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Anal. Chem. 1988, 60, 2254-2258
Oscillometric Detector for Ion Chromatography. A Note on Detection Limit and Detector Sensitivity Ferenc Pll a n d Ern0 Pungor* Institute for General and Analytical Chemistry, Technical University of Budapest, Technical Analytical Research Group of Hungarian Academy of Science, Gell6rt tbr 4, H-1111 Budapest, Hungary Ervin sz. Kovlts Laboratoire de Chimie-technique de l’Ecole, Polytechnique Federale de Lausanne, CH-1015 Lausanne, Ecublens, Switzerland
Ion chromatography has become a wldely accepted analytlcal technlque. As a contrlbtiiion to this field, an Ion chromatographlc system uslng a new type of osclllometrlc detector has been tested. The performance of the detector has been tested flrst by calibration wlth solutions of known conductlvlty and pemdttlvlty (dielectrk constant) and then under chromatographlc condltions. Relationships were derived for the lnterconverslon of the sensltlvlty functions determlned in the two types of experiments.
MHz together with the heating and temperature control system comprises a single unit separated from the power supply and the measuring-amplifier unit. The instrument can be operated at four sensitivity settings, which enable it to be used in a wide concentration range. Cells of different volumes (e.g. 6 and 12 pL) can be matched to the oscillator by means of a variable capacitor, which enables measurements to be carried out within the most sensitive range of the oscillator. The output of the measuring amplifier, a dc voltage signal, is fed to a radio frequency noise fiiter, then to the sensitivity range switch, then to a compensator, and to the digital display. Apparatus. All the calibrations and measurements were made with the chromatographicsystem schematicallyshown in Figure
The common feature of conductivity detectors described in the literature is that there is a direct galvanic contact between electrodes and column effluent (1-8). In contrast to that, in the measuring cell of the detector developed for flow injection studies (9),the electrodes are separated from the electrolyte. They are isolated by a methylsilicone or Teflon coating, chemically resistant to most electrolytes used in chromatography. It has been demonstrated that this construction ensures a better reproducibility of the detector signal compared to that of cells with direct galvanic contact, both for the measurement of conductance and for that of the permittivity (dielectric constant) of the liquid. The ensemble described in ref 9 was developed for flow injection analysis. In the present paper its adaptation is described for ion chromatography. Its electronics were slightly modified but again based on the oscillometric measuring principle. It will be shown that the adapted cell performs at least as well as most existing detectors concerning sensitivity, having in addition a higher reproducibility and long-term stability. EXPERIMENTAL SECTION Reagents. Salicyclic acid, benzoic acid, and their Na salts, NaHC03, Na2C03,and dioxane were research grade chemicals from Fluka (Buchs, Switzerland). Double distilled water was prepared by distilling deionized water over KMn04 in a Pyrex glass still from Buchi (Flawil, Switzerland). Column Materials. Laboratory-madeanalytical columns were used. The synthesis of the column materials is described in ref 10. In our case the silicon dioxide was Zorbax (Du Pont, Wilmington, DE) with particle diameter of 7 pm. The hydrated (11) Zorbax was treated with a mixture of N-[(5-(dimethylamino)3,3-dimethylpentyl)-4-dimethylsilyl]-N,N-dimethylamine (DMP-A) and N- [ (3,3-dimethylbutyl)dimethylsilyl]-N,N-dimethylamine (DMB). The product contains 0.51 pmol m-* DMP-A and 3.08 pmol m-* DMB on the surface. Columns. Columns were stainless steel tubes of 0.46 cm i.d. and 25 cm length. The columns were packed by the slurry method with a solution of 1%NaCl in methanol. The concentration of the slurry was 10% and the packing time was 30 min at 400 bar. Detector. The detector cell has been described in detail in ref 9. The block diagram of the instrument is shown in Figure 1. The quartz crystal oscillator operating at a frequency of 42.37
2.
