,
Polychromator System for Multielement Determination by Gas Chromatography with Helium Plasma Atomic Emission Spectrometric Detection Michael A. Eckhoff, Thomas H. Rldgway, and Joseph A. Caruso” Department of Chemistry, University of Clncinnati, Clncinnati, Ohio 4522 1
A polychromator/mlcrocomputer system has been developed to slmultaneously monitor four atomlc emlsslon wavelengths throughout an entlre chromatographlc run. A refractor plate mounted on a stepper motor Is used to acqulre background Information at each emlsslon wavelength. By appropriately stepplng the refractor plate, one also obtalns slgnal plus background Information at each wavelength. Thus, as the chromatography proceeds, dynamlcally background corrected data polnts are plotted yleldlng multichannel chromatograms. The Importance of such background correctlon IS clearly demonstrated through markedly enhanced selectlvltles and elimination or mlnlmizatlon of “false peaks”. This multlchanne1 detector approach can reduce chromatographlc amblguHies and with He plasma elemental emlsslon lead to determination of emplrlcal formulas for the elutlng substances. Detection limits, llnear dynamlc ranges, and preclslons available by this technlque are reported for a serles of envlronmentally Important compounds. Absolute detectlon llmlts are at nanogram levels and preclslons are on the order of 5 % relative standard devlatlon. These compare well with those reported for single-channel emlsslon methods whlle retalnlng the high selectivity that the background corrected experiment affords.
The recent resurgence of atomic emission spectrometry as a method for elemental analysis may be attributed to the use of the inductively coupled plasma (ICP), direct current plasma (DCP), and microwave induced plasma (MIP) as excitation sources. The high temperature (5000-7000 K), stability, and inert atmosphere provided by these types of plasmas result in an improved analytical performance a t trace levels when compared to traditional excitation sources such as flames and electrical arcs or sparks. Given the excellent analytical performance achieved with these excitation sources, it is not surprising that considerable interest has been shown in using plasma emission spectrometry for element-selective chroma-
tographic detection. The ---st reported use of F.-isma emission spectrometry for this purpose appeared in 1965 with the interface of a gas chromatograph and a reduced pressure MIP by McCormack et al. (I). Over 50 papers have subsequently been published. Overviews of previous chromatographic applications of plasma emission spectrometric detection, as well as details on the interface between chromatography and plasma emission spectrometry, have appeared in two reviews (2, 3). Plasma emission spectrometry is capable of providing selective and sensitive chromatographic detection for virtually any element in the periodic table. Many investigators have demonstrated the use of plasma emission spectrometric detection as a sequential multielement detector for chromatography by varying the single wavelength monitored for each successive injection. Simultaneous multielement detection for chromatography may greatly extend the scope and utility of this technique. To this end, several papers have appeared in which polychromator systems were used to monitor a number of atomic emission wavelengths throughout a chromatographic run. Windsor et al. used a polychromator system to evaluate the ability of the ICP to perform simultaneous, multielement analyses on each component of a mixture separated by gas chromatogaphy (4). A commercial low-pressure MIP and polychromator system, the MPD 850 (Applied Research Laboratories) has been used to determine the elemental ratios of compounds separated by gas chromatography (5,6). Other investigations have used polychromator systems as plasma emission detectors for high-performance liquid chromatography (HPLC). Applications have included HPLC determinations of metalloproteins (7), metal chelates (8), and metal salts of dialkylbenzenesulfonates (9). Clearly, these publications demonstrate the feasibility and utility of using polychromator systems as multielement plasma emission spectrometric detectors for chromatography. Unfortunately, these polychromator systems either lacked background correction capabilities or were not capable of rapid data acquisition rates. Background correction is particularly important when monitoring many of the nonmetal plasma
0003-2700/83/0355-1004$01.50/00 1983 American Chemical Society
