802
Anal. Chem. 108% 61, 802-805
Thermally Labile Salts for Aiding Quantitation of Reactive Decomposition Products John J. Morelli,* Michael A. Grayson, and Clarence J. Wolf McDonnell Douglas Research Laboratories, P.O. Box 516,St. Louis, Missouri 63166
Quantltatlve analytlcal procedures udng external standards were devdoped for NH, and HF evdutlon observed durlng evolved-gas mass spectrometric analysts. The callbratlon standards cond8ted of thermally labile salts dlsperred In an Inert d i d powder. For NH, quantltlcatlon, mlxtures having varylng concentrations of NH,Br dkpersed In 120/14O-mesh glass beads were used, whereas HF quanttficatbn requlred NaHFdNaF mMures. The preclslon of the measurements on unknowns was determined from the carllbratlon curves and was generally wlthln f30% (rdatlve standard devlatbn). The quantltathre technique was applied to evaluate HF evoMkn from fmulsted poly(ethylene-tetrafluoroethylene) wire Insulatlone. The source of the HF" slgnal observed in the evdved-gas-analyds mass spectra was detemdned to be due to direct electron Impact lonlzatlon of HF gas evolved from the wlre Insulations during heatlng.
INTRODUCTION Commercial plastics consist of a wide variety of materials (1)whose exact compositions and formulations are frequently complex and difficult to analyze. In many cases, polymers may evolve toxic or reactive species during extremes encountered in the use-environment. Furthermore, relatively minor changes in plastics formulations may promote the evolution of hazardous species at normal environmental conditions. Quantitative analysis of reactive compounds either indigenous or produced pyrolytically within a polymeric material is difficult. One common analytical technique for quantitative analysis, combined gas chromatography/mass spectrometry (GC/MS) (2-4), may not be amenable to polymer systems because the analyte within the polymer has low volatility or is highly reactive. Mass spectrometry (MS) coupled with controlled heating to pyrolysis temperatures, Le., evolved-gas-analysis mass spectrometry (EGAMS) (5),can circumvent both of these drawbacks, provided a means of instrument calibration can be devised. Because cross-linked polymers are generally intractable, quantitative schemes employing internal standards are not practical. We have developed a procedure for the quantitative analysis of two reactive and potentially toxic compounds, hydrogen fluoride (HF) and ammonia (NH,), released from an otherwise intractable matrix, the analysis employs external standards. The quantification of H F release from an irradiated ethylenetetrafluoroethylene (ETFE) electrical insulation is presented as an example. EXPERIMENTAL SECTION Instrumental Methods. Mass Spectrometry. All EGAMS analyses were obtained by using the first two sectors of a VGZAB-3F instrument (magnetic and electrostatic sectors, respectively). The solids probe for the VG mass spectrometer, equipped with a resistance heater and thermocouple, was used for temperature measurement and control. The sample was heated from 40 to 333 OC at a linear rate of -10 OC/min. The mass spectrometer was continuously scanned during the analyses at a rate of one spectrum (from 10 to 800 daltons) every 10 s. At the 0003-2700/89/0361-0802$01.50/0
conclusion of the heating process, the sample temperature was held at 333 "C for approximately 15 min. The evolution of a particular compound can be monitored by plotting the intensity of a characteristic ion as a function of scan number. Such plots are called extracted ion current profiles. In addition, the complete mass spectrum can be examined at any temperature to obtain information about the mixture of materials evolved. When more information is required for positive identification of a compound in an evolved mixture, a variety of tandem mass spectrometric techniques can be applied with the VG-ZAB-3F instrument. More information on the EGAMS technique can be found in a review by Langer (5). Thermograuimetric Analysis (TGA). The decomposition temperature and range of several materials were monitored by TGA. All TGA analyses were performed by using a Du Pont 951 equipped with a Du Pont 9900 computer for control and data manipulation/storage. Samples were subjected to a linear heating rate of -10 OC/min and a Nz purge rate of 25 mL/min. Preparation of Calibration Standards. HF Quantification. The controlled thermal decomposition of NaHF? was used to generate HF. Calibration standards were prepared by combining NaHFz (sodium bifluoride) with NaF in a clean mortar. Typical standards contained 1-10 ppt (parts per thousand) NaHF? These materials were gently ground with a clean pestle for 1to 2 min. One to two milligrams of the standard were placed into a glass capillary for EGAMS analysis. Typical samples of NaHFz yield 1-10 pg of HF during EGAMS analysis. The mass range from 10 to 800 amu was scanned and the total area under the HFtemperature (m/z 20.01) curve was determined by using the integration software available with the VG-ZAB-3F. The system was calibrated