Chromatographic analyses were made with or without a suppressor column. A high-pressure pump from Waters (Milford, MA, Model 6000A) was used throughout the measurements. The pressure drop was -90 bar at a flow rate of 1 mL/min. The samples were applied to the column by using an injector from Rheodyne (Berkeley, CA, Model 7125) with lo-, 20-, and 100-pL loops. Columns where thermostated at 25.0 O C with a thermostat from Haake (Model D3). Before experiments doubledistilled water was pumped through the system,the base line was allowed to stabilize. At the beginning of the experiment eluent was pumped and the oscillator output was adjusted to a maximum by a variable capacitor. Calibration. In the calibrationprocedurethe analyticalcolumn was bypassed by a “zero column” (V, = 6 p L ) and the detector signal was recorded for solutions with different conductivitiesand permittivities, respectively. Eluent solutions were prepared with double-distilledwater. The eluents were fitered by use of a 22-pm membrane filter and degassed by purging with helium. Samples were prepared similarly, but were not degassed. RESULTS AND DISCUSSION The base line of the measuring system of ref 9 with the flow-through cell connected to a commercial oscillotitrator from Radelkis (Budapest, Hungary, Model OK-302/1) was sensitive to even very slight temperature changes. They affected not only the conductance of the solution but also the oscillator hardware. To overcome this problem, a device was constructed in which the oscillator was incorporated in a thermostated aluminum block inside the instrument box. The measuring cell was also placed here and it was connected directly to the oscillator. The eluent was thermostated separately. The base-line drift and the thermal noise of the chromatograms produced by the system (Figure 2) are determined by the rate of establishment of thermal equilibrium and by the extent to which ambient temperature changes affect the detector signal. By thermostating the system as discribed, a t a 3 “Cchange in the ambient temperature the liquid temperature remained constant within *O.l O C , and the detector signal within 0.5%. The oscillator circuit consists essentially of a quartz crystal oscillator that determines the measuring frequency and of the
0003-2700/88/0360-2254$01.50/0 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 20, OCTOBER 15, 1988
2255
OETfCTOR
SfNSOR
SILGi
I
COMRNSATING AMPLI F l f R
I
L
________________
J
.w
-1Y
1%
-5v
POWiR S U R Y
Flgure 1. Block diagram of the oscillometric detector.
I H JLC T O R
GUAROCOLUMN
20
Flgure 3.
60
40
B o €
Detector signal vs conductivity and permittivity curves.
In chromatographic practice the performance of the chromatographic procedure as a whole is characterized by the detection limit (DL) defined as follows:
DL fLUEN1
U
Figure 2. Chromatographic measuring system.
"indicator" circuit. The cell attached to the oscillator causes a conductivity and/or capacitance loss, the latter giving rise to a change in the resonance frequency. The system can be retuned to the original frequency by a variable capacitor. The conductivity change caused by the cell and depending essentially on the composition of the solution in the cell gives rise to a change in the amplitude of the signal. The dependence of the detector signal on conductivity and permittivity is shown in Figure 3. In a first series of experiments potassium chloride solutions of known conductivity were prepared. The measuring circuit was tuned to maximum sensitivity with double-distilled water in the cell and calibration was made with the prepared solutions. In a second series of experiments the system was tuned with dioxane in the cell and the permittivity calibration curve was determined with waterldioxane mixtures with different ratios. Actually these measurements fully characterize the sensitivity of the system as a chromatographicdetector if the signal measured is compared to the base-line noise, 6, of the detector which is observed under chromatographic conditions. The detector sensitivity is then calculated for the linear part of the calibration curve a5 follows:
where Ac is the concentration change at a given point on the line and Ah(c) is the corresponding change in detector signal in the same units as the base-line noise. In eq 1 DS is expressed in concentration units, actually it is the concentration change that generates a signal change of 56. Obviously, DS can also be given in terms of conductance or permittivity. In the example of Figure 3 the following values were observed: DS(KC1) = 1.6 pmol dm-l; DS (conductivity) = 0.1 p S DS(dioxane) = 1.4 mmol dm-3; DS (permittivity) = 0.01.
