Some time ago there was a discussion about the two methods for using EPMA to determine water in hydrated glasses. First there is the method for simply adding up all the elements (and their associated stoichiometric oxygen), and then subtracting that sum from 100 to obtain the so called water by difference. This water is then included in the matrix correction.
Note that one *cannot* simply perform this water by difference calculation in a spreadsheet, as the value of H2O obtained by difference needs to be iterated in the matrix correction for the other elements. Why? Because the oxygen (in the water) absorbs the emission lines of the other elements, particularly Si. Think about it: by not including the H2O (by difference) in the matrix correction, the Si ka absorption correction will be underestimated for the simple reason that oxygen absorbs Si Ka more than silicon does!
This water by difference and the need for including it in the sample matrix correction is nicely demonstrated in the following post:
http://smf.probesoftware.com/index.php?topic=11.msg235#msg235
The other method is to calculate water in glass as suggested by Barbara Nash, where one measures oxygen in addition to the other cations, and then subtracts the oxygen calculated by stoichiometry (for the measured cations) from the measured oxygen to obtain an oxygen excess or deficit, that can then be converted into OH or H2O as desired. This method is described in more detail here (starting with point 9):
http://smf.probesoftware.com/index.php?topic=307.msg5427#msg5427
But this morning I decided to take the above water by measured oxygen run file, containing the data for the Withers glasses, and modify the calculation in Probe for EPMA to ignore the measured oxygen and instead perform the first method described above for water by difference (and included in the matrix correction).
First I made a copy of the Withers run and disabled the measured oxygen channel in the Elements/Cations dialog using the Disable Quant checkbox. Then I added oxygen as a specified (not measured) element, also in the Elements/Cations dialog. Then in the Calculation Options dialog I specified hydrogen by difference and then clicked the Calculate With Stoichiometric Oxygen option.
That done, here are the results for the first Withers glass which has little or no water (all analyses 15 keV, 10 nA, 20 um):
Un 12 Withers-NSL, Results in Oxide Weight Percents
ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O-D H2O O SUM
514 5.763 4.302 .213 -.046 .111 .163 4.122 .033 .156 74.653 10.439 .013 --- .080 .000 100.000
515 5.119 4.352 .234 -.079 .108 .239 4.144 .109 .212 74.253 10.410 .009 --- .890 .000 100.000
516 5.508 4.109 .215 .054 .038 .228 4.135 .082 .210 74.471 10.473 .018 --- .459 .000 100.000
517 5.285 4.351 .291 -.009 .164 .168 4.120 .122 .166 74.723 10.499 .013 --- .106 .000 100.000
518 5.686 4.351 .234 .018 .069 .152 4.157 .114 .163 74.671 10.435 .014 --- .000 .000 100.063
519 5.560 4.501 .153 .007 .091 .168 4.087 .082 .185 74.357 10.382 .015 --- .412 .000 100.000
520 5.889 4.450 .210 .011 .097 .249 4.054 .088 .180 74.729 10.422 .012 --- .000 .000 100.393
521 5.795 4.548 .210 .022 .085 .222 4.128 -.004 .173 74.646 10.426 .023 --- .000 .000 100.273
522 5.771 4.466 .221 -.034 .056 .211 4.090 .069 .175 74.947 10.506 .016 --- .000 .000 100.494