every day. The first EGAMS analysis each day used 4040 pg of NaHFz to 'condition" the ion source prior to calibration run. Standards were run before and after each unknown; however, the standards were always analyzed from highest concentration of NaHFz to lowest to minimize variations in the ion source performance at low concentrations. At least four standards were used each day. Ions at m/z 19.02 (HDO"), 19.98 (A$-+), and 20.01 (HF'+) were monitored with a spectral resolution (M/M.I) of 1250. The response factor (slope of calibration plot) for m/z 20.01 (HF'+) was monitored daily and was found to decrease linearly over the 4 days required for the EGAMS analyses of the ETFE wires, reaching 60% of ita initial value. This decrease probably results from filament aging which is typical of electron-impact ion sources and is accelerated by the presence of HF. The total amount of HF evolved from the sample was determined by the relation (1) HFbd = AREA,~RF,/,~,ol + c where HF is the weight in micrograms, AREAbd is the total HF signal area, RFm,r20.01 is the response factor (slope of calibration plot) of the ion at m/z 20.01, and c is a constant (interceptof plot). The quantity of HF released in a given temperature interval was determined from the ratio of the area in that interval to the total area times the total quantity of HF, i.e. HF,, = (AREA,,/AREA,M)*HF,d (2) Our ability to determine HF evolution within a temperature interval relative to the total is much more precise than our capability for absolute quantification of HF. NH, Quantification. NH4Br thermally decomposes to NH3 and HBr; hence, NH4Br was used to calibrate the mass spectrometer's response to NH3 Standards were prepared by combining 1-10 ppt of NH4Br with glass beads (120/140 mesh) by using the same methodology described above for the NaHFz/NaF 0 1989 American Chemical Society
ANALYTICAL
CHEMISTRY. VOL. 61. NO. a. APRIL
is. 1989
1m.5
1m.o
803
0.12 '1.
E g x 2 m " d d l y ( N
0.10
LEMI Shield
Figure 1. Schematic representation of wire with ETFE insulator and jacket. calibration standards. The ion signals at m/z 17.00 (OH') and m/z 17.03 (NH,") were extracted hv usine the VG-supplied software. The &curacy of the procedure w& checked by ;sing a standard NH,Br sample. Preparation of Samples. Wires. Irradiated ETFE i n d a t e 3 wires were obtained from three manufacturers. Each of the wires, schematically illustrated in Figure 1, consisted of two or more irradiated ETFE insulated conductors wrapped with shield and covered with an irradiated ETFE jacket. For EGAMS analysis the wire bundle was disassembled and 1- to 2-mg sections of insulation were placed in a glass capillary (2-mm 0.d.).
RESULTS AND DISCUSSION Quantification. Calibration techniques were developed by utilizing external standards. T o minimize the effects of variability in instrumental performance from sample-tc-sample and from day-to-day, it was important to devise calibration standards amenable to analysis with the VG solids prohe. The salts chosen for calibration must he obtainable in pure form and must generate the species of interest in sufficient quantity. For example, NaHF, produces H F when heated to 150 O C and above (6) NaHF,(s) NaF(s) + HF(g) (3) and ammonium hromide (NH,Br) decomposes to form ammonia (NH,) and hydrogen hromide (HBr) NaMs) NH&) + H B r k ) (4) Fischer measured the dissociation of sodium hifluoride hetween 157 and 269 "C and reported an enthalpy change of 6.73 X 104 J/mol(16.1 kcal/mol). Most importantly, both reactions shown in eq 3 and 4 proceed to completion, and the amount of gaseous product found is a direct function of the amount of salt used. The . oreparation of solid calibration standards reouired the . selection of inert solid diluents and a homogenization procedure for combining the diluent and calihrant. NaF was used as the diluent for NaHF,. The thermal stability of NaF was determined by TGA. NaF exhibited no measurable weight loss even up to 500 "C. NaF was not a suitable diluent with the NH,Br because it reacts a t elevated temperatures to produce HF. Glass heads were used for the NH,Br diluent. The TGA data from 120/140-mesh glass-head chromatographic support are shown in Figure 2. Both weight loss (dashed curve) and derivative weight loss (solid curve) are plotted as a function of temperature. At approximately 100 "C, a 2% weight loss is observed and is likely due to physisorbed water; the heads were not dried prior to analysis. Important to our quantitative applications is the thermally inactive region observed between 200 and about 500 "C. A t 500 "C a second weight loss hegins; however, this temperature is significantly heyond our region of interesL A control sample of 21.0 wt % "$1 on glass heads lost 21.1 wt % during TGA analysis,indicating that the glass heads are inert for calibration purpcees. TGA was also used to evaluate the mixing efficiency of the mortar and pestle. The homogeneity of the solid mixture used to calibrate our EGAMS response directly affects the precision of our quantification.