= 56-
G U
hmax
(2)
where C, is the solute concentration in the sample and h, is the peak height generated by the injection of the normal sample volume (same units as 6). The signal-to-noise ratio and the detection limit (DL) are dependent, in addition to detector sensitivity (DS), on the properties of the analytical column, the volume of sample injected, composition and pH of the eluent, the efficiency of thermostating, pump pulsation, and possibility of bubble formation in the detector. Consequently, DL is not a characteristic number for the detector. The detector sensitivity, DS, is a property of the detector only for a given solute ion. It can be determined from a chromatographic experiment following the proposal of T6th et al. (12). Let us assume that a known amount of solute, n,, (mmol), is injected and that the chromatographic signal generated can be approximated by a Gaussian (or double Gaussian) frequency curve. The concentration of the solute at the peak maximum, cm=, is given by
(3) where V,(mL) is the standard deviation of the Gaussian signal or the average standard deviation of the double Gaussian curve. (The value of 2Vu is found as the peak width at the height h = e-lI2hmar or 4Vu is found between the intercepts of the tangents of the peak with the base line. See Figure 4.) The detector sensitivity is given by
DS =
cmax56 (mol dm-3) hmax
(4)
Combination of eq 3 and 4 gives eq 5 for the experimental determination of DS
2256
ANALYTICAL CHEMISTRY, VOL. 60, NO. 20, OCTOBER 15, 1988
~
4
b
1
r \ -,-l
-
-.rz--c
Flgure 4. Determination of the minimum detectable concentration, characteristics of the chromatographic peak. The relationship between DL and DS is calculated as follows. The amount of solute, n,u(66),necessary to generate a detector signal with h, = 56 is calculated from eq 5. After rearrangement, eq 6 results
r~,,(~*)= DS Vg(27r)li2(mmol) (6) The injected amount is related to the injected sample volume, Vinj,following eq 7 : r ~ , , ( ~ *=) DL Vinj (mmol) (7) Combination of eq 6 and 7 gives the desired relationship DS V,(~T)'/~ DS Vu DL = -2.5 -(mol dm-3) (8) Vinj Vinj Finally, let us derive a relationship between column performance, characterized by the "number of theoretical cells" and DL. The variance of the signal is given by V,2 = Vu,02+ Vu,inj2(mL2)
(9)
where Vu,ozis the variance dur to peak broadening in the column and is the variance of the input signal. If the solute is injected as a dilute solution, the input signal can be approximated by a Heaviside step function with an approximate variance of (Vhj/2)2. Inserting this approximation into eq 9, eq 10 results
The number of equivalent theoretical cells, N , calculated by eq 11 is a measure of the column performance
N+) where VR (mL) is the gross retention volume. Combination of eq 10 and 11 gives eq 12 for the peak variance as a function of V, Vinj,and N
Insertion of eq 12 into eq 8 gives eq 13 for the detection limit DL=DS-
vi
(27r)1/2( 1 + N(".)' - )'" Vinj N vR
"(
2.5DSL(VinjN112 1 +. -
y)"'
"nj vR
-
(mol dm-3) (13)
30
2c
0
20
10
0
Figure 5. Comparison of chromatograms: (A) eluent, 1 mM sodium salicylate/3 mM salicyclic acid eluent; (6)eluent, 1 mM sodium benzoate/3 mM benzoic acid eluent; separator column, see text; flow rate, 2.5 mL/min; injection volume, 10 pL; solute concentrations, 50 ppm (1.4 X M NaCI, 6.2 X lo4 M NaBr, 8 X lo-' M NaN03, 3.9 X M NaI) in the case of each anion. The second term under the square root is small compared to 1 if small volumes are injected. In this case eq 13 simplifies to give DL =
2.5DSVR VinjN1/'
(mol d m 3
