Rapid Gas Chromatographic Determination of 2-Hydroxybenzoic and 2,4=DihydroxybenzoicAcids in Copper and Lead Compounds Bernard J. Alley and Hiram W. H. Dykes U S .Army Missile Command, Redstone Arsenal, Ala., 35809 COMPOUNDS CONTAINING 2-hydroxybenzoic and 2,4-dihydroxybenzoic acids in various structural combinations with lead and copper are used as additives in certain double-base propellants ( I ) to modify ballistic properties, and the acid contents of such additives must be determined for complete characterization and evaluation of materials. The known analysis methods for determining acid ligands in copper and lead compounds (2, 3), based on spectrophotometry and titrimetry, are not only involved, time-consuming, and subject to interferences from the metals, but also lack the degree of precision and accuracy required for propellant evaluation. Gas-liquid chromatography (GLC) was investigated as an alternate approach to circumvent these problems. The GLC methods reported for analyzing derivatives of free hydroxy aromatic acids (4-6) were unsuitable for determining 2-hydroxybenzoic and 2,4-dihydroxybenzoic acids combined with metals in salts and chelates. However, continuing experimentation with modifications to these procedures succeeded in developing a rapid, precise GLC procedure optimized for the propellant applications of interest. The method generally consists of removing the acids from the compounds and forming their trimethylsilyl (TMS) derivatives (6, 7) in a single step, followed by separation of the derivatives on a short, lightly loaded column. The derivatives are detected by a flame ionization detector (FID), and accurate quantitative results are provided by an internal standard technique and an electronic digital integrator for measuring peak areas. EXPERIMENTAL
Apparatus. The chromatograph used was a HewlettPackard 7624A equipped with a 3370A electronic digital integrator. The dual columns were 1/8-in. x 2-ft stainless steel tubing packed with 3% by weight UCW-98 stationary phase on 80-100 mesh Gas-Chrom Q solid support (Applied Science Laboratories). The packing was prepared by a filtration-fluidization procedure (8) that produced a uniform, high quality coating of the stationary phase on the solid support. Before use, the packed columns were conditioned overnight in the chromatograph at 250 “C. Helium carrier gas at the rate of 10-15 mljmin was passed through the columns during conditioning. The carrier gas and the air and hydrogen for the dual FID were passed through molecular sieve driers before they entered the chromatograph. Chemicals. The silylation reagents TRI-SIL, TRI-SIL/ BSA Formula P, and N,O-bis(trimethylsily1)acetamide (BSA) were purchased from Pierce Chemical Co. The n-hexadec(1) A. T. Camp and F. C. Crescenzo, US.Putem 3138499 (1964). (2) S. W. Bowen, “Procedure for Analysis of All Combinations of Lead and Cupric Salts of Salicylic and 0-Resorcylic Acids,” Hercules, Inc., Radford, Va., April 1970. (3) MIL-STD-286B, Method T225.1, July 1969. (4) E. R. Blakley, Anal. Biochem., 15, 350 (1966). (5) C. M. Williams, ibid., 11, 224 (1965). (6) R. F. Coward and P. Smith, J. Chrornnrogr., 45, 230 (1969). (7) A. E. Pierce, “Silylation of Organic Compounds” Pierce Chemical Company, Rockford, Ill., 1968, p 161. (8) Applied Science Laboratories, Inc., State College, Pa., Bull. 2A (1967).