-
97.5 Tmpoatvn ('0
Flgure 2. TGA ansiysis of inert glass bead support. Table I. TGA Results on Neat and 130 ppt NaHF, in NaF theoretical
obad wt,
concn, %
w t loss, %
70
1day
100.0
months 5months
100.0 100.0
32.27 32.27
31.19 30.56 30.02
sample neat powder
age 5
32.27
30.6' 0.6' 12%* 130 ppt Std
1dsy 1day 1der 1month
13.0 13.0 13.0 13.0
4.2
3.9
4.2
4.0 4.1 4.0
4.2 4.2
AM
O.lb
15%'
"Mean value.
byr
value.
'Percent error.
The thermal decomposition data obtained by TGA for neat NaHF, and 130 ppt NaHF,/NaF are summarized in Table I. The age of the sample a t the time of the analysis, sample weight, theoretical weight loss (based on the stoichiometry in eq 2), observed weight loss, and percent relative error hetween the observed and theoretical weight loss are listed in the table. The precision observed between individual determinations made on neat NaHF, and that of the 130 ppt standard was both between 2% and 3%. The accuracy of the averaged observed weight logs relative to the theoretical weight loss for neat NaHF, was similar to that observed in the 130 ppt standard, i 5 % . These data indicate that the standards were well mixed and stable over a 1-month period. The consistently low estimation of the ohserved weight logs relative to the theoretical loss is likely due to the reactivity of the H F with the glass, quartz, and other surfaces of the TGA equipment. Nonvolatile, thermally inert impurities might also contribute to the inaccuracy of the TGA measurements. No evidence of water was found in the sodium salts. Determining precision and accuracy is integral to understanding the quality of any chemical measurement (7). The precision of our EGAMS determinations were estimated from the data used to construct the calibration curve according to the procedure outlined by Mandel et al. (8). We assume that the uncertainty of the gravimetric measurements is much smaller than that of the EGAMS analysis. We also assume that errors in determining the area under the curves of ion yield versus temperature during calibration are independent; however, this latter assumption has not heen validated and some concern is justifiable in light of observed chemical effects of reactive decomposition gases on the ion source. Nevertheless, the precision estimate is useful and does not require replicate determinations on unknowns as described hy Shard et al. (9). Replicate determinations are not practical in
804
ANALYTICAL CHEMISTRY, VOL. 61, NO. 8, APRIL 15, 1989 t
n
NH. umc. rhw. = 0.284.
L*
..-
.
I
I
I
I
SO
I
0
0
3
6
9
12 15 Time (min)
18
21
24
Flgure 3. Generation of ammonia during EGAMS analysis of NH,Br.
Table 11. Data Used during NRs and €IF Calibration of Mass Spectrometer Response
NHS calibration signal area, arb units mass NH,, 15.32 9.47 1.18 2.99 0.85
3.52 2.45 0.52 2.13 0.67
pg
-
HF calibration signal area, arb units mass HF, pg 14.96 14.34 6.28 5.46 1.40
5.76 5.76 2.40 2.40 1.06
EGAMS analyses for several reasons: (1)the time for analyses including instrument tuning requires about 20 min for each calibration run and each unknown requires approximately 1 h; (2) EGAMS is a destructive analytical technique, therefore, separate samples are required for each replication; (3) the heterogeneity of the sample studied by using EGAMS often complicates interpreting the sources of error. Mandel's procedure eliminates the need for replicate determinations from several samples while allowing the evaluation of sample homogeneity when necessary (8). The high-resolution extracted-ion profile of NH3(mlz 17.03 dalhns) from the decomposition of NHIBr is shown in Figure 3. The theoretical concentration of NH3 is determined from the dilution factor and 100% decomposition of the NH4Br is assumed. Table I1 presents the data used to calibrate the response of the mass spectrometer. Presented are the signal areas obtained and theoretical weight of NH3 evolved from each calibrant. A least-squares fit of the data presented in Table I1 allows the determination of the weight of NH3 evolved from an unknown and, importantly, a graphical determination of the precision of a measurement on an unknown can be estimated by using the procedures of Mandel et al. (8). To estimate one standard deviation of the unknown determinations, confidence curves (a= 0.68) were generated about the fitted data. Several internal checks on the entire procedure were conducted by an operator who did not know the concentrations at the time of analysis. The theoretical concentrations of N H , the observed concentration of NH3, and the standard deviation of the method are given in Figure 3. The percent error associated with the observed NH3 concentration determination relative to the theoretical concentration is also Table 111. Hydrogen Fluoride Released during the EGAMS of Irradiated ETFE Electrical Wire Insulation
HF released, wt 90 supplier A A
B B C C
15 min isotherm 350
construction componenta
below 200 " C
200-250 "C
250-300 " C
300-350 "C
"C
total
jacket insulation jacket insulation jacket insulation
0.01 0.02 0.01 0.01
