470
Anal. Chem. 1984, 56,470-473
a small fraction of the 0.2 pH sampling interval. The reiterative least-squares method appears to be well suited to the spectral resolution of major components in a mixture of organic acids, even where the overlap in both p H and spectral dimensions is severe. Further work will be required to characterize the performance of the method on minor components, where the IND function might be less successful. Application of the method to multiprotic acids would require some adjustment of the model for the C matrix, particularly for ionization steps which are not well separated. Extension of the technique to other equilibrium-based analytical methods, complexation or redox, for example, would be straightforward.
ACKNOWLEDGMENT The authors acknowledge the fine work of D. Heisler who constructed the spectrometer interface.
LITERATURE CITED Hlrschfeld, T. Anal. Chem. 1980,5 2 , 297A-312A. Kowalskl, 8. R. Anal. Chem. 1980,5 2 , 112R-122R. Shoenfeld, P. S.;DeVoe, J. R. Anal. Chem. 1976,48, 403R-411R. Sternberg, J. C.; Stlllo, H. S.;Schwendeman, R. H. Anal. Chem 1980.32.84-90. - . Hirschfeld, T. Anal. Chem. 1978,48, 721-723. Brown, C. W.; Lynch, P. F.; Obremski, R. J.; Lavery, D, S. Anal. Chem. 1982,5 4 , 1472-1479. Kralj, Z.; Sirneon, V. Anal. Chlm. Acta 1982, 138, 191-198. Ohta, N. Anal. Chem. 1973,45, 553-557. Connors. K. A.: Eboka. C. J. Anal. Chem. 1979. 51. 1262-1266. Spj~tvoll, E.; Martens,’ H.; Volden, R. rechnomefrics 1982, 2 4 , 173-180. Knorr, F. J.; Thorsheirn, H. R.; Harris, J. M. Anal. Chem. 1 ~ 8 15,3 , 821-825.
(12) Knorr, F. J.; Harris, J. M. Anal. Chem. 1981,5 3 , 272-276. (13) Bevington, P. R. “Data Reductlon and Error Analysis for the Physical Sciences”; McGraw-Hill: New York, 1969. (14) Scott, J. F. I n “Physical Techniques in Blological Research”; Oster, G., Pollister, A. W., Ed.; Academic Press: New York, 1955; Chapter 3. (15) Leggett, D. J. Anal. Chem. 1977,4 9 , 276-281. (16) Leggett, D. J.; McBryde, W. A. E. Anal. Chem. 1975,47, 1065-1070. (17) Leggett, D. J. Talanta 1977,2 4 , 535-542. (18) Leggett, D. J.; McBryde, W. A. E. Talanta 1974,2 1 , 1005-1011. (19) Leggett, D. J.; McBryde, W.A. E. Talanta 1975,2 2 , 781-789. (20) Gordon, W. E. Anal. Chem. 1982,5 4 , 1595-1601. (21) Shrager, R. I.; Hendler, R. W. Anal. Chem. 1982, 5 4 , 1147-1152. (22) Strang, G. “Applied Linear Algebra”; Academic Press: New York, 1976. (23) Golub, G.; Kahan, W. J. SIAM Numer. Anal., Ser. B 1985, 2 , 205-224. (24) Peters, G.; Wilkinson, J. H. Comput. J. 1970, 13, 309-316. (25) Nelder, J. A.; Mead, R. Comput. J. 1965, 7 , 308-313. (26) O’Neill, R. Appl. Stat. 1971,2 0 , 338-345. (27) Deming, S. N.; Morgan, S. L. Anal. Chem. 1973, 45, 278A-283A. (28) Morgan, S. L.; Deming, S. N. Anal. Chem. 1974, 4 6 , 1170-1181. (29) Malinowski, E. R. Anal. Chem. 1977,4 9 , 606-612. (30) Mallnowski, E. R. Anal. Chem. 1977,4 9 , 612-617. (31) “pH Ranges and Color Changes of Kodak Indicators”; Eastman Kodak Co.: Rochester, NY, 1978; Kodak Publication JJ-13. (32) McCue, M.; Mallnowski, E. R. Appl. Spectrosc. 1983,3 7 , 463-469. (33) Draper, N. R.; Smith, H. “Applied Regression Analysis”; Wiley: New York, 1981.
- - - - .--.
for September l2, 1983. Accepted DfX€dXr 6, 1983. This research was supported in part with funds provided by the National Institutes of Health through Biomedical Research Support Grant No. RR7092. Additional funding by the donors of-the Petroleum Research Fund, administered by the American Chemical Society, is acknowledged.