ane (Eastman Organic Chemicals) used as an internal standard was practical grade. The 2-hydroxybenzoic and 2,4-dihydroxybenzoic acids (J. T. Baker Chemical Co.) used for preparation of calibration standards were reagent and practical grades, respectively, and the 2,4-dihydroxybenzoic acid was recrystallized from water before use. The lead and copper compounds of the acids were commercial products obtained from Shepherd Chemical Co., National Lead Co., and American Cyanamid Co. Procedure. The calibration factors for quantitative determinations were established by first preparing and analyzing a standard mixture of the internal standard and TMS derivatives of the free acids. Since previous experimentation had established that the FID response as a function of acid concentration was linear with zero intercept over the concentration range used, the calibration factor, F, could be calculated for each acid derivative from the expression F =
(n-hexadecane peak area) (mg of acid) (TMS-acid peak zrea) (mg of n-hexadecane)
(11
The calibration factors were essentially constant over a period of several weeks of operation: analyses of 25 different Calibration mixtures during a one-month period gave factors of 0.862 i 0.005 and 0.797 i 0.006 for 2-hydroxybenzoic and 2,4-dihydroxybenzoic acid derivatives, respectively. The analysis procedure was the same for the standard calibration mixture and the propellant additive samples. Preparation consisted of the following steps: 3-4 mg of each free acid or 3-5 mg of the additive sample was weighed, to a precision of *1 pg by a Cahn Gram Electrobalance, into a DSC (differential scanning calorimeter) aluminum pan (Perkin-Elmer); 2-3 mg + 1 pg of n-hexadecane was also weighed into the pan and the pan was placed in a dried 1-dram polyethylene-stoppered glass vial; 1 ml each of TRI-SIL and BSA was added, in that order, to the vial and the sealed vial was shaken for 5 minutes on a No. 5000 mixer/ mill (Spex Industries); the reaction mixture was then centrifuged and allowed to stand at room temperature for 30 minutes to ensure complete reaction prior to analyses. A 1-p1 aliquot of either the calibration or sample reaction mixture was injected on the column at a temperature of 70 OC and held at this temperature for 1 minute. The oven temperature was then programmed to 190 “C at the rate of 15 “C/min. The F I D temperature was maintained at 200 “C, and the helium flow was set at 40 mlimin. Peak areas were measured with the 3370A integrator. A typical chromatogram, recorded at a full-scale sensitivity of 4 X lo+’ A, is shown in Figure 1. Retention temperatures for TMS-2hydroxybenzoic acid, n-hexadecane, and TMS-2,4-dihydroxybenzoic acid were, respectively, 129, 140, and 162 “C. The acid derivatives gave well-resolved symmetrical peaks. The percentage of acid in an analyzed additive sample was calculated by the linear expression Wt
of acid
=
(TMS-acid peak area) (mg of n-hexadecane) F(100) --__ (2) (n-hexadecane peak area) (mg of sample) Supplementary experiments showed that the TMS derivatives of such acid impurities as phenol and resorcinol eluted before
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Table I. Accuracy of GLC Metbod for Analysis of Free Acid Mixtures Mix2,CDihydroxybenzoic ture 2-Hydroxybenzoic acid, mg acid, mg No. Taken Found Error Taken Found Error 1 3.42 3.38 -0.04 3.33 3.34 0.01 2 3.11 3.12 0.01 3.17 3.17 0.00 -0.03 3 3.39 3.36 3.52 3.50 -0.02 4 3.31 3.27 -0.04 3.93 3.90 -0.03 5 3.27 3.25 -0.02 3.29 3.29 0.00 0.02 6 3.11 3.13 3.62 3.68 0.06 7 3.52 3.52 0.00 3.62 3.63 0.01 8 3.51 3.48 -0.03 3.30 3.28 -0.02 9 2.98 2.99 0.01 3.90 3.90 0.00 10 3.14 3.12 -0.02 3.15 3.13 -0.02 Mean error -0.01 0.00 0.3 Relative error, 0.0
0
2 4 6 8 RETENTION TIME (min)
Figure 1. Gas chromatogram for the separation of TMS-2-hydroxybenzoic acid, n-hexadecane, and TMS-2,4-dihydroxybenzoicacid the acid derivatives and could also be quantitatively determined by the same procedure. RESULTS AND DISCUSSION
Silylation Reagents. A series of preliminary analyses selected a reaction mixture of TRI-SIL and BSA, containing 1 ml of each, as the optimum silylation reagent. The TRISIL alone proved to be effective in dissolving the additive samples but did not achieve 100% conversion (generally in the 90-100% range) of the acids to their TMS derivatives. The BSA alone was a poor solvent for the samples, but when combined with TRI-SIL provided quantitative conversion. The mixture proved to be more effective in 2-ml quantities (1 ml of each) than in smaller amounts. Injecting reaction mixtures of TRI-SIL and the propellant additives, which contained small amounts of unreacted free acids because of incomplete conversion, caused decomposition of the acids at the head of the column. This in turn produced some decomposition of TMS derivatives (particularly noticeable in the case of the 2,4-dihydroxybenzoic acid derivative) and rapid deterioration of column performance. Neither column life nor performance was affected by analysis of the reaction of the 1 :1 mixture of TRI-SIL and BSA with the propellant additives, which left no free acids. Precision and Accuracy. The accuracy of the GLC procedure was demonstrated by analyzing 10 mixtures containing known weights of the two free acids. The results, listed in 184