To prove the validity of the above theoretical considerations, chromatograms recorded under different experimental conditions were evaluated by using the above equations. The DS value calculated as 1.55 pmol dm-3using eq 5 for chloride from chromatographic data is in good agreementwith DS calculated by eq 1 and given earlier in this paper. DL was calculated from DS (from eq 5) by using eq 8 and eq 14 in which column parameters are also included. The injection volumes were 100 pL. DL obtained in both cases are equal to 3.5 X lo4 and 3.6 X lo4 mol dm-3, respectively, which are in sufficiently good agreement with DL calculated by using eq 2. After calibration of the system, a series of chromatographic experiments were performed for ion detection. In such experiments changes in conductance of aqueous solutions of practically equal permittivities were followed. In order to ensure a low limit of detection, the background conductivity of the eluent was reduced either by using a suppressor column (13,14) or by application of low-conductivity eluents, such as slightly dissociating organic acids (15, 16). In most experiments chloride, bromide, nitrate, and iodide were used as test solutes. With eluents of a concentration between low3and 5 x mol dm-3 and at flow rate of 1 mL/min, the highest retention volume was about 15 mL. At lower eluent concentrations the background conductivity is lower; however retention times and peak widths increase. The specific conductivity of a sodium benzoate/benzoic acid eluent is low. Unfortunately, this eluent has a low elution strength, as shown in Figure 5. The conductivity of sodium carbonate/sodium hydrogen carbonate eluents is between those of benzoic acidlsodium benzoate and salicyclic acid/ sodium salicylate at equal concentration. A drawback of this eluent is that, due to its alkaline pH, it causes a deterioration of silica substrate columns. On the
ANALYTICAL CHEMISTRY, VOL. 60, NO. 20, OCTOBER 15, 1988
2257
K.
Table I. Effect of t h e Suppressor Column in the Case of Different Eluents specific conductivity measured after the analytical column, FS cm-I without with suppressor suppressor
eluent 3 mM NaHC03/0.3 mM NazC03 4 mM benzoic acid 3 mM benzoic acid/l mM sodium benzoate 1 mM benzoic acid/3 mM sodium benzoate 2 mM salicylic acid/2 mM sodium salicylate
253 165 142
19 165 163
215
167
279
370
other hand, its conductivity can be effectively reduced by passing it through a suppressor column, whereas with the benzoate and salicylate eluents suppression results in no significant reduction in the conductivity, as shown in Table
I.
2
0
Chromatograms obtained with 3 mM NaHC03/0.3 mM Na2C03as eluent are shown in Figure 6. As clearly shown by the figure, eluent suppression with a hollow-fiber suppressor results in a remarkable increase in sensitivity. This increase is partly due to the reduction of the background conductivity, but to a greater extent to the fact that the counterions (e.g. alkali ions) of the anions are exchanged for H+in the suppressor, which has a higher mobility. This exchange and the mechanism of the formation of the elution peaks described by Kuwamoto et al. (17)are proved by the absence of a peak that, in the chromatogram obtained without suppressor,is due to the cations in the sample. The height of this peak depends on the total salt concentration of the sample and appears at a retention volume less than the V , value of the column. The chromatogram shown in Figure 7 was obtained with sodium salicylahalicylic acid as eluent, without a suppressor. Sodium salts of seven anions are injected. Similar chromatograms were taken with solutions containing increasing amounts of the anions and the peak heights were measured. The calibration curves for chloride, bromide, and nitrate are linear in the range of 0.5-30 ppm. DL and DS values calculated using eq 2 and 5, respectively, from actual chromatograms are summarized in Table 11. For comparison of the performance of our detector with a commercially available conductivity detector, chromatograms were run for the same samples using the same apparatus except that the detector was replaced by a conductivity detector from Gynkotek (Munich, West Germany, Model Lambda 1). The chromatograms obtained were very similar, as shown by the example in Figure 7 .
CONCLUSIONS The experimentalresults described in this paper clearly slow that the oscillometric detector and the analytical column designed and constructed in our laboratories are well suited for ion chromatographic purposes. For the chromatographic system constructed from the detector and columns, the minimum detectable amount of chloride, n-, calculated from
6
L
Bml
2
0
6
4
Em!