Characterization and Determination of Formaldehyde Oligomers by Capillary Column Gas Chromatography David F. Utterback,’ David S. Millington: and Avram Gold* Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina, Chapel Hill, North Carolina 27514.
A method has been developed for characterization of formaldehyde ollgomers In methanol-water solutions as their trlmethylsllyl derlvatlves, utlllzlng capillary gas chromatography wlth ammonia chemical lonlratlon mass spectrometry. The Me$ ollgomers form stable adducts with the ammonium Ion, (M NH,)’, permtltlng asslgnment of their molecular weights and structures. Quantitative analysis of formalin was accomplished by callbratlng the flame lonlratlon detector for molar response to the oligomers by use of closely related compounds. By thls method, the total formaldehyde content of formalin solutions was accounted for and the dlstrlbutlon of formaldehyde In the lndlvldual ollgomers accurately determined.
+
Despite decades of research, poly(oxymethy1ene) glycols, the oligomers of formaldehyde, and the monomethyl ethers Present address: D e p a r t m e n t o f H e a l t h Sciences, California State University-Fresno, Fresno, CA. Present address: D i v i s i o n of Genetics a n d Metabolism, D u k e U n i v e r s i t y M e d i c a l Center (Box 3028), Durham, NC 27710. 0003-2700/84/0356-0470$01.50/0
of poly(oxymethy1ene) glycols that form upon reaction with methanol, have not been well characterized. The existence of poly(oxymethy1ene) glycols has been established by thermodynamic calculations (1-3) and nuclear magnetic resonance (4) while the silylated oligomers have been partially separated and their relative distribution in formalin solution determined by gas chromatography (4). The analysis of formalin was based on the assumption that the first peak in the series eluting after solvent in the gas chromatogram was the bis(trimethylsilyl) derivative of methylene glycol. Unequivocal evidence for the validity of this assumption and for the structures of the two series of compounds requires additional physicochemical characterization of these compounds once they have been separated. We present here the resolution of the trimethylsilyl derivatives of the poly(oxymethy1ene)glycols and their monomethyl ethers by gas chromatography, their definitive characterization by chemical ionization mass spectrometry, and their quantitation by application of the flame ionization detector (FID) principle of equal per carbon response (5). The genesis of oligomeric oxymethylene glycols and the corresponding series of monomethyl ethers is represented formally in the following equilibria (6, 7): 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO.3, MARCH 1984
471
CH2=O + ROH + ROCH2OH (+ ROH -ROCH,OR) CH2=O + ROCHZOH + ROCH2OCH20H CH2=O + ROCHzOCH2OH * RO(CH20)2CH2-OH,etc. type I oligomers: R = H, oxymethylene glycols type I1 oligomers: R = CH3, oxymethylene glycol monomethyl ethers The type I1 oligomers have been referred to in earlier publications as "hemiacetals" or "acetals of poly(oxymethylene) glycols" (4).Since both types of oligomer are in fact hemiacetals of formaldehyde, we have adopted the less ambiguous nomenclature suggested above. Oligomers of both types I and I1 are thermally unstable and depolymerize readily when separation is attempted by gas chromatography. Replacement of hydroxyl protons by trimethylsilyl (Me3Si) groups stabilizes these formaldehyde polymers and prevents their decomposition on the GC column ( 4 ) . Earlier attempts to characterize the Me,Si derivatives by mass spectrometry were frustrated by the lack of molecular ions and differentiating fragment ions in their electron impact (EI) mass spectra (4),which prevented unequivocal assignment of molecular weight and elemental composition to the components. We have determined that chemical ionization mass spectrometry (CIMS) with ammonia as the reactant gas produces highly informative spectra from each of the Me3Si derivatized oligomers separated by capillary GC. Generation of molecular ions in initial GC/CIMS experiments led to the observation that, contrary to the assumption of previous workers, retention times of the derivatized glycols and monomethyl ethers overlapped on nonpolar columns (even capillaries), the glycol containing n formaldehyde units coeluting with the monomethyl ether containing n + 1 units. By the use of a suitable capillary column, the series was completely resolved for accurate quantitation on a gas chromatograph equipped with an FID. FID response factors for the derivatized oligomers were estimated by investigation of a series of Me3Si-derivatized alcohols and glycols.
r
,
l
r
l
r
8
l
l
~
l
I
I
24
1.5
~I
Il 32
I
~I
I
'
I
40
Figure 1. Resolution of formaldehyde Oligomers as Me@ derivatives on a 30-m DB-5 caplllary column wlth FID: (series G) poly(oxymethylene) glycols, n = 1-6 (series E) glycol monomethyl ethers, n = 1-7; M, methylal.
