Hi Fernando,
To cut to the chase: L-type monochromator are better to my opinion.
L-type stands for larger area monochromator. They allow for a higher collection rate of X-ray (bigger solid angle), yet they fully preserve a peak-to-background ratio equivalent to any “normal” 140 mm Rowland circle spectrometer.
H-type spectrometer (smaller R circle) offers certainly a higher X-ray count rate, but this includes also a higher count rate on the background. Hence the peak-to-background ratio is LOWER than the achievable peak-to-background ratio on L-type
monochromator. As you mention, there is also a loss of energy resolution, hence a higher risk of peak or background interference issues. I have a few data on my paper on REE phosphate analysis, comparing notably REE analysis on LiF-L vs. LiF-H, or Pb on PET-L,
PET-VL [Cameca only] or PET-H (check notably table 5 and figure 2):
On another note, there has been several reports on H-type monochromator being more likely prone to fracturing over time. As the Rowland circle is smaller, the monochromator must be bent more (= more tension on the 2d-lattice and risk of failure).
Other important point: H-type spectrometer have some limitation on their range (~85 to 235 mm on H-type spectrometer against ~70 to 250 mm for regular 4-crystal spectrometer or for L-type monochromator). There are thus several X-ray lines you
cannot reach. For instance, Ti on PET-H is just at the limit and you cannot place a low background (but this is not a problem if you use a mean atomic number background correction), and you cannot reach Si Ka on TAP-H (not that I would do this anyway in silicate
as the count rate will be too way too high > 50k cps).
L-type monochromator comes to a price, but you will not regret your purchase.
Cheers,
Julien
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Dr. Julien Allaz
SEM/EPMA lab manager
Department Erdwissenschaften
Inst. für Geochemie und Petrologie
ETH Zürich
NW E 84
Clausiusstrasse 25
8092 Zürich
Switzerland
Tel: +41 44 632 37 20
Fax: +41 44 632 16 36
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Hi all,
there is a possibility that we can add two spectrometers to our microprobe, and we are trying to decide which could be the best combination. We perform mostly analyses on geological samples.
I would be grateful if you could tell me the difference between the L and H crystals (e.g. LiFH vs LiFL). The JEOL brochure reads that the L type is for "trace element overlaps in geology" and the H type is for "high
X-ray intensity". As expected, we noticed some loss of resolution when going from the "normal" (e.g. PETJ, LIF) to H crystals (PETH, LIFH) in the spectrometers we already have. Do L-type crystals offer a better resolution than the regular crystals? How about
detection limits?
I will appreciate your comments and experiences.
Thank you very much in advance,
Fernando Colombo
National University of Cordoba
Argentina
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