523 5.578 4.507 .288 -.046 .079 .217 4.018 .049 .185 74.065 10.424 .014 --- .621 .000 100.000
524 5.697 4.485 .231 .031 .084 .260 4.085 .080 .185 74.333 10.413 .011 --- .104 .000 100.000
AVER: 5.605 4.402 .227 -.006 .089 .207 4.104 .075 .181 74.532 10.439 .015 --- .243 .000 100.111
SDEV: .231 .126 .038 .040 .033 .038 .041 .037 .018 .258 .038 .004 --- .306 .000 .185
Note that it calculated 0.243 +/- 0.306 H2O. So that is a zero within statistics and the measured water by FTIR was 0.13 wt%, so again, within statistics. I say within statistics, because being within 0.1 wt% absolute is what we call "spurious accuracy"! :)
For N1 glass we obtain:
Un 13 Withers-N1, Results in Oxide Weight Percents
ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O-D H2O O SUM
526 5.428 4.385 .258 .067 .082 .233 4.247 .091 .196 72.389 10.504 .023 --- 2.096 .000 100.000
527 5.779 4.227 .248 .014 .037 .277 4.139 .077 .208 72.751 10.459 .016 --- 1.767 .000 100.000
528 5.849 4.808 .161 .090 .076 .212 4.214 .053 .176 72.137 10.455 .023 --- 1.747 .000 100.000
529 5.214 4.672 .242 .054 .052 .212 4.123 .098 .198 72.407 10.434 .024 --- 2.271 .000 100.000
530 5.664 4.566 .245 -.009 .092 .266 4.215 .046 .206 72.259 10.506 .017 --- 1.927 .000 100.000
531 5.366 4.392 .239 .010 .064 .359 4.189 .046 .185 72.042 10.500 .015 --- 2.592 .000 100.000
532 5.343 4.509 .186 -.003 .039 .250 4.179 .029 .201 72.244 10.492 .018 --- 2.514 .000 100.000
533 5.272 4.460 .231 .014 .095 .223 4.124 .091 .208 72.276 10.562 .014 --- 2.431 .000 100.000
534 5.343 4.409 .229 .006 .055 .294 4.112 .054 .189 72.349 10.513 .019 --- 2.429 .000 100.000
535 5.067 4.216 .242 -.017 .108 .196 4.156 .102 .194 72.522 10.447 .016 --- 2.752 .000 100.000
536 5.646 4.537 .169 -.022 .123 .282 4.183 .096 .227 72.480 10.461 .017 --- 1.800 .000 100.000
537 5.281 4.385 .172 .009 .069 .261 4.143 .087 .187 72.534 10.433 .015 --- 2.423 .000 100.000
AVER: 5.438 4.464 .219 .018 .074 .255 4.169 .073 .198 72.366 10.480 .018 --- 2.229 .000 100.000
SDEV: .242 .170 .035 .035 .027 .045 .043 .025 .013 .193 .039 .003 --- .350 .000 .000
so a little higher than the expected 1.16 wt % by FTIR. For the N3 glass we obtain:
Un 14 Withers-N3, Results in Oxide Weight Percents
ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O-D H2O O SUM
538 5.172 4.310 .269 .003 .117 .261 4.034 .089 .181 72.031 10.287 .018 --- 3.226 .000 100.000
539 5.383 3.983 .213 .005 .097 .256 4.059 .105 .178 71.788 10.252 .024 --- 3.659 .000 100.000
540 5.213 4.288 .213 .031 .029 .196 3.988 .092 .208 72.293 10.276 .027 --- 3.147 .000 100.000
541 5.796 4.384 .215 .049 .027 .250 4.110 .060 .213 71.777 10.212 .020 --- 2.887 .000 100.000
542 5.215 4.571 .210 .084 .092 .234 4.032 .044 .176 72.471 10.287 .024 --- 2.562 .000 100.000
543 5.974 4.128 .188 .047 .045 .250 4.118 .106 .186 72.015 10.254 .016 --- 2.672 .000 100.000
544 5.324 4.479 .248 .003 .116 .109 3.980 .088 .199 71.832 10.241 .019 --- 3.361 .000 100.000
545 5.356 4.092 .164 -.004 .066 .201 4.122 .013 .216 72.204 10.257 .020 --- 3.293 .000 100.000
546 5.303 4.122 .224 .029 .055 .174 4.095 .075 .185 72.459 10.249 .020 --- 3.011 .000 100.000
547 5.487 4.422 .213 -.028 .059 .201 4.037 .023 .179 71.855 10.223 .011 --- 3.318 .000 100.000
548 5.259 4.382 .261 .035 .100 .245 4.053 .100 .186 72.281 10.242 .024 --- 2.832 .000 100.000