IB0
'0°1
R.1.
I
100-
I 30
( b) C H30 (CH20)4TMSI 6 'N H: 90 152
R*l Int
. 182 242 212
EXPERIMENTAL SECTION The derivatized formalin solution for analysis was prepared by homogenizing dimethylformamide (5 pL) with N,O-bis(trimethylsily1)trifluoracetamide (BSTFA, 25 pL) in a screw-top reaction vial and then adding 37% formalin solution (3 pL) at room temperature. The mixture was shaken until homogeneous and then analyzed by injection (1pL, split ratio 101) onto a 30-m fused silica capillary GC column (J & W Scientific, Rancho Cordova CA) coated with a chemically bonded methyl phenyl silicone liquid phase (DB-5). After a 2-min isothermal delay at 60 OC, the GC oven was programmed to 280 "C at 6 "C/min. The carrier gas was helium and the column was directly connected to the ion source of a double focusing mass spectrometer (Model 7070F, VG-Analytical Ltd., England) operated in the chemical ionization (CI) mode with ammonia (0.3 torr) as reactant gas. Under these conditions, ammonium ion (NH4)+is the dominant species (8). Data reduction was accomplished with a VG 2035F/B data system and mass spectra were recorded on a Versatec 800A printer/plotter. Quantitative analysis of derivatized formalin solution was performed utilizing the same column and temperature program, with a Varian Model 3700 gas chromatograph equipped with a flame ionization detector. The detector was maintained at 300 "C and the Grob type split/splitless injector (at a nominal 10:1 split ratio), at 250 "C. Helium was used as the carrier gas (1.5 ml/min) and a hydrogen (30 mL/min)-air (300 mL/min) mixture was supplied to the FID. Detector response was determined by peak area with a Hewlett-Packard Model HP3380 integrator. Molar FID response factors were determined by the derivatization of methanol (0.55 pL), ethanol (0.80 pL), 1-propanol(0.55 pL), ethylene glycol (0.75 pL), and 1,2-propanediol (10 p L ) as
'
,,me. ml"",..
300
Mass spectra of the Me,Si derivatives of (a) tris(oxymethylene) glycol and (b) tetrakis(oxymethy1ene)glycol monomethyl ether. The spectra are-consistentwith formation of ammonium ion addducts in the CI source from homologous poly(oxymethy1ene)glycol and glycol monomethyl ether derivatives. Flgure 2.
described above. Aliquots of the derivatization solutions were chromatographed with the following temperature program: 30 "C for 6 min followed by 10 "C/min temperature rise to 100 "C. Calibration solutions for methylal were generated by dilution of reagent grade methylal (Fisher Scientific) in methanol and analyzed on the Varian capillary gas chromatography system.
RESULTS AND DISCUSSION Resolution of the Me3Si derivatives of the poly(oxymethylene) glycols and their monomethyl ethers as well as methylal by capillary chromatography column is shown in Figure 1 and permits identification of the compounds by chemical ionization mass spectrometry. The ammonia CI spectra of the MeaSi derivatives of the poly(oxymethy1ene) glycols and their corresponding monomethyl ethers obtained during capillary column GC/MS are characterized by the presence of abundant complex molecular ions, (M + NH4)+,and fragment ions corresponding to the successive loss of CH,O units from the (M + NHJ+ ions. Less diagnostically useful but also prominent are signals a t m / z 30,47, and 90, ubiquitous in mass spectra of MeBSiderivatives. Typical examples of mass spectra are shown in Figure 2, which
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
472
Table I. Detector Factors for Derivatized Formaldehyde Oligomers compd as Me& ether CH,OH CH;CH,OH CH3(CH2)20H HOCH,CH, OH a
moles analyzed 1.29 x 1.24 x 6.83 X 1.24 x 1.21 x
10-7 10-7 10-7 10-7
av detector response x 1.004 1.337 0.935 2.113 2.308
effective carbonsa 3 4 5 6 7
mole effective C/unit area 3.85 x 3.70 x 3.65 x 3.52 x 3.66 x
10-13 10-13 10-13
10-13 10-13
Reference 10, p 94. Table 11. Comparisoii of Analytical Yield with Published Concentrations for Formaldehyde Oligomers in Formalin Solution
compound
Figure 3. Composite reconstructed Ion current chromatogram of Me,Siderivatized serles of poly(oxymethy1ene)glycols (G) with m l z 210 and !heir monomethyl ethers (E)with m / z 152. Subscripts refer to the number of formaldehyde units in each oligomer.