Figure 6. Effect of hollow fiber suppressor column: (A) chromatogram without suppressor column; (B) with suppressor column; separator column, Zorbax (7 pm) covered with DMP-A (3.7 pmol/m2); flow rate, 0.5 mllmin; injection volume, 10 pL; eluent, 3 mM NaHCO3/0.3 mM Na,CO,; solute concentration, 25 ppm (7 X lo4 M NaCI, 3.1 X lo-' M NaBr, 2 X lo4 M NaI) in both cases.
n
0
2
4
6
0
10
12
0
2
4
6
8
10
12
ml
-
mi
-
Figure 7. Separation of seven anions using (A) oscillometric detector and (B) conductivity detector: column: see text; flow rate, 1 mL/min; injection volume, 20 pL; eluent, 2 mM sodium salicylate/2 mM salicylic acid; solute concentration, 3 ppm CH,COO-, CI-, Br0,-; 8 ppm Br-, NO^-, I-, SO,*-.
Table 11. Values of the Minimum Detectable ConcentrationsCalculated Different Ways (Injected Volume, 100 ML) no.
eluent
C1-
106DL,mol/L BrNO3-
SO4'-
Cl-
106DS,mol/L BrNO3-
SO-:
1 2 3 4
1 mM sodium benzoate, 3 mM benzoic acid 1 mM sodium salycilate, 1 mM salicylic acid
3.5 2.2 3.0 0.5
4.0 3.4 3.8 0.62
3.7 2.8 3.4 0.58
1.7 1.3 1.6 0.25
1.9 2.0 2.0 0.35
2.0 1.9 2.1 0.34
0.3 mM NazC03/3.0 mM NaHC03 same as 3 but with suppressor
5.2 4.7 4.9 0.81
3.1 2.8
2.6 0.55
Anal. Chem. 1988, 6 0 , 2258-2263
2258
DL (eq 2) was found to be 5 X mmol (2 ng) in the case where a suppressor was used and between 2 X lo-' and 5 X lo-' mmol(8 and 20 ng) without a suppressor, depending on the eluent used. nmincalculated from DS (eq 1) was found to be in the range 4 X to 7 X mmol (150-250 pg). The value of DL, calculated by eq 8 or eq 14 from DS characteristic of the detector and from column parameters is in good agreement with DL calculated by eq 2 from actual chromatograms, which indicates that the value of DL, most important analytically, can be predicted from knowledge of the detector sensitivity and column parameters. Registry No. I-, 20461-54-5;NO3-, 14797-55-8;Br-, 24959-67-9; C1-, 16887-00-6;CH3COO-, 71-50-1; Br09-, 15541-45-4.
LITERATURE CITED (1) Tesarlk, K.; Kallb, P. J. Chromatogr. 1973, 78, 357-361. (2) Svobcda, V.; Marsal, J. J . Chromatogr. 1978, 148, 111-116.
Molnlr, I.; Knauer, H.; Wiik, D. J. Chromatogr. 1980, 201, 225-240. Johnson, D. E.; Enke, C. G. Anal. Chem. 1970, 42, 329-334. Jackson, P. E.; Haddad, P. R. J . Chromatogr. 1986, 355, 87-97. Glatz. J. A.: Girard, J. E. J. Chromtogr. Sci. 1982, 20,266-273. Jenke, D. R.; Pagenkopf, G. K. Anal. Chem. 1982, 54, 2803-2604. Cassidy, R. M.; Elchuk, S. J. Chromatcgr. Sc;. I983#21, 454-459. Pungor, E.; Pli, F.; T6th, K. Anal. Chem. 1883, 55, 1728-1731. Amati, D.; sz. KovBts, E. Langmuir 1987, 3, 887. Gobet, J.; sz. Kovlts, E. Adsorpt. Sci. Technol. 1984, 1 , 77. T6th, P.; Kugler, E.; sz. Kovlts, E. Helv. Chim. Acta 1959, 42, 2519-2530. Gustafson, F. J.; Markell, C. G.; Simpson, S. M. Anal. Chem. 1985, 57,621-624. Jenke, B. Anal. Chem. 1981, 53, 1535-1536. Okada, T.; Kuvamoto, T. Anal. Chem. 1983, 55, 1001-1004. Gjerde, D.T.; Fritz, J. S. Anal. Chem. 1981, 53,2324-2327. Okada, T.; Kuvamoto, T. Anal. Chem. 1984, 56, 2073-2078.
RECEIVED for review October 14, 1987. Accepted June 13, 1988. P. F. gratefully acknowledges the financial help of the Hamilton Foundation (S.E.A.).
Acoustic Signal as an Internal Standard for Quantitation in Laser-Generated Plumes Guoying Chen and Edward 5.Yeung* Ames Laboratory-USDOE
and Department of Chemistry, Iowa State University, Ames, Iowa 50011
To correct for puke-te.puke v(rrktlww In lasergenerated plumes, one needs to monltoT th. total mount of materlal vaporized durlng each puke. We f p M that the magnitude of the acoustic wave arsoalated with @utwgeneration Is linearly related to the OmWon lntenritles of both major and minor elements In the rl(l Over L wkle range of vaporkatbn pretrea signals the acoustlc slgnal can be used as an lnterw3I standard for normallzlng analytlcal signals derived frdm lasergenerated plumes.
The laser microprobe analyzer (LMA) was originally developed to enable qualitative analysis with high spatial resolution for samples of diverse natures (1). LMA has established itself as an appropriate tool for this purpose. Efforts have been made to improve its reproducibility so that it can be used as a reliable quantitative tool. However, so far this goal has not been fully realized. The reason is multifaceted. Firstly, as expected, the analytical precision is highly dependent on fluctuations in the laser power on a shot-to-shot basis (2). Secondly, matrix effects contribute to the irreproducibility of the result of analysis. This includes two factors, namely physical and chemical matrix effeds (3). The former refers to the influence exerted by the mechanical, physical, and also chemical nature of the sample surface on the vaporization process. Examples are grain size ( 4 ) , mechanical tension (4),crystal orientation ( 5 ) ,chemical composition (6),and so on. The latter, on the other hand, refers to the effects of foreign elements on the chemical composition, the total electron density, and the subsequent excitation process in the plasma, and consequently 0003-2700/88/0360-2258$01.50/0
on the spectroscopicsignals. This effect is closely tied to the composition of the solid sample itself (7). Finally, variation in experimental conditions is another factor to consider. These include laser focusing, angle of incidence of the vaporization laser, surface condition changes due to etching, different sample treatment procedures, oxidation conditions, contamination, and so on. Unfortunately, these factors generally cannot be precisely controlled. So, LMA remains only a qualitative or semiquantitative tool (3, 8), with a relative standard deviation usually in the range of 10-30%. Improved precision can be obtained by signal averaging over a number of laser pulses (9). However, the cost here is throughput and spatial resolution, since as different spots or different depths are being sampled, the physical conditions and chemical compositions might not be identical. This approach is thus not applicable to inhomogeneous solid samples. Other alternatives have been suggested to improve the analytical precision. These include the use of standard material with closely matched composition (IO),monitoring the power fluctuations of the vaporization laser ( I I ) , monitoring the size of the crater on the sample surface produced by the laser shot (12),and the use of internal standards (13). Among these methods, the use of standard material is highly recommended to improve the analytical accuracy (14). But, this cannot take into account the variations of experimental parameters, including laser power fluctuations, so the improvement in precision is quite limited. Besides, it is not always possible to find a standard sample close enough in composition to that of the sample under study. Monitoring the laser power results in, unfortunately, no improvement in precision of analysis (11, 15),since higher power does not necessarily produce a higher signal intensity. Instead, it can generate more ions, especially multiply charged and more energetic ions (16). Besides, reflectivity and thus absorption of the target material is intensity dependent. Crater size measurement is extremely tedious and 0 1988 American Chemical Society