549 5.385 4.193 .237 -.050 .090 .131 4.108 .134 .222 72.044 10.227 .022 --- 3.258 .000 100.000
AVER: 5.405 4.280 .221 .017 .074 .209 4.061 .077 .194 72.087 10.251 .020 --- 3.102 .000 100.000
SDEV: .244 .177 .029 .036 .032 .050 .049 .036 .017 .251 .024 .004 --- .317 .000 .000
which is quite close to the FTIR value of 3.30 wt%. For the N3.35 glass we obtain:
Un 15 Withers-N3.35, Results in Oxide Weight Percents
ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O-D H2O O SUM
550 5.296 4.343 .229 -.078 .096 .283 4.280 .093 .208 70.317 10.258 .013 --- 4.663 .000 100.000
551 4.849 4.032 .170 -.009 .108 .251 4.249 .086 .181 70.492 10.184 .014 --- 5.394 .000 100.000
552 5.240 4.328 .213 -.050 .108 .212 4.212 .075 .178 70.826 10.242 .014 --- 4.403 .000 100.000
553 4.964 4.326 .218 .046 .084 .207 4.156 .087 .199 70.550 10.246 .014 --- 4.904 .000 100.000
554 5.336 4.339 .213 -.009 .033 .174 4.251 .045 .179 70.821 10.330 .014 --- 4.274 .000 100.000
555 5.670 4.488 .229 .032 .086 .310 4.193 .124 .225 70.945 10.261 .011 --- 3.428 .000 100.000
556 5.011 4.152 .213 .018 .063 .278 4.184 .106 .170 70.532 10.387 .016 --- 4.869 .000 100.000
557 4.997 4.268 .240 .021 .046 .261 4.248 .142 .214 70.633 10.233 .022 --- 4.674 .000 100.000
558 5.600 4.252 .156 -.037 .078 .262 4.164 .094 .170 69.915 10.287 .018 --- 5.041 .000 100.000
559 4.658 3.848 .208 -.004 .067 .076 4.304 .149 .211 70.511 10.255 .011 --- 5.705 .000 100.000
560 5.169 4.583 .215 .013 .092 .180 4.327 .096 .193 70.203 10.303 .027 --- 4.599 .000 100.000
561 5.279 4.671 .153 .016 .076 .142 4.218 .121 .202 70.217 10.314 .017 --- 4.574 .000 100.000
AVER: 5.172 4.302 .205 -.003 .078 .220 4.232 .101 .194 70.497 10.275 .016 --- 4.711 .000 100.000
SDEV: .295 .225 .029 .036 .023 .068 .054 .029 .018 .297 .053 .005 --- .571 .000 .000
which is a little higher than the 3.51 wt% from FTIR. And for N4.6 we obtain:
Un 16 Withers-N4.6, Results in Oxide Weight Percents
ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O-D H2O O SUM
562 4.976 4.283 .234 -.027 -.009 .338 4.011 .100 .184 71.539 10.240 .017 --- 4.115 .000 100.000
563 5.113 4.355 .180 .012 .115 .240 3.965 .106 .196 71.564 10.260 .020 --- 3.874 .000 100.000
564 5.773 4.116 .215 .042 .107 .228 3.967 .145 .207 71.525 10.220 .019 --- 3.436 .000 100.000
565 4.816 4.508 .223 .025 .064 .191 4.115 .090 .211 71.138 10.202 .019 --- 4.400 .000 100.000
566 5.336 4.332 .156 .051 .082 .272 3.943 .095 .226 70.918 10.280 .025 --- 4.284 .000 100.000
567 4.879 4.218 .234 .000 .048 .163 3.973 .060 .195 71.469 10.341 .019 --- 4.401 .000 100.000
568 5.053 4.147 .218 .019 .037 .191 3.909 .068 .198 70.828 10.319 .020 --- 4.993 .000 100.000
569 4.963 3.944 .259 -.019 .080 .245 3.892 .097 .198 70.910 10.234 .022 --- 5.174 .000 100.000
570 5.395 4.282 .250 -.049 .067 .114 3.891 .078 .201 71.323 10.241 .015 --- 4.191 .000 100.000
571 4.943 4.125 .256 -.018 .152 .153 3.831 .083 .214 71.425 10.176 .015 --- 4.646 .000 100.000
572 4.615 4.141 .251 .029 .033 .278 3.870 .096 .175 71.119 10.214 .018 --- 5.161 .000 100.000
573 5.047 4.244 .232 .031 .078 .174 3.939 .075 .221 71.371 10.242 .022 --- 4.323 .000 100.000
AVER: 5.076 4.225 .226 .008 .071 .216 3.942 .091 .202 71.261 10.247 .019 --- 4.417 .000 100.000
SDEV: .304 .144 .031 .031 .042 .063 .074 .022 .015 .268 .047 .003 --- .517 .000 .000