compares the ammonia CI spectrum of the Measi derivative of the tris(oxymethy1ene) glycol with that of the tetrakis(oxymethylene) glycol monomethyl ether. The fragmentation of (M + NH4)+ions in the CI spectra of all oligomers with greater than one oxymethylene unit results in a common m/z 210 ion in the spectrum for each of the bis (Measi) glycol derivatives and an m / z 152 ion in the spectrum for each of the MeaSi monomethyl ether derivatives. Therefore, composite reconstructed mass chromatograms for these two ions may effectively be used to identify the parallel oligomeric series as shown in Figure 3. Oligomers of up to eight formaldehyde units were detected by injection of large sample volumes onto the column but earlier eluting peaks seriously overloaded the system. The ammonia CI mass spectrum of methylal, the dimethyl acetal of formaldehyde, contained a complex molecular ion (M NH4)+a t m / z 94 with fragment ions at m/z 75,45,32, and 30. Definitive evidence for the proposed structures was provided by verification of the elemental composition of selected ions by peak matching a t a resolution of 7000 (10% valley definition) during GC/MS. By use of the protonated molecular ion of dimethyl phthalate as the internal reference (m/z 195.0657, introduced continuously from a reference inlet), the accurate mass of (M NH4)+ from the methylene glycol derivative eluting at 9.8 min (Figure 3) was confirmed as m / z 210.1346 (C7Hz4NOzSiz).Similarly, an identical composition for the ion corresponding to (M + NH4 - CH20) from the bis(oxymethy1ene) glycol derivative eluting at 13.3 min was confirmed. With the same internal reference, the proposed (M + NH4)+ ion corresponding to the tris(oxymethy1ene) glycol monomethyl ether derivative was confirmed as C7HzzN04Si( m l z 212.1318). Since the accuracy of this comparison is on the order of f 5 ppm, the assigned elemental compositions of the salient ions are unequivocal assuming that the elements present are C, H, N, 0, and Si. In our experience, other CI reactant gases including methane and isobutane did not yield mass spectral data any more informative than the E1 spectra. The success of the ammonia CI technique apparently depends on the formation of stable adducts of the derivatized poly(oxymethy1ene) glycols
+
+
CH,OCH,OCH, HOCH,OH HO(CH,O),H HO(CH,O),H HO(CH,O),H HO(CH,O),H HO(CH,O),H HOCH,OCH, HO(CH,O),CH, HO(CH,O),CH, HO(CH,O),CH, HO(CH,O),CH, HO(CH,O),CH, HO(CH,O),CH,
concn, mo1/100 g previously reporteda found
mol of formaldehyde as CH,O
10-3 1.17 x 10-3 10" 7.94 X 10.' l o - ' 1.84 X lo-' lo-' 1.04 X 1 0 - l 10.' 4.60 X lo-' lo-' 1.94 X lo-' N/R 10-3 8.64 x 10-3 10.' 2.23 X 10-' 2.16 X 10-l 1.07 X lo-' 2.14 X 10-' 9 . 3 X 10" N/RC 3.85 X 10.' 1.16 X 10" N/RC 1.40 X lo-' 5.60 X 4.90 X 2.45 X N/RC N/RC 2.94 X lo-' 1.77 X 1.77 X l o M 31.24 X lo-, N/RC total formaldehyde analyzed 1.11 mol total formaldehyde expected 1.23 mol
N/R
1.04 X 10" 9..1 X 3.9 X 9.0 X 5.0 x lo-'
1.17 x 7.94 X 9.18 X 3.48 X 1.15 X 3.88 X 1.44 x 2.23 X
a Source: ref 4. Determined by NMR analysis. Concentration of individual monomethyl ethers with greater than two formaldehyde units could not be determined.
and their monomethyl ethers with the ammonium ion. To account for this, we propose a cyclic structure for these adducts in which the charge is delocalized and suggest a mechanism that accounts for the formation of the common stable m / z 210 and 152 ions as shown in Figure 4. These ions might also arise by successive elimination of CHzO units from (M + NH,)+. Similar structures have been proposed to account for the adducts of ammonium ions with some carbohydrates under CI conditions (8, 9). Since the derivatized poly(oxymethy1ene) glycols and monomethyl ethers cannot be isolated, a standard curve plotting detector response vs. oligomer mass cannot be generated. However, a series of related compounds can be derivatized by BSTFA and analyzed to approximate the molar response factors for the trimethylsilylated formaldehyde oligomers. Investigation of the flame ionization detector output for a variety of hydrocarbons has indicated an equal per carbon response by the detector (5). Atoms other than carbon and hydrogen will reduce the molar flame response to hydrocarbons. Ether oxygens have been shown to decrease molar flame response by an amount equivalent to reducing the number of carbon atoms by one (IO). On the basis of these observations, expected detector response for each of the derivatized poly(oxymethy1ene) glycols and monomethyl ethers was calculated to be principally a function of the trimethylsilyl groups attached to the oligomers. Verification of this approximation was obtained by the derivatization and analysis of the series of primary alcohols and glycols in Table I. The peak area per mole of carbon (calculated according to Sevcik (IO)) is essentially the same for all the derivatized alcohols mol and glycols with an average molar response of 3.70 X
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
m/z
R.R’=TMS R=CH3,R’=TMS
R,R’=TMS m/r 210 R=CH3.R1=TMS mi. 152
240,270,etc.