Table I, gave a mean error of -0.01 mg for the 2-hydroxybenzoic acid and of 0 for the 2,4-dihydroxybenzoic acid determinations. A truer test of the precision and accuracy of the method, however, is the determination of the chemically combined acids in chelates and salts of copper and lead: since the acids must be removed from the compounds before the TMS derivatives can be formed, an additional error source is introduced. The reproducibility of the method for determining both acids in a single copper chelate is illustrated by the data in Tables I1 and I11 obtained from five samples. To ensure that the variability among samples included all major sources of error, each of the five samples was analyzed on a different day using a different calibration mixture to determine the calibration factors. The estimated standard deviations (Table 11) are for replicate analyses of aliquots of the same sample. The pooled relative standard deviations with 13 degrees of freedom were 0.531 % for 2-hydroxybenzoic acid and 0.320% for 2,4-dihydroxybenzoic acid. The analysis of variance ( 9 ) outlined in Table 111 compares the variances within and among the same five samples. The variance among samples is significantly greater than the variance within samples, as would be expected, but is nevertheless small and represents an improvement over other types of methods (2,3). To obtain an estimate of the accuracy of the GLC method for determining the acids in the chelate, the calculated total carbon percentage from the GLC average of samples was compared with a carbon determination by elemental analysis. The resulting values were 28.3% for the GLC calculation and 28.2 =t0.2 %for the elemental analysis. To establish the general applicability of the method, determinations were made of 2-hydroxybenzoic and 2,4-dihydroxybenzoic acids in a wide variety of commercial lead and copper compounds that are used as propellant ballistic modifiers, with the results given in Table IV. In all cases, the differences between GLC and elemental determinations are well within the experimental errors of the two methods. CONCLUSIONS
The results of these series of experiments demonstrated that the GLC method presented here is both precise and accurate, and that it can be applied without modification to (9) C. R. Hicks, "Fundamental Concepts in the Design of Experiments," Holt, Rinehart and Winston, New York, N.Y., 1964,
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Table 11. GLC Determination of 2-HydroxybenzoicAcid and 2,4-Dihydroxybenzoic Acid in a Copper Chelate 2-Hydroxybenzoicacid 2,LCDihydroxybenzoic acid Number of Sample No. replicates Wt Std dev Wt Std dev 1 4 11.81 0.019 38.86 0.117 2 4 11.74 0.046 38.81 0.080 11.81 0.115 38.81 3 3 0.215 4 3 11.80 0.076 38.84 0.110 5 4 12.02 0.043 38.38 0.088
z
z
Table 111. Analysis of Variance for Determination of Acid Ligands in a Copper Chelate Degrees of 2-Hydroxybenzoicacid 2,CDihydroxybenzoic acid Source of variation freedom Sum of squares Mean square Sum of squares Mean square Among samples 4 0.17762 0.04440 0.63777 0.15944 Within samples 13 0.05142 0.00396 0.20038 0.01541 Total 17 0.22904 0.83815 Table IV. GLC Determination of 2-Hydroxybenzoic and 2,4-DihydroxybenzoicAcids in Copper and Lead Compounds Compound Bis(2,4-dihydroxybenzoato)lead(II)
2,4-Dihydroxybenzoatolead(II) Bis(2-hydroxybenzoato)lead( 11) 2-Hydroxybenzoatolead(II)
GLC duplicates ,' 59.03
159.09 149.94 i49.78 157.10 157.72 139.32 139.38
2,4-Dihydroxybenzoatocopper( 11)
Bis(2-hydroxybenzoato)copper( 11)
178.26 178.82
2-Hydroxybenzoatocopper(II) (Type 1)
2-Hydroxybenzoatocopper(II) (Type 2)
!66.40 166.14
Acid, wt GLC average
Elementala
Difference
59.06
59.04
0.02
49.86
49.65
0.21
57.41
57.34
0.07
39.35
39.68
63.76
63.47
78.54
79.04
-0.50
61 .OO
61.58
-0.58
66.27
66.38
-0.11
(GLC- elemental)
-0.33 0.29
Calculated from elemental carbon analysis.
copper and lead compounds containing all possible cornbinations of the two acids. In addition, the method is simpler, more rapid, and less subject to interferences than the techniques currently used for determining acids in propellant ballistic modifiers. Additional experimentation on a limited scale has indicated that the method can also be applied to other copper and lead compounds containing ligands with oxygen coordinating atoms, and to direct determination of the ligands in nitro-
cellulose-base propellants. Applicable GLC procedures have recently been reported (10) for determining volatile components of these propellants.
RECEIVED for review May 12, 1972. Accepted August 21, 1972. (10) B. J. Alley and H. W. H. Dykes, J. Chrornafogr.,71,23 (1972).
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