which is quite close to the FTIR value of 4.11 wt%. And finally for N5 we get:
Un 17 Withers-N5, Results in Oxide Weight Percents
ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O-D H2O O SUM
574 4.151 4.261 .205 .033 .086 .180 4.044 .078 .227 70.035 10.241 .028 --- 6.430 .000 100.000
575 4.936 4.282 .207 .018 .084 .207 4.018 .102 .207 70.694 10.169 .020 --- 5.056 .000 100.000
576 4.308 3.994 .181 .036 .100 .229 4.039 .108 .180 70.560 10.079 .010 --- 6.178 .000 100.000
577 4.622 4.275 .259 .046 .110 .164 4.078 .075 .164 70.409 10.304 .008 --- 5.486 .000 100.000
578 4.645 4.219 .259 -.015 .083 .120 4.092 .104 .162 70.132 10.171 .024 --- 6.004 .000 100.000
579 4.725 4.288 .240 -.088 .070 .207 4.111 .106 .182 70.305 10.285 .018 --- 5.550 .000 100.000
580 4.362 4.254 .224 .018 .088 .169 4.030 .070 .184 70.304 10.113 .007 --- 6.177 .000 100.000
581 5.017 4.163 .248 -.021 .120 .196 3.992 .064 .212 70.749 10.158 .018 --- 5.083 .000 100.000
582 4.488 4.222 .164 -.028 .080 .224 4.122 .065 .167 70.989 10.186 .024 --- 5.297 .000 100.000
583 4.580 4.138 .218 -.069 .033 .246 4.080 .040 .199 70.575 10.218 .014 --- 5.726 .000 100.000
584 4.596 4.073 .213 .004 .033 .207 4.028 .083 .185 70.887 10.096 .015 --- 5.579 .000 100.000
585 4.022 4.346 .191 .033 .071 .202 4.060 .079 .224 70.688 10.186 .019 --- 5.879 .000 100.000
AVER: 4.538 4.210 .217 -.003 .080 .196 4.058 .081 .191 70.527 10.184 .017 --- 5.704 .000 100.000
SDEV: .294 .101 .030 .043 .026 .034 .039 .021 .022 .295 .070 .007 --- .442 .000 .000
which is also close to the FTIR value of 5.06 wt%. Here a summary of these glasses which contain water measured by FTIR of the following values:
glass FTIR EPMA (H2O by diff)
NSL 0.13 0.24
N1 1.16 2.23
N3 3.30 3.10
N3.35 3.51 4.71
N4.6 4.11 4.42
N5 5.06 5.70
I think this water by difference method is useful, but one does need to account for all minor (and trace?) cations (and CO2?), and also the FeO/Fe2O3 ratios for correctly calculating the stoichiometric oxygen from the cations.
john
By the way, the water by difference calculations shown above (that is, *not* estimating water by measuring oxygen), were performed without the blank correction (which was used to improve oxygen accuracy, based on the NIST mineral glasses, for the water by oxygen measurement method).
And in case anyone is interested in playing with this data themselves, I've attached the MDB files for both methods below. Also since the original data file was from 2008, it does not contain the standard compositions as is the case for more recent MDB files. So I'm also attaching my standard.mdb file for reprocessing. So you should browse to this standard database first from the Standard | Select Standard Database menu, before opening either of the two Withers glass files.
Let me know if you have any questions.
john
This may be a subtle point but I think it could be important when measuring very small water contents using the WBD (water by difference) method.
When measuring very small concentrations of water, the totals will tend to be very close to 100% if all cations are measured. In fact if zero water was present and all the traces measured and stoichiometric oxygen is properly specified, the average total of all elements should be close to 100%. And of course some points will total less than 100%, and some more than 100% just due to statistics.
For points with totals less than 100% we perform the WBD method which gives us a positive concentration of water. If the total is more than 100%, one generally does not calculate water by difference- but when averaging the water content for all points, this procedure causes a bias in the average water by difference because we are adding in the statistical variance for low totals but not the variance for high totals. This will bias our small concentrations of water in a positive (higher concentration) direction when the WBD is performed and the points are subsequently averaged. See here:
Un 12 Withers-NSL, Results in Oxide Weight Percents
ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O-D H2O O SUM
514 5.763 4.302 .213 -.046 .111 .163 4.122 .033 .156 74.653 10.439 .013 --- .080 .000 100.000
515 5.119 4.352 .234 -.079 .108 .239 4.144 .109 .212 74.253 10.410 .009 --- .890 .000 100.000
516 5.508 4.109 .215 .054 .038 .228 4.135 .082 .210 74.471 10.473 .018 --- .459 .000 100.000
517 5.285 4.351 .291 -.009 .164 .168 4.120 .122 .166 74.723 10.499 .013 --- .106 .000 100.000
518 5.686 4.351 .234 .018 .069 .152 4.157 .114 .163 74.671 10.435 .014 --- .000 .000 100.063
519 5.560 4.501 .153 .007 .091 .168 4.087 .082 .185 74.357 10.382 .015 --- .412 .000 100.000
520 5.889 4.450 .210 .011 .097 .249 4.054 .088 .180 74.729 10.422 .012 --- .000 .000 100.393
521 5.795 4.548 .210 .022 .085 .222 4.128 -.004 .173 74.646 10.426 .023 --- .000 .000 100.273
522 5.771 4.466 .221 -.034 .056 .211 4.090 .069 .175 74.947 10.506 .016 --- .000 .000 100.494
523 5.578 4.507 .288 -.046 .079 .217 4.018 .049 .185 74.065 10.424 .014 --- .621 .000 100.000
524 5.697 4.485 .231 .031 .084 .260 4.085 .080 .185 74.333 10.413 .011 --- .104 .000 100.000
AVER: 5.605 4.402 .227 -.006 .089 .207 4.104 .075 .181 74.532 10.439 .015 --- .243 .000 100.111
SDEV: .231 .126 .038 .040 .033 .038 .041 .037 .018 .258 .038 .004 --- .306 .000 .185
The lines in red have totals greater than 100% so no water by difference was calculated. But if we want an accurate *average* of the WBD, then we really should calculate a "negative" water concentration for these high total points, which then can be included in the calculation of the average water by difference.
Does this make sense? It's a little bit analogous to the problem when measuring a zero concentration element. Simply due to statistics, some trace element points will give a small positive number and some points will give a small negative number. Both positive and negative results are equally valid for averaging the concentration. If one insists on zeroing out negative k-ratios or concentrations (as some do), this will bias the average trace concentration in a positive (higher concentration) direction, because only the values zero or higher will be averaged.
This is not a problem for the WBD method if the totals for all points are less than 100% (just as it's not a problem to zero negative k-ratios or concentrations, if all trace element values are greater than zero!), but it could be a problem for glasses that contain zero or very small amounts of water.
I'm going to try this "negative water by difference" correction when I get home from EMAS and will let you all know what I find. Please chime in with your own thoughts!
john
Edit by John: come to think of it, this point about "negative" WBD concentrations for more accurate averaging, also applies to *any* element by difference (not just hydrogen!), if the concentration of that element by difference approaches zero...
After taking a nice slow walk around the "old town" of Konstantz, I got back to the hotel and decided to try the "negative" water by difference idea, and here are the results for the same sample in the previous post:
Un 12 Withers-NSL, Results in Oxide Weight Percents
ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O-D H2O O SUM
514 5.763 4.302 .213 -.046 .111 .163 4.122 .033 .156 74.653 10.439 .013 --- .080 .000 100.000
515 5.119 4.352 .234 -.079 .108 .239 4.144 .109 .212 74.253 10.410 .009 --- .890 .000 100.000
516 5.508 4.109 .215 .054 .038 .228 4.135 .082 .210 74.471 10.473 .018 --- .459 .000 100.000
517 5.285 4.351 .291 -.009 .164 .168 4.120 .122 .166 74.723 10.499 .013 --- .106 .000 100.000
518 5.685 4.350 .234 .018 .069 .152 4.157 .114 .163 74.665 10.434 .014 --- -.055 .000 100.000
519 5.560 4.501 .153 .007 .091 .168 4.087 .082 .185 74.357 10.382 .015 --- .412 .000 100.000
520 5.883 4.449 .210 .011 .097 .249 4.052 .088 .180 74.692 10.414 .012 --- -.338 .000 100.000
521 5.791 4.546 .210 .022 .085 .222 4.127 -.004 .173 74.620 10.421 .023 --- -.236 .000 100.000
522 5.764 4.464 .220 -.034 .056 .211 4.087 .069 .175 74.901 10.496 .016 --- -.425 .000 100.000
523 5.578 4.507 .288 -.046 .079 .217 4.018 .049 .185 74.065 10.424 .014 --- .621 .000 100.000
524 5.697 4.485 .231 .031 .084 .260 4.085 .080 .185 74.333 10.413 .011 --- .104 .000 100.000
AVER: 5.603 4.401 .227 -.006 .089 .207 4.103 .075 .181 74.521 10.437 .015 --- .147 .000 100.000
SDEV: .230 .126 .038 .040 .033 .038 .042 .037 .018 .246 .037 .004 --- .412 .000 .000
As you can see, the lines in red had totals higher then 100%, so the water by difference becomes "negative" thus slightly lowering the average WBD. The average water by difference for this Withers glass is now a little closer to the FTIR value of 0.13 wt%. In fact it's so close I'm calling it "spurious accuracy". That is: too good to be true... either way, all within a single standard deviation of 0.13 wt% H2O.
john
Hello hydrous glass analysists,
I have been studying volatiles in magmatic systems and silicate glasses for my PhD work at University of Oregon. One thing that I have been discussing with Probeman about is how to best apply a water-by-difference correction to hydrous silicate glasses. I have two comments on this front (apologies if these topics have been mentioned previously):
1) Water in silicate glasses does not necessarily occur as H2O (molecular water). At high temperature (magmatic), water is actually incorporated into glass as hydroxyl (OH). During cooling, or re-hydration of glasses at lower temperatures, then molecular water dominates. Below is a figure showing the OH fraction of total water in rhyolitic glasses as measured from OH vs H2O vibrational spectroscopy (FTIR). I've colored in temperature regimes that one might expect for different processes.
Water speciation in rhyolite, modified from Ihinger et al.1999
(https://smf.probesoftware.com/gallery/1344_05_02_20_4_27_25.jpeg)
The final OH vs molecular H2O speciation in measured magmatic glass will be dependent on magmatic temperature and water re-speciation during cooling (cooling rate is important). Glass re-hydration during hydrothermal circulation may also occur, where mostly molecular water (H2O) is incorporated. These factors should be considered when choosing an approach for water-by-difference because different H:O ratios can be specified when doing water corrections.
H:O can be edited in the sequence shown below: From the Analyze! Window > select "Elements/Cations" button > select "h" element > enter # of H / O in "Cations / Oxygens" windows for the number of H you want per each O. Example: 1:1 for OH, 2:1 for H2O, 3:2 for 50% OH + 50% H2O [H3O2, which might be realistic for some rapidly quenched, hydrous magmatic glasses]
(https://smf.probesoftware.com/gallery/1344_05_02_20_5_09_56.jpeg)
(https://smf.probesoftware.com/gallery/1344_05_02_20_4_25_41.jpeg)
(https://smf.probesoftware.com/gallery/1344_05_02_20_4_26_25.jpeg)
2) [this has been discussed previously in this topic] When doing any water-by-difference correction, water is added until the analysis total = 100%. This effectively assumes that your sample consists entirely of the elements analyzed (+ oxygen) and water (in whatever H:O form you choose to include it). However, we all know that natural samples can contain appreciable amounts of trace elements, which are often not measured. Depending on the sample type, these unmeasured trace species may end up totaling to 0.5 to >1.0 wt% total, so that the maximum sample total % we should expect to see would be ~99 wt% rather than 100 wt%. Consequently, the water-by-difference approach to sum to 100 wt% total ignores these unmeasured trace elements, and would OVER-CORRECT the amount of water in your glass.
The best technique would be to include actually SPECIFY the amount of water expected in your sample (know independently through FTIR, SIMS, or otherwise), which avoids the issue I just laid out (see "Specified Concentrations" button in the Analyze! panel).
However, water content of glasses is often not independently known. In these cases, for hydrous glasses with > 2wt% water likely present, then the water-by-difference correction to 100 wt% still probably does more good than harm (particularly for very hydrous samples).
Hope this was helpful!
Allan Lerner