m/z 152,182.etC.
Figure 4. Suggested mechanism for genesis of stable daughter ions from ammonium adducts of poly(oxymethy1ene)glycol derivatives. Monomothvl Ethers
Formaldahyde Unit,
q , I , , j:i 1 I
a
¶
3
Foimmld.hyd.
.
1
Unit8
,
0
,
O’
1
2
determined from Table I was divided by 6 to obtain the proper molar response factor for the derivatized poly(oxymethy1ene) glycols and by 3 for the molar response factor of the monomethyl ethers. In Table 11, the concentrations of the oligomers found in this study are compared to those previously reported for the partial analysis of a similar solution. It must be borne in mind that the concentrations of glycols reported in ref 4 include the monomethyl ether containing one additional formaldehyde unit. The distribution of the formaldehyde oligomers and total formaldehyde as CH20 units per oligomer is visualized in Figure 5. The total formaldehyde content as moles of CHzO determined from quantitation of the BSTFA-derivatized mixtures is virtually identical with the nominal content expected for the 37% formalin solution. Registry No. CH20, 50-00-0; CH30CH20CH3,109-87-5; HOCHZOH, 463-57-0; HO(CHz0)2H, 4407-89-0; HO(CH,O),H, 3754-41-4;HO(CHZO),H, 28317-12-6; HO(CH20)6H,28317-13-7; HO(CH20),H, 28317-14-8; HOCH20CH3, 4461-52-3; HO(CH2O)&H3, 19942-08-6;HO(CH2O),CH3,87728-58-3;HO(CH,O),CH, 87728-59-4; HO(CH20)5CH3, 87728-60-7; HO(CH20)8CH3, 87728-61-8; HO(CHZO)$HS, 87728-62-9; TMSO(CH20)3TMS, 87728-63-0; CH30(CHz0)4TMS, 87728-64-1.
LITERATURE CITED
O‘
,
473
3
1
I
5
,
8
,
7
Form.ldehrde Unlll
Figure 5. Relative dlstrlbution of formaldehyde In aqueous-methanollc Solution In terms of oligomer concentration and total formaldehyde content per oligomer determined by the BSTFA derivatlzation method.
C per unit area. The data clearly indicate reliability of this method in determining molar response factors for the flame ionization detector. The response factor for methylal was determined to be 5.66 X mol/unit area. Concentrations of the oligomers in formalin solution are given in Table 11. The molar effective carbon response factor
(1) Ileceto, A. Gazz. Chim. Ita/. 1954, 8 4 , 536-552. (2) Bryant, W. M.; Thompson, J. B. J. Polym. Sci., Polym. Chem. Ed. 1971, 9 , A - I , 2523-2540. (3) Williams, I. H.; Maggiora, G. M.; Schowen, R. L. J. Am. Chem. SOC. 1960, 102, 7a31-7a39. (4) Dankelman, W.; Daeman, J. M. Anal. Chem. 1976, 48, 401-404. (5) Blades, A. T. Chromafogr. Sci. 1973, 7 7 , 251-255. (6) Walker, J. F. “Formaldehyde”, 3rd ed.; Relnhold: New York, 1964;p 66. (7) Walker, J. F. ”Formaldehyde”; Relnhold New York, 1964;p 96. (8) Hunt, D. F. A&. Mass. Spectrom. 1974, 6 , 517-522. (9) Winkler, F. J.; Stahl. D.J. Am. Chem. SOC. 1979, 707, 3687-3688. ( I O ) Sevcik, J. “Detectors in Gas Chromatography”; Elsevier: New York, 1976;p 94.
RECEIVED for review August 12, 1983. Accepted October 7, 1983. This work was supported by the University of North Carolina Biomedical Research Support Grant No. 2507 RR05450